The failures during oxidation of electron beam physical vapor deposition (EBPVD) and air plasma spray (APS) yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) on different bond coats, namely, platinum-modified aluminide and NiCoCrAlY, are described. It is shown that oxidation of the bond coats, along with defects existing near the TBC/bond coat interface, plays a very important role in TBC failures. Procedures to improve TBC performance via modifying the oxidation characteristics of the bond coats and removing the as-processed defects are discussed. The influence of exposure conditions on TBC lives is described and factors such as cycle frequency and thermal gradients are discussed.

No abstract

The high‐temperature behavior of high‐purity, low‐density (HP‐LD) air plasma sprayed (APS) thermal barrier coatings (TBCs) with NiCoCrAlY bond coats deposited by argon‐shrouded plasma spraying is described. The high purity yttria‐stabilized zirconia resulted in top coats which are highly resistant to sintering and transformation from the metastable tetragonal phase to the equilibrium mixture of monoclinic and cubic phases. The thermal conductivity of the as‐processed TBC is low but increases during high temperature exposure even before densification occurs. The porous topcoat microstructure also resulted in good spallation resistance during thermal cycling. The actual failure mechanisms of the APS coatings were found to depend on topcoat thickness, topcoat density, and the thermal cycle frequency. The failure mechanisms are described and the durability of the HP‐LD coatings is compared with that of state‐of‐the‐art electron beam physical vapor deposition TBCs.