Rare earth silicates have been investigated to evaluate their potential as an advanced environmental barrier coating (EBC) having higher temperature capability than current EBCs. Volatility data in high steam environments indicate that rare earth monosilicates (RE 2 SiO 5 ; RE: rare earth element) have lower volatility than the current barium strontium aluminum silicate (BSAS) EBC top coat in combustion environments. Rare earth silicates also exhibit superior chemical compatibility compared to BSAS. The superior chemical compatibility and low volatility are key attributes to achieve higher temperature capability. The chemical compatibility is especially critical for EBCs on Si 3 N 4 because of the high chemical reactivity of some of the oxide additives in Si 3 N 4 . In simulated combustion environments, EBCs with a rare earth monosilicate top coat exhibit superior temperature capability and durability compared to the current state-of-the art EBCs with a BSAS top coat.

Rare earth silicate environmental barrier coatings for SiC/SiC composites and Si3N4 ceramics

Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2O3-2SiO2) or mullite + BSAS (1-xBaO-xSrO-Al2O3-2SiO2) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, silica volatility, phase stability, and thermal conductivity. Variants of this family of EBCs were applied to monolithic SiC and melt infiltrated SiC/SiC composites. Reaction between BSAS and silica results in low melting (approx. 1300 C) glasses at T > 1400 C, which can cause the spallation of the EBC. At temperatures greater than 1400 C, the BSAS top coat also degrades by formation of a porous structure, and it suffers significant recession via silica volatilization in water vapor-containing atmospheres. All of these degradation mechanisms can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. There was evidence of rapid sintering at temperatures as low as 1300 C, as inferred from the sharp increase in thermal conductivity.

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Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

Functional coatings are widely used in energy generation equipment in industries such as renewables, oil and gas, propulsion engines, and gas turbines. Intelligent thermal spray processing is vital in many of these areas for efficient manufacturing. Advanced thermal spray coating applications include thermal management, wear, oxidation, corrosion resistance, sealing systems, vibration and sound absorbance, and component repair. This paper reviews the current status of materials, equipment, processing, and properties’ aspects for key coatings in the energy industry, especially the developments in large-scale gas turbines. In addition to the most recent industrial advances in thermal spray technologies, future technical needs are also highlighted.

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Advances in Thermal Spray Coatings for Gas Turbines and Energy Generation: A Review

Silicon carbide fiber reinforced silicon carbide matrix composites (SiC/SiC CMC's) are attractive materials for use in gas turbine hot sections due to the potential for high temperature mechanical properties and overall lower density than metals. Potential SiC/SiC CMC gas turbine components include combustion liners, and turbine shrouds, vanes, and blades. Engine design with SiC/SiC CMC's will allow optimization for performance, efficiency, and/or emissions. However, SiC/SiC CMC's are silica formers under oxidizing conditions and have been shown experimentally to undergo accelerated oxidation due to exposure to steam in high temperature combustion environments such as found in the gas turbine hot section. Oxidation by steam in a flowing gas stream has been shown to exhibit paralinear behavior and result in unacceptable recession of the surface. Thus, prior to the successful introduction of SiC/SiC CMC's for long life use in gas turbines, the problem of accelerated oxidation needs to be addressed and resolved. To this end, one approach has been the development of the environmental barrier coating (EBC) to prevent accelerated oxidation by limiting oxidant access to the surface of the silica former. This paper will review the accelerated oxidation of silica formers such as silicon carbide, the experimental testing confirming the problem, and EBC approaches resolving the problem.

Accelerated oxidation of SiC CMC's by water vapor and protection via environmental barrier coating approach

At high temperatures SiC and Si3N4 react with water vapor to form a silica scale. Silica scales also react with water vapor to form a volatile Si(OH)4 species. These simultaneous reactions, one forming silica and the other removing silica, are described by paralinear kinetics. A steady state, in which these reactions occur at the same rate, is eventually achieved, After steady state is achieved, the oxide found on the surface is a constant thickness and recession of the underlying material occurs at a linear rate. The steady state oxide thickness, the time to achieve steady state, and the steady state recession rate can all be described in terms of the rate constants for the oxidation and volatilization reactions. In addition, the oxide thickness, the time to achieve steady state, and the recession rate can also be determined from parameters that describe a water vapor-containing environment. Accordingly, maps have been developed to show these steady state conditions as a function of reaction rate constants, pressure, and gas velocity. These maps can be used to predict the behavior of silica formers in water-vapor containing environments such as combustion environments. Finally, these maps are used to explore the limits of the paralinear oxidation model for SiC and Si3N4

