TIE UNIVERSTIY OF MICHIGAN INDUSTRY PROGRAM OF THE COLLEGE OF ENGINEERING THE INFLUENCE OF SURFACE ON THE -PROPERTIES OF MATERIALS Mo Jo Sinnott September i-960 IP-465

LIST OF FIGURES Figure Page 1 Identical Brass Tensile Specimens. The left sample was tested in air, the right in mercurous chloride solution............... o o..o 3 2 The Wear Rate of Graphite as a Function of the Water Vapor Content of the T st Environmento (After Savage)............e...oo.......... 4 3 The Variation in Elastic Modulus of Copper as a Function of the Grain-Boundary to Volume Ratio for Three Sizes of Test Specimens. (Floreen and Hucke)........................... o 8 4 The Variation in Elastic Modulus of Copper as a Function of the Surface Area to Volume Ratio for Specimens of Three Different Grain Sizes. (Floreen and Hucke).........o...........o.....o 9 5 The Variation in Elastic Modulus of Copper as a Function of the Grain-Boundary to Volume Ratio for Three Sizes of Test Specimens. (Floreen and Hucke)........... o o......................... 10 6 Changes in the Surface Concentration of the Elements in Stainless Steels as a Function of Their Prior Treatment. (After Rhodin)....o. 19 7 Changes in the Surface Concentration of the Elements in Stainless Steels as a Function of Their Prior Treatmento (After Rhodin)..,.... 20 8 The Composition of the Gases Desorbed from Nickel Powders as a Function of the Temperature of Desorptiono The upper curves are for steam scattered nickel into a nitrogen atmosphere; the lower curves are for steam scattered nickel into an air atmosphere..o........o...oeo 22 ii

THE INFLUENCE OF SURFACE ON THE PROPERTIES OF MATERIALS While the contributions of solid state science to the understanding of the behavior of materials are many and varied. there are some problems which are not susceptible to solution from solnid state principles; these are regions which are specifically excluded from their analyses, the surfaces. These surfaces can be of two types; interior, the grain boundaries, or exterior, the free surfaces. These surface A.toms are in. an unbalanced force state relative to the atoms in the solid and their coordination is on the average much less As a result these surfaces have a marked affinity for environmental atoms or for one another giving rise to what is normnally termed surface t[ension, surface activity, surface energy, etc. These surfaces can and will influence all the properties of the materials we use as engineers In many cases the influence will be slight while in others it may be very great, To illustrate the importance of internal surfaces we might look at the determination of the mechanical, properties of a brass. A standard test specimen is made up and this fixes the amount of external surface, We are aware that grain size is a variable and this is controlled. to a given range to fix this variable and the mechanical. properties are determined. We may change the grain size and determine the resulting change in properties and. we do to this extent recognize the influence of internal surfaces although it is intuitively believed, that what we are measuring are bulk properties and, the surfaces play a subordinate roleo If now we repeat the tests but this time merely ch.arngS-1 -

-2 -the environment and in place of air substitute a rnercurou.s chloride solution we obtain qui te markedly different resu.ltso The yield and tensi.le strengths are markedly lowered as well. as the duictil.ity as measured by percent elongation and reduction in area In this environment the bulk properties play a subordinate role and the grain boundary surfaces and-. free surfaces are much more important;o Figure I is an illustration of how different identical brass test specimens appear after testing in air and i.n a mercurous nitrate environmento Admittedly this is a severe change i.n environment and. as a matter of fact is a stand.ar. ASTM test to determi.ne the presence of residual stresses i.n copper and copper-base alloys b-ut it does il.l.ustrate very graphically the importance of internal surfaces. An, example of where free surfaces and their adsorbed fi lms produce a desired property is to be found in graphite. Most of us are aware of the lubricating properties of this material and at;temptis are sometimes made to relate this property to the layer-lattice structure and t'he disparity in strength between layers and. across layers, The work of Savage is quite definitive in. proving that this is not the case, He has shown that it is the presence of adsorbed films of water vapor or oxygen that produces the low coefficients of friction as shown in Figure 2, Granted then. that surfaces play an important part in. determining the properties of materials9 what is the status of research in this field? To be perfectly candid one must admit that the entire pi.cture of research on surface is quite confusingo Mathematicians, Physicists, Chemists, Ceramists and Engineers5 to mention just a few) are

