High-efficiency Polishing of Polycrystalline Diamond
Polycrystalline diamond (PCD) is extremely hard for mirror finishing and so the large polishing time and the cost become serious problems. Previous wet polishing tests proved that the stock removal was large (0.04–0.05 mm3/min). However, high polishing pressure was necessary (100–200 MPa) and the surface roughness was too coarse (Ra=40–60 nm). This is because the machining mechanism of PCD is based on a micro fracture (micro cutting) of diamond. In this experiment, a dry polishing of PCD is attempted, for which a polishing machine and new types of wheel (polishing plate) have been manufactured. In the dry polishing process, the polishing temperature between the polishing plate (wheel) and PCD becomes higher, so that thermo-chemical reactions such as graphitization, oxidation and diffusion of diamond will become main mechanisms of polishing instead of the micro cutting action.
Three types of metal bond for diamond wheel were prepared. The first is an iron bond (Fe), which wears diamond thermo-chemically at high temperature. The second is a cobalt bond (Co), which has a wear property similar to the Fe bond and a slight tendency of wetting with diamond. The third is a bronze bond (B). Diamond wheels (grain size; #800, concentrations; 25, 100) were made from these bond materials.
Before polishing tests, precise truing of wheel was carried out on a machine using the GC-cup method. Polishing pressure in operation was controlled by a micro-computer to be constant Rising of the polishing temperature makes an oxidation of the bond surface progressive. In cases of Fe and Co bonds, especially, overlapping of the oxides on their surfaces induces the reduction of thermo-chemical reaction and the adhesion of the oxides onto the polished surface, so that the stock removal reduces. Therefore, in-process shaving (IS) was carried out to remove the oxide layer on the bond surface. On the conditions of Fe bond, V=6000 rpm and P=2.5 MPa, mirror polished surfaces of Ra=8 nm and flatness =96 nm/mm were obtained.