Luis Isern¹, Koldo Almandoz Forcen¹, Christine Chalk¹, Gyaneshwara Brewster², Alan Johnstone², John Nicholls^1
1Cranfield University, Cranfield, United Kingdom
²Rolls-Royce Plc, Derby United Kingdom
Key aero-engine components are subject gas stream temperatures above the melting point of their metal alloy, a demanding environment that can only be survived thanks to the combination of cooling and protection from Thermal Barrier Coatings (TBCs). Electron-Beam Physical Vapour Deposition (EB-PVD) can deposit TBCs with a unique columnar microstructure that is strain compliant and ideal to survive in cyclic, high-strain, high-thermal load environments, such as those of the rotating parts of the high temperature turbine. TBCs based on yttria-stabilised zirconia (YSZ) are tough and effective, but they are also susceptible to sintering and chemical attack by calcium magnesium alumino-silicates (CMAS) at higher operating temperatures, which are required to improve engine efficiency. Rare Earth Zirconates (REZ) are postulated as potential YSZ substitutes due to their higher resistance to CMAS attack, lower thermal conductivity and high phase stability, although they also exhibit a lower toughness and more manufacturability challenges. This work focuses on two known systems (Gadolinium and Lanthanum Zirconate – GZ and LZ), a novel system (Neodymium Zirconate - NZ), and YSZ references and explores co-evaporation and use of mixed-ingot oxides to overcome the manufacturability challenges in EB-PVD. The columnarity, general microstructure, and uniformity of all systems has been evaluated, with special emphasis on the LZ system that, traditionally, results in heterogeneous composition and lack of columnarity. The use of simple computer models has helped to understand the underlaying mechanisms of these challenges. The performance of the resulting TBCs has been evaluated for CMAS attack and high velocity erosion, considering depth of infiltration and reactive formation of protective compounds for the former, and erosion rates, damage mechanisms and proposed erosion testing alternatives for the latter. Overall, NZ seems a promising system on-par or better than GZ.