Tribology of thin coatings

ReviewTribology of thin coatingsK. Holmberga,*, H. Ronkainena, A. MatthewsbaVTT Manufacturing Technology, PO Box 1704, FIN-02044 VTT, FinlandbThe Research Centre in Surface Engineering, The University of Hull, Hull HU6 7RX, UKReceived 24 July 1999; received in revised form 31 August 1999; accepted 4 September 1999AbstractThe fundamentals of coating tribology are presented in a generalised holistic approach to friction and wear mechanisms of coatedsurfaces in dry sliding contacts. This is based on a classification of the tribological contact process into macromechanical, micro-mechanical, tribochemical contact mechanisms and material transfer. The tribological contact process is dominated by the macro-mechanical mechanisms, which have been systematically analysed by using four main parameters: the coating-to-substrate hardnessrelationship, thefilm thickness, thesurface roughnessesand thedebrisin thecontact. The descriptioncovers both softand hard coat-ings with thicknesses typically in the range 0.150 mm, where the interaction between the coating and the substrate is essential to thetribological behaviour. The concept is supported by experimental observations. The important influence of thin tribo- and transferlayers formed during the sliding action is shown. Optimal surface design both regarding friction and wear can be achieved by newmultilayer techniques giving reduced stresses, improved adhesion to substrate, more flexible coatings and harder and smoother sur-faces. The diC128erences in contact mechanisms in dry, water- and oil lubricated contacts with coated surfaces is illustrated by experi-mentalresultsfromdiamond-likecoatingsslidingagainstasteelball.Themechanismsoftheformationofdrytransfer-andtribolayersandlubricatedboundaryandreactionfilmsarediscussed. # 2000 Elsevier Science Limited and Techna S.r.l. All rights reserved.Keywords: A. Films; C. Friction; C. Wear resistance; D. Carbon0272-8842/00/$ - see front matter # 2000 Elsevier Science Limited and Techna S.r.l. All rights reserved.PII: S0272-8842(00)00015-8Ceramics International 26 (2000) 787795Contents1. Introduction.7882. Tribological contact mechanisms.7882.1. Tribological contact mechanisms.7882.2. Macromechanical friction and wear mechanisms.7892.3. Micromechanical tribological mechanisms.7902.4. Tribochemical mechanisms of coated surfaces.7902.5. Formation of thin microfilms on hard coatings.7902.6. Nanophysical contact mechanisms .7902.7. Mechanisms of material transfer.7913. Multilayer and multicomponent coatings.7913.1. Multilayer coatings.7914. Water and oil lubricated DLC coated sliding contacts.7924.1. The eC128ect of water lubrication.7934.2. The eC128ect of oil lubrication.7935. Conclusions.794* Corresponding author. Tel.: +358-9-456-5370; fax: +358-9-456-7002.E-mail address: kenneth.holmbergvtt.fi (K. Holmberg).1. IntroductionThere has been a large interest in studying thin sur-face coatings for wear protection and friction reductionboth in the scientific community and in industry. Recentadvances in coatings technologies now permit thedeposition of films with properties which were unac-hievable even a decade ago. Examples are multilayeredand metastable coatings with extreme mechanical andchemical properties. The plasma and ion-based vacuumcoating techniques have been at the forefront of thesenew developments. They allow the coating/substratesystem to be designed in such a way that the combina-tion performs in an optimal manner. This objective isfurther aided by improvements in our fundamentalunderstanding of contact mechanisms between surfaces,at the macro, micro and nano level.The paper presents a review of the present level ofunderstanding of contact mechanisms, especially forthin coated surfaces. This includes an appreciation ofstress states, mechanical properties and chemical influ-ences.Thepossibilitiesopenedup byusingmultiplelayersof coatings are discussed; this includes the possibility tograde the functional properties from the surface to theinterface between coating and substrate. Special mentionismade of recent developments in carbon-based coatingswhich combine excellent frictional properties with goodwear resistance. Their behaviour in dry and lubricatedconditions is discussed.2. Tribological contact mechanismsThe tribology of a contact involving surfaces in rela-tive motion can be understood as a process with certaininput and output data 1. Input data that are used as astarting point for the analysis of a tribological contactare the geometry of the contact, both on a macro andmicro scale, the material properties based on the che-mical composition and structure of the diC128erent partsinvolved and the environmental parameters. Otherinput data are the energy parameters such as normalload, velocity, tangential force and temperature.The tribological process takes place as the two sur-faces are moving in relation to each other, and bothphysical and chemical changes occur in accordance withthe physical and chemical laws with respect to the inputdata. As a function of time the tribological processcauses changes in both the geometry and the materialcomposition and results in energy related output eC128ects:friction, wear, velocity, temperature, sound and dynamicbehaviour.2.1. Tribological contact mechanismsThe complete tribological process in a contact inrelative motion is very complex because it involvessimultaneously friction, wear and deformation