High-rate synthesis of ultra-nanocrystalline diamond in an argon-free hot filament chemical vapor deposition atmosphere for tribological films

In the present study, we demonstrate that the ultra-nanocrystalline diamond (UNCD), which has a mirror-like surface finish with a thickness-independent surface roughness of Ra ∼5 nm and an average grain size as low as 2.7 nm, could be deposited at a growth rate up to 1.5 μm/h in an argon-free hot filament chemical vapor deposition (HFCVD) condition. The growth mechanism is attributed to the high content of energetic carbon precursors, which is enabled by a low-pressure (600 Pa) and a high CH4:H2 ratio (≤1:10) environment, resulting in a substantially high renucleation rate. The hardness and Young's modulus of the deposited UNCD film are 28.8 GPa and 225 GPa respectively. In the ball-on-plate friction tests, the UNCD film exhibits a stable COF stabilizing at 0.063–0.076, which is 43%–55% lower than that for the MCD and NCD film. Besides, its wear rate is in the order of 10-8 mm3N-1m-1 with the counterbody of Si3N4 balls, comparable to that of the MCD and NCD film. We believe that the synthesis method presented in this study is a practical and economical approach for the deposition of UNCD films as a tribological film in a variety of applications. [Display omitted] •High-rate deposition of ultra-nanocrystalline diamond (UNCD) films.•Surface morphology, chemical phase and microstructure of the as-deposited UNCD film.•Mechanism of the UNCD deposition in the Ar-free HFCVD atmosphere.•Mechanical and tribological performance of the UNCD film compared to the micro- and nanocrystalline diamond film.