Effect of WC morphology on dry sliding wear behavior of cold-sprayed Ni-WC composite coatings

Cold spray is a relatively new method used to deposit WC reinforced composite coatings, where its low temperature is advantageous for avoiding oxidation and carbide decomposition. Previous studies demonstrated that using powders made from agglomerated WC resulted in higher WC retention in a sprayed coating when compared to that of cast WC. However, the influence of the morphology of the starting powders on the coating's microstructure, properties, and wear performance is not well understood. Here, we report cold spray deposition of Ni with two types of WC particles, i.e. cast and agglomerated. In both cases, similar to 30 vol% WC was retained in coatings, allowing for a side-by-side comparison. Coatings with cast WC featured a multi-modal distribution of WC particles ranging from 0.2 to 20 mu m with a mean free path (MFP) between particles of 8.5 +/- 0.7 mu m. In comparison, coatings with agglomerated WC had WC size range of 0.3 to 1.3 mu m and an MFP of 31 +/- 4 mu m. The sliding wear behavior of coatings was studied with a sliding speed of 3 mm/s under normal loads of 5 and 12 N. Coatings with cast WC were found to be more wear resistant than coatings with agglomerated WC. The multimodal size distribution of cast WC with significantly lower MFP minimized adhesive wear and helped to develop a higher coverage of protective mechanically mixed layers (MMLs) that typically formed near WC particles. For coatings with cast WC, subsurface microstructure and chemical analysis suggested higher oxidation for MMLs with shallower depths of deformation in the metal matrix beneath the MMLs compared to coatings with agglomerated powder. The main factors for improved wear resistance of cast WC coatings compared to agglomerated WC coatings were the stability of the MMLs, and the wider size distribution with lower MFP, which offered better load supporting properties.