Environmental effects on the friction of hydrogenated DLC films

Abstract

We have investigated environmental effects on hydrogenated diamond-like carbon (H-DLC) films under various pressures of H 2 O, O 2 , and N 2 by ultrahigh vacuum (UHV) tribometry. The H-DLC film exhibits an ultralow coefficient of friction (l=0.004 in UHV). The l value increases with increasing pressure of H 2 O and O 2 . Specifically, l increases up to 0.07 under 10 Torr of H 2 O, and up to 0.03 under 150 Torr of O 2 ; these are typical H 2 O and O 2 contents respectively in ambient air. Our results are consistent with similar environmental effects previously reported. But, we have also discovered that these friction changes are reversible, returning to the ultralow value when UHV is restored. The reversibility of the friction behavior in both environments, coupled with the lack of evidence of tribochemical changes by Auger electron spectroscopy, suggest that the observed friction changes are due to the weakly adsorbed gas molecules that influence the friction property by physically separating the H-DLC interface. Speed-dependent tribometry also supports this argument. In addition, two DLC films with different hydrogen contents and with widely different friction coefficients in UHV are shown to exhibit identical l values under humid environments, further demonstrating that the frictional properties of these DLC films are essentially determined by the surface layer of adsorbed gas molecules.

Origin of Ultralow Friction and Wear in Ultrananocrystalline Diamond

“…2(b)]. Results agree with other tribological studies, which infer that dissociative passivation either by water [3,5,24] …”

“…The remarkably low friction and wear of diamond, particularly in humid environments, is postulated to be due to either rehybridization [1][2][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 temperature and ambient pressure, and is lubricious due to its layered structure.…”

“…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.…”

Understanding the Atomic Scale Mechanisms that Control the Attainment of Ultralow Friction and Wear in Carbon-Based Materials

“…More recent research, including by the both PI's, points toward specific atomic-scale mechanisms strongly affecting the sliding behavior of these materials, including: rehybridization from sp 3 to sp 2 -bonding of the C atoms 20 , formation of bonds across the interface and subsequent atomic bond breaking 21 , and the strong passivating influence of small quantities of adsorbates 9,22 . However, many of these observations are indirect or inferred, and others have not been studied in a systematic way that allows understanding of the run-in transition (namely the contact interactions on the surface separation and the real contact area for an asperity) to be investigated.…”

Friction Characteristics of DLC and WC/C

“…However, previous studies mainly focused on the effects of materials and coating conditions of solid lubricating films [4][5][6][7][8], materials and physical properties of mating Journal of the KSTLE surfaces [9,10] and environmental conditions [11][12][13][14][15] on the friction characteristics of solid lubricating films. The studies on the effects of sliding velocity and normal load [16,17] were not enough.…”