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先贴一部分吧，摘要加简介和最后结论
Abstract
Organic friction modifiers (OFMs) are important additives in the lubrication of machines and especially of car engineswhere performance improvements are constantly sought-after. Together with zinc dialkyldithiophosphates (ZDDPs)antiwear additives, OFMs have a predominant impact on the tribological behaviour of the lubricant. In the current study,the influence of OFMs on the generation, tribological properties and chemistry of ZDDP tribofilms has been investigatedby combining tribological experiments (MTM) with in-situ film thickness measurements through optical interferenceimaging (SLIM), Alicona profilometry and X-ray photoelectron spectroscopy.OFMs and antiwear additives have been found to competitively react/adsorb on the rubbing ferrous substrates in atribological contact. The formation and removal (through wear) of tribofilms are dynamic processes which result fromthe simultaneous interaction of these two additives with the surface of the wear track. By carefully selecting thechemistry of OFMs, the formulator can achieve lubricants that generate ZDDP antiwear films of optimum thickness,morphology and friction according to the application-specific requirements.
KEY WORDS: Organic friction modifiers, Tribofilm, ZDDP, Friction, Boundary lubrication, XPS
1. Introduction
Modern engine oils contain a large number of additives, but the most influential on the tribological performance of thelubricant are antiwear zinc dialkyldithiophosphates (ZDDPs) and friction modifiers (FMs).
ZDDPs have been used in engine oils for more than 70 years and are probably the most successful antiwear additivesever discovered. In addition to being good antioxidants [1], they reduce wear by rapidly forming relatively thick,sacrificial boundary tribofilms with hardness characteristic to soft polymeric materials [2, 3].
The generation of ZDDP tribofilms on steel surfaces and their nature has been the main focus of published research. Ithas been shown that they form only on the rubbing tracks and are dependent on temperature [1]. On steel, tribofilmscan rapidly grow to a thickness >100 nm and have an uneven, pad-like distribution (typically 5 to 20 μm across),separated by deep fissures. A two layer model was proposed, where a soft polyphosphate film covers a more rigidoxide-sulphide layer chemisorbed onto the steel surface [2]. The chemistry of the ZDDP oil solution was reported toaffect the chemical make-up, thickness and mechanism of formation of the antiwear film [4]. The antiwear filmsformed by ZDDP have high boundary friction coefficients in the range of 0.11 to 0.14, which are maintained up to muchhigher sliding speeds than is normally the case. This is thought to be due to the unusual morphology of the reactionfilm [5]. To reduce the high boundary friction of the ZDDP tribofilms, it is especially important to use efficientfriction modifiers when formulating lubricants.
2Friction modifiers (FMs) can improve lubricity and thus energy efficiency by reducing the coefficient of friction (COF)in the boundary, and in some cases, also in the mixed lubrication regimes [6]. A direct application of this can be foundin engine oils, which contain FMs for fuel economy purposes.
Presently, there are two main types of FMs: organic friction modifiers and molybdenum compounds. Organic frictionmodifiers (OFMs) are surfactant-like molecules with long chains and polar groups (e.g. alcohol, amide, carboxylic acidand ester groups), which adsorb or chemically react on the polar metal surfaces to form dense monolayers (2 nm thick)or thick reacted viscous layers [7, 8]. Organic molybdenum compounds, such as molybdenum dithiocarbamate(MoDTC), generate nanosized single sheets of MoS2 dispersed in a carbon [9] or FeS2 [10] matrix. The MoS2 sheetsfacilitate sliding and thus lower friction between rubbing asperities.
Both antiwear and friction modifier additives work by reacting or adsorbing on the lubricated contact surface. Therefore, since these two types of additives are used together, it is important to understand the mechanism of action forgenerating the tribofilm and how these additives interact during this process. Unfortunately, at the present there is nosystematic understanding of the interaction of FMs with other lubricant additives. For this reason, the selection ofoptimal additive combinations for lubricant formulation primarily depends upon trial and error or past experience,rather than knowledge of fundamental chemical interactions.
Previously published work investigating the interaction between ZDDP and FMs mainly focused on ZDDP and MoDTC.The reason for the high interest in MoDTC is that aside from being a FM, it is also one of the most efficientnon-phosphorous antiwear additives [11], shown to synergize well with ZDDP to reduce wear and friction [10, 12-14].However, the downside of using MoDTC alone or in combination with ZDDP in oils is the formation of abrasive MoO3,which is conducive to high friction [15]. One study investigated the synergism between ZDDP and MoDTC andreported that there is a competitive adsorption on the rubbing steel surface between ZDDP and MoDTC, which resultsin a thinner ZDDP tribofilm [16].
The only published study which investigated OFMs besides MoDTC has proposed a different mechanism of action, inwhich FM additives form a friction-reducing film not on the ferrous surface, but on the zinc phosphate. Therefore, itwas recommended for FMs to be tested and optimized for effectiveness on the ZDDP films [17].
The objective of the current study was therefore to explore the interaction between ZDDPs and OFMs and to determinewhether this influence is susceptible to variations of the OFM additive type. To accomplish this objective, the workemployed various techniques to study the growth kinetics, the physical (thickness, morphology) and tribologicalproperties (friction and wear) and the chemical composition of tribofilms generated by ZDDP and three types of OFMs.
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4. Conclusions
This study has investigated the influence of three organic friction modifiers (OFMs) of different chemistries on theZDDP tribofilm formation, composition and properties. The results show that the tribological properties of the fullyformulated engine oils significantly depend on their composition.
Tests carried out with fully formulated oils without OFMs (BLO) showed that the generated ZDDP tribofilm isnon-uniform and thinner (as shown by optical interference images), rich in calcium and poorer in zinc concentration(XPS spectra) than the BO+ZDDP films.
Whether in the presence or absence of other additives, OFMs can greatly influence the reaction of the ZDDP antiwearadditive with the steel substrate and consequently, the kinetics, thickness, composition and tribological properties of thetribofilm generated in the rubbing contact.
Depending on their chemical composition, OFMs can react with the wear track in rubbing contacts to generate owntribofilms (as in the case of BO+OFM B and BO+OFM C) which have kinetics and thickness comparable to the ZDDP tribofilms (BO+ZDDP).
In a tribological contact, the formation and removal (through wear) of tribofilms are dynamic processes controlled bythe interaction between the surface active additives and the lubricated contact surfaces. This is especially the case ofantiwear and OFM additives, which compete to react/adsorb on the rubbing ferrous substrates. The generation oftribofilms is the result of the simultaneous interaction of these two additives with the surface of wear track.When mixed into the fully additized oil (BLO), the three OFMs of various chemistries influenced the tribofilmgeneration and properties in particular ways. OFM B produced thin tribofilms (110 nm) and was very efficient atreducing friction (COF=0.09), while OFM A and C generated thick tribofilms (160 nm) but reduced friction to a lesserextent (COF=0.11).
Despite their different behaviour influenced by chemistry, all three OFMs have potential value for tribologicalapplications. In automotive transmission, where emphasis is placed more on wear protection than friction reduction,OFM C and OFM A, which generate thicker tribofilms could be more useful. Engine oils require high levels offriction reduction and fuel efficiency and could be formulated with friction modifiers similar to OFM B.The investigation of the effect of OFMs chemistry on ZDDP antiwear film performance will guide the formulator inselecting OFMs according the application-specific requirements.
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楼主: 网轮碟刹
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发表于 四川省 2018-4-11 13:54
