Boundary Friction of ZDDP Tribofilms

Abstract

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 boundary friction values that vary with alkyl group structure in a similar fashion to rolling-sliding MTM tests. These variations in friction occur immediately on rubbing, before any measurable tribofilm can develop. This study suggest that ZDDPs control boundary friction by adsorbing on rubbing steel or tribofilm surfaces in a fashion similar to organic friction modifiers. However it is believed that, for primary ZDDPs, residual alkoxy groups still chemically bonded to the phosphorus atoms of newly-formed polyphosphate/phosphate tribofilm may also contribute to boundary friction. This understanding will contribute to the design of low friction, fuel efficient crankcase engine oils. Graphical Abstract

Influence of Black Oxide Coating on Micropitting and ZDDP Tribofilm Formation

“…BO/BO friction shows greater fluctuations during the 180 minute test than that of the steel/steel contact. It is well known that the growth of ZDDP tribofilm can increase friction coefficient in mixed lubrication conditions since its pad-like structure can increase the roughness of the rubbing surfaces, particularly if the latter are very smooth to start with, resulting in a reduction of the effective lambda ratio and consequent transition from mixed to boundary lubrication [33]. Since both steel/steel and BO/BO tribopairs show an increase in friction from an initially lower value to 0.11 -0.12, which corresponds to friction of ZDDP tribofilms in boundary lubrication, this behaviour suggests that ZDDP tribofilms Based on the increase of friction coefficient (Fig.…”

Mitigation of False Brinelling in A Roller Bearing: A Case Study of 4 Types of Greases

“…It is well document that ZDDP lms can form with a combination of increased temperature ( ash temperature) and shear of the lubrication lm in oil lubricated system [8,25,40]. These factors are described as 'mechanochemical reactions', which drive the ZDDP tribo lm formation under the rubbing surface at low environmental temperatures 25ºC to 85 ºC [25].…”

“…These factors are described as 'mechanochemical reactions', which drive the ZDDP tribo lm formation under the rubbing surface at low environmental temperatures 25ºC to 85 ºC [25]. Furthermore, in these oil lubricated systems, the ZDDP tribo lm forms even if there is no asperity contact due to EHL (Elastohydrodynamic Lubrication) regime, on the proviso that the shear stress is su ciently high [40]. However, the shear of the lubrication lm is unlikely to be the contributing factors for the ZDDP tribo lm formation in the current result.…”

The effect of anion architecture on the lubrication chemistry of phosphonium orthoborate ionic liquids

“…Currently lacking is an understanding of (i) the manner in which phosphonium ionic liquids facilitate lubrication, (ii) the mechanism of their breakdown, and, particularly, (iii) the relationship of the structure of the ionic liquid components to both of the above. On point (i), both traditional sacrificial tribofilms 32 and ionic boundary layers 33 have been invoked to explain the tribological properties of ionic liquids. Where referred to, breakdown of ionic liquids has focussed upon the cation of the ionic liquid and the resulting decomposition products.…”

“…0.14 with no pronounced adhesion spike. This lower value reflects the presence of antiwear additives such as zinc dialkyldithiophosphates (ZDDPs) 32 ; this additive also provides a degree of wear protection as observed in the smaller, more well-defined wear scar. It thus appears that, under these test conditions, lubricant viscosity is of much less significance than the chemical composition of the lubricant for these systems.…”