Abstract
In the research of a gear, it was mainly surveyed when defect or extraordinary phenomena was found out. Therefore, the research data studying a gear which was operated for long period under stable condition was rare. In terms of extending maximum life-span, we liked to take a hint to predict a life expectancy from good status of use. We made a detailed observation by TEM, focusing on the structure under sliding surfaces of final drive gear in an automobile transmission, which was used in a real driving car for approx 300,000 km. As a result, it was found that the grain sizes were very fine in the subsurface area down to 0.3μm, and cracks, which cannot be seen from the surface, were present.
