Plasma Electrolytic Carburizing of Metal and Alloys
Plasma Electrolytic Saturation of Steels with Nitrogen and Carbon
P.N. Belkin1, A. Yerokhin2[1], S.A. Kusmanov1
1Nekrasov Kostroma State University, Kostroma, 156961, Russia
2University of Manchester, UK
Abstract. Studies in plasma electrolysis resulted in development of various surface treatments for metal components. These treatments include formation of protective ceramic layers on some metals (e.g. oxide coatings), saturation of metal surfaces with interstitial elements (e.g. nitrogen, carbon and boron), and plasma electrolytic deposition of extrinsic compounds, heat-treatments (e.g. hardening and annealing), surface cleaning and polishing. The main advantages of plasma electrolytic treatments are high processing speeds and low costs. The treatments enable production of surface nanostructures and local area processing. This review examines recent results in plasma electrolytic carburising, nitriding, and nitrocarburising (as the most common diffusion-based treatments), including treatment modes, electrolyte compositions, structures, and properties of hardened materials. Analysis of the results obtained up to date indicates that pulse plasma electrolytic saturation treatments leading formation of surface nano-structures appear to be the most promising to advance further this type of electrolytic plasma technology. Moreover, electrolytic plasma treatments provide considerable research interest in terms of fundamental science, in particular for development of models of heat transfer on flat vertically or horizontally oriented surfaces and electrochemical processes occurring in the studied systems. These processes include stages of liberation of saturating components, adsorption of active atoms, and their diffusion into the metal surface; therefore understanding associated kinetics and limiting factors is important for gaining proper control over these surface treatments.
Keywords: Plasma electrolysis; Carburising; Nitriding; Nitrocarburising; Phase composition; Protective properties
Contents
1. Introduction 3
2. Physical features of plasma electrolysis 3
2.1. Formation of vapour-gas envelope 3
2.2. Heat exchange in system electrolyte–envelope–metal electrode 8
3. Electrochemical features of plasma electrolysis 11
4. Carburising 13
4.1. Electrolyte compositions 13
4.2. Structure and phase composition of carburised layers 15
4.3. Carbon diffusion kinetics 16
4.4. Tribological behaviour 19
4.5. Corrosion resistance 21
5. Nitriding and nitrohardening 23
5.1. Electrolyte compositions 23
5.2. Structure and phase composition of nitrided layers 24
5.3. Nitrogen diffusion kinetics 26
5.4. Hardness, strength, and toughness of PEN modified steels 27
5.5. Fatigue strength and residual stresses 29
5.6. Tribological behaviour 30
5.7. Corrosion resistance 33
6. Nitrocarburising 35
6.1. Electrolyte compositions 35
6.2. Structure and phase composition of nitrocarburised layers 38
6.3. Mechanical properties 42
6.4. Tribological behaviour 43
6.5. Corrosion resistance 44
7. Future challenges 48
8. Conclusions 50
Acknowledgements 51
References 52
1. Introduction
Various techniques of electrolytic-plasma surface treatment technology were classified in the review by Yerokhin et al. [1] who also discussed physical and chemical features associated with phenomena and obtained results. By now, a number of research papers on plasma electrolytic treatment (PET) has increased significantly. For this reason, the analysis of relatively new publications seems topical.
Cathode and anode plasma electrolytic carburising (PEC) processes have been studied since the 1960s [2], and later applied for industrial use. Anode PEC units were produced by the Academy of Sciences of Moldova and used industrially in Russia, Ukraine, Belarus, Kazakhstan, Moldova and Romania [2, 3]. A new step in development of plasma electrolytic saturation (PES) was provided by pulse technology which enabled formation of nanostructured layers on the surface of metallic substrates. Fundamentals of plasma-assisted cathode electrolysis, including pulse current modes, electrolyte compositions and mechanical and electrochemical properties of resulting coatings were reviewed by Aliofkhazraei et al [4]. Selected physical, chemical and technological features of PES were discussed elsewhere [3, 5, 6].
In this article, we review examples of PES of steels. In Section 2, we consider key concepts and physical fundamentals of electrolytic plasma initiation and its existence while no dielectric film is formed on the electrode surface and no substances are deposited from the electrolyte on the substrate. In Section 3, we analyse oxidation and anode dissolution of steel workpieces. We cover PEC conditions, characteristics and properties of carburised layers in Section 4 which is the main section of the review. Sections 5 and 6 are devoted to the nitriding and nitrocarburising of steels. Finally, Section 7 discusses challenges and development prospects of PEC.
2. Physical features of plasma electrolysis
2.1. Formation of the vapour-gaseous envelope
Conventional electrolysis is known to be interrupted at a certain critical voltage U1 by formation of a vapour-gas envelope (VGE) around one of the electrodes (Fig. 1) [1]. The main cause of VGE initiation is electrolyte boiling [7] while the gaseous products of electrolysis are secondary consideration. The VGE is formed at the electrode where optimum conditions for electrolyte boiling exist [8], e.g. at the electrode with smaller surface area. Therefore, the first critical voltage U1 is associated with the thermal energy required for local electrolyte boiling around the electrode [2]. Conventional electrolysis shifts to the current oscillation mode when the cell operates as a pulse generator because of instabilities in the VGE which is periodically destroyed by pulsed discharges [9]. Critical field strength causing VGE breakdown determines discharge inception. At this stage, temperature of the electrode with the small surface area does not exceed 100 (C due to the periodic contact renewal between the electrode and the electrolyte. Thus continuous but unstable VGE is a characteristic feature of the oscillation mode.
The second critical voltage U2 determines transition from the current oscillation mode to the stable VGE state, in which current oscillations are relatively small. The steady-state mode is described by decreasing current–voltage characteristics (CVC) typical of stationary electric discharges. The VGE stabilisation occurs in the region U2–U3 while U2 depends sufficiently on the active electrode polarity. The minimal voltage to maintain the true glow discharge is considerably lower on the cathode (160 V) than on the anode (420 V) [8]. It is likely that the marked difference of the voltages can be associated with possible electron emission from the metallic cathode. The CVC falling section is explained as follows. The voltage increase results in evaporation intensification, an increase in VGE thickness and decrease in its electrical resistance [10, 11]. This mode can be realised under conditions of both natural convection and controlled hydrodynamic flow of the electrolyte. As a rule, this stationary mode promoting the electrode heating up to high temperatures is used for PES.
In the case of cathode treatment, a critical voltage U3 corresponds to transformation of the normal glow discharge to intensive arcing with specific low-frequency acoustic emission [1]. In the case of anode treatment, arcing is impossible owing to the absence of a metal cathode providing intense emission of electrons. The mode of stationary anode heating up to maximum temperature of 1000 (C exists in the range of 100–320 V. With increasing voltage, anode temperature drops sharply to 200–250 (C probably due to increased possibility of local contact recovery between the anode and the electrolyte [2].
The situation is different when electrolyte flows through the perforated anode onto the workpiece (cathode) [12]. An intermediate mode appears in the region U2–U2(, where the current decreases but plasma intensity increases when the voltage raises (Fig. 1c). The bulk sample temperature remains relatively low (less than 200–300 (C) due to the simultaneous cooling by the electrolyte solution. However, the local surface temperature around the plasma bubbles is very high. A specific feature of this intermediate mode is a continuous and unstable VGE with local disturbances of its continuity. The oscillation mode can be promoted by application of pulsed current [13].
The intermediate mode is used for cleaning steel surfaces (oxides and contamination) using the perforated anode [12]. Surface cleaning of a steel band moving through electrolyte can be carried out in the current oscillation mode when the bulk metal temperature does not exceed 80–100 (C [14–16]. The authors assume that the cause of an oxide layer (mill-scales) removal is short-term (0.001 s) electrical discharges which provide significant pressure and high temperature gradient in the electrode surface layer.
Regularities of cleaning including formation of micro-craters and spheroids on the workpiece surface are explained by Gupta et al by effects attributed to the collapse of plasma bubbles [5]. These bubbles can melt micro-volumes of processed metal within fractions of a second but these volumes are cooled and quenched after the bubbles collapse. The surface morphology is characterised by two unique features, namely, micro-craters and spheroids. These features are the result of plasma bubble implosion and localised quenching of melted surface layer.
This hypothesis can probably explain the discrete nature of discharges, but needs to be clarified. Gas bubbles can be formed in liquid or in micro-craters on the metal surface between the workpieces and the electrolyte. Electrical field in these bubbles can cause their breakdown and heating of metal surface up to its melting, whereupon the bubble will collapse and the metal micro-volume would be cooled. This process is not possible in a continuous VGE. A stable and continuous VGE can exist only due to its own electrical conductivity. In this case, the VGE will receive the energy to maintain stable interface between the electrolyte and the vapour-gaseous phase. Concentration of positive charges in the bubble in a continuous VGE presumed by the authors of [5] appears to be impossible, the charges will move and discharge on the electrode surface. Formation of bubbles and emergence of electrical discharges in the bubbles are possible only outside the continuous VGE. For example, the VGE continuity may be destroyed by electrolyte moving with sufficient speed. It should be noted that the combination of short-term local overheating of the metal surface with its cooling by the electrolyte could be achieved by switching voltage. This method is proposed for steel hardening with the use of plasma electrolytic quenching [17].
Traditional methods are hardly applicable in experimental studies on physical characteristics of plasma electrolysis because of small VGE thickness. Electrical discharge formation in the cathode VGE is confirmed by the data of optical emission spectrum which contains almost all elements of the system [18]. However, the optical emission spectrum of glow discharge in the anode VGE contains only lines of alkali or alkaline earth metals with low ionization potentials. These elements are not necessary in provision of PES which requires the use of other electrolytes.
Electrical conductivity of the anode VGE in the voltage range of 100–300 V can be provided by anion emission from boiling electrolyte in the VGE followed by transfer of these ions to the anode under the influence of electric field. This conclusion is based on modelling of electrolytic plasmas through film boiling [2, 19]. An electrode (nickel-chromium alloy wire) is immersed in an aqueous solution of ammonium nitrate and heated by the electric current passing it through until a vapour film is formed completely surrounding the electrode. Thereafter, the voltage is applied between the wire and the other electrode that is placed in the electrolyte. In the case of positive wire polarity current flow through the vapour film starts at 40–50 V (ammonium nitrate concentration (25–30 wt.%). The current flow does not change the observed glow color; its emission spectrum is appropriate to that of a hot body with a maximum at the wavelength of 700 nm. Conversely, when the wire is connected as the cathode, an unstable current develops at 30 V, with bright blue flashes at the orange-red background. At 57–58 V, a stationary blue glow is observed, the current increases sharply and the vapour film is broken [20].
Evaporation does not provide electric current. The current through the anode VGE may be provided by means of the field and thermal evaporation of the electrolyte ions [21]. Critical voltage U2 promoting the anion emission is calculated on the base of Gouy–Chapman model and Tonks–Frenkel aperiodic instability [22]. Theoretical dependence of U2 on the electrolyte concentration is confirmed experimentally.
The anion mechanism of the anode VGE conduction accounts for practically all the experimental data. These data include the influence of electrolyte composition on the anode mass change (anodic dissolution and oxidation) [3], heating temperature limit (( 1000 (C) due to limited emissive capacity of the electrolyte [19], and the observed deposition of solution components on the surface of the anode and on the walls of the electrolyser [2]. Discrete current character in the case of small anode surface area was found in [19], which may be related to both low- [23] and high-frequency [19, 21] current pulses.
Some authors believe that the transition from the current oscillation mode to the stable electrode heating can be explained by transition from bubble to film boiling. Then the second critical voltage U2 is associated with the power supplied to the system [1, 5, 16]. However, it should be clarified that the true bubble boiling in this system is impossible. The bubble boiling occurs on the surface heated by an external energy source that is capable of providing sufficient heat flux into the liquid. In the system under study, such a source does not exist. Electrical energy is liberated in the VGE where the electrical resistance is maximum. It is the envelope that appears to be the primary source of heat transferred to the electrode and the electrolyte.
2.2. Heat exchange in the system electrolyte–VGE–metal electrode
Nevertheless, the stationary heating mode is often considered analogous to the film boiling in liquid. Common features of conditions that exist during PES and film boiling are the liquid-vapor interface and the heat flux into the liquid, which ensures the interface stability. Essential difference between the film boiling and the PES is a presence of internal heat sources in the VGE caused by the passage of electric current. Fig. 2 shows the scheme of energy transfer in the electrode region for the stationary PES condition.
Heat transfer in the studied system can be considered with a number of assumptions [11]. Electric energy is dissipated in the bulk of the VGE since its thickness exceeds the mean free path of the particles in the vapour-gaseous medium by two or three orders of magnitude at atmospheric pressure. In the simplest case, it is assumed that the current is described by Ohm's law, that is specific VGE conductivity is constant. A more sophisticated case takes into account the space charge in the anode VGE, which appears to be caused by electrolyte anions. Evidently, the space charge in the VGE affects the quantitative values of calculated current and temperature but does not change the CVC type. A physically justified general description of the current flow through the VGE has not yet been identified. It is shown that the heating conditions of a cylindrical workpiece may be described by two complexes: the dimensionless energy released in the envelope, and the ratio of the densities of heat fluxes from the envelope into the electrolyte and into the sample [24].
For the stationary anode PES, the heat flux per unit area from the VGE to the liquid is described by a linear dependency of ql on the voltage and the electrolyte flow rate along the cylindrical sample [25]. According to the experimental data for anode temperatures 600–910 (C, the heat flux per unit area from the VGE to the electrolyte ranges from 1.3 to 2.4 MW/m2. The share of heat transferred to the electrolyte can reach 90% of the total power consumption [2].
The description of heat exchange between the VGE and the metallic electrode is difficult because of the absence of material surface that transfers heat from the VGE to the anode. Heat transfers from the VGE to the electrolyte and the workpiece; therefore, the maximum VGE temperature is reached not on its boundaries but somewhere in the VGE (Fig. 2) [3]. The mathematical surface of the maximum VGE temperature is analogous to the heating surface which may coincide with the outer workpiece surface at its top. For this reason, the formal use of the heat transfer coefficient from the VGE to the workpiece results in a number of contradictions owing to inhomogeneous distributions of temperature and heat flux on the workpiece surface [26]. Description of the heat transfer using the heat flux density from the VGE to the workpiece qA is more adequate. The method for determining qA is developed by solving the inverse heat conduction problem with the use of stepping regulation [27]. There is also a less accurate method for the determination of qA by numerical differentiation of the experimental temperature distribution on the workpiece surface.
The heat flux density from the VGE to the metal electrode is linearly dependent on voltage and is equal to 0.1–0.21 MW/m2 in the range of 140 to 270 V. The share of heat transferred to the anode is less than 10% of the total power consumption but it can reach 30% in the case of treating cylindrical workpieces.
The share of heat consumed by electrolyte evaporation does not exceed 8% of the total power [28]. In the range of 120 to 280 V, the heat flux density from the VGE to atmosphere qs associated with evaporation depends linearly on the voltage and increases with it from 0.02 to 0.15 MW/m2. An increase in the sample size results in this heat flux density to decrease.
When the end of a cylindrical sample is heated, the heat flux distribution becomes significantly different. The fraction of heat consumed by evaporation can reach 60%. In this case, vapor pathway into the atmosphere is significantly reduced. Conversely, the vapor motion along the sample surface submerged into electrolyte increases, the fraction of molecules returning to the electrolyte, which increases ql and reduces qs.
The knowledge of heat fluxes allows us to calculate the temperature of the treated cylindrical sample as a function of voltage by solving the equation of heat conduction in the VGE [11]. Obtained increase in temperature–voltage characteristics (TVC) is qualitatively consistent with the experimental data up to the voltage range of 240–250 V. Specific conductivity of the VGE was established to increase from 1.66∙10-3 to 2.20∙10-3 S/m with the increment in NH4NO3 concentration from 1 to 3 mol/l. Based on these results, an increasing dependency of the average VGE thickness on the voltage was obtained. The voltage rise results in the increase in energy supplied to the VGE and its volume expansion due to intensified evaporation. Moreover, the calculation confirms the increase in the VGE thickness with the electrolyte temperature rise that leads to a reduction in the sample-anode temperature. Confirmed experimentally [7, 29], this dependence accounts for the need of the electrolyte temperature stabilisation and consequently, the saturation conditions.
