Relating Tribological Performance and Tribofilm Formation to the Adsorption Strength of Surface-Active Precursors
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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.
KeywordsCarbon tribofilm Adsorption strength Tribochemistry
The authors would like to thank the support from the US National Science Foundation (Grant No. CMMI-1662606). We thank Valvoline for providing PAO lubricants and Prof. Seong H. Kim for critical reading of this manuscript. This work made use of the Keck-II Facility of Northwestern University’s NUANCE Center, which has received support from the Keck Foundation, the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the Materials Research Center (NSF DMR-1121262), the McCormick Research Catalyst Awards Fund, Grant No. 10038293, and the International Institute for Nanotechnology (IIN) at Northwestern University. We would like to thank Dr. Xinqi Chen for his help in conducting FTIR spectroscopy. Hongxing Wu and Qiang Ma would also like to acknowledge the scholarship support from China Scholarship Council (CSC, Nos. 201606280181 and 201806280152 respectively). This research was supported in part through the computational resources and staff contributions provided for the Quest high performance computing facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology.
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