Langmuir 1996, 12, 4501-4504
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Issues in Practical Tribology P. R. Norton Interface Science Western and the Department of Chemistry, University of Western Ontario, London, Ontario, N6A 5B7, Canada Received January 3, 1996 Introduction and General Comments1,2 The remarks in this essay draw on material presented during the course of the entire workshop and not just the opening session, of which the present author was Chair and Commentator. The complexity and interdisciplinarity of the field of tribology makes this inevitable. The only generalization one might make about tribology is that it is not possible to generalize anything! This would be an unnecessary counsel of despair, since progress can be (and has been) made. The following remarks suggest some fruitful areas for research and reflect the bias of the author as a surface and materials scientist who has recently entered the field of “nanotribology”. Equally inevitably, in the role of Commentator (who were chosen as people broadly based in fields other than tribology), some of the remarks might appear naive or show a lack of awareness of aspects of current knowledge. For this the author begs patience; it seems to him that sometimes progress arises from being able to see the wood instead of the trees and that such a perspective arises when one has not yet entered the forest! Ex-Situ Analysis Singer and Hsu (this issue) covered many of the practical issues in a manner that demonstrated how complex the “real world” really is. They clearly demonstrated that tribological behavior is not a simple property of the materials but rather one of the system as a whole. Given this complexity, should we expect a reductionist approach of physics and chemistry to work? By this I mean, “Can we build up the properties of a system one variable at a time and expect to achieve a basic (even atomic level) understanding?” My own view is that we have to approach tribology research from as many rational directions as possible, including a strictly reductionist approach, but we should pick the system for study rather carefully. Some may be too complicated to make much progress if they show the whole gamut of behavior including the involvement of particles, wear debris, etc. (From a practical viewpoint in tribology, there is presumably always wear, however miniscule, but we might for a start ignore the effect of wear debris). At the two simplest extremes one might therefore imagine the elastohydrodynamic (EHD) limit in which rheology determines everything (one still has to worry about surface roughness and conformity); the other limit is the (imaginary) perfect antiwear film from which either no material is transported or it is transported at such a small rate as to be insignificant. Great progress is being made in the former area with techniques such as the surface forces apparatus (SFA) being extended to studies of surfaces in relative motion. Special mention must be made of the work of Spikes (this issue), and this Commentator regards the progress made in controlling and measuring the properties of films of thickness down to 1 nm, at relative velocities of the order (1) Houston, J. E.; Michalske, T. E. Nature 1992, 356, 266. (2) Warren, O. L.; Houston, J. E.; Michalske, T. E.; Wan, W. K.; Sheasby, J. S.; Norton, P. R. Poster presented at the Workshop on Physical and Chemical Mechanisms in Tribology, held at Bar Harbor, ME, August 27 to September 1, 1995.
S0743-7463(96)00019-4 CCC: $12.00
meters per second and pressures of gigapascals, as outstanding. While artificial, it might be helpful to divide the establishment of a tribological contact into different regimes that occur from the onset of sliding motion and to consider the mechanics, physics, and chemistry that could occur and to try to relate this to real situations and suggest research approaches. Static Contact: No Load. At first sight this situation is of little importance. However the “prehistory” of the system might be of overriding importance, and certainly chemical effects could occur such as corrosion of a system in mechanical and electrical contact. If the system has been shut down from a previous sliding/wearing/lubricating environment, modification of whatever film had developed could be expected during shutdown and thus the start-up properties might well be different until some kind of constant behavior has been established. A real practical example of this can be found in automobiles in the cam shaft-tappet interaction. Antiwear films are essential for controlling wear during start-up. Static Contact: Loaded. In this case the contact region might deform through “creep” during shutdown. Measurements on the mechanical and chemical stability of surfaces and films are very important. This is simply another way of restating the question raised by Hsu and Singer about ex-situ examination of worn surfaces and films: “How do we know that what we study ex situ is representative of the situation as lubricated wear is occurring?” We must work a little harder to determine whether changes occur after cessation of sliding motion. Certainly one can envisage methods of measuring creep or other dimensional changes. Initiation of Motion under Load. Again we have to carefully define our starting surfaces. There are two extreme situations: “virgin” surfaces that have never been slid against one another and those which have reached some reproducible steady state. (I recognize that there is really no such thing as steady state in a tribological system.) If the materials are complex alloys (e.g., steels), then complete microstructural and microchemical characterization is required. This characterization must be available as a function of depth into the solids to at least the expected deformation depth. In addition it is necessary to know the mechanical properties of the interfaces on a length scale at least equivalent to the average grain size. Methods developed by Pethica, Oliver (this issue), and Houston (interfacial force microscope, IFM)1 offer promise. The question is: “If we are going to do ex-situ pathology on complicated systems, shouldn’t we devote the same attention to the starting materials?” As the system is loaded, we must understand how the load is being carried. This translates into knowing the topography of the surfaces. Asperities are believed to involve rather gentle slopes (few degrees) which could be measured with laser interferometric techniques, but what about the registry of topography between the two surfaces before sliding wear? One way of approaching this problem is to actually control the morphology by lithographic methods in which a photoresist could be spun onto a © 1996 American Chemical Society
