Failure Analysis - University of Rhode Island
Failure Analysis.
Introduction.
When a component fails, why it failed requires understanding so it is not repeated. This brings up the area of “Failure Analysis” which is basically looking at failures and attempting to determine what caused them. An important tool in the process is the Scanning Electron Microscope, or SEM. In this laboratory, an SEM will be employed to examine the regions where several samples of material failed to determine the process by which they failed.
The Scanning Electron Microscope.
As noted previously in the Metallography laboratory, the SEM has a higher resolution than an optical microscope and importantly, it also has a good depth of focus, which means that rough surfaces can be examined in an SEM and all the surface will be in focus, even at high magnifications. Detail on surfaces that cannot be observed by optical microscopes can be observed in an SEM, so features on a surface that provide information on the mechanism of failure can be found.
Sample for the SEM.
For conventional SEM’s, samples must be conductive. For metal samples there is not an issue, but for nonmetals, then a thin coating of gold is applied in a sputtering machine. The gold coating is very thin, around 50A, but provides enough conductivity for imaging. An electrical connection between the sample and the SEM is required. One method is to just grip the sample in metal holders on the SEM specimen stage, the other is to use an adhesive backed copper tape to hold down the sample. This last method is used for conductive samples coated with gold. Other methods include placing the sample in a metal holder and sputter coating both the holder and sample as the same time for a continuous coating over both. To examine powders, the powder can be placed on one side of double sided adhesive tape and sputter coated.
The SEM specimen stage.
The specimen stage on an SEM has several directions of motion. It can move in X, Y and Z directions, it can rotate and it can tilt the sample. There are limits to the movement, for example in the Z direction, the sample needs to be a distance away from the objective lens, called the “Working Distance”. The X and Y movement are limited by the size of the specimen stage relative to the vacuum chamber in which it sits.
The Vacuum System.
The sample sits in the sample chamber on the specimen stage. When operating a vacuum surrounds the sample, but at other times when the sample needs to be changed, then air needs to surround the sample, Therefore, a vacuum system is required to pump the specimen chamber from atmospheric pressure down to a vacuum. This is achieved by a two pump system, the first a rotary vacuum pump, the second a diffusion pump. The rotary pump can start in air and create a moderate vacuum, not high enough for SEM operation. The diffusion pump operates from moderate vacuum to low vacuum pressures. If an operating diffusion pump is opened to air, it will backstream and fill the sample chamber with oil, therefore a computer controlled vacuum pumping system is present on SEM’s.
Sample Changing.
Normally a sample will be under vacuum and to observe a different sample the vacuum must be removed and then brought back up to atmospheric pressure. On the SEM in the laboratory, three buttons control the vacuum system once it is operational. The first is “EVAC” the second” SHUT” and a third “AIR”. To remove the sample, the AIR button is depressed and valves isolate the sample chamber from the vacuum system. Once these valves are closed, another valve opens and bleeds air into the sample chamber. When the chamber is at atmospheric pressure, the chamber door can be opened and the sample replaced on the specimen stage. The door is closed and the button “EVAC” is depressed. The air bleed valve closes, and valves connecting the diffusion pump and rotary pump close to avoid the diffusion pump experiencing atmospheric pressure. A different valve opens which connects the rotary pump to the sample chamber and it begins to pull a vacuum on the sample chamber. A vacuum gage monitors the pressure and when sufficiently low, the valve between the sample chamber and the rotary pump is closed, and two different valves, one between the rotary pump and diffusion pump and a second between the diffusion pump and the sample chamber open, and the sample chamber is then reduced further in pressure until a low enough to operate an electron beam. The gage that monitors sample chamber pressure senses when a low enough pressure is obtained by the vacuum system and the electron beam is activated.
Sample Observation.
Often a sample has been optically examined in a low power microscope and either regions of interest identified, or if the sample is too large to place in the SEM, a region of interest is sectioned from the larger sample for observation. The sample is usually brought into focus at low magnification. If there are no interesting features at this magnification, then the magnification is increased so smaller features on the surface can be observed. If this reveals nothing of interest, then the sample can be move by the X and Y controls to a new area and the process continued.
Failure Analysis Examples.
Below are two examples of failure analysis from the lecture portion of the class. The first shows crack initiation on a slip line in a material. The second shows a fracture surface.
[pic] [pic]
Figure 1. Crack on surface of metal. Figure 2. Fracture surface of a metal.
Discussion.
1. What mechanical process initiated the crack shown in figure 1?
2. What process was responsible for creating the fracture surface shown in figure 2?
Questions to ask.
Is the material single phase or two phase?
What processes that lead to failure start on the surface of a metal?
What processes that lead to failure start internally?
What features of a fracture surface identify an overload failure from other types of failure?
Which failure processes can be ruled out by these figures?
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