On the influence of the Cr/(Cr + Ta) ratio on the microstructure, mechanical properties and thermal stability of magnetron sputtered CrxTa1-xN coatings

CrxTa1-xN coatings were deposited by magnetron sputtering and the influence of the Cr/(Cr + Ta) ratio on microstructure, mechanical properties, thermal stability and oxidation resistance was investigated. X-ray diffraction revealed that all coatings exhibit an fcc-CrxTa1-xN crystal structure with an increasing size of coherently diffracting domains as the Cr/(Cr + Ta) ratio increases. A balanced Cr/(Cr + Ta) ratio of 0.46 results in the highest hardness of 27.4 ± 1.0 GPa, whereas binary fcc-TaN was determined to have the highest Young's modulus of 417 ± 20 GPa. All Ta-containing coatings exhibit a KIC value of 2.40 ± 0.10 MPa × m1/2 and thus exceed the fracture toughness of CrN (1.53 ± 0.21 MPa × m1/2). While vacuum annealing at 1000 °C already provokes a decomposition of the binaries, N-deficient compounds only form in the ternaries at temperatures ≥1200 °C. The oxidation onset and end temperature as well as the phase composition of the oxides were found to depend significantly on the Cr/(Cr + Ta) ratio. Among the investigated coatings, Cr0.75Ta0.25N exhibits the most promising thermal stability, both in inert and oxidizing atmosphere.