Improvement of Cutting Performance of Silicon Nitride Tool by Adherent Coating of Thick Diamond Film

A thick diamond film was coated using two‐stage microwave plasma CVD in the COH, system on a pretreated silicon nitride substrate for a cutting tool. The effects of acid treatment and microflawing treatment of the substrate on adherence of the film and cutting performance were investigated as well as the effects of two‐stage CVD conditions. The combination of substrate pretreatment in a hot, strong acid solution of HF and HNO3 and subsequent ultrasonic microflawing pretreatment with diamond grains resulted in the anchored deposition of CVD diamond into the micropores in the silicon nitride substrate. An excellent adherence of the diamond film to the substrate was attained by the two‐stage CVD, which consists of a first CVD of fine diamond grains into the micropores and a second higher rate CVD of thick diamond film (thickness >30 pm). A dense layer composed of diamond‐like carbon and silicon nitride was formed deep in the boundary region of the substrate during long CVD treatment. Long tool life of the silicon nitride chip coated with a thick diamond film was verified by a milling test using Al‐20 wt% Si alloy as the work material.