Calculations of the VGE profile formed during the treatment of the vertical surfaces were carried out in [30]. The VGE expansion in the vertical direction results in the non-uniform temperature distribution on the workpiece surface, thus leading to non-uniform thickness of the modified layer, differential hardness, and other characteristic features. This disadvantage can be offset by promoting special hydrodynamic conditions that would equalise the temperature distribution on the workpiece surface [31–33] and even raise the temperature of individual sections where an increased hardness is required.
Physical and chemical mechanisms underlying development of electrolytic plasmas are of significant interest to researchers. Qualitatively a different state of the electrochemical system compared to conventional electrolysis allows new information about laws of nature to be obtained. In practical terms, insights into mechanisms of plasma electrolytic modification are important when justifying a selection of treatment modes and electrolyte compositions, as well as developing technological equipment. Depending on the hydrodynamic conditions in the electrolyser, heat transfer regularities in the electrode region affect significantly the liberation of saturating components and their transfer to the workpiece surface. Associated regularities depend on the electrolyte composition which, together with treatment modes, determines the material properties after PES.
The PES temperature is affected by applied voltage, electrolyte composition, temperature and flow rate as well as workpiece shape and size. The composition of saturating medium (VGE) is determined by the electrolyte composition and the PES temperature. Therefore, the main processing parameters influencing anodic treatments comprise PES temperature controlled by voltage, electrolyte composition and flow rate, treatment time and post-saturation cooling method (solution or air).
Physical mechanisms of electrolytic plasmas and their theoretical description are found only for the PES version of steady-state anode heating. However, there are unexplored aspects in this simplest case. For example, effects of electrolyte concentration on the sample temperature are not explained and the reasons behind impossibility of anode heating for some metals (aluminium, tantalum, and zirconium) are not clear.
More complex conditions of transient heating are even less studied. Regimes of thermal cycling found empirically enable to reduce significantly the heat transfer from VGE to the electrolyte and to control the surface temperature of flat components. However, the lack of understanding of the regularities of transient heating makes it impossible to use these developments for treatments of complex shape components. Moreover, the most complex processes are observed at the pulse cathode saturation where short-term discharges migrating along the treated surface provide its local heating followed by cooling. It is assumed that such regimes lead to the formation of nanoscale structures. In this case, the control of the results is carried out only by empirical selection of the pulse current parameters without understanding the mechanisms underlying this phenomenon.
3. Electrochemical features of plasma electrolysis
Anode dissolution and high-temperature oxidation of the sample surface occur in parallel with oxide layer formation [2, 3]. Many regularities of anodic dissolution established for conventional electrolysis remain unchanged under the PES conditions. According to experimental data, iron dissolves more actively in chloride, than in nitrate or ammonium sulfate solutions. Similarly, the nature of anions determines the dissolution rate of titanium [3], chromium [34], and copper [35]. Iron dissolution rate evaluated based on the specimen weight loss due to iron transfer to the electrolyte always increases with voltage and processing temperature.
Pores in the oxide layer aid not only anodic dissolution, but also adsorption of carbon, nitrogen and boron on the surface followed by their diffusion into the material. Fig. 3 shows the surface morphology and elemental composition of the oxide layer formed during anode carburising. Weight of the steel sample always decreases after anodic PES since iron dissolution prevails over its oxidation [36]. It should be noted that oxide formation is also observed during cathodic nitrocarburising for AISI 316 stainless steel in an aqueous electrolyte containing urea [37].
Figure 4 shows dependences of the sample weight and the weight of iron found in the electrolyte on the treatment time [38]. The sample weight loss is less than the amount of iron in solution due to the oxygen transfer from the VGE to the sample. Iron oxidation may occur by the following reaction:
3Fe + 4H2O ( Fe3O4 + 8H+ + 8e– (–0.025 V) (1)
Hereinafter, the potentials are referred to the standard hydrogen electrode. The iron dissolution may occur with the formation of both Fe2+ and Fe3+. However, Fe2+ ions were not found in the solution. It is possible that several electrochemical reactions proceed simultaneously. The anodic reaction of steel may be expressed in the following form [38]:
Fe + 3H2O ( Fe(OH)3 + 3H+ + 3e–. (2)
This reaction cannot proceed in one-step, it is likely to be a combination of one or two anodic and one chemical reactions:
Fe ( Fe2+ + 2e– (–0.44 V) (3)
Fe2+ ( Fe3+ + e– (0.77 V) (4)
Fe3+ + 3OH– ( Fe(OH)3 (5)
Formation of Fe(OH)3 may also occur following reaction (1):
Fe3O4 ( FeO + Fe2O3 (6)
Fe2O3 + 3H2O ( 2Fe(OH)3 (7)
Fe(OH)3 is an idealised description of multiple forms Fe2O3(H2O)x and FeOOH(H2O)x. Proton liberation in reactions (1) and (2) suggests a possibility of electrolyte acidification which is observed experimentally.
Information on dissolution and oxidation of PES treated steels is available only for selected electrolyte compositions. Influence of electrical discharge on erosion of workpieces at cathode PES is also studied insufficiently. Meanwhile, these processes affect very important properties of the surface layers, including wear and corrosion resistance.
4. Carburising
4.1. Electrolyte compositions
In most cases, binary aqueous electrolytes are used for PET. The first component provides to the solution a sufficiently high electrical conductivity for possible electrolyte boiling with VGE formation. The second component is the source of diffusing species (in this case carbon). Known electrolyte compositions for carburising and associated results are shown in Tables 1 and 2.
It can be assumed that carbon-containing compounds in the form of short, easily cleavable molecular chains are more preferable for PEC. For example, dissociation of ethanol occurs in the case of cathode PEC as follows:
C2H5OH + H+ ( C2H5+ +H2O (8)
The C2H5+ cation is then discharged at the cathode:
C2H5++ e– ( C2H5 (9)
Under sparking conditions, ethyl radical dissociates liberating atomic carbon which can be adsorbed on the cathode surface and diffuse into its crystal lattice:
2C2H5 ( 4C + 5H2 (10)
A hypothesis regarding glycerol decomposition in plasma discharge zone with formation of the H2, CO and some fragments of hydrocarbon radicals has been put forward in [44]. Hydrocarbon groups such as CH, CH3 have priority to be absorbed on cathode surface through particle collisions [55]. In the end, the intermediate products are decomposed into C, H reactive species on the sample surface. The active radicals and ionic components accelerated by electric field, bombard and heat the sample, providing enhanced diffusion of atoms in the iron lattice [44].
Chromatographic analysis of the organic volatile components formed in the VGE during anode carburising at 1000 (C in the electrolyte containing glycerol (10%) and ammonium chloride (10%) showed prevailing formation of formaldehyde and methanol with minor amounts of acetaldehyde, acetone and ethanol [53]. Decomposition mechanism of glycerol during anode carburising can be described as follows. The glycerol is oxidised to glyceraldehyde and dihydroxyacetone that are involved into the VGE and decomposed into low molecular weight compounds detected by chromatography. These compounds are absorbed at the anode surface and subjected to electro-oxidation and dissociation to form active atomic carbon.
Apart from the components providing electrical conductivity and supplying carbon, additives are recommended to improve some electrolyte properties. Surfactants (isoamyl alcohol, oxyethylated higher fatty alcohols, etc.) result in significant reduction of current density together with a small decrease in temperature, which provides electric energy savings [56]. To increase viscosity of the electrolyte solution, additives of soluble inert substances, such as gelatine or starch are also proposed [57].
4.2. Structure and phase composition of carburised layers
4.2.1. Low-carbon steels.
The maximum surface carbon concentration (1.3 wt.%) was obtained through cathodic carburising using potassium acetate solution in glycerol [40]. The surface carbon concentration obtained by anode carburising was 0.9 wt.% in aqueous solution of acetone, 0.8, 0.7 and 0.6 wt.% – in solutions of glycerol, sucrose and ethylene glycol, respectively [3, 49]. When the sample was cooled in electrolyte after carburising, a martensite layer with high hardness is formed (Fig. 5) [39]. In the case of air-cooling, a slight quenching or its absence are possible with perlite layer formation which may contain cementite network [3, 49]. The phases usually obtained in carburising (cementite, martensite or pearlite, ferrite and austenite) are confirmed by X-ray diffraction (XRD) analysis [39].
According to XRD results, the oxide layer formed after the anode carburising contains FeO, Fe2O3 and Fe3O4 (Fig. 6). Oxygen distribution in the surface layer determined by proton nuclear backscattering shows that the thickness of the oxide layer is several micrometres; deeper – within tenths of micrometres – an iron zone enriched with its oxides is formed [3, 51, 58].
Anode PEC of powder iron-graphite results in perlite layer formation without carbon diffusion from graphite pores [54]. This layer with thickness of 0.5 mm was obtained after a four-stage anode carburising at 950 (C, each lasting for 5 min, followed by air-cooling.
4.2.2. Low-alloy steel.
Noteworthy are the results of AISI H13 steel carburising using high-frequency voltage. The current at frequency of 10 kHz is shown to reduce the crystal size to the nanometre scale and to improve the wear resistance of the material [43].
4.3. Carbon diffusion kinetics
4.3.1. Low-carbon steels.
Dependence of layer thickness on treatment time is adequately described by a parabolic function (Fig. 7). Cathodic PEC of pure iron in glycerol electrolyte also yields a parabolic dependency of the carburised layer thickness on processing time (line 5, Fig. 7), as derived from data of [39]. These dependencies confirm the diffusive nature of the process and indicate the constant carbon concentration on the sample surface. Furthermore, these data indicate a rather rapid stabilisation of conditions in the VGE, which is an important advantage of PEC.
The layer thickness increases expectedly with the carbon saturation temperature. It is important to note that different electrolyte compositions result in different maximum achievable temperatures of treated sample [59]. Therefore, the thickness of the layer obtained in a glycerol-based electrolyte at the temperature of 1000 (C is not significantly different from the thickness observed after carburising in the electrolyte with acetone at 900 (C, in spite of a higher saturating capacity of the latter.
A layer thickness formed by the anode PEC depends not only on the concentration of the carbon-containing components in the electrolyte but also on dissolution and oxidising rates of the sample material [3, 51]. Increase in concentration of ammonium chloride containing no carbon results in the thickening of carburised layer owing to intensified anodic dissolution and reduction in the thickness of the oxide layer which retards carbon diffusion.
Dependence of the carburised layer thickness on the concentration of carbon components has a maximum which is associated with the competition between the following two processes. When the concentration increases from 0 to 2%, the saturating capacity of the VGE rises because of the flow increase of carbon compounds into the VGE from the electrolyte. Further concentration increase leads to reduction in the carburised layer thickness due to the oxide layer growth owing to the decrease in the rate of anodic dissolution. Accordingly, the current density decreases as the concentration of carbon compounds is increased.
The measured carbon distribution in the carburised layer can be approximated by a well-known equation [36]:
|[pic], |(11) |
where С(x) is the final carbon concentration at depth x, С0 is the initial carbon concentration in steel, СS is the final carbon concentration on the sample surface, D is the carbon diffusion coefficient in steel, t is time, and erf(x) is the Gaussian error function. This equation represents the solution to the problem of nonstationary diffusion for a half-infinite body with the following boundary conditions: С(0, 0) = С0, C(∞, t) = C0, and C(0, t) = CS. Thickness dependence of the carburised layer on processing time as well as СS enables rough estimation of the carbon diffusion coefficient using Eq. (11). The data obtained for different electrolytes containing 10% ammonium chloride and 10% of one of the above mentioned carbon components are presented in Table 3.
The increase in diffusion coefficient of carbon in steel with increasing temperature is common since it follows exponential law. The influence of carbon component on carbon diffusion coefficient can be explained by the change in the thickness of the layer enriched in iron oxides. Oxidising ability of electrolytes determined by XRD analyses according to the intensity of the iron oxide peaks increases in the sequence of acetone – glycerol – sucrose [54]. Reduction in carbon diffusion coefficient with increasing oxygen content proves inhibitory effects of the oxide layer.
The influence of electrolyte composition on the carbon diffusion coefficient highlights different structure and phase composition of obtained layers. For this reason, it is preferable to refer to an effective diffusion coefficient in the multiphase system. Glycerol concentration fall or ammonium chloride concentration rise lead to an increase in the estimated value of carbon diffusion coefficient. These facts confirm both the prevention of carbon diffusion in the bulk of the sample by the surface oxide layer and the influence of electrolyte composition on the oxygen content in it. Increases in both temperature and treatment time also lead to an increase in the carbon diffusion coefficient, but to a different extent. Increasing processing time from 5 to 10 min is more effective than raising sample temperature to 1000 (C to increase both the layer thickness and carbon content in it. The results presented in Table 4 indicate that this fact is due to an increase in the carbon surface concentration, rather than in diffusion coefficient. Substitution of acetone for glycerol results in an increase in the effective diffusion coefficient and higher carbon surface concentrations, which is consistent with the data in Table 3 demonstrating a higher saturation capacity of the acetone electrolyte.
4.3.2. High-carbon steel.
Enhanced carbon diffusion is discovered under the conditions of pulse cathode PEC of high-carbon steel forming a carburised layer containing diamond-like carbon [44]. In this case, the carbon concentration on the sample surface reached 13.25 wt. %. Carbon diffusion coefficient is (1.39(0.14)(10−7 cm2/s. Plasma discharge provides the activation energy reduction of carbon diffusion from 151 kJ/mol for conventional carburising process to 132.8 kJ/mol. More accurate values of carbon diffusion coefficients were obtained using spectral emission analysis of carburised surfaces after PEC (Table 4) [53].
4.4. Tribological behaviour
4.4.1. Low-carbon steel.
Cathode carburising of pure iron is carried out in a solution containing about 10% ammonium chloride and 20% glycerin [41]. The highest hardness is obtained at 750 (C when martensite and cementite are detected in the surface layer by XRD analysis. Cementite is not identified at higher processing temperatures (900 and 950 (C). An increase in the saturation time results in the hardness rise probably owing to carbon content increment due to diffusion. As a rule, their surface roughness Ra increases from 0.18 (m (untreated sample) to 0.25–0.98 (m as a result of cathode carburising; no effect of PEC duration and temperature on the surface roughness is identified.
Anode carburising of low-carbon steel (0.08% C) in a solution containing 10% ammonium chloride and 10% glycerol (treatment mode: voltage 200 V, current 3 A, 950 (C) demonstrates similar results. Surface hardness of cooled in electrolyte carburised layers exceeds 850HV50 owing to martensite phase formation. Microhardness is not significantly affected by treatment temperature and time. In this case, the modified layer thickness increases with treatment time.
Cathode carburising may result in the increment of friction coefficient of the samples. The untreated and carburised samples were tested against 6-mm Al2O3 ball counterparts under 10N load in dry sliding conditions. The mean average friction coefficients increased from 0.6 to 1.0 (maximum value). The wear rate of carburised samples was lower than that of the untreated one.
Anode carburising usually results in decreased surface roughness due to anodic dissolution. For example, surface roughness of mild steel specimens treated at 900 (C decreases from 1.2 (m (untreated sample) to 0.2 (m during the first 2 minutes of carburising and remains constant afterwards [50].
4.4.2 Medium carbon steel.
Anodic carburising improves the mechanical properties of steel [50]. Nominal tensile strength increases from 420(20 MPa (untreated sample) to 930(50 MPa (carburised with quenching). The toughness (impact test) of the material is the same in comparison with the traditional method of heat treatment.
4.4.3. High-carbon steel.
Enhanced carbon diffusion is discovered under the conditions of pulse cathode PEC of high-carbon steel forming a carburised layer containing diamond-like carbon [44]. In this case, the carbon concentration on the sample surface reached 13.25 wt.%. Carbon diffusion coefficient is (1.39(0.14)(10−7 cm2/s. Plasma discharge provides the activation energy reduction of carbon diffusion from 151 kJ/mol for conventional carburising process to 132.8 kJ/mol. More accurate values of carbon diffusion coefficient were obtained using spectral emission analysis of carburised surfaces after PEC (Table 4) [53].
