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Essays Fundamental Mechanisms of Friction and Lubrication of Materials† Stephen M. Hsu National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received October 17, 1995. In Final Form: March 5, 1996X The fundamental mechanisms of friction and lubrication are reviewed from an engineering perspective. The review integrates information from mechanical engineering, chemistry, and contact mechanics research results to provide a universal view of the area of tribology at this time. This view is to be contrasted by subsequent views from the molecular perspective.
Introduction Friction can be defined as the net resistant force for one surface to move against another. The coefficient of friction is defined by the ratio of the tangential force over the normal force acting on the surface. The frictional force depends on whether the motion starts from rest or the motion is already underway. Static friction refers to the force necessary to start the motion. Steady state friction refers to the force necessary to sustain the motion. This essay will focus on the steady state friction and will define friction as such. Friction is influenced by the interfacial layer that exists between the two surfaces. Therefore to understand the nature and mechanisms of friction, one needs to understand the basic contact phenomena, deformation, wear, and the physical and chemical interactions between the two surfaces. Lubricants, atmosphere, and wear debris at the interface also affect friction. This essay describes the various types of frictional processes, identifies the factors that influence friction, and assesses the current understanding. The influence of lubricants and material deformation processes and their interactions on friction are discussed. From these discussions, future research needs in this area are proposed. Surface Compositions and Contact Conditions For iron-based alloys, the surface is usually covered with the oxides, FeO, Fe3O4, and Fe2O3. The subsurface layer is often a deformed or case-hardened layer which is a result of the machining and polishing actions or heat treatments the material has received during the manufacturing process. The microstructure of this layer generally is a microcrystalline phase dispersed in an amorphous iron phase. Specific compositions depend on the particular alloying elements being present. The surface has a microscopic roughness which is random in nature. Each microscopic hill is termed an asperity. Under concentrated contact, these asperities are deformed either elastically or plastically to form the interface. Therefore, the initial interface depends on the relative hardness and the relative roughness of the two surfaces in contact. If the two surfaces conform perfectly † Presented at the Workshop on Physical and Chemical Mechanisms in Tribology, held at Bar Harbor, ME, August 27 to September 1, 1995. X Abstract published in Advance ACS Abstracts, June 1, 1996.
to each other and all the asperities are deformed 100%, then the real contact area is equal to the apparent contact area. Of course, this is not the case. For most of the engineering surfaces, the surface has been machined and polished extensively, so the microasperities take on a random spherical shape. For highly loaded, steel-bearing surfaces under sliding conditions, the real area of contact may be only 15-25% of the apparent area of contact3 depending on other parameters such as the relative surface roughness and the relative surface hardness. If the normal force acting on the surface is very high, then some plastic deformation will also occur. Greenwood and Williamson and others have studied this topic extensively.1,2 In considering the asperity contact, the issue of scale needs to be discussed. On each asperity, there are subasperities which are smaller in scale. On each subasperity, there are sub-subasperities, and so on. At what scale should the friction of two surfaces be considered? For most engineering applications, the critical scale for friction is at the micrometer scale. The sub-subasperities need not be considered. Can one describe fully the asperity deformation process from the deformation of subasperities, sub-subasperities... down to the molecular or atomic level? The asperity deformation process is a complex system containing many stress domains, defects, and various dislocation zones. Within each zone, there are many energy levels existing at the molecular level. It is not clear how atomic simulations can reach the proper scale factor to simulate what is happening at the interface. But if a constitutive relationship can be developed from the molecular dynamic models, it may help to explain phenomena difficult to observe experimentally. Frictional Processes under Sliding Conditions Once the surfaces begin to move, the contact phenomena are controlled by asperity interactions and the interface layer, whether the interface layer is a liquid lubricant or soft oxide layer. At very light loads, the friction is controlled by the surface forces, which include van der Waals force, hydration force, and electrostatic or doublelayer forces (depending on the materials), by the elastic (1) Greenwood, J. A.; Williamson, J. B. P. Contact of Nominally flat rough surfaces. Proc. R. Soc. London, 1966, A295, 300-319. (2) McCool, J. I. Comparison of models for the contact of rough surfaces. Wear 1986, 107, 37-60. (3) Wang, F. X.; Lacey, P.; Gates, R. S.; Hsu, S. M. Study of the Relative Surface Conformity Between Two Surfaces in Sliding Contact. J. Tribol. 1991, 113, 755-761.
