Analysis of the spallation mechanism suppression in plasma-sprayed TBCs through the use of heterogeneous bond coat architectures

This paper critically examines the use of heterogeneous bond coats to increase the durability of plasma-sprayed thermal barrier coatings under spatially-uniform cyclic thermal loading. A major failure mechanism in these types of coatings involves spallation of the top coat caused by the top/bond coat thermal expansion mismatch concomitant with deposition-induced top/bond coat interfacial roughness, oxide film growth and creep-induced normal stress reversal at the rough interface’s peaks. The reduction of the top/bond coat thermal expansion mismatch aimed at increasing coating durability can be achieved by embedding alumina particles in the bond coat. Herein, we analyze the evolution of local stress and inelastic strain fields in the vicinity of the rough top/bond coat interface during thermal cycling, and how these fields are influenced by the presence of spatially uniform and non-uniform (graded) distributions of alumina particles in the metallic bond coat. The analysis is conducted using the higher-order theory for functionally graded materials which accounts for the high-temperature creep/relaxation effects within the individual TBC constituents. In the presence of two-phase bond coat microstructures, both the actual and homogenized bond coat properties are employed in the analysis in order to highlight the limitations of the prevalent homogenization-based approach applied to graded materials. The results reveal that the use of heterogeneous, two-phase bond coats, with spatially uniform or graded microstructures, while slightly suppressing the normal stress component evolution in the interfacial peak region, increases the magnitude of the shear stress component as well as the inelastic strain evolution in this region, thereby potentially promoting delamination initiation. The analysis based on homogenized bond coat microstructure produces misleading results relative to how the bond coat heterogeneity affects the magnitude of the normal and shear stress, and inelastic strain, components.