Analysis on mechanism of thin film lubrication

ARTICL .ESChinese Science Bulletin 2005 Vol. 50 No. 22 2645- -2649TFL as a research intention.Essentially, TFL is ascribed to a lubrication regimeAnalysis on mechanism of thin where nhe tiqud fim uhcknes is of the order of hanofilm lubricationmeters or molecules. In literature, super or partial or mo-lecular thin film lubrication is used to describe this newlubrication featured with sub-micron or nanometer scaleZHANG Chaohui', LUO Jianbin2 & HUANG Zhiqiang3film thicknessesb.4. However, the term‘thin film lubrica-1. School of Mechanical, Electronic and Control Engineering, Bejingtion' comes into being dominant, for instance, Wen'sl andJiaotong University, Bijing 100044, China;Tichyl6. To date, it is widely accepted that TFis a dis-2. State Rey Labortory of Tibology. TSinghua University. Beijig tinctive lubrication state, bridging the gap between ehl100084. China;3. NSK China Technial Center and Basic Technology Research & De-and BL, which has its own distinctive lubrication traits.velopment Center, Kunshan 215335, ChinaWith the lubricant film thickness stepping down, lubricat-Correspondence should be addressed to Zhang Chaohuiing state will undergo following changes: hydrodynamic(email: zhangchaohui@ tsinghua.org.cn)lubrication -→elasto-hydrodynamic lubrication -→thin filmAbstract It is an important concern to explore the pro-lubrication- +boundary lubrication- →dry friction. Recently,perties and principles of lubrication at nano or molecularlubrication map was proposed accordingly", asscale. For a long time, measurement apparatus for film shown in Fig. 1, where h is the lubrication film thickness,thickness of thin flm lubrication (TFL) at nano scale have R the combined roughness of the two surfaces in relativebeen devised on the basis of superthin interferometry tech- motion, and Rg the efective radius of lubricant molecules,nique. Many experiments were carried out to study the lu-respectively. TFL bridges the gap between EHL and BL.brication principles of TFL by taking advantages of afore-mentioned techniques, in an attempt to unveil the mechanismh/R2↑of TFL. Comprehensive experiments were conducted to ex-MLplore the distinctive characteristics of TFL. Results show thatTFL is a distinctive lubrication state other than any knownDry contact, BLEHL and HDLlubrication ones, and serves as a bridge between elastohy-l TFL and EHI.drodynamic lubrication EHL) and boundary lubrication10- -15---------(BL). Two main influence factors of TFL are the solid surface( Dry contact、TFLeffects and the molecular properties of the lubricant, whose(BL and TFL)combination effects result in alignment of liquid moleculesTDry Contactnear the solid surfaces and subsequently lubrication with_Bordered film emerged. Results of theoretical analysis consid-ering microstructure are consistent with experimental out-Fig. 1. Lubrication map].comes, thus validating the proposed mechanism.Keywords: thin fim lubrication, super thin interfrometry, lubrica-In the 1990s of last century, researchers extendedtion property, ordered fim, microstructure.conventional optical interference technique so as to providemeasurement ability of film thickness down to a few na-DOI: 10.1360/982004-431nometers or molecular scale. With the help of these meth-It is a main concern to interpose a lubrication film ofods, profound experiments were carried out to investigatelow shear strength between two solid surfaces in relativethe specific features of TFL, some comprehensive refer-motion intending to efficiently reduce friction and wearences were summarized by the first authorl'l. At present,and to prevent seizure. There is a veiled regime in lubrica-extensive investigations have been conducted to experi-tion theories: film thicknesses of elastohydrodynamic lu-mentally study the properties of TFL, however, the otherbrication (EHL) are typically of the order of 0.1-1 um,side of the same coin is, it is a far cry to obtain mathe-matically feasible predictive model of TFL, leaving asidewhile film thicknesses of boundary lubrication (BL) arethe ordered model has been put forward for a long time. Itusually of the order of several nano metersh. Thus theis the main purpose to give a preliminary analysis onunknown span of film thickness constitutes the study ob-mechanism of TFL on the basis of previous works.ject of TFL investigation. Furthermore, it is widely ac-1 Superthin interferometry makes it possible to studycepted that EHL is featured with viscous fluid films, inaddition to piezo-viscous