Atomistic origins of material removal rate anisotropy in mechanical polishing of diamond crystal

In this work, molecular dynamics simulation is employed to represent the diamond polishing. Radial distribution function and coordination number analyses are further performed to reveal the underlying atomistic origins of the removal rate anisotropy. The results show that the lattice distortion is inevitable as the diamond substrate suffers from the mechanically induced effects, which produces an amorphous layer on the surface. In the amorphization, the perfect diamond cubic transforms to some non-diamond phases, including the amorphous sp0, sp1, sp2 and sp3 hybridized structures and well-arranged sp2 structures. However, the dominant phases are sp2 and amorphous sp3 phases. More interestingly, it is found that the removal rate strongly depends on the proportion of sp2 hybridizations to amorphous sp3 structures. In the 'hard' direction, phase transformation from amorphous sp3 to sp2 is difficult, and therefore a low proportion of sp2 to amorphous sp3 appears, which results in a small removal rate. In the 'soft' direction, phase transformation from amorphous sp3 to sp2 has less resistance, and a higher proportion output, which gives a greater removal rate. The variation laws as revealed above confirm that the removal rate anisotropy in diamond polishing is derived from the concentration of sp2 hybridizations in the as-created amorphous layer and debris.