Film Thickness and Friction of ZDDP Tribofilms

Joanna Dawczyk,Hugh Spikes,Neal Morgan,Joe Russo

doi:10.1007/s11249-019-1148-9

Joanna Dawczyk, Hugh Spikes + Show 2 more

Open Access

https://doi.org/10.1007/s11249-019-1148-9

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Abstract

Tribofilm formation by several zinc dialkyl- and diaryldithiophosphate (ZDDP) solutions in thin film rolling-sliding conditions has been investigated. A primary, a secondary alkyl and a mixed alkyl ZDDP show similar rates of film formation and generate films typically 150 nm thick. Another secondary ZDDP forms a tribofilm much faster and the film is partially lost after extended rubbing. An aryl ZDDP forms a tribofilm much more slowly. The films all have a pad-like structure, characterised by flat pad regions separated by deep valleys. Three different techniques have been used to analyse the thickness and morphology of the tribofilms: spacer layer imaging (SLIM), scanning white light interferometry (SWLI) of the gold-coated film and contact mode atomic force microscopy (AFM). The SLIM method measures considerably thicker films than the other two techniques, probably because of lack of full conformity of a glass disc loaded against the rough tribofilm. No evidence of a highly viscous layer on top of the solid tribofilm is seen. SWLI and contact mode AFM measure similar film thicknesses. The importance of coating the tribofilm with a reflective layer prior to using SWLI is confirmed. As noted in previous work, the formation of a ZDDP tribofilm is accompanied by a marked shift in the Stribeck friction curve towards higher entrainment speed. For a given ZDDP this shift is found to correlate with the measured tribofilm roughness, proving that it results from the influence of this roughness on fluid entrainment in the inlet.

Highlights

Zinc dialkyl- and diaryldithiophosphates (ZDDPs) were first introduced as lubricant anticorrosion, extreme pressure and oxidation inhibitor additives in the early 1940s, with their remarkable wear preventive properties being recognised a decade later in the 1950s [1]
In the 1990s, exhaust after-treatment systems were introduced in motor vehicles in order to reduce harmful emissions such as NOx, soot and unburnt hydrocarbons, and it was soon observed that sulphur and phosphorus compounds originating in part from ZDDP reduced the useful life of these systems [2]
In the current study we explore two main issues of ZDDP tribofilm formation

Summary

Introduction

Zinc dialkyl- and diaryldithiophosphates (ZDDPs) were first introduced as lubricant anticorrosion, extreme pressure and oxidation inhibitor additives in the early 1940s, with their remarkable wear preventive properties being recognised a decade later in the 1950s [1]. In the 1990s, exhaust after-treatment systems were introduced in motor vehicles in order to reduce harmful emissions such as NOx, soot and unburnt hydrocarbons, and it was soon observed that sulphur and phosphorus compounds originating in part from ZDDP reduced the useful life of these systems [2]. This led to a progressive reduction in the levels of metallic elements, P and S allowed in engine oils. As well as enabling continued use of ZDDPs, this requirement has led to the increased choice of those ZDDPs having relatively low volatility [5]

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