effect and solid elastic deforma-properties of TFLtion, and it is based upon continuum mechanism. Bound-A key prc中国煤化Ieceristis of TFLary lubrication, however, featured with adsorption films, is to devisef measuring filmeither due to physisorption or chemisorption, is based on thickness purYHcNMHGuesbasedonop-surface physical/chemical properties-. It will be of great tical interferometry is one of the most important contribu-importance to bridging the gap between EHL and BL in tors to experimental research. Conventional optical inter-aspects of work mechanism and study methods, by taking ferometry has two major limitations that prevent it beingCh数糖ience Bulletin Vol. 50 No. 22 November 20052645ARTICLESable to measure very thin films. One is that, for optical temperature control range: room-temperature ~120°C, etc.interference to occur, the optical path difference must be at These successes laid the foundation of investigating prop-least half a wavelength of light, which means that it is noterties and lubrication principles of TFL.practical to measure films less than about one-quarter ofThese experimental methods and techniques and theirwavelength of light thick, i.e, about 50 nm. The second is utilization have brought out fruitl outcomes, which pro-an inherent inability of the human eye accurately to dis- vide some valuable data of properties and mechanism oftinguish interference colors, which means that measure-TFL， and paved a feasible way for lubrication andment resolution is limited to about +10 nm. The limita-tribo-system at nano scale.tions of optical interferometry give impetus to further ex-2 TFL is a transition state between EHL and BLtensions so as to enable it capable of measuring lubricantfilm thickness falling down to a few nanometers. In 1991,Size effects can be seen in TFL and is the deviation ofJohnston et al!. used a method of spacer layer in combi- TFL from EHLIO, ie, the film thickness Vs. velocity,nation with the spectrum analysis of reflective light toviscosity, pressure etc. relations are no longer linear onesexpand the optical interference technique, enabling it in log-log coordinates. Fig. 3 is a schematic view of theavailable to measure thin film thickness down to nano typical test curve from experimental data, wherein thescale. Soon later, in 199 Hartl and co-workersI0- 12] axes are logarithm coordinates. Three regions can be seenmeasured film thickness down to 5 nm and further ex-from the figure: thick film region (section I), thin filmpressed the film thickness distribution as intuitionisticregion (section II), and failure region (section II). It canimage by applying colorimetric interferometry technique.be seen that, the film thickness various linearly with ve-Reently, ntertence method with the help of multi-beam locity (or viscosity and pesse)) in thick film region. Thistechnique also reported the measured film thickness valueis the EHL regime, where film thickness variation com-down to about 1 nml3,141.plies with EHL theory predictions. While in thin film re-Luo and co-workers affiliated to Tsinghua Universitygion，with the decreasing of film thickness, film thicknessdevised NGY-3 film thickness measurement rig (Fig.2)of curve levels out prgesively difering from that of HLTFL, by making use of a technique of relative optical in-predictions. The thinner the film, the more the difference.tensity interferencells (ROII). The apparatus can provideWith further decreasing film thickness, either due to thedynamic measurement of film thickness of the order ofshear limit of lubricant is arrived, or due to the solid sur-nano meter with stable precision. The main qualificationsface cease to sustain an available absorption, the lubricantof the system are: measure range: 0- 500 nm, verticalcan no longer hold mobility and bear loads. This heralds aresolution: 0.5 nm, horizontal resolution: 1 μm, velocityfailure region.range: 0.2- - 1900 mm/s, friction force resolution: 0.1 mN,Actual curve30- -40|!1EHL curve4050v/mm.s-'Fig. 3. Schematic view of various regimes of film thickness (Lubricant:Ci6Hs; Pressure: 185 MPa).The failure of TFL only means a mobility loss here. Infact, monolayers can stay on solid surfaces to separate thesolid surfaces in relative motion, and subsequently sustaina feasible bol中国煤化Ise film thicknessof TFL is onfYHr order, from me-chanic pointcNMHGilityTFLcanberegarded as the last one of lubrication fields where Rey-Fig.2. Measure apparatus for lubrication film thickness at nano scalenolds equation is valid.