-53 -~:: Figure 1. Influence of Surfaces on Properties of Materials~

-4-.08 w.06 u w.04 rrw.02 I 0. 0 2 3 4 WATER VAPOR PRESSURE,MM. HG. Figure 2. The Wear Rate of Graphite as a Function of the Water Vapor Content of the Test Environment. (After Savage)

-5 -all active investigating various facet's of s.urface phenomena With so many and, such varied. i.nterests investigating surfaces.'it -'.s na..tu.ral. to expect that, the picture would. be confusing particular.y whern ne recognizes that there is no standard way of describing or characterizing a surface that is acceptable to all groupso If we cannot agree on what are examininr or investigating it is quite obvious that the results are apt to be confusing. The Mathematicians Thermodynamicists and Theoretical Physicists are of course unhampered by these di.fficulties since one can always postulate a clean surface of known area and proceed, from. this pointo Work of this type is quite important si.nce it is of considerable aidl to the experimentalist i:n at;tempting to correlate or interpret his results. To date however the correspondence between the experimental and theoretical approaches is rather poor and. at best is onl.y qualitative in natureo I do not intendd.in this paper to go into a review of the theoretical studies since there are several. excelent reviews and. texts that summarize these contributions (2.39 h,',- 7) Let us look at the properties of materials from. art engineering standpointo In general. we are interested, in mechanical. electrical. and. chemical, properties Phenomena such as elasticity'5 strength,5 plasticity9 conruction or semi.-conduction 2 oxidation and corrosion resistance, catalytic effects, adsorption, lubrication9 wetting, etco occur in everyday engineering practice and, are supposed, to be und.er the control, of the engineer. Are surfaces involved in these operations? The answer is yes in all cases but they are not necessarily the controlling factor in many applicationso

One would not normally anticipate that the elastic properties of a solid woul.d be a function of the internal or external surfaces yet there are many references in the literature which show that this is indeed the caseo Table I is a listing of a few of these datao Th.ese are very large changes in moduli but; this is to be expected when very small. diameter specimens are used since it is only in these specimens that the surface atoms become an appreciable proportion of the total volume or cross-section of the sampl.e These data would indicate that E, the TABLE I ELASTIC MODULI IN VARIOUS MEDIA Material Environment E,Dynes/CMW Glass Air 5.22 x 1011 Menthanol 4 56 x lOl1 Water 3o46 x 1011 Platinum Air 8 35 x 10-10 Water 3 59 x 1010 Steel Air 2359 x 0 11 Water 19ol x 1011 Quartz (100lO) Air 4 9 x 1011. (lk[) Air 1537 x 1.01 elastic modulus should be a function of the surface to vdoume ratio as the specimen size changes and, the internal surface to volume change as the grain size changeso Floreen and Hucke(8) have sho'wn that there are changes in the elastic modulus when the external surface to volume ratio and the internal. surface to volume ratio for a given material is varied. using sheet speci.mens of various sizes machined from the same stock or

