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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
M A R T I N
R.
ADAMS
ROBERT L. ADAMCZAK
FREDERICK F. LING
FOCAL POINT FOR M A N Y SCIENCES
Friction and wear cost the chemical industry more than $400 million each year.
Yet, these
phenomena still are not well understood
he chemical industry alone spends more than $400
Tmillion per year to replace mechanical parts which either have failed or may fail because of wear. Add to this the money that other industries spend and that the average man spends on his home and car. The figure amounts to billions of dollars. W f y , then, has not more progress been mad?? Friction and wear are complex phenomena (70) and our knowledge in this area is still extremely limited, because basically it is an amalgamation of several sciences. Any young graduate going into this field is necessarily ill prepared. He must become a Hydraheaded scientist-mechanical, electrical, and chemical engineer, as well as a chemist, ceramist, metallurgist, physicist, and mathematician. Then, as he gains experience in the field, this list of titles must grow. Despite this diversity, most of the literature has been provided by chemists and mechanical and chemical engineers because of the assumption that these disciplines have more to contribute. Consequently, no attempt has been made to entice other scientists to participate. Another factor, more difficult to define, is involved. For years, the term lubrication has had an unpleasant psychological connotation for engineers. Chemical plant lubrication engineer or grease monkey sounds decidedly unprofessional. To remedy this, perhaps our thinking should be reorientated. Perhaps we should think in terms of “phenomena occurring at a sliding surface,” and thus move ourselves immediately into a
scientific world. The study of interfacial phenomena is necessarily highly specialized. For example, in decomposition of grease at elevated temperatures, heat generated at the moving interface contributes significantly to degradation. Unquestionably this change in composition is a chemical process. Yet, little is known about the reaction mechanism, kinetics, or the quantitative influence of interfacial temperatures. In 1948, Shaw found that organometallics can be prepared continuously by cutting a reactive metal in the presence of suitable liquid or gas reactants ( 7 7). These reactions were much faster than those where the samf constituents are heated in a glass apparatus. Shaw attributed this to the high local temperature and pressure, and the stressed metal at the face of the cutting tool. However, the fast rates could not be attributed solely to exposing clean, freshly cut surfaces continuously. Shaw, a chemical engineer himself, urged other chemists and chemical engineers to explore this process in more detail. The interlocking aspects of stress corrosion and chemical reaction need further exploration. I n 1957, a serious problem developed when landing gears on large aircraft, parked on runways in the northern latitudes, began to buckle under static loading. Later investigation showed that reaction of salt, used to melt snow on the runways, with the highly stressed metal surfaces, contributed to the failures. Can an analogy be drawn here, between reactions occurring on a mechanistically stressed surface, and reactions catalyzed or otherwise supported by a surface which has been stressed during sliding? Even more than most sciences, lubrication technology has been forced into strange environments. For example, in terms of temperature, it has been assumed traditionally that operational limits were about -65’ to some 500’ F. Now, however, even if only near future requirements are considered, these limits have moved down to near absolute zero and up to more than 3000’ F. Obviously, no single science or even several sciences can deal with all the problems encountered within these VOL. 5 4
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Controversial: Do flash temperatures occur at interfaces?
