Effect of nanodiamond reinforcement and heat-treatment on microstructure, mechanical and tribological properties of cold sprayed aluminum coating

Nanodiamond (ND) is a superhard nanoparticle reinforcement for metal matrix composites with its superior hardness and wear properties. A homogeneous dispersed ND aluminum metal matrix composite coating was successfully synthesized by the cold spray process in the present study. TEM investigations of the as-sprayed coating corroborate nanodiamond's uniform distribution, an array of dislocations formed within the Al matrix and along the grain boundaries. A coating density of 99% was achieved upon heat-treatment due to the reduced porosity and good inter-splat bonding. Superhard ND reinforcement in the Al metal matrix coating contributed to a similar to 96% increase in microhardness compared to pure Al coating, which increased to 135% after heat treatment. The coefficient of friction for heat-treated Al-2 wt% ND coating decreased to similar to 35%, and the wear rate decreased by similar to 34% compared to the as-sprayed coating. This study demonstrates that a small (2 wt%) amount of ND addition in the Al matrix inhibits grain growth during heat treatment and increases microhardness and tribological properties.