Oxidation resistance of (Ti,Al,Zr,Si)N coatings in air

Abstract

We prepared TiN-based multicomponent hard coatings by combined d.c. and r.f. magnetron sputtering with different contents of Ti, Al, Zr and Si on high speed steel substrates at 300°C. These coatings, with thicknesses ranging from 1 to 3 μm, were annealed in air at temperatures between 500 and 850°C in order to obtain information on their oxidation behaviour. The composition-depth profiles of heat-treated coatings were then measured by Rutherford backscattering spectrometry (RBS). The results show that the introduction of aluminium improves the oxidation resistance in all cases. During the heat treatment of Ti 0.62Al 0.38N at 600°C, a mixed oxide of Ti and Al is formed. At temperatures between 700 and 850°C, a protective superficial layer of Al 2O 3 with traces of Ti is formed, which is followed by an aluminium-depleted zone. However, for Ti 0.57Al 0.38Zr 0.05N coatings, no protective Al 2O 3 layer was found on the surface after heat treatment. At 600°C, the oxidation resistance of Ti 0.57Al 0.38Zr 0.05N is similar to that of (TiAl)N, but is about 30 times poorer at 700°C due to the absence of the protective Al 2O 3 layer. The Ti 0.62Al 0.26Si 0.12N system shows a slightly lower oxidation resistance than (TiAl)N ( K p = 9.0 × 10 −12 kg 2 m −4 s −1 and K p = 6.4 × 10 −12 kg 2 m −4 s −1 at 800°C respectively). It also forms a two-phase scale as in (TiAl)N, but the amount of Ti in the Al-rich outer layer is about 10 at.%, instead of 4at.% found in the (TiAl)N system. In the temperature range 700–850°C, oxidation is thermally activated with activation energies of 187 kJ mol −1 and 296 kJ mor −1 for (TiAl)N and (TiAlSi)N coatings respectively.