A comparison between a chromate and a non chromate surface treatment for adhesive bonding of a carbon fiber composite to titanium alloys

Presently, conventional surface treatment for adhesive bonding of Ti-6Al-4V to carbon fiber composites employs a hydrofluoric acid surface etch followed by strontium chromate coating. Both the acid-etch and chromate coating are environmentally unfriendly. The main objective of this research is to find an effective replacement to the conventional surface treatment that is environmentally friendly but maintains bond strength and bond durability. A two-step process involving an alkaline hydrogen peroxide treatment followed by a titanium-based treatment was proposed to replace the conventional treatment. Lap shear data from the these surface treatments indicated this new acid-free, chromate-free treatment had improved residual strength compared to conventional treatment after six weeks of salt spray exposure. In addition, inter-ply failure in the composite was the mechanism of failure for the new treatment while the conventional treatment separated at the substrate/chromate interface. The change in mechanism was thought due to surface modifications by the new treatment.