Sliding wear behavior of sub-microcrystalline pure iron produced by high-pressure torsion straining

Sliding wear behavior of sub-microcrystalline pure iron disks produced by high-pressure torsion (HPT) straining was studied using a ball-on-disk configuration, and the effects of the strain in the HPT process and of the duration of wear tests were investigated. It was found that the wear amount decreased with increasing number of turns during HPT, and that the specific wear rate of the initial stage was inversely proportional to the Vickers hardness of the disks. However, for long wear test times, the wear rate became very small for both non-deformed and HPT-processed specimens, because of transition from adhesive shearing wear to sliding process which was caused by (i) grain refinement at the wear-induced layer, (ii) submicron wear particles adhered to the worn surface, and (iii) surface flattening by local asperity deformation. There was no significant difference in microstructure and hardness beneath the worn surface between non-deformed and HPT-processed specimens after the wear tests. Thus HPT improved the wear resistance only at the early stage, and became less effective for long wear tests. This study is the first to show that submicron grains produced by HPT straining were further refined to a size of 100-150 nm by friction shear straining while grain refinement of iron is saturated upon further straining in HPT processing.