NbC-Ni coatings deposited by DC reactive magnetron sputtering: Effect of Ni content on mechanical properties, thermal stability and oxidation resistance

Abstract

Hard coatings have been widely used in the machine tool industry to increase the wear resistance and hence to improve their service lifetime, as a result of their better mechanical properties. Most of the studies focused more in nitrides and less in carbides. In this study, we evaluated the effect of Ni additions on the properties of NbC films deposited by magnetron sputtering. In particular, we investigated the microstructure, chemical composition, mechanical properties, oxidation resistance and thermal annealing of the films. The properties of Ni rich coatings were compared to a NbC film deposited as reference. All the films displayed a columnar morphology with columns extending from the substrate up to the surface. Increasing the Ni content decreases the NbC columns diameter and their level of compactness. Excluding the substrate contribution, all the diffraction peaks could be generally assigned to the fcc NaCl type structure. A progressive decrease of grain sizes, down to 2.5nm is observed, with Ni additions higher than 17at.% Ni, leading to a quasi-amorphous structure. Precipitation of a hexagonal NiCx phase led to an antagonistic shift of the (111) emphases and (200) NbC peaks to the right and to the left, respectively. The hardness of the films increased with increasing Ni content in the range of 0<at.% Ni<13 and then progressively decreased with further Ni additions. Annealing performed at 600 and 800°C increased the hardness and Young's modulus of the films. Plastic deformation resistance H3/E2 parameters also showed a growing trend with Ni additions. TGA measurements showed that Ni additions increased the oxidation performance of the coatings. The onset point of oxidation of the films is higher by 100°C for Ni containing films independently of the Ni amount, when compared to NbC nickel free films.