In other circumstances, roughness rise after cathode carburising does not lead to an increase in the coefficient of friction. For example, pulse carburising of T8 steel in a glycerol electrolyte results in the surface roughness increase from 0.1 to 0.3 μm, but the coefficient of dry friction against ZrO2 decreases from 0.4 to 0.3–0.35 [44]. The wear rate of PEC-treated steel is about 1.13(10−5 mm3/Nm which is less than half of that for T8 steel. High wear resistance of PEC-treated steel is caused by high hardness of a diamond-like carbon containing carburised layer and possibly graphitisation of this surface during friction process. Moreover, high oxygen content (18.98 wt.%) confirmed by energy dispersive X-Ray spectroscopy (EDS) analysis indicates that the wear mechanism of PEC-treated steel transforms to oxidation wear. The friction heat causes steel oxidation in the contact region, and iron oxides act essentially as lubricants.
4.4.4. Low-alloy steel.
An increase in wear resistance is achieved using pulse processing. This result is obtained at the cathode carburising of steel AISI H13 in electrolyte containing 50% glycerol and 2% NaOH at 150 V and the pulse duty cycle, 90% [43]. Weight loss was measured at the sliding distance of 250 m. The minimal weight loss equal to 0.022 g was observed for the carburised layer obtained at a frequency of 10,000 Hz. Worse data were obtained at 100 Hz (0.033 g) and at DC (0.028 g). The weight loss of the untreated sample was 0.043 g. It should be noted that the highest temperature of the sample 944 (C was obtained at the frequency of 10 kHz, whereas the frequency of 100 Hz corresponded to 918 (C. Yet lower temperature was achieved at DC for the same voltage of 150 V. These results are consistent with the hardness measurements, where samples processed at high and low frequencies had the maximum and the minimum surface hardness, respectively.
Information available indicates that friction mechanisms of steels subjected to PEC treatments have been studied insufficiently. Typically, cathodic PEC leads to increases in the surface roughness and friction coefficient, but the surface hardness also increases owing to the formation of martensitic layer, providing improved wear resistance. Anode PEC usually results in surface smoothing and causes friction coefficient to decrease, but it is always accompanied by the formation of an oxide layer. In this case, an improvement in the wear resistance is not always associated with increased hardness, being determined by a combination of the hard martensitic sublayer and a porous oxide surface layer which can retain lubricants. In addition, a certain amount of austenite retained after quenching can improve the running-in ability of carburised steels and provide localised plastic deformation in the thin surface layer.
4.5. Corrosion resistance
4.5.1. Low-carbon steels.
Corrosion properties of samples after the anodic PEC are improved. One of the qualitative characteristics of the oxide layer is the corrosion behaviour of samples after processing. The open circuit potential decreases during the first 10 min [36] and then oscillates, which can be associated with the saline passivation and development of diffusion limitations in the porous oxide layer. Polarisation curves indicate a stronger tendency for passivation for the samples cooled in air. The cathode branches contain nearly linear regions, indicating the only the reaction of hydrogen reduction taking place. The absence of such regions in the anode branches can be associated with the formation of a relatively dense surface oxide layer which limits the kinetics of anodic process.
4.5.2. Medium carbon steels.
Treatment of medium carbon steel (0.45% C) in a solution of ammonium acetate provides a positive potential shift for the specimens in 0.5 M sulfuric acid solution [60]. The best results are obtained after oxidation of samples at voltages of 200–240 V, which corresponds to the processing temperature of 850–950 (C. In this case, the sample potential of OCP vs a saturated silver–chloride electrode is in the range of 0.20–0.35 V and the layer conductivity is 10–50 (S/cm. Extension of treatment time over 3 minutes leads to expanding of surface which negatively affects the workpiece corrosion characteristics.
4.5.3. Stainless steels.
Similar results were obtained for anodically carburised stainless steels (austenitic and martensitic) studied by the electrochemical impedance method [61]. Corrosion rate of samples treated in glycerol electrolyte decreased by a factor of 7 (AISI 403) to 30 (AISI 304) with the use of boric acid or borax solutions as the corrosion medium.
The effect of carburising on the corrosion behaviour of three different stainless steels (AISI 304, 316L and 430) was studied in [62] by scanning potentiodynamic and electrochemical impedance spectroscopies. Less compact structures and/or smaller amounts of alloying elements (as occurs with the body centred cubic structure in ferritic AISI 430 steel) are affected stronger by this modification. Conversely, stainless steels with a larger amount of alloying elements and/or more compact structures show only slightly changed corrosion behaviour. Corrosion rates of the carburised samples are slower than of the untreated one, and most of the treated samples maintains the best behaviour for longer periods of time. Carburising of AISI 304 increases the localised corrosion resistance. Increasing applied voltages above 260 V strongly affects the corrosion mechanism by producing a surface layer, which could favour a more passive behaviour. In contrast, carburising produces a harmful effect on AISI 430 by decreasing its pitting corrosion resistance.
The most common component for carburising of steels is glycerol. However, comprehensive studies of the properties of the base-glycerol electrolytes include their saturation ability, resources, and energy consumption does not carry out. There are some positive examples of PEC steels for improving their protective properties, such as resistance to wear and corrosion. Relationship between the test results and structural features of the modified layers must be found for the scientific justification and practical application. Similar, still unresolved, problems arise when studying corrosion behaviour of carburised steels where the increase in their resistance may be due to the formation of the martensitic phase or the presence of iron oxides.
The protective properties of iron oxides are most clearly manifested after the anode PEC of steels. In this case, the oxide layer is easily detectable. A positive role is also presumed for spinel oxides formed during cathode PEC [62]. Another phase leading to increased corrosion resistance is martensite. Currently, PEC regimes and electrolyte compositions contributing to the increase in corrosion resistance have been established for various steels. However, the study of the structural features of surface oxide layers is in the initial stage. The pore size in the oxide layer has been determined and the possibility of its peeling off due to sharp changes in temperature noticed. Nevertheless, the effects of anode PEC conditions on oxide layer structure and its protective properties have not been evaluated comprehensively, which complicates justification of processing modes.
5. Nitriding and nitrohardening
5.1. Electrolyte compositions
Ammonia is widely used in the processes of gas and plasma nitriding as a source of atomic nitrogen. At the specimen surface, ammonia molecules are adsorbed and dissociate by stepwise removal of hydrogen atoms. The adsorbed nitrogen atoms can diffuse into the bulk of the metal [63]. Under the condition of plasma electrolytic nitriding (PEN), volatile ammonia is evaporated from the electrolyte to the VGE and adsorbed on the workpiece surface.
Besides ammonia, some ammonium salts can serve as a source of nitrogen. The feasibility of nitriding of austenitic stainless steel in the aqueous electrolysis of 5 wt.% solution of ammonium carbonate is shown [64]. The hydrolysis of ammonium carbonate is suggested to occur as follows:
(NH4)2CO3 ( 2NH4+ + CO32– (12)
H2O ( H+ + OH–.
At low potentials, these ions are neutralised at the cathode and ammonia and hydrogen are produced. However, at high potentials, plasma generated at the cathode may also contain other ions, like N+, NH+, NH2+, similar to plasma nitriding.
The following reaction paths could lead to formation of iron nitrides via NHx species [65]:
Fesolid +NHx,gas → FeNH2-3 → Fe2-4N + N2 + H2 + NH3. (13)
A mixture of ammonium chloride and nitrate can be used as a source of nitrogen for the anode PEN which is followed by hardening [6]. In this electrolyte, the ammonium salts are hydrolysed to ammonium hydroxide that liberates ammonia to the VGE at the electrolyte boiling temperature [66]:
NH4Cl + H2O ( NH4OH + HCl (14)
NH4NO3 + H2O ( NH4OH + HNO3 (15)
NH4OH ( NH3 + H2O (16)
It should be noted that gaseous nitrogen added into PES process can serve as a source of active nitrogen too [67]. Known electrolyte compositions for nitriding processes and associated results are shown in Tables 5.
5.2. Structure and phase composition of nitrided layers
5.2.1. Medium carbon steel.
The cathode PEN of cast steel S0050A allowed nitride and diffusion layers to be obtained, with thicknesses of 45 and 100 μm, respectively [69]. This treatment was carried out in an aqueous solution of carbamide during 8 min at 280 V (sample surface temperature (600 (C). The optical micrograph of cross-section of a cast iron G3500 nitrided for 5 min at 250 V (sample surface temperature 570 (C) (Fig. 8) shows that the thickness of the nitride layer is 10 μm and that of the diffusion layer – 40 μm, with both layers being not uniform because of the effect of graphite blocking nitrogen diffusion.
No nitride layer was formed at all on the surface areas where the graphite flakes were located. The diffusion of nitrogen was also blocked by graphite flakes underneath the surface layer (i.e., subsurface). It was found that both of the XRD patterns, obtained from the treated cast iron and cast steel, matched the reference of Fe2.4N.
Surface saturation of steel with nitrogen results in both formation of nitrides (or solid solution of nitrogen in iron) and decrease in austenitisation temperature. According to the Fe–N phase diagram, the eutectoid transformation temperature decreases from 727 °C (Fe–C phase diagram) to 700 °C in presence of 0.07 wt.% N and becomes 590 °C when steel contains 0.1 wt.% N or more [63]. Therefore, the saturation with nitrogen at 650–750 °C during several minutes results in austenitisation of the surface layer and formation of nitrogenised martensite in the case of rapid cooling. PEN combines easy with quenching in the same electrolyte; therefore, this process can be termed nitrohardening.
The SEM analysis shows a laminated structure of the steel surface after nitrohardening. It is composed of the following layers (Fig. 9):
– surface oxide layer;
– nitride–martensite layer (including martensite, retained austenite and Fe4N и Fe2–3N nitrides);
– martensite–ferrite layer (including martensite, retained austenite and solid solution of nitrogen);
– original pearlite–ferrite structure of the substrate material.
Formation of nitrides in the surface layer after the PEN treatment was confirmed by XRD analysis, with Fe4N и Fe2–3N nitrides, FeO and Fe3O4 oxides, martensite, retained austenite and ferrite detected. The Fe2–3N phase was detected only after PEN at 750 °C. The anode PEN of the steel samples resulted in thicker nitride layers using the electrolyte containing ammonia and ammonium chloride [6].
Measurements of the surface nitrogen concentration carried out by different methods showed similar results. According to EDS analysis the maximum concentration of nitrogen (5.6 wt.%) was observed beneath the oxide layer and further down into the bulk of the sample [66]. The nitrogen concentration in the nitride zone after PEN in the electrolyte containing ammonia and ammonium chloride was 4.6 wt.% according to the X-ray probe analysis [73] or 7.3 wt.% according to the Auger electron spectroscopy [6].
5.2.2. Stainless steel.
Generally, the microstructure of a nitrided layer formed at low temperatures is similar to that obtained by conventional methods [64]. XRD patterns of plasma treated austenitic stainless steel show diffraction peaks corresponding to nitrides of iron and chromium, i.e. Fe4N, Fe2N and Cr2N. The peak intensity of Fe2N increases with nitriding temperature, which is associated with the gain of the VGE nitriding potential. It should be noted that the processing conditions in this research were not stable, as current variation during the process was random making temperature control difficult. In order to maintain the temperature within the desired range, the voltage was controlled between 135 and 200 V and the electrolyte was not cooled during the treatment.
After the cathode PEN, a 2–3 μm thick iron nitride (FeN0.076) layer was formed in the outmost surface, resulting in hardness values of about 800 HV20 [64]. However, the cathode treatment led to erosion of the sample surface. The surface porosity generated by discharge coarsened when the amount of KOH in the electrolyte has increased from 10g/l to 40g/l.
5.3. Nitrogen diffusion kinetics
Under the stationary PEN conditions, the thickness of the compound layer on low-carbon steels increases parabolically with the treatment time (Fig. 10) [6], which testifies a constant nitrogen concentration on the surface. All layers enlarge with the increase in temperature owing to the intensification of the nitrogen diffusion. Fig. 11 shows the diffusion layer thickness versus treatment time. More precisely, this dependency represents only a part of the diffusion layer containing excessive Fe4N nitrides that were identified through their coagulation after aging of the nitrided samples. The PEN treatment in a chloride-nitrate electrolyte leads to a more developed diffusion zone. It can be assumed that the rate of ammonia evaporation from nitrate solution into the VGE increases with temperature rising to 700 (C and becomes equal to its formation rate. Therefore, the thickness of the nitride layer formed in the chloride-nitrate solution at 650-750 (C remains the same. On the contrary, ammonia evaporation rate from the solution containing ammonia into the VGE increases with temperature monotonically. This explains both increasing in the nitride layer thickness and a relatively low resource of this solution due to the rapid depletion with ammonia in the operation.
5.4. Hardness, strength, and toughness of PEN modified steels
5.4.1. Medium carbon steels.
Similar values of hardness were obtained by PEN of medium carbon steel (0.45% C) followed by quenching in chloride-nitrate electrolyte [66]. The surface microhardness profile is shown in Fig. 12. It is associated with the phase composition of the modified layers. The maximum value of microhardness and the greatest width of the hardened zone were observed after nitriding at 750°C (Fig. 12a). The microhardness of the modified layer increased with the increase in treatment time from 2 to 10 min (Fig. 12b). At various electrolyte concentrations, the maximum values of microhardness and depth of the hardened zone correlated with the thickness of the modified layer (Fig. 12c and d). The best results were obtained after the PEN treatment in a solution of ammonium nitrate (10%) and ammonium chloride (15%) at 750(С for 5 min. This is due to an increase in nitrogen content in the surface layer with increased ammonium chloride concentration in the electrolyte, although the maximum concentration of nitrogen on the surface did not change. Adjustment of quenching temperature after nitrogen saturation may provide additional means of controlling thickness and hardness of the modified surface layer. Decreasing quenching temperature compared to that of nitriding leads to reduction of layer thickness and its maximum microhardness.
Maximum microhardness of the nitrided layer obtained by anode PEN followed by quenching is observed at the some depth where the martensite concentration is maximum [49]. The outer part of the nitride layer contains Fe2–3N nitride and retained austenite with a lower hardness. A similar decrease in hardness is observed for gas-nitrided steels (74(. The PEN temperature rise results in increased surface hardness of medium carbon steels due to the growth of nitrogen concentration in martensite. The surface hardness of the nitrided steel increases with processing time prolonged from 1 to 5 min owing to increases of nitrogen concentration in both nitride and martensite phases. However further saturation with nitrogen does not increase the hardness.
The modified layer thickness can be controlled using the three-step processing that includes heating for the nitrogen saturation, cooling, and second heating for hardening in the same electrolyte. For example, a medium carbon steel sample (0.45% C) was treated in the electrolyte containing 11 wt.% ammonia chloride and 11 wt.% ammonia nitrate at 750 (C (160 V) during 2 min. Further, the voltage dropped to 80 V and the sample temperature decreased to 520 (C during 30 s. Then, before quenching the voltage increased for a short period of time to 160 V and switched off. When the second step decreased from 6.5 s to 2.7 s, the hardened layer thickness reduces from 2.2 mm (900–950 HV50) to 1.05 mm (800–850 HV50).
It was established that the anode PEN of a medium carbon steel sample (0.45% C) leads to an increase in its tensile strength similarly to nitrocarburising followed by quenching [58] (Table 6). Fracture toughness of round samples with U-type notches was measured after three-step PEN in the aqueous solution of ammonium chloride (11 wt.%) and ammonium nitrate (11 wt.%) at 770 (С (130 V) during 2 min. Further, the voltage was dropped to 97 V at the sample temperature decreased to 550 (C during 15 s. Then, before quenching, the voltage was increased to 180 V and after a short exposure, switched off (Table 7). Fracture toughness decreases from 1.11 MJ/m2 (untreated sample) to 0.19 MJ/m2 (one-step PEN) but only to 0.52 MJ/m2 (three-step PEN) with the second heating exposure during 5.2 s.
