ROLE OF DEPOSITION TEMPERATURE IN THE MANUFACTURE OF PLASMA SPRAYED THERMAL BARRIER COATINGS

In the broad field of advanced materials, plasma sprayed Thernal Barrier Coatings (TBC's) are galning more and more importance as they are in the unique position of combinlng the toughness of a metallic substrate with the thermal lnsulatlon, wear, erosion and corrosion resistance of the ceramic overlayer. Moreover such desired properties can be taylored to the specific engineering application at hand, e.g. in heat engines, through adjustrnent of the manufacturing paraneters. Nowadays TBC's are more consolidated than monolithic new ceramics, but research efforts are still needed to improve their reliabÍtity with special reference to thermal shock resistance. In this paper original work concerning the effect of deposition temperature on both metal/ceramic adhesion and coating microstructure is presented. Investigations are carried out on an archetype sistem consisting of a Nimonlc 80A substrate and a NiCoCrAlY bonding layer and a CeO2 /Y2O3 partially stabilized zirconia top coat. Results indlcate that adhesion strength increases with temperature until a preferred range ls obtained. The microcracking pattern of the bulk of the coating, as dlsclosed by metallographic sections, is also affected by'the depositfon temperature thus interfering with crack propagation in servÍce. Additionally X-Ray Photoelectron Speotroscopy reveals that high cooling rates give rlse to Ce2O3 whereas Ce02 is favoured by increasing temperatures. Chemical reactions taking place in the coating during service can thus be pre-determined.