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Forum on New Ideas in Tribology Richard J. Colton* Chemistry Division, Code 6177, Naval Research Laboratory, Washington, D.C. 20375-5342 Received January 3, 1996X The “Forum on New Ideas” was held at the end of the workshop to give participants an opportunity to express new ideas and directions in tribology. In addition, it gave an opportunity to review the highlights of the workshop and to continue discussion on important topics. It is the latter point that occupied most of the session. These pages contain a summary of these discussions and my views on what is new in tribology.
I was asked by the organizers of this workshop, as an outsider in the field of tribology, to serve as moderator for the final session on “Forum for New Ideas”. The commentators for the other sessions were also not tribologists, but experts in related fields of surface science, polymer rheology, mechanical properties, and theory. One must ask why the organizers would choose a group of nontribologists to conduct the workshop. Clearly, the commentators would not have a biased viewpoint and may be able to listen to the presentations and discussions with an open mind. They may also be able to comment on the relevance and importance of various areas. On the other hand, one must also ask why these nonexperts would undertake this assignment. A subsidized vacation to Maine you saysnay!sbut a genuine interest in the field, and an opportunity to gain some insight into the complex mechanism(s) of tribology. The purpose of the concluding session was to give participants an opportunity to express new ideas on tribology. In addition, the final session could be used to review the highlights of the workshop and to continue discussion on important topics. Therefore, each session commentator was asked to review (from their perspective) their session highlights. These presentations and ensuing discussion took more time than anticipated, so unfortunately we never got to the key issue of identifying new ideas and directions in tribology. While my colleagues were enjoying some time-off during the workshop, I found myself locked in my room contemplating the complexities of tribology. To organize my thoughts I came up with an outline which I use here to organize this essay. I. What to do? There were a number of formats considered, e.g., panel discussion, commentator feedback, and organizer summary, but in the end I followed the outline below. The information is assembled from a combination of lecture notes, personal impressions, and notes from various discussions. In most cases, I try to identify the person (names usually given in parentheses) who introduced the topic and whose paper can be found in this volume, but I do not identify individuals who added to the discussion. The technical details can be found in the papers and references published by the authors in this volume. While I cannot take full credit for the ideas presented in this essay, I do offer my perspective of tribology. * E-mail:
[email protected]. Tel: 202-767-0801. Fax: 202-767-3321. X Abstract published in Advance ACS Abstracts, September 1, 1996.
II. Guidance Limited guidance was given to the moderator and participants. They were told that the workshop would conclude with an open discussion of issues raised during the week including opportunities for new research. They were also asked to make a presentation if they thought that an important issue remained unresolved, a viewpoint was not adequately represented, an important topic was not covered, or they have a new idea or approach. Only a few people came forward with new topics (see section IV). Some researchers were also asked in advance to prepare a few viewgraphs to emphasize an important point raised in the poster session or during the discussion. III. Review of Important Issues Tribology is the study of friction and wear of materials.1-6 The word “tribology” is derived from the Greek words “tribos” meaning rubbing and “logos” meaning reason. While the derivation of the word tribology is simple, the study of friction and wear is not. The tribosystem is highly complex, with many variables and phenomena that occur at many solid-liquid-gas interfaces. The complexity of the system requires today’s tribologist to seek multidisciplinary approaches to understand these phenomena or solve tribological problems. The advent of new analytical tools in surface science and nanoscience plus the development of miniature mechanical systems have led to unique approaches at the microscopic and atomic scale. Modern terms such as microtribology and nanotribology, which imply measurements on a different length scale, have been introduced. The methods begin to provide new insight into the fundamental mechanisms of friction and wear that challenge our classical description of these phenomena. To begin the discussion I asked each commentator to review briefly the important issues or highlights from their respective session. The participants were also asked to consider if there had been a serious omission. While the commentators will give a detailed account of the technical (1) Fundamentals of Friction: Macroscopic and Microscopic Processes; Singer, I. L., Pollock, H. M., Eds.; NATO ASI Series, Series E: Applied Science, Vol. 220; Kluwer Academic Publishers: Dordrecht, 1992. (2) Materials Tribology. Nichols, F., Ed. MRS Bull. 1991, 16 (Oct), 30. (3) Fischer, T. E. Research Needs and Opportunities in Friction; Special Publication CRTD, Vol. 28; The American Society of Mechanical Engineers: New York, 1994. (4) Microtribology: Proceedings of the 1st International Workshop, October 12 & 13, 1992, Morioka, Japan; Kaneko, R., Enomoto, Y., Eds. Wear 1993, 169. (5) Nanotribology; Belak, J. F., Ed. MRS Bull. 1993, 18 (May). Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (6) Handbook of Micro/Nano Tribology; Bhushan, B., Ed.; CRC Series: Mechanics and Materials Science; CRC Press: Boca Raton, FL, 1995.
