Influence of Bond Coat Roughness on the Microstructure and Mechanical Properties of EB-PVD TBCs

The surface condition of the bond coat is an important factor governing the growth and microstructural development of electron-beam physical vapour deposition (EB-PVD) TBCs. In this study, 7 wt.% yttria-stabilised zirconia (7YSZ) coatings were deposited by EB-PVD on vapour-phase aluminide bond coats with three distinct surface conditions, ranging from the as-coated state to ground and polished surfaces. The effect of bond coat roughness was evaluated through surface profilometry, scanning electron microscopy, X-ray diffraction, quantitative analysis of columnar architecture and porosity, scratch testing, and Vickers microindentation. Reducing the bond coat roughness resulted in a more uniform columnar architecture, reduced intercolumnar spacing, and a lower degree of column inclination, accompanied by a decrease in the coating surface roughness. XRD confirmed the formation of the expected tetragonal/cubic 7YSZ phase assemblage for all investigated conditions. Scratch testing revealed no delamination or spallation up to the maximum applied load of 200 N, indicating high interfacial integrity irrespective of bond coat roughness. Vickers measurements revealed a statistically significant through-thickness hardness gradient for all coatings, with hardness decreasing from the bond coat interface towards the column tips. Importantly, bond coat roughness had no statistically significant effect on hardness near the interface or in the central region, whereas significant differences were observed near the column tips. These results demonstrate that bond coat roughness strongly governs EB-PVD columnar architecture and surface morphology, while its influence on hardness is predominantly confined to the uppermost region of the coating.