Highly efficient polishing of polycrystalline CVD diamond via atmosphere inductively coupled plasma

Objectives: As a typical difficult-to-machine material, polycrystalline CVD diamond often requires sophisticated and time-consuming polishing methods to achieve a smooth surface. In this study, a non-contact polishing method based on atmospheric inductively coupled plasma is investigated for the polishing of polycrystalline CVD diamond. Several indicators, including surface roughness, surface chemical composition, inner crystal structure, and material removal rate (MRR), are measured during the polishing process to evaluate the surface quality and polishing efficiency. Methods: The atmospheric inductively coupled plasma device, consisting of a plasma working zone, radio frequency (RF) power, electric sparker, RF match, mass flow controller, and water cooler, is used for polishing experiments. Oxygen is used as a reaction gas and added to the pure argon plasma, generating highly active oxygen radicals. The polycrystalline CVD diamond sample is fully exposed to oxygen-containing plasma irradiation to gain sufficient activation energy and react with the oxygen radicals. Once the processing is completed, the RF power is turned off, the oxygen supply is stopped, and the diamond is raised and cooled with pure argon shielding to protect the sample from being etched by ambient oxygen at high temperatures. Laser scanning confocal microscopy and scanning electron microscopy (SEM) are used to observe the diamond morphology, while the S_a roughness at different scales is characterized by scanning white interferometry and atomic force microscopy (AFM). The material removal rate is measured using an ultra-precision balance, and surface temperature is detected by an infrared imager. Raman spectroscopy and X-ray diffraction (XRD) are separately used to characterize the surface chemical composition and crystal orientation of the diamond before and after polishing. Results: The results of diamond treatment under different plasma irradiation conditions show that pure argon plasma tends to deteriorate the surface quality, while oxygen-containing plasma achieves significant polishing performance, removing grain tips and protruding structures. The morphology evolution of diamonds during the polishing process reveals that the initial surface is extremely rough, with severe surface irregularities and angular grain structures. After exposure to oxygen-containing plasma, the elevated grain structures are gradually removed, and the sharp edges are rounded. As the plasma irradiation time increases, some protruding sites are further eliminated, revealing the remaining cavities formed during the crystal growth process. Ultimately, the various crystal grains on the diamond surface maintain a consistent height, forming a relatively smooth surface. The fluctuating grain boundaries, which are widespread among the crystal grains, inhibit the formation of a smoother surface. The polishing effect can be explained by the principle of plasma-based atom-selective etching (PASE). During the polishing process, carbon atoms with different bonding states are randomly distributed on the diamond surface. Among them, carbon atoms at the grain tips, which only bond with atoms below and possess less stable bonds compared to those in the substrate, have lower activation energy. Consequently, tip carbon atoms preferentially react with active oxygen radicals due to their lower activation energy, accounting for the removal of grain tips during the initial polishing stage. As the polishing process continues, carbon atoms with fewer covalent bonds fade away, and the bonding states become equivalent in local areas, contributing to a progressively smoother surface. However, carbon atoms at the crystal boundary zones have complex arrangements and bonding characteristics, which restrict the differential removal of carbon atoms and hinder the overall polishing effect. The roughness evolution results show that the final S_a roughness of polycrystalline CVD diamond is reduced to 93.70 nm over a 400 μm × 400 μm area and 21.40 nm over a 20 μm × 20 μm area after polishing for 30 minutes. In some smooth areas, the roughness is as low as 2.53 nm, while in the crystal boundary zones, it reaches 31.30 nm. The presence of the crystal boundary zone hinders the global polishing performance. After 30 minutes of polishing, the surface roughness stabilizes, with the MRR also stabilizing at 34.4 μm/min. Surface composition analysis indicates a tensile stress of 1.5309 GPa on the diamond surface, and the polishing process dose not introduce new stress or amorphous carbon contaminants. XRD spectra show that there is no change in the crystal grain orientation of the diamond before and after polishing, demonstrating that PASE acts on all crystal planes of polycrystalline diamond (PCD) without preference for any specific crystal plane. Conclusions: Atmospheric inductively coupled plasma (ICP) can be used to efficiently smooth polycrystalline CVD diamond based on the principle of PASE. During the polishing process, oxygen is introduced as a reaction gas, generating oxygen radicals that selectively etch the diamond by preferentially removing carbon atoms with low activation energy, thus rapidly smoothing the surface. The S_a roughness of diamond is reduced from 10.10 μm (400 μm × 400 μm) and 338.00 nm (20 μm × 20 μm) to 93.70 nm and 21.40 nm, respectively, after 30 minutes of polishing. As polishing progresses, the MRR sharply decreases and eventually stabilizes at around 34.4 μm/min. The surface chemical composition and crystal grain orientation of polycrystalline CVD diamond remain consistent before and after polishing. Overall, atmospheric ICP can be an efficient pre-polishing method to significantly improve the overall polishing efficiency of polycrystalline CVD diamond.