OXIDATION BEHAVIOR OF CHROMIUM-BASED NITRIDE COATINGS

Chromium-based nitride coatings (Cr2N, (Cr,Pd)N, (Cr,Al)N, (Cr,Al,Pd)N) were deposited on high-speed steel substrates by reactive magnetron sputtering. Annealing of the coatings was performed in air over the temperature range between 300 and 800 degrees C. The annealing time was varied from 5 min up to 4 h. The oxidized coatings were analysed using AES sputter depth profiling. The Cr2N coatings show a diffusion-controlled formation of Cr2O3 layer with an activation energy of 1.27 eV (outward diffusion of Cr). The (Cr,Pd)N coatings show, besides a Pd enrichment below the Cr2O3 layer, a very similar oxidation behaviour with a slightly lower activation energy of 1.16 eV as compared with Cr2N. In the case of(Cr,Al)N and (Cr,Al,Pd)N a sequence of different oxide layers (Cr-oxide, Al/Cr-oxide, oxynitride) is built up owing to complicated diffusion processes (outward diffusion of Cr and Al, inward diffusion of O), In (Cr,Al)N a wide oxygen interdiffusion region is observed, whereas in (Cr,Al,Pd)N this region is not very large owing to the small oxygen affinity of Pd. In both the oxidation processes of(Cr,Al)N and (Cr,Al,Pd)N, the diffusion of chromium is identified as the rate-limiting process, with activation energies of 0.62 eV for (Cr,Al)N and 0.77 eV for (Cr,Al,Pd)N.