-7 -heat treated to produce various grain sizeso They also showed that varying the environment in which the specimens were tested, produced. changes in the elastic modulus for a given internal or external. surface to vol.ume ratio A summary of these results is given in Figures 35 49 and 5o It is true that in these larger specimens, where the surface to volume ratios are less than in wire or film specimens, the changes i.n the elastic mod.uli are not as great since the surface makes u.p a smaller proportion of the total volume, nevertheless the influences are there and. are significantO Conceivably other testing media may produce even. more significant changes in the surface energy and. still more strongly affect the elastic behavior than the data shown. Another piece of confirmatory information along this same line are the data of Clauss(9) who showed that a change i.n the torsional modulus of wires was a direct function of the degree of gas adsorption when helium, argon or carbon dioxide is adsorbed. on sillver wires o When one studies the influence of surfaces on the plastic properties of solids and such phenomena as fractu.re, yield strength, fatigue strength and creep strength. one encounters an. enormous volume of published. worko It is a well established fact that there is a specimen size effect in. dealing with these properties and. many recognize that there are environmental effects which in some cases are equal.ly as important as size and bulk properties. In preparing this survey I had intended to avoid involvement in mechanisms; not that they are unimportant but in surface research there are as many mechanisms as there are investigators and, one can loose

-8 -0.040" 21.0 14.5 20.6 - 20.2 14.0 19.8 - _ 19.8 ~0.069 " aS. Q ^^^"^ -13.5 w 19.4 - 19.0 0.250" - WJ 13.0 '0 18.6 - ^ x 18.2 - 12.5 17.8 17.4 - Air Environment 12.0 17.0 O l 1 1 0 200 400 600 800 1000 Co /V I INCHES Figure 3. The Variation in Elastic Modulus of Copper as a Function of the Grain-Boundary to Volume Ratio for Three Sizes of Test Specimens. (Floreen and Hucke)

-9 -21.4 21.0 600 14.5 1200 20.6 1500 20.2 14. 19.8 - mbY/^ - 13.5 w 19.4 Z w 0 19.0 -18.6 - 18-2 12.5 178 174 Air Environment 12.0 I I I I I Ii I I I I 0 10 20 30 40 50 60 a /V IIINCHES Figure 4. The Variation in Elastic Modulus of Copper as a Function of the Surface Area to Volume Ratio for Specimens of Three Different Grain Sizes. (Floreen and Hucke)

-10 - 21.0 14.5 20.6 20.2 140 a. 19.8 0 a 19.8 -..040" 13.5 'lo 19.4 - _.o- "-Z IQA w n^ 19.0 l _ o-.069" _ 13.0 18.6 _ O 18.2 o 250" 12.5 i 17.8 17.4 Water and Stearic Acid Environment - 12.0 17.0,, I I I I Ia I 0 200 400 600 800 1000 a/V I a/V INCHES Figure 5. The Variation in Elastic Modulus of Copper as a Function of the Grain-Boundary to Volume Ratio for Three Sizes of Test Specimens. (Floreen and Hucke)

-11 -sight of the data in attempting to interpret it, I had envisioned this paper as one that would present, the fact that there are important surface phenomena but would leave their interpretation to others I might say in passing that in surface phenomena we do not have the equivalent of that universal cause of all solid state behavior, the dislocationo In order to bring some coherency to the large volume of data on the plastic behavior of solids as influenced by their surface it will be convenient to go into mechanisms and. to subdivide solids into classes of glasses, ceramics and metalso To explain the mechanical behavior of glasses one is usually forced to a Girffith crack mechanism which essentially postulates the existence of microcracks or imperfections in the surface of the glasso This then relates the strength to the size in that with decreasing sizes there is a lowered probability of finding a crack which would produce failure to give the relationship observed experimentally that the strength is inversely related to the sizeo The influence of environment on the strength is usually explained by either of two alternate mechanisms. Stress corrosion at the root of the surface cracks, or a change in surface energy due to the adsorption of the environment onto these surfaceso Note that both of these mechanisms involve surface phenomena rather than any intrinsic property of the glass as a solid although the bulk properties can be modified to produce property changes. In the case of ceramics, A1203, MgO and ionics such as NaCl the same type of size effect as noted in glasses is observed, i.ee, the tensile strength is inversely proportional to sizeo Environmental factors are probably more significant in affecting the plasticity and

