Principle and Practice to Achieve Improvements in TBC Thermal Cycle Lifetime

Thermal barrier coatings (TBC) are a critical enabling technology for both aero engines and industrial gas turbines. TBCs are commonly deposited by either EB-PVD on Pt modified nickel aluminide bond coats or air plasma sprayed (APS) on thermally sprayed MCrAlY bond coats. Detailed examination of crack paths in cross-sections of thermally cycled samples combined with statistical image analyses of fracture surfaces of APS TBCs has shown that APS TBCs predominately fail in the top coat. Cracks initiate and propagate along splat boundaries near the bond coat / top coat interface. Based on these observations, a simple phenomenological failure mode of APS TBC has been proposed: when the normal stress is larger than the strength of the top coat (σ>σ_f ), cracks initiate and propagate along splat boundaries during each thermal cycle, and the accumulated damage finally leads to coating spallation . TBC lifetime can be increased by either increasing the top coat strength, or reducing the normal stress, or both. The normal stress in a top coat during thermal cycling results from the CTE mismatch between metals and oxides (including grown TGO) and instability of bond coat by creep and martensitic phase transformations. Three new bond coat alloys have been developed that reduce normal stresses in the top coats. These new bond coats have achieved extremely long TBC coating system lifetimes in FCT testing.

Key words TBC, failure analysis, FCT, MCrAlY, stress