Direct Observation of Tribochemically Assisted Wear on Diamond-Like Carbon Thin Films
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Friction represents a major energy wastage, with typical estimates in the range of 2–5 % of the GDP of developed countries; 15 % of the energy losses in a new automobile engine are due to friction (Uchida et al. J. Cryst. Growth 114:565–568, 1991). While the macroscopic laws of friction have been known for centuries, the exact nanoscale processes taking place are less clear. It is established that friction involves small asperities sliding on a locally flat surface, but the exact mechanisms of slip as well as energy dissipation are still unclear and sometimes controversial. In many ways even less is known about chemical reactions occurring during sliding, what is called tribochemistry. We report here direct in situ observation at the nanoscale of tribochemically assisted wear for a tungsten tip sliding on diamond-like carbon films in wet hydrogen, nitrogen and compare these to similar experiments in vacuum. Differences in the wear directly indicate passivation of the films in hydrogen and accelerated wear in wet nitrogen. The results are surprisingly similar to what one would expect at the macroscale, indicating that in many respects there is little difference between the processes taking place across many length scales.
KeywordsNanotribology Friction mechanisms Wear mechanisms Corrosive wear Solid lubrication mechanisms TEM EELS
This study was supported by the National Science Foundation on grant number CMMI-1030703. Additional support was provided by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program, under Contract No. DE-AC02-06CH11357. Research carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
Zero loss energy-filtered video shows the development and progression of visible wear tracks on the DLC film during sliding in wet N2. The scale is identical to Fig. 1. Analysis of the estimated wear is shown in Fig. 3
Several sliding passes in 1.2 torr of wet H2. No measurable amounts of chemical wear were observed during these tests despite the presence of water vapor
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