University of Pennsylvania engineers reveal what makes diamonds slippery at the nanoscale

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.

Source: University of Pennsylvania

Related stories:

'Heftier' atoms reduce friction at the nanoscale
A research team led by a University of Pennsylvania mechanical engineer has discovered that friction between two sliding bodies can be reduced at the molecular, or nanoscale, level by changing the mass of the atoms at the surface. “Heavier” atoms vibrate at a lower frequency, reducing energy lost during sliding.
Ultrasmooth carbon for ultrahigh data storage density
Ultra-smooth diamond-like carbon surfaces are key to enable the ultimate storage density of 1 terabit/ inch² in next generation ultra-high storage density hard disks. Cambridge researchers shed light on how these surfaces grow, the key to enabling design engineers and manufacturers to optimize the structure of these surfaces for this and other applications.
From a sheet of ice to a hailstorm
Diamond-like carbon films have a low coefficient of friction because they are extraordinarily smooth. This is why they are applied to almost all PC hard disks and many machine parts. A scientific paper explains conclusively for the first time why deposited layers do not grow rough.
Argonne researchers create new diamond-nanotube composite material
Researchers at the U.S. Department of Energy's Argonne National Laboratory have combined the world's hardest known material – diamond – with the world's strongest structural form – carbon nanotubes. This new process for “growing” diamond and carbon nanotubes together opens the way for its use in a number of energy-related applications.
Diamonds are a Scientist's best Friend: Research into Building Better Small Machines
Do diamonds really last forever? That's the hope of University of Wisconsin-Madison researchers who are trying to solve the problems associated with building extremely small machines and having them withstand the test of time, wear and tear.
The problem is that these machines are so small - microscopic or smaller - that their moving parts cannot be assisted by lubricants; instead, they have to function in a dry state, like a car with no oil.
A really, really small car with no oil.
Energy Technology researchers solve energy and medical problems
Argonne's Energy Technology Division (ET) provides innovative materials and engineering solutions to national energy challenges that range from energy production and conservation to transportation. Researchers also find creative ways to re-use and extend the value of their discoveries.

The division's innovation has been recognized in the past two years by three R&D 100 Awards, given annually by R&D Magazine to the world's "100 most technologically significant new products."
Nanocoatings Can Save Energy, Costs
Argonne's Energy Technology Division (ET) provides innovative materials and engineering solutions to national energy challenges that range from energy production and conservation to transportation.

In 2003, nanostructured carbide-derived carbon (CDC) technology for sliding and rotating equipment received an R&D 100 award. CDC is grown with graphite, diamond, amorphous carbon and carbon "nano-onions" -- small carbon structures with concentric rings, resembling an onion. These components vary from 2 to 10 nanometers in thickness (one nanometer is one-billionth of a meter).
Nanopencil Can Provide Terabit Data Storage Density
(PhysOrg.com) -- Scientists have fabricated a 'nanopencil' with a tip so small that it can be used as a scanning probe in ultrahigh-density computer data storage systems.

News discussion:

Nanotechnology news