Production and tribological properties of friction-induced ultra-fine and nanocrystalline structures on metal surfaces

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Sliding friction tests of pin-on-disc type were carried out for carbon steel, pure iron and pure copper, and the microstructure and hardness near the sliding surfaces were investigated in detail. It was found that patchy transfer layers with ultra-fine structures were produced on the disc surfaces. Nanocrystalline of 30-50nm was identified for carbon steel, and submicron size grains of 100-150nm were observed in pure copper. The thicknesses of the ultra-fine structures were in the range of 10-50μm, depending on the specimen material, sliding speed and applied load. The hardness near the sliding surface of pure iron was increased compared with the matrix. It was suggested that the hardening was due to the very fine structure by severe plastic deformation, but not due to phase transformation caused by thermal effects.