Precise and efficient surface flattening of polycrystalline diamond by normal-irradiated trochoidal femtosecond laser machining

The widely used polycrystalline diamond generally requires high smoothness and high efficiency in surface flattening. However, conventional machining methods, such as grinding and polishing, are not capable of processing the polycrystalline diamond with the desired high material removal rate owing to its high hardness, wear resistance and thermal stability. Femtosecond laser is considered an effective machining approach with high precision, but the machining efficiency is not satisfactory in terms of large-scale surface flattening. To further improve the processing efficiency with good surface smoothness, a strategy of normal-irradiated trochoidal femtosecond laser machining (NTFM) was developed in this study to process the polycrystalline diamond with Co binder. The theoretical analysis model, including the accumulated laser pulse number and the distribution of laser energy density, was first deduced for NTFM. The key influent factors were found to be the amplitude of galvanometer and the space between two adjacent laser scanning passes. Then, through machining microgrooves, it is proved that the material removal rate of NTFM was almost twice as high as the conventional nontrochoidal femtosecond laser milling (CFM). Besides, short pulse delay in the NTFM strategy can reduce the heat penetration and inhibit the falling-off of diamond grains. Thus, NTFM strategy is more favorable for generating smooth surface when compared to CFM. It is also found that improving the heat accumulation in NTFM is beneficial to decrease the graphitization level on the machined surface. Finally, a mirror-like large-scale surface of the polycrystalline diamond was successfully processed, with an average roughness of Ra 63 nm and P-V value of 0.741 mu m by NTFM, taking 30 min or only one-sixth of the time required by CFM. The results indicate that the proposed NTFM strategy is more practical to precise and highly efficient surface flattening of diamond compared to CFM.