mechan-isms at diC128erent scale levels and of diC128erent types. Toachieve a holistic understanding of the complete tribo-logical process taking place and to understand theinteractions it is useful to separately analyse the tribo-logical changes of four diC128erent types: the macro andmicro scale mechanical eC128ects, the chemical eC128ects andthe material transfer taking place, as shown in Fig. 1. Inaddition there has recently been an increasing interest instudying tribological behaviour on a molecular level; i.e.nanomechanical or nanophysical eC128ects 2.A better and more systematic understanding of themechanisms involved in a tribological contact is neces-Fig. 1. Tribological contact mechanisms are related to macromechanical changes, material transfer, micromechanical, tribochemical and nanophy-sical changes in the contact.788 K. Holmberg et al./Ceramics International 26 (2000) 787795sary for the optimisation of the properties of the two con-tacting surfaces in order to achieve the required frictionand wear performance. Approaches to the tribologicaloptimisationofsurfaceshavebeenpresentedbyMatthewsetal.3andFranklinandDijkman4whointroduceeightwear design rules in an expert system for assisting theselection of metallic materials, surface treatments andcoatingsduringtheinitialstagesofengineeringdesign.2.2. Macromechanical friction and wear mechanismsThe macromechanical tribological mechanismsdescribethefrictionandwearphenomenabyconsideringthestressandstraindistributioninthewholecontact,thetotal elastic and plastic deformations they result in, andthe total wear particle formation process and its dynam-ics. In contacts between two surfaces of which one orboth are coated, four main parameters can be definedwhich control the tribological contact behaviour. Theyare the coating-to-substrate hardness relationship, thethickness of the coating, the surface roughness, and thesize and hardness of any debris in the contact which mayoriginate from external sources or be produced by thesurface wear interactions themselves.The relationship between these four parameters willresult in a number of diC128erent contact conditions char-acterised by specific tribological contact mechanisms.Fig. 2 shows, schematically, 12 such very typical tribo-logical contacts, with diC128erent mechanisms influencingfriction, when a hard spherical slider moves on a coatedflat surface 5. The corresponding wear mechanismshave been described in a similar way 6.An important parameter is the coating hardness andits relationship to the substrate hardness. It is commonto consider hard coatings and soft coatings separately7,8. The advantages of using a soft coating to reducefriction are well known, owing to the work of BowdenandTabor9andothers.SoftcoatingssuchasAgandAumay also have the function of reducing sliding-originatedsurface tensile stresses, that contribute to undesirablesubsurface cracking and subsequently to severe wear10. A hard coating on a softer substrate can decreasefriction and wear by preventing ploughing both on amacro scale and a micro scale 6,1113. These coatingstypically exhibit residual compressive stresses which canprevent the likelihood of tensile forces occurring. Fur-ther decreases in friction and wear can be achieved byimproving the load support, that is by increasing thehardness of the substrate to inhibit deflections andploughing due to counterpart load.For soft coatings the thickness of the coating influ-ences the ploughing component of friction, while forrough surfaces it aC128ects the degree of asperity penetra-tion through the coating into the substrate as shown in(a), (b), (e) and (f) in Fig. 2 10,14,15. A thick hardcoating can assist a softer substrate in carrying the loadand thus decrease the contact area and the friction. Thinhard coatings on soft substrates are susceptible to coatingfracture because of stresses caused by substrate deforma-tion. Rough surfaces will reduce the real contact area,Fig. 2. Macromechanical contact conditions for diC128erent mechanisms which influence friction when a hard spherical slider moves on a coated flatsurface.K. Holmberg et al./Ceramics International 26 (2000) 787795 789although the asperities may be subject to abrasive orfatigue wear 7,13,1619.Loose particles or debris are quite often present insliding contacts. They can either originate from the sur-rounding environment or be generated by diC128erent wearmechanisms. Their influence on friction and wear maybe considerable in some contact conditions, dependingon the particle diameter, coating thickness and surfaceroughness relationship and the particle, coating andsubstrate hardness relationship. Particle embedding,entrapping, hiding and crushing represent typical con-tact conditions involving the influence of debris, asshown in Fig. 2 (i)(l) 5.2.3. Micromechanical tribological mechanismsThe origin of the friction and wear phenomena thatwe observe on the macro level is found in the mechan-isms that take place at the micro level. The micro-mechanical tribological mechanisms describe the stressand strain formation at an asperity-to-asperity level, thecrack generation and propagation, material liberationand particle formation. In typical engineering contactsthese phenomena are at a size level of about 1 mm or lessdown to the nanometre range as shown in Fig. 1.Shear and fracture are two basic mechanisms for thefirst nucleation of a crack and for its propagation untilit results in material