Recuperated, high-temperature microturbines (< 1 MW) could be a key enabler for hybrid powertrains of tomorrow’s small aircraft. To achieve competitive thermal efficiencies, turbine inlet temperature (TIT) must increase to 1550 K, well beyond conventional metallic microturbine limits. This calls for high-temperature refractory ceramics, which call for a new ceramic-specific, microturbine design like the Inside-Out Ceramic Turbine (ICT). This study focuses on the applicability of a refractory thermal barrier coating (TBC) to the internal surface of the ICT cooling ring. By cutting the heat transfer from the main flow to the structural rim-rotor, the use of a refractory TBC coating in an ICT enables higher TIT and lower cooling air mass flow.
A preliminary experimental assessment is done at room temperature on 1 mm-thick coatings of 8% yttria-stabilized zirconia (8YSZ), air plasma sprayed (APS) TBC, applied to Inconel 718 and Ti64 test coupons. Results show that the strongly orthotropic behaviour of the tested TBC fits perfectly with the deformation mechanics of the ICT configuration under load. First, large in-plane strain tolerance allows the large tangential deformation imposed by the structural shroud under centrifugal loading. Second, high out-of-plane stiffness and compressive resistance combine to support extreme compressive loads with no apparent damage to the TBC even at more than 3 times blade indentation average loading. An experimental demonstration on a small-scale prototype shows a reduction of 40% in cooling flow in a, 8-minute ICT test, with no damage to the TBC, proving the effectiveness and potential of the proposed TBC design.