While I am sure many readers of Nobel Intent are aware that diamond is the hardest known naturally occurring material—a full 10 on the Mohs scale—what many may not know is that it is also an extremely slippery material. The surface of diamond has a remarkably low coefficient of friction, which gives the diamond an added level of utility in industrial applications. This low friction coefficient may not be surprising given diamond's relationship to graphite, a well known lubricant.
While science and engineering have a good explanation for the slipperiness of graphite, namely its layered structure, the cause of diamond's slickness is not so readily explained. Its crystal structure is composed of two interspersed, face-centered cubic structures, something that will not necessarily lead to the molecularly smooth surface found in graphite. While there is no general molecular theory of friction, previous hypotheses for diamond's slippery surface suggested that a phase change to graphite played a role.
The prior line of thought was that energy transfered between two diamond surfaces sliding over one another was enough to turn the diamond into graphite, which lubricated the motion. To date though, no detailed spectroscopic measurement had been taken to confirm this idea. As reported in the June 11th edition of Physical Review Letters, researchers used a technique known as photoelectron emission microscopy to examine the passivation of the atomic bonds on the diamond surface. They found that the previous ideas were wrong; the surface atoms are not being rearranged into graphite. Instead, they are being passivated through the dissociative adsorption of water molecules. The researchers also found that the coefficient of friction went up when the surrounding environment was less humid.
The experiment was carried out by sliding ultrananocrystalline diamond films across one another. Even in dry conditions (relative humidity near one percent), the films are extremely slick—with a friction coefficient of approximately 0.01, the films are slicker than ice. Using a host of spectroscopic and force measurement techniques, the experimenters looked at the wear patterns after the surfaces were slid across one another. They found no detectable amounts of graphite; they did find that some of the diamond rearranged to amorphous carbon with oxygen atoms bonded in along the wear tracks.
This discovery led them to conclude that the water from the environment plays a large role in the low friction coefficient seen in diamond films. The researchers imply that this knowledge can be used to help direct research to a next generation of super hard, low friction materials for a variety of industrial applications.
Physical Review Letters, 2008. DOI: 10.1103/PhysRevLett.100.235502