Influence of structural factors on the tribological performance of organic friction modifiers

Febin Cyriac,Xin Yi Tee,Pui Shan Chow,Sendhil K Poornachary

doi:10.1007/s40544-020-0385-0

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.

Highlights

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]
We focused on the influence of structural factors on the frictional performance of organic friction modifiers (OFMs)
Based on the experimental results, the following conclusions can be drawn: 1) An OFM plays a vital role in mitigating friction in the boundary and mixed lubrication regimes

Summary

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]. 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 high. Friction modifiers (FMs) are an important class of lubricant additives for reducing friction when there is no sufficient liquid at the contact interface [2]. 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, and sulfur (SAPS). With the tightening of environmental regulations in recent years, OFMs (primarily made up of carbon, hydrogen, oxygen, and nitrogen) are starting to replace the former

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