PLASMA SPRAY COATINGS AS TREATMENTS FOR ALUMINUM, TITANIUM AND STEEL ADHERENDS

Plasma spray coatings have been evaluated as surface treatments for al uminum, titanium and steel substrates prior to adhesive bonding. These treatments are environmentally benign in that they involve no chromat es and emit no liquid or gaseous wastes. The coatings can be engineere d for specific applications and are better suited for localized repair than chemical processes. For aluminum adherends, a 60Al-Si/40polyeste r 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 pe rformance to match that of phosphoric acid anodization. A Ti-6Al-4V co ating on titanium substrates exhibits identical initial strength and d urability to the best chemical controls under moderate temperature con ditions. At high temperatures, the plasma spray coating continues to e xhibit excellent durability while oxide-based treatments readily fail due to oxygen dissolution into the metal. For steel adherends, an Ni-C r-Zn coating provides enhanced corrosion resistance and bendability ev en after exposure to aggressive environments or ambient conditions ove r long periods of time. Additionally, rubber bonds with the plasma spr ay coating are more tolerant to surface contamination than those with grit-blasted surfaces. These investigations indicate that the plasma s pray process is more robust than conventional processes and can give e quivalent or (in some cases) superior performance. (C) 1997 by John Wi ley & Sons, Ltd.