Generation and oxidation of wear debris in dry running tests of diamond coated SiC bearings

Diamond coated silicon carbide (SiC) bearings can be used in pumps to enable dry running or mixed lubrication conditions. Dependent to different test parameters the coefficient of friction and the wear behaviour shows big differences. In this work α-SiC bearings were coated in a hot filament chemical vapour deposition process (HF-CVD) with a diamond layer thickness of 4 μm. Diamond coated rings were tested in a tribometer under dry running conditions in a ring–ring geometry. During tribological tests under various atmospheres (oxygen, argon, nitrogen, ambient air) the coefficient of friction shows values in a range from 0.1 to 0.4. These values can be ascribed to different chemical and mechanical processes between the mating surfaces. Tribological tests of self mating diamond layers generate wear debris between the mating surfaces. This diamond wear debris can convert from sp3 hybridized to sp2 hybridized carbon. Against common expectations tribological tests at ambient air with high sliding velocities and normal loads did not show wear debris, despite of high wear rates. Under inert gas (argon or nitrogen) the sliding surface shows agglomerated areas with sp2 hybridised carbon. The reason is the absence of oxygen, which is necessary for combustion of carbon species. The coefficient of friction remains nearly constant during the test in oxidizing atmosphere, but shows high fluctuations in inert gas atmospheres. The authors explain this tribological behaviour by a new combustion wear model for self mating diamond coated bearings.