The influence of low concentrations of chromium and yttrium on the oxidation behaviour, residual stress and corrosion performance of TiAlN hard coatings on steel substrates

Ti0.43Al0.52Cr0.03Y0.02N films, which have been shown to exhibit a fine grain near equiaxed microstructure were found to exhibit a compressive residual stress of - 6.5 GPa in contrast to conventional columnar Ti0.44Al0.53Cr0.03N coatings which demonstrated - 3.8 GPa compressive stress. Novel coatings with this modified microstructure were also found to possess improved resistance to both dry oxidation and wet aqueous corrosion. Glancing angle parallel beam geometry X-ray diffraction (GAXRD) studies showed that in conventional Ti0.44Al0.53Cr0.03N films, severe oxidation initiated above 850 degrees C whilst oxidation of Ti0.43Al0.52Cr0.03Y0.02N started close to 950 degrees C. In an alkaline aqueous medium, Ti0.43Al0.52Cr0.03Y0.02N coatings deposited on steel showed an extended passive potential range and a significantly lower passive current compared with Ti0.44Al0.53Cr0.03N films of similar thickness. A similar improvement was evident in sulphuric acid where yttrium containing coatings passivated at high potential (Ti0.44Al0.53Cr0.03N films did not passivate). These effects may be ascribed to reduced porosity in the fine-grained Ti0.43Al0.52Cr0.03Y0.02N as well as the well-known effects of low concentrations of yttrium on high-temperature oxidation performance. (C) 1999 Elsevier Science Ltd. All rights reserved.