On the Crystallinity and Durability of ZDDP Tribofilm

Ueda, Mao; Kadiric, Amir; Spikes, H. A.

doi:10.1007/s11249-019-1236-x

Cited by 51 publications

(63 citation statements)

References 30 publications

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“…This loss of some ZDDP tribofilm when rubbed in base oil has been previously reported and shown to occur when the film has formed over a relatively short rubbing time under mild conditions [5,25]. In this case the outer part of the tribofilm has an amorphous structure and is relatively weak.…”

Section: Replacement By Base Oilsupporting

confidence: 63%

“…XPS analysis was carried out without any sputtering. The XPS instrument and conditions employed are described in [5], with charging of the specimen corrected by referring aliphatic carbon binding energy to 284.8 eV (C 1 s signal). Figure 24a and b shows carbon 1 s XPS spectra from inside and outside the rubbed track of a CYC-1° tribofilm on a disc after rubbed in base oil.…”

Section: Discussionmentioning

confidence: 99%

“…This behaviour appears to be characteristic of this ZDDP under these test conditions since it was seen in three repeat tests. It is believed to reflect the interplay between tribofilm formation, its removal by rubbing, and its gradual conversion during rubbing to a more wear-resistant, nanocrystalline structure [5].…”

Section: Prolonged Mtm Testsmentioning

confidence: 99%

“…In this case the outer part of the tribofilm has an amorphous structure and is relatively weak. During prolonged rubbing the tribofilm becomes nanocrystalline and much stronger and thus not significantly removed during rubbing in base oil [5]. Figure 16a and b shows the effect on boundary friction of replacing DOD-1° solution by MPE-2° solution after two hours rubbing and vice versa.…”

Section: Replacement By Base Oilmentioning

confidence: 99%

“…The tribofilm has a quite complex structure, with a thin sublayer rich in iron sulphide mixed with and overlain by a much thicker film of zinc and iron phosphate/polyphosphate [3]. When formed initially, the film appears to be mainly polyphosphate with a predominantly amorphous structure, but during prolonged rubbing this depolymerises into nanocrystalline ortho-and pyrophosphate, possibly driven by diffusion of iron from the substrate [4,5]. XPS analysis suggests that the immediate surface layer contains a high proportion of polyphosphate even after prolonged rubbing.…”

Section: Introductionmentioning

confidence: 99%

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Boundary Friction of ZDDP Tribofilms

et al. 2020

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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

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Section: Replacement By Base Oilsupporting

confidence: 63%

Section: Discussionmentioning

confidence: 99%

Section: Prolonged Mtm Testsmentioning

confidence: 99%

Section: Replacement By Base Oilmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Boundary Friction of ZDDP Tribofilms

et al. 2020

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Structural characterisation of iron sulfide tribofilms formed on carbon and stainless steels

Miyajima

Yonemura

Kitamura

et al. 2023

Lubrication Science

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This study investigated tribofilm layers formed under boundary lubrication using a polysulfide additive on S45C and SUS304 steel surfaces. In the tribotests, the friction coefficient of the SUS304 disc fluctuated and increased more gradually in comparison to that of the S45C disc. The tribofilm on the S45C disc comprised the outermost and inner layers of FeS2 (pyrite) and FeS (troilite), respectively. Notably, all the tribofilm layers on the SUS304 disc surface contained pyrite. The tribofilm on the SUS304 disc was weakly crystalline in the outermost layer and thinner than that on the S45C disc. The poor formation of the tribofilm on the SUS304 disc was potentially caused by the unstable friction coefficient and scuffing.

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