Thermal Barrier Coatings for the 21st Century

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) fabricated via electron beam physical vapor deposition (EBPVD) provide some unique properties for aerofoil applications. Such coatings are usually deposited on diffusion aluminide or MCrAlY bond coats on superalloy substrates. During deposition of the YSZ-TBC on the bond coat, a thermally grown oxide (TGO) consisting primarily of -Al2O3 is formed between the TBC and bond coat. The lives of these TBCs in oxidizing environments is determined by the interplay of the stored elastic energy driving spallation versus the interfacial toughness of the TGO/bond coat interface since failure of the TBC system often occurs at this interface. The microstructures of EBPVD-TBCs in the as-processed and exposed conditions have been documented using a variety of techniques to determine and describe the failure mechanisms. It is shown that the failure mechanisms for TBC’s are dependent on the bond coat fabrication procedure and the exposure environment. Finally, possible schemes for improving TBC systems are outlined.