OXIDATION RESISTANCE OF (TI,AL,ZR,SI)N COATINGS IN AIR

We prepared TiN-based multicomponent hard coatings by combined d.c. an d r.f. magnetron sputtering with different contents of Ti, Al, Zr and Si on high speed steel substrates at 300 degrees C. These coatings, wi th thicknesses ranging from 1 to 3 mu m, were annealed in air at tempe ratures between 500 and 850 degrees C in order to obtain information o n their oxidation behaviour. The composition-depth profiles of heal-tr eated coatings were then measured by Rutherford backscattering spectro metry (RES). The results show that the introduction of aluminium impro ves the oxidation resistance in all cases. During the heat treatment o f Ti0.62Al0.38N at 600 degrees C, a mixed oxide of Ti and Al is formed . At temperatures between 700 and 850 degrees C, a protective superfic ial layer of Al2O3 with traces of Ti is formed, which is followed by a n aluminium-depleted zone. However. for Ti0.57Al0.38Zr0.05N coatings, no protective Al2O3 layer was found on the surface after heat treatmen t. At 600 degrees C? the oxidation resistance of Ti0.57Al0.38Zr0.05N i s similar to that of(TiAl)N, but is about 30 times poorer at 700 degre es C due to the absence of the protective Al2O3 layer. The Ti0.62Al0.2 6Si0.12N system shows a slightly lower oxidation resistance than (TiAl )N (K-p = 9.0 x 10(-12) kg(2) m(-4) s(-1) and K-p = 6.4 x 10(-12) kg(2 ) m(-4) s(-1) at 800 degrees C respectively). It also forms a two-phas e scale as in (TiAl)Nt but the amount of Ti in the Al-rich outer layer is about 10 at.%, instead of 4 at.% found in the (TiAl)N system. In t he temperature rang 700-850 degrees C, oxidation is thermally activate d with activation energies of 187 kJ mol(-1) and 296 kJ mol(-1) for (T iAl)N and (TiAlSi)N coatings respectively.