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 gra in near equiaxed microstructure were found to exhibit a compressive residua l stress of - 6.5 GPa in contrast to conventional columnar Ti0.44Al0.53Cr0. 03N coatings which demonstrated - 3.8 GPa compressive stress. Novel coating s with this modified microstructure were also found to possess improved res istance to both dry oxidation and wet aqueous corrosion. Glancing angle par allel beam geometry X-ray diffraction (GAXRD) studies showed that in conven tional Ti0.44Al0.53Cr0.03N films, severe oxidation initiated above 850 degr ees C whilst oxidation of Ti0.43Al0.52Cr0.03Y0.02N started close to 950 deg rees C. In an alkaline aqueous medium, Ti0.43Al0.52Cr0.03Y0.02N coatings de posited on steel showed an extended passive potential range and a significa ntly lower passive current compared with Ti0.44Al0.53Cr0.03N films of simil ar thickness. A similar improvement was evident in sulphuric acid where ytt rium 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 eff ects of low concentrations of yttrium on high-temperature oxidation perform ance. (C) 1999 Elsevier Science Ltd. All rights reserved.