Oxidation behavior of CrN_x and (Cr,Al)N_x hard coatings

Abstract

Abstract Reactively magnetron‐sputtered CrN x and (Cr,Al)N x hard costings were oxidized in air at temperatures between 500°C and 900°C for periods up to 16 h. The original and the oxidized samples were characterized by scanning electron microscopy, X‐ray diffraction and scanning Auger electron spectroscopy. The results show that CrN x coatings oxidize by growth of chromium oxide layer on top of the coating governed by outward diffusion of Cr. Below the oxide layer, a Cr‐depleted and N‐enriched intermediate nitride layer is left. The ternary (Cr,Al)N x nitride coating is about five times more oxidation resistant than CrN x at all temperatures between 500°C and 900°C. The oxide layer on (Cr,Al)N x shows a strong variation in composition with depth, which can be interpreted as a main oxide layer of a Cr‐rich (Cr,Al) oxide followed by an Al‐enriched (Cr,Al) oxynitride. In contrast to CrN x , a wide interdiffusion zone of N and O is found. In both cases the oxidation process is mainly controlled by Cr diffusion.