A TEM investigation of crack formation mechanism on chrome-molybdenum steel tested under real driving conditions

Many automotive components of gears, power transmissions, and engines are made of chrome molybdenum steel, which has high wear resistance. The life of automobiles depends on the life of such components. In order to extend the life of components, it is important to know the mechanism of wear of metallic parts. Many studies have been conducted on the wear mechanisms. Recently, we found a unique crystalline structure that cannot be explained by the established metallurgical theory in the course of transmission electron microscope (TEM) examination of the grain structures 0.2 to 1 μm deep from the sliding surfaces of a tested transmission part. The original body-centered cubic (BCC) crystal changed to very fine face-centered cubic (FCC) crystal in the region just under the sliding surfaces. It is still not known why such a unique change was caused, but this is an intermediate report of our findings. This article discusses the relation between the unique transformation of the crystalline structure and formation of cracks, which led to production of fish-scale thin wear particles.