Oxidation and Volatilization of Silica-Formers in Water Vapor

SiC and Si3N4 materials were tested under various turbine engine combustion environments, chosen to represent either conventional fuel-lean or fuel-rich mixtures proposed for high speed aircraft. Representative CVD, sintered, and composite materials were evaluated in both furnace and high pressure burner rig exposure. While protective SiO2 scales form in all cases, evidence is presented to support paralinear growth kinetics, i.e. parabolic growth moderated simultaneously by linear volatilization. The volatility rate is dependent on temperature, moisture content, system pressure, and gas velocity. The burner tests were used to map SiO2 volatility (and SiC recession) over a range of temperature, pressure, and velocity. The functional dependency of material recession (volatility) that emerged followed the form: exp(-Q/RT) * Px * vy. These empirical relations were compared to rates predicted from the thermodynamics of volatile SiO and SiOxHv reaction products and a kinetic model of diffusion through a moving ...

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SiC and Si3N4 Recession Due to SiO2 Scale Volatility Under Combustor Conditions

Two commercially available additive-containing silicon nitride materials were exposed in four environments which range in severity from dry oxygen at 1 atm pressure, and low gas velocity to an actual turbine engine. Oxidation and volatilization kinetics were monitored at temperatures ranging from 1066 to 1400 C. The main purpose of this paper is to examine the surface oxide morphology resulting from the exposures. It was found that the material surface was enriched in rare earth silicate phases in combustion environments when compared to the oxides formed on materials exposed in dry oxygen. However, the in situ formation of rare earth disilicate phases offered little additional protection from the volatilization of silica observed in combustion environments. It was concluded that externally applied environmental barrier coatings are needed to protect additive-containing silicon nitride materials from volatilization reactions in combustion environments. Introduction Si3N4 is proposed for use as components, such as vanes, in turbine applications. Tens of thousands of hours of life are needed for both land-based turbines and aeropropulsion applications. Additive-containing SisN4 materials are

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Additive Effects on Si3n4 Oxidation/Volatilization in Water Vapor

Three SiC/BN/SiC composite specimens reinforced with different SiC fibers (Sylramic, Sylramic-iBN, and Hi-Nicalon Type S) were exposed in a combustion environment. Exposures were carried out for 151 h in a fuel-lean high pressure burner rig at 0.9 MPa total pressure, sample temperatures near 1573 K, and a gas velocity of 15 m/s. Weight loss of all three composites was observed. Extensive oxidation of SiC fibers was observed in cracked locations. A mechanism based on borosilicate enhanced oxidation coupled with volatilization of boria is described. Ramifications of this degradation mechanism are discussed for long-term applications of SiC/BN/SiC composites in combustion environments.

Borosilicate Glass‐Induced Fiber Degradation of SiC/BN/SiC Composites Exposed in Combustion Environments

Vane subelements were fabricated from a silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composite and were coated with an environmental barrier coating (EBC). A test configuration for the vanes in a gas turbine environment was designed and fabricated. Prior to testing, finite element analyses were performed to predict the temperatures and stress conditions present in vane during rig testing. This paper discusses the test configuration, the finite element analysis predictions, and results of the vane testing.

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Ceramic Matrix Composite Vane Subelement Testing in a Gas Turbine Environment

The use of ceramic matrix composites (CMCs) as vanes for the next generation of turbine engines is under evaluation for improving engine performance, such as lowering emissions and enabling higher cycle efficiency, relative to today's engines with superalloy hot section components. Because of the high-temperature capability of this class of materials, CMC vanes would be able to operate with higher combustion exit temperatures than today's engines can. Alternatively, a potential vane cooling requirement reduction of 15 to 25 percent for a CMC, such as SiC/SiC, relative to a single-crystal superalloy would be realized if the combustion operation was not altered.

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R. Craig Robinson

Papers

SiC and Si3N4 Recession Due to SiO2 Scale Volatility Under Combustor Conditions

Additive Effects on Si3n4 Oxidation/Volatilization in Water Vapor

Borosilicate Glass‐Induced Fiber Degradation of SiC/BN/SiC Composites Exposed in Combustion Environments

Ceramic Matrix Composite Vane Subelement Testing in a Gas Turbine Environment

Ceramic Matrix Composite Vane Subelements Tested in a Gas Turbine Environment