Diamond is extensively used in industrial applications, such as cutting tools and sliding parts, due to its exceptional properties, including high hardness, low friction, and excellent wear resistance. Conventional methods for synthesizing diamond films include plasma-enhanced chemical vapor deposition and hot filament chemical vapor deposition (HFCVD). Although HFCVD can coat three-dimensional (3D) metallic surfaces, achieving uniform deposition on complex geometries remains difficult. The microwave sheath–voltage combination plasma (MVP) method can deposit diamond-like carbon films on rod-shaped substrates, with successful diamond film deposition on flat substrates already demonstrated. This finding demonstrates the potential of the MVP method for synthesizing nanocrystalline diamond films on complex 3D surfaces. In this study, we investigated the effect of CH4 concentration on carbon film deposition using the MVP method. A grounded-type MVP setup was employed, where the substrate was electrically grounded and a positive voltage was applied to an external electrode. Films were deposited from CH4–H2 gas mixtures onto rod-mounted substrates. At low CH4 concentrations, needle-like structures were observed, whereas higher concentrations resulted in granular film morphologies. X-ray diffraction suggested the presence of diamond-related phases at higher CH4 concentrations, and the deposited films exhibited a maximum hardness of 46.2 GPa. These results indicate that the films likely contain nanocrystalline diamond components, although contributions from amorphous carbon cannot be completely excluded.
TANAKA et al. (Thu,) studied this question.