Application of a simple surface nanocrystallization process to a Cu–30Ni alloy for enhanced resistances to wear and corrosive wear

A simple surface nanocrystallization process combining repeated hammering and recovery treatment was applied to Cu–30Ni alloy for improved resistance to corrosive wear in a NaCl solution. This study was motivated by aquaculture applications of copper alloys, which require not only the resistance to pure corrosion but also that to corrosion involving wear such as scratching. The synergy of corrosion and wear may significantly accelerate surface failure of copper alloys in the marine environment. We nanocrystallized the surface of the copper alloy by repeatedly hammering the surface followed by a recovery treatment that turned dislocation cells into nano-sized grains, which considerably strengthened the surface. The high-density grain boundaries largely enhanced atomic diffusion and thus improved the passivation capability of the nanocrystallized surface. As a result, the nanocrystallized surface possessed higher resistances to corrosion, wear and corrosive wear. In this work, particular emphases were put on effects of the hammering duration on the thickness of nanocrystallized surface layer and variations in corrosion, wear and corrosive wear rates as a function of the distance from top surface to bulk. Variations in other relevant properties with the distance were also investigated, including the adhesive force, friction coefficient and hardness. The relationships between the properties and depth help to estimate the durability or service life of the nanocrystallized surface layer. Efforts were made to clarify the mechanisms responsible for observed phenomena and relationships.