Influence of coating morphology on adhesive bonding of titanium pre-treated by plasma electrolytic oxidation

The effect of plasma electrolytic oxidation (PEO) pre-treatments on the adhesive bonding of titanium is investigated using lap-shear tests. Two pre-treatments are compared, using aluminate-phosphate and silicate-phosphate electrolytes respectively, which result in different compositions and morphologies of coatings. Specimens were joined with an epoxy adhesive. The PEO-treated specimens revealed mainly cohesive failure within the titanium-rich inner coating regions, which contrasted with the adhesive failure of untreated titanium. The silicate-phosphate pre-treatment resulted in the infiltration of a highly porous outer region of the coating by the adhesive. The fracture of the coatings occurred close to the titanium/coating interface and intersected the bases of relatively large pores, which in the case of the aluminate-phosphate coating lay beneath nodules that were evident at the coating surface. The pre-treatments did not significantly increase the lap-shear strength in comparison with untreated titanium.