Correlation of particle impact conditions with bonding, nanocrystal formation and mechanical properties in kinetic sprayed nickel

Owing to the specific high-strain-rate thermomechanical characteristics of Ni particle impact in kinetic spraying, the rebound phenomenon of the impacting particles hinders the formation of the first layer and impedes successful build-up of the coating. Even at higher impact velocities, the deposition efficiency of the coating is quite low because of excessive kinetic energy, which induces the rebound and/or erosion of the highly flattened particles. This paper reports noticeably improved bonding and deposition characteristics of Ni particles resulting from suppressed equivalent (von Mises) flow stress and enhanced interface heat-up as a result of powder preheating. Experimental observations coupled with finite-element modeling (FEM) corroborate the fact that the thermally softened Ni particle is very effective for enhanced adhesive and cohesive bonding. Based on the FEM results, the thermal boost-up zone, increased by thermally accelerated adiabatic shear instability, is proposed as a crucial factor for enhancing bonding between the particles, which is essential in producing better coating properties. Moreover, nanocrystal formation (<100 nm) in the coating was more pronounced than cases previously reported in the literature, mainly because of the enhanced thermal activation and straining of the severely deformed particles, which was verified by transmission electron microscopy investigations and nanoindentation tests.