Ion-beam-deposited surface layers of diamond-like carbon (DLC) on fine-grain chemical-vapor-deposited (CVD) diamond have been found to be effective in reducing friction and wear in a variety of environments, including ultrahigh vacuum. This discovery opens the possibility of taking fuller advantage of the properties of CVD diamond and DLC to manufacture protective coatings to provide solid lubrication and resistance to wear, erosion, and corrosion. Such coatings could be applied to surfaces of bearings, valves, cams, gears, and magnetic recording disks and tapes, for example. Notwithstanding the high costs of natural diamonds of gem quality, the costs of DLC and CVD diamond are similar to those of CVD and physical-vapor-deposited (PVD) carbide and nitride films. The one major disadvantage of DLC is its lack of resistance to high temperature; its use must be restricted to temperatures ≤250 °C in air and ≤350 °C in vacuum.
CVD diamond offers some solid lubrication and resistance to wear. However, friction and wear rates of as-deposited CVD diamond depend on the environment; in particular, they are greater in vacuum than in humid air or dry nitrogen. CVD diamond can be modified by carbon- or nitrogen-ion implantation to obtain an amorphous, nondiamond carbon surface layer that reduces friction and wear regardless of the environment. However, the surface layer is usually ≤.5 μm thick; consequently, endurance is limited in the sense that use must typically be limited to light-load and/or short-term operations. In contrast, DLC films can be deposited to thicknesses as great as 5 μ m, with concomitant potential for increasing endurance.
The wear-reducing and self-lubricating properties of specimens of ion-beam-deposited DLC on fine-grain CVD diamond were investigated in a series of experiments. For comparison, specimens of ion-beam-deposited DLC on silicon and specimens of both as-deposited and carbon-ion-implanted fine-grain CVD were also included in the experiments. The specimens were fabricated on flat disk silicon substrates, then coefficients of friction were measured while the specimens were rotated in sliding contact with CVD-diamond-tipped hemispherical pins.
The sliding-contact tests were performed in humid air, dry nitrogen, and vacuum. Surface profilometry was performed to characterize surface features and determine surface roughnesses and depths of wear. The results of the sliding-contact tests (summarized in the figure) indicate that ion-beam-deposited DLC can effect significant reductions in the friction and wear of fine-grain CVD diamond.
This work was done by Kazuhisa Miyoshi of Lewis Research Center and Richard C. Wu and William C. Lanter of K Systems Corp.
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