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surface, patterned, and used as a mask in a chemical etching process. It would be possible to control both the magnitude and relative orientation of surface features on each side of a sliding couple. After wear-in, one might have a template on which to build a detailed picture of wear processes. Also by controlling the regions of the surface at which loads are carried and therefore at which other tribological effects will occur, ex-situ pathologists will know where to look. By examining the development of changes in surface topography, mechanics, and chemistry as a function of sliding motion, correlated with measurements of the efficacy of lubrication and reduction of wear, it might be possible to make progress. Singer and Hsu are all too aware of the difficulties of making such progress; what I’m suggesting (perhaps naively) is that we control the morphology of the surfaces on a submicron scale. Now we initate the sliding motion. At the outset it seems to me that it is essential to carry out even model experiments at “realistic” sliding velocities if one wants to make contact with practical tribology. This is not to say that low-speed experiments are not useful. Far from it, they will contribute to the necessary body of knowledge, but we should not expect close correlation with practical situations. This is because in my view when when one looks at the kinetics and dynamics of the possible physical, mechanical, and chemical processes, the relative importance of each will change with change in sliding velocity. For example at low speeds, tribochemistry might not be as important as at higher speeds (lower “flash temperatures”). (I reserve the word “kinetics” to refer to overall processes occurring on a time scale much slower that atomic motions, although of course atomic motions underly all the processes. “Dynamics” is taken to refer to the fast events such as electron emission, bond breaking, etc.) As the asperities interact with each other (we note here the very nice single collision experiments of Hsu), what will happen at the beginning is conceptually clear. If the vertical forces exceed the mechanical strength of any surface films on (e.g.) a metal surface and break them, then we have to take into account the environment. Here we meet tribochemistry. Although chemistry occurring at lubricated contacts happens in an extreme and complex environment, one should be able to explain the products by kinetic and/or thermodynamic control. In this regard, I think the observations reported by Singer are highly significant: the thermodynamically expected product is normally formed. Provided that this formation cannot be explained by chemistry occurring after the tribological experiment, this indicates “normal” chemistry. In the sliding contact region very high temperatures and pressures can be generated locally and for very brief periods. During these temperature and pressure excursions reactive gases that can gain access to the region because of a breakdown of protecting films will immediately react, forming a stable (oxide, sulfide, etc.) compound. The role of pressure is uncertain. It might simply play the role of concentrating reactants at the “hot-spots”. This would be important for the polymerizations observed by Hsu, perhaps less so for the thermodynamically stable phases observed by Singer. This brings me to perhaps an obvious message, but one that has apparently been regarded either as not important or as too difficult to control: the environment in which a tribological experiment is carried out must be controlled. This is particularly obvious in the case of oxygen and formation of oxides, but the same is true for water vapor in otherwise inert atmospheres. Most surfaces (including oxides) are very reactive toward water and will form oxides with hydroxylated surfaces. Such surfaces could easily form a strong H-bonded network
Norton
between them. At high humidities, water will adsorb on surfaces, the coverage diverging as the humidity approached 100%. Such a layer will convey its own lubricating properties. In my opinion, failure to control the atmosphere in which experiments are carried out will render the results meaningless. Singer’s observations of “thermodynamic” products were made on microscopic wear debris and thin surface films. The significant fact here is that the length scale appropriate to the material is very small: i.e. to make the products, the reactants did not have to diffuse far. Under the mechanical stresses in the contact, the reaction might well be facilitated by the “pestle and mortar” picture of Singer. Under these circumstances, “normal” diffusion might be irrelevant. Given the uncertainties in the temperatures at contact and the relevant diffusivities in a highly defected material, this does not seem an area ripe for theoretical investigation. However, by contrast, experiments utilizing isotopes could be very helpful. For example implantation of 18O into at least one of the substrates, or use of 18O in the gas phase, followed by ex-situ examination of the surfaces and debris would give some information on the chemistry in the contact region and the depth of region involved in the tribochemistry. This in turn might give some feeling as to whether “normal” diffusion is rate controlling. What about the surface film? Clearly the surface chemistry has been modified, but so have the mechanical properties. The hardness of the (e.g.) oxide film