S0743-7463(95)00885-7 This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society
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contacts of the asperities, and the viscous drag if a lubricating oil film is present. The elastic contacts will sometimes produce chatter or stick slip phenomena as observed by frictional force traces. When the load is increased, the asperity contacts will result in plastic deformation of the asperities. This changes the surface features of the interface. Often, very small wear particles are generated.22 Friction in this regime is then controlled additionally by the energy necessary to deform the asperities. The asperity-asperity contacts lead to deformation of the asperities, and this process provides the majority of the frictional resistance.22 As the asperities deform, the surface roughness decreases if adhesion is absent. A further increase in load causes additional factors that contribute to the friction. These are adhesion, plowing, and the presence of the wear particles (third-bodies effect). If the deformation is large, nascent metal surfaces will come into direct contact under relative motion. The bonding energy at the interface is often stronger than the original strength of the material. As the asperity continues to move, the softer material will undergo subsurface fracture, which contributes to the frictional energy. At the same time, the asperity experiences very high localized temperatures, generally referred to as the flash temperature, a short duration, high intensity temperature burst.23 Under the temper(4) Hsu, S. M.; Shen, M. C.; Ying, T. N.; Wang, Y. S.; Lee, S. W. Tribology of Silicon-Based Ceramics. Ceram. Trans. 1994, 42, 1892054. (5) Frost, H. J.; Ashby, M. F. Deformation-Mechanism Maps: The Plasticity and Creep of Metals and Ceramics; Pergamon Press: New York, 1982. (6) Ying, T. N.; Hsu, S. M. Asperity-Asperity contact mechanisms simulated by a Two-ball Collision Apparatus. Wear 1993, 169, 33-41. (7) Hsu, S. M. Boundary Lubrication of Advanced Materials. MRS Bull. 1991, 16 (10), 54-58. (8) Morecroft, D. W. Reactions of Octadecane and Decoic Acid with Clean Iron Surfaces. Wear 1971, 18, 333. (9) Mori, S.; Imaizumi, Y. Adsorption of Model Compounds of lubricant on Nascent Surfaces of mild and stainless Steels under dynamic conditions. STLE Trans. 1988, 31 (4), 449. (10) Hsu, S. M.; Klaus, E. E. Some Chemical Effects in Boundary Lubrication: Base oil-metal interaction. ASLE Trans. 1979, 22 (2), 135. (11) Hsu, S. M.; Klaus, E. E. Estimation of Molecular Junction Temperatures in a Four-ball Wear Contact by Chemical Reaction Rate Studies. ASLE Trans. 1978, 21, 3, 201. (12) Gates, R. S.; Jewett, K. L.; Hsu, S. M. A Study on the Nature of Boundary Lubricating Film: Analytical Methods Development. Tribology Trans. 1989, 32 (4), 423. (13) Hsu, S. M.; Klaus, E. E.; Cheng, H. S. A Mechano-Chemical Descriptive Model for Wear under Mixed Lubrication Conditions. Wear 1988, 128, 307. (14) Hsu, S. M.; Shen, M. C.; Klaus, E. E.; Cheng, H. S.; Lacey, P. I. Mechano-Chemical Model: Reaction Temperatures in a concentrated Contact. Wear 1994, 175, 209. (15) Gates, R. S.; Hsu, S. M. Silicon Nitride Boundary Lubrication: Lubrication Mechanism of Alcohols. Tribol. Trans. 1995, 38 (3), 645653. (16) Simoi, C.; Hrianca, I.; Cracium, P. Exoemission of electrons without photostimulation. Phys. Status Solidi 1968, 29, 761. (17) Ramsey, J. A. The Emission of electrons from Aluminium abraded in atmosphere of air, oxygen, nitrogen, and water vapour. Surf. Sci. 1967, 8, 313. (18) Rosenblum, B.; Braunlich, P.; Himmel, L. Spontaneous emission of Charged particles and Photons during Tensile Deformation of Oxidecovered metals under ultrahigh vacuum conditions. J. Appl. Phys. 1977, 48, 5262. (19) Nakayama, K.; Hashimoto, H.; Triboemission from various materials in atmosphere. Wear 1991, 147, 335. (20) Lenahan, P. M.; Curry, S. E.; First Observation of the 29Si hyperfine spectra of silicon dangling bond centers in silicon nitride. Appl. Phys. Lett. 1990, 56, 157. (21) Dickenson, J. T.; Langford, S. C.; Jensen, L. C. Recombination on fractal networks: Photon and electron emission following fracture of materials. J. Mater. Res. 1993, 8, 2921. (22) Ying, T. N. Wear mechanisms for ductile and brittle materials in a microcontact. Ph.D. Thesis, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD, 1994. (23) Hsu, S. M. The effects of chemical reactions in boundary lubrication. Ph.D. Thesis, Department of Chemical Engineering, Pennsylvania State University, University Park, PA, 1976.
