2020 年 59 巻 1 号 p. 304-308
Ultra–precision cutting has its own problems that are not taken into the consideration in ordinary cutting. One of such problems is grain boundary step （GBS） formation. The GBS is a step forming along a grain boundary on ultra–precision finished surface of polycrystalline metals. The height of GBS can be higher than that of cutting marks, resulting in reducing the quality of the finished surface. Although grain boundary sliding is considered as the origin of GBS formation, the growth mechanism of GBSs is still not clear. In the studies of grain boundary sliding in creep phenomenon, the apparent activation energy was calculated from the temperature dependence of creep strain rate, and the mechanism of grain boundary sliding was proposed based on the calculated activation energy. The apparent activation energy will be effective measure to study the growth mechanism of GBSs. In this study, in order to calculate the apparent activation energy of GBS growth process, the change in the height of GBS at －30°C, 25°C, 60°C and 90°C was measured. The height of GBS saturated in shorter time at higher temperature. The height increasing with time was higher at higher temperature. The value of the apparent activation energy was about 20 kJ/mol, which was significantly smaller than the apparent activation energy of the grain boundary diffusion （104 kJ/mol）. Such difference in the apparent activation energy indicates that the growth of grain boundary steps on ultra–precision finished surface originates from mechanisms different from the grain boundary diffusion.