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Accelerated oxidation of SiC CMC’s by water vapor and


Journal of the European Ceramic Society 22 (2002) 2741–2747 www.elsevier.com/locate/jeurceramsoc

Accelerated oxidation of SiC CMC’s by water vapor and protection via environmental

barrier coating approach
Harry E. Eaton*, Gary D. Linsey
United Technologies Research Center, 411 Silver Lane, East Hartford, CT 06108, USA

Abstract Silicon carbide ?ber 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, e?ciency, 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 ?owing 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 con?rming the problem, and EBC approaches resolving the problem. # 2002 Published by Elsevier Science Ltd.
Keywords: Composites; Corrosion; Engine components; Environmental barrier coating; SiC

1. The problem—accelerated oxidation of SiC in steam In 1949 and 1951, A.C. Lea1 studied and reported on the rate of oxidation of silicon carbide and noted that steam caused the oxidation rate to be accelerated compared to the rate in dry oxygen. The volatilization of silica in an air-steam atmosphere was presented as a possible mechanism to explain the observation. Almost 50 years later, Opila and Hann,2 Opila et al.,3 and Opila et al.4 studied in detail and presented a model explaining the accelerated oxidation of silicon carbide due to high temperature exposure to steam. Their work showed that, on exposure to steam, oxidation of silicon carbide occurs by a paralinear rate process involving oxidation of silicon carbide by H2O to form silica and then volatilization of the silica by reaction with H2O to form Si–O– H(g) species [see Eqs. (1) and (2)]. For overall paralinear behavior, the silica layer which forms on the substrate due to oxidation occurs by a parabolic rate process while the volatilization of the silica occurs by a linear
* Corresponding author. Tel.: +1-860-610-7414; fax: +1-860-6107879. E-mail address: eatonhe@utrc.utc.com (H.E. Eaton).

rate process. Over long times the growth rate process of silica formation is balanced by the volatilization process at which time further oxidation of the substrate is controlled simply by the linear volatilization rate. The silica scale reaches a steady state thickness of approximately 10 microns for temperatures in the range of 1200– 1400  C. This is contrasted with oxidation under dry conditions, whereby oxidation is governed by a parabolic rate process involving the formation and growth of a stable, protective silica scale on the surface of silicon carbide. SiC ? 3H2 O?g? ? SiO2 ? 3H2?g? ? CO?g? SiO2 ? H2 O?g? ? Si-Ox -Hy?g? ?1? ?2?

Further work by these authors modeled the volatilization process and showed that the ?ux of the volatile species is controlled by di?usion through a boundary layer. For a ?at plate geometry and for laminar ?ow, the ?ux is predicted to be dependent on the gas velocity and pressures as shown in Eq. (3) for when the volatile silicon species is Si(OH)4 which is the predicted dominant species for fuel lean, gas turbine combustion environments.

0955-2219/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S0955-2219(02)00141-3

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? ?2 JSi?OH?4 / v1=2 ? PH2 O =?Ptotal ?1=2

?3?

where J is the ?ux, v is the gas velocity and P is either the total system pressure or partial pressure of steam. Experimental work by Robinson and Smialek5 in a high pressure combustion test facility measured SiC recession versus test conditions and con?rmed Eq. (3) for lean combustion conditions. Fig. 1 summarizes the results for lean burn conditions over the temperature range of approximately 1200–1450  C. Table 1 provides predictions based on the experimental data for long term recession of SiC in a combustion environment. The predicted recession at 1200  C of 270 microns in 1000 h in a combustion environment for SiC is too signi?cant to ignore for a material with expected useful life 30,000 h in a proposed application such as a industrial gas turbine. Con?rmation of this behavior in actual engine environments has been observed in a Solar Turbines, Inc. Centaur 50S industrial gas turbine6 and in an Allison M501-K industrial gas turbine.7 The Solar Turbines, Inc. engine was run with SiC CMC combustion liners at nominally 1200  C for 5018 h. Recession values of up to $ 2200 microns were measured. This recession rate is roughly 0.44 m per hour to result in a 1000 h calculated recession of 440 m which is similar to the predicted loss of 270 m in 1000 h at 1200  C from Table 1. Fig. 2 is a view of the liner in cross section at two di?erent locations.

Fig. 3 shows a plot of predicted (lines) and measured recession (points) at the mid-span position along the trailing edge of Allison silicon nitride ?rst turbine vanes versus time in an Allison M501-K industrial gas turbine.7,8 The predicted recession values are based on the work from Refs. 5 and 9. This is another example of the e?ect of steam exposure based on actual engine experience. The fact that it covers silicon nitride illustrates the generic problem probably facing all silica formers in a high temperature, high steam combustion environment. Additional supporting documentation of the e?ects of steam exposure can be found in Refs. 10–16.

