The influence of bias voltage on structure, thermal stability and mechanical properties of arc evaporated Cr0.69Ta0.20B0.11N coatings

CrTaN coatings grown by cathodic arc evaporation have recently been reported to exhibit high hardness, good thermal stability and promising tribological properties. Despite the known grain refining and thus hardening effect of boron on transition metal nitrides, detailed studies on quaternary CrTaBN coatings are lacking in the literature. Thus, within this work, Cr _0.69Ta _0.20B _0.11N coatings were grown by cathodic arc evaporation and the effect of the applied bias voltage on the microstructure and mechanical properties of the coatings was investigated. All coatings were grown on cemented carbide substrates and crystallize in an fcc-Cr _xTa _1-xN solid solution without indication for additional crystalline boron containing compounds. Increasing the negative bias voltage results in a decreased surface roughness, higher compressive residual stress and improved hardness. Starting at ~1200 °C, the powdered fcc-Cr _xTa _1-xN solid solution decomposes into h-Cr ₂N and h-Ta ₅N ₄, followed by the formation of h-Ta ₂N and bcc-Cr at 1400 °C. An annealing treatment at 1100 °C for 15 min has no significant influence on the phase composition of the coatings on cemented carbide substrates. Annealing at 1200 °C however induces a reaction between the coating and the substrate. As a result, fcc-TaC and bcc-Cr form, which is accompanied by loss of nitrogen and deterioration of the mechanical properties.