S0743-7463(96)00018-2 This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society
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papers and discussion in their own essays, I will highlight the important issues brought out by the discussion. A. Overview: Issues in Friction and Adhesion from the Practitioner’s Viewpoint [Commentator: Peter Norton, University of Western Ontario]. Professor Norton is a well-known surface scientist. He is schooled in the use of numerous surface analytical tools. Hence his viewpoint comes from a surface science perspective or “surface science opportunities” as he would portray. He divides the issues raised in his session on the fundamental aspects of friction and adhesion into two broad categories: (1) ex situ pathology and (2) surface chemistry and tribochemistry. 1. Ex situ Pathology. Ex situ pathology, as the name implies, involves the analysis of surfaces before and after some tribological operation or experiment. For example, establishing the morphology of a surface prior to a wear experiment is essential in describing the characteristics and behavior of asperities. The analyses are usually conducted in separate instruments that may require the test components to be dissembled prior to analysis. There are numerous examples of using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and other techniques to study wear tracks or particle debris in tribology. These techniques are still evolving such that improved imaging modes of operation and higher spatial resolution are available. Synchrotron or other light sources have enhanced the capabilities of older techniques such as X-ray absorption spectroscopy and Raman spectroscopy/microscopy that are well suited for chemical analysis. In addition to chemical analysis, detailed microstructure and mechanical property evaluation of substrates and wear particle debris are equally important. New proximal probes such as atomic force microscopy (AFM) are able to quantitatively measure the adhesive, frictional, and mechanical properties of surfaces on the nanoscale. 2. Surface Chemistry and Tribochemistry. Surface chemistry also contributes to the complexity of tribological systems. Even in the absence of sliding, surfaces are interacting with the environment or numerous substances that may be present in a formulated lubricant. During sliding, new surface films can be created by various tribochemical mechanisms leading to films that may or may not have beneficial properties. Singer7 and others have described various tribochemical reactions that can occur between solids. They employ a variety of ex situ surface analysis techniques to determine the composition and structure of these films. The results are also compared to thermochemical models that predict the reaction products. However, the phase diagram becomes complex with the addition of three or more components. In order to simplify the problem, model studies of “clean” surfaces under controlled conditions and environments (e.g., O2, H2O, etc.) are needed. For example, water (i.e., humidity) can have a profound effect on surface chemistry and hence the frictional and adhesive properties of many materials. Furthermore, determining the reaction dynamics in complex environments is extremely difficult. Simplifying the environment is helpful. Applying well-known isotopic labeling techniques to these studies would also be helpful. One excellent example of using a controlled environment to study low friction films is an ultrahigh vacuum (UHV) tribometer equipped with in situ AES and XPS [ref 8, Singer’s paper in this volume]. While surface science has contributed significantly to the study of tribology and adhesion, there are still many
fundamental problems to solve. From an engineering perspective, one problem raised in the discussion is the age old questionswhat is the real area of contact in a multiasperity contact and can it be measured experimentally? Electrical conductivity measurements do not work. One simplification is to work with single asperities as can now be done with AFM or other proximal probe instruments. However, are they really relevant to the problem? B. Properties of a Single Asperity and the Interface between Molecular Dynamics and Continuum Mechanics [Commentator: Michael Baskes, Sandia National Laboratories]. While the subtitle appears to be a strange grouping of topics, one common relationship between asperities and atomistic models is that the models (or the number of atoms treated) are becoming large enough to simulate an asperity. Dr. Baskes, a world renown theorist, did an admirable job summarizing several critical issues related to the modeling of tribological processes. One recurring question however pertains to the problem of length scale. Is there a fundamental length scale in tribology? We find that the frictional properties of materials measured on the nanoscale do not correspond to macroscale measurements, i.e., the “coefficient of friction” measured with an AFM does not agree with that measured with a conventional tribometer. To a first approximation this result is not surprising considering the complexity of the tribological process and the expected difficulty in determining appropriate length/time scales. As mentioned earlier, it is important to design simpler experiments so that these differences can be sorted out. In this section, the length and time scale issues associated with both experimental and modeling work are summarized. The schematic diagram (created by Goddard9 and modified by Singer1,10) of the non-overlapping time/length scale relations between theoretical models and experimental approaches (Figure 1) still holds although recent progress shows improvement in the time response of some proximal probe experiments.11
(7) Singer, I. L. Surf. Coatings Technol. 1991, 49, 474. (8) Martin, J. M.; le Mogne, Th. Surf. Coatings Technol. 1991, 49, 427. Donnet, C. Condens. Matter News 1995, 4, 9.
(9) Tribology of Ceramics; National Materials Advisory Board Report 435; National Academy Press: Washington, DC, 1988; p 88. (10) Singer, I. L. J. Vac. Sci. Technol. 1994, A12, 2605.
Figure 1. Time and length scales of present-day models and experiments in tribology (from refs 1 and 10). Labels 1-4 refer to recent experiments that come closest to matching the time/ length scale of various models. Method 1 uses a fast IR detector to measure flash temperatures during high speed sliding.29 Method 2 (Spikes) uses a real time optical technique to investigate the physical behavior of EHL films.30 Method 3 (Krim) uses a quartz crystal microbalance to measure the velocity dependence of solid-solid interfacial friction.31 Method 4 is one of the first STM experiments to show high temporal resolution.32
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1. Single Asperity and Multiasperity Contacts. It is curious that among the more than six single/ multiasperity experiments or models presented at the workshop, no experiments or models were similar enough to compare. All the asperities had different lengths, heights and aspect ratios. Furthermore, as far as I could tell, the “model asperity” in both experiment and simulation did not correspond to the size of asperities considered relevant by the engineering community. Is there a fundamental length scale in tribology? This question was not answered. While the mechanical properties of nanoasperities can be measured, it may be more important to ask if this scale applies since their mechanical properties are expected to differ from bulk properties. 2. Modeling. Many papers have been published on atomistic and continuum modeling of adhesion and tribological phenomena.5 On the atomic scale, molecular dynamics (MD) is becoming a powerful and insightful method for examining the mechanisms of friction. However, the MD methods are limited by the availability of interaction potentials that are accurate, fast computationally, and describe realistic metal-metal, metal-metal oxides, and oxide-organic interactions. Moreover, MD models are only able to simulate short time-scale events (
