Oxidation resistance of titanium–aluminium–silicon nitride coatings

The demands for cutting tools tend to still longer service life and higher speed in cutting processes. To fulfil these demands, hard and wear resistant thin coatings have to be deposited on cutting tools. The functionality of the coating must be ensured at the high temperatures reached at the cutting edge during machining. At these temperatures, oxidation of the coatings may lead to problems. Titanium–aluminium nitride coatings show an oxidation behaviour superior to the widely used titanium nitride coatings. To further enhance the oxidation resistance, the addition of silicon to titanium–aluminium nitride films is analysed in this work. The investigated coatings were deposited by reactive DC magnetron sputtering on high-speed steel. The composition of the coatings could be varied by the type of target used and the applied voltage to each target and was determined by glow discharge optical spectroscopy (GDOS). The coating thickness is derived from ball cratering and ranges between 3 and 6 μm. The adhesion of the coatings is analysed by the scratch test. The coated samples are subjected to an oxidation test, where they are exposed to a temperature of 800 or 1000 °C and an oxygen partial pressure ranging between 1 and 100 mPa for 1 h. The results of the TiAlSiN coatings are compared with TiAlN coatings to evaluate the influence of the silicon. Ultra-microhardness measurements are performed prior and after the oxidation tests. Changes in the composition due to diffusion processes are measured with GDOS over the whole coating thickness and the coating/substrate interface. X-ray photoelectron spectroscopy reveals changes of composition and binding energy in the near surface zone.