5.4.2. Stainless steel.
Cathode PEN of austenitic stainless steel in the solution containing 7.5 wt.% ammonium carbonate permits high microhardness of the surface layer to be obtained [65]. Higher concentration of ammonium carbonate did not lead to an increase in hardness due to a decrease in the voltage required to maintain a certain temperature. The voltage decrease results in the reduced velocity of the ions responsible for supplying nitrogen to the sample surface. The highest hardness was obtained at a temperature of 550 (C. The nitride layer thickness increases from 20 μm (478 (C) to 40 μm (379 (C) when the processing time is prolonged from 20 to 60 min.
5.5. Fatigue strength and residual stresses
5.5.1. Endurance limit of nitrided medium carbon steels
The practical importance of nitriding follows from the pronounced increase of fatigue strength, wear and corrosion resistance of medium carbon steels [63], which can be achieved by application of PEN. The anode PEN followed by quenching promotes sufficient increases in the endurance limit of samples with stress concentrators, which is in agreement with the experimental data obtained by other nitriding techniques (63(. Table 8 shows the examples of the anode PEN of medium carbon steel (0.35% C) samples, 10 mm in diameter with various stress concentrators:
1. V-shaped circumferential groove (d/D = 0.8; ( = 60(; R = 0.1 mm);
2. U-shaped circumferential groove (d/D = 0.8; R/d = 0.0625);
3. Transverse hole (d/D = 0.3);
4. Keyway (b ( t ( l = 4 ( 2.5 ( 20 mm);
5. Groove for woodruff (b ( t = 3.5 ( 3.5 mm), cutter diameter is 13 mm.
The samples were saturated with nitrogen and quenched in the electrolyte containing 11 wt.% ammonium chloride and 11 wt.% ammonium nitrate.
5.5.2. Residual stress distribution in the surface layer
The improvement of the fatigue properties of steels after the anode PEN is associated with the increase in their strength and formation of residual compressive stresses in the surface layer [6]. The observed stress state is caused by a combination of thermally induced plastic flow and structural (phase transformation) changes. These include high-speed heating and rapid cooling, formation of nitride phases with differential volume and martensitic transformation. The residual compressive stress is shown to always form in the surface layer when nitrided samples are cooled down in the electrolyte.
The shift in the maximum compressive stress to the depth of the hardened layer is also observed after carburising followed by quenching [75]. Such stress profile has a positive effect on the fatigue strength since the fatigue cracks observed at the tests initiated in the subsurface layer. A similar regularity is found in fatigue tests of the samples that were treated by gas or plasma nitriding [74].
5.6. Tribological behaviour
5.6.1. Medium carbon steels and cast iron.
It was found that the PEN treatment could significantly increase hardness and wear resistance of the cast iron and cast steel [69]. Cathode nitriding was carried out in the aqueous solution of carbamide at voltages of 200–300V, resulting in sample surface temperature of about 400–600 (C. The PEN-treated cast iron (G3500) and steel (S0050A) had about three times higher hardness than the untreated samples of G3500 and S0050A. The treated cast iron and steel had a similar coefficient of frictions that was higher than those of original untreated samples due to increasing of surface roughness (Fig. 13). Lubrication significantly reduced the friction coefficient μ of cast steel S0050A (μ=0.109) to the level of that of the cast iron G3500 (μ=0.105). Under the lubricated testing condition, the PEN treated cast iron and steel samples had only slightly higher friction coefficient μ=0.12, than the untreated samples.
The cast iron G3500 (Fig. 13a) showed a much higher wear resistance than cast steel (Fig. 13c) since graphite in G3500 acted as a solid lubricant. Both of the treated samples G3500 and S0050A (Fig. 13b and d) had an improved wear resistance compared to corresponding untreated substrates (Fig. 13a and c). The wear rates of nitrided samples decreased from 120 to 40 (m3/(N(m) for S0050 steel and from 23 to 8 (m3/(N(m) for G3500 cast iron. It should be mentioned that the surface of cast iron G3500 has become rougher after the EPN treatment, the pin-on-disc test however seemed to make the surface of the wear track smoother rather than deeper. Moreover, the samples G3500 and TG3500 exhibited very low wear rates at the dry testing condition. Particularly at a lubricated testing condition, all the treated and untreated samples appeared to have a negligible wear rate.
SEM micrographs of wear scars of the untreated and treated samples after pin-on-disc dry tests against WC pins show a large fatigue cracking occurred on the wear track of untreated G3500. The cracks were initiated at the edges of graphite flakes by stress concentrations when the WC pin counterface was rubbed against the G3500 sample. After 200 m of sliding, only a small amount of localised peeling was observed. Although cracks developed in the wear track, the cracked material was still attached to it, which appeared to have a low wear rate when the wear track was measured by a surface profilometer. However, this may change after testing at longer sliding distances and the cracked material would detach, causing severe wear. Interestingly, no fatigue cracking but only slight wear tracks and asperity smoothing was observed on the EPN-treated G3500 sample under the same dry testing condition. Severe wear and plastic deformation were observed on the untreated S0050A cast steel (Fig. 13c). The EPN treatment improved wear resistance of the cast steel; however, the wear mechanism did not change.
In tests under lubricated conditions, cracking and localised peeling still occurred in a relatively narrow wear track on the untreated cast iron. Contrarily, no cracking was observed on the EPN-treated cast iron. Surface polishing appeared on both the treated cast iron and cast steel. Those observations implied that all treated and untreated samples had similar low wear rates under the applied loads, lubrication and sliding distances. However, a longer sliding distance could cause cast iron to suffer from severe localised wear, also due to the fatigue cracking and peeling.
The friction characteristics of these layers in dry pin-on-disc sliding against sintered TiC (hardness 2900 HV50) were studied [49]. The normal load on the sample ranged from 0.5 to 4 MPa at an average sliding speed of 1.25 m/min. The PEN of steel (0.4%C and 1.0% Cr) was carried out during 5 min. Figure 14 shows that the friction coefficient of the nitrided samples decreases when the normal load increased. Hence, elastic contact takes place during friction of nitrided samples. On the contrary, implantation of the counter-body in the untreated sample is accompanied by plastic deformation, which increases the area of real contact and facilitates development of adhesive wear. Consequently, the friction coefficient increases with increasing load. In this case, wear tracks and secondary structures are formed in the initial period of the test, which results in reduced linear wear rate. The wear is localised in the thin surface layer owing to a combination of hard martensite with oxides and nitrides. The high hardness of martensite prevents the subsurface layer from plastic deformation.
Friction characteristics of the medium carbon steel samples (0.4% C, 1% Cr) after their anode PEN treatments were compared with those of tool steel samples (1% C, 1.5% W, 1% Mn, 1% Cr) treated by thermal diffusion boronising or thermal hardening.
The anode PEN followed by quenching in the solution of sodium nitrite (45 wt.%) shows similar results [76]. Wear tests were carried out under dry sliding conditions at the normal load of 1 MPa, sliding speed of 15 m/min and sliding distance of 500 m, against hardened medium carbon steel (HRC 50) counterface. The friction coefficient was found to decrease when the normal load increased. Table 9 shows further details of the test results.
5.6.2. High-speed steels.
It is established that cathode PEN of high-speed steel R6M5 in the carbamide solution results in the increase of abrasive wear resistance [71]. The anode PEN followed by quenching also increases the wear resistance of medium carbon steels [2]. This treatment creates the conditions for localisation of contact stress in thin surface layers. A thin layer of nitrides Fe2–3N and Fe4N with retained austenite is formed on the surface, a solid or "carrier" phase (martensite) is placed lower. The surface nitride zone is characterised by a sufficient plasticity and good wear-in. High hardness of martensite allows plastic deformation in subsurface layers to be prevented.
At the moment, the information on wear mechanisms of steels subjected to PEN treatments is rather limited. SEM studies of wear tracks have been carried out only for cathode PEN treated samples of cast steel and cast iron tested at a fixed normal load and sliding velocity. Effects of the loading on the friction coefficient under dry sliding conditions have been established only for medium carbon steel treated by anode PEN followed by quenching. Tribological properties of nitrided high-speed steel have been studied only under conditions of abrasive wear. Thus, different wear tests employed by various authors prevent comparative analysis of data obtained be performed.
5.7. Corrosion resistance
5.7.1. Medium carbon steels.
It is known that corrosion properties of steels are influenced by the possibility of formation and stability of passive surface layers [69]. Additional protection can be provided by the oxide layer thermally grown in special atmospheres containing oxygen or air [77, 78]. This process is recommended for manufacture of automobile engine valves made from high-temperature grades of steel [79]. It was found that dispersed inclusions of Fe2O3, along with excessive nitrides in the diffusion zone, increase the microhardness of diffusion layer [80].
The positive role of protective oxide layers is also confirmed for the samples subjected to anode PEN treatments [2, 3]. The treatment of medium carbon steels in the aqueous solution of ammonium chloride (10 wt.%) and ammonia (5 wt.%) at 750 (С during 5 min resulted in the decrease in corrosion current density by 4–5 orders of magnitude in neutral media due to passivating effects provided by a combination of the oxide layer and the nitrided zone [81]. The nitrided samples were tested by full immersion in an aqueous solution of sodium sulfate (0.1N) or their exposure above the surface of 3% sodium chloride solution [6]. The maximum corrosion potentials and minimum corrosion current densities were observed for steels nitrided at 750 °C in the aqueous solution of 11 wt.% ammonium chloride and 11 wt.% ammonium nitrate during 5 min, followed by air cooling [82].
The maximum corrosion rate was observed during the first few hours of the test; then it reduced and stabilised for all studied samples. The steady-state corrosion rate of the nitrided steel was 3 times less than the untreated one. After the oxide layer has been removed, the nitrided steel showed an intermediate corrosion rate. After the tests, the untreated samples were covered totally in a continuous brown layer of corrosion products. The nitrided samples exhibited localised corrosion which can be explained by the presence of pores in the nitrided layer. The area affected by corrosion damage after a 96-hour test was about 21% on the nitrided samples, 70% on those without oxide layer, and 100% on the untreated steel samples.
The anode PEN of other steels in the electrolyte comprising NH4Cl and NH4OH usually leads to increased corrosion resistance measured by electrochemical impedance spectroscopy in solutions containing boric acid and borax [61].
The corrosion resistance of nitrided steels can be increased by additional oxidation in an aqueous solution of sodium nitrite (10–30 wt.%) [83], wherein the final quenching could also be carried out. The resultant 2- to 3-fold increase in the thickness of the oxide layer and its densification, which improve significantly the corrosion resistance of the workpiece, can be explained by high oxidative capacity of NO2– ions:
2Fe + NaNO2 + 2H2O [pic] γ-Fe2O3 + NaOH (17)
It was established that the additional oxidation of the nitrided medium carbon steel (0.45% C) in the solution of sodium nitrite reduces anodic dissolution current. When the anode PEN was carried out in an aqueous solution of NH4Cl (11 wt.%) and NH4NO3 (11 wt.%), the corrosion rate of samples in 0.05 M Na2SO4 solution reduced from 6.3 to 0.85 g/(m2(day). The additional oxidation of steel parts after the anode PEN is patented and recommended in situations, when enhancement in corrosion resistance of machine parts and tools is required [84].
5.7.2. Stainless steel.
Corrosion behaviour of stainless steels (AISI 304, 316L and 430) in solutions of sodium chloride was improved significantly by the cathode PEN, in particular the 304 grade with lower contents of alloying elements [62]. Changes in the electrochemical response of nitrided AISI 304 and 430 steels could be due to the spinel oxide sublayer formed in the outermost region of the passive layer, which improves the corrosion resistance. The potentiodynamic polarisation curves for all steel samples show that PEN shifts Ecorr towards higher potentials and reduces corresponding values of icorr. Accordingly, the samples’ resistance to pitting corrosion increases significantly. Bode diagrams of nitrided AISI 430 samples shows that higher PEN voltages cause corrosion resistance to increase. In this case, the effect of PEN treatment lasts longer than for other stainless steels, so that the corrosion mechanism remains the same for all treated samples, although with time most of the surface layer becomes damaged due to a partial dissolution in the electrolyte. By contrast, the untreated AISI 430 sample shows response characteristic of pitting corrosion.
Efficient sources of nitrogen are much scarcer than the carbon-containing organic compounds suitable for PES. The most common component is ammonia which provides a sufficient concentration of nitrogen in the surface layer. However, the nitrogen generation capacity of base ammonia electrolytes is quite low. The search for new nitrogen-containing compounds is highly challenging. It is also of interest to supply the saturating components, in particular gaseous nitrogen [67] or powdered materials [85], directly in the processing zone.
Although high anitcorrosion properties of iron nitride Fe2-3N are well known, iron oxides may play a positive role in provision of corrosion resistance by PEN treatments too. This is confirmed by evaluation of corrosion rates of nitrided samples with the outer oxide layer removed or further oxidation in air or in a solution of sodium nitrite performed. In addition, alloying elements in stainless steels may also have effects on corrosion behavior of nitrided samples.
6. Nitrocarburising
6.1. Electrolyte compositions
Known electrolyte compositions for the nitrocarburising process and its results are shown in Tables 10 and 11. As a rule, appropriate electrolytes contain components similar to those used for carburising or nitriding individually.
Hypothetical reactions of decomposition of electrolyte components as sources of nitrogen and carbon are presented in some articles [37, 97, 98, 105, 107]. For example, it is suggested that monoethanolamine decomposition can yield carbon monoxide, methane, hydrogen and hydrogen cyanide according to the following reaction [97]:
C2H5ONH2 ( CO + HCN + H2 + CH4 (18)
CH4 ( 2H2 + [C ] (19)
2CO ( CO2 + [C] (20)
H2 + CO2 ( CO + H2O (21)
2HCN ( H2 + 2[C] + 2[N] (22)
Thermal decomposition of triethanolamine and formamide is also possible as follows [107]:
HCONH2 ( NH3 + CO (23)
HCONH2 ( HCN + H2O (24)
(C2H4OH)3N ( 2CH4 + 3CO + HCN + 3H2 (25)
The most common component in electrolytes for plasma electrolytic nitrocarburising (PEN/C) is carbamide. This component is shown to decompose to N2, CO2 and nitrate or nitrite anions by anodic oxidation under condition of conventional electrolysis [108]. Therefore, carbamide is believed to decompose forming ammonia and carbon dioxide [37]. However, the chemical inertness of carbon dioxide gives grounds for a doubt that this is a final product in plasma-assisted electrolysis. Carbon dioxide is suggested to decompose in strong electric fields but it should be noted that carbamide is an effective source of carbon in the case of anode PEN/C even with no visible electric discharges. According to data of ultra violet spectroscopy, the groups of atoms present in the electrolyte following processing point out to the formation of carbon monoxide, atomic carbon and cyanides [98]. Carbamide may be decomposed in an aqueous solution at temperatures below 50°С via the formation of isocyanic acid [105]:
(NH2)2CO ( NH3 + HNCO (26)
HNCO + H2O ( NH3 + CO2 (27)
These reactions are confirmed by the detection of isocyanate-ions in the electrolyte. Nitrogen diffusion is associated with decomposition of the adsorbed ammonia and oxidation of isocyanic acid:
2НNCO + O2 ( Н2O + CO2 + CO + 2N (Feγ (N)) (28)
The anode PEN/C of low carbon steel leads to increase in ammonia concentration in the electrolyte and decrease in that of carbamide (Fig. 15) [109]. Consequently, carbamide is decomposed in the adjacent to VGE electrolyte region. The sample weight decreases at a rate of 0.024±0.001g/(cm2(min). Accordingly, the concentration of iron ions in the solution is observed to increase.
Apparently, ammonia as a product of carbamide decomposition is accumulated in the electrolyte faster than evaporated in the VGE wherein the limiting pressure of ammonia is reached. Accumulation of ammonia in the solution results in its buffering, which is confirmed by an increase in pH from 5.5 to 7.7 during 250 minutes of PEN/C. During 300 min of PEN/C processing at 220 V, changes in the electrolyte composition result in surface temperature increasing from 830 to 880 (C. In parallel, the current increases from 8 to 10 A, which indicates higher energy liberated in the VGE, probably due to its thinning. The reason for the VGE thinning is likely to be due to the increase in surface tension at the electrolyte/VGE interface induced by the changes in electrolyte chemical composition.
