Study on the mechanism of ultra-low friction coefficient of CVD diamond in vacuum

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Recently, super-lubricity phenomenon showing low coefficient of friction of 0.1 or less were found in the friction test performed in vacuum for the combination of CVD diamond and metals. This phenomenon can't be explained by conventional theory, since almost all matals are thought to be show high coefficient of friction under vacuum condition. To clarify the mechanism of this phenomenon, friction tests for the combination of CVD diamond and many kinds of metals were carried out in air, vacuum, and argon. In this experiment, three types of stainless steels with different Ni contents were used for mating material of CVD diamond. From the XPS analysis of sliding surfaces, carbon and oxygen was found for combination of CVD diamond and metals, which show especially low coefficient of friction. From these results, it was clarified that the oxygen in the oxide film play a role as a catalyst for graphitization of diamond, and showing a low coefficient of friction in vacuum.