Investigating Concepts for Multilayered Thermal and Environmental Barrier Coating Systems for Porous Matrix Oxide Fiber Ceramic Composites

Increasing the efficiency of gas turbine engines, both for aviation and commercial power generation, means higher operation temperatures and new materials capable of withstanding the increasingly extreme environments in the hot sections of these engines. Ceramic matrix composites have increased temperature capabilities, and particularly porous matrix oxide fiber ceramic composites (OFCCs) are promising materials due to their innate oxidative stability, resistance to thermal shock, cost, and ease of manufacture. However, OFCCs are not without limitations. The fibers and porous matrices have temperature limitations for microstructural stability, and the primary oxide constituents, Al2O3 and especially SiO2, have issues with corrosion and volatilization in high temperature water vapor, a major constituent of combustion atmospheres. Components require thermal and environmental barrier coatings (T/EBCs) with engineered structures and properties to maximize protection. This work studied techniques for building multilayered T/EBC systems for OFCCs and tests their efficacy. Two generations of air plasma sprayed (APS) coated OFCC coupons were supplied by Siemens Corporate Technology for durability assessment. These were compared against in-house coated OFCC coupons using electron beam-physical vapor deposition (EB-PVD) in thermocyclic testing from room temperature to 1200°C. EB-PVD coated samples performed better, with significant delamination seen in the APS coated specimens. The porous matrices of the OFCCs, while necessary for bulk damage tolerance and fiber pullout, complicates adhesion of these barrier coatings. Precursor impregnation and pyrolysis was used to infiltrate uncoated OFCC coupons with both alumina and yttria stabilized zirconia precursor solutions to create a gradient in density to selectively strengthen the matrix region near the surface to be coated, without sacrificing bulk toughness. The gradient was characterized using Vickers microhardness indentation tests of matrix pockets in cross-section. Further EB-PVD coating was performed on hardened, uncoated OFCCs, with additional compositions from 7wt% yttria stabilized zirconia (7YSZ) to more yttria rich compositions of Y4Zr3O12 (YZO) and Y2O3 with better matched thermal expansion to the OFCC. The latter compositions were deposited in bilayer configurations with a thin 7YSZ interlayer to prevent diffusional interaction with the OFCC. Despite morphological irregularities observed in 7YSZ and Y2O3 depositions, especially at deposition temperatures above 1080°C, adherent coatings with desirable columnar microstructures were grown in all three compositions, with deposition temperatures below 1035°C. Finally, select EB-PVD coated OFCCs were thermally cycled in flowing water vapor at 1200°C to assess durability of the coatings and amount of damage to the SiO2 – containing fibers from water vapor ingress through the porous coating. All coatings remained adherent, fiber damage from SiO2 volatilization was observed, especially around coating defects. Specimens with denser interlayers did mitigate some ingress, coating and OFCC surface defects still provided pathways for water vapor. The multilayer thermal and environmental barrier coating systems for OFCCs assembled and tested in this work provides insight into creating needed effective barrier coatings for the implementation of OFCCs into hot gas components of gas turbine engines