2. Environmental barrier coatings (EBC’s) Work to understand the mechanisms of accelerated oxidation of SiC and to document the behavior in experimental testing and actual engine environments led

Fig. 2. Cross sectional view of Solar Turbines, Inc SiC CMC Centaur 50S combustion liner after 5018 h operation (see Ref. 6). Fig. 1. Experimental recession measurements of SiC in a high pressure combustion environment (see Ref. 5).

Table 1 Predicted recession of SiC under lean burn combustion conditions (see Ref. 5) Predicted lean burn recession (mm) T ( C) 1000 1100 1200 1300 1400 1500 1000 h, 10 atm, 90 m/s 70 140 270 480 790 1230

Fig. 3. Predicted and measured 1st turbine vane loss vs time in an Allison M501-K industrial gas turbine (Refs. 7 and 8).

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to the conclusion in the mid 19900 s that SiC based systems are not suitable for long term use at high temperature in high steam environments such as that found in gas turbines unless the problem of accelerated oxidation is solved. As a result, e?orts were undertaken to identify a near term solution involving an environmental barrier coating (EBC) applied to the surface of the SiC substrate to provide protection from high temperature steam. This research was conducted with NASA under the High Speed Research Program. From a manufacturing viewpoint, the goal of the an EBC development program should be to provide su?cient protection to the surface of SiC such that surface recession or a surface a?ected zone resulting from accelerated oxidation at 1200  C, the nominal operating temperature for SiC composites, is not the life limiting factor for the component. If the development program is successful, the accelerated oxidation e?ect no longer controls the life of the component in the combustion environment. Thus, for example, the predicted recession of 270 m at 1200  C in 1000 h needs to be reduced to probably no more than 25–50 m of either recession or a?ected zone at the surface. The actual acceptable value needs to be determined by engineering analysis of the component based on either thermo-mechanical stress state and/or airfoil performance for the particular component. Based on an acceptable 25–50 m e?ect, however, the EBC development goal is to reduce the accelerated oxidation problem by 5–10?. 2.1. Baseline mullite/zirconia EBC Considerable e?ort was undertaken in the 19800 s to develop corrosion coatings for Si based ceramics to provide corrosion protection. The coating system consisting of a layer of mullite on SiC followed by a top layer of zirconia was developed to address the corrosion issue.17 This system was also one of the ?rst to be considered as an EBC candidate system. The advantages of

the system lie with the coe?cient of thermal expansion of the mullite in that it closely matches the expansion of SiC. However, the expansion of the top layer, zirconia, is almost twice that of SiC. This leads to cracking in the coating on thermal cycling and eventual delamination. Additionally, thermal spray processing of mullite generally causes it to dissociate and deposit as alumina and amorphous silica which on subsequent exposure to high temperature reverts back to mullite. This leads to cracks in the mullite. Fig. 4 is a summary of the problems associated with the mullite–zirconia EBC system. 2.2. Improved EBC systems: CAS, yttrium silicate, and BSAS The mullite-zirconia based EBC pointed out the importance of matching the thermal expansion coe?cient of the coating layers to the expansion of the substrate. In addition to expansion coe?cient, it was also important to consider only EBC candidates exhibiting the ability to act as a barrier to steam along with phase stability and low volatility over the temperature range of interest. With these goals in mind, at least three EBC systems have been discussed in the open literature to date. 2.3. Calcium aluminosilicate The calcium aluminosilicate system18 was examined based on a chemistry of 40% by weight Al2O3, 36% silica, and 24% CaO which has been referred to as nonstoichiometric calcium aluminosilicate (ns-CAS) and which is slightly de?cient in silica compared with anorthite (CaO Al2O3 2SiO2). This chemistry was chosen over the anorthite composition based on testing that indicated instability of anorthite in steam. Fig. 5 summarizes the work with ns-CAS. Thermal cycle testing of the ns-CAS system in steam showed that it was at least an order of magnitude more resistant to steam than SiC after 250 cycles and 500 h at 1200  C. The expansion of ns-CAS

Fig. 4. The mullite–zirconia EBC system problem (see Ref. 17).

