Study on surface finishing strategy of ultrahard nanotwinned diamond

The newly developed nanotwinned diamond (ntD) has a hardness of as twice as monocrystal diamond excellent fine toughness and high onset oxidization temperature. With these excellent properties, the ntD bulk becomes a promising solution for machining difficult-to-machine materials as a cutting tool. Therefore, surface finishing strategies of this extremely hard material were studied. Mechanical lapping and laser polishing of ntD surface were investigated which were widely known available for surface finishing of diamond. In the experiment, mechanical lapping was an effective approach for smooth surface generation with surface roughness Ra lower than 5nm. But due to low material removal rate, it was difficult and time consuming to eliminate all the remained ripples and cracks of raw surface. On the other hand, laser polishing generated a homogeneous ntD surface without pits and cracks, and the surface roughness Ra was lower than 40 nm. Raman spectra with D-band (∼1332 cm^-1) only indicated the depth of phase translation layer on laser polished ntD surface was small. But the surface waviness was unacceptable because of the inevitable marks by laser beam ablation. Then the laser polished ntD surface was machined by mechanical lapping. The result indicated that laser marks were easily removed by mechanical lapping and the surface was very smooth without any regenerated pits and cracks. The final surface roughness reached Ra 0.34 nm. In all, combining laser polishing with mechanical lapping was a proper surface finishing solution for ntD bulk.