Effect of grain boundary on polycrystalline diamond polishing by high-speed dynamic friction

Abstract

In-situ micro-area analysis of precisely polished polycrystalline diamond (PCD) film by high-speed three-dimensional dynamic friction polishing (3DM-DFP) is carried out in this work, continuing from the previous works focusing on the subsurface cleavage and polishing mechanism. Multi-orientational grains with variable hardness anisotropy are generally considered the reason for the surface irregularity of PCD after smoothing. The unreached surface ultra-smoothness in local region after polishing is mainly resulted from the height difference at grain boundary (GB) area (e.g., a selected GB micro-region is of around Ra roughness of 5.7 nm and altitude intercept of 6.6 nm). However, different individual grains nearby showing (100), (110), (111) orientations are all of roughness <0.5 nm, demonstrating these places are contiguously polished. The observed local roughening at GBs region after mechanical sliding accompanying with friction heat is the major factor to cause the surface irregularity, which is affected by the inhomogeneity of crystal thermal expansion and contraction associated with the defects in multi-orientational crystals generated by mechanical process. Among multi-crystals the low-position (110) region has a Ra roughness of 0.1 nm but with great tensile stress and relatively weak distortion compared with other orientational areas. An insight of the inhomogeneous volume change associated with thermal expansion and contraction affected by defects condition was proposed to straightforwardly illuminate the rationale of GB effects on the ultra-smoothing of PCD by 3DM-DFP.