Shear Induced Dynamic Grain-Refinement in Sliding Polycrystalline Metal Surfaces

Tribological shearing of conventional polycrystalline metal surfaces typically leads to grain refinement near the sliding interface, however, how and why grain refinement occurs in metallic surfaces under tribological shear remains poorly understood. Here, employing large scale atomistic simulations ( $$\sim $$ ∼ 5 to $$\sim $$ ∼ 44 million bcc iron atoms on 100 to 500 HPC cores) and ultra high vacuum microtribometry, we capture the mechanisms leading to the formation and evolution of the refined-grain layer and discuss its implications on the ongoing sliding motion. For pure bcc iron, the simulations showed that fundamentally the initial grains are refined through the generation of numerous dislocations and twin boundaries that quickly agglomerate and start forming non-crystalline walls, which eventually leads to the formation of relatively stable grain boundaries. As in the simulations, experimentally sheared pure iron surfaces showed significant grain refinement in the near surface region within the first two reciprocating sliding cycles with only minor changes in grain structure and very slow growth of the refined layer into the rest of the substrate for increasing number of sliding cycles. The simulations, more importantly, revealed that the evolution of the refined-grain structure is a dynamic continuously evolving process where after the initial grain structure is refined and a nanocrystalline layer is established, the refined grains are then continuously coarsened and refined by creating and moving grain boundaries, twin boundaries and other lattice defects. The motion of the grain boundaries is the main shear accommodating mechanism in the refined layer as the sliding motion continues. In essence, the generated refined layer hardens the surface and minimizes plastic flow towards the surface. Simultaneously, the dynamic coarsening and refining of the grains in the refined layer via grain boundary migration, allows the near surface material to undergo the necessary plastic shear deformation to be able to accommodate the imposed sliding motion by the counter surface.