PRL2008 - PRL 100 235502(2008 PHYSICAL REVIEW LETTERS week...

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Origin of Ultralow Friction and Wear in Ultrananocrystalline Diamond A. R. Konicek, 1 D. S. Grierson, 2 P. U. P. A. Gilbert, 3, * W. G. Sawyer, 4 A. V. Sumant, 5 and R. W. Carpick 6 1 Department of Physics & Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania, 19104, USA 2 Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA 3 Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA 4 Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida, 32611, USA 5 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, 60439, USA 6 Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania, 19104, USA (Received 16 October 2007; published 11 June 2008) The impressively low friction and wear of diamond in humid environments is debated to originate from either the stability of the passivated diamond surface or sliding-induced graphitization/rehybridization of carbon. We find ultralow friction and wear for ultrananocrystalline diamond surfaces even in dry environments, and observe negligible rehybridization except for a modest, submonolayer amount under the most severe conditions (high load, low humidity). This supports the passivation hypothesis, and establishes a new regime of exceptionally low friction and wear for diamond. DOI: 10.1103/PhysRevLett.100.235502 PACS numbers: 81.40.Pq, 46.55.+d, 62.20.Qp The remarkably low friction and wear of diamond, particularly in humid environments, is postulated to be due to either rehybridization [ 1 3 ], or passivation [ 4 , 5 ] of dangling bonds formed during sliding. Rehybridization to ordered sp 2 bonding is plausible because graphite is the thermodynamically stable form of carbon at room tem- perature and ambient pressure, and is lubricious due to its layered structure. Rehybridization may also involve the formation of lubricious amorphous sp 2 -containing carbon [ 6 ]. The significant energy barrier to convert diamond to graphite or amorphous carbon ( 1 : 0 eV = atom) [ 7 ] may be lowered by shear, frictional heating, and oxygen and water vapor. However, passivation is proposed by others [ 4 , 5 , 8 ] because friction and wear for diamond are lower, compared to vacuum, in environments containing H 2 or H 2 O . Desorption, induced mechanically, creates dangling carbon bonds that increase friction and wear due to interfacial bonding [ 9 ]. A sufficient supply of passivating species overcomes this by preemptively terminating the dangling bonds. However, no previous studies presented spectro- scopic evidence to validate either hypothesis. Ultrananocrystalline diamond (UNCD), one of the smoothest diamond films available, has a thickness- independent RMS roughness of < 12 nm over a 1 m 2 area [ 10 ]. UNCD has equiaxed diamond grains 2 5 nm in diameter with atomically abrupt grain boundaries, and shares many of the properties of diamond [ 11 , 12 ]. UNCD films 1 m thick were deposited onto silicon flats and Si 3 N 4 spheres [ 13 ]. Substrates were ultrasonically pre- treated as in [ 10 ] except with functionalized nanodiamond
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