-12 -strength of these materials. The work of Joffe(l1) on testing of NaCl in water and obtaining greatly increased strength and the work of Polanyi and Ewald(llyl2) showing increased plasticity when surfaces are wet, are well known. Other media also have quite as marked effects; Parker and others(l' l4) work on gas adsorption for example shows the same phenomena. There are three mechanisms proposed for the behavior of these materials The healing or dissoluttion of microcracks by wetting of the surfaces, the penetration or wetting of the internal boundaries by the environment, or the barrier quality that the surface offers to dislocation flow or generationo Turning to metals these same size and. environmental effects that have been noted in glasses and. ceramics have been detected. The size effect is more generally believed to be due to surface restrictions to the emergence of dislocations while the environmental. effects can either be explained. by dislocation theory which, postulates the blockage of their flow by surface films of oxides, hydroxides, sulfides or even synthetic plastics,(l5516) or by a Russian concept known as the Rebinder Effect.o This latter theory, based on the studies of Rebi.nder and his associates (1l7^18,l9) showed. that when materials are immersed in surface active environments the environment adsorbs onto the surfaces to lower the surface energy which in turn produces a marked decrease in the extent of plastic flow and. lowers the fracture stresso Both of these mechanisms involve surface fil.ms but there are data which show environmental effects even in the absence of a filmo The testing in acid, for example, of bismuth and germanium produces marked increases in strengtho(20A

-15 -In some respects the less ductile metals or metalloids are more sensitive to surface conditions than the ductile metalso Turning now to electrical properties once again one can show that in many applications the influence of surfaces, either internal or external is quite significanto The earlier work of Langmuir and Volmer on the study of surface diffusion showed that the work function was definitely related to the surface concentration of impurity atomso The development of the field emission microscope has enabled significant advances to be made in studies of surface diffusion in. the last few years (22,23) It should be marked in passing however that most of the surface diffusion data is concerned with metallic surfaces; the phenomenon has been noted on glasses, plastics, polymers and other types of surfaces, but quantitative data are absent. Electronic conduction in metals is largely a volume controlled process but the interfering effects of grain boundaries are well knowne If however the dimensions of the conductor are reduced to where the size is of the order of the mean free path of the conduction electrons, the external surface once again plays an increasingly more important roleo Thi.s effect is apt to become quite significant from an engineering standpoint as we drive further towards miniaturization of our electronic gearo The theory of what should happen when conduction occurs in thin films has been quite thoroughly presented; effects on thermal conductivity, thermoelectric power, influence of magnetic fields, Hall effects, optical property changes, etco On the other hand the experimental verification of any of these studies is quite scarce. What data are available

-14 -seem to indicate that if extreme care is taken to prepare films of high purity with smooth and parallel faces, one can then show that these have the same properties as the bulk metalo Unfortunatel.y however, the introduction of impurity atoms, adsorption effectss, non-coherency of the films, etc,, all produce what are termed "anomalous behavior", i.e., the data don't agree with the known behavior of the massive metalo As Dr. Mayer has stated "Among 5000 papers dealing with thin film since the beginning of their study, a great number tell of the great pains the authors took and still take, not beforehand, but afterwards, to trace out extraneous physical factors and. to relate them to certain. observed, properties " (24) In the field of semiconductors the presence of interfaces is even more important than in conductorso It was the study of surface state theory in semiconductors which, led, to the invention of the transistoro Surface states on germanium were suggested by Bardeen to account for the observed facts that the contact potential, the work function and the rectification characteristics appeared, to be independent of the metal that was used for the reference or contacto The exact character of the surface state of the semiconductor is not usually specified. they may be merely work-function differences between metal and semiconductor, chemical barrier layers, surface contamination due to adsorption,, or combinations of these factors, etc. As in the case in so many phenomena involving surfaces, ways have been devised to analytically handle the description of their behavior without actually determining basic mechanisms. One cannot tolerate internal surfaces in semiconductors since