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ature and pressure, many materials will undergo phase transformations shifting into a harder phase through the martensitic phase transformation. The harder asperity will plow into the softer opposing surface creating a groove.22 The energy necessary to produce the groove contributes to the increase of friction. Another factor is the effect of the wear particles at the interface. As the groove is being created, wear particles are produced. Now the contact stresses are much more complicated as the wear particles themselves begin to plow and deform the surfaces. Eventually, the contact situation becomes asperities riding on the opposite surface. Here the concept of surface conformity is important. In lubrication, the surface roughness is an important indicator as well as a significant parameter in the calculation of oil film thickness. Hsu3 suggested that in a lubricated case, even though the surfaces may be rough, if they conform to each other under plastic yielding, then the relative surface roughness may be quite small. Oil film calculations based on elastohydrodynamics may yield a significantly smaller film thickness than actually present. Conformity has been demonstrated to change with time, materials, additive chemistries, and wear modes. The stress distribution inside a clean contact without debris can be described by continuum mechanics. The response of the materials to different stress cycles has been mapped by some researchers.4,5 Many models exist today to describe friction and wear. Unfortunately, these models are basically correlational in nature and lack predictive power. Formulation of stress calculations to take into account the third-body effects is lacking. This represents a significant roadblock to a priori modeling. Lubrication Single asperity collision experiments have been performed under carefully controlled conditions.6 Results from dry sliding, paraffin oil, and antiwear additives treated oil-lubricated cases suggest that the overall sequence of events and failure modes are the same in all cases, but the time it takes to reach the failure point is drastically different. This difference in part can be explained by the mechanical properties of the thin lubricating films formed under boundary lubrication conditions. Adhesion to the surface, cohesion of the film, and the reaction rate to generate the film are the most critical parameters.7 The nature of the chemical reactions and the origin of the tribochemistry hold the key to understand lubrication. Tribochemistry refers to the chemistry that occurs when two surfaces rub against one another. There are reactions that occur only under rubbing conditions and reactions that would occur independently under the temperatures and pressures existing in the contact. The reactions that take place only during rubbing will be defined as the tribochemistry for the purpose of this discussion. These reactions usually involve direct chemical interactions with the surface. The reactions that would occur independently will be defined as the contact reactions or the contact chemistry. These reactions usually involve oxidation and thermal degradation reactions. The two sets of reactions are often intricately intertwined, and one set of reactions leads to the other set of reactions. Yet as far as the fundamental process of lubrication is concerned, we need to focus on the chemistry that lubricate (i.e., tribochemistry). The lack of this distinction over the years has caused confusion in the literature. In the past, researchers have studied the nature of “friction polymers” to surface reactions with water and oxygen. In 1971, Morecroft8 studied the effect of nascent
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iron surface on surface reactions with simple hydrocarbons and found that a newly deposited iron surface decomposed the hydrocarbons. More recently, Mori in Japan9 studied the reactions between vibrating particles of metal and the adsorption characteristics of gases passing through the column. He found that when the particles were ultrasonically energized and rubbed against each other, more organic gases were adsorbed and decomposition of the hydrocarbon took place. To some extent, this result confirmed Morecroft’s observation of decomposition. From these studies, one might conclude that the nascent surfaces produced during rubbing would decompose the hydrocarbon lubricant molecules and the decomposition reaction would be the dominant reaction. Hsu and Klaus10,11 first identified the presence of oilsoluble organoiron species in the lubricant after wear, and using gel permeation chromatography, they also identified the presence of high molecular weight species. Hsu later used a coupled analytical technique, gel permeation chromatography and graphite furnace atomic absorption spectroscopy, to positively identify the high molecular weight species as organometallic polymers of variable molecular weights (1000-100000).12 Subsequent studies further suggested that the “polymers” were formed as a result of the reactions between the carboxylic acids from the lubricant oxidation reactions and the iron surface (covered with iron oxides).13 The polymerization reactions were primarily condensation reactions. When the molecular weight of the polymers reached 100 000, the reaction products became insoluble in oil. These high molecular weight polymers then deposited themselves on the worn surface. This was the brownish “sludge” commonly observed near the exit of the wearing contacts. Careful analysis of this organiciron on worn surfaces as a function of time suggested that the molecular weight increase was counterbalanced by the dynamic shear process from rubbing. The polymerization process was influenced by the asperity temperature and the adsorbed species on the rubbing surface. Depending on the molecular structure of the adsorbed species, different molecular structures had different kinetic constants for the polymerization reaction. This might explain why some structures were effective and some were not. The average molecular weight observed in the contact when the wear was moderated by the lubrication process was about 3000. When no high molecular weight products were observed, lubrication was generally not effective. The link between effective lubrication and the polymerization reaction was significant. The decomposition reactions from nascent iron surfaces were interesting, but the linkage to lubrication had not been demonstrated. So there was a fundamental difference between the tribochemistry and the basic chemistry occurring between the hydrocarbon and the pure iron surface freshly deposited in a vacuum. When antiwear additives such as zinc dialkyl dithiophosphate and tricresyl phosphate were added to base oils, inorganic species were observed on the surface in addition to the organometallic compounds, which were formed between the iron surface and the base oil. When the same additives were used without base oils, lubrication was not effective. This suggested that the organometallic compounds were essential to lubrication and the role of some antiwear additives was to form a much tougher film by reinforcing the organometallic film. Therefore, the combination was effective to achieve lubrication. Once these organometallic compounds had been identified, researchers attempted to simulate these reactions under controlled conditions without rubbing. In tribochemistry research, the biggest barrier was the need to
