The incorporation of silicon and oxygen into hydrogenated amorphous carbon (a-C:H) is an effective approach to decrease the dependence of the tribological properties of a-C:H on the environment. Here, we evaluate the effect of hydrogen and oxygen partial pressures in vacuum on the tribological response of steel pins sliding against films consisting of silicon- and oxygen-containing a-C:H (a-C:H:Si:O). Experiments are conducted in the low-friction/low-wear regime, where sufficient gas pressure prevents steel from adhering to the a-C:H:Si:O, with the velocity accommodation mode being interfacial sliding between the tribotrack formed in the a-C:H:Si:O film and the carbonaceous tribofilm that is formed on the countersurface. The experiments indicated a decrease (increase) in friction and wear with the hydrogen (oxygen) pressure (hydrogen pressures between 50 and 2000 mbar; oxygen pressures between 10 and 1000 mbar). Characterization by X-ray photoelectron and absorption spectroscopies indicated the occurrence of tribologically induced rehybridization of carbon-carbon bonds from sp3 to sp2. This mechanically induced structural transformation coincided with the dissociative surface reaction between hydrogen (oxygen) gas molecules and sp2 carbon-carbon bonds that are highly strained, which results in the formation of carbon-hydrogen groups (carbonyl or ether groups together with silicon atoms having higher oxidation states). On the basis of variations of the fraction of these surface functional groups with gas pressure, a phenomenological model is proposed for the gas pressure dependence of friction for steel when sliding on a-C:H:Si:O films: while the decrease in friction with hydrogen pressure is induced by an increase in the percentage of carbon-hydrogen groups, the increase in friction with oxygen pressure is caused by a progressive increase in the relative fraction of silicon atoms having higher oxidation states and an increase in surface oxygen concentration.
Keywords: NEXAFS; X-ray photoelectron spectroscopy; XPS; carbon-based materials; imaging near-edge X-ray absorption fine structure spectroscopy; silicon- and oxygen-containing hydrogenated amorphous carbon; solid lubrication; tribochemistry.