ULTRALOW FRICTION, TRIBOCHEMICAL WEAR, AND THE HUMIDITY-INDUCED FRICTIONAL TRANSITION OF ULTRANANOCRYSTALLINE DIAMOND INTERFACES
Andrew R. Konicek, David S. Grierson, Anirudha V. Sumant, Matthew A. Hamilton, W. Gregory Sawyer, and Robert W. Carpick
Polycrystalline diamond films exhibit extremely low friction coefficients (≤ 0.01) and wear rates (< 10-9 mm3N-1m-1) in environments with sufficient levels of passivating species (e.g. water). Tribometry on ultrananocrystalline diamond (UNCD) films shows that even low relative humidity (RH) levels (0.8 %) provide adequate lubrication to sustain low friction (μ < 0.007). Our results confirm this is due to passivation of surface bonds, not rehybridization. We also observe a dramatic transition in friction when the RH drops below a velocity-dependent threshold. At 1 mm/s, for example, the friction coefficient increased by an order of magnitude when the RH dropped below 1.7 %, and recovered when the RH was raised above 2.0 %. To explore this slightly hysteretic but otherwise reversible behavior, self-mated UNCD interfaces were subjected to sliding in Ar and N2 atmospheres with controlled H2O partial pressures to understand how tribochemical mechanisms are related to gas adsorption kinetics. Ex situ measurements by optical profilometry and X-ray photoelectron emission microscopy (X-PEEM) with near-edge X-ray absorption fine structure spectromicroscopy characterized the wear scars. X-PEEM was used to determine the degree of hydrogenation, oxidation, and rehybridization. We will discuss the correlations between friction, wear, and tribochemistry.
This work was partially supported by the US Department of Energy, BES-Materials Sciences, under Contract W-13-109-ENG-38 and the Air Force grant FA9550-05-1-0204. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.