Influence of bias voltage on microstructure, mechanical properties and thermal stability of arc evaporated Cr0.74Ta0.26N coatings

Abstract

CrTaN coatings were deposited by cathodic arc evaporation and the influence of the bias voltage on the microstructure, mechanical properties and thermal stability was investigated. Independent of the applied bias voltage (−40, −60 and −80 V), all coatings crystallize in an fcc-Cr0.74Ta0.26N solid solution, while in contrast a significant influence on the residual stress and grain size was observed. Hardness, Young's modulus and fracture toughness of the as-deposited coatings are enhanced by an increasing bias voltage due to grain refinement and higher compressive residual stress. Powdered CrTaN coatings are stable in inert atmosphere up to ~1200 °C, where the formation of h-Cr2N, bcc-Cr and h-TaN0.8 provokes N2 release. An annealing treatment at 1000 °C for 15 min does not affect the phase composition of the coatings on cemented carbide. Annealing the samples however at 1000 °C for 2 h induces a reaction between coating and cemented carbide substrate, which results in the formation of fcc-TaC and h-Cr2N. As the bias voltage increases, the thermal stability of coating powders and the solid CrTaN coatings on cemented carbide substrates decreases. The lower thermal stability with increasing bias voltage can be attributed to the smaller grain size and thus higher number of diffusion pathways.