Development of WC-based metal matrix composite coatings using low-pressure cold gas dynamic spraying

A low-cost, low-pressure cold spray unit was used to fabricate tungsten carbide (WC)-based metal matrix composite (MMC) coatings. A sintered and crushed WC-based powder was mixed with nickel powder, forming various compositions of mechanical blends. Scanning electron microscopy (SEM), coupled with image analysis, Vickers micro-hardness, and X-ray diffraction (XRD) was used to characterize the coatings. Image analysis was used to determine the WC content in the coating and the mean free path between the reinforcing particles. The WC content in the coating increased as the WC content in the powder blend increased. Each coating fabricated by the various mechanical blend compositions had a non-homogeneous distribution of the hard WC reinforcing particles embedded in the nickel matrix. However, as the WC content in the powder increased, the coating porosity decreased and the WC deposition efficiency increased. An inversely proportional relationship between hardness and mean free path between the reinforcing WC particles was observed, where a decrease in mean free path led to an increase in hardness. The hardness of each MMC coating was predicted using the rule of mixtures (ROM) method and compared to the experimental hardness values. However, the ROM method did not completely describe the strengthening mechanism of the MMC coatings because densification, plastic deformation, and adiabatic shear instability played a role, especially for the coatings with a lower WC content.