Surface analysis of disbonded titanium/sol-gel/polyimide joints

Disbonded lap-shear specimens were analyzed to determine the locus of failure within bonded titanium (Ti) sol-gel polyimide joints. Bonded Ti alloys are being evaluated for use at an operating temperature of 175 °C. Determining the locus of failure for bonded Ti lap-shear specimens is part of a larger effort to develop durable, environmentally safe surface treatments for Ti alloys. Surface-treated Ti alloy (Ti-6Al-4V) plates are bonded in a standard lap-shear specimen configuration and exposed to temperature for specified intervals. The lap-shear bond joint consists of two etched Ti panels coated with a silicon and zirconium containing sol-gel, primed with a polyimide, and then bonded together with adhesive and supporting scrim material. The lap-shear specimens are tested for overall strength and failure modes. Specimens with adhesive failure modes were examined with x-ray photoelectron spectroscopy (XPS) to determine the composition of the bond joint failure layer. Analysis shows that the failure was located closer to the sol-gel/polyimide interface than to the Ti/sol-gel interface. Transmission electron micrographs (TEM) of the cross-sectioned joint confirmed the chemical distribution determined from the XPS data.