Cold spray deposition of a Ni-WC composite coating and its dry sliding wear behavior

Abstract

Ni-WC composite are ideal protective coatings against wear that are often fabricated using laser cladding. However, high temperature of laser cladding and associated materials melting may result in non-homogenous distribution of WC particles, as well as decarburization and considerable residual stress and a deterioration of the wear resistance. An alternative is to use cold spray to deposit WC based composite coatings. The lower temperature allows one to retain the composition of initial WC feedstock but the cold spray process is only recently being researched for development of composite coatings. In this study, Ni and Ni-WC coatings were fabricated by cold spray. The WC and Ni powders were fed to a de Laval nozzle from separate hoppers with independent feed rates. By adjusting feedrates, a blend of Ni-36.2vol% WC was sprayed, which resulted in a composite coating of Ni-10.5vol.% WC. The influence of WC on Ni deposition was examined via microstructural characterization, including morphology of the coating's top surface and polished cross sections. Mechanical properties of coatings were improved by incorporation of WC into the Ni matrix. The wear behavior of coatings was studied with reciprocating sliding wear tests using a 6.35mm diameter WC-Co ball. All tests were conducted in dry air with a sliding speed of 3mm/s, a track length of 10mm, and normal load of 5N. WC-Ni coatings were more wear resistant than cold-sprayed Ni coatings. The correlations between worn surface morphologies, subsurface microstructure induced by wear and the wear behavior of the coatings was discussed. Microstructural analyses showed a mechanically mixed layer (MML) on the top of worn surfaces consisting of compacted oxides. It was revealed that the presence of hard particles in the Ni-WC coating facilitated fast development of the MML, as well as stabilized the MML, characterized by less plastic flow, fewer cracking and higher hardness.