(NGY-3).Boundary lubrication can also evolve into TFLU8.26月方数据Chinese Science Bulletin Vol. 50 No. 22 November 2005ARTICL ESCompared to BL, TFL has a thicker film. In the vicinity ofsolid walls, the liquid molecules take the states of that of0g■CB/hexadecanethe boundary film, the ordered one, and the disordered one,-0- CALhexadecanefrom the wall surfaces to the center of the gap. From a-一CA/hexadecanemechanic point of view, the existence of an ordered film20--0- CP/hexadecanemakes lubricant film differs strongly from the boundary-0- - hexadecanefilm, which can form a glassy state or solid-like statel91On the other hand, lubricant of TFL can only sustain someviscosity, expressing mobility to some extent.3 Study on film-forming of TFL号o-4■The property of TFL is strongly associated with theproperty of solid wall surfaces (Fig. 4). To investigate the105202530influences of solid surface energy, experiments were car-Film thickness/nmried out with four types of materials of various solid sur-Fig. 5. Effctive viscosity vs. film thickness relaionl21.face energy. From the thickness difference among differ-ent friction surfaces, it can be seen that the film with TiO2，properties of liquid molecules. As mentioned before, EHLwhich has the highest surface energy, is over than that is featured with viscous fluid films. As to a particular lubewith any other surfaces. The films with Ti and Cr, whose system, if any other conditions are same, the film charac-surface energy are nearly of the same, are nearly of the teristics of the lube system will be same for different solidsame thickness. The film with Al, whose surface energy is surface and different lubricant (same viscosity, viscos-the least, holds the smallest thickness. It can be deduced ity -pressure relation and viscosity-temperature relation arethat the higher the surface energy is, consequently theassumed). In TFL, due to surface adsorption potentialshigher the surface tension is, the thicker the film and the and/or shear induction, the orientation of liquid moleculescritical values are2021.tends to being ordered from each other, and conclusivelyresulting in different characteristics. Luo et a1.2s con-cluded that TFL is essentially dominated by the orderedEHLfilm.4 Ordered model of TFL (analysis on physical model)IFLIn TFL, because the lubricant film is confined in nar-row gap separating the solid surfaces in relative motion of10-tribo system, the molecules of lubricant film tend to bestructurally ordered in friction shear process, due to theload and surface energy. Then lubrication film at thesub-micro or nano scales will comprise three types of filmwith different structures and properties: i.c. the adsorbedfilm, the ordered film, and the fluid film (Fig. 6). The or-dered liquid film bordering between the viscous fluid film100and the adsorbed film is thus formed that the liquid mole-Speed/mm .s'Fig.4. Film thickness of TFL with solid surface energy21. Lubricant:cules undergoes surface forces during friction and shearprocess. From the viscous fluid film to the adsorbed film,13604; Load:4N,T= 25"C.the ordering degree increases gradually. This orientationThe property of TFL is linked closely to the lubricant ordered molecular film is called ordered liquid film, whichproperties. Fig. 5 shows experimental results of hexadec-has a better ordering degree than that of the bulk fluids,ane with various liquid crystal additives. The abscissa is and thus subsequently being less mobile. However, thefilm thickness while the ordinate is effective viscosity. Itother side of the same coin is, the ordered liquid film has atells that the effective viscosities are almost the same mobility to some extent, thereby it can bear loads and leadwhile film thickness is high (EHL regime). However, into a decrease of side leakage due to hydrodynamic effects.TFL regime, effective viscosities vary with film thickness,In TFL, the ordering degree of lubricant molecules de-and the variations are overt. This clearly shows that, even creases from中国煤化工lee, which resultsthough the apparent viscosities (i.e.. the bulk viscosity)are in differentbehavior andalmost same in TFL, film forming characteristics are dif-size-dependerMHCNMHGferent, indicating a different mechanism of TFL.Experimental results show that the transition from EHLThe above -mentioned experiments show two main in- to TFL is closely linked to the rearrangement structure offluential factors of TFL: the solid surface effects and the liquid molecules and transition conditions are also relatedCh数糖ience Bulletin Vol. 50 No. 22 November 20052647ARTICLESorder. It lies in surface force field. The surface force usu-Fluid fimally does not work long away from the solid walls.