Principle and Practice to Achieve Improvements in TBC Thermal Cycle Lifetime

Abstract

As a critical technology; thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades; however; the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats; three new bond coat alloys have been designed and developed; and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys; such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase; absence of mixed / spinel oxides; and TGO self repair ability; which cannot be achieved by the existed MCrAlY alloys.