Caution: Diamonds slippery when wet

Tag : coefficient of friction

While I am sure many readers of Nobel Intent are aware that diamondis the hardest known naturally occurring material—a full 10on the Mohs scale—what many may not know is that is is alsoan extremely slippery material. The surface of diamond has aremarkably low coefficient of friction, which gives the diamond anadded level of utility in industrial applications. This lowfriction coefficient may not be surprising given diamondsrelationship to graphite, a well known lubricant.
While science and engineering have a good explanation for theslipperiness of graphite, namely its layered structure, the causeof diamond's slickness is not so readily explained. Its crystalstructure is composed of two interspersed, face-centered cubicstructures, something that will not necessarily lead to themolecularly smooth surface found in graphite. While there is nogeneral molecular theory of friction , previous hypotheses for diamond's slippery surface suggested thata phase change to graphite played a role.
The prior line of thought was that energy transfered between twodiamond surfaces sliding over one another was enough to turn thediamond into graphite, which lubricated the motion. To date though,no detailed spectroscopic measurement had been taken to confirmthis idea. As reported in the June 11th edition of Physical Review Letters , researchers used a technique known as photoelectron emissionmicroscopy to examine the passivation of the atomic bonds on thediamond surface. They found that the previous ideas were wrong; thesurface atoms are not being rearranged into graphite. Instead, theyare being passivated through the dissociative adsorption of watermolecules. The researchers also found that the coefficient offriction went up when the surrounding environment was less humid.
The experiment was carried out by sliding ultrananocrystallinediamond films across one another. Even in dry conditions (relativehumidity near one percent), the films are extremelyslick—with a friction coefficient of approximately 0.01, thefilms are slicker than ice. Using a host of spectroscopic and forcemeasurement techniques, the experimenters looked at the wearpatterns after the surfaces were slid across one another. Theyfound no detectable amounts of graphite; they did find that some ofthe diamond rearranged to amorphous carbon with oxygen atoms bondedin along the wear tracks.
This discovery led them to conclude that the water from theenvironment plays a large role in the low friction coefficient seenin diamond films. The researchers imply that this knowledge can beused 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