urface phenomena be best explained in t e r m of S electronic and structural configurations of the surface. Accordingly, there are three d8erent aspects can
of surfaces to examine: chemical behavior, structural propertiep, electronic properties. With respect to chemical behavior, the relation between unsaturated bonds and lattice configuration is of prime importance. In a diamond cubic configuration, the degree of unsaturation or density of free bonds varies anisotropically. In a (111) surface, only one unsaturated bond goes upward, as shown in the model of Figure 2, while three bonds go into the bulk, or downward. In the case of the (100) surface, two unsaturated bonds go upward and two go into the main bulk. In the (110) surface, the very top atoms have two bonds with two other surface atoms, one with the bond up and one dangling bond. In general, one can state that the more dangling bonds there are on the surface, the more .eactive the surface will be. This variation of the un-
I
saturated bonds with the various surfaces permits-draw-. ing conclusions with respect to the chemical behavior of different surfaces. Gatos has shown this in his experimental work with indium antimonide by etching the material in various crystallographic planes (see Figure 1). Using the model shown in Figure 2, he also determined the number of free h n d s and calculated the relative work functions and dissolution r a t e in the three crystallographic directions as shown in Table I. To study and analyze surfaces, new tests, new equipment, and new tools must be developed. Ultrahigh vacuum equipment which permits us to obtain pressure of lo-” mm. H g is of prime importance in order to obtain “dean” surfaces. Electron diffraction equipment is another requirement. However, the presently used fast electrons (having energies of approximately 50 kilovolts) penetrate the material too deeply and give little information on the surface configurations. Germer has shown that this
ANALYSIS L
1
In the Second Annual Symposium on Fundamental Phenomena in the Material Sciences,sponsoredannually by The llikon Corporation and held in Boston earlier this year, the importance of the solid surface was emphasized from both the theoretical and the practical aspects. The characteristics of surfaces, their effects on bulk properties, and the interactions between solids are not clearly understood, but with the advent of research utilizing ultrahigh vacuum and low energy electron diffractiontechniques reported in the symposium there should be accelerated progress in the near future. In sponsoring the symposium, llikon believes that the most vital area of investigation in the material sciences is that dealing with interfaces between dissimilar materials. Such interfaces con be studied most effectively if they are treated as participating in surface phenomena in the broadest sense. In this manner the great variety of manifestations of surface activity can be explained most consistently. Cochairmen of the symposium were Professors F. V. lenel of the Rensseloer Polytechnic Institute and P. 1. de Bruyn of the Massachusetts Institute of Technology. Speakers included Dr. H. C. Gatos of MIT, Dr. L H. Germer of Cornell, Dr. P. M. Ku of the Southwest Research Institute, Dr. F. F. ling of RPI, Dr. E. Rabinowicz of MIT, I. Farkass of Ilikon, Dr. G. S. Springer of MIT, Dr. S. Rou of RPI, Dr. F. M. Fowkes of Sprague Electric Co., Dr. J. J. Bikerman of MiT, and Dr. C. M. Adorns, Jr., of MIT. 40
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
1. BONIS
TABLE 1. SURFACE CHARICTERICTICS OF , ANISOTROPIC~lRDiUMANTIMONIDE
lLloriw Orimtdisn
(100) (110) (111)
Free Bondrper , Sq. Cnr
Frar Bond DC&J
1.25X10E 8.83 X 10"
1.00 0.71
7.22 X IO"
0.53
Rel&'uI Work Fmlbn
1.00 0.95 0.93
&I&'V#
DimIuth Rorrr
1.00 0.89 .
