EFFECT OF PALLADIUM-BASED UNDERCOAT ON THE FORMATION, STRUCTURE AND PROPERTIES OF DIFFUSION ALUMINIDE COATINGS

Coatings with Pd-Ni predeposit alloys have been processed using variou s aluminizing treatments: low and high activity cements, pack and vapo r phase coatings. Their metallurgical structure has been investigated with a special emphasis on the repartition of the various metallic ele ments throughout the coating section. During a first step, aluminizing of the palladium rich underlayer takes place, followed by simultaneou s diffusion of nickel and aluminum through this first aluminized layer . Palladium concentration profiles - which significantly differ when u sing L.A. or H.A. cements - provide qualitative indications concerning the overall ''activity'' of aluminum in the aluminizing process. Pall adium, if present at a sufficient level, enhances Al diffusion in the beta phase. In the case of L.A. processes, a consequence of this incre ase of Al diffusion flux is the location of NiAl reaction zone inside the coating.