E噩≈S Journal of the European Ceramic Society 22(2002)2741-2747 www.elsevier.com/locate/jeurceramsoc Accelerated oxidation of SiC CMCs by water vapor and protection via environmental barrier coating approach Harry e. e United Technologies Research Center, 411 Silver Lane, East Hartford, CT06108, US.A Abstract ilicon 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/ 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 CMC's are silica formers under oxidizing conditions and have been shown experimentally to undergo accelerated oxidation due to exposure to steam in high tem- perature combustion environments such as found in the gas turbine hot section Oxidation by steam in a fowing gas stream has been shown to exhibit paralinear behavior and result in unacceptable recession of the surface. Thus, prior to the successful intro- duction 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. C 2002 Published by Elsevier Science Ltd Keywords: Composites: Corrosion; Engine components: Environmental barrier coating: SiC 1. The problem-accelerated oxidation of SiC in steam rate process Over long times the growth rate process of silica formation is balanced by the volatilization process In 1949 and 1951, A.C. Lea' studied and reported on at which time further oxidation of the substrate is con the rate of oxidation of silicon carbide and noted that trolled simply by the linear volatilization rate. The silica steam caused the oxidation rate to be accelerated com- scale reaches a steady state thickness of approximately pared to the rate in dry oxygen. The volatilization of 10 microns for temperatures in the range of 1200- silica in an air-steam atmosphere was presented as a 1400C. This is contrasted with oxidation under dry possible mechanism to explain the observation. Almost conditions, whereby oxidation is governed by a parabolic 50 years later, Opila and Hann, Opila et al., and Opila rate process involving the formation and growth of a et al. studied in detail and presented a model explaining stable protective silica scale on the surface of silicon the accelerated oxidation of silicon carbide due to high carbide temperature exposure to steam. Their work showed that, on exposure to steam, oxidation of silicon carbide SIC 3H2O(g)= SiO2+ 3H2(g)+CO(g) (1) occurs by a paralinear rate process involving oxidation of silicon carbide by h2o to form silica and then volati 1O2 +H2 O(g=Si-Ox-Hylgl ization of the silica by reaction with H20 to form Si-O- H(g) species [see Eqs. (1)and(2)]. For overall paralinear Further work by these authors modeled the volatili- behavior, the silica layer which forms on the substrate zation process and showed that the flux of the volatile due to oxidation occurs by a parabolic rate process species is controlled by diffusion through a boundary while the volatilization of the silica occurs by a linear layer. For a flat plate geometry and for laminar flow, the flux is predicted to be dependent on the gas velocity and Corresponding author. Tel :+1-860-610-7414: fax: +1-860-610 pressures as shown in Eq. (3)for when the volatile silicon species is Si(oH)4 which is the predicted dominant spe E-mailaddress:eatonhe(@utrcutc.com(HE.Eaton) cies for fuel lean, gas turbine combustion environments 0955-2219/02/S. see front matter C 2002 Published by Elsevier Science Ltd PII:S0955-2219(02)00141-3
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 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. # 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 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 þ 3H2OðgÞ ¼ SiO2 þ 3H2ðgÞ þ COðgÞ ð1Þ SiO2 þ H2OðgÞ ¼ Si-Ox-HyðgÞ ð2Þ Further work by these authors modeled the volatilization process and showed that the flux of the volatile species is controlled by diffusion through a boundary layer. For a flat plate geometry and for laminar flow, the flux 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 Journal of the European Ceramic Society 22 (2002) 2741–2747 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +1-860-610-7414; fax: +1-860-610- 7879. E-mail address: eatonhe@utrc.utc.com (H.E. Eaton)
HE.Eaton,GD. Linsey/ Journal of the European Ceran ocher22(2002)274-2747 Jsi(oh, ov. (PH, 0)/(Ptotal) (3) 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 first turbine vanes where J is the flux, v is the gas velocity and P is either versus time in an Allison M50l-K industrial gas tur the total system pressure or partial pressure of steam bine.7,The predicted recession values are based on the o, Experimental work by robinson and Smialek in a high work from Refs.5 and 9. This is another example of the oressure combustion test facility measured SiC recession effect of steam exposure based on actual engine exp versus test conditions and confirmed Eq. (3)for lean ence. The fact that it covers silicon nitride illustrates the combustion conditions. Fig. I summarizes the results for generic problem probably facing all silica formers in a lean burn conditions over the temperature range of high temperature, high steam combustion environment. approximately 1200-1450C. Table I provides predic- Additional supporting documentation of the effects of tions based on the experimental data for long term reces- steam exposure can be found in Refs. 10-16. sion of SiC in a combustion environment. The predicted recession at 1200 oC of 270 microns in 1000 h in a com bustion environment for SiC is too significant to ignore 2. Environmental barrier coatings (EBCs) for a material with expected useful life 30,000 h in a pro- posed application such as a industrial gas turbine. Work to understand the mechanisms of accelerated Confirmation of this behavior in actual engine eny oxidation of sic and to document the behavior in onments has been observed in a solar Turbines Inc experimental testing and actual engine environments led Centaur 50S industrial gas turbine and in an Allison M501-K industrial gas turbine. 