OXIDATION RESISTANCE OF PVD CR, CR-N AND CR-N-O HARD COATINGS

The plasma-beam-sputtering process was used to deposit coatings 3 mum thick of various compositions from pure chromium to stoichiometric CrN and a few CrN(x)O(y) coatings on steel and sapphire substrates at a temperature of 200-degrees-C. The formation of the coatings, their microstructure, morphology, composition and microhardness were determined. Their oxidation behaviour in an oxygen flow in an oven was studied at temperatures between 500 and 800-degrees-C by the weight gain method, X-ray diffraction and Auger electron spectroscopy depth profile analysis. The surface morphology and crystallite growth were observed by scanning electron microscopy and roughness measurements. The results show a pronounced minimum in weight gain of Cr-N(x) coatings as a function of partial pressure of nitrogen after heating in oxygen at 700 and 800-degrees-C. The protective layer on all the Cr-based coatings studied was a stable Cr2O3 oxide. Of all the hard coatings investigated, the stoichiometric CrN coating had the lowest oxidation rate in pure oxygen, also after a long-term test of 200 h. Oxidation in oxygen also induces the growth of crystallites, 0.05 mum in size in as-deposited coatings. After heating for 4 h at 700-degrees-C in oxygen, the grain size increased to 0.4 mum, while at 800-degrees-C it increased to 0.75 mum. There was no essential difference in grain growth between tool steel and sapphire substrates.