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

Titanium substrates coated with silicate/zirconate sol-gel and plasma sputt ered chromium have been adhered using a combined PETI-5 polyimide psuedo-th ermoplastic primer/adhesive system. Composite laminates were exposed to the rmal aging up to 2000 hours at 194 degreesC, subsequently interface analysi s was performed using nanoindentation to determine material modulus degrada tion and plastic deformation changes. Inhomogeneities at the interface mand ated that both low loads (as low as 25 muN) and a 90 degrees cube-corner di amond tip be utilized to obtain sub-micron resolution. Sol-gel coated and c hromium coated titanium substrates exhibited a pronounced step-wise gradien t across the interface dependent upon the indent load level and correspondi ng 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 i ncreased by approximately 15% with environmental exposure and at an exposur e 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 criti cal in predicting the overall life in adhesion joints within mission critic al aerospace structures. (C) 2001 Kluwer Academic Publishers.