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@article{Zhang:2021:10.1007/s11249-020-01389-4,
author = {Zhang, J and Ueda, M and Campen, S and Spikes, H},
doi = {10.1007/s11249-020-01389-4},
journal = {Tribology Letters},
pages = {1--17},
title = {Boundary friction of ZDDP tribofilms},
url = {http://dx.doi.org/10.1007/s11249-020-01389-4},
volume = {69},
year = {2021}
}
TY - JOUR
AB - The frictional properties of ZDDP tribofilms at low entrainment speeds in boundary lubrication conditions have been studied in both rolling/sliding and pure sliding contacts. It has been found that the boundary friction coefficients of these tribofilms depend on the alkyl structure of the ZDDPs. For primary ZDDPs, those with linear alkyl chains give lower friction those with branched alkyl chain ZDDPs, and a cyclohexylmethyl-based ZDDP gives markedly higher friction than non-cyclic ones. Depending on alkyl structure, boundary friction coefficient in rolling-sliding conditions can range from 0.09 to 0.14. These differences persist over long duration tests lasting up to 120 h. For secondary ZDDPs, boundary friction appears to depend less strongly on alkyl structure and in rolling-sliding conditions stabilises at ca 0.115 for the three ZDDPs studied. Experiments in which the ZDDP-containing lubricant is changed after tribofilm formation by a different ZDDP solution or a base oil indicate that the characteristic friction of the initial ZDDP tribofilm is lost almost as soon as rubbing commences in the new lubricant. The boundary friction rapidly stabilises at the characteristic boundary friction of the replacement ZDDP, or in the case of base oil, a value of ca 0.115 which is believed to represent the shear strength of the bare polyphosphate surface. The single exception is when a solution containing a cyclohexylethyl-based ZDDP is replaced by base oil, where the boundary friction coefficient remains at the high value characteristic of this ZDDP despite the fact that rubbing in base oil removes about 20 nm of the tribofilm. XPS analysis of the residual tribofilm reveals that this originates from presence of a considerable proportion of C-O bonds at the exposed tribofilm surface, indicating that not all of the alkoxy groups are lost from the polyphosphate during tribofilm formation. Very slow speed rubbing tests at low temperature show that the ZDDP solutions give boundar
AU - Zhang,J
AU - Ueda,M
AU - Campen,S
AU - Spikes,H
DO - 10.1007/s11249-020-01389-4
EP - 17
PY - 2021///
SN - 1023-8883
SP - 1
TI - Boundary friction of ZDDP tribofilms
T2 - Tribology Letters
UR - http://dx.doi.org/10.1007/s11249-020-01389-4
UR - https://link.springer.com/article/10.1007%2Fs11249-020-01389-4#Abs1
UR - http://hdl.handle.net/10044/1/85430
VL - 69
ER -