Microstructure and mechanical performance of nickel coating on copper sheet via laser shock processing

Abstract

A novel technique of laser shock processing (LSP) was proposed to consolidate Ni powder into coating at room temperature to protect Cu substrate. This is a combined process of shock compaction of powder, shock micro-welding of particle/particle and particle/substrate. The required pressures of each process were calculated and the effect of laser energy on the aspect of microstructure, surface morphology, and mechanical properties of coatings were investigated. Experimental results showed that a fully dense (relative density 99.81%), smooth and uniform Ni coating without oxidation was fabricated due to the high-speed, high-pressure, and cold processing characteristic of LSP. The coating/substrate interface presented curved convex feature by mechanical locking and a 1.2 μm thick atomic diffusion layer by metallic bonding under adiabatic shear instability. The relative density and mechanical performance of coating was improved with a proper increase of laser energy. The maximum hardness of coating reached 332 HV, presenting approximately three times increment of substrate. The wear rate of coating decreased sharply with increasing laser energy, with the minimum value being 20.7% of substrate. The improvement of mechanical properties was attributed to the coupled effect of void collapse by high-speed plastic or viscoplastic deformation and the shockwave-induced microstructure refinement.