-15 -they also produce anomalous effects; they may form a highly conductive region if the bulk is highly insulating, or it may act as a high resistance layer in a conducting bulk phaseo In addition they also act as regions of high recombinationo Since they are a disordered region they also act as scattering centers and lower the electron and hole mobilitieso It is for these reasons that semiconductor devices are almost invariably single crystals. These same effects can of course be generated at external surfaces but since these cannot be eliminated, their presence has to be tolerated but they can also introduce some extraneous effects of their own, so much so that environmental conditions such as humidity, cleanliness etc.o must be controlled in order to produce commensurable componentso Materials which are dielectrics or insulators are also subject to surface property changeso Many of the engineering failures of insulators are not due to intrinsic breakdown of the dielectric but are primarily caused by structural discontinuities which produce internal surfaces, subsequent internal heating chemical deterioration and eventual loss in insulating propertieso The external surface is particularly susceptible to chemical. change by way of adsorption, aging effects and eventual chemical decomposition which results in tracking failures, crazing, and finally failure. Many of the effects which are noted in the electrical, magnetic and optical properties of materials are really traceable to the changed chemical properties of internal and external surfaceso By their very nature these surfaces are bound to produce anomalous effectso Since they consist of poorly coordinated bonds

-16 -which are satisfied, either by physical or chemical adsorption of environmental atoms, are points of high stress, are preferential loci of impurity atoms, their resulting influence on the properties of materials should not be surprisingo The fact that we cannot accept in an imperfect manner characterize these regions is no reason for ignoring them. External surfaces more strongly affect the chemical behavior of materials than any other propertyo This is to be expected since the surface a solid presents to a liquids gas or to another solid actually consists of a barrier or series of barriers which must be penetrated before chemical, reactions can occur. Most of us are aware of this from the extensive studies that have been made and continue to be made on the oxidation and corrosion of metal.s Corrosion is generally thought of an occurring by one of three basic mechanisms~ direct chemical, attacks galvanic attack; or oxygen depolarizationO In direct chemical attack the metal reacts with the medium and supposedly goes directly into sol.u.tion. While some systems are this simple many of them are not. The solution of copper, for example, in acid ferric chloride is the basis of the treatment of plates for the photoengraving industry and in preparing printed circuitry in the electronic industryo A person simply immerses copper in the acid solution and the copper is dissolvedo Because the etching obtained. commercially was not of sufficiently high quality an investigation was made of the actual mechanism with a view to improving it. This study showed that the dissolution of copper is not a simple reaction but consists of two separate and distinct steps. First a monofilm of Cu.C12 forms on the surface of the copper and i.s insoluble in

-17 -the depleted acid film near the copper. As the solutlion is replenished via diffusion from the bulk of the liquid the Cu2Cl2 dissolves and. forms CuC12 which is soluble and the process proceeds. A study of the metallic variables such as purity, orientation, grain size, cold work, etcO, showed that none of these variables had any effect on the dissolution rate. The controlling factor is the kinetics of the formation and dissolution of the surface films, not the properties of the underlying metal. In corrosion by galvanic attack most people are aware that a change in environment or merely the concentration of the environment will change the positions of the elements relative to one another in the electromotive serieso Here again this is obviously a surface phenomenon since nothing has been done to the metal. only the environment, yet the stability of the metal has been changed. The very mechanism of corrosion by oxygen depolarization, the removal of oxygen from solution to react with the hydrogen released at the anode, is intimately tied. to the surface The voluminous and seeming never-ending flow of papers, symposia, research proposals and programs dealing with corrosion is due principally to the fact that the problem is inherently one that is surface controlled and simply cannot be handled by considering only the bulk metal. as the system; the bulk composition is only one variable and generally not the most important oneo It is somewhat naive to believe that the chemical analysis or the physical structure of a solid alone determines its sjbsequent behavior in various media. The strong feelings that engineers have relative to the merits of one type of stainless steel over another; 316 vso 347, 304 vso straight chromium steels, etco, are based on the