After the PEN/C in the fresh electrolyte, the compound layer thickness is 68 μm, which is diminished to 60 μm when the electrolyte served for 30 min was used. On the contrary, the thickness of diffusion layer increases from 93 μm to 125 μm due to the temperature rise at the same rate of anode dissolution. It can be expected the nitrogen diffusion to decrease and carbon to increase. The microhardness profiles in the surface layers produced during 300 min of electrolyte operation remain unchanged.
Acetonitrile can be used for anode PEN/C as a source of nitrogen and carbon [106]. Acetonitrile and ethanol were found in the VGE during PEN/C treatments at 850 °С by chromatography techniques. The data obtained permit us to propose the following description of the transport mechanism of saturating components. During PEN/C, acetonitrile is evaporated in the VGE where it is adsorbed and subjected to thermal decomposition to form atomic nitrogen and carbon on the anode surface. Ethanol found in the VGE can be obtained as follows:
CH3CN [pic] C2H5NH2 [pic] C2H5OH + NH3 (29)
Ammonia is adsorbed on the anode and decomposed to atomic nitrogen. Ethanol may be subjected to both thermal decomposition on the anode to atomic carbon and multistep oxidation:
C2H5OH ( CH3CHO + 2H+ + 2e– (30)
CH3CHO ( CH3CO• + H+ + e– (31)
CH3CO• ( CO + CH2• + H+ + e– (32)
CH2• ( C + 2H+ + 2e– (33)
In this case, carbon monoxide and atomic carbon provide the sources for carbon diffusion in the surface.
6.2. Structure and phase composition of nitrocarburised layers
6.2.1. Low carbon steel.
Surface layer composition and elemental distributions are similar to those observed following conventional thermochemical nitrocarburising of steels. The cathode PEN/C of Q235 steel in all studied electrolytes results in formation of iron carbides and nitrides.
The modified layer contains Fe3C, Fe5C2, Fe4N, Fe2–3N, Fe(C,N). The compound layer formed during nitrocarburising treatment includes various iron nitrides and/or carbonitrides such as Fe4N or Fe4(N,C) and Fe2-3N or Fe2-3(N,C). In some cases, martensite [92, 97] and retained austenite [89, 90] can be found when processing temperature is adequate for hardening. Fig. 16 shows the carbonitrided layer obtained in an aqueous solution of ethanolamine using pulsed voltage with magnitude and frequency of 150V and 50 Hz respectively [89]. It was found that the layer growth rate decreased and the quality worsened after 75 s of treatment. For example, when the treatment time increased to 120 s, the layer formed had a thickness of only about 90μm, and a continuous network of carbides appeared in it. According to the results of EDS analysis, carbon concentration on the surface of the sample treated in an aqueous solution of monoethanolamine and potassium chloride was 1.06 wt.% [97].
The surface modified by the anode PEN/C is characterised by the oxide layer and reduced roughness. According to the XRD analysis, the nitrocarburised surface layer on low carbon steel (0.2% С) contains iron oxides FeO, γ-Fe2O3 [105], ζ-Fe2O3 [106] and Fe3O4; carbonitride (-Fex(NC); nitrides FeN0.05, and Fe2-3N; carbide Fe4C; martensite and austenite.
According to data of the nuclear backscattering of protons the oxygen concentration in the surface layer of nitrocarburised low carbon steel (0.1% C) increases significantly when processing temperature rises up to 950 (C and reaches to 50 at.% at the depth of 5 μm [104]. As usual, nitrogen diffusion dominates over that of carbon, at low temperatures.
According to SEM analysis, the surface microstructure of the treated steel is composed of several layers (Fig. 17) [105]:
– oxide layer containing iron oxides;
– outer nitrocarburised layer containing nitrides, martensite and retained austenite, when the sample is quenched in the electrolyte, or nitrides and perlite, after cooling in air:
– inner diffusion layer containing solid solution of nitrogen and/or carbon, depending on the treatment temperature and cooling conditions;
– original pearlite-ferrite substrate structure.
The iron oxide FeO and FeN0.05 phases were detected between 650 and 850°C, the latter suggesting development of nitrogen diffusion into steel surface. Iron oxides Fe2O3 and Fe3O4 were detected between 750 and 950°C. The concentration of iron oxides increases with treatment temperature, which is explained by the intensification of high-temperature oxidation.
According to the results of EDX analysis, nitrogen concentration reaches the maximum value underneath the oxide layer, decreasing further into the bulk of the sample (Fig. 18a). Kinetics of steel saturation with nitrogen during PEN/C are virtually identical at all treatment temperatures, except for 650 °C, where an increase in nitrogen concentration is only observed around the interface with the oxide layer.
The carbon concentration in the diffusion layer increases with the saturation temperature (Fig. 18b). Pearlite structure is clearly observed in the internal diffusion layer after PEN/C at 850 and 950 °C [105]. At 950 °C the fragments of pearlite structure appear throughout the modified surface layer. The temperature of 650 °C can be considered as a threshold below which no appreciable carbon diffusion is observed under the studied conditions.
6.2.2. Medium carbon steel.
Nitrocarburising of medium carbon steel AISI 1045 in a saturated aqueous solution of urea results in the formation of iron nitrides in the surface layer [95]. The compound layer on the surface of the samples treated for a shorter time of 6 min and at voltages of up to 180 V had dense appearance (while those treated at 220 V for 6 min and for the longer time of 9 min contained some fine porosity dispersed in the layers). The voltages around 220 V in plasma electrolytic process are equal to temperatures in the region of 900 oC and air quenching from that temperature results in bainite or martensite structures.
The cathode DC PEN/C of nitriding steel DIN1.8509 in the electrolyte containing carbamide and sodium carbonate leads to the formation of iron oxides and nitrides in equal proportions, in the surface layer [110]. When the carbamide concentration was above 90%, the Fe4N and Fe2–3N nitrides are detected. The latter nitride phase dominates in the surface layer when the PEN/C treatment was carried out in the electrolyte containing >84% carbamide, whereas the former prevails when the electrolyte containing 60–80% carbamide was used.
Nitrocarburising of nitriding steel DIN1.8509 using pulsed bipolar plasma electrolytic techniques results in a submicron-size surface layer [91]. The treatments were performed for 600 s in an electrolyte composed of 85.8 wt.% urea, 12 wt.% water and 2.2 wt.% sodium carbonate at various frequencies with duty cycle of 80%. It was found that the treatment at 5000 Hz reduces the grain size in the compound layer to 110 nm. XRD analysis confirmed the presence of Fe3O4 phase on the surface of the samples treated at lower frequencies and Fe4N rather than Fe2-3N nitride was found to be the dominant phase at higher frequencies. The results also suggest that the size of the cavities formed on the surface is reduced to 300 nm.
The influence of the pulsed PEN/C on the phase composition can be explained as following. In the case of DC mode, the gaseous envelope surrounding the workpiece is stable. The stability of this envelope causes a sudden increase in the workpiece temperature. However, if the voltage regime is changed to pulsed bipolar, short-term voltage interruptions may partially destroy the envelope, enabling brief contacts with electrolyte that would temporarily reduce the workpiece temperature. At lower frequencies, the resulting mean average temperature still keeps increasing and the phase Fe2–3N becomes dominant. At higher frequencies, there is less tendency for the temperature to rise and the Fe4N phase remains prevailing.
6.2.3. Stainless steel.
The cathode PEN/C of AISI 316L steel results in formation of oxides and nitrides of iron and alloying elements. The sample microstructure consists of an outer thin (2–3 μm) magnetite-chromite layer and an expanded austenite diffusion layer which possesses fine recrystallised grains [37]. Because of the relatively high temperature on the sample surface and rapid quenching by the electrolyte after the power supply is switched off, nitrogen and carbon diffused inward cannot precipitate as grain-boundary nitrides or carbides and remain within the recrystallised grains as a solid solution (expanded austenite). A lower, compared to the sample surface, bulk temperature prevents recrystallisation (or chromium carbide precipitation) in the metal substrate.
6.2.4. Cast iron.
The PEC/N treatment was applied to cast iron using an aqueous solution of acetamide and glycerine as the electrolyte [94]. After the treatment, the carbonitrided layer consisting of martensite, austenite, Fe2C, Fe3C, Fe5C2, FeN and Fe2−3N was produced on the sample surface.
6.3. Mechanical properties
6.3.1. Low-carbon steel
The PEN/C of low carbon steels causes surface microhardness to increase from 600 to 1000 HV (Tables 10 and 11). The higher values of microhardness are observed for the medium carbon or stainless steel. The nitrocarburising of AISI 304 stainless steel with a diamond-like carbon coating should also be noted [99].
6.3.2. Medium carbon steel
It is shown that anodic PEN/С results in increase in tensile strength of the medium carbon steel from 360(20 to 1200(50 MPa [104]. Therefore, KCU impact tests were performed after the anodic PEN/C at the same conditions, yielding similar values of toughness as for the samples subjected to conventional hardening.
6.3.3. Stainless steel
Comparative fatigue-wear behaviour of untreated and PEN/C treated AISI 316L austenitic stainless steel was investigated in [111] under different combinations of cyclic loading and contact pressure. As a result, the PEN/C treated specimens exhibited a higher resistance (about 40% for 15.6 N contact load and about 60% for 25 N contact load) to simultaneous cyclic stress and contact pressure. Also it was shown that under a range of combined fatigue and wear loads, the specimens exhibit a better endurance than under wear or fatigue loads applied separately, with this effect being significantly stronger for the PEN/C-treated samples. In these types of fatigue-wear tests, the surface plastic deformation induced by the superposition of compressive and bending stresses work-hardens the surface region of the specimen. Also the favourable residual compressive stresses in the surface layer may postpone or even prevent the propagation of primary fatigue cracks.
6.4. Tribological behaviour
6.4.1 Low-carbon steels.
The PEN/C of steels increases in their wear resistance almost in all cases. Fig. 19 shows wear behaviour of untreated low carbon steel (0.2% C) samples and those treated using the anode PEN/C in the electrolyte containing 10–15% ammonium chloride, 15% carbamide [105]. It can be seen that the nitrocarburised samples exhibited lower wear rates and friction coefficient than the untreated one (μ = 0.16 vs 0.19). Similarly, the cathode PEN/C of AISI 1020 steel in a urea electrolyte shows that the wear properties of the nitrocarburised samples were increased significantly in relation to the untreated sample [92, 98]. It was concluded that the adhesive wear has developed on the original sample; on the contrary, the abrasive wear mainly appeared on the PEN/C treated one.
6.4.2 Cast iron.
The cathode PEN/C of cast iron in an aqueous solution of acetamide and glycerin results in the increase in the wear resistance [94]. It was shown that the tracks created on the original sample surface are wider and deeper compared to those on the PEN/C treated sample. The wear rate of the treated sample drops down 6.5 times compared to the untreated sample.
6.4.3 Stainless steels.
Considering the effects of PEN/C treatments in the carbamide containing electrolyte on the tribological performance AISI 316L steel [37], it was noticed that despite of the sharp increase in surface roughness from 0.002 to 0.15 μm the average friction coefficient of the treated samples was slightly lower than that of the untreated one. The sample treated at 250 V during 1 min demonstrated a much higher wear resistance than both the untreated sample and those treated under different conditions, for which severe ploughing through the surface was observed. Additional increase in wear resistance of AISI 304 stainless steel can be obtained using a diamond-like carbon coating on the PEN/C pre-treated substrate [99]. This treatment results in a simultaneous reduction of the friction coefficient and wear rate due to changes in the wear mechanism from adhesion/abrasion to asperity deformation and polishing. The primary wear mechanism observed is one of mild asperity deformation and polishing and, in extreme cases, localised delamination of the DLC coating. The volumetric wear rate was almost constant for loads up to 15 N, particularly when sliding against WC–Co, where material loss from the ball counterface is less severe than from the SAE 52100 steel ball, despite the higher contact pressure at an equivalent load. No failure of the DLC coating was observed for 10 and 15 N loads against WC–Co and at up to 25 N against SAE 52100 steel [112].
Fatigue-wear behaviour of the PEN/C 316L steel was studied under a range of combined fatigue and wear stresses [111]. This test was carried out at the contact stress of 6.25 MPa and the bending stress of 87 MPa, in Ringer’s solution used as both a corrosive medium and a lubricant. It is established that the induced surface plastic deformations due to the superposition of compressive and bending stresses work-harden the sample surface. Favourable residual compressive stresses in the surface layer may postpone or even prevent the propagation of primary fatigue cracks. The weight loss at the first stage (up to 16.000 cycles) for PEN/C-treated samples was significantly lower than that of the untreated sample.
There are attempts to clarify wear mechanisms of PEN/C treated steels. The cathode PEN/C of low-carbon steel was established to promote transition from adhesive to abrasive wear. Moderate abrasive/adhesive wear has been observed in the case of cathode PEN/C of stainless steel. In general however, effects of loading and sliding speed on tribological behavior of PEN/C treated steels have also been studied insufficiently.
6.5. Corrosion resistance
6.5.1 Low carbon steel.
The corrosion resistance of nitrocarburised samples is usually increased compared to the untreated substrate. The values of corrosion current densities and corrosion potentials are often determined by Tafel analysis (extrapolation of linear segments of anodic and cathodic branches of polarisation curves towards their intersection in the semi-logarithmic, E vs log I, plane). Disk specimens of AISI 1020 steel were treated in the aqueous solution of urea and sodium carbonate [92]. The corrosion potential of the sample nitrocarburised for 5 min at 240V (715 (C) increased from −795mV (for the untreated sample) to −545 mV vs Ag-AgCl saturated electrode; corrosion current densities of the treated samples were also reduced.
Although Lee et al. [113] reported that the corrosion resistance of AISI 1020 steel samples nitrocarburised at 220 and 240 V has been improved in comparison with the untreated sample, a further increase in this property is only possible using oxidation post-treatments. An adherent Fe3O4 film with 0.2 to 1 μm thickness formed on the top of the compound layer during the oxidation post-treatment, enhances the corrosion resistance of other nitrocarburised low carbon steels [113, 114].
6.5.2 Medium carbon steels.
The corrosion resistance of nitriding steel (DIN1.8509) can be enhanced by the PEN/C in the solution containing urea (61–92 wt.%), sodium carbonates (0.7–9 wt.%) and water (7–30 wt.%) [110]. The EIS technique was used for corrosion evaluation. The samples were exposed to a solution of 3 wt.% NaCl for 1 h, 4 h, 2 days and 7 days. It was found that the corrosion resistance of samples processed in electrolytes with higher water contents is reduced. Developing a submicron-size surface layer on a nitriding steel using pulsed bipolar plasma electrolytic nitrocarburising results in improvement of the corrosion resistance [110]. All the treatments were performed for 600 s in the electrolyte containing 85.8 wt.% urea, 12 wt.% water and 2.2 wt.% sodium carbonate. According to the results of EIS analysis, the sample corrosion resistance increases with increasing pulse frequency in the PEN/C treatment.
Similar results were obtained for the anode PEN/C of medium carbon steel [51]. Corrosion tests were performed by immersion in decinormal solution of sodium sulphate. The weight loss of specimens treated in the carbamide containing electrolyte for 3 and 5 min decreased more than fivefold and eightfold respectively; however, the decrease was only fourfold following the 10-min treatment. It can be assumed that the increase in treatment time over 5 min impairs the integrity of the surface oxide layer which plays an important role in metal passivation. The increase in the saturation temperature to 950 °С significantly increases the thickness of the oxide layer, but this does not necessarily result in increased corrosive resistance. This can be attributed to the pores and microcracks developed in the oxide layer after hardening. Thicker oxide layers obtained by cooling specimens after saturation in air can stratify. As a rule, the corrosion rate decreases which can be attributed to the gradual filling of the pores with corrosion products. The minimum corrosion rate is observed after saturation with nitrogen and carbon at 850 °С for 5 min. The decrease in corrosion rate was attained by an increase in saturation time from 3 to 5 min regardless of temperature, which can be attributed to an increase in the thickness of the modified layer. As suggested previously, the increase in corrosion rate after saturation for 10 min is associated with the increase in porosity of the oxide layer.
