Friction and wear mechanisms of smooth diamond films during sliding in air and dry nitrogen

Under the influence of extreme contact pressure and high frictional heating, the real contact areas of diamond films may undergo phase transformation and gradual wear during long-duration dy sliding contacts. The wear debris particles that accumulate at the contact interface can then dominate the long-term sliding friction and wear performance of these films. In this study, employing a combination of transmission electron microscopy, electron diffraction, Raman spectroscopy, and electron energy loss spectroscopy, the authors explored the structural chemistry of the diamond debris particles and the sliding contact interfaces of smooth diamond films (surface roughness: 20-40 nm, root mean square (RMS)) and described their friction and wear mechanisms in open air and dry nitrogen (N-2) The results of tribological tests indicated that the friction coefficients of Si3N4 balls against smooth diamond films were 0.04 in dry N-2 but 0.1-0.15 in air. Friction fluctuated substantially in dry N-2, especially during long-duration tests. The wear rates of Si3N4 balls were by factors of 5 to 6 lower in dry N-2 than in air, but a reverse situation was observed for diamond films; their wear rates were significantly higher in dry N-2 than in air. The results of the surface and structure analytical studies have suggested that the sp(3)-bonded crystalline diamond had transformed to a sp(2)-bonded amorphous slate, but not to crystalline graphite.