Coupling effects of methane concentration and nitrogen addition level on morphologies and properties of MPCVD diamond films on WC-Co substrates

In the present study, at fixed hydrogen flow rate of 400 sccm, we demonstrate the coupling effects of methane/ hydrogen ratio eta C (7.5- 15 vol%) and nitrogen flow rate fN (0- 0.9 sccm) on morphologies, mechanical properties and tribology behaviors of the diamond films deposited on cemented carbide (WC-Co) substrates. At lower eta C (7.5 vol%), increasing fN from 0 to 0.9 sccm leads the diamond film to transform from (110)-oriented to (110)- & (100)-oriented. Besides, increasing eta C accelerates the orientation transition, i.e., {110} and {100} crystallographic planes can be formed at even lower nitrogen addition level. Furtherly increasing eta C and fN to 15 vol% and 0.9 sccm, the film morphology transforms to nanocrystalline diamond (NCD). Moderate nitrogen addition acting as a catalyst at the substrate surface increases the growth rate, by the mechanism of promoting the individual grains growth. However, supersaturated nitrogen will suppress the growth of individual grains by generating masses of secondary nucleation on the crystal faces, resulting in a slight reduction of the growth rate. The (110)& (100)-oriented diamond film deposited at eta C = 7.5 vol% and fN = 0.9 sccm and the NCD film both present low small surface roughness and coefficient of friction (COF) when sliding against the Si3N4 balls, while the former has higher hardness and Young's modulus, along with lower wear rate and better adhesive strength, which has great potential for the application as wear-resistant and anti-friction coating.