Substituent effects on tribochemical processes James P. Ewen, Jie Zhang, Carlos Ayestarán Latorre, Daniele Dini and Hugh A. Spikes Department of Mechanical Engineering, Imperial College London, UK Tribometer experiments using high-friction and low-friction base oils have shown that the tribofilm formation by the lubricant additive zinc dialkyldithiophosphate (ZDDP) increases exponentially with shear stress [1]. The rate of this mechanochemical process can be described using the Bell model. Commercial ZDDPs containing mostly secondary alkyl groups form tribofilms much faster than those with primary alkyl groups, with the main difference being the pre-exponential factor (rather than the activation energy or activation volume) [2]. The same observation has recently been made from reactive nonequilibrium molecular dynamics (NEMD) simulations of trialkyl phosphates. Here, using carefully designed tribometer experiments under well-controlled conditions, we show how the mechanochemical reactivity of ZDDP can be altered by tailoring the additive's alkyl substituents. Specific examples include the observations that additives with long alkyl chains only form tribofilms at high temperature, while those containing bulky cycloaliphatic groups are particularly effective at high stress. We expect these findings to be useful for the rational design of high-performance lubricant additives for specific applications [4]. References 1. Zhang, J; Spikes, H.A. On the Mechanism of ZDDP Antiwear Film Formation. Tribol. Lett. 2016, 63, 24. https://doi. org/10.1007/s11249-016-0706-7 2. Zhang, J.; Ewen, J.P.; Ueda, M.; Wong, J.S.S.; Spikes, H.A. Mechanochemistry of Zinc Dialkyldithiophosphate on Steel Surfaces under Elastohydrodynamic Lubrication Conditions. ACS Appl. Mater. Interfaces 2020, 12, 6662–6676. https://doi. org/10.1021/acsami.9b20059 3. C. Ayestarán Latorre, J.E. Remias, J.D. Moore, H.A. Spikes, D. Dini, J.P. Ewen. Mechanochemistry of Phosphate Esters Confined between Sliding Iron Surfaces. Commun. Chem. 2021 4, 178.https://doi.org/10.1038/s42004-021-00615-x 4. Zhang, J.; Ewen, J.P.; Spikes, H.A. Substituent Control of Mechanochemical Tribofilm Formation. Mol. Syst. Des. Eng. 2022. https://doi.org/10.1039/D2ME00049K
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