Materials Science and Engineering, 83 (1986) 175-176 175

EXECUTIVE SUMMARY

JOHN R. SMITH (PANEL CHAIRMAN)

Physics Department, General Motors Research Center, Warren, M148090-9055 (U.S.A.)

In this report a summary is given of the discussions and recommendations of a panel which met on August 11-16, 1985, in Aspen, CO, to review the field of bonding and adhe- sion at interfaces. This s tudy was commis- sioned by the Council of Materials Sciences for the Division of Materials Sciences, U.S. Department of Energy. Our purpose was to assess what is known and to make recom- mendations for future research programs in the field. The attendees included 23 panel members whose backgrounds cross many dis- ciplines, consistent with the nature of the field.

Our discussions covered adhesion aspects of such diverse subjects as contact adhesion and wear, grain boundary energetics and crack for- mation, polymer adhesion in the presence of solvents, ceramic crystal-glass-crystal grain boundary characteristics and even semicon- ductor heterojunction band discontinuities. These subjects have suprisingly similar adhe- sive considerations.

The panelists agreed that adhesion and bonding at interfaces are problems of con- siderable practical importance. Some even felt that adhesion is engineering related and too complex for fundamental research. We initially struggled on the definition of the word adhe- sion. It was in the resolution of that struggle that we began to understand better the place of fundamental research in adhesion. We noted that practical or experimental adhesion in- volved the testing of not only bond strengths at interfaces but also the effects of elastic and plastic deformation, impurity effects, surface processing and the speed of the adhesion or debonding process. We then defined a basic adhesion process which is an appropriate start- ing point for calculation as well as an appro- priate ideal limit for the understanding of experimental or practical adhesion. Two solids are brought together in an adiabatic and re- versible manner. At each separation between the two surfaces, the total energy is a minimum

functional of the electron density distribution and atomic coordinates. The value of the total energy at the equilibrium separation then determines the basic adhesive energy. As such, it could include the effects of elastic deforma- tion as well as impurity and surface roughness effects but would not include inelastic effects. On this basis we could then proceed to de- lineate what is known and what needs to be known about the fundamentals of adhesion and bonding at interfaces.

It seems that at present the detailed mech- anisms of basic adhesion are largely missing. This is true even though the field has a long history of practical importance. It may be due in part to the complexities mentioned above and also to the small number of active re- searchers in the field, a significant fraction of whom were on the panel. There clearly is a need to separate and quant i fy adhesive vari- ables. Some examples of basic questions can be listed. Where does an interface fail, and how does this depend on the chemical struc- ture, mechanical behavior and stress levels within the interface? What are the effects of interfacial species? Do they affect the adhesive bonding directly, or is their effect primarily on the yield stress?

The answering of these and other questions would be greatly facilitated by interdisciplin- ary collaboration. As discussed below, those who consider the mechanical properties of interfaces and those who study structural and chemical make-up of internal interfaces have much in common. It seems that the time is ripe for collaborations between first-principles con- densed-matter theorists and experimentalists in order to a t tempt to separate the variables of adhesion. Finally, it would be important to try to combine in s i tu spectroscopic and microscopic imaging of the interfacial zone during the adhesion experiment.

Experimental force versus interfacial sepa- ration measurements, such as are now available for some polymer interfaces, would provide

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invaluable information for metallic, ceramic and semiconductor interfaces.

There are some differences between internal interfaces and those formed by bringing sur- faces into contact. Properties specific to the latter are surface preparation and real area of contact (asperity contact, and elastic and in- elastic deformation). There are numerous common considerations, however. The basic adhesion process as defined above is the same. Impuri ty layers deposited onto the surface or segregated from the bulk to an internal inter- face may or may not have similar effects. Dislocations play a role in both cases as the lattices tend to come into commensurat ion at the interface. Bimetallic contacts have similarities to grain boundaries in alloys where there is elemental segregation at the internal interface. Solid overlayers, whether deposited via vapor or chemical deposition, have a real area of interfacial contact which is like that of an internal interface.

These common features between internal interfaces and those formed by bringing solids into contact p rompted the suggestion of nu- merous collaborative experiments, as can be seen in the chapters to follow. Basic to many of these suggestions is the use of bicrystals to correlate with single-crystal adhesion studies. Such bicrystals may in some cases be made thin enough to allow transmission electron microscopy through the interface.

Only very recently have first-principles theories of basic adhesion appeared. These theories involve the self-consistent calculation of total energies as a function of interfacial spacing. To date, calculations have been done only for bimetallic interfaces. A universality in adhesive energy v e r s u s separation was dis- covered which was subsequently found to extend to cohesion, to chemisorption and even to nuclear matter energetics. It seems appropriate to carry out such calculations on semiconductor, ceramic and polymer inter- faces. These calculations should be done in

conjunction with ongoing calculations on internal interfaces such as those involving grain boundary structure and energy, as sug- gested above. The effects of surface impurity layers and of impurity layers at grain bound- aries should be treated. These theoretical calculations must be brought closer to the practical adhesion experiments discussed above. It is perhaps more likely that such effects as elastic deformation and even inter- facial defect formation will be more readily included in the theory than isolated in the experiment. Future progress for adhesion theory would certainly be facilitated by ex- pected improvements in computers. In the last 15 years, surface theory has progressed from the treating of jellium surfaces then to transition metal overlayers and semiconductor interfaces now. Most of that surprising pro- gress was due to new theoretical methods rather than to advances in computer speed, however. I suspect that the same will be true for future progress.

The interdisciplinary nature of the field described above is evident from the chapters to follow. The first chapter is on solid surfaces brought into contact , often involving single crystals under controlled conditions. The second chapter is devoted to practical adhesive processes, including the wear and failure of atomistically deposited films and coatings. The subject of the third chapter is internal interfaces or grain boundaries in ceramics, while the fourth is on internal interfaces in metals. The last two chapters are devoted to polymer interfaces and semiconductor inter- faces respectively.

Although our report is obviously quite broad, it certainly does not provide every- thing that we would want to know about adhesion. In fact, I have tried to portray that very little is known about this subject which is so important in a practical sense. There is much to do in this field, and I suspect that the field is ripe for expansion and progress.