They call diamonds "ice," and not just because they sparkle. Engineers
and physicists have long studied diamond because even though the material is
as hard as an ice ball to the head, diamond slips and slides with remarkably
low friction, making it an ideal material or 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
of Pennsylvania, and his group led a collaboration with researchers from
Argonne National Laboratories, the University of Wisconsin-Madison and the University
of Florida to determine what makes diamond films such slippery customers, settling
a debate on the scientific origin of its properties and providing new knowledge
that will help create the next generation of super low friction materials.
The Penn experiments, the first study of diamond friction convincingly supported
by spectroscopy, looked at two of the main hypotheses posited for years as to
why diamonds demonstrate such low friction and wear properties. Using a highly
specialized technique know as photoelectron emission microscopy, or PEEM, the
study reveals that this slippery behavior comes from passivation of atomic bonds
at the diamond surface that were broken during sliding and not from the diamond
turning into its more stable form, graphite. The bonds are passivated by dissociative
adsorption of water molecules from the surrounding environment. The researchers
also found that friction increases dramatically if there is not enough water
vapor in the environment.
Some previous explanations for the source of diamond's super low friction and
wear assumed that the friction between sliding diamond surfaces imparted energy
to the material, converting diamond into graphite, itself a lubricating material.
However, until this study no detailed spectroscopic tests had ever been performed
to determine the legitimacy of this hypothesis. 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.
The team tested a thin film form of diamond known as ultrananocrystalline diamond
and found super low friction (a friction coefficient ~0.01, which is more slippery
than typical ice) and low wear, even in extremely dry conditions, (relative
humidity ~1.0%). Using a microtribometer, a precise friction tester, and X—ray
photoelectron emission microscopy, a spatially resolved X-ray spectroscopy technique,
they examined 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.
Already used in industry as a mechanical seal coating to reduce wear and improve
performance and also as a super-hard coating for high-performance cutting tools,
this work could help lead to increased use of diamond films in machines and
devices to increase service life, prevent wear of parts and save energy wasted
by friction.
The study was published in the June issue of the journal Physical Review Letters
and was conducted by A.R. Konicek of the Department of Physics and Astronomy
at Penn, D.S. Grierson of the Department of Engineering Physics at Wisconsin-Madison,
P.U.P.A. Gilbert of the Department of Physics at Wisconsin-Madison, W.G. Sawyer
of the Department of Mechanical and Aerospace Engineering at Florida, A.V. Sumant
of the Center for Nanoscale Materials at Argonne National Laboratory and Carpick.
Funding was provided by the U.S. Air Force and the U.S. Department of Energy.