Thermal Barrier Coating Applied to the Structural Shroud of the Inside-Out Ceramic Turbine: A First Experimental Assessment of the Benefits of Orthotropic Behaviour

The Inside-Out Ceramic Turbine (ICT) rotor configuration enables the use of monolithic ceramic blades in a sub-MW turbine rotor. The ICT rotor is comprised of individual ceramic blades that slide along grooves in a hub, supported at their tip by simple contact against a structural shroud. Supporting the blades at the tip and freeing the root converts centrifugal forces to compressive loading, thus capitalizing on ceramics’ compressive strength and refractory properties, meanwhile avoiding tensile loading. This should eventually enable a recuperated cycle ICT to withstand temperatures up to 1275 °C, allowing it to reach a thermal efficiency of 40 % at 3 kW/kg power density, paving the way to high-efficiency hybrid aircraft at the sub-MW scale.

The structural shroud in the ICT consists of two layers: a) an outer, high-strength, filament-wound, carbon-polymer composite rim, cooled by b) an inner, shrink-fit, metallic air-cooled ring. The high-temperature ceramic blades are in direct contact with the cooling ring, therefore the channels in the cooling ring act as a thermal buffer zone between the high-temperature ceramic blades and the low-temperature composite rim. The cooling ring must force a radial temperature drop of roughly 1000 °C between the hot gases and the composite rim. The use of a cooling ring presents a twofold design challenge: 1) the heat flux requires an important amount of cooling air, estimated at 3 % of the main flow at 500-kW scale, which costs net efficiency points and cools the ceramic blade tips, causing 2) important contact forces between the relatively cool ceramic blades and the very hot metallic cooling ring, inducing local tensile loads at the blade tip due mainly to the thermal expansion mismatch at the ceramic-to-metal interface. The addition of a third layer to the shroud, i.e. c) a thermal barrier coating (TBC) applied to the inner skin of the cooling ring, significantly reduces the total heat transfer to the outer layers of the shroud. By impeding heat transfer at the interface between the blades and cooling ring, TBC helps in 1) cutting down on the thermal gradient required in the metallic cooling ring, thus reducing cooling flow for a given operating temperature and 2) keeping the cooling ring cooler and the blades hotter, thus reducing the thermal expansion mismatch.

TBC is by and large the simplest way to integrate some form of ceramics in traditional full-metal turbine rotors. The mechanical loading is typically a body force or thermally-induced strain. In the ICT, TBC is applied to a single surface, the cooling ring inner skin, and thus undergoes the same hoop strain and the high-pressure radial contact with the ceramic blade tips. In order to gain a first appreciation of how well TBC could perform under these loadings, air plasma spray (APS) TBC was applied to two substrate samples, Ti-6Al-4V and Inconel 718, for testing tensile properties and indentation resistance at room temperature. The platelet structure that confers APS TBC its orthotropy was found to be tailored to the ICT application: low in-plane equivalent modulus of elasticity means it can undergo significant hoop strain without spalling, and high out-of-plane compressive strength means it can support the blade’s centrifuged weight without failing. Preliminary testing in the ICT rotor at a TIT of 900 °C corroborates these results. Further work is needed to identify whether electron beam physical vapour deposition (EB-PVD) TBC could be a better candidate than APS TBC, as well as truly quantify TBC applicability limitations and its benefits to the ICT configuration, in terms of decreasing cooling mass flow rate and probability of failure in the monolithic ceramic blades.