21 Background

TBCs are complex engineering material systems consisting of 4 layers (Figure 1): substrate, bond coat, TGO and ceramic layer.[1-4] The composition and processing of each layer affect the durability of the TBC. A comprehensive review of the state-of-the-art has been carried out in preparation for this program. It has been concluded that significant improvement in TBC reliability and durability can be achieved by focusing on the bond coat and the TGO layers. Significant bond coat and TGO factors that determine TBC reliability and durability include:

Bond coat composition1-5-7 Bond coat deposition method[3'4] Post-bond coat processing[8'9] Bond coat creep strength[3'10] Bond coat defects[11,12]

Surface roughness[13,14]

TGO-bond coat residual stress[11,12,15-17] Composition / crystal structure of TGO[8'17-20] TGO to bond coat adherence (active elements effects)[21-34]

Of these bond coat and TGO factors, it is concluded that significant improvements in TGO durability and reliability can be achieved by focusing on (a) TGO perfection, (b) TGO stress and (c) TGO adherence in the program.

Bond Coat

Superalloy

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