Abradable Seal Development for Ceramic Matrix Composite (CMC) Shrouds in High Performance Turbine Engines

Currnet Ni-based metallic turbine shrouds, coated with ceramic abradable materials, are limited to temperatures not exceeding 2,100 F. Integrated High Performance Turbine Engine Technology (IHPTET) demands gas turbine hot sections with operational temperatures up to 2,500 F (1,370 C). Ceramic fiber reinforced ceramic matrix composites (CMCs) such as SiC/RBSN are candidate materials for these high temperature turbine shroud applications. However, slight rub of the shrouds with rotor blades can cause fiber damage/breakage leading to a premature component failure. An abradable coating on the CMC shroud will wliminate this problem and miantain the integrity of both the stationary and rotating parts. We propose three candidate coating materials/systems for evaluation as abradable seals. All three coating materials have low densities, high temperature capability, good oxidation resistance, and thermal expansion match to the SiC/RBSN substrate material. Low pressure plasma spraying (LPPS) will be used to deposit these coatings on the CMC substrates. LPPS is a proven, cost-effective technology extensively employed in the turbine industry. Compared with conventional air plams spray, LPPS offers the advantages of large plasma volume, no contamination and high particle velocities. As a result, the substrate material, placed at relatively larger spray distances, will not degrade thermally during the plasma spray deposition, and the coatings will have strong adhesion to the CMC substrate. With a careful control of the porosity, composition and themicrostructure, these coatings are expected to havce excellent abradability and good thermal shock resistance and thus meet the atated performance goals.