0.62
information can be by using low energy e l e e ~ mdiaFraction (5 kilovolts or less). The method itself is not new but has been used very little in previous years because of experimental difficulties. Only during the last few years has the method been improved to such an extent that commercial equipment is now available. During the last four years, low energy electron diffraction has been used for the study of thermal vibration of surface atoms without
Interdisciplinary Aspects of Surface Phenomena
A
A
the results being altered by the different vibrations of the bulk atoms. I t has been found that the thermal vibrations of the surfam atoms di5x conxiduaL4y in the various directions of the crystallogmphic yes. Information with respect to the adsorption and reaction e n o r of the surfaces can he obtained in thia way. Gaprrr applied low energy electron dfiaction metbada to study the &ation of metal s u r k e s . In opder bo k p the d a t i o n rate slow, tests wexe made at preaaum near 10- mm. Hu. Germer concluded
# c-, even if the adsorption concerns a fraction of a monolayer only. He found that, for example, oxygen on the (110) face of nickel has produced four or five M e r e n t structures succaulively, by keeping the oxidation rate low. Following this process was possible only by the application of low energy electron diffraction. The friction between solids is largely a surkce phenomenon and has long beenrecognized, but there is still no complete agreement about the o r i ~ nof frictional force. The-early iiew of Coulomb, friction is due to the interlmking of surface irregularities is still held by some, frictional work being believed to reprenent the work done in lifting one such irregularity over another. Ling presented new ideas on the deformational and geometrical aspects of surfaces in sliding contact. Based on the measurement of f r i i o n between two cones of similar or dissimilar materials, of which one cone is rotating and the second one is stationary, he developed a method of calculation showing the effects of the cone angles on the friction characteristics between a variety of materials. This kind of experimental and mathematical treatment permitted the speaker to develop a modfied equation which could be useful for selecting materials for a variet). of friction conditions. Ling's findings permit calculations of the heat flux generated in the moving as well as the stationary body based on the dissipation capacities of these bodies. One usually better understands an effect if he can correlate some data concerning the effect with the result of another, somewhat similar effect. Rabinowicz has used this treat of correlating M e r e n t effects for his experimental work on the influence of surface energy on sliding contact. He emphasized that the real lubrication problem in any sliding contact is the wear of the sliding parts and the effect of the lubricant on this wear. Wear can best be expressed in terms of the size and number of particles which are taken off during this sliding action. The speaker found that ball milling of solid materials with a lubricant added resulted in the formation of fine particles, the& of which is practically identical with that of particles produced by the sliding action. In both cases, the addition of the lubricant reduces the surface energy of the solids and the lowered surface energy reduces the size of the wear particles. There is a close correlation between the roughness of the surface due to friction and the size of the wear particles. ~
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Figure 7 .
Indium antimonide single crystal after chemical etching in various crystallographic planes
Materials which produce low surface energy in the solid are those which result in low friction coefficient. Any experiment designed to study the physical or mechanical properties of a surface whose composition is representative of the bulk material requires the preparation of the surface free of contamination and its maintenance in that condition during the experiment. Even the simplest experiment requires the utilization of ultrahigh vacuums since the adsorption time is long. The period of time one can maintain a "clean" surface depends on the pressure and the sticking probability. This is shown in Figure 3 which relates the surface coverage time constant to the pressure in the system for two values of the sticking probability. The sticking probability is defined as the fraction of the molecules striking a surface in a given time which adsorb on the surface. According to Dushman, an atomically clean surface is "one free of all but a few per cent of a single monolayer of foreign atoms either adsorbed on or substitutioni
1
E
g
3
Figure 2. Saturated and unsaturated bonds in the surface of a diamond, cubic lattice configuration 42
ally replacing surface atoms of the parent lattice." mm. Hg Systems capable of maintaining 1 x are commercially available and were described in detail by Farkass. He indicated that in adsorption studies the production of clean surfaces was usually accomplished by heating the specimen in vacuum. During these studies, the outgassing process-the desorption-was seldom investigated. The reasons for this are, the author indicated, first, when the test specimen is heated, it is hard to separate the effects of the chamber wall surrounding the test specimen from the degassing effects of the specimen itself. Second, the amount and composition of the gas on the surfaces, both on the test specimen and on the chamber surface, are unknown. It was also pointed out, on the other hand, that when adsorption experiments were performed, the surface was supposed to be clean, the amount and composition of the test gas was known, and the surface effects of the chamber wall were carefully eliminated. In his report. Farkass
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
Figure 3. Relation between the surface coverage time conslant (t,,,) and the absolute pressure (P)Jor two values of the stickinq coeficient (7).Such plots are obtained in evacuating systems in which clean surfaces are to be prepared