-18 -behavior of these materials in various corrosive media, While all engineers will admit that the passivation, (doing something to the surface), is the principal reason for the corrosion resistance, they seem to associate this passivity with the base analysis of the metal~ The fine work of Rhodin(25) in analyzing the surface of stainless steels, as shown in Figures 6 and 79 clearly indicates that while there are changes in the major constituents of Fe, Cr^ and Ni. the minor constitutents show an even greater change and in this fashion as he states "a minor constituent can become a major constituent of the surface." Undoubtedly the composition of the bulk phase determines the composition of the surface films but the straight bulk analysis is not the only criterion since the treatments used to produce different types of surface films are equally as important o In the fields of adsorption and absorption on. surface there has been and. there continues to be a great deal. of activity The major portion of work in this field is of the variety where the solid, surfaces are cleaned, then subsequently exposed to a given media, usually a gas" and the process of adsorption followed through pressure changeso This is of course the basis of the famous BET technique for the measurement of surface area. While studies of this type are informative and important they unfortunately do not shed too much light on. the behavior of real materialso As engineers we are forced to work with surfaces before they are cleaned and in this respect we are more interested in what comes off the surfaces than what is readsorbedo An example of this is the material graphite which is findig considerable use ins the nuclear fields as well as in extreme temperature applicationso The

-19 - 90 80 70 60 0 50 I i E - S.S. 316 40 20 ROMIUM ALLOY STOICHIOMETRIC ROLLED PICKLED PASSIVATED COMPOSITION OXIDE SURFACE SURFACE SURFACE OR HIGH TEMP OXIDE SURFACE TREATMENT Figure 6. Changes in the Surface Concentration of the Elements in Stainless Steels as a Function of Their Prior Treatment. (After Rhodin)

15 20 __ 'S.S. 304, 10 / 7 SILICON 316,347 0 / o~~~~~~~~~~~~~~~ H I / ) / ~~~/ ~COLUMBIUM <S.S.347 5 -_______ --- —-- -^ --- —------ 5~~~~~~~~~~~~~~~~~~~~~~ / I ^ 7<>S.S. 316 - -AI / I MOLYBDENUM 0 ALLOY STOICHIOMETRIC ROLLED PICKLED PASSIVATED COMPOSITION OXIDE SURFACE SURFACE SURFACE OR HIGH TEMP. OXIDE SURFACE TREATMENT Figure 7. Changes in the Surface Concentration of the Elements in Stainless Steels as a Function of Their Prior Treatment. (After Rhodin)

-21 -properties of this material are quite well known but it is not generally recognized that this material has a very extensive surfaceo It's theoretical density is 2o25 g/cc but this is never obtained; most graphites are of a density of lo85 g/cco Some of this defect in density is due no doubt to point and planar defects but *the great bulk is internal porosityo While the quantity of gas adsorbed per square centimeter on these internal surfaces may be small, the total square centimeters of internal surface is enormous, This means that considerable quantities of gases can and are ad.sorbed in graphite and. can and will. change the properties of the graphite particularly where graphite is being used for its chemical, stabilityo These are not lightlyy held gases since desorption is still occurring at temperatures as high as 2200 OC and furthermore the desorption is not irreversible since on re-exposure to the atmosphere the graphite readily readsorbs approximately 0% of the gas desorbed. The principal gas desorbed i.n this material. i.s water vapor This adsorption is not something which. is restricted, tno a defect material such as graphite. Powdered metals and ceramic materials particularly when in a size range where there is a very large exten.ded, surface, readily adsorb or absorb gases on their active surfaceso In some cases the bonding is rather weak and this is known as physica.l adsorption while in other cases strong bonding occurs and, this is termed chhemir.sorption. Subsequent release of these adsorbed atoms during such operations as sintering causes problems in dimensional stability, density and, the various physical properties Here again the properties of the bulk material do not determine what composition the surface ad.sorption will take. Figure 8 shows the desorption analysis obtained, on nickel powders

-22 -1.0 soI2 - H0 2 0 0 200 600 1000 1400 1800 TEMPERATURE,F 3D sorption The upper urves are CO2 (/ 2.0 H20 IJ 0 > 1.0,-co S02 200 600 1000 1400 1800 TEMPERATURE, F Figure 8. The Composition of the Gases Desorbed from Nickel Powders as a Function of the Temperature of Desorption. The upper curves are for steam scattered nickel into a nitrogen atmosphere; the lower curves are for steam scattered nickel into an air atmosphe re.

