On the microstructure-dependency of mechanical properties and failure of low-pressure cold-sprayed tungsten carbide-nickel metal matrix composite coatings

This study improves upon understanding of the effects of reinforcing particle content and microstructure on the mechanical properties and failure of low-pressure cold spray fabricated tungsten carbide-nickel (WC-Ni) metal matrix composite (MMC) coatings. Image analyses were performed on scanning electron microscope (SEM) micrographs of the coatings to characterize the microstructure and measure the total interfacial area between the reinforcing WC particles and the Ni metal matrix, the mean free path between the particles, the average particle size, and the porosity of the coating. Uni-axial quasi-static tensile testing was conducted on the as-sprayed coatings. The tensile stresses that were measured were coupled with the evolving strains that were calculated by using the digital image correlation (DIC) technique. The results showed that mechanical properties, namely tensile strength and Young's modulus, of the composite coatings increased with increasing WC content in the coatings. The increases were due to the refined microstructural features that occurred with the decrease in the porosity of the coating that was caused by significant consolidation of the Ni metal matrix. Furthermore, this increase in strength and Young's modulus was also related to the increase in the interfacial area between the reinforcing carbide particles and the metal matrix that was caused by the increase in carbide content in the coating, a decrease in the average size of the carbides in the coating, and a reduction of the mean free path between the carbide particles. A framework was presented to elicit the micro-macro relationships between the microstructure and the tensile strength of the cold-sprayed composite coatings. Based on the developed framework, two distinct coating mechanical strength regimes were observed, indicating a significant increase in the tensile strength of the coatings that were comprised of more than 30 wt% WC due to the refined microstructure of the coatings. The higher value tensile strength regime was also confirmed by the larger average mechanical energy absorbed to failure under tensile loading and lower damage accumulation. This collection of data of mechanical properties and the microstructures presented in this study are important to validate numerical-based failure models for cold-sprayed composite coatings. Altogether, the results of this study will enhance understanding of the sensitivity of mechanical response of a composite coating to microstructural changes.