58972 - Thermal Barrier Coating Applied to the Structural Shroud of the Inside-Out Ceramic Turbine 

Recuperated, high-temperature microturbines (< 1 MW) could prove to be a key component for the hybrid powertrains of tomorrow’s small aircraft. To achieve competitive thermal efficiency, a turbine inlet temperature (TIT) of 1550 K must be achieved, which is much higher than conventional metallic microturbine TIT. This calls for the introduction of refractory ceramics in microturbines and drives the development of the Inside-Out Ceramic Turbine (ICT). The ICT is a novel high-temperature turbine rotor design which relies on two key components: (i) monolithic ceramic blades held in compression against (ii) a structural rotating shroud assembly. The shroud is comprised of a tape-wound carbon-polymer hoop – used for its high specific strength – maintained at operating temperature by a shrink fit cooling ring. The cooling ring is air-cooled and must achieve a radial temperature gradient greater than 800 °C. To assist in achieving this gradient, thermal barrier coating (TBC) is evaluated to reduce cooling flow requirements and ultimately increase TIT.

This study focuses on the applicability of TBC to the internal surface of the ICT cooling ring. The cooling ring undergoes two main loadings: (i) significant hoop strain and (ii) blade indentation as centrifugal loading increases. A preliminary experimental assessment was done at room temperature on 1 mm-thick coatings of 8% yttria-stabilized zirconia (8YSZ), air plasma sprayed (APS) TBC, with a NiCrAlY bond coat applied to Inconel 718 test coupons. Strain resistance was evaluated through tensile testing with digital image correlation, and indentation resistance was tested with macro-indenters under compressive loading, followed by µCT and confocal microscopy. Finally, an Inconel 718 cooling ring was coated with 0.75 mm 8YSZ TBC for validation in a downscaled ICT operating environment.

Results show that the strongly orthotropic behaviour of TBC is tailored for use in the ICT. In-plane strain tolerance allows large tangential deformation of the structural shroud, with complete spalling of the TBC occurring at a strain of 1 %, or substrate yield. High out-of-plane stiffness and compressive resistance combine to support extreme compressive loading, with no apparent damage to the TBC under 1 GPa, and failure occurring at 4 GPa, much greater than the typical 300 MPa blade indentation average loading. A reduction of 50 % in cooling flow was noted in the 5-minute ICT field test, with no damage to the TBC.