Water, not Graphite, Makes Diamond Slippery

Experiments performed at Penn State, 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.
The abstract of the study, “Origin of ultralow friction and wear in ultrananocrystalline diamond,” is available here http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000100000023235502000001&idtype=cvips&gifs=Yes

Ultrananocrystalline Diamond Thin Films Advance

Left: UNCD deposited onto a 200 mm silicon wafer shown on top of an uncoated wafer. Right: High-resolution transmission electron microscopy image of UNCD thin film showing nano-sized grains.

Thin film diamond chemical vapor deposition (CVD) technology was developed about 30 years ago. Despite advances over the years, CVD diamond has failed to meet lofty industry expectations based on the perfection of natural diamond. Instead of becoming the material of choice for demanding applications, thin film diamond has been commonly referred to as the material of last resort; engineers consider CVD diamond only if every other commercial material fails to do the job.

In addition to cost, a number of technological problems, such as poor reproducibility, lack of mature deposition technology, relatively small deposition areas and poor film properties, have restricted the application of thin film diamond to cutting tools, heat sinks and other niche markets. Over the past two decades, research at Argonne National Laboratory led to the discovery of ultrananocrystalline diamond (UNCD®), a new technology that overcomes previous limitations related to thin film diamond. Advanced Diamond Technologies (ADT) was formed in 2003 to commercialize the ultrananocrystalline diamond technology developed at Argonne National Laboratory.

The UNCD innovation is rooted in the chemistry used to synthesize the material. In the past, the growth chemistries used to synthesize pure CVD diamond resulted in rough films characterized by very large diamond grains (microns in size) and weak, low-angle grain boundaries. These large-grained, microcrystalline diamond (MCD) films were nearly impossible to integrate with other materials. Efforts to develop smoother films led to work on nanocrystalline diamond (NCD). Argonne researchers discovered that by adding argon gas, along with methane and hydrogen, to the vapor mixture, the radicals generated during growth changed and yielded diamond films consisting of approximately 5 nanometer grains without any graphitic phases (i.e., phase-pure, diamond-bonded carbon with high-energy, atomically abrupt grain boundaries).

Since 5 nm grain UNCD is phase-pure, diamond-bonded carbon, it retains the desirable characteristics of natural diamond, including hardness, modulus, refractive index, acoustic velocity and surface chemistry. UNCD's electronic, thermal and optical transport properties, however, are different from natural diamond. It also has a low thermal conductivity compared to single crystal diamond (10-20 vs. ~2200 W/m°K) and is not optically transparent. From an electronic structure point of view, UNCD has diamond's bandgap (5.5 eV), but the bonding states within the grain boundaries create a material that is more electrically conductive with lower electron mobility. UNCD features a number of unique properties, including low as-deposited film roughness, low stress, higher fracture toughness and higher strength compared to natural diamond. Unlike previous NCD technologies, UNCD films can be grown at lower temperatures (400°C compared to 600-900°C). All of these differences result in a thin film diamond that can be easily integrated with other materials, such as other thin film technologies that are the basis for microelectronics and microelectromechanical systems (MEMS). More on UNCD applications coming soon. http://www.ceramicindustry.com/CDA/Articles/Cover_Story/BNP_GUID_9-5-2006_A_10000000000000177645