Influence of structural factors on the tribological performance of organic friction modifiers
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ISSN 2223-7690 CN 10-1237/TH
RESEARCH ARTICLE
Influence of structural factors on the tribological performance of organic friction modifiers Febin CYRIAC*, Xin Yi TEE, Sendhil K. POORNACHARY, Pui Shan CHOW Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research, Singapore 627833, Singapore Received: 26 November 2019 / Revised: 17 February 2020 / Accepted: 15 March 2020
© The author(s) 2020. Abstract: The influence of structural factors on the lubrication performance of organic friction modifiers (OFMs) formulated in Group V (polyol ester oil) base oil was studied using a ball-on-disk tribometer. The results show that OFMs can mitigate friction under heavy loads, low sliding speeds, and high temperatures. These conditions are commonly encountered in internal-combustion engines between cylinder liners and piston rings. The reduction in friction is ascribed to the boundary lubrication film containing the OFM. The chemical composition analysis of the metal disk surface using energy dispersive X-ray spectroscopy (EDS) confirmed the presence of a protective film of OFM on the wear track, albeit inconsistently deposited. Although the adsorption of the OFM on the metal surface was observed to be dependent on the chemical reactivity of the functional groups, levels of unsaturation, and hydrocarbon chain length of the OFM, the frictional performance was not always directly correlated with the surface coverage and tribofilm thickness. This implies that the friction reduction mechanism can involve other localized processes at the interface between the metal surface and lubricant oil. The occasional variation in friction observed for these OFMs can be attributed to the stability and durability of the boundary film formed during the rubbing phase. Keywords: organic friction modifiers; friction; wear; film thickness
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Introduction
Growing concerns regarding climate change caused by greenhouse gas emission as well as the need for improved energy efficiency in automotive engines have led to a shift toward the use of low-viscosity lubricants [1]. Although low-viscosity lubricants contribute to improved energy efficiency by reducing fluid friction, they can have a detrimental impact on the boundary and mixed lubrication regimes where friction and wear at the contact interfaces are particularly high. Therefore, conventional lubricating oils (transmission fluids, engine oils, etc.) are formulated with a variety of additives for minimizing the parasitic energy loss and for achieving
the desired performance under different lubrication regimes. Friction modifiers (FMs) are an important class of lubricant additives for reducing friction when there is no sufficient liquid at the contact interface [2]. Typically, the FMs used in engine oil formulations belong to the class of organomolybdenum compounds and organic friction modifiers (OFMs). The former type is mostly based on sulfur- and phosphoruscontaining compounds that can produce hazardous lubricant discharge consisting of sulfated ash, phosphorus
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