Oxidation behavior of CrNx and (Cr,Al)Nx hard coatings

Reactively magnetron‐sputtered CrNx and (Cr,Al)Nx 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 CrNx 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)Nx nitride coating is about five times more oxidation resistant than CrNx at all temperatures between 500°C and 900°C. The oxide layer on (Cr,Al)Nx 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 CrNx, a wide interdiffusion zone of N and O is found. In both cases the oxidation process is mainly controlled by Cr diffusion.