Microstructural Evolution of Environmental Barrier Coatings in Combustion Environments
Abstract
SiC-based ceramic composites are now being utilized in hot-section components of aircraftturbine engines. Environmental Barrier Coatings (EBCs) are required to protect the underlyingceramic composite from reactions with water vapor in combustion environments. These coatingsare typically fabricated from complex silicates for chemical and thermal expansion compatibilitywith the SiC. The EBCs themselves, however, are still prone to react with steam in thecombustion environment, depleting the surface in silica, and leaving a porous oxide that is morethermochemically stable. The microstructure of the porous oxide reaction layer evolves withtime, increasing in depth, with corresponding changes in crystallographic faceting, sintering, andcoarsening. These processes have not yet been studied and provide a strategy for design of morestable EBC surfaces.The objective of this work is to characterize the microstructural evolution of three silicates(Yb2Si2O7, HfSiO4, and barium strontium aluminosilicate) in high temperature (1200-1400~C),high velocity (150-170m/s), 1 atm steam utilizing a steam-jet furnace. The oxides are allrelevant as EBCs and were chosen as the expected silica-depleted reaction products span a rangeof melting temperatures, thermochemical stabilities, compositional complexity, and crystalstructures. Scanning electron microscopy, energy dispersive spectroscopy, x-ray diffractionanalysis, transmission electron microscopy, and x-ray photoelectron spectroscopy will all beused to characterize observed phase changes, compositional changes, nucleation rates, andmorphological changes of the surface oxide. These changes will be related to temperature, time,water vapor pressure, and gas velocity to elucidate reaction mechanisms and enable improvedlife prediction. Volume change due to the surface reaction, rates of sintering and coarsening,faceting, and crystallographic orientation will all be monitored to determine those factors thatmost affect microstructural evolution. The relative importance of these factors will be assessedas parameters for design of EBCs with improved durability in combustion environments.Development of EBCs with improved thermochemical stability is relevant for future navalcapability of aircraft turbine engines with increased efficiencies, longer lifetimes, and moreaccurate life prediction of hot-section components. The project will involve one graduate studentwho will be trained in the area of high temperature materials, an area of expertise critical to thefuture needs of Navy and original equipment manufacturers.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Mar 03, 2017
- Source ID
- N000141712280
Entities
People
- Elizabeth Opila
Organizations
- Office of Naval Research
- United States Navy
- University of Virginia