Plasma Spray Coatings as Treatments for Aluminum, Titanium and Steel Adherends

Abstract

Plasma spray coatings have been evaluated as surface treatments for aluminum, titanium and steel substrates prior to adhesive bonding. These treatments are environmentally benign in that they involve no chromates and emit no liquid or gaseous wastes. The coatings can be engineered for specific applications and are better suited for localized repair than chemical processes. For aluminum adherends, a 60Al-Si/40polyester coating gives a performance equivalent to that of the best chemical treatment (phosphoric acid anodization) for some epoxy adhesives. With stronger, tougher adhesives, a Ti-6Al-4V coating provides improved performance to match that of phosphoric acid anodization. A Ti-6Al-4V coating on titanium substrates exhibits identical initial strength and durability to the best chemical controls under moderate temperature conditions. At high temperatures, the plasma spray coating continues to exhibit excellent durability while oxide-based treatments readily fail due to oxygen dissolution into the metal. For steel adherends, an Ni-Cr-Zn coating provides enhanced corrosion resistance and bondability even after exposure to aggressive environments or ambient conditions over long periods of time. Additionally, rubber bonds with the plasma spray coating are more tolerant to surface contamination than those with grit-blasted surfaces. These investigations indicate that the plasma spray process is more robust than conventional processes and can give equivalent or (in some cases) superior performance.