Harry E. Eaton

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

Abstract Silicon carbide fiber reinforced silicon carbide matrix composites (SiC/SiC CMCs) 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 CMCs will allow optimization for performance, efficiency, and/or emissions. However, SiC/SiC CMCs 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 CMCs 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.

Evaluation of CFCC liners with EBC after field testing in a gas turbine

Abstract Under the Ceramic Stationary Gas Turbine (CSGT) Program sponsored by the U.S. Department of Energy (DOE), a team led by Solar Turbines Incorporated has successfully designed engines, utilizing silicon carbide/silicon carbide (SiC/SiC) continuous fiber-reinforced ceramic composite (CFCC) combustor liners. Their potential for low NO x and CO emissions was demonstrated in eight field-engine tests for a total duration of more than 35,000 h. In the first four field tests, the durability of the liners was limited primarily by the long-term stability of SiC in the high steam environment of the gas turbine combustor. Consequently, the need for an environmental barrier coating (EBC) to meet the 30,000-h life goal was recognized. An EBC developed under the National Aeronautics and Space Administration high speed civil transport, enabling propulsion materials program was improved and optimized under the CSGT program and applied on the SiC/SiC liners by United Technologies Research Center (UTRC) from the fifth field test onwards. The evaluation of the EBC on SiC/SiC liners after the fifth field test with 13,937-h at Texaco, Bakersfield, CA, USA is presented in this paper.

EBC Protection of SiC/SiC Composites in the Gas Turbine Combustion Environment: Continuing Evaluation and Refurbishment Considerations

Silicon carbide fiber reinforced silicon carbide composites (SiC/SiC CMC’s) are attractive for use in gas turbine engines as combustor liner materials because the temperature capability allows for reduced cooling. This enables the engine to operate more efficiently and enables the design of very stringent emission goals for NOx and CO. It has been shown, however, that SiC/SiC CMC’s and other silica formers can degrade with time in the high steam environment of the gas turbine combustor due to accelerated oxidation and subsequent volatilization of the silica due to reaction with high pressure water (ref.s 1, 2, 3, & 4). As a result, an environmental barrier coating (EBC) is required in conjunction with the SiC/SiC CMC in order to meet long life goals. Under the U.S. Department of Energy (DOE) sponsored Solar Turbines Incorporated Ceramic Stationary Gas Turbine (CSGT) engine program (ref. 5), EBC systems developed under the HSCT EPM program and improved under the CSGT program have been applied to both SiC/SiC CMC coupons and SiC/SiC CMC combustion liners which have been evaluated in long term laboratory testing and in ground based turbine power generation. This paper discusses the continuing evaluation (see ref. 6) of EBC application to SiC/SiC CMC’s and the results from laboratory and engine test evaluations along with refurbishment considerations.Copyright

Evaluating Environmental Barrier Coatings on Ceramic Matrix Composites After Engine and Laboratory Exposures

SiC/SiC continuous fiber-reinforced ceramic matrix composite (CFCC) combustor liners having protective environmental barrier coatings (EBCs) applied to the liner working surfaces have been field-tested in a Solar Turbines’ Centaur 50S SoLoNOx engine at the Chevron, Bakersfield, CA engine test site. This latest engine test ran for a total of 13,937h. The EBCs significantly increased the lifetime of the in-service liners compared with uncoated CFCC liners used in previous field-tests. The engine test was concluded when a routine borescope inspection revealed the formation of a small hole in the inner liner. Extensive microstructural evaluation of both the inner and outer liners was conducted after removal from the engine. Post-test analysis indicated that numerous degradation mechanisms contributed to the EBC and CFCC damage observed on the liners, including EBC volatilization, sub-surface CFCC oxidation and recession, and processing defects which resulted in localized EBC spallation and accelerated CFCC oxidation. The characterization results obtained from these field-tested liners have been compared with the analyses of similarly-processed CFCC/EBCs that were laboratory-tested in a high-pressure, high temperature exposure facility (the ORNL “Keiser Rig”) for >6000h.Copyright

The Evaluation of CFCC Liners After Field Testing in a Gas Turbine — III

Under the Ceramic Stationary Gas Turbine (CSGT) Program and the Advanced Materials Program, sponsored by the U.S. Department of Energy (DOE), several silicon carbide/silicon carbide (SiC/SiC) combustor liners were field tested in a Solar Turbines Centaur 50S gas turbine, which accumulated approximately 40000 hours by the end of 2001. To date, five field tests were completed at Chevron, Bakersfield, CA, and one test at Malden Mills, Lawrence, MA. The evaluation of SiC/SiC liners with an environmental barrier coating (EBC) after the fifth field test at Bakersfield (13937 hours) and the first field test at Malden Mills (7238 hours) is presented in this paper. The work at Oak Ridge National Laboratory (ORNL) in support of the field tests was supported by DOE’s Continuous Fiber-Reinforced Ceramic Composite (CFCC) Program.Copyright