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Fig. 5. The ns-CAS EBC system (see Ref. 18).

was measured to be 5.9 ppm/ C between RT and 1200  C which compares reasonably well with the expansion of SiC which was measured to be approximately 4.9 ppm/  C. The system was fabricated into an EBC on SiC CMC by plasma spray technology and shown to provide protection. The most signi?cant problem with this system, however, is the heat treatment ($ 68 h) necessary after thermal spraying. The coating was found to form pores unless an elaborate heat treat was used (see Ref. 18). Additionaly the maximum use temperature was probably limited to roughly 1300  C. Although the targeted use temperature of 1200  C is well below this maximum, there is little room for growth with this system. 2.4. Yttrium silicate The yttrium silicate system19 was examined based on several yttria–silica ratios. The thermal expansion is sensitive to the amount of silica present. Higher silica content results in lower expansion. This is shown in Fig. 6. The steam behavior of yttrium silicate appears to be more than an order of magnitude more stable than SiC based on 250 thermal cycles and 500 h exposure time at 1200  C. The system was fabricated into an EBC coating on SiC CMC by thermal spray processing and shown to provide protection. 2.5. Barium strontium aluminosilicate The barium strontium aluminosilicate (BSAS) system17,20?22 has received the most development and test-

ing to date. The barium aluminosilicate (BAS) composition, BaO Al2O3 2SiO2, exhibits a high temperature, hexagonal phase (hexacelsian) which transforms to the monoclinic phase (celsian) at nominally 1550  C. The transformation, however, is quite sluggish and as a result the hexacelsian phase is generally observed at room temperature23. Hexacelsian has an expansion of roughly 8–9 ppm/ C over the range RT to 1200  C. The celsian phase is desired in appplications with SiC since its expansion is approximately 5.4 ppm/ C over the range RT to 1200  C. When strontium is substituted for barium at a 25% by mole level, the BSAS system now transforms much more readily. As a result the chemistry of 0.75BaO 0.25SrO Al2O3 2SiO2 is used for the BSAS based EBC system. Fig. 7 shows the celsian phase ?eld for the alumina, baria, and silica ternary system (no quartenary diagram is known that includes strontium) and the high temperature steam exposure results for the BSAS system. The BSAS system is shown to be roughly an order of magnitude more stable in the steam environment after 250 thermal cycles and 500 h at 1200  C than is SiC. Fig. 8 presents the thermal properties of BSAS. As shown the RT to 1200  C expansion is a?ected by the hexacelsian to celsian ratio. The celsian phase is required to closely match the expansion behavior of the SiC substrate. The thermal conductivity of BSAS was measured over the range of 100  C to 1300  C and is approximately 1.6W/mK at 1200  C. This is similar to the value of zirconia based thermal barrier coatings and as a result it is expected that the BSAS based EBC will

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Fig. 6. The yttrium silicate system (see Ref. 19).

Fig. 7. The BSAS EBC system (see Ref. 20).

Fig. 8. Thermal properties of BSAS (see Refs. 6, 20, and 21).

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Fig. 9. Thermal spray processing of BSAS at high temperature produces crystalline structure (see Ref. 22).

Fig. 10. BSAS EBC system successfully applied to SiC CMC (see Ref. 20).

exhibit thermal barrier properties as well as environmental barrier protection. Fig. 9 shows the phase content of thermal sprayed BSAS versus time at temperature after depositing the BSAS onto the SiC substrate held at elevated temperature during the deposition process. As fabricated the BSAS exhibits an amorphous structure which on large surfaces tends to crack when ?rst heat treated. Holding the substrate at a temperature of between 1000 and 1100  C for a period of time prior to ?rst cooling to RT after thermal spraying results in a crystalline structure which is stable to post coating heat treatment. Fig. 10 shows the BSAS coating system. The system consists of a silicon metal bond coat followed by a mixed layer of mullite and BSAS with a top layer of BSAS. The system has been successfully applied to SiC CMC combustion liners for a Solar Turbines, Inc. industrial gas turbine engine. In this case the liners were 760 mm in

diameter for the outer liner and 330 mm diameter for the inner liner. To date these liners have received almost 30,000 h total testing with the longest test being 13,937 h. Improvement in life of an EBC coated component versus an uncoated component is at least 3? based on this engine testing.

3. Summary ‘‘Accelerated’’ oxidation of SiC in a high temperature, high steam environment is now well documented by experiments, engine testing, and modeling. Current understanding of the problem and development of SiC technology leads one to the conclusion that long life use of SiC in gas turbine engines requires an environmental barrier coating to provide protection for SiC. Environmental barrier coatings for SiC have been developed,

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applied to SiC substrates, and tested in rig facilities and actual engine environments. Testing has shown that these EBC’s provide protection to the substrate and that at least a 3? improvement in life is achieved at this time.

12.

