Study on the effect of Ar-containing work gas on the microstructure and tribological behavior of nanocrystalline diamond coatings

Using a combined pretreatment of sand-blasting and two-step etching, nanocrystalline diamond coatings with primarily diamond crystallites and good adhesion were deposited onto carbide cement by tuning the Ar/H-2 ratio in the hot filament chemical vapor deposition (HFCVD) work gas. The microstructure, bonding structure, surface morphology and roughness, and wear scar were characterized using a scanning electron microscope, an atomic force microscope, a white light interferometer, and a Raman spectrometer. The dry sliding behavior of the coating against Si3N4 was tested using a pin-on-disc instrument. It was found that the addition of Ar to the HFCVD work gas can enhance grain refinement and tune the morphology of the nanocrystalline coatings. The roughness was reduced from 87.7 nm for the coating prepared without Ar to 6.21 nm for the coating prepared with 90% Ar. With the addition of Ar, the friction coefficient and wear rate decrease monotonically. The wear rate of the coating prepared with 90% Ar was less than 5% that of the coating prepared without Ar, and the wear mechanism transforms from spallation to be limited by the stress-induced sp(2) tribo-layer.