Role of bond coat processing methods on the durability of plasma sprayed thermal barrier systems

Abstract

Metallic alloy bond coats are used as an interlayer to promote adhesion of ceramic Thermal Barrier Coatings (TBCs) to the superalloy substrates. These bond coats can be applied through a variety of processing means such as Air Plasma Spray (APS), High-Velocity Oxy Fuel spray (HVOF), and Vacuum Plasma Spray (VPS). While widely adopted in the industry, some uncertainties remain as to the specific microstructural attributes which are influenced by the processing routes and their implications on durability. This paper seeks to elucidate these subtleties between bond coat deposition processing routes through an integrated study of processing to connect to furnace cycle durability on a single bond coat chemistry of NiCoCrAlYHfSi. A standard porous APS TBC top coating based on yttria stabilized zirconia (7YSZ) was consistently applied on these distinct bond coats and tested for durability in cyclic furnace testing at 1100 °C. The results point to substantial differences in durability which are not readily explained only by the variances in coating roughness. Detailed assessment of deposited bond coat properties (modulus, thermal expansion, chemistry) provided some explanation relative to durability. The results suggest a complex interplay among roughness, compliance, aluminum diffusion pathway, and thermal expansion mismatch. Photo-stimulated luminescence piezospectroscopy (PLPS) provided additional insights enabling a comprehensive assessment of processing-performance linkages.