Abstract
The influence of a series of Ethomeens (ethoxylated alkylamine organic friction modifiers) on the durability and friction of tribofilms formed by a commercial blend of primary and secondary ZDDP in sliding/rolling contact has been studied. When pre-formed ZDDP tribofilms are rubbed in Ethomeen solution, boundary friction is reduced and some of the ZDDP film is removed. Ethomeens having just two ethoxy groups give lower boundary friction on ZDDP than those with 15 ethoxy groups, but result in much greater removal of the tribofilm itself. Based on XANES analysis, the film removed by both types of Ethomeen consists primarily of nanocrystalline orthophosphate. The level of boundary friction and its dependence on sliding speed, coupled with the dimensions of the molecules, suggests that the Ethomeens with two ethoxy groups may form quite closely packed vertical monolayers on ZDDP tribofilm surfaces, but that those with fifteen ethoxy groups cannot be close packed; yet they still reduce boundary friction significantly. The study shows that selection of an appropriate aminic friction modifier for use with ZDDP is a balance between its ability to reduce friction and its potentially harmful effect on a ZDDP tribofilm.
Highlights
Zinc dialkyl- and diaryldithiophosphates (ZDDP) are used as antiwear and extreme pressure additives in almost all current crankcase engine oils
ZDDP tribofilms gradually transform from polyphosphate to short-chain phosphate during extended rubbing [42, 43] and it is quite likely that most of the tribofilm formed in the current study was, depolymerised orthophosphate
Focused Ion Beam (FIB)-TEM and X‐ray Absorption Near Edge Structure (XANES) analysis shows that the ZDDP tribofilms, which are formed during 3-h rubbing at 100 °C, have a nanocrystalline structure and consist principally of zinc orthophosphate, with some iron disulphide in regions close to the steel substrate
Summary
Zinc dialkyl- and diaryldithiophosphates (ZDDP) are used as antiwear and extreme pressure additives in almost all current crankcase engine oils. Hoshino et al used a rolling–sliding contact to compare the friction properties of three ZDDPs, two primaries, linear n-octyl and branched 2-ethylhexyl, and a secondary ZDDP based on 4-methylpent-2-yl [8] They showed that boundary friction varied in the order octyl < ethylhexyl < methylpentyl. The ZDDP with linear alkyl groups showed friction that increased linearly with log(sliding speed), a phenomenon generally seen with effective organic friction modifiers [9]. These studies were made with simple solutions of ZDDP in base oil, suggesting that the observed boundary friction dependence probably resulted from the alkyl groups of partially alkylated phosphate at the outermost surface of the ZDDP tribofilm acting in a similar fashion to an organic friction modifier
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