Hsu
analyze a microgram quantity of a very complex mixture of organic and inorganic products. Isolating the key reaction pathway and identifing the products were very difficult. So if the simulation was successful, it would allow more detailed analysis for understanding the reaction mechanisms. This attempt met with partial success. Thermally induced reactions on an iron surface (oxide covered) were able to generate analytical patterns similar to those observed for the tribochemical reaction products. The issue was that the temperature necessary to generate these products appeared to be too high (150 °C higher than predicted).14 When the same procedure was repeated for ceramics, thermally induced reactions produced distinct different reaction products than those from the rubbing contacts.15 In all of these attempts, there were many interesting reactions identified, but the rubbing surfaces appeared to produce only a certain kind of reaction that led to effective film formation and lubrication. What was in the rubbing that could induce different chemical reactions? The surface state, the nascent surface, the pressure (several GPa), the asperity temperatures, and the strained state of the surface were all possible answers. Today, the origin of the tribochemistry remains unsolved. Simoi16 suggested that the rubbing of metal surfaces produced highly localized electron clouds, referred to as the “exoelectrons”. The presence of these electrons had also been detected by others.17,18 How this electron cloud could induce reactions that otherwise would not occur remained to be resolved. Lenahan19 reported the measurement of dangling bonds on silicon nitride. This suggested that rubbing might produce dangling bonds on crystalline surfaces, and these dangling bonds were highly energetic and reactive for silicon-based materials. Nagayama20 detected the emissions of charged particles, electrons, and photons from scratched silicon-based ceramics. Dickenson observed the emission of particles and electrons during the fracture of crystalline surfaces.21 In repeated single-pass experiments, Ying observed the surface to be highly strained and ordered for various metals.22 Strain-induced fracture and deformation appeared to be the dominant mechanism under lubricated conditions as far as materials were concerned. Could this mechanism also cause the surface energy to change? Would this highly strained state upon fracture produce electrons and charged particles? Research Needs On the basis of these discussions, many gaps of knowledge emerge. Friction is a key issue; so is wear. To control friction and wear in engineering applications, we need to understand the fundamental mechanisms of friction and wear. From this understanding, how lubrication can be used to alter the processes will offer opportunities for technological advances. At the same time, the tribochemistry, the nature of the chemical reactions, the film formation mechanisms, and the strength and durability of the films become additional issues. The following sections describe the research needs in more details. (A) The Evolution of Surface Roughness and Surface Features During Sliding. For a given system, how will the surface chemistry and lubrication affect the surface roughness during contact? Can we begin to link molecular structures to film properties and surface roughness changes? Lubrication by design will require the critical linkage be established. (B) Definition of Surface Reaction Sites. Many instances have been observed indicating that the chemistry occurring under rubbing conditions is very different
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from that under static conditions. What changes the surface to produce different chemical reaction paths? Can we determine and quantitatively measure the surface active sites to be either exoelectrons or dangling bonds? In fact, can we determine the specific influence of exoelectrons or dangling bonds on specific model compound reactions? (C) Formation of Lubricating Films. What constitutes an effective film? Under what conditions (i.e., reactions and starting materials) will an effective film be formed? Under what conditions will the film not form? What are the effects of nascent surfaces? Can we determine the kinetic rates of film formation? (D) Properties of the Lubricating Films. Can we measure accurately the elastic modules, shear strength, adhesive and cohesive strength, and the tendencies to form ordered structures of molecules that form such a film? How does the film break down? What are the basic processes and mechanisms of film degradation? What are the molecular structural effects on film formation in terms of functional groups, reactivity, chain length, crosslinking tendencies, self-assembling tendencies?
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(E) Film Formation Modeling. Can molecular dynamic calculations be used to simulate the effect of the surface energy, active sites, defect sites, unpaired electrons, dangling bonds on reactivity and film formation? Implications If we can understand the origins of the chemical reactions that occur under lubricated conditions, we will be in a position, for the first time, to understand the basic steps of lubrication. The understanding of how surfaces react with lubricants and the ability to control the lubrication process will afford much better system design practices. The information will also allow intelligent monitoring of device durability under service conditions. Many new materials are currently being explored for various components, yet the lubrication requirements and the state of the surfaces are not clearly understood. Having the basic knowledge will significantly improve introduction rate of the new materials as well as system performance and efficiency. LA9508856