will in general be different from that of the substrate on which it grew. The distribution of loads, stresses, and strains will change, and perhaps most importantly, there will be a new interface between the substrate and the film. This interface will probably not be abrupt, as the processes leading to its formation are by their nature chaotic or at least stochastic. At interfaces there are normally regions of nonstoichiometry; these might be very important in controlling the shear strength at the interface. In sliding motion, shear forces are transmitted to the interface and if they exceed the strength, detachment will occur. One postulate arises naturally from this simple picture: the typical size of debris might be expected to scale with the typical thickness of the wear film. Thus careful comparison of debris size and substrate film thickness might be instructive. This loss mechanism is but one of the many outlined by Singer and Hsu (this issue). However it is apparent again that very careful pathology on the debris is important. Questions that occur to me include whether the particles are homogeneous or does their ultrastructure reveal the mechanism of their formation? Is a given particle formed in one event (e.g. junction plucking) or by an accumulation of smaller events? Of course if the debris recycles into the contact (as it must), then it will be modified. Both Singer and Hsu believe such recycling is very important. Evaluation of debris as a function of time would then be crucial to any understanding as to its formation. Staying with the theme of tribochemistry, the possibilities become much more complex if hydrocarbons or other more complex additives are present. In EHD the viscoelastic properties are paramount and the study of how these properties are modified at high pressures and in the presence of viscosity modifiers was the subject of the beautiful work described by Spikes (this issue). We will restrict our discussion to situations in which chemistry can occur. If bare metal is exposed transiently in a sliding contact involving a transition metal, surface science has shown that a hydrocarbon can be cracked (carbon-carbon bonds are broken) to produce reactive carbon species and hydrogen. What happens then will depend upon the
Issues in Practical Tribology
relative rates of surface processes, and we must be guided by the observations. Reactive carbon species will have a short lifetime under the high-pressure conditions in the contact region, and dynamics of molecular motion will be important. Hsu has made important studies which indicate that polymeric materials are formed. Certainly if reactive hydrocarbon species are formed at the metal surfaces, then chain growth can occur. Iron for example, is a good Fischer-Tropsch catalyst. Hsu also observes the formation of organometallic (OM) complexes containing Fe. The question is not whether these processes can occur (they obviously can) but whether they are relevant to lubricated wear. If one draws a lesson from the surprising chemistry of zinc dialkyl dithiophosphates (more on that later), then one should not dismiss the role of OM complexes in controlling further surface chemistry, and this is a fruitful (if complex) area for future work. What about the role of the chemical products? In some cases this is very clear. For example in the use of MoS2 the physical properties of the material itself are crucial (might this be the one example where tribology is a material property?). In the UHV experiments described by Singer, it is clear that it is possible to transform “virgin” surfaces into the appropriate compound by the presence of H2S. These are very promising experiments, and the relative kinetics of the processes can be worked out. Tracer (isotopic) studies will be very useful. An important question to resolve is whether wear is occurring in these very low friction systems. Again it seems possible that by modulating the isotopic composition of the cover gas and then using (e.g.) SIMS to evaluate the composition one could unequivocally answer this question. In this latter case, the ready cleavage of the material, and its ready re-formation by surface chemistry (at least in the Mo + H2S experiments) are obvious reasons for its efficacy. In the absence of materials to resynthesize the antiwear film, the observation that most of the film can be worn away in the first 5 -10% of the sliding life (Singer) and still confer excellent antiwear properties is perhaps an example of the effect of the interphase region in the chemically transformed film. As the interface between the unmodified substrate and the film is approached, the material properties will change; perhaps the material is more resistant to shear forces and it is the recycled material that conveys the lubricating action, the substrate/ thin film providing a suitable template that matches the “pure” MoS2 properties with the substrate. The transformed film might also act to spread the contact load. It would be very interesting to look for continual material transfer between the apparently stable adherent film and debris. Eventually all films fail, and so there must be transfer. The question is rather: “How much and how fast?” Hsu indicated that the polymers formed by tribochemistry might form “rubbery” pads that separate asperities and spread the load. The chemical presence of such material is not of itself a sufficient criterion; it seems to me that a significant amount must be present to provide the appropriate mechanical properties. The mechanical properties of successful antiwear films should be measured at length scales below 1 µm. Houston’s interfacial force microscope has measured unusual mechanical behavior in transformed ZDDP films.2 They appear to be highly deformable with effective Young’s moduli of less than annealed gold. Even after very high loads and indentation depths they appear to recover without any permanent deformation. These are known to be phosphate glasses, but the situation appears more complicated than that. The obvious problem with such films is that they are thin and laterally inhomogeneous.
