Thermal barriers to help handle the heat
Thermal barrier coatings (TBC) protect the metal surfaces in advanced gas turbine engine components from high temperature gases. They are bilayer or multilayer configurations comprising advanced ceramics which exhibit very low thermal conductivity plus the so-called bond coat between the ceramic and the underlying metal. The bond coat provides protection to the superalloy substrate from oxidation and hot corrosion attack. The oxidation of aluminium that is contained in the bond coat forms a thermally grown oxide (TGO). Yttria stabilized zirconia (YSZ) is used as the insulating layer, deposited by plasma spraying and electron beam physical vapour deposition (EBPVD) processes. Rare earth zirconates and co-doped zirconias have emerged as alternatives to YSZ due to their high melting point, low thermal conductivity, high temperature phase stability and good sintering resistance. The HIPERCOAT interdisciplinary research group united their forces in an effort to light the complex mechanisms and dynamics of these materials seeking enhanced erosion resistance and microstructure stability. Spallation (coating loss) is a common failure mechanism of TBC and occurs after a critical number of thermal cycles, due to thermal stress, erosion, and/or corrosion. Spallation failures often occur by crack propagation at or immediately above the TGO/TBC interface. Due to this an interlayer of YSZ of 50 µm thickness has been used between a novel TBC material and the TGO, acting effectively as a diffusion barrier at temperatures as high as 1200 °C. Analytical transmission electron microscopy demonstrated no significant bulk or boundary inter-diffusion between novel TBC materials based on rare-earth zirconates and the TGO. Moreover, the zirconate grows epitaxially on the YSZ columns, arising from EB-PVD deposition, causing no perturbations on the microstructure. The integrity of this interface suggests its durability in thermal cycling. HIPERCOAT advances the science base for multifunctional high temperature coatings guiding the evolution in the field. Applications are numerous throughout industry.
