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Mechanochemical reactions induced by external stress provide a unique approach for in situ synthesis of carbon tribofilms that can improve friction and wear performance. In this work, we studied how tribofilm formation and tribological performance might be related to the adsorption strength of three additives in polyalphaolefin (PAO4) as base oil, viz., cyclopropanecarboxylic acid (CPCa), cyclopropanemethanol (CPMA), and 1-cyclopropylethanol (CPEA) as characterized by two different surface-active groups –COOH and –OH. Tribo-testing results reveal that addition of 2.5 wt% CPCa to PAO4 gave the lowest friction coefficient and wear volume. FTIR and Raman analysis demonstrate substantial tribofilm formation only in the case when CPCa was used as the oil additive, not CPMA or CPEA, in spite of the fact that all three additives contain the same metastable cyclopropane ring. Thermogravimetric analysis and molecular dynamics simulations indicate the stronger adsorption of CPCa on the iron oxide surface compared with CPMA and CPEA. Weak adsorption of the latter molecules results in their desorption from the surface due to flash heating during tribotesting before they have the chance to participate in mechanochemical reactions required for tribofilm formation. The stronger binding of CPCa to the steel surface is a necessary condition for this type of surface mechanochemistry and appears critical to the efficient formation of carbon-containing tribofilms under our tribo-testing conditions. [ABSTRACT FROM AUTHOR]

Extreme pressure (EP) lubricant additives form protective tribofilms at the site of contact using the heat and pressure of contact and relative motion. Common EP additives contain undesirable elements such as phosphorus and sulfur. A novel EP lubricant additive, which contains no phosphorus and sulfur, is presented for generating lubricious carbon films. The additive consists of a surface-active molecule with a metastable cycloalkane ring, which dissociates readily during tribological contact to form lubricious carbon films. Friction and wear performance of PAO4 with this additive under a range of loads and speeds were shown to be superior to that without the additive. Optical and scanning electron microscopy and Raman spectroscopy were used to analyze the tribofilms formed on post-test contact surfaces, providing direct evidence for the formation of carbon films. Quantitative kinetics for the carbon tribofilm formation was analyzed as a function of temperature and stress, from which the activation energy for carbon tribofilm formation was obtained. [ABSTRACT FROM AUTHOR]