Nanoscale mechanical degradation of titanium/PETI-5 adhesive interface due to thermal exposure

Titanium substrates coated with silicate/zirconate sol-gel and plasma sputtered chromium have been adhered using a combined PETI-5 polyimide psuedo-thermoplastic primer/adhesive system. Composite laminates were exposed to thermal aging up to 2000 hours at 194°C, subsequently interface analysis was performed using nanoindentation to determine material modulus degradation and plastic deformation changes. Inhomogeneities at the interface mandated that both low loads (as low as 25 μN) and a 90°cube-corner diamond tip be utilized to obtain sub-micron resolution. Sol-gel coated and chromium coated titanium substrates exhibited a pronounced step-wise gradient across the interface dependent upon the indent load level and corresponding depth. Thermal aging produced an increase in both the PETI-5 primer and adhesive modulus by 20% and upwards of 30%, respectively. Sol-gel modulus increased by approximately 15% with environmental exposure and at an exposure level at 1000 hours, the chromium modulus increased approximately 20%. An decrease in plastic deformation resulting from thermal aging observed and reported, combined with material modulus alteration, is thought to be critical in predicting the overall life in adhesion joints within mission critical aerospace structures.