liberation and formation of a wearscar and a wear particle. These mechanisms have beendiscussed by for example Argon 20 and Suh 7, butstill today there is only a very poor understanding ofthese quite fundamental phenomena. Another approachis to study the tribological micromechanical mechan-isms by using the velocity accommodation mode con-cept developed by Berthier et al. 21.2.4. Tribochemical mechanisms of coated surfacesThe chemical reactions taking place at the surfacesduring sliding contact and also during the periodsbetween repeated contacts, change the composition ofthe outermost surface layer and its mechanical proper-ties. This has a considerable influence on both frictionand wear because they are determined to a great extentby the properties of the surface, where phenomena suchas shear, cracking and asperity ploughing take place22. The chemical reactions on the surfaces are stronglyinfluenced by the high local pressures and the flashtemperatures, which can be over 1000C, at spots whereasperities collide.2.5. Formation of thin microfilms on hard coatingsVery low coe cients of friction (down to 0:1)have been reported for a hard titanium nitride coatingsliding against itself 23 and even lower values (down toabout 0:01 but more typically 0.05) have been mea-sured for diamond-like hard carbon (DLC) coatingssliding against diC128erent counter materials (reviewed byDonnet 8) and diamond coatings sliding against dia-mond and ceramics 13,16,24,25. This can be explainedby the formation of low-shear microfilms on the hardcoating or perhaps only on the asperity tips of the coat-ing. Thus, if we consider the contact on a micro scale,there is eC128ectively a soft coating on a hard substrate,althoughnowthecoating(e.g.diamond)playstheroleofhard substrate and the soft microfilm formed plays therole of a coating, as shown in Fig. 3. It is obviouslyadvantageousifthesubstrateunderthehardcoatingisashard as possible, to avoid fracture of the brittle coatingby deformation, to improve the load support and todecrease the real area of contact. The very low coe -cients of friction of polished diamond and diamond-likecoatings are further explained by the extreme smooth-ness of the surface excluding eC128ects such as interlockingand asperity ploughing as well as of the hard coatingreducing the ploughing component of friction.A transfer layer is soon built up on the counterfacewhen a steel or ceramic surface slides against a dia-mond-like hard carbon coating 26,27. The explanationfor the low shear strength between the two surfaces is,however, the formation of an extremely slippery micro-film between the surfaces. DiC128erent explanations for thestructure of the microfilms have been presented and it isvery di cult to analyse them conclusively 28.Recently Erdemir et al. 29,30, Liu et al. 31 andSchouterden et al. 32 have published convincing evi-dence in the form of Raman spectra of the surfaces andthe wear products which indicate that graphitization istaking place and a low shear strength graphitic micro-film is formed between the surfaces.2.6. Nanophysical contact mechanismsRecently emerging technologies such as the atomicforce microscope and the surface force apparatus 33Fig. 3. A hard coating on a soft substrate inhibits ploughing and asoft microfilm on the hard coating results in decreased friction.790 K. Holmberg et al./Ceramics International 26 (2000) 787795have opened the possibility to study friction and wearphenomena on a molecular scale and to measure fric-tional forces between contacting molecules at the nanoNewton level. Increased computational power has madeitpossibletostudyfrictionandassociatedphenomenabymolecular dynamic simulations of sliding surfaces and toinvestigate the atomic scale contact mechanisms. Thefriction that arises from slippage between solid to solidinterfaces34andbetweencloselypackedfilmsinslidingcontact 35 have been investigated. The atomic scalemechanisms of friction when two hydrogen-terminateddiamond surfaces are in sliding contact have been stu-died and the dependence of the coe cient of friction onload, crystallographic sliding direction and roughnesshave been investigated 36. In another study the atomicscale roughness eC128ect on friction when two diamondsurfaces are placed in sliding contact was examined 37.The increased understanding of the origin of frictionat the atomic scale and even why friction exists hasresulted in an examination of the relationship betweenthe commonly used laws of friction at a macro scale andthe molecular frictional behaviour on a nano scale.There have been suggestions that friction arises fromatomic lattice vibrations occurring when atoms close toone surface are set into motion by the sliding action ofatoms in the opposing surface. Thus, some of themechanical energy needed to slide one surface over theother would be converted to sound energy, which isthen eventually transformed into heat 38. Today weare only at the very beginning of the understanding ofthe nanomechanical tribological contact eC128ects thatexplain the origin of friction and wear and there is nodoubt that in the near future many new theories andexplanations for the origin of tribological phenomenawill become available.The scaling up of the nanophysical explanations ofcontact mechanisms to practically useful conclusions ona macro scale is a most challenging and complex taskand will take many years. Already there are practicalapplications on a nano scale