temperature ranges. Only by concerted and well organized interdisciplinary efforts can a satisfactory degree of perspective be obtained. Such approaches are being used, and the need for them is constantly increasing. For solid films capable of withstanding high temperatures, ceramic binders have been used. Does this area belong to ceramics? What has been done to find new applications or even develop new configurations usable with solid film or powder lubricants? In the past this has been done the other way around-i.e., hydraulic fluids have been developed for a given system. Today, pumps and fluids often are developed simultaneously. Problem Areas
D r y Friction. Some types of bearings are designed to operate without a lubricant-e.g., in certain space applications involving oscillating temperatures and zero gravity fields. o w € ! should the bearings be designed and which special features should they contain? Can seizing be prevented? Will the design differ markedly from the conventional bearing which incorporates a grease or fluid lubricant? Which materials of construction will be used? In the domain of friction and wear, interfacial temperature, its prediction and measurement, is highly controversial. Some investigators contend that temperature “flashes” occur in the surface asperities, during which instantaneous point temperatures surge several hundred degrees. Others contend that such flashes do not occur (5). Is this flash temperature responsible for the effectiveness of extreme pressure additives or degradation of the lubricant itself? O r is the bulk, mean interfacial temperature responsible? Perhaps other phenomena operate as well. Little is known about the energy state of a metal surface in sliding contact. Certainly the surface energy is increased. Does an ordinary metal surface become catalytic when it is rubbed? If so, which types of reactions does it promote and what are their kinetics and mechanisms? How does interfacial temperature influence this excited state and which reactions occur? Recent work indicates that temperature at an interface is discontinuous-i.c., it is higher at the interface of a stationary surface than at the interface of a moving, tan-
AUTHORS Ma r t i n R. Adams aad Robert L. Adamczak are members of the Air Force Materials Central, Research and Technology Division, Wright-Patterson Air Force Base, Ohia. Frederick F. Ling is an dssociate Professor of Mechanical Engineering at Rensselaer Polytechnic Institute, Troy, N . Y . 42
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
gential ring-type specimen (7). In this sense, the interface experiences an ill-defined temperature zone instead of asimple point value. How does the thermal condition at the interface influence physicochemical changes in metal substructure? Interfacial phenomena definitely involve solid state physics, mechanical engineering, and metallurgy. Physics and mechanical engineering are basic in predicting and measuring temperature. Initially at least, the excited interface involves application of physics and metallurgy to the solid state. Changes or alterations in substructure are of interest to the metallurgist. Chemical Synthesis. In practically all commercially available lubricants, chemical synthesis and characterization are involved. What should be the molecular structure of a good base fluid for lubricant preparation, and how can it be synthesized? After the fluid has been synthesized, what can be done with it? Which base fluids are best for formulation? For example, polyphenyl ethers are oxidatively stable at temperatures to about 500’ F.; however, their usefulness as lubricants is limited by pour point or poor performance at low temperatures. Therefore, the problem arose of finding fluids with better pour points, and to do this, compounds with two and three nitrogen atoms in the ring (pyrazine and triazine) were synthesized. Pyrazine was selected because of prior experience, and triazine was selected because the reactions are simple tc3 conduct. Neither compound was chosen on the basis of clear-cut correlations between structure and properties. Successful synthesis paves the way for additional research, development, formulation, and evaluation. For example, incorporation of a new fluid into a grease formulation involves colloid chemistry. Furthermore, commercial lubricants are composed of many different ingredients which control viscosity, improve oxidative stability, and provide for extreme pressure action. The base fluid or grease must function as a carrier for all these additives and still have desirable thermodynamic and rheological properties and lubricity in itself. Basically, the synthesis and formulation problem belongs to organic and inorganic chemists, and evaluating performance belongs to other technologists. Metallurgists are needed to study corrosion and lubricant compatibility with pipes, pumps, seals, bearing, and containers. Chemically Treated Surfaces. In this technique, chemical reactions between selected gaseous or liquid environments and metal bearing surfaces produce a protective film which reduces friction and wear. The lubricating film rnay be deposited either prior to use or continuously in situ. The reaction, sustained by heat generated during rolling or sliding, deposits the protective film where it is needed. Extreme pressure additives operate in this manner. Most in situ reactions