stated that, in experiments performed on surface outgassing in his laboratory, the specimen and the vacuum chamber were usually a t the same temperature-mostly at room temperature. The author concluded that the existing data and empirical equations available in the literature about surface outgassing cannot be used to describe the phenomenon. An entirely new approach to the problem should be found which must take into account that the gas in the vacuum system is not a perfect gas, the collisions between the gas molecules and the wall of the chamber are not perfectly elastic, and there is a possibility that the gas molecules will be adsorbed on the wall or on the test specimen itself. The author described acommercially available vacuum system in which he had done his work. Pressure of 1X mm. Hg can be reached and maintained in these ultrahigh vacuum chambers without bakeout during the pumpdown period. When liquid helium cryogenic pumping is used, pressures of 1 X mm. Hg were reached. On a closely related matter, Ross spoke of adsorption on heterogeneous and homogeneous solid surfaces. He differentiated between localized and mobile adsorbed films. Special emphasis was given to the phenomenon where, with van der Waal’s type of two dimensional gas, one would expect a phase change of the adsorbed layer as the temperature decreases. Ross disclosed that in an alkali halide with ethane adsorbed at 90’ K., he actually had observed this phenomenon, theoretically foreseen by DeBoer. A discontinuity in the adsorbed film was observed with krypton and alkali halide when the amount of adsorbate corresponded to less than the close packed monolayer. If the coverage was more than 607,, the first monolayer began to develop into a multilayer on heterogeneous surfaces and the experimental data hardly permitted any theoretical analysis. Fowkes discussed the determination of intermolecular forces between adjacent materials by surface chemical techniques. The author gave an insight into the determination of intermolecular forces between these adjacent materials. Reference was made to Hilhebrand’s work in which he predicted intermolecular interactions from the solubility of one material into the other. The intermolecular forces between the adjacent surfaces include the ever present dispersion forces, dipole and ionic interactions of various kinds, hydrogen bonding, and metallic bonding. Surface tension can be used as a measure of intermolecular forces, Fowkes said. Means were offered to evaluate the magnitude of interacting forces in the measurement of contact angles, interfacial tension, or free energy of immersion or adsorption. Existence of surface forces should bring about adhesion of two solid surfaces whenever such surfaces are brought within atomic range and the effects can be made manifest
L. J . B o n i s i s President nnd Technical Director of T h e Ilikon Corp.
AUTHOR
in a variety of ways. Two freshly drawn fibers of glass, for example, will adhere to one another if they are brought into contact. This can be demonstrated by holding a fiber between the thumb and the forefinger of each hand and then drawing the upper fiber gently across the lower so that the point of contact is about an inch away from the thumb. The motion will be found to be snatchy and it is clearly seen that the two fibers stick together for an appreciable time before being pulled apart by the deflection of the lower fiber. Also thin pieces of celluloid will stick together when they touch, if new. Bikerman analyzed this problem of solidto-solid adhesion, and pointed out that the force needed for the separation of two adhered solids is due to one or a combination of three effects-viscosity effect, capillary pressure, or electrostatic attraction. If the pressing occurs in air at atmospheric pressure, the two solid surfaces would be covered by adsorbed air and moisture and no solid-to-solid contact would exist, but merely contact between the adsorbed layers of contamination on the surfaces. If, however, breaking and pressing occur in a vacuum of lo-“ torr or better, as already discussed by Farkass, solid-to-solid contact would occur since it takes a very long time until a solid surface is contaminated (see Figure 3). Adhesion between solids actually depends on the viscosity of the medium between them and/or the transfer of charge between the solid surfaces. Long time pressing at higher temperatures will result in mutual diffusion, sintering, or recrystallization, and separation cannot be achieved without cohesional break of one of the solids. Another subject of practical importance is spreading and capillary flow. When a drop of liquid is placed on a solid, it sometimes spreads out immediately to cover the whole surface of the solid, but more often it stays bunched up in the form of a lens. In general, it is clear that whether or not it spreads is determined by the relative attractions of the liquid for the surface of the solid and for itself. If the former exceeds the latter spreading will occur. Adams offered a new concept for spreading of liquid metal on a solid surface and developed equations which strongly indicated the similarity in the thermodynamics of wetting and penetration. In partial wetting, there will be no spreading. If the interfacial energy is low enough for spreading, it certainly will be low enough for penetration. Observed velocities of a pure copper liquid spreading on a solid copper surface are in the range of several hundred centimeters per second. Low velocities of spreading were observed in the system liquid tin on solid copper. This system is controlled by “surface” diffusion. For example, an alpha solid solution of tin and copper has to be formed before reasonable spreading can be observed. In addition, the author reported that it was found that the process depends on the presence of a very small concentration of oxygen that seems to be vital for this type of wetting. Complete absence of oxygen inhibits wetting. This was somewhat contrary to our customary belief that oxygen is detrimental to some processes, such as brazing. VOL. 5 6
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JULY 1 9 6 4
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