Fracture behavior of an epoxy/aluminum interface reinforced by sol-gel coatings

The strengths of epoxy/aluminum joints reinforced with a zirconium-silicon based sol-gel adhesion promoter were investigated using an ADCB (Asymmetric Double Cantilever Beam) wedge test. The fracture energies and loci of failure of these joints were shown to depend upon the mixity of the normal and shear modes of stress acting at the crack. The ADCB geometry enabled the crack to propagate along the epoxy/aluminum interfaces so that the effect of surface pretreatment and the processing conditions of the adhesion promoter on adhesion strength could be directly evaluated. The dry strength of these joints depends on the thickness of the sol-gel film derived from different concentrations of the precursors. Thinner films are more fully crosslinked and thus give higher adhesion strengths than those obtained with thicker films. The differences in the wet strengths of the sol-gel reinforced joints for various surface pretreatments suggest that the sol-gel films are subject to moisture degradation with certain surface pretreatments. The loci of failure of many of these joints alternate between the sol-gel/aluminum and epoxy/sol-gel interfaces. This behavior is similar to that observed more generally in adhesively-bonded joints tested in DCB (Double Cantilever Beam) geometry. The brittle versus ductile behavior associated with the failure process reveals important information about how the sot-gel films affect the adhesion strength.