produce compounds that involve the metal surface chemically (4),but recently, formation of a n independent MoS2 on a steel bearing surface was demonstrated ( 9 ) . Several preformed surface films have been prepared by careful reaction of various metals and gases at moderately high temperatures (3)-e.g., a MOSSfilm by reaction of a molybdenum specimen with HsS, which seemed to have excellent lubricating properties at temperatures to 800’ C. Other types of preformed surface films in various bearing geometries and under wide ranges of load, speed, temperature, and vacuum are needed. Metallurgists are interested in the reaction itself and properties of the resulting film. Statistical analysis, particularly the design of experiments and the analysis of variance can be a valuable tool in interpreting lubrication, friction, and wear data ( 7 ) . Behavior of bodies in sliding or rolling contact frequently is erratic and data are difficult to reproduce with reliability. A chemical engineer, with his mathematical background and knowledge of surface-film testing techniques, and a statistician are an excellent combination for focusing on development of chemically-treated lubricating surfaces and their evaluation from minimum data. Bonded Dry-Film Lubricants for Extreme VacuumThermal Environments. This is one of several techniques developed for extreme environments such as those encountered in aerospace where temperatures are cryogenic to 3000” F. and pressures are very low (less than 10-9 mm. of mercury). The technique can be illustrated by a simple example-all of us are familiar with use of graphite for lubricating automobile fan belts and generators. However, the problem of keeping it where we want it arises, and for more sevsre applications, bonding techniques are needed. A solid lubricant may be painted on a surface, using a material such as a ceramic or a phenolic-resin binder. What guide lines are available for developing a high temperature dry lubricant for vacuum operation? Which factors must be considered? For certain types of films, free energy relationships have been used (8). If a ceramic binder is selected, many ceramic materials are available to choose from. Which properties do we look for and what criteria should be used? The ceramic should have some innate lubricity and not be abrasive. The coefficient of expansion should approximate that of the substrate material. Adhesion to the metal surface must be acceptable. T h e binder should not be considered an entity in itself, because it must be physically and chemically compatible with the lubricant which is yet to be selected. Classic examples of ceramic binding materials are SiOz, BZ03, and high temperature borate glasses. Next, the lubricant is selected. Many solid materials besides graphite are known to lubricate (2). T h e lubricant must have a low inherent coefficient of friction plus relatively high melting and decomposition temperatures. Some promising materials are the alkali earth halides (CaF2 and MgF2), metal sulfides and oxides (WSp, PbS, PbO), and other compounds such as nitrides and intermetallics.
DISCIPLINES CONTRIBUTING T O T H E LITERATURE ON LUBRICATION, FRICTION, AND WEARDiscipline ._ Articles, Lhem. Lub. Mech. Aero. No. Phys. Chem. Eng. Eng. Eng. Eng. ~
Wearb 84 7 Journal of A@plied Phys. 2 6 6 Lubrication 66 Engineering ASME Trans138 actions Industrial and Engineering Chemistry 23 J . Chem. Eng. Data Ser. 13
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Five journals were covered for the period, 1957-61. Only those articles were included which were considered to have a direct bearing on lubrication, friction, and wear. Only senior authors were listed; however in some znstances it was dzficult to define the diciplines of some foreign authors, based on academic titles. Not included in this tabulation is a civil and an automotive engineer who could well be a chemical or mechanical engineer. Also, the lub. cng. could be anyone of several disciplines. Foreign bimonthly. First volume published June 1.957. 0
I n studying the performance of any lubricant-binder combination, a reasonable simulation of environment is necessary. Characteristics of interest include coefficient of friction, sliding or rolling speed, temperature, load, wear rate, and vacuum; however, they cannot be measured accurately by simple means. Thus sophisticated electromechanical instrumentation becomes necessary, such as strain gage dynamometers, transducers, thermistors, vacuum gages, and ultrasensitive control and recording devices. The ideal man to work on this type of problem in dryfilm lubricants is a ceramacist or ceramics engineer with strong knowledge in electronics. How many men with such specialized knowledge coupled with an interest in lubrication can be found? SUGGESTED READING
(1) Adams, M . R., Lum, M . D., ASME Spring Lubrication Symposium, 62-Lubs-6, Miami, Fla., 1962. (2) Amateau, M. F., Nicholson, D. W., Glaescr, W. A., U. S. Dept. Commerce, Office Tech. Services, PB 171625, May 1 2 , 1961. (3) Baldwin, D. J., Rowe, G . W., ASLE-ASME Lubrication Symposium, Miami, Fla., 1960. (4) Bowden, F. P., Tabor, D., “Friction and Lubrication of Solids,’’ Oxford University Press, New York, 1954. (5) Chem. Week (June 10,1961). (6) Kingsbury, E. P., Reichenbach, G. S., ASME Spring Lubrication Symposium, ASME 62-Lubs 18, Miami, Fla., 1962. (7) Ling, F. F., Ng, C. W., Proc. 4th Nat. Congr. Appl. Mech., 1962. (8) Orcutt, F. K., Krause, H. H., Allen, C. M., ASME Spring Lubrication Conf., Miami, Fla., 1961. (9) Perilstein, W. L., ASD Tech. Rept. 61-87. (10) Riley, M. W., Mat. in Design Eng. (June 1961). (11) Shaw, M. C., J. Appl. Mech. 15, 1, 37 (1948). (12) Sprowls, D. O., Brown, R. H., Metal Progr. 81,4 (1962). VOL. 5 4
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