EBC Protection of SiC/SiC Composites in the Gas Turbine Combustion Environment

Silicon carbide fiber reinforced silicon carbide composites (SiC/SiC) are attractive for use in gas turbine engines as combustor liner materials, in part, because the temperature capability allows for reduced cooling. This enables the engine to operate more efficiently and to meet very stringent emission goals for NOx and CO. It has been shown, however, that SiC/SiC and other silica formers can degrade with time in the high steam environment of the gas turbine combustor due to accelerated oxidation and subsequent volatilization of the silica due to reaction with high pressure water (ref.s 1 & 2). As a result, an environmental barrier coating (EBC) is required in conjunction with the SiC composite in order to meet long life goals. Under the U.S. Department of Energy (DOE) sponsored Solar Turbines Incorporated Ceramic Stationary Gas Turbine (CSGT) engine program (ref. 3), EBC systems developed under the HSCT EPM program (NASA contract NAS3-23685) were applied to both SiC/SiC composite coupons and SiC/SiC combustion liners which were then evaluated in long term laboratory testing and in ground based turbine power generation, respectively. This paper discusses the application of the EBC’s to SiC/SiC composites and the results from laboratory and engine test evaluations.Copyright

Development and Evaluation of Environmental Barrier Coatings for Silicon Nitride

Environmental barrier coatings (EBCs) are required for applications of silicon nitride (Si3 N4 ) and silicon carbide (SiC) based materials in gas turbine engines because of the accelerated oxidation of Si3 N4 and SiC and subsequent volatilization of silica in the high temperature high-pressure steam environment. EBC systems for silicon carbide fiber reinforced silicon carbide ceramic matrix composites (SiC/SiC CMC’s) were first developed and have been demonstrated via long-term engine tests. Recently, studies have been carried out at United Technologies Research Center (UTRC) to understand the temperature capability of the current celsian-based EBC systems and its suitability for silicon nitride ceramics concerning thermal expansion mismatch between the EBC coating and silicon nitride substrates. This paper will present recent progress in improving the temperature capability of the celsian –based EBC systems and discuss their effectiveness for silicon nitride.Copyright

Advanced Environmental Barrier Coatings for SiC/SiC Composites

Environmental barrier coatings (EBCs) are being developed for silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composites to protect against accelerated oxidation and subsequent silica volatilization in high temperature, high-pressure steam environments encountered in gas turbine engines. Engine testing of three-layer barium strontium aluminosilicate (BSAS) has demonstrated a life of over 15,000 hours in a combustor liner application at a nominal temperature of 2200°F (1204°C). The engine field tests have shown that useful engine life is limited by BSAS recession and potential eutectic reactions between BSAS and silica. BSAS based coatings have also been shown to survive severe thermal gradient burner rig tests with 2700°F (1482°C) surface temperature and a 300°F (167°C) gradient through the coating. Promising EBC candidates for longer life and/or higher temperature applications include strontium aluminosilicate (SAS) based coatings.Copyright

Human Elements in the Recovery Program

PRESIDENT ROOSEVELT’S SecIretary of Agriculture, Henry Wallace, has set forth not only the general nature and purpose of the New Deal but also its implications and its difficulties, all in one brief paragraph. He did this about a month ago in a most appealing address before the Federated Council of Churches. Here is the paragraph: To enter the Kingdom of Heaven brought to earth and expressed in terms of rich material life it will be necessary to have a Reformation even greater than that of Luther and Calvin. I am deeply concerned in this because I know that the social machines set up by this Administration will break down unless they are inspired by men who in their hearts catch a larger vision than the hard driving profit motives of the past. More than that, the men in the street must change their atti-

Development and Evaluation of Environmental Barrier Coatings for Si-Based Ceramics

Environmental barrier coatings (EBCs) are being developed for silicon carbide (SiC) based composites and monolithic silicon nitride (Si3 N4 ) to protect against the accelerated oxidation and subsequent silica volatilization in high temperature, high-pressure steam environments encountered in gas turbine engines. While EBCs for silicon carbide (EBCSiC ) have been demonstrated in combustor liner applications, efforts are ongoing in the development of EBC systems for silicon nitride (EBCSiN ). The challenges of adapting EBCSiC to monolithic Si3 N4 are discussed in this paper. Progress in the area of EBCSiN including development and performance during field tests and tests simulating engine conditions are reviewed.Copyright