Plasticity- Controlled Friction and Wear in Nanocrystalline SiC

Wear resistance of ceramics can be improved by suppressing fracture, which can be accomplished either by decreasing the grain size or by reducing the size of the deformation zone. We have combined these two strategies with the goal of understanding the atomistic mechanisms underlying the plasticity-controlled friction and wear in nanocrystalline (nc) silicon carbide (SiC). We have performed molecular dynamics simulations of nanoscale wear on nc-SiC with 5nm grain diameter with a nanoscale cutting tool. We find that grain-boundary (GB) sliding is the primary deformation mechanism during wear and that it is accommodated by heterogeneous nucleation of partial dislocations, formation of voids at the triple junctions, and grain pull-out. We estimate the stresses required for heterogeneous nucleation of partial dislocations at triple junctions and shear strength of GBs. Pile up in nc-SiC consists of grains that were pulled out during deformation. We compare the wear response of nc-SiC to single-crystal (sc) SiC and show that scratch hardness of nc-SiC is lower than that of sc-SiC. Our results demonstrate that the higher scratch hardness in sc-SiC originates from nucleation and motion of dislocations, whereas nc-SiC is more pliable due to additional mechanism of deformation via GB sliding.