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This makes other than a microchemical analysis difficult, while what is required is a micro- (even nano-) structural analysis. It will be essential to be able to evaluate the porosity, interconnectedness, etc. of the materials. It seems that modern field emission SEM or TEM is essential, although “local” structural methods such as EXAFS might be helpful. The high-pressure properties of porous materials are known to be modified by inclusion of low molecular weight species, and it will be necessary to measure the contents of the pores. This will be very difficult ex situ, as the species might be volatile. At the root of my conviction that studies of effective films will lead to important discoveries is that nature has designed effective antiwear films that have exactly the properties one wants. A similar philosophy has led to progress in understanding the properties of biological membranes. Antiwear films have invariably been discovered serendipitously. In the case of ZDDP-derived films, ZDDP was originally added as an antioxidant. Characterization techniques have so far used the “standard” armory of surface analysis: XPS, SEM, AES, TEM, etc. Tribologists must use the new generation of methods based on the bright light sources that can chemically speciate films in a way not possible by XPS. X-ray absorption near edge fine structure (XANES) exhibits excellent chemical state sensitivity, and differential depth resolution is possible via different detection methods (e.g. total electron yield or fluorescence yield). In the near future, spectromicroscopy lines will be available with which micron or submicron features can be examined. It is essential to correlate lubrication and wear behavior under realistic conditions with microscopic studies of chemistry and mechanical properties. I believe that correlations made from data from experiments run under widely different sliding conditions are at the least suspect and might be worthless. This section has concentrated on the film. What about the substrate? Obviously its microstructure and composition could have been altered during sliding contact. Outside the chemically transformed region it is likely that the mechanically transformed region plays a key role in the response of the system. Ex-situ measurements of the variation of composition, microstructure, and micromechanical properties with depth are crucial, and interpretation of the changes from the “virgin” material should be very revealing. It might be that what is needed for effective antiwear behavior is a “graded” mechanical behavior with depth from the interface. This might aid in distributing the load from the local asperity contacts over the entire interfacial region. Indeed it might be instructive to attempt to produce materials with graded mechanical properties as test materials. The initial section of the workshop also contained excellent lectures by Gent and De Gennes on the effects of energy dissipation on adhesion. This basically is a dynamical effect related to the characteristic relaxation times for molecular motion which can greatly enhance the adhesion energy at high deformation rates. One can imagine scenarios in which such effects could be important in deadhesion in tribology experiments, but it is by no means certain that they are relevant. In the case of antiwear films that are glassy, the temperature at which the films are effective is not well-known, but relaxation effects could be very important in determining their adhesion energies. It is therefore clear that the viscoelastic properties of films should be measured, again a challenge when the small amounts of poorly characterized materials are taken into account.
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In-Situ Methods It is essential that the community attempt to carry out meaningful in-situ measurements. For progress in practical tribology, I again emphasize that in my view it is essential to use realistic velocities. Spikes and Israelachvili have shown that it is possible to control the separation of sufaces sliding against one another with a precision ,1 nm even with relative velocities of a few meters/second. The properties of the confined high-pressure fluids can be directly measured. The use of a transparent component of the couple is a necessary simplification and for lowwear situations could even be modified by deposition of a metal film. Methods are now available to carry out timeresolved IR spectroscopy on a lateral scale of