(2) Molecular orientation is ordered, and tends to beparallel to someone axis, which is named“easy axis" andOrderedexpressed by a unit vector, the director, e.film(3) The direction of the easy axis in space is arbitrarilyAdsorbeddistributed, which can be influenced by the solid surfaceSolid surlaceforces.Fig. 6. Ordered liquid film model for TFL".(4) The directions e and -e are indiscernible.(5) The director is supposedly fixed on solid surfaceto the property of the liquid as well as the film thickness. due to the surface potentials.The critical point of film thickness varies with liquid types.The ordered molecules are apparently similar to that ofTFL is featured with ordered liquid film, wherein the ori-nematics. Velocity field of TFL can be analyzed with theentation of the molecules are aligned. The thickness of the help of liquid crystal theories accordingly (Fig. 7).ordered film is closely associated to the interface adhesiveenergy and its work range. The relation between the inter-1.0←-do/dr°=2face adhesive energy and liquid molecular structure, mo-+ do2/dr' =-2lecular weight, and environment temperature is similar to、0.8that of the effective viscosity.da's/dr'=0」5 Verification of the ordered film modelTFL is essentially a lubrication regime where the or-dered film takes a dominant role. Even though it is very昌0.4-difficult to give an in situ measurement of this orderingstate and up to date there is hardly report on the observa-tion of it during real TFL process, the ordered property ofliquid molecules has drawn more and more broad con-.0一0.40.60.80.00.cerns. Ehara et al.!24 showed that molecular alignmentFluid velocity, v”will take place if the molecules are adsorbed onto a solidFig. 7. Velocity distribution under different normal pressure gradient.surface with an appropriate surface structure. Their inves-tigation shows that the surface with high energy can makethe liquid molecules ordered. For low energy surfaces, if itBecause lubrication film of TFL is very thin, ap-undergoes a macroscopic processing such as mechanical proaching the length scale of the particles of lubricants,shear brushing, molecular alignment will also take place, the size of the particles will contribute much to lubricationwhich means that the shear induction effects account forproperty. The force and moment transition of lubricantthe molecular alignment. The lyotropic liquid crystallinelayers confined by particular boundary conditions shouldcan also express this characteristicl251. These provide some be considered. And the microstructure and the corre-indirect verifications of the ordered liquid film model.sponding deformation should also be taken into account.It should be point out that, the ordered orientation of For instance, three types of deformation can occur inliquid molecules is prevailing in BL", whilst they usu- nematics: splay, twist, and bend. By reducing the problemally cease to be mobile. However, the ordered film of TFLcomplexity, Zhang et al.127.28.31]1 supposed that the lubricanthas the characteristic of a liquid, and thus sustains mobil- particles are rigid and randomly ordered in viscous media,ityand performed numerical simulation of lubrication fea-6 Theoretical analysis on the basis of the orderedtures of TFL by using micropolar theory. Fig. 8, verysimilar to Fig. 5, shows the relation between the effectivemodelviscosity and the minimum film thickness, taking intoTheoretical analysis on TFL properties lags far behind account of the micropolar effects (with different charac-experimental investigations, partly because it has close teristic lengths and couple numbers).relation to the solid surface adsorption potentials and theThese investigations are useful for quantitatively mod-property of the lubricants, which obviously contribute toeling the lubrication features of TFL. The results indicatethe complexity of the problem. Zhang et al.26- 0l focused that the orde中国煤化工:s have an importheir endeavors on mathematical modeling and theoretical tant influence|YHJcteristics and lu-prediction of the lubrication features of TFL.brication propCNMHGaldthatitisper-The orientation of the ordered film has the following fect, however. Further exploration on the basis of the or-characteristicsl2o:dered film model will lay the foundation for the mecha-(1) Molecular weight center does not have long range nism of TFL.26月方数据Chinese Science Bulletin Vol. 50 No. 22 November 2005ARTICL .ES3.5non-polar5. Tichy,J. A.. Thin film lubrication, in Proc. 1st Int. Symp. Tribol,=!nm,Ns=0.5Beiling: Tsinghua University Press, 1993, 48- -53.0-..