Tribochemical wear in a transmission electron microscope
Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge February 2013
In situ transmission electron microscopy studies reveal how chemical reactions affect the wear of diamond-like carbon.
ONE OF THE CENTRAL CHALLENGES IN TRIBOLOGY is observing asperity contacts in a sliding interface. Electron microscopy can be used to image a contacting tip that mimics an asperity sliding against a surface. In a previous column, we showed how in situ electron microscopy could follow the fate of fullerene nanoparticles in the sliding contact.
A group comprised of professor Laurie Marks from Northwestern University, Dr. James Ciston from the Brookhaven National Laboratory and Drs. Ali Erdemir and Osman Eryilmaz from Argonne National Laboratory, has extended this approach to examine the effects of the environment on the wear of diamond-like carbon (DLC). Films of DLC grown by the Argonne group are some of the lowest-friction materials known when measured in a vacuum or an inert environment but are susceptible to wear when water vapor is present. Insights into the chemical effects that might contribute to wear were obtained by measuring electron-energy loss spectra (EELS) of the samples.
In this technique, the energies of the electrons are analyzed after interacting with the sample. The characteristic energy losses of the electrons can be used to measure the ratio of sp2 to sp3 carbons in the sample, as well as to identify the elements that are present.
To prepare very thin, electron-transparent DLC samples, films were grown on a sodium- chloride substrate. The water-soluble substrate allowed the films to be floated off and deposited onto a transmission electron microscope (TEM) sample holder. The DLC sample was rubbed against a tungsten tip with a radius of curvature of ~100 nm to mimic the asperity contact in a TEM that could operate with ~1.5 Torr of a gas present. Wear was followed by measuring changes in the contrast of the images, since thinner samples transmit more electrons and produce a brighter image.
Nanoscale wear tracks were found on the sample after rubbing in the presence of wet nitrogen. The wear volume was found to increase with the number of sliding passes, but almost no wear debris was found. This suggested that wear had occurred by a tribochemical reaction to form volatile species.
The EELS spectra revealed both oxygen on the surface, which increased significantly as the samples were rubbed, as well as a growth of the proportion of sp2-hybridized carbon, indicating that the surface becomes graphitized during rubbing. In contrast, when the experiment was performed with wet hydrogen, no wear was found, although oxygen was still detected on the surface.
Evidently, the gas-phase environment has a significant influence on the wear of DLC. It was proposed that sliding produces chemically activated carbon atoms in the surface region. The activated atoms are passivated by hydrogen but not by less reactive nitrogen. The activated carbon atoms are then oxidized by water from the gas phase to produce carbon monoxide, thereby leading to wear without debris formation. Such a combination of in situ imaging with chemical analyses of a sliding nano-asperity provides unique insights into the interaction between tribochemistry and wear.
REFERENCES
1. Tysoe, W. and Spencer, N.D. (2011), “Imaging Rolling Nanoparticles,” TLT, 67(6), p. 96.
2. M’ndange-Pfupfu, A., Ciston, J., Eryilmaz, O., Erdemir, A. and Marks, L.D. (2012), “Direct Observation of Tribochemically Assisted Wear on Diamond-Like Carbon Thin Films,” Tribology Letters, DOI 10.1007/s11249-012-0074-x.
Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.
Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.