To create tribosystems that are environmentally-friendly and free from health hazards, many researchers and engineers have tried to apply vegetable oils as practical lubricants. Our preliminary findings have shown that rapeseed oil and soybean oil exhibit a low friction coefficient and superior wear durability when used for the combination of a bearing steel ball and a phosphor bronze plate under temperature conditions of 40°C and 100°C. However, the long-term performance of tribosystems using vegetable oils has not yet been fully clarified. In our research, we have used an accelerated test method to investigate the transition of lubricity of a rapeseed oil related to its oxidative deterioration. The wear durability of the combination of a bearing steel ball and a phosphor bronze plate was improved together with the increased acid number (AN) of the rapeseed oil at both 40°C and 100°C. On the other hand, though the combination of a bearing steel ball and a bearing steel plate showed the same tendency at 40°C, the tendency was reversed at 100°C. It may have been induced by differences between the bearing steel and the phosphor bronze on both adsorptive capacity with fatty acid and corrosion-resistant characteristics.
Hydrogen-free diamond-like carbon (DLC) films with many sp3 bonding sites have a highly smooth surface, but very few studies have been carried out on the surface morphology of DLC films affected by the amorphous structure. In this study, the surface morphology of the DLC films was investigated on the basis of classical molecular dynamics simulations. The DLC samples composed of ten thousand carbon atoms were generated by the Melt-Quench scheme. We prepared 4 samples of various densities in order to control the ratio of sp2 / sp3 sites in the structure. As the calculation results, the ratios of sp3 sites of each structure were in proportion to the densities. We also found that the surface topography got rougher with a decrease in the sp3 sites in the structures. Sp2-rich structure had huge graphite clusters which were the series of many graphite rings over the structure, making curved surfaces, resulting in the formation of rougher surface. We discussed that the relaxation process at sp2-rich sites in the amorphous carbon structure promoted graphitization and clustering to stabilize the entire amorphous structures, and that sp3-rich films made a smooth surface because sp3 sites prevented carbon atoms from clustering.