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6 STATE OF THE ART: INTERFACES (INTERPHASES) Recent studies have demonstrated the importance of surface morphology in determining the integrity of metal-to-polymer bonds [Venables et al. 1979, Kinloch 1980]. In the case of aluminum and titanium, certain etching or anodization pretreatment processes produce oxide films on the metal surfaces that are extremely rough and porous on a microscopic scale. For example, using the high-resolution capabilities of the scanning transmission electron microscope (STEM), it has been observed that the Forest Products Laboratories (FPL) and phosphoric acid anodization (PAA) processes used for preparing aluminumoproduce surface oxide structures having microporous cells approximately 400 A wide. For titanium, the chromic acid anodization process developed by Boeing produces an oxide structure that is intermediate between the FPL and PAA oxide on aluminum in both thickness and roughness [Natan and Venables 1983]. Because of their porosity and microscopic roughness, these surfaces mechanically interlock with polymeric coatings, forming much stronger bonds than if the surfaces were smooth. Indeed, evidence has been obtained that this type of bond fails (in the absence of environmental effects) only when the polymer itself fails by viscoelastic deformation [Venables, in press]. In contrast, when the oxide lacks these morphological features and the bond strength is determined solely by chemical forces across the interface, separation can occur rather cleanly at stress levels that may be totally inadequate for structural applications. The long-term durability of metal-to-polymer bonds is determined to a great extent by the environmental stability (or lack of stability) of the same oxide that is responsible for promoting good initial bond strength (Kinloch and Abbey 1982; Venables, in press; Davis and Venables 1983). For aluminum, moisture intrusion at the bondline causes the oxide to convert to a hydroxide, with accompanying drastic changes in morphology. The resulting hydrated material adheres poorly to the aluminum beneath it, and therefore, once it forms, the overall bond strength may be severely degraded. For titanium, the evidence suggests that its oxides are much more stable than those of aluminum. However, preliminary evidence suggests that in very harsh environments the oxide, which is originally amorphous, undergoes a 17
18 polymorphic transformation to anatase; because of volume changes and accompanying morphology changes, this may lead to bond degradation, just as the oxide-to-hydroxide conversion process does for bonds to aluminum [Venables et al. 1979]. This transformation, which is temperature- and humidity-dependent, occurs much more slowly than that of the oxide-to- hydroxide transformation process on aluminum under similar environmental conditions. Nonetheless, since titanium may be used at much higher temperatures than aluminum, it would be of considerable interest to determine the importance of this potential degradation mechanism in the harsher environments in which titanium-to-polymer bonds may be expected to operate. Although it has been possible to define important factors that govern bond integrity and durability for bonds to aluminum and titanium, no similar concentrated effort appears to have been made to characterize other systems of potential interest to DOD or NASA applications. For example, even though there is considerable interest in bonding parasitic armor or low observable coatings to ship superstructures, Army tanks, etc., very few studies designed to improve the durability of bonds to steel have been made. Some recent investigations, however, have demonstrated that the wedge test, which has been used so successfully to evaluate the durability of aluminum- to-polymer and titanium-to-polymer bonds, is also applicable for steel adherends. In fact, preliminary work using the wedge test has demonstrated significant improvements in bond durability when certain conversion coatings are employed rather than the more conventional method of applying adhesives or primers directly over a grit-blasted steel surface [Trawinski et al. 1984]. Furthermore, it is of interest to note that, for steel, investigators have thus far emphasized surface preparation techniques that remove the oxide (e.g., grit-blasting or etching) rather than techniques that form microporous oxides as on aluminum or titanium. This emphasis undoubtedly arises because oxide coatings on most steels are quite unstable in the presence of moist environments. This great difference in stability between oxides on steel and those on aluminum or titanium therefore suggests that a completely different approach to surface preparation is needed for steel, and a better understanding of the behavior of steel-to-polymer bonds will be needed before consistent and reliable structural bonds can be made to this technologically important material. Polymer composites are another important class of materials for which adhesive bonding is used as a joining technique. Here, the nature of the material dictates a completely different approach for surface preparation than is used for metals. Currently, in the absence of an understanding of those factors that are important for developing good reliable bonds between composite structures, or between composites and metals, no standard procedures have been developed for optimizing the process. The situation has led to difficulties in which extremely small surface concentrations of contaminants have resulted in debonding at low stress levels. For example, it has been found that 5 percent monolayer coverage of silicone mold release agents (used to allow separation between the composite and the tool on which it is formed) can lead to severe reductions in the strength of composite-to- composite bonds [Hatienzo et al. 1983]. Considerable effort is clearly needed to reduce this sensitivity to contaminants.
19 CURRENT RESEARCH Within the past several years, there has been a dedicated effort to raise the scientific level of understanding of those factors that govern the stability of adhesive bond interfaces. The power of high-resolution STEMs for revealing the importance of microporous oxides in determining bond integrity has been demonstrated. The importance of sophisticated surface spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS), Auger, Rutherford back scattering (RBS), and Fourier transform infrared (FTIR) to study the stability of surfaces and to investigate means of further improving their stability has also been demonstrated. In addition, new approaches for examining interface stability, such as the surface behavior diagram (SBD), have been specifically developed as tools to aid in examining the interactions between interfaces, the environment to which they are exposed, and inhibitors designed to improve interface stability [Davis and Venables 1983]. REFERENCES Davis, G. D., and J. D. Venables. 1983. Surface and Interfacial Analysis in Durability of Structural Adhesives. Ed. by A. J. Kinloch. London, England: Applied Science Publications. Kinloch, A. J. 1980. The science of adhesion I. J. Mat. Sci. 15:2141. Kinloch, A. J., and W. Abbey. 1982. The Service Performance of Structural Adhesive Joints, p. 181. In Adhesion As a Basis for Joints. Bremen, West Germany. Matienzo, L. J., T. K. Shah, and J. D. Venables. 1983. Detection and Transfer of Release Agents in Bonding Processes, p. 604. Proceedings of the 15th National SAMPE Conference, Cincinnati, Ohio. Natan, M., and J. D. Venables. 1983. The stability of anodized titanium surfaces in hot water. J. Adhesion 15:125. Trawinski, D. L., D. K. McNamara, and J. D. Venables. 1984. Adhesive bonding to conversion coated steel surfaces. SAMPE Quarterly 15(3):6. Venables, J. D. 1984. Adhesion and durability of metal/polymer bonds. J. Mat. Sci. (In press). Venables, J. D., D. K. McNamara, J. M. Chen, T. S. Sun, and R. L. Hopping. 1979. Oxide morphologies on aluminum prepared for adhesive bonding. Appl. Surf. Sci. 3:88.
