Ageing of adhesive bonds with various surface treatments, part 1: Aluminium-dicyandiamide cured epoxy joints

Ageing studies on adhesive-bonded aluminium-dicyandiamide cured epoxy joints prepared using a variety of different surface treatments were carried out using 100% humidity and cycling the joints between 42-48-42 degrees C every hour. The pretreatments include a chromic acid, silica/siloxane pretreatment, nonrinse chrome pretreatment, titanium/zirconium (Ti/Zr) pretreatment, and anodised substrate with no pretreatment. Dielectric spectroscopy measurements were used to characterise the rate of the water uptake and monitor the conversion of the surface oxide to hydroxide. These data were correlated with the changes that occurred in the mechanical strength of the bonds. The changes in the surface structure were observed using electron microscopy and elemental analysis conducted using X-ray photoelectron spectroscopy. The dielectric permittivity changes observed were similar for all the different pretreatments, indicating that the predominant process was water absorption. However, small differences were observed that reflect the different surface treatments used. Ageing at an elevated temperature of 70 degrees C provided definite evidence of hydration of the surface oxide layer. Electron microscopy of the fracture surface indicated oxide-to-hydroxide conversion and was reinforced by X-ray photoelectron spectroscopy. Predominantly, the changes in the mechanical strength observed at low temperature are consistent with the plasticization of the adhesive. However, at the elevated temperature of 70 degrees C, evidence for weakening of the interfacial layer by hydration becomes evident as a reduction in the mechanical strength. The dielectric measurements allowed the changes in the bond to be followed nondestructively. The mechanical strength of the etched-only pretreatment aged surprisingly well and at the lower temperature of ageing was comparable with the no-rinse chrome and titanium/zirconium pretreatments. The least durable was the PT2Cr-free treatment; however, all treatments showed a significant level of durability at low temperature.