Our goal here is to determine how the damage is taking place. Are there hard, sharp particles that are scraping material away? Are two surfaces coming into contact so that small bumps or projections (asperities) are actually being microwelded together and then torn apart, leaving craters in one or both of the components? Are projections being smeared (deformed) and then flaking off? Is a localized oxidation of asperities creating an abrasive condition (fretting) that may lead to fatigue cracks in unexpected locations? Once you have determined which of these scenarios – or perhaps others not listed here – is allowing the damage, you have the basic information you need to look for a “fix.”
Unless you have access to a nano-indentation hardness tester, it may be difficult to determine the hardness of the debris particles. The use of an EDS microchemical analyzer (part of many scanning electron microscope accessory attachment equipment) is essential in such cases. The EDS will provide insight into the composition of the debris particles, while examination of the shapes of metallic debris particles may provide information about whether the wear particles are a result of subsurface cracking, smearing and flaking, or scraping or gouging action.
Examining both the worn component surface and the debris particles will allow the analyst the opportunity to think about whether both sets of data are self-consistent and telling the same story about the damage evolution.
Case Study 1: Steel Plate in a Conveyor (Machinery) Application
The upper image in Figure 1 does not have a scale marker bar. It’s from a project I worked on years ago and the particulars are gone from my records. However, the image was obtained with a normal film camera (Canon AE1 Program) with a 2x macro lens. What is depicted was visible to the eye. The application involved another component sliding over this plate. Area A was the as-manufactured surface of the plate. Area B is seen to be relatively more highly reflective. Perhaps the surface asperities have been smoothed out. Area C is the problem. It is very rough. The circled areas give a hint that material is being smeared, and the overall appearance of Area C indicates that material has been removed.
The lower image does have a scale marker bar. It was obtained with a scanning electron microscope. Here, the white arrows show several areas that have clearly been smeared out into flake shapes, waiting to be broken away. The dark arrow indicates a large crater than has presumably been created by the flaking mechanism. The small circled area with the white arrow shows a less severe area with the same basic scenario. This gives some confidence that we’re seeing one basic sequence of damage in different degrees of progress. Of course, it would be preferable to examine more than one area. The area depicted by the lower image is approximately 0.2 mm by 0.15 mm. Here we see the limitations of the scanning electron microscope. The magnification setting of the SEM was less than 500x, a very moderate situation. It is always important to make some effort to determine how representative your image data is when working with microscopic images.
This damage was clearly a combination of deformation followed by fracture. We call it wear. I don’t remember what fix I recommended, if any. But looking back on the data with the lens of the years, I don’t think lubrication would have helped. Maybe a smoother surface to start or a higher-hardness material might have been potential solutions.
We will look at two more case studies next time.