On the subject of tooth damage, many of my teeth have gum-line grooves. A friend who is a retired dental technician insisted that gum-line grooves are a result of brushing horizontally and using an excessively hard toothbrush. I never admitted to her that I was suspicious of that theory because I didn’t brush my teeth that often for a long time, and that was when the grooves appeared. I became convinced that this was a great example of crevice corrosion. She was unimpressed with my theory. Last year while researching my failure-analysis book, however, I found out that dentists are now admitting that this can be a source of gum-line grooves. This new Cerec dental technician informed me that it is now thought that gum-line grooves may be exacerbated by flaking of the enamel due to bending stresses. This location, which is near the bone socket, is subject to higher stresses than the rest of the tooth.

Figure 3 showed the burr used to machine my molar. The technician announced that the CNC machining instrument had determined that my ceramic overlay was the last to be made with that burr because the dimensions were no longer within specification. I asked if I could have it for my “museum of failure.”

Figure 5 shows the tip in a view obtained using a scanning electron microscope (SEM). Figure 6 shows a higher-magnification view. We can see the protruding abrasive particles. We can see that a few are missing. It’s hard to believe that the instrument was able to detect such a minimal amount of wear. I did not inspect the entire working surface with the SEM.

Figure 7 shows the EDS data for the surface composition of a portion of the surface of the tool. The electron beam penetrates a little less than 1 micron into nickel (there are 25.4 microns in 0.001 inches) and to a significantly greater depth into carbon at the instrument settings I used. The main elements present are nickel (Ni) and carbon (C).[1] Note that the EDS method is notoriously imprecise for low-atomic-weight elements (including carbon), especially when the surface is not flat, which this clearly is not. However, EDS gives us a good idea of what elements are present. I have additional data (additional blue spectra not included here, similar to Figure 7) that show almost pure nickel in the bright areas and almost pure carbon in the dark areas.

Figure 8 shows an EDS element map. This is a powerful tool for materials characterization and failure analysis. We have a gray-scale image of the part. The EDS micro-chemical analyzer creates a color to represent the composition at each pixel and overlays it onto the gray-scale topological image. In this case, I “told” the EDS software to make the nickel-rich areas red, the carbon-rich areas green, the silicon-rich areas pink and the oxygen-rich areas blue. The fact that most of the area shown is either red or green shows us that the nickel and carbon are mostly separate. If you are colorblind and want to see the individual element maps, please write to me to ask.

Note that the green is associated with the protruding particles that are actually cutting the ceramic blank. See the red “hole” at the center of the image. One of the carbon-rich particles has been pulled out. This appears to be how the wear is degrading the tool over time. The topological images did not show obvious evidence of scraping of the nickel itself.

Part 1 of this blog can be read here; Part 2 is here.