Tools coated with diamond film (diamond-coated tools) are used for difficult-to-machine materials. A typical example of difficult-to-machine materials is carbon fiber-reinforced plastic (CFRP), which is used as a light and hard material to reduce the weight of aircraft and automobile bodies.

Tungsten carbide with cobalt as a bonding agent is usually used as a tool for diamond film deposition. Hot filament chemical vapor deposition (HF-CVD) is used as the film deposition technique. The following figure shows the film deposition process. When diamond film is deposited on cemented tungsten carbide by HF-CVD, it is known that the quality of diamond-coated tools deteriorates as the diamond film becomes non-uniform due to cobalt grains. Therefore, a liquid pretreatment is required to remove the cobalt on the cemented tungsten carbide. However, there are concerns about the environmental impact of liquid waste from this treatment (wet processing), so there is a need to develop a process that does not use liquids (dry processing). Our research team initially revealed the mechanism by which cobalt makes the diamond film surface non-uniform with the aim of developing a pretreatment that does not require wet processing. There are two types of diamond films deposited on tools: microcrystalline diamonds and nanocrystalline diamonds. Previous research has proposed a mechanism for non-uniform formation only for microcrystalline diamonds. However, there were no findings about nanocrystalline diamonds, which are considered to be smooth, less frictional, and be more suitable for tools. Secondary, the research team revealed the non-uniform formation mechanism for nanocrystalline diamonds as well as microcrystalline diamonds with time-resolved observation of the deposition process for each. In particular, we found that in nanodiamond, the non-uniform formation is caused by carbon filaments lifting particles on the carbide substrate and making the substrate non-uniform.

The research team also showed that it is important to inhibit carbon filament growth in the initial stage of deposition in order to inhibit the non-uniform formation of nanocrystalline diamond films in dry processing. In future, we plan to develop a deposition technique using HF-CVD in a high-temperature environment that can inhibit the growth of carbon filaments. Furthermore, after establishing a uniform formation process for nanocrystalline diamond films, we plan to apply the process to actual tools and investigate their machining performance.