Grinding characteristics of CVD diamond grits in single grit grinding of SiC ceramics

Single crystalline diamond (SCD) grits are usually made by a high-pressure and high-temperature (HPHT) method and exhibit cuboctahedral shapes and smooth surfaces. In this paper, we synthesized new kinds of SCD grits and nanocrystalline diamond (NCD) grits by a chemical vapor deposition (CVD) method and explored their grinding performance by single grit grinding of SiC ceramics. Grinding by HPHT grits was also conducted for comparison. The CVD-SCD grits exhibit cuboctahedral shapes, but surfaces are covered by protruded twin crystals of several microns, while the NCD grits are spherical agglomerations of nano-grains. We demonstrated that these morphological features have profound influences on material removal behavior. Results show that the wear of NCD grits is dominated by wear flat and intergranular cracks. The worn NCD grits can exert intense rubbing and ploughing effects on the workpiece. In contrast, the wear of CVD-SCD grits is dominated by fracture of the protruded twin crystals. It is proved that the twins on the grit surface are effective cutting edges to remove the material in a micro scale and thus lead to the formation of fine grinding striations and smaller cracking pits, but this process has little influence on specific grinding energy. Besides, grits with rough twins, especially on the flank face, tend to exhibit lower grinding force ratios and thus better sharpness. Moreover, for grits with smooth rake faces, there exists an explicit critical undeformed chip thickness value around 0.4-0.6 mu m, above which the material removal mechanism changes from ductile to brittle rapidly. However, for grits with rough twins on rake faces, this transition process becomes vague.