Adhesion and fracture of a sol-gel reinforced polymer/metal interface

The Boeing sol-gel coating (Boegel-EPII), derived from an acid-catalyzed aqueous solution of organofunctional silane and zirconium alkoxide precursors, is being used as an adhesion promoter for adhesive bonding and painting applications in the aerospace industry. In this study, a fracture mechanics approach is used to investigate the adhesion properties and fracture behavior of a sol-gel reinforced epoxy/aluminum joint. An asymmetric double cantilever beam (ADCB) wedge test is employed which illustrates that both the fracture energy and locus of failure depend upon the mode mixity. The study starts with investigating the quasi-static fracture behavior of the sol-gel reinforced epoxy/aluminum interface under dry and wet conditions. Various loci of failure are observed for different surface pretreatments and sol-gel processing conditions. Strong experimental evidence shows that the room-temperature-dried sol-gel film is partially cross-linked and subject to moisture degradation. To further investigate the hydrolytic stability of the sol-gel, the ADCB test is modified such that the critical fracture energy, crack growth kinetics, and threshold fracture energy can be measured on a single sample. The effect of surface pretreatment on moisture-assisted crack growth is then studied in detail. It is found that the macro-roughness of the substrate enhances the initial strength of the sol-gel reinforced interface, whereas the micro-roughness is more effective than the macro-roughness in enhancing the durability. The fractal dimension of the macro-rough substrates shows a good correlation with the fracture energies. The following studies on sol-gel processing conditions show that the sol-gel drying time, film thickness and drying humidity all have a considerable effect on moisture-assisted crack growth of the sol-gel reinforced joints. Analysis of locus of failure reveals that most of the observed trends on adhesion kinetics can be explained in terms of interdiffusion of the sol-gel film and epoxy. The interdiffusion strongly depends on the degree of condensation of the sol-gel film, which is directly affected by the sol-gel processing conditions. Lastly, the environmental effects on crack growth at the sol-gel reinforced joint are investigated.