Water Makes Diamond Coatings Slippery

Crystalline diamond is prized as a durable, low-friction coating for the contacting surfaces of high-performance tools and certain machine parts. The reason for diamond's slippery surface, however, has been the subject of much debate. One hypothesis has been that when two surfaces rub together, the carbon surface structure of diamond morphs into a graphite structure. A group of researchers led by Robert W. Carpick of the University of Pennsylvania now provide spectroscopic evidence that the diamond structure is maintained (Phys. Rev. Lett. 2008, 100, 235502). After rubbing together two objects coated with ultrananocrystalline diamond films, which contain diamond grains smaller than 5 nm in diameter, the researchers used X-ray techniques to analyze the worn regions of the surfaces. They found no sign of graphite, but they did find signs of carbon oxidation. They also observed lower friction and less wear in humid conditions. The team proposes that as the surface carbon atoms are stripped off by mechanical friction, water dissociatively adsorbs on the worn surfaces to produce C–H and C–OH bonds that provide lubrication.