Washington, Jun 24: University of Pennsylvania engineers say that they have found out what causes diamonds to slip and slide with remarkably low friction at the nanoscale, something that makes it an ideal material and coating for seals, high performance tools and high-tech moving parts.
Robert Carpick, associate professor in the Department of Mechanical Engineering and Applied Mechanics at the university, says that the new findings may help create the next generation of super low friction materials. The researchers looked at two of the main hypotheses posited for years as to why diamonds demonstrate very low friction and wear properties. Using a highly specialized technique know as photoelectron emission microscopy (PEEM), they found that the slippery behaviour came from passivation of atomic bonds at the diamond surface that were broken during sliding.
According to them, the bonds are passivated by dissociative adsorption of water molecules from the surrounding environment.
The researchers also found that friction increased dramatically where there was not enough water vapour in the environment.
Until these findings, most scientists believed that the friction between sliding diamond surfaces imparted energy to the material, converting diamond into graphite, itself a lubricating material.
The Penn researchers say that their findings do not lack legitimacy because they are the first of their kind to be supported by spectroscopic tests.
The PEEM instrument, part of the Advanced Light Source at Lawrence Berkeley National Laboratory, allowed the group to image and identify the chemical changes on the diamond surface that occurred during the sliding experiment.
Working with his colleagues, Carpick tested a thin film form of diamond known as ultrananocrystalline diamond, and found super low friction and low wear even in extremely dry conditions.
The researchers used a precise friction tester called microtribometer, and a spatially resolved X-ray spectroscopy technique called X-ray photoelectron emission microscopy, to examine wear tracks produced by sliding ultrananocrystalline diamond surfaces together at different relative humidities and loads.
They found no detectable formation of graphite, and just a small amount of carbon re-bonded from diamond to amorphous carbon.
However, oxygen was present on the worn part of the surface, indicating that bonds broken during sliding were eventually passivated by the water molecules in the environment.
Carpick and his colleagues believe that their study may help increase the use of diamond films in machines and devices to increase service life, prevent wear of parts, and save energy wasted by friction.
A research article on their findings has been published in the journal Physical Review Letters.