Zinc dialkyldithiophosphates (ZDDPs) form antiwear tribofilms at sliding interfaces and are widely used as additives in automotive lubricants. The mechanisms governing the tribofilm growth are not well understood, which limits the development of replacements that offer better performance and are less likely to degrade automobile catalytic converters over time. Using atomic force microscopy in ZDDP-containing lubricant base stock at elevated temperatures, we monitored the growth and properties of the tribofilms in situ in well-defined single-asperity sliding nanocontacts. Surface-based nucleation, growth, and thickness saturation of patchy tribofilms were observed. The growth rate increased exponentially with either applied compressive stress or temperature, consistent with a thermally activated, stress-assisted reaction rate model. Although some models rely on the presence of iron to catalyze tribofilm growth, the films grew regardless of the presence of iron on either the tip or substrate, highlighting the critical role of stress and thermal activation.

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Highly sp3 -bonded, nearly hydrogen-free carbon-based materials can exhibit extremely low friction and wear in the absence of any liquid lubricant, but this physical behavior is limited by the vapor environment. The effect of water vapor on friction and wear are examined as a function of applied normal force for two such materials in thin film form -one that is fully amorphous in structure (tetrahedral amorphous carbon, or ta-C) and one that is polycrystalline with <10 nm grains (ultrananocrystalline diamond, or UNCD). Tribologically-induced changes in the chemistry and carbon bond hybridization at the surface are correlated with the effect of the sliding environment and loading conditions through ex-situ, spatially resolved near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. At sufficiently high relative humidity (RH) levels and/or sufficiently low loads, both films quickly achieve a low steady-state friction coefficient and subsequently exhibit low wear. For both films, the number of cycles necessary to reach the steady-state is progressively reduced for increasing RH levels. Worn regions formed at lower RH and higher loads have a higher concentration of chemisorbed oxygen than those formed at higher RH, with the oxygen singly-bonded as hydroxyl groups (C-OH). While some carbon rehybridization from sp 3 to disordered sp 2 bonding is observed, no crystalline graphite formation is observed for either film. Rather, the primary solid-lubrication mechanism is the 2 passivation of dangling bonds by OH and H from the dissociation of vapor-phase H 2 O. This vapor-phase lubrication mechanism is highly effective, producing friction coefficients as low as 0.078 for ta-C and 0.008 for UNCD, and wear rates requiring thousands of sliding passes to produce a few nanometers of wear.

Wear in tetrahedral amorphous carbon (ta-C) and diamond is studied by molecular dynamics and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy after self-mated sliding. Both theory and experiment observe the formation of a soft, mainly sp 2 hybridized amorphous carbon (a-C) tribolayer which grows faster for ta-C than for diamond surfaces sliding under otherwise similar conditions. The faster sp 3 → sp 2 transition in ta-C is explained by easy breaking of prestressed bonds in a nanoscale ta-C region triggered by plasticity in the adjacent a-C while the diamond/a-C transition occurs at an atomically sharp interface.

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