7 The Solar Turbines, Solar Turbines, Inc. engine testing showed more than 90% Inc engine was run with SiC CMC combustion liners at oxidation of sic Cmc combustor liner wall thickness nominally 1200C for 5018 h Recession values of up to 5018hrs test (@ nominally 1200C 2200 microns were measured. This recession rate is roughly 0.44 H per hour to result in a 1000 h calculated recession of 440 H which is similar to the predicted loss of 270 H in 1000 h at 1200C from Table 1. Fig. 2 is a view of the liner in cross section at two different locations SiC Specic Weight Loss Rate NASA HPBR-Lean Bum [), T(K, P(atm), ms] SiC CMC combustor liner wall cross sections showing (a )no oxidation. k=206(e.w-50 00 um loss/5018 hrs =-0.44 um/hr recession rate or 440 um per 1000 hrs 866264666.8 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 measurements of Sic in a high pressure combustion environment (see Ref. 5) Alison M501-K 1st stage ceramic(AS-800)van recession observed vs time and predicted Table I Predicted recession of Sic under lean burn combustion conditions(see Predicted lean burn recession (um) g§ 1000h,10atm,90m/s 9?c 80010001200 time(hrs) 1500 1230 Fig. 3. Predicted and measured Ist turbine vane loss vs time in an Allison M50l-K industrial gas turbine(Refs. 7 and 8)
JSiðOHÞ4 / v1=2 PH2O 2 =ð Þ Ptotal 1=2 ð3Þ where J is the flux, 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 confirmed 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 significant to ignore for a material with expected useful life 30,000 h in a proposed application such as a industrial gas turbine. Confirmation 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 different 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 first 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 effect 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 effects 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. 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 h, 10 atm, 90 m/s 1000 70 1100 140 1200 270 1300 480 1400 790 1500 1230 Fig. 2. Cross sectional view of Solar Turbines, Inc SiC CMC Centaur 50S combustion liner after 5018 h operation (see Ref. 6). Fig. 3. Predicted and measured 1st turbine vane loss vs time in an Allison M501-K industrial gas turbine (Refs. 7 and 8). 2742 H.E. Eaton, G.D. Linsey/ Journal of the European Ceramic Society 22 (2002) 2741–2747
H E. Eaton, G D. Linsey /Journal of the European Ceramic Society 22 (2002)2741-2747 2743 to the conclusion in the mid 1990s that SiC based sys- the system lie with the coefficient of thermal expansion tems are not suitable for long term use at high tem- of the mullite in that it closely matches the expansion of perature in high steam environments such as that found SiC. However, the expansion of the top layer, zirconia in gas turbines unless the problem of accelerated oxida- is almost twice that of SiC. This leads to cracking in the tion is solved. As a result, efforts were undertaken to coating on thermal cycling and eventual delamination identify a near term solution involving an environ- Additionally, thermal spray processing of mullite gen mental barrier coating(EBC) applied to the surface of erally causes it to dissociate and deposit as alumina and the Sic substrate to provide protection from high tem- amorphous silica which on subsequent exposure to high perature steam. This research was conducted with Nasa temperature reverts back to mullite. This leads to cracks under the High Speed Research Program in the mullite. Fig. 4 is a summary of the problem From a manufacturing viewpoint, the goal of the an associated with the mullite-zirconia EBC system ebC development program should be to provide suffi cient protection to the surface of Sic such that surface 2. 2. Improved EBC systems: CAS, yttrium silicate, and recession or a surface affected zone resulting from BSAS accelerated oxidation at 1200C, the nominal operating temperature for SiC composites, is not the life limiting The mullite-zirconia based EBC pointed out the factor for the component. If the development program importance of matching the thermal expansion coeffi is successful, the accelerated oxidation effect no longer cient of the coating layers to the expansion of the sub- controls the life of the component in the combustion strate. In addition to expansion coefficient, it was also environment. Thus, for example, the predicted