High temperature oxidation of thin CrN coatings deposited on steel

Abstract

Chromium nitride coatings were deposited on steel substrates by the tetrode plasma-beam sputtering technique at 200 °C. The coatings exhibit a dense, smooth, and homogeneous structure with the appearance of occasional pinhole defects, as shown by scanning electron microscopy (SEM) and atomic force microscopy. The thickness of the coatings was sufficiently small (0.34 μm) to allow the study of the compositional changes induced by the high temperature oxidation throughout the oxide and nitride phases, as well as at the inner interface between the nitride coating and the substrate. The CrN coatings were oxidized in an oxygen flow at temperatures up to 750 °C. After oxidation at temperatures higher than 450 °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 progressive replacement of nitrogen by oxygen. As the temperature is raised the thickness of the Cr2O3 layer increases and its grain structure becomes more pronounced. Besides oxidation of CrN to Cr2O3 another parallel process is taking place, namely the oxidation of iron from the substrate. This process leads to the formation of FeO islands, which represent defect spots on the surrounding Cr2O3 surface. The number of these 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 spectroscopy, energy dispersive x-ray spectroscopy, and SEM. It is suggested that the penetration of iron through the coating starts at the pinholes 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 between the substrate and the nitride coating. Thermal stress thus established leads to the loss of adhesion at the inner substrate/coating interface and therefore creates additional paths for the diffusion of iron.