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SUMMARY OF DISCUSSION: INTERFACES In any adhesive bond, the interface between adhesive and adherend is an integral functioning component of the joint. Interactions between the adhesive and the adherend at the interface occur that are a necessary precursor to satisfactory adhesive joint performance. This interface must transfer loads between adhesive and adherend during the useful lifetime of the adhesively bonded joint without substantial degradation. Recommendations that address adhesive bond performance in severe environments require that the interface and its role in bond reliability be addressed. The collective knowledge of the scientific and technical community suggests that any study of the interface begins with a realization of the fact that the traditional picture of the two-dimensional nature of the interface is outdated. Realistically, the interface is a three-dimensional "interphase" (Figure 1) extending from some point in the adherend where the local properties begin to change from the bulk Thermal.Mechanical, Chemical Environment Bulk Adhesive Polymer Surface Layer Adsorbed Material Adherend Surface Layer Bulk Adherend Figure 1 Interphase. 31
32 properties, through the "interface," and into the adhesive where the local properties approach the bulk properties. Depending on the actual adhesive-adherend system, this interphase can extend from a few to a few hundred nanometers. On the adherend side it includes morphological alterations in the adherend near the surface as well as oxides either deliberately constructed or native to the adherend surface. This oxide can vary in porosity and microstructure from the regular columnar anodized type to natural oxides having a disordered morphology. Adsorbed gases may be present on the oxide surface, giving rise to unfavorable thermodynamic wetting of the surface by the adhesive, and/or these volatile species may be the source of voids in the formed adhesive bond. The polymeric adhesive is likewise affected by the presence of the adherend surface. Certain nonreactive adhesive components may be free to diffuse to the oxide surface in higher concentration than in the bulk. Primers used to protect the surface prepared for bonding can create structurally different regions and may contain corrosion-inhibiting components present in particulate forms. The polymeric network may also be altered in the region near the adherend surface, giving rise to a material different in composition and mechanical properties. This interphase is formed under the dynamic conditions existing during the processing of the adhesive bond. It is an integral part of the formed bond and is subject to the mechanical, chemical, and thermal environment experienced by the adhesive joint during its useful lifetime. Acceptance of this "interphase" as a working model of the actual joint interface serves as a framework for the systematic enumeration of the research needs that must be addressed to make the adhesive interface a functioning component of an adhesive bond and not a limiting factor to its performance in severe environments. SURFACE PREPARATION The logical beginning of the discussion of the interface is the surface preparation of the adherend. Because this step occurs before actual bond formation, characterization of the surface is straightforward. A spectrum of surface and bulk analytical techniques currently available can be used to characterize the adherend surface. Further research is required mainly to address certain generic questions that are applicable to a greater or lesser degree in all classes of potential adherends (i.e., metals, ceramics, and polymers), namely, what is the effect of adherend topography on bond performance, and can alter- native surface preparation techniques be used for adherend preparation? It is generally agreed that some form of topographical alteration in an adherend surface is necessary to achieve optimum bond performance. Traditionally this has meant electrochemical anodization for metals and surface roughening for nonmetals. Research is required to identify the topographical features that enhance adhesive joint performance and permanence. For example, what is the proper combination of oxide thickness, pore size, wall thickness, and oxide surface chemistry for a given application? Can nonaqueous methods be used to anodize and create a
33 mechanically stable and corrosion-resistant surface? Do conversion coatings offer a viable alternative to anodization methods for the generation of a mechanically, chemically, and thermally durable oxide? In a more short-term vein, can the currently used anodization methods be applied successfully to the new generation of structural materials, such as metal matrix composites, adherends prepared with powder metallurgical techniques, lithium alloy aluminums, titanium alloys for high-temperature applications, and structural steels? NATURE OF THE INTERPHASE Once the adherend surface has been prepared, actual formation of the bond occurs. During the application of temperature and pressure the adhesive becomes fluid and the interface forms. Fundamental research must be directed at quantifying the effect of the adherend on the adhesive and the adhesive on the adherend. Isolated investigations of the adhesive are not directly relatable to the performance of the adhesive in the bond. Specifically, what is the gradient of properties thought to exist in the adhesive, and how can it be quantified? Are chemical forces necessary for optimum adhesion or are physical forces sufficient? Can concepts such as the acid-base interaction approach or the use of surface-behavior diagrams be utilized to predict polymer-substrate interaction? How do adhesion promoters and/or coupling agents function structurally in the joint and as an enhancement to bond durability? The corollary to these materials questions is, from a micromechanics viewpoint, what effect can materials of differing composition existing in an interphase region only a few hundred nanometers thick have on the mechanical, chemical, and thermal responses of the adhesive joint? Although complete characterization of the interphase is not available, investigation of the micromechanics of a region of this size is highly desirable. Implicit in this need to identify the nature of the interphase is the ability to characterize the interphase as it exists in the joint. Some analytical characterization techniques exist that can provide the atomic and molecular information required to characterize the interphase. These, however, usually require the destructive separation of the joint for success. It is necessary to have the ability to interrogate nondestruc- tively the interphase as it exists in the joint. Some new approaches to this problem appear to have potential and warrant further development. A partial listing of these techniques would include neutron beam techniques, Rutherford backscattering, polarization modulation FTIR, nuclear magnetic resonance (NMR) tomography, and ultrasonic Rayleigh waves. Other analytical techniques not capable of in situ characterization but offering information for potential interphase characterization are internal reflec- tion spectroscopy, ultraviolet visible labeling, and ultramicrotomy in conjunction with analytical electron microscopy. Significant accomplishments in this area not only offer the possibility of providing the necessary information for materials characterization of the
34 interphase but also offer the hope of leading to a nondestructive method for inspecting and evaluating the adhesive joint during its lifetime. BOND DURABILITY The events that lead to bond formation occur within a very short time span. The functioning of the adhesive joint in a severe environment is expected to be on the order of years. Mechanisms of degradation with long time constants can be very important to a bond's durability in an adhesive joint. Research needs in this area involve kinetic and thermodynamic phenomena. Four factorsâwater, stress, temperature, and adhesive compositionâmust be addressed. The transport of water to the interface must be considered on two levels. First, the question of the importance of interfacial versus bulk diffusion of water must be addressed; second, at the molecular level are chemical factors or morphological considerations important in water transport. Of equal importance is the effect of water on the destruction of interfacial bonds. Identification of these mechanisms offers the possi- bility of creating a moisture-insensitive interface through proper materials selection and alteration of the adhesive-adherend combination. The effect of mechanical stress on the durability of an adhesive joint needs to be explored further. Research should explore the change in moisture transport with different stress levels and types. A better understanding of the mechanics of load transfer through the interphase region is required in order to optimize the interphase structurally. Elevated temperatures allow degradation mechanisms to operate that might not be favorable under ambient conditions. The synergism between water, stress, and temperature and its effect on the kinetics of interfacial degradation must be emphasized if a viable predictive model of long-term interfacial bond durability is to be developed. Finally, the components of the adhesive itself must be investigated further to determine their effect on bond durability. This investigation should include the effects of impurities, of processing additives, and of fillers.
