Mechanical properties, thermal stability and oxidation resistance of Ta-doped CrAlN coatings

The thermal decomposition of metastable CrAlN coatings into wurtzite AlN and cubic Cr accompanied by N-loss limits their advanced machining applications. Here, the element of Ta is introduced into CrAlN coatings to enhance the mechanical properties and thermal stability. Incorporation of Ta into CrAlN coatings enhances the hardness from 30,8 +/- 0.9 GPa for Cr0.48Al0.N-52 to 32.7 +/- 0.9 GPa for Cr0.36Al0.59Ta0.05N and 34.4 +/- 0.7 GPa for Cr0.31Al0.59Ta0.10N. Doping with Ta optimizes the thermal stability of CrAlN coatings by postponing the dissociation of Cr-N bonds, where the N-loss process is delayed by similar to 127-144 degrees C according to DSC results. Correspondingly, the Ta-containing coatings invariably exhibit higher hardness upon annealing. Nevertheless, the Ta-addition brings about an adverse impact on the oxidation resistance of CrAlN coatings, due to the earlier formation of Ta-containing oxides. After oxidation of 10 h at 1000 degrees C, the Cr0.48Al0.52N, Cr0.36Al0.59Ta0.05N and Cr0.31Al0.59Ta0.10N coatings present an oxide layer of similar to 0.19, 0.27 and 0.30 mu m, respectively.