Diamond films were deposited on silicon nitride (Si₃N₄) substrates with three different roughnesses using the method of hot-filament chemical vapor deposition (HFCVD). The tribological properties of the film were studied by changing the deposition time, deposition distance, and methane (CH₄) concentration. The friction coefficient, delamination threshold load, and wear rate of the diamond films were tested and calculated using the reciprocating friction and wear test under dry friction conditions. The results show that, when the deposition time is 12 h, the bonding force of the film is the lowest and the friction coefficient is the largest (0.175, 0.438, and 0.342); the deposition distance has little effect on the friction performance. The friction coefficients (0.064, 0.107, and 0.093) of nano-diamond films (NCD) prepared at a 40 sccm CH₄ concentration are smaller than those of micro-diamond films (MCD) prepared at a 16 sccm CH₄ concentration. The load thresholds before delamination of R_a 0.4 μm substrate diamond film are as high as 40 N and 80 N, whereas the diamond films deposited on R_a 0.03 μm substrates have lower wear rates (4.68 × 10⁻⁴ mm³/mN, 5.34 × 10⁻⁴ mm³/mN) and low friction coefficients (0.119, 0.074, 0.175, and 0.064). Within a certain load range, the deposition of a diamond film on a R_a 0.03 μm Si₃N₄ substrate significantly reduces the friction coefficient and improves wear resistance. Diamond film can improve the friction performance of a workpiece and prolong its service life.

In this paper, diamond coatings with different microstructure were prepared on cemented carbide substrates by HFCVD technique. The changes of grain growth mode and size of diamond coating under different substrate temperature (800 °C, 850 °C, 900 °C) and carbon source concentration (2%, 4%, 6%) were studied, and the differences of coating performance were explored. Both substrate temperature and carbon source concentration have an effect on grain size. When the grain size is in micron scale (0.5 ∼ 6 μm), the growth mode is columnar crystal competing growth. When the substrate temperature reaches 900 °C and the carbon source concentration is 6%, the diamond grain will be fully secondary nucleated, and the grain size will reduce to the nanometer scale (20 ∼ 50 nm). The growth mode is particle cluster accumulation growth and the rate of growth increases significantly. The increase of grain size will increase the adhesion of the coating and its resistance to wear and external loading. However, it may lead to larger friction coefficient, which is about 0.1. Due to the small grain size, the adhesion force of nanocrystalline diamond coating is obviously smaller than micrystalline diamond coating, but the friction coefficient is only 0.03.