Influence of the grain size of CVD diamond on the thermal conductivity, material removal depth and surface roughness in nanosecond laser machining

Due to the exceptional thermal conductivity and material hardness, CVD diamonds have been ideal materials for cutting tools; however, the specific properties of CVD diamonds vary with the grain size. An experimental study was carried out on chemical vapor deposition (CVD) diamonds with varying grain sizes in laser processing. This study aims to analyze the behavior of polycrystalline CVD diamonds under different thermal processing based on its thermal conductivity model. The pit formed under a single laser pulse on the material with a small grain size is shallow and tiny, because of its small thermal conductivity, a value calculated using the established model. The results show that for laser-machined surface, the material with a small grain size can obtain a better machined surface but a small material-removal depth; conversely, material with a large grain size provided opposite experimental results. Additionally, reasonable laser parameters achieved the desired material-removal depth of up to or even exceeding 180 mu m and surface roughness Sa as low as 0.2 mu m. Overall, both the grain sizes of polycrystalline CVD diamonds and laser parameters affect the machining result. However, there are considerable obstacles in simultaneously achieving a good surface and massive material removal. Hence, a tradeoff between the two aspects is essential in an actual machining process, and this study provides some useful guidance to practitioners and academicians.