Friction and fretting on quasicrystals under vacuum

In order to investigate friction and fretting on quasicrystals at room temperature in a reliable manner, we have subjected sintered samples to tests in a vacuum chamber evacuated to a residual pressure lower than 10(-5) mbar. Pin-on-disc tests and fretting tests were ran using hard steel spherical counterparts. Experiments in vacuum reveal that the variation of the coefficient of friction with cumulated sliding distance is very different when one considers Cr-hard steel riding on alumina, on a (true) quasicrystal or on a high-carbon steel. Sharp minima in friction coefficients and adhesive forces occur when the quasicrystalline surface is loaded in contact with a metallic surface. The characteristic friction values measured in high vacuum are smaller than in ambient atmosphere by at least a factor 2, thus indicating that the oxide layer is involved to a significant extent in determining the tribological mechanisms. The discussion focuses on an interpretation of the differences observed between samples, based on their chemical composition and possible bonding to steel. For the first time, an estimate of the true surface energy of a quasicrystal is derived from measurements. (C) 2004 Elsevier B.V. All rights reserved.