Influence of the testing parameters on the tribological behaviour of self-mated PVD-coatings

Friction and wear of self-mated PVD-coated tribo-couples were investigated using different testing parameters. By means of pin-on-disc geometry, tribological tests with coatings like TiN, CrN and (Ti,Al)N were performed. Fundamental investigations on these sliding pairs show that the tribological behaviour is strongly influenced by the testing conditions. Depending on the ambient medium, relative humidity, sliding distance or velocity etc., very different friction and wear coefficients can be observed before the coating of at least one tribo-partner is worn out. The variation of a single test parameter can lead to a fundamental change of the functional behaviour of the whole tribo-system. While TiN- and CrN-coatings reveal very good results at high relative humidities, (Ti,Al)N-coatings show its best performance at dry environments or higher ambient temperatures. After the tests, the worn surfaces and wear debris were investigated by means of microanalysis, e.g. Auger electron spectroscopy (AES), small-spot electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy (SEM). Small-spot-ESCA gives valid information about the chemical composition of the surface before and after tribological stressing. The results of microanalytic investigations show that tribochemical reactions are the dominating wear phenomena. The extent of these tribochemical reactions and the chemical composition of the wear debris depends on the ambient conditions and the testing parameters. The tribological behaviour of TiN/TiN sliding pairs gets worse when TiO 2 is built as a result of tribochemical reactions. On the other side, the formation of Cr 2 O 3 and CrO 3 during tribological stressing do not lead to a significant change of friction and wear of CrN/CrN sliding pairs. For (Ti,Al)N/(Ti,Al)N sliding pairs tribo-oxidation is the dominating wear mechanism. The wear debris consist of titanium and aluminium oxide, but the chemical structure is not clear yet.