References
1. Lea, A. C., Oxidation of silicon carbide refractory materials. J. Soc. Glass Technol., 33(150) 27–50T (1949); Ceram. Abstr., 1951, November, pp. 198f. 2. Opila, E. J. and Hann Jr., R. E., Paralinear oxidation of CVD SiC in water vapor. J. Am. Ceram. Soc., 1997, 80(1), 197–205. 3. Opila, E. J., Fox, D. S. and Jacobson, N. S., Mass spectrometric identi?cation of Si-O-OH(g) species from the reaction of silica with water vapor at atmospheric pressure. J. Am. Ceram. Soc., 1997, 80(4), 1009–1012. 4. Opila, E. J., Smialek, J. L., Robinson, R. C., Fox, D. S. and Jacobson, N. S., SiC recession caused by SiO2 scale volatility under combustion conditions: II, thermodynamcis and gaseousdi?usion model. J. Am. Ceram. Soc., 1999, 82(7), 1826–1834. 5. Robinson, R. C. and Smialek, J. L., SiC recession caused by SiO2 scale volatility under combustion conditions: I, experimental results and empirical model. J. Am. Ceram. Soc., 1999, 82(7), 1817–1825. 6. Eaton, H. E., Linsey, G. D., More, K. L., Kimmel, J. B., Price, J. R. and Miriyala, N., EBC Protection of SiC/SiC Composites in the Gas Turbine Combustion Environment. ASME 2000-GT-631, Munich, 2000. 7. Ferber, M. K., Lin, H. T., Parthasarathy, V. and Wenglarz, R. A., Degradation of Silicon Nitrides in High Pressure, Moisture rich Environments, ASME 2000-GT-661, Munich, 2000. 8. Wenglarz, R. A. and Kouns, K., Ceramic Vanes for a Model 501K Industrial Turbine Demonstration. ASME 2000-GT-73, Munich, 2000. 9. Smialek, J. L., Robinson, R. C., Opila, E. J., Fox, D. S. and Jacobson, N. S., SiC and Si3N4 recession due to SiO2 scale volatility under combustor conditions. Adv. Composite Mater., 1999, 8(1), 33–45. 10. More, K. L., Tortorelli, P. F., Ferber, M. K. and Keiser, J. D., Observations of accelerated silicon carbide oxidation at high water-vapor pressures. J. Am. Ceram. Soc., 2000, 83(1), 211–213. 11. Schenk, B. J., Strangman, T., Opila, E. J., Robinson, R. C., Fox,

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D.S., Klemm, H., Taut, C., More, K. and Tortorelli, P., Oxidation Behavior of Prospective Silicon Nitride materials for Advanced Microturbine Applications. ASME 2001-GT-459, New Orleans, 2001. Yuri, I., Hisamatsu, T., Etori, Y. and Yamamoto, T., Degradation of Silicon Carbide in Combustion Gas Flow at High-Temperature and Speed. ASME 2000-GT-664, Munich, 2000. Filsinger, D., Schulz, A., Wittig, S., Taut, C., Klemm, H. and Wotting, G., Model Combustor to Assess the Oxidation Behavior ¨ of Ceramic Materials under Real Engine Conditions. ASME 1999GT-349, Indianapolis, 1999. Miriyala, N., Fahme, A. and van Roode, M., Ceramic Stationary Gas Turbine Program-Combustor Liner Development. ASME 2001-GT-512, New Orleans, 2001. Eaton, H. E., Linsey, G. D., Sun, E. Y., More, K. L., Kimmel, J. B., Price, J. R. and Miriyala, N. EBC Protection of SiC/SiC Composites in the Gas Turbine Combustion Environment—Continuing Evaluation and Refurbishment Considerations. ASME 2001-GT-513, New Orleans, 2001. Corman, G., Dean, A., Brabetz, S., McManus, K., Brun, M., Meschter, P., Luthra, K., Wong, H., Orenstein, R., Schroder, M., Martin, D., De Stefano, R. and Tognarelli, L., Rig and Gas Turbine Engine Testing of MI-CMC Combustor and Shroud Components. ASME 2001-GT-593, New Orleans, 2001. Lee, K. N., Current status of environmental barrier coatings for Si-based ceramics. Surface and Coatings Technology, 2000, 133– 134, 1–7. United States Patents 6,284,325. Silicon Based Substrate with Calcium Aluminosilicate Environmental/Thermal Barrier Layer and 6,296,942, Silicon Based Substrate with Calcium Aluminosilicate Environmental/Thermal Barrier Layer. United States Patents 6,296,941. Silicon Based Substrate with Yttrium Silicate Environmental/Thermal Barrier Layer and 6,312,763, Silicon Based Substrate with Yttrium Silicate Environmental/Thermal Barrier Layer. European Patent Application EP 1 044 943, Silicon Based Substrate with Environmental/Thermal Barrier Layer. Eaton, H. E. et al., 24th Annual Conf. On ceramics, Metal & Carbon Composites, Materials and Structures. January 2000. United States Patent 6,254,935. Method for Applying a Barrier Layer to a Silicon Based Substrate. Eldridge, J., Phase Evolution Study of BSAS in Environmental Barrier Coatings. Ceramic Engineering and Science Proceedings, 2001, 22(4), 383–390.


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