-23 -that had been manufactured by the same technique but in different atmospheres. To still further complicate the picture if these same powdered metals are given different surface treatments, either chemically or mechanically, their subsequent readsorption characteristics are modifiedo Copper powders for example when cleaned in ammoniacal solutions, rinsed and dried, are stable in air for weeks while the same powder cleaned in nitric acid, rinsed and, dried, will interact almost immediately with the atmosphere to produce a surface staino Vacuum cathodic etching of uranium also produces this stabilization of the surface to atmospheric corrosion whi.le untreated uranium will tarnish almost immediately on exposure to the atmosphereo One of the most important and at the same time most complicated of all phenomena associated. with surfaces is that of heterogeneous catalysis. These are chemical reactions which are carried out in the liquid or gaseous state by contacting a solid catalysto Catalysts are usually defined as promoters of a chemical reaction which do not appear in either the feed or product streams. They don't contribute energy to the system, they do not change the equilibrium of the system, they are capable of regeneration, and their essential characteristic is that they change the kinetics of a reactiono While catalysts have been recognized and used for well over a hundred years and the last ten. years have seen an enormous amount of effort put into research to understand. the phenomenon, it is still safe to say that other than recognizing that it is a surface phenomenon, the basic mechanisms are unknowno There are many reasons that can be given for this lack of understand.ingj

-24 -the phenomenon is entirely surface controlled, not all the catalyst surface is active, addition agents called promoters and poisons can markedly change the characteristics, temperatures, pressures, compositions, dispersions, prior treatments, types of carriers, modes of regeneration are but a few of the many variables which are known to affect the catalytic behavior. Many concepts have been investigated in order to gain some insight into the mech.anisms involved. Relationships between contact potentials, work functions, electronic configurations, paramagnetic properties, atomic sizes, lattice imperfections, influence of impurity atoms, conducting or semi-conducting properties, particle sizes and distributions, orientations or crystal faces, presence of adsorbed films, etc. have all been investigated. Weak interrelationships can sometimes be found but for the most part the problem is still unsolved from a mechanism standpoint. I have mentioned three or four fields in which the presence of surface is of considerable significance but this by no means is a complete listing nor even the most important but they are field-s inri which active research, is being carried forward at an accelerated rate. Other fields which are concerned with surfaces are those dealing with fibres and films, polymers and plastics, coatings, paints, emulsions, adhesives, lubricants and cements. All solids, liquids and gases that we handle as engineers are contained within some confined volume and produce interfaces where surface phenomena can occur. In symposia such as the present one there is a tendency to put our best foot forward and show how we are making real progress.

-25 -You. will note that in my discussion I have 'been taking what might be termed a negative attitude. There are advantages in seeing where you have been but I feel that it is more important to look forward. to see where we should be going. While there is no doubt that progress has been made and will continue to be made along the line of current research programs I have the strong feeling that more time, efforts and. funds should be devoted to basic studies on surface phenomena if we ever expect to obtain a real understanding of the behavior of real materials in engineering practiceo

