Effect of Surface Morphology on Crack Growth at a Sol-Gel Reinforced Epoxy/Aluminum Interface

Abstract

The Boeing sol-gel conversion 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. A unique advantage of the sol-gel process is that strong and durable bonds are produced without the hazardous chemical usage and rinse-water requirements of conventional anodizing or etching processes. In this study, a fracture mechanics method was used to investigate the adhesion properties of sol-gel-reinforced epoxy/aluminum joints. The Hugh Brown asymmetric double cantilever beam (ADCB) wedge test was employed, which allowed the measurements of the critical energy-release rate, subcritical crack-growth kinetics, and threshold energy-release rate on a single sample in a reasonably short period of time. These experiments were carried out with aluminum substrates on which the surface morphology was systematically varied by polishing, sanding, grit-blasting, and chemical etching. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to identify the locus of failure. The surface morphology of the substrates was characterized with SEM, optical profilometry, and spreading kinetics. The macrorough structures drive the crack to within a thin epoxy layer close to the polymer/metal interface, which enhances the initial strength of the sol-gel-reinforced interface. The microroughness of the substrate is, however, more effective than the macroroughness in enhancing the durability. Lastly, an attempt has been made to correlate the energy-release rate with the fractal dimension for sol-gel-reinforced joints with macrorough substrates.

Keywords:

• ADCB wedge test
• Crack growth
• Durability
• Epoxy/aluminum
• Sol-gel coating
• Surface pretreatment
• Surface roughness

ACKNOWLEDGMENTS

Financial support for this work, provided by the Boeing Company and the Polymer Interface Center at Lehigh University, is gratefully acknowledged. We also thank R. A. Pearson of Lehigh University for helpful discussions, A. C. Miller of Lehigh University for his assistances in XPS studies, and Jacob Grob of the Boeing Company for helpful discussions and training on processes.

Notes

^a The substrates treated by this method are denoted as “grit-blasted + FPL” in the text.

^a R _a is the arithmetical mean deviation of the assessed profile. , where l is the sampling length and y is the altitude of the profile. R _t is the total height of profile, which is the sum of the height of the largest profile peak height and the largest profile valley depth within the evaluation length [32]

¹The most generally accepted definition of macroroughness and microroughness is given by Venables [27], where he defines macrorough surfaces as having characteristic bumps and jagged features about 1.0 µm or greater and microrough surfaces as having fine structure with dimensions 0.1 µm or less.

^a Calculated value based on epoxy formulation.

Note: Data were obtained on one sample for each surface preparation.

One of a collection of papers honoring Hugh R. Brown, who received The Adhesion Society Award for Excellence in Adhesion Science, Sponsored by 3M, in February 2006.