6.5.3 Stainless steel.
The corrosion resistance of the AISI 316L austenitic stainless steel can be increased by more than 50% using the PEN/C treatment in the electrolyte containing urea and sodium carbonate [93]. It is established that the percentage contributions of the applied voltage, treatment time, electrolyte conductivity and urea concentration to the corrosion resistance are 78.24%, 19.05%, 2.41% and 0.3%, respectively. The optimised treatment parameters for the maximum corrosion resistance include 1150 g/l for urea concentration, 360 mS/cm for electrolyte conductivity, 260 V for applied voltage, 6 min for treatment time.
Dramatic improvements in corrosion resistance by the PEN/C treatment may be due to the combination of thick, fine-grained layer of expanded austenite and a thin yet dense magnetite-based iron-chromium oxide layer [37]. Potentiodynamic polarisation curves for the treated and untreated samples are shown in Fig. 20. The corrosion potential of –270 mV exhibited by the untreated substrate has increased –500 mV for the sample treated at 230 V during 30 s. Corrosion potentials of samples treated at 230 V (60 s) or 250 V (30 or 60 s) also increased, while both icorr and the limiting current of passive state significantly decreased, providing significant improvements in corrosion resistance. For the samples treated at 230 V (60 s) and 250 V (30 s), the layers of expanded austenite were significantly thicker than that in the sample treated at 230 V during 30 s, resulting in increased corrosion resistance. The sample treated for a longer time at the relatively high process temperature, showed the best performance in all corrosion tests, which may be due to a combination of oxide layer and a thick (and finer-grained) diffusion layer of expanded austenite.
It is shown that addition of gaseous nitrogen during the PEN/C treatment of AISI 316L steel in a saturated aqueous solution of carbamide and sodium carbonate can significantly increase nitrogen diffusion rate and result in a higher corrosion resistance of the treated material [67]. Among the samples nitrocarburised in gas free conditions, only one processed at 500 V has shown slight increases in both corrosion potential and polarisation resistance compared to those for the untreated substrate (Ecorr = – 200 mV vs SCE; Rp = 126.5 kΩ/cm2). In contrast, for the samples treated with nitrogen purged through plasma, these characteristics increased significantly following treatments in the range of 600 to 700 V. The highest values of Ecorr = +3 mV and Rp = 296.5 k(/cm2 were observed for the samples treated at 650 and 600 V respectively, which is probably due to the formation of dense iron-oxi-nitrocarburised layers on the surface under these conditions.
Finally, results of rapid anodic hardening of austenitic stainless steel 12Cr18Ni10Ti, consisting in saturation with nitrogen and carbon in a chloride–carbamide electrolyte followed by quenching, are discussed in [51]. The treatment at 850 °С for 10 min yields a hardened surface layer of up to 80 μm thick with microhardness of 450 HV50. Corrosion tests revealed no evidence of intercrystalline corrosion along the grain boundaries, although a localised corrosion was observed, with depths of corrosion spots approximately equal to their widths. These did not exceed 30 and 60 μm for all specimens treated for 5 and 10 min respectively. The average corrosion rates probably did not exceed the corresponding value for the untreated substrate.
Simultaneous saturation of steels with nitrogen and carbon has a number of well-known advantages; consequently, carbonitriding and nitrocarburising are being developed successfully. New possibilities emerge for other multicomponent saturation processes, such as borocarburising and borocarbonitriding [115, 116]. It should be noted that electrolyte components added to adjust solution conductivity affect sufficiently intensity of electrical discharges and characteristics of oxide layer formed on the surface, which determines surface roughness, wear and corrosion resistance. Hence, careful selection of these components can significantly improve electrolyte properties and quality of treatment.
The corrosion resistance of PEN/C treated steels has been paid more attention to compared to nitriding or carburising, including pulse treatment. Protective properties of PEN/C layers on steels may be enhanced by grain refinement in the porous layer. Increased pulse frequency and duty cycle facilitate formation of nanocrystalline carbonitrides, which improves corrosion resistance. In the case of anodic nitriding, a positive role of iron oxides has also been noticed.
7. Future challenges
Further advancement of PES technology requires a better understanding of its physical features. Thermal potential of PES was studied only for simplified steady-state conditions of a continuous vapour layer on vertical cylindrical surfaces. More complex, practically important but theoretically challenging, systems with unsteady conditions of heating and cooling are expected to be addressed. Particularly challenging issues include modelling of heat transfer during PES treatments of immersed flat vertically or horizontally oriented surfaces as well as those treated by electrolyte jets.
At present, PES treatments are not standardised, which makes it difficult to compare results obtained by different authors. Various hydrodynamic conditions are used in the apparatuses for PES. Natural convection in a cylindrical vessel without electrolyte cooling [70] or cooling electrolyte in a jacketed vessel do not provide adequate control over electrolyte temperature and therefore processing stability. Conditions of forced convection with electrolyte stirring [4] or longitudinal flow are more preferable. The latter provides both stable [39, 41, 53] and more uniform [31] temperature distribution over the treated surface. Moreover, PETs can be implemented in stagnant electrolytes cooled by airflow.
It is necessary to find the optimum hydrodynamic conditions in the electrolyser, which can provide a stable heating up to the temperatures practically suitable for saturation with interstitial elements. A unified electrolyser design would facilitate comparison between results of different studies, development and scaling up of technology and equipment. Studies into hydrodynamic conditions and regularities of heat transfer for jet processing using perforated counter-electrodes are also important.
Up to date, most of the results have been obtained on samples convenient for analysis and testing. At the same time, processing features of real components could be different because the shape of the working electrode would influence local hydrodynamic, thermal and chemical conditions on the surface [16]. Systematic studies into these practically important aspects of PES technology would therefore be of particular interest.
Erosion of the workpiece, its oxidation and dissolution should be investigated more deeply because they determine important characteristics of state surface, crucial for its tribological behaviour. The thickness and microstructure of the surface oxide layer influence diffusion characteristics of carbon and nitrogen, thus affecting structure and composition corresponding diffusion layers. Furthermore, iron oxides contribute to the corrosion resistance of steels, under certain conditions. In the case of the cathode PES, electrical discharges are more important than the oxidation process, but they may cause surface erosion, which characteristics depend also on electrolyte composition. This influence should be studied too.
Pulse PES has already demonstrated certain advantages, but further research of this method is required. Effects of pulse parameters (amplitude, frequency, and duty cycle) on the wear and corrosion resistance were studied. However, the formation of nanostructures is explained by different hypotheses which require confirmation.
There is also theoretical and practical interest in studies of electrochemical processes that always occur in the studied systems and determine many aspects of PET technology. These include conditions of current flow across the VGE, liberation of saturating components, adsorption of active species, and their diffusion into the bulk of the working electrode. A limited number of publications addresses transport mechanisms of saturating components from the electrolyte to the VGE and further into the treated material. There are only assumptions about possible reactions in the system without sufficient experimental confirmation. It is desirable to investigate chemical and electrochemical reactions in the system electrolyte–VGE–metal electrode, nature of charge carrier in the VGE, and transport mechanisms of nitrogen, carbon, boron and oxygen from the electrolyte to the VGE and further to the modified material.
The obtained results make it possible to choose the electrolyte composition with regard to its saturating capacity, service life and energy consumption. Attention should be paid to the electrolyte components that determine both solution conductivity and properties of the treated surfaces. A range of such components should be expanded to enable efficient optimisation of electrolyte composition. Opportunities should also be sought to improve electrolyte properties, e.g. by adding components that reduce energy consumption, or using non-aqueous solutions.
Finally, new counter-electrode designs, e.g. enabling purging plasma supporting gases through the electrolyte flow directed to the sample [67], may provide new technological opportunities, including extension of the range of saturating species. Moreover, the diffusion saturation can be facilitated using a supply of particulate precursor materials (boron carbide, aluminium or nickel) directly in the VGE [85].
8. Conclusions
Modern studies have led to the development of a number of electrolyte compositions and processing regimes enabling hardness, wear and corrosion resistances of a wide range of steels to be enhanced using PES. The possibility of formation of surface nanostructures with promising properties by pulsed cathodic PES was shown. One of the main advantages of electrolytic plasma treatments, such as carburising, nitriding or carbonitriding, is the possibility to combine PES with hardening by means of interrupting polarisation and cooling in the same electrolyte without reheating. Continuous motion of the medium in the VGE provides rapid supply of saturating components to workpieces and removal of reaction products. Owing to small VGE thickness, high workpiece heating rates (up to 100 deg/s) can be achieved, which allows grain growth and associated deterioration of material’s properties to be avoided. In addition, small VGE thickness contributes to rapid stabilisation of processing conditions as the equilibrium concentration of adsorbed diffusant is achieved in several seconds. As a result, processing time is reduced to several minutes. Pulse PES treatments appear to be of particular significance as a means of obtaining nanoscale surface structures with promising properties.
Surface modification can be carried out by jetting electrolyte onto the workpiece surface as well as by conventional immersion technique. The jetting scheme opens up new technological opportunities, including localised processing, treatments of inner surfaces and tailoring in service performance by processing separate areas of the component under different conditions.
It has been established that the stationary heating mode is analogous to the film boiling in liquids distinguished by the presence of internal heat sources in the VGE. These sources are caused by energy liberation associated with electrical discharges or ion emission from the electrolyte.
The mechanism of nitrogen and carbon liberation from the electrolyte has been proposed. The saturating components are evaporated from the electrolyte to VGE where they are oxidised, decomposed, subjected to electro-oxidation to low-molecular products, and dissociated to form active atomic carbon and nitrogen species. It was shown that carbon and nitrogen diffusion is affected by the surface oxide layer formed in the case of anode PES. The layer thickness is determined by anode oxidation and dissolution processes that are influenced by electrolyte composition.
These results convincingly demonstrate significant potential of PES technology for scientific research and various industrial applications.
Acknowledgements
The research leading to these results has received funding from the Russian Science Foundation (Contract #15-13-10018) (Sections 1, 3, 5, 6, 7) and the European Research Council under the ERC Advanced Grant (#320879 ‘IMPUNEP’) (Sections 2, 4, 8).
References
[1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93.
[2] P.N. Belkin, V.I. Ganchar, A.D. Davydov, A.I. Dikusar, E.A. Pasinkovskii, Anodic heating in aqueous solutions of electrolytes and its use for treating metal surfaces, Surf. Eng. Appl. Electrochem. (2) (1997) 1–15.
[3] P.N. Belkin, Anode electrochemical thermal modification of metals and alloys, Surf. Eng. Appl. Electrochem. 46 (6) (2010) 558–569.
[4] M. Aliofkhazraei, A. Sabour Rouhaghdam, P. Gupta, Nano-Fabrication by Cathodic Plasma Electrolysis, Critical Reviews in Solid State and Materials Sciences, 36 (2011) 174–190. .
[5] P. Gupta, G. Tenhundfeld, E.O. Daigle, D. Ryabkov, Electrolytic plasma technology: Science and engineering—An overview. Surf. Coat. Technol., 201 (21) (2007) 8746–8760.
[6] P.N. Belkin, E.A. Pasinkovskij, Heat treatment and case hardening of steels subjected to heat in electrolytic solutions, Met. Sci. Heat Treat. 31(5-6) (1989) 331–337.
[7] H.H. Kellogg, Anode effect in the aqueous electrolyses, J. Electrochem. Soc. 97(4) (1950) 133–142.
[8] S.K. Sengupta, O.P. Singh, Contact glow discharge electrolysis: a study of its onset and location, J. Electroanal. Chem. 301 (1991) 189–197.
[9] T. Mizuno, T. Ohmori, T. Akimoto, A. Takahashi, Production of heat during plasma electrolysis in liquid, Jpn. J. Appl. Phys. 39 (2000) 6055–6061.
[10] T. Paulmier, J.M. Bell, P.M. Fredericks, Development of a novel cathodic plasma/electrolytic deposition technique. Part 2: Physico-chemical analysis of the plasma discharge, J. Mat. Proc. Tech. 208 (2008) 117–123.
[11] S.Yu. Shadrin, P.N. Belkin, Analysis of models for calculation of temperature of anode plasma electrolytic heating, Int. J. Heat and Mass Transf. 55 (2012) 179–186.
[12] E.I. Meletis, X. Nie, F.L. Wang, J.C. Jiang, Electrolytic Plasma Processing for Cleaning and Metal-coating of Steel Surface, Surf. Coat. Technol. 150 (2002) 246–256.
[13] A. Yerokhin, A. Pilkington, A. Matthews, Pulse current plasma assisted electrolytic cleaning of AISI 4340 steel, J. Mat. Proc. Technol. 210 (2010) 54–63.
[14] Yu.A. Koshelev, G.A. Kitaev, E.I. Bazhin, Use of thyristor converters in the electrolytic cleaning of continuous wide strip, Surf. Eng. Appl. Electrochem. (6) (1984) 93–95.
[15] V.M. Rudim, Electrolytic descaling technology for strip steel, Surf. Eng. Appl. Electrochem. (5) (1986) 100–104.
[16] A.Ya. Zanin. Yu.I. Nazarenko, M.I. Serdyuk, Heating a continuously moving circular plate in an electrolyte plasmotron, Surf. Eng. Appl. Electrochem. (3) (1984) 36–38.
[17] Yu.N. Tyurin, A.D. Pogrebnjak, Electric heating using a liquid electrode, Surf. Coat. Technol. 142–144 (2001) 293–299.
[18] J. Garbarz-Olivier, C. Guilpin, Etude des discharges electriques produites entre l(electrode et la solution lors des effects d(anode et de cathode dans les electrolytes aqueux, J. Chim. phys. 72 (2) (1975) 207–214.
[19] P.N. Belkin, V.I. Ganchar, Yu.N. Petrov, A study of vapor film conduction at anodic electrolytic heating, Sov. Phys. Dokl. 31 (1986) 1001–1004.
[20] P.N. Belkin, V.I. Ganchar, Passage of a current through a vapor-gas sheath during anodic electrolytic heating, Surf. Eng. Appl. Electrochem. (5) (1988) 97–102.
[21] S.O. Shiryaeva, A.I. Grigor’ev, V.V. Morozov, On the Appearance of Ions Near the Charged Surface of an Intensely Evaporating Electrolyte, Technical Physics, 48 (7) (2003) 822–828.
[22] S. Yu. Shadrin, A. V. Zhirov, P. N. Belkin, Formation Regularities of Gaseous Vapour Plasma Envelope in Electrolyser, Surf. Eng. Appl. Electrochem. (1) (2016) 110–116.
[23] T. Mizuno, T. Akimoto, K. Azumi, T. Ohmori, Y. Aoki, A. Takahashi, Hydrogen Evolution by Plasma Electrolysis in Aqueous Solution, Jpn. J. Appl. Phys., 44 (1A) (2005) 396-401.
[24] I.G. D’yakov, V.S. Belkin, S.Yu. Shadrin, P.N. Belkin, Peculiarities of Heat Transfer at Anodic Plasma Electrolytic Treatment of Cylindrical Pieces, Surf. Eng. Appl. Electrochem. 50(4) (2014) 346–355.
[25] P.N. Belkin, A.B. Belikhov, Stationary Temperature of an Anode Heated in Aqueous Electrolytes, J. Eng. Phys. Thermophys. 75 (6) (2002) 1271–1277.
[26] P.N. Belkin, T.L. Mukhacheva, I.G. Dyakov, Features of the distribution of heat fluxes in the anode–vapor-gas sheath system in anodic electrolytic heating, J. Eng. Phys. Thermophys. 81 (6) (2008) 1069–1075.
[27] A.V. Zhirov, S.Yu. Shadrin, Determination of heat flux from vapor-gas envelope into short anode heated by plasma electrolysis, 6th International conference on material science and condensed matter physics, Chisinau, 2012, P. 279.
[28] I.G. Dyakov, S.Yu. Shadrin, P.N. Belkin, Features of anode heating in movement of electrolyte in a free convection regime, Surf. Eng. Appl. Electrochem. (4) (2004) 7–12.
[29] V.I. Ganchar, I.M. Zgardan, A.I. Dikusar, Anodic dissolution of iron in the process of electrolytic heating, Surf. Eng. Appl. Electrochem. (4) (1994) 69–77.
[30] I.G. Dyakov, P.N. Belkin, Thickness of vapour/gas envelope during anode heating of vertically immersed cylinder, Surf. Eng. Appl. Electrochem. (4) (2002) 44–51.