REFERENCES 1o Savage, Ro Ho "Graphite Lubrication." J, App. Physo 19, (1948) 1, 2. Herring, Conyers, "The Use of Classical Macroscopic Concepts in Surface Energy Problems " Structure and Properties of Solid Surfaces, University of Chicago Press, 19535 35 Ewald, Po Po and Juretschke, Ho "Atomic Theory of Surface Energyo" Ibid.o 4o Boudart, Mo "Surface Structure from the Standpoint of Chemisorption and Catalysis " Ibido 5~ Evans, Uo Ro "Chemical Behavior as Influenced by Surface Conditionso" Properties of Metallic Surfaces, Inst, of Metals Monograph 15, 19535 6, Honig, J. M. "Adsorbent-Adsorbate Interactions and Surface Heterogeneity in Physical Adsorption," Annals of New York Academy of Sciences, 58, (1954) 741L 7o Ehrlich, Go "Molecular Processes at the Gas-Solid Interface." Structure and Properties of Thin Films, New York- John Wiley and Sons, Inc,, 1959~ 80 Floreen, S. and Hucke, Eo E "The Effect of Strain on the Surface Energy of Solids'" In publicationo 9o Clauss, Ao Compto Rendue "Influence of Gas Adsorption on Elastic Moduli," 259, (1954) 25; "Influence of Gas Adsorption on Strength of Metals," 242, (1956) 1578o 10o Joffe, A. 1st Confo on Appo Mech,, Delft, 664, 1.924, Z, Phys,, "Deformation and Strength of Crystals," 22, (.1924) 286, The Physics of Crystals, New York- McGraw-Hill Co,, 1.928o 11o Polanyi, Mo and Ewald, W, "The Plasticity and Strength of Salt Under Water," Zeito Phys,, 28, (1924) 290 12o Polanyi, M. and Ewald, W. "The Strength and. Elastic Limit of NaClo" Zeito Phys,, 31, (1925) 746, 13o Gorum, Ao E,, Parker, Eo Ro and Pask, Jo A. "Effect of Surface Conditions on Room Temperature Ductility of Ionic Crystals," J, Am, Cero Soc., 41, (1.958) 161l 14 Parker, E. R., Pask, J, A., Washburn, J,, Gorum, Ao E and Luhman, W, "Ductile Ceramics, A High Temperature Possibilityo" Je of Met,, 10, (1958) 351o -26 -

-27 -15e Gabrera N. and Price P B Growth and. Perfection of Crystals. New York~ John Wiley and Sons, (1958) 204. 1,6o Weiner, Lo Co and, Gensamer, Mo "Effect of Solid. Environmrent on Brittle Fracture of Zinc Single Crystalso' Jo Insto of Met., 85, (1.956) 467y 17. Rebinder, Po Ao "Adsorption Layers and. Their Effects on the Properties of Disperse Systems"' Izvo Akad, Nauk.USSR, Otd,.a t. i. Estesttv. Nauk. (.1956) 639-706. 18o Rebi.nd.er, P. A "The Decrease in Scratch-Hard.ness on Boundary Laver Adsorption." Z o Physik, 729 (1931.) 191: 19o Rebinder, Po Ao and Kalinowskaya, No "'Reducti.on of Surface Layer Strength of Solidso" Zhuro TekhO Fix.o, 2 (1932) 726-75o 20 Classen-Nekludova M. Tech Phys o USSR, 5 (1938) 827 21o Briedt. P,, Hobsetter, J. No and Ellis, Wo Co "Some Effects of Environment on Fracture Stresso" J, Appl. Phys, 29, (1958) 226. 220 Gomer, R., Wortman, Ro and. LunyP R. "Mobili.ty and, Alsorption of Hydrogen on Tungsten." J. Chemo Phys,, 26, (1957) 1.147o 23o Becker, J, A and Brandes, R Go "On the Adsorp+tion of Oxygen on Tungsten by Field. Emission Microscopeo" JO Chemo Physo, 235 (1955).1323 2 24,o Mayer, HI "Recent Developments in Conduction Phenomaneo" Stru.cture and. Properties of Thin Films, New York- John Wiley and, Sons, Inc, 19590 25, Rhodzin, To No "Oxide Film Composition Studies." Annals of NoY. Academy of Sciences, 58, (1954) 721o