时.. 1=5 nm, Ns= 0.57. Luo,J. B., Shen, M. W., Shi, B. et al., Thin film lubrication and lu-=1 nm, Ns= 0.65brication map, Chinese J. Mechanical Engineering (in Chinese),..0.. 1=5 nn, Ns= 0.658. Zhang, C. H. Research on thin film lubrication: State of the art,-v- 1= 1 nm, Ns=0.99. Johnston, G. J., Wayte, R., Spikes H. A,The measurement and .-切-- l=5 nm, Ns-0.92.0-study of very thin lubricant fims in concentrated contacts, Tribol.Trans., 1991, 34(2): 187- 194.0. Hartl, M., Krupka, I, Poliscuk, R. et al, An automatic system for.5-,real-time evaluation of EHD film thickness and shape based on thecolorimetric interferometry, Tribol. Trans., 1999, 42(2): 303- -309.Krupka, L, Hart, M., Poliscuk, R. et al, Experimental study of1.0-orimetric interferometry technique, Tribol. Trans, 2000, 43(4):611- -618.20408C102. Hartl, M.. Krupka, I, Poliscuk, R. et al, Thin fim colorimetric in-h/nmterferometry, Tribol. Trans., 2001, 44(2): 270- 276.3. Guo, F, Wong, P. L. A multi-beam intensity- based approach forFig. 8. Effective viscosity vs. minimum film thickness relation (theo-retical analysis)2wortcanttilm, measurepents 1n non-contorma.contacsoptroignst:28-291. Engneers7 Conclusions4. Glovnea, R. P， Forrest, A. K. et al, Measurement ofTFL is an important branch of nano tribology. TFL insub-nanometer lubricant flms using ultra-thin film interferometry,Tribol. Lett.. 2003, 15: 217- -230.ultra thin clearance exists extensively in micro/nanoLuo,J. B., Wen, S. z, Huang, P, Thin film lubrication, Part I: Thecomponents，integrated circuit (IC)， micro-electrome-transition between EHL and thin film lubrication, Wear, 1996, 194:chanical system (MEMS), and computer hard disks, etc.6. Zhang, C. H., Numerical Analysis on Tribological Performances ofThe impressive developments of these techniques presentLubricating Film in the Nano Scale (in Chinese), Beijing: Tsinghuaa rosy future of TFL with an ordered structure at nanoUniversity, 2002. .7. Dorinson, A.. Ludema, K. C., Mechanics and Chemistry in Lubri-scale.cation, Berlin: Elsevier, 1985.The mechanism of TFL can be summarized to the or-8. Glovnea, R. P, Spikes, H. A., Elastohydrodynamic film formationdered film lubrication, concluding from preceding re-ahe s:u_fhsmotion,n Proc. Instn. Mech. Engrs (Parsearches. It should be noted that the property and mecha-9， Homola, A. M, Israelachvili, J. N, Gee, M. L. et al, Measure-ments of and relation between the adhesion and friction of two sur-nism of TFL are not fully understood to date, and therefaces separated by molecularly thin liquid films, ASME, J. Tribol,remains a wide blank area to be covered between experi-20. Luo,J. B., Wen, S. Z, Sheng, X. Y. et al., Substrate surface energyments and theoretical predictions. Some key problems toeffet on liguid lubricant film at nanometer scale, Lubricationbe addressed are1) measurement technique on lubrication film distribu-I. Shen, M. W., Luo, J. B.. Wen, S. Z, Influence of friction pair sur-face's physicochemical properties on nano scale film thickness, J.tion;Tsinghua University (in Chinese), 2000, 40(4): 103- 106.2) the relation of the efective viscosity and the prop- 22. Shen. M. w. LIO. I B. Wen, s. z etal. Ivestigation of the lig-uid crystal additive' s influence on film formation in nano scale,erty of the lubricant molecules; .3) influence factors relating to transition between the3. LUO.J.B..Huang.P.Wen.s.Z.etal.Characteristics of fluid lubricant films at the nano-scale, ASME, J. Tribol, 1999, 121(4):liquid phase and the solid-like phase;4) further perfection of the mathematical model based4. Ehara, T, Hirose, H, Kobayashi, H. et al., Molecular alignment inorganic thin films, Synthetic Metals, 2000, 109: 4on the ordered film;25. Nesrullajev, A., Tepe, M., Kazanci, N. et al, Surface-induced tex-5) the property of electro-rheology, magneto-rheology,tures in lyotropic liquid crystalline mesophases, Materials Chemis-intelligibility and controllability, etc of ordered liquids and6. Zhang, C. H, Wen, S. z, Luo, J B.. Lubrication theory. for thinso on.film lubrication accounting for the ordered film model, Int. J.Nonlinear Sciences and Numerical Simulation, 2002, 3(3-4): 481-Acknowledgements This work was supported by the College ResearchFund of Beijing Jiaotong University (2004SM041).27. Zhang, C. H., Wen, s. Z, Luo, J. 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