Examination of wear mechanisms of hard coatings

PVD-CrN and -TiN coatings are abraded in a wear test equipment which simulates the sliding-rolling friction of a screw rotor used in screw compressors. The coatings show significant differences concerning their durabilities. The worn-out coatings as well as the debris particles produced by abrasion are analyzed by means of X-ray induced photoelectron spectroscopy and Auger electron spectroscopy. The failure mechanism of TiN coatings can be explained by spalling off small debris particles, whereas the failure mechanism of CrN coatings has to be attributed to tribooxidation, because in this case the analyzed particles exclusively consist of chromium oxide. Nevertheless, CrN coatings show the lowest wear rate. When changing the environment (inert atmosphere or water lubrication) in the test equipment, the tribooxidation of CrN can be reduced or totally stopped, but a decrease in wear rate cannot be achieved. In this case tribooxidation leads to a better wear resistance.