Plasma surface treatment to improve fracture toughness of aluminium/aluminium foam composites

In the present study, the effect of a plasma treatment using nitrogen gas on the fracture characteristics of adhesively bonded aluminium foam/aluminium composites was investigated. Fracture tests were performed on three types of single lap shear test samples: untreated aluminium foam/aluminium (case 1), plasma treated aluminium foam/untreated aluminium (case 2) and untreated aluminium foam/plasma treated aluminium (case 3). Fracture toughness was determined by the work factor method. Results showed that the plasma treatment of aluminium foam and aluminium materials using nitrogen gas has a positive effect on the fracture toughness of aluminium foam/aluminium composites. Specifically, the fracture toughness values in cases 2 and 3 improved by about 150 and 200% respectively compared with those in case 1. The fracture surface analysis showed that an adhesive fracture occurred in both cases 1 and 2, whereas a cohesive failure occurred for case 3. The occurrence of cohesive fracture was a result of increased hydrophilicity on the aluminium surface caused by the plasma treatment, which led to a greater adhesion of epoxy to the surface.