Crystal grain size effects and crystallinity dynamics during supersonic particle impacts

We examine the effect of crystal grain size on particle impacts with substrates at supersonic speeds, which are commonly used in producing metal and ceramic coatings. Large-scale atomistic simulations are utilized to examine the impact outcomes of 60 nm nickel particle impacts on nickel substrates, with mean crystal grain sizes varied from 5 nm to 30 nm, representing nanoparticles commonly formed in vapor phase synthesis processes. We observe that smaller grain particles have higher critical strains and lower Young's moduli than their larger grain counterparts due to having a larger fraction of grain boundaries. Smaller grain particles also experience a greater degree of heating and amorphization during higher speed impacts, while defect formation and propagation are hindered. We also provide insight into experimentally observed losses in crystallinity during high-speed deposition processes; in all cases a rapid loss of crystallinity occurs at impact, followed by recrystallization, with the net result being a decrease in crystal grain sizes.