Abstract
Abstract Tribofilms are activated using precision sliding strip microscale tribometry with a base and a fully formulated lubricant with a ZDDP anti-wear additive. The employed tribometry uses combined pressure, shear and temperature activation. The chemical compositions of the formed tribofilms are ascertained through use of Photoelectron X–ray Spectroscopy (XPS). Nanoscale frictional measurements of the tribofilms are reported using fluid cell lateral force microscopy (LFM). The measured coefficient of interfacial boundary shear strength is used with analytical contact mechanics to relate the in-situ conditions to the activation energy components of the Eyring potential cage model. The paper shows that combined LFM and the Eyring model can explain the variations in the frictional characteristics of formed tribofilms.
Highlights
Automotive lubricants typically comprise 85–90% base oil and 10–15% chemical additives [1], including anti-wear agents, friction modifiers, dispersants, detergents and oxidation stabilisers, as well as corrosion inhibitors [2]
Wil lermaet et al [8] studied various mechanisms and steps in the formation of Zinc dialkyldithiophosphates (ZDDP)-based tribofilms on metallic surfaces. They showed that an anti-wear tribofilm forms by adsorption of ZDDP onto a surface, chem ically reacting in order to form phosphothionates and phosphate-based compounds which chemically bond to the surface
The fluid cell lateral force microscopy is conducted with the lubricated samples after the formation of a ZDDP-based tribofilm activated through tribometry
Summary
Automotive lubricants typically comprise 85–90% base oil and 10–15% chemical additives [1], including anti-wear agents, friction modifiers, dispersants, detergents and oxidation stabilisers, as well as corrosion inhibitors [2]. Watkins [4] used a highly oxidised iron surface in the presence of a ZDDP solution in order to study the mechanism of tribofilm formation on typical metallic engine components which inherit oxide layers. They showed that zinc phosphate physically adsorbs, whilst iron sulphide chemically reacts with surface materials. The nanoscale friction was measured using LFM and relative comparison of thermal, shear and pressure activation energy components of the thermally activated cage model were determined in an attempt to explain the observed frictional characteristics of the fully formulated lubricants. Shear and thermal activation en ergies in the Eyring energy model are shown to vary with the test con ditions, primarily due to the nature of the molecules within the sliding contact
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