Wear-induced microstructure in Ni/Cu nano-multilayers

Sliding wear tests were carried out in order to investigate wear resistance and resultant microstructure of Ni/Cu multilayers. The Ni/Cu multilayers having the component layer thickness h ranging from 5 to 100 nm were fabricated on copper substrates using the elecrodeposition technique. It was found that the wear depths in the multilayers were less than one-fifth of that of a conventional nickel coating at a high load condition. The wear resistance of the multilayer was almost independent of the component layer thickness, except the multilayer of h = 100 nm whose resistance was lower than those of the others. The observation of cross section revealed that the grains were generated locally near the worn surface in the Ni/Cu multilayers. Surface cracks were grown in such grained areas. The multilayer having a large grained area showed relatively low wear resistance. From the TEM observation, there were many equiaxed grains without the laminated structure. It is conceivable that the equiaxed grains without the laminated structure were formed due to dynamic recrystallization occurring after the laminated structure was annihilated by severe deformation. Assuming that the annihilation period is required for the wear of the Ni/Cu multilayer, the high wear resistance can be obtained regardless of the strengths of the multilayers.