Grain coarsening of nano laminated structure in martensite steel under sliding wear

The sliding wear-induced grain coarsening of nano laminated structure in a martensite steel was investigated by using Transmission Electron Microscopy (TEM) and transmission Kikuchi diffraction (TKD). Microstructure was initially refined into nano laminated structure by dry sliding. In the subsequent lubricated sliding, the nano laminated structure became coarsened instead of being continuously refined. Equiaxed grains and subgrains were observed in the laminated structure under 60 min dry + 60 min lubricated sliding, as compared to the coarsened but still remained lamellar structure under 120 min dry + 60 min lubricated sliding. In contrast to the grain refinement, which was mechanically driven but thermally assisted, the grain coarsening was mechanically dominated without involving the thermal effect. The nano laminated structure did not show strong texture before and after coarsening. The continuous grain growth occurred even if the stress was reduced in the lubricated sliding. The high density of triple junctions in the nano laminated structure promotes grain boundary motion, facilitating the grain coarsening.