GROWTH MECHANISMS AND HOT CORROSION-RESISTANCE OF PALLADIUM MODIFIED ALUMINIDE COATINGS ON SUPERALLOYS

Recently palladium has generated interest as a precoating for aluminizing superalloys used in gas turbine blades. Little work has however been done on the mechanisms of formation of palladium modified aluminide coatings, and also on the constitution of the ternary Ni-Pd-Al system. The present work was initiated to fill this gap. It was first devoted to a study of the ternary phase diagram, with synthesis of alloys in order to evaluate the stability domains of the different known phases: Ni3Al, NiAl, Ni2Al3, Pd2Al, PdAl, Pd2Al3. Annealing of diffusion couples made with different alloys of the two binary Pd-Al and Ni-Al systems enabled to complete this study. Formation mechanisms of the coatings were also determined, with aluminization of binary PdNi alloys in different conditions (low and high aluminum activity). Resistance to hot corrosion of palladium modified aluminide coatings was evaluated with burner rig tests, and was compared to that of simple and platinum modified aluminide coatings.