Tribological Properties of Sulphur-Free Antiwear Additives Zinc Dialkylphosphates (ZDPs)

Hoshino, Kazuki; Yagishita, Kazuhiro; Tagawa, Kazuo; Spikes, H. A.

doi:10.4271/2011-01-2132

Cited by 10 publications

(11 citation statements)

References 3 publications

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“…Furthermore, in the MTM the linear alkyl ZDDPs have boundary friction that increases linearly with log(SRR) and thus, at fixed entrainment speed, with log(sliding speed). This is commonly seen with linear, saturated organic friction modifiers (OFMs) [21] and was also noted by Hoshino et al for a linear ZDDP [20]. Boundary friction coefficient is higher in the more severe, pure sliding HFRR tests than in the rolling-sliding MTM tests but the overall dependence on alkyl structure is the same, while the very slow speed linear sliding tests also show this order.…”

Section: Discussionsupporting

confidence: 65%

“…(i) a shift of the mixed lubrication region of the friction curves to higher entrainment speed and also (ii) an increase in slow speed boundary friction for some ZDDPs (EHX and CYC). Figure 7 shows that the four ZDDPs give quite different boundary friction values after two hours rubbing, in the order; DOD-1° < OCT-1° < EHX-1° < CYC-1°F or DOD, OCT and EHX this order is similar to that seen in previous studies [19,20]. Figure 8a-c shows how MTM friction coefficient versus entrainment speed evolves with rubbing time for the three secondary ZDDPs, while Fig.…”

Section: Mtm Friction Curvessupporting

confidence: 74%

“…Much more recently, using a very low speed cylinder on ring sliding test rig, Aoki et al showed that a secondary ZDDP gave higher boundary friction than primary ZDDPs based on linear alkyl groups and that ZDDPs with longer, linear alkyl groups gave lower boundary friction than those based on shorter chains [19]. Hoshino et al used a rolling-sliding ball on disc 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 [20]. They showed that boundary friction varied in the order octyl < ethylhexyl < methylpentyl.…”

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: Discussionsupporting

confidence: 65%

Section: Mtm Friction Curvessupporting

confidence: 74%

Section: Introductionmentioning

confidence: 99%

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Boundary Friction of ZDDP Tribofilms

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“…In EHD lubrication, branched hydrocarbons tend to give higher EHD friction than linear chain ones since they cannot easily slide past one another during shear [53] and Hoshino et al [54] found that a di-ethylhexyl-based ZDDP gave faster film formation that a di-n-octyl-based one, despite both being primary ZDDPs. It should also be noted that very earliest ZDDP patent was based on cyclic alcohols rather than linear or branched chain ones [55] and cyclic hydrocarbons form the basis of almost all traction fluids.…”

Section: Discussionmentioning

confidence: 99%

On the Mechanism of ZDDP Antiwear Film Formation

2016

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Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid-solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high-pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.

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“…Also, ZDDPs with longer, linear alkyl groups gave lower boundary friction than those based on shorter chains [7]. 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.…”

Section: Introductionmentioning

confidence: 99%

Ethoxylated Amine Friction Modifiers and ZDDP

2019

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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.

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Tribological Properties of Sulphur-Free Antiwear Additives Zinc Dialkylphosphates (ZDPs)

Cited by 10 publications

References 3 publications

Boundary Friction of ZDDP Tribofilms

Boundary Friction of ZDDP Tribofilms

On the Mechanism of ZDDP Antiwear Film Formation

Ethoxylated Amine Friction Modifiers and ZDDP

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