[31] Pat. 124 682 U1 RU, (C23D 1/00). Device for Electrolyte Treatment of Metal Products, A.O. Komarov, T.L. Mukhacheva, P.N. Belkin. 07.03.2012.
[32] A.O. Komarov, T.L. Mukhacheva, I.G. Dyakov, P.N. Belkin, Influence of Hydrodynamical Peculiarities of Electrolyte Flows on Temperature of Cylindrical Workpiece by Plasma Electrolysis, Surf. Eng. Appl. Electrochem. 48(2) (2012) 141–186.
[33] I.G. Dyakov, S.A. Kusmanov, A.R. Naumov, Dynamics of 12X18H10T steel and VT1-0 alloy dissolving in the anodic electrolytic heating, Book of the abstracts of the International Conference dedicated to the 50th anniversary from the foundation of the Institute of the Chemistry of the Academy of Sciences of Moldova, Chisinau, 2009, P. 232.
[34] V.I. Ganchar, I.M. Zgardan, A.I. Dicusar, Anodic dissolution of chromium during electrolytic heating, Surf. Eng. Appl. Electrochem. (5) (1996) 13-19.
[35] I.M. Zgardan, V.I. Ganchar, A.I. Dikusar, Abnormal Anodic Dissolution of Copper under Electrolysis Heating Conditions, Russian J. of Electrochem., 35 (4) (1999) 494–495.
[36] S.A. Kusmanov, P.N. Belkin, I.G. D’yakov, A.V. Zhirov, T.L. Mukhacheva, A.R. Naumov, Influence of Oxide Layer on Carbon Diffusion during Anode Plasma Electrolytic Carburizing, Prot. Met. Phys. Chem. 50 (2) (2014) 223–229.
[37] X. Nie, C. Tsotsos, A. Wilson, A.L. Yerokhin, A. Leyland, A. Matthews A, Characteristics of a plasma electrolytic nitrocarburising treatment for stainless steels, Surf. Coat. Technol. 139 (2–3) (2001) 135–142.
[38] I.G. Dyakov, A.R. Naumov, Electrochemical Reactions on Anodic Heating in Aqueous Ammonium-Chloride Electrolytes, Surf. Eng. Appl. Electrochem. (6) (2006) 1–4.
[39] M. Tarakci, K. Korkmaz, Y. Gencer, M. Usta, Plasma electrolytic surface carburized and hardening of pure iron, Surf. Coat. Technol. 199 (2-3) (2005) 205–212.
[40] Pat. 44–1049 Japan, (12А34, 12А31, 12А32). Solution for Electrolytic Thermal Treatment / Inoue Kiyoshi, Kaneko Hideo; 18.01.1979.
[41] F. Çavuşlu, M. Usta, Kinetics and mechanical study of plasma electrolytic carburizing for pure iron, Appl. Surf. Sc. 257 (9) (2011) 4014–4020.
[42] A. Rezaei, A. Shokuhtar, M. Asadi, M.M. Hosseinzadeh, S. Ahmadi, Carburizing of Low Carbon Steel with Plasma Electrolysis in Aqueous Solution, Defect and Diffusion Forum Vols, 258–260 (2006) 26–31. DOI: 10.4028/scientificnet/DDF.258–260.26.
[43] M. Yaghmazadeh, C. Dehghanian, Hardening of AISI H13 Steel Using Pulsed Plasma Electrolytic Carburizing (PPEC), Plasma Process. Polym. 6 (2009) S168–S172.
[44] J. Wu, W. Xue, B. Wang, X. Jin, J. Du, Y. Li, Characterization of carburized layer on T8 steel fabricated by cathodic plasma electrolysis, Surf. Coat. Technol. 245 (2014) 9–15.
[45] M. Skakov, E. Batyrbekov, L. Zhurerova, M. Scheffler, Microstructure and Microhardness Changes of 30CrMnSiA Steel Modified Surface Layers by Electrolyte-Plasma Processing, Appl. Mech. Mater. 423-426 (2013) 824–827.
[46] S. Kurbanbekov, M. Skakov, M. Scheffler, A. Naltaev, Changes of Mechanical Properties of Steel 12Cr18Ni10Ti after Electrolytic-Plasma Cementation, Adv. Mat. Res. 601 (2013) 59–63.
[47] W. Xue, Q. Jin, Q. Zhu, X. Wu, Characterization of Plasma Electrolytic Carburized Stainless Steel in Glycerin Aqueous Solution, J. Aeronautical Mat. 30(4) (2010) 38–42.
[48] W. Xue, J. Qian, R. Liu, X. Jin, B. Wang, J. Du, Influence of glycerin concentration on plasma electrolytic saturation process of stainless steel surface, The Chinese Journal of Nonferrous Metals, 23(3) (2013) 882–887.
[49] P. Belkin, A. Naumov, S. Shadrin, I. Dyakov, A. Zhirov, S. Kusmanov, T. Mukhacheva, Anodic Plasma Electrolytic Saturation of Steels by Carbon and Nitrogen, Adv. Mat. Res. 704 (2013) 37–42.
[50] A.V. Zhirov, A.O. Komarov, V.V. Danilov, S.A. Shorokhov, Effect of Glycerine Concentration on Dissolution and Oxidation of Mild Steel During Anodic Cementation, Surf. Eng. Appl. Electrochem. 48(3) (2012) 289–291.
[51] P.N. Belkin, I.G. Dyakov, A.V. Zhirov, S.A. Kusmanov, T.L. Mukhacheva, Effect of Compositions of Active Electrolytes on Properties of Anodic Carburization, Prot. Met. Phys. Chem. 46 (6) (2010) 715–720.
[52] V.N. Duradji, A.M. Mokrova, T.S. Lavrova, Carbon distribution in steel that underwent case hardening in electrolytic plasma, Surf. Eng. Appl. Electrochem. (5) (1984) 43–46.
[53] S.A. Kusmanov, S.Yu. Shadrin, P.N. Belkin, Carbon transfer from aqueous electrolytes to steel by anode plasma electrolytic carburizing, Surf. Coat. Technol. 258 (2014) 727–733.
[54] A.B. Belikhov, P.N. Belkin, Features of iron-graphite anode carburising, Surf. Eng. Appl. Electrochem. (6) (1998) 9–17.
[55] J. Wu, W. Xue, X. Jin, B. Wang, J. Du, Z. Wu, Preparation and characterization of diamond-like carbon/oxides composite film on carbon steel by cathodic plasma electrolysis, Appl. Phys. Lett. 103, 031905 (2013); .
[56] A.O. Komarov, P.N. Belkin, The effect of surfactants on characteristics of anodic cementation of structural steels, Powder Metallurgy and Functional Coatings, (2) (2008) 46–49.
[57] Pat. 3,098,151 USA (Cl. 219–71). Electrical Discharge of Metals in Electrolytes / Inoue Kiyoshi, 16.07.1963.
[58] P. Belkin, S. Kusmanov, A. Naumov, Yu. Parkaeva, Anodic Plasma Electrolytic Nitrocarburizing of Low-Carbon Steel, Adv. Mat. Res. 704 (2013) 31–36.
[59] P. Belkin, I. Dyakov, S. Kusmanov, A. Naumov, Composition and properties of structural steel surface after plasma electrolytic carburising, Proceedings of the International Conference BALTRIB, Kaunas, 2011, pp. 184–188.
[60] E.P. Grishina, A.V. Zhirov, P.N. Belkin, A.I. Dikusar, Influence of anodic electrothermochemical oxidation on the corrosion stability of steel 45, Surf. Eng. Appl. Electrochem. 44 (5) (2008) 390–395.
[61] V. Andrei, Gh. Vlaicu, M. Fulger, C. Ducu, C. Diaconu, Gh. Oncioiu, E. Andrei, M. Bahrim, A. Gheboianu, Chemical and structural modifications induced in structural materials by electrochemical processes, Romanian Rep. Phys. 61(1) (2009) 95–104.
[62] M.Kh. Aliev, A. Sabour, P. Taheri, Study of corrosion protection of different stainless steels by nanocrystalline plasma electrolysis, Prot. Met. Phys. Chem. 44 (4) (2008) 402–407.
[63] E.J. Mittemeijer, Fundamentals of Nitriding and Nitrocarburising ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes.
[64] A. Roy, R.K. Tewari, R.C. Sharma, R. Sherhar, Feasibility study of aqueous electrolyte plasma nitriding, Surf. Eng. 23(4) (2007) 243–246.
[65] A. Roy. Aqueous electrolyte plasma nitriding of austenitic stainless steel, MT thesis, Indian Institute of Technology, Kanpur, DC, 2006.
[66] S.A. Kusmanov, A.A. Smirnov, Yu.V. Kusmanova, P.N. Belkin, Anode plasma electrolytic nitrohardening of medium carbon steel, Surf. Coat. Technol. 269 (2015) 308–313.
[67] L. Cenk Kumruoglu, A. Yerokhin, A. Ozel, A. Matthews, Effect of nitrogen gas addition onto the process of plasma electrolytic nitrocarburising of AISI 316L stainless steel, Proceed. 1st ISTS International Surface Treatment Symposium, Istanbul, 2011, pp. 295–310.
[68] J. Li, D. Shen, Z. Tian, Y. Wang, K. Liu, Hot working technology (2006)-01.
[69] X. Nie, L. Wang, Z.C. Yao, L. Zhang, F. Cheng, Sliding wear behaviour of electrolytic plasma nitrided cast iron and steel, Surf. Coat. Technol. 200 (5–6) (2005) 1745–1750.
[70] J.H. Kong, M. Okumiya, Y. Tsunekawa, T. Takeda, K.Y. Yun, M. Yoshida, S.G. Kim, Surface modification of SCM420 steel by plasma electrolytic treatment, Surf. Coat. Technol. 232 (2013) 275–282.
[71] M. Skakov, B. Rakhadilov, M. Scheffner, G. Karipbaeva, M. Rakhadilov, Electrolyte plasma nitriding of high-speed steel, Appl. Mech. Mater. 379 (2013) 161–166.
[72] Pat. 621799 USSR, (C23C 9/12). Electrolyte for Anode Nitriding of Steel Products. P.N. Belkin, A.A. Factorovich, E.A. Pasinkovski. 09.03.1977.
[73] P.N. Belkin, A.A. Burbelko, E.A. Pasinkovski, A.V. Rabinovich, A.A. Faktorovich, Nitriding kinetics of technical iron and steel 40Kh under electrolytic heating conditions, Surf. Eng. Appl. Electrochem. (2) (1984) 68–70.
[74] J.M. Cowling, J.W. Martin, Effect of internal residual stresses on the fatique behavior of nitrided En41b steel, Heat. Treat. 79 Proc. Int. Conf. Birmingham, 1980, pp. 178–181.
[75] L.J. Ebert, The role of residual stresses in the mechanical performance of case carburized steels, Met. Mat. Trans. A 98(11) (1978) 1537–1551.
[76]. S.E. Kuzenkov, B.P. Saushkin, Borating of steel 45 in electrolytic plasma, Surf. Eng. Appl. Electrochem. (6) (1996) 10–15.
[77] Pat. 4,881,983 USA, C23C 8/34; C23C 11/08. Manufacture of Corrosion Resistant Components, John D. Smith, Stephen E. Vanes.
[78] Pat. 4,131,492 USA, C23C 11/16. Steel Article Having a Nitrided and Partly Oxidizes Surface and Method for Producing Same, Shingi Fushimi, Takeshi Miyata.
[79] Sh. Fuji, S. Fushimi, M. Hashimoto, Analysis of the surface of the heat-resistant steel subjected oxy nitriding, J. Jap. Soc. Heat. Treat. 22(2) (1982) 105–110.
[80] Z. Li, D. Yu, H. Pau, D. Wang, Mechanism study of increasing the depth and hardness of diffusion layer on steel after its liquid nitriding in urea, J. Zhejiang Univ. (2) (1982) 91–104.
[81] V.G. Revenko, V.V. Parshutin, G.P. Chernova, N.L. Bogdashkina, N.D. Tomashov, P.N. Belkin, E.A. Pasinkovskii, A.A. Faktorovich, Influence of nitriding during electrolytic heating on electrochemical and corrosion behaviour of steel St 45, Electronnaya Obrabotka Materialov (5) (1985) 56–59.
[82] P.N. Belkin, S.A. Kusmanov, A.A. Smirnov, Plasma electrolytic hardening and nitrohardening of medium carbon steels, Mat. Sci. Forum. 844 (2016) 146–152.
[83] V.V. Parshutin, E.A. Pasinkovslii, Promotion of Steel Corrosion Resistance by Chemical-Thermal Treatment in Electrolytes, Surf. Eng. Appl. Electrochem. 43 (6) (2007) 431–433.
[84] Patent 2959 Moldova, С23C 20/00, С23C 20/06, С23C 20/08. Method of steel parts treatment for formation of surface layer with high corrosion resistance. V. Parshutin, E. Pasincovschi. 2006.01.31.
[85] Pat. 870 486 USSR, (C23C 9/00). Method of Thermochemical Treatment of Metals and Alloys. A.K. Tovarkov, V.N. Duradji. 07.10.1981.
[86] V.S. Vanin, G.A. Semenova, Cyaniding of steel by heating in an electrolyte, Met. Sci. Heat Treat. 7(10) (1965) 670–672.
[87] P. Taheri, Ch. Dehghanian, A phenomenological model of nanocrystalline coating production using plasma electrolytic saturation (PES) technique, Transaction B: Mech. Eng. 16 (1) (2009) 87–91.
[88] P. Taheri, Ch. Dehghanian, M. Aliofkhazraei, A.S. Rouhaghdam, Nanocrystalline Structure Produced by Complex Surface Treatments: Plasma Electrolytic Nitrocarburizing, Boronitriding, Borocarburizing, and Borocarbonitriding, Plasma Process. Polym. 2007, 4, S721–S727. DOI: 10.1002/ppap.200731805.
[89] D.J. Shen, Y.L. Wang, P. Nash, G.Z. Xing, A novel method of surface modification for steel by plasma electrolysis carbonitriding, Mat. Sc. Eng. A 458 (2007) 240–243.
[90] J. Li, D. Shen, Y. Wang, K. Liu, Research on the formation conditions of plasma electrolytic carbonitriding in liquid, Chin. Surf. Eng. 18(4) (2005) 31–33
[91] H. Tavakoli, S.M. Mousavi Khoie, S.P.H. Marashi, S.A. Hosseini Mogadam, Characterization of submicron-size layer produced by pulsed bipolar plasma electrolytic carbonitriding, J. Alloys Comp. 583 (2014) 382–389.
[92] M.K. Zarchi, M.H. Shariat, S.A. Dehghan, S. Solhjoo, Characterization of nitrocarburized surface layer on AISI 1020 steel by electrolytic plasma processing in an urea electrolyte, J. Mater. Res. Technol. 2013. 2(3) 213–220.
[93] M. Aliofkhazraei, P. Taheri, A.Sabour Rouhaghdam, Ch.Dehghanian, Systematic Study of Nanocrystalline Plasma Electrolytic Nitrocarburising of 316L Austenitic Stainless Steel for Corrosion Protection, J. Mater. Sci. Technol. 23(5) (2007) 665–671.
[94] H. Pang, G.-L. Zhang, X.Q. Wang, G.-H. Lv, H. Chen, S.-Z. Yang, Mechanical Performances of Carbonitriding Films on Cast Iron by Plasma Electrolytic Carbonitriding, Chin. Phys. Lett. 28(11) (2011) 118103.
[95] A.R. Rastkar, B. Shokri, Surface modification and wear test of carbon steel by plasma electrolytic nitrocarburizing, Surf. Interface Anal. 44 (2012) 342–351. DOI 10.1002/sia.3808.
[96] N. Afsar Kazerooniy, M.E. Bahrololoom, M.H. Shariat, F. Mahzoon and T. Jozaghi, Effect of Ringer’s solution on wear and friction of stainless steel 316L after plasma electrolytic nitrocarburising at low voltages, J. Mater. Sci. Technol. 27(10) (2011) 906–912.
[97] Y.F. Jiang, Y.F. Bao, K. Yang, Effect of C/N concentration fluctuation on formation of plasma electrolytic carbonitriding coating on Q235, J. Iron Steel Res. 19(11) (2012) 39-45.
[98] Y.F. Jiang, T. Geng, Y.F. Bao, Y. Zhu, Electrolyte–electrode interface and surface characterization of plasma electrolytic nitrocarburizing, Surf. Coat. Technol. 216 (2013) 232–236.
[99] A.L. Yerokhin, A. Leyland, C. Tsotsos, A.D. Wilson, X. Nie, A. Matthews, Duplex surface treatments combining plasma electrolytic nitrocarburising and plasma-immersion ion-assisted deposition, Surf. Coat. Technol. 142–144 (2001) 1129–1136.
[100] L.C. Kumruoglu, A. Ozel, Plasma electrolytic saturation of 316 L stainless steel in an aqueous electrolyte containing urea and ammonium nitrate, Materials and technology 47(3) (2013) 307–310.
[101] Pat. 461 161 USSR, (C23C 9/10). Method of Thermochemical Treatment of Metals, B.R. Lazarenko, V.N. Duradji, A.A. Factorovich, I.V. Bryantsev. 25.02.1975.
[102] Pat. 922 177 USSR, (C23C 9/16). Electrolyte for Treatment of Steel Products, V.N. Duradji, A.K/ Tovarkov. 23.04.1982.
[103] Pat. 618 447 USSR, (C23C 9/16). Electrolyte for Nitrocarburising of Steel Products, V.N. Duradji, I.V. Bryantsev, A.K. Tovarkov. 05.08.1978.
[104] P. Belkin, B. Krit, I. Dyakov, V. Vostrikov, T. Mukhacheva, Anode saturation with nitrogen and carbon in aqueous solution of carbamide-bearing electrolytes, Met. Sci. Heat Treat. 52(1-2) (2010) 20–24.
[105] S.A. Kusmanov, Yu.V. Kusmanova, A.R. Naumov, P.N. Belkin, Features of Anode Plasma Electrolytic Nitrocarburising of Low Carbon Steel, Surf. Coat. Technol. 272 (2015) 149–157.
[106] S.A. Kusmanov, Yu.V. Kusmanova, A.R. Naumov, P.N. Belkin. Formation of diffusion layers by anode plasma electrolytic nitrocarburising of low carbon steel, J. Mater. Eng. Perform. 24(8) (2015) 3187–3193.
[107] X.-M. Li, Y. Han, Porous nanocrystalline Ti(CxN1–x) thick films by plasma electrolytic carbonitriding, Electrochem. Commun. 8 (2006) 267–272.
[108] N.V. Osetrova and A.M. Skundin. Products of Anodic Oxidation of Carbamide: Effect of Anionic Composition of Solution, Rus. J. Electrochem. 38(3) (2002) 266–269.
[109] S. A. Kusmanov, Yu. V. Parkaeva, I. S. Frolov, A. R. Naumov, and P. N. Belkin, Effect of Electrolyte Depletion on Characteristics of Anodic Plasma Electrolytic Nitrocarburizing, Surf. Eng. Appl. Electrochem. 51 (3) (2015) 213–219.
[110] H. Tavakoli, S.M. Mousavi Khoie, S.P.H. Marashi, and O. Bolhasani, Effect of Electrolyte Composition on Characteristics of Plasma Electrolysis Nitrocarburizing, J. Mater. Eng. Perform. 22(8) (2013) 2351.
[111] F. Mahzoon, S. A. Behgozin, M. E. Bahrololoom, and S. Javadpour, Study the fatigue-wear behavior of a plasma electrolytic nitrocarburized (PEN/C) 316L stainless steel, J. Mater. Eng. Perform. 21(8) (2012) 1751–1756.
[112] C. Tsotsos, A.L. Yerokhin, A.D. Wilson, A. Leyland, A. Matthews, Tribological evaluation of AISI 304 stainless steel duplex treated by plasma electrolytic nitrocarburising and diamond-like carbon coating, Wear 253(9–10) (2002) 986–993.
[113] K.H. Lee, K.S. Nam, P.W. Shin, D.Y. Lee, Y.S. Song, Effect of post-oxidizing time on corrosion properties of plasma nitrocarburized AISI 1020 steel, Mater. Lett. 57 (2003) 2060–2065.
[114] T. Bell, Y. Sun, A. Suhadi, Environmental and technical aspects of plasma nitrocarburising, Vacuum 59 (2000) 14–23.
[115] B. Wang, W. Xue, J. Wu, X. Jin, M. Hu, and Z. Wu, Characterization of surface hardened layers on Q235 low-carbon steel treated by plasma electrolytic borocarburizing, J. Alloys Comp. 578 (2013) 162–169.
[116] P. Taheri, Ch. Dehghanian, M. Aliofkhazraei, and A.S. Rouhaghdam, Nanocrystalline Structure Produced by Complex Surface Treatments: Plasma Electrolytic Nitrocarburizing, Boronitriding, Borocarburizing, and Borocarbonitriding, Plasma Process. Polym. (4) (2007) S721–S727.
Figure captions
Fig. 1. Typical current-voltage (a) and temperature-voltage (b) characteristics for the process of anode plasma electrolysis without dielectric film on the anode surface [2] and current-voltage characteristics of cathode plasma electrolysis (c) [12].
Fig. 2. Heat flow distribution in the vapour-gaseous envelope. Nomenclature: x – Cartesian coordinate, ( – thickness of the envelope T – temperature, TA –anode temperature, TS – water saturation temperature, W – heat generation rate per unit volume, qA – heat flux density from the envelope to the anode, ql – heat flux density from the envelope to the liquid, qs – heat flux density from the envelope to air [11].
Fig. 3. SEM image of the low-carbon steel surface after carburising in aqueous solution containing 10% NH4Cl and 10% glycerol (900 (С, 210 V, 5 min) followed by cooling in air [36].
Fig. 4. Kinetics of iron transfer from anode to an aqueous solution (10% NH4Cl) during stationary anode heating (200 V, 850 °С) [38].
Fig. 5. SEM image of pure iron carburised at 850 (C for 30 min, showing the carbon rich layer (1), the transition layer (2), and the base metal (3) [39].
Fig. 6. Cross-section of the sample (diameter 12 mm, length 14 mm) treated at 850 (C for 10 min in an aqueous solution of ammonium chloride (10%) and glycerol (10%). 1 – oxide layer, 2 – carburised layer [49].
Fig. 7. Dependence of the square of layer thickness on carburising time in aqueous electrolytes with ammonium chloride concentration, 10%. It presents the martensite layers on steel with 0.2% C obtained at 900 (C (1–4) and carburised layer on pure iron obtained at 850 (C (5). Carbon containing components (10%): 1– acetone, 2 and 5 – glycerol, 3 – sucrose, 4 – ethylene glycol.
Fig. 8. Microscopic images of (a, b) the untreated sample and (c, d) cross sections of the treated ones [69].
Fig. 9. SEM image of cross-section of the medium carbon steel surface after anode PENH in a solution of ammonium nitrate (10%) and ammonium chloride (10%) at 750 °C for 5 min. 1 – surface oxide layer; 2 – nitride-martensite layer; 3 – martensite-ferrite layer; and 4 – initial pearlite-ferrite structure [66].
Fig. 10. Dependence of the square of nitride layer thickness on processing time in a solution of ammonium nitrate (11%) and ammonium chloride (11%) for different treatment temperatures [6].
Fig. 11. Dependence of the square of diffusion layer thickness on processing time in a solution of ammonium nitrate (11%) and ammonium chloride (11%) for different treatment temperatures [6].
Fig. 12. Microhardness distribution in the modified layer after anode nitriding at different treatment temperatures (a) and time (b), concentration of ammonium chloride (c) and ammonium nitrate (d) at: (a) 10% NH4NO3, 10% NH4Cl, 5 min; (b) 10% NH4NO3, 10% NH4Cl, 750 °C; (c) 10% NH4NO3, 5min, 750 °C; and (d) 10% NH4Cl, 5 min, 750 °C [66].
Fig. 13. Appearance of wear tracks after pin-on-disc tribological tests. Untreated samples: (a) G3500 and (c) S0050A. PEN-treated samples: (b) G3500 and (d) S0050A [69].
Fig. 14. Dependence of friction coefficient on the normal load. Nitriding time is 5 minutes, temperature ((C): 650 (3) and 750 (2, 4). 1 – untreated sample. Solutions: 10% ammonium chloride and 5% ammonia (2, 3); 11% ammonium chloride and 11% ammonium nitrate (4) [49].
Fig. 15. Change of electrolyte composition during PEN/C. Initial composition is 10% ammonium chloride and 15% carbamide [109].
Fig. 16. Cross-sectional SEM micrographs of Q235 steel PES treated for different times: (a) 30 s, (b) 75 s and (c) 120 s [89].
Fig. 17. Cross-sectional SEM image of steel surface after anode PEN/C treatment for 5 min in the electrolyte containing 15% (NH2)2CO, 10% NH4Cl, at 850oС. 1 – oxide layer, 2 – external nitrocarburised layer, 3 – internal diffusion layer, 4 – initial pearlite-ferrite structure [105].
Fig. 18. EDX analysis of distribution of nitrogen (a), carbon (b) and oxygen (c) in the surface layers of samples treated by anode PEN/C in a solution of carbamide (10%) and ammonium chloride (10%) during 5 min for different treatment temperature [105].
Fig. 19. Wear behaviour of untreated substrate of low carbon steel (0.2% C) and samples treated in a solution of carbamide (15%) and ammonium chloride (10%) at 850 °С for 5 min [105].
Fig. 20. Potentiodynamic polarisation curves in a 3% NaCl solution of untreated AISI 316 steel sample and those subjected to PEN/C treatments under different conditions: (1) – 230 V, 30 s; (2) – 230 V, 60 s; (3) – 250 V, 30 s; (4) – 250 V, 60 s. [37].
[pic]
Fig. 1. Typical current-voltage (a) and temperature-voltage (b) characteristics for the process of anode plasma electrolysis without dielectric film on the anode surface [2] and current-voltage characteristics of cathode plasma electrolysis (c) [12].
[pic]
Fig. 2. Heat flow distribution in the vapour-gaseous envelope. Nomenclature: x – Cartesian coordinate, ( – thickness of the envelope T – temperature, TA –anode temperature, TS – water saturation temperature, W – heat generation rate per unit volume, qA – heat flux density from the envelope to the anode, ql – heat flux density from the envelope to the liquid, qs – heat flux density from the envelope to air [11].
[pic]
Fig. 3. SEM image of the low-carbon steel surface after carburising in aqueous solution containing 10% NH4Cl and 10% glycerol (900 (С, 210 V, 5 min) followed by cooling in air [36].
[pic]
Fig. 4. Kinetics of iron transfer from anode to an aqueous solution (10% NH4Cl) during stationary anode heating (200 V, 850 °С) [38].
[pic]
Fig. 5. SEM image of pure iron carburised at 850 (C for 30 min, showing the carbon rich layer (1), the transition layer (2), and the base metal (3) [39].
[pic]
Fig. 6. Cross-section of the sample (diameter 12 mm, length 14 mm) treated at 850 (C for 10 min in an aqueous solution of ammonium chloride (10%) and glycerol (10%). 1 – oxide layer, 2 – carburised layer [49].
[pic]
Fig. 7. Dependence of the square of layer thickness on carburising time in aqueous electrolytes with ammonium chloride concentration, 10%. It presents the martensite layers on steel with 0.2% C obtained at 900 (C (1–4) and carburised layer on pure iron obtained at 850 (C (5). Carbon containing components (10%): 1– acetone, 2 and 5 – glycerol, 3 – sucrose, 4 – ethylene glycol.
[pic]
Fig. 8. Microscopic images of (a, b) the untreated sample and (c, d) cross sections of the treated ones [69].
[pic]
Fig. 9. SEM image of cross-section of the medium carbon steel surface after anode PENH in a solution of ammonium nitrate (10%) and ammonium chloride (10%) at 750 °C for 5 min. 1 – surface oxide layer; 2 – nitride-martensite layer; 3 – martensite-ferrite layer; and 4 – initial pearlite-ferrite structure [66].
[pic]
Fig. 10. Dependence of the square of nitride layer thickness on processing time in a solution of ammonium nitrate (11%) and ammonium chloride (11%) for different treatment temperatures [6].
[pic]
Fig. 11. Dependence of the square of diffusion layer thickness on processing time in a solution of ammonium nitrate (11%) and ammonium chloride (11%) for different treatment temperatures [6].
[pic][pic][pic][pic]
Fig. 12. Microhardness distribution in the modified layer after anode nitriding at different treatment temperatures (a) and time (b), concentration of ammonium chloride (c) and ammonium nitrate (d) at: (a) 10% NH4NO3, 10% NH4Cl, 5 min; (b) 10% NH4NO3, 10% NH4Cl, 750 °C; (c) 10% NH4NO3, 5min, 750 °C; and (d) 10% NH4Cl, 5 min, 750 °C [66].
[pic]
Fig. 13. Appearance of wear tracks after pin-on-disc tribological tests. Untreated samples: (a) G3500 and (c) S0050A. PEN-treated samples: (b) G3500 and (d) S0050A [69].
[pic]
Fig. 14. Dependence of friction coefficient on the normal load. Nitriding time is 5 minutes, temperature ((C): 650 (3) and 750 (2, 4). 1 – untreated sample. Solutions: 10% ammonium chloride and 5% ammonia (2, 3); 11% ammonium chloride and 11% ammonium nitrate (4) [49].
[pic]
Fig. 15. Change of electrolyte composition during PEN/C. Initial composition is 10% ammonium chloride and 15% carbamide [109].
[pic]
Fig. 16. Cross-sectional SEM micrographs of Q235 steel PES treated for different times: (a) 30 s, (b) 75 s and (c) 120 s [89].
[pic]
Fig. 17. Cross-sectional SEM image of steel surface after anode PEN/C treatment for 5 min in the electrolyte containing 15% (NH2)2CO, 10% NH4Cl, at 850oС. 1 – oxide layer, 2 – external nitrocarburised layer, 3 – internal diffusion layer, 4 – initial pearlite-ferrite structure [105].
[pic][pic]
[pic]
Fig. 18. EDX analysis of distribution of nitrogen (a), carbon (b) and oxygen (c) in the surface layers of samples treated by anode PEN/C in a solution of carbamide (10%) and ammonium chloride (10%) during 5 min for different treatment temperature [105].
[pic]
Fig. 19. Wear behaviour of untreated substrate of low carbon steel (0.2% C) and samples treated in a solution of carbamide (15%) and ammonium chloride (10%) at 850 °С for 5 min [105].
[pic]
Fig. 20. Potentiodynamic polarisation curves in a 3% NaCl solution of untreated AISI 316 steel sample and those subjected to PEN/C treatments under different conditions: (1) – 230 V, 30 s; (2) – 230 V, 60 s; (3) – 250 V, 30 s; (4) – 250 V, 60 s. [37].
-----------------------
[1] Corresponding author: Tel.: +44 (0) 161 306 4114; fax: +44 (0) 161 306 4114
E-mail address: Aleksey.Yerokhin@manchester.ac.uk (A. Yerokhin).
School of Materials, University of Manchester, Oxford Road, Manchester, UK, M13 9PL.
................
................
In order to avoid copyright disputes, this page is only a partial summary.
To fulfill the demand for quickly locating and searching documents.
It is intelligent file search solution for home and business.
Related download
- 2 experimental methods university of manchester
- title of abstract goes here title of abstract goes
- electrochemistry
- biocompatibility of materials
- microsoft word perry nature reviews
- structure and electrochemical behavior of
- plasma electrolytic carburizing of metal and alloys
- synthesis and characterization of novel electrodeposited
- anodic stripping voltammetry
Related searches
- photos of metal buildings
- interior pictures of metal homes
- metal and nonmetal on periodic table
- four non metal and their symbol
- metal and nonmetal periodic table
- properties of metal elements
- specific heat of metal calculator
- taste of metal in mouth
- metal and wood wall art
- metal and wood decor
- periodic table metal non metal and metalloids
- similarities and differences of debit and credit