HIGH-TEMPERATURE OXIDATION OF THIN CRN COATINGS DEPOSITED ON STEEL

Chromium nitride coatings were deposited on steel substrates by the te trode plasma-beam sputtering technique at 200 degrees C. The coatings exhibit a dense, smooth, and homogeneous structure with the appearance of occasional pinhole defects, as shown by scanning electron microsco py (SEM) and atomic force microscopy. The thickness of the coatings wa s sufficiently small (0.34 mu m) to allow the study of the composition al changes induced by the high temperature oxidation throughout the ox ide and nitride phases, as well as at the inner interface between the nitride coating and the substrate. The CrN coatings were oxidized in a n oxygen flow at temperatures up to 750 degrees C. After oxidation at temperatures higher than 450 degrees C the formation of a Cr2O3 layer as a separate phase on the top of the nitride coating is observed. It is suggested that the mechanism of CrN oxidation proceeds by the progr essive replacement of nitrogen by oxygen. As the temperature is raised the thickness of the Cr2O3 layer increases and its grain structure be comes more pronounced. Besides oxidation of CrN to Cr2O3 another paral lel process is taking place, namely the oxidation of iron from the sub strate. This process leads to the formation of FeO islands, which repr esent defect spots on the surrounding Cr2O3 surface. The number of the se defects depends on the temperature and time of oxidation. In order to study the failure mechanism, various surface analytical techniques were applied, i.e., x-ray photoelectron-spectroscopy, Auger electron s pectroscopy, energy dispersive x-ray spectroscopy, and SEM. It is sugg ested that the penetration of iron through the coating starts at the p inholes present in the coating. At higher temperatures, however, this is not the dominating mechanism, and the diffusion process is further facilitated by the mismatch in the thermal expansion coefficients betw een the substrate and the nitride coating. Thermal stress thus establi shed leads to the loss of adhesion at the inner substrate/coating inte rface and therefore creates additional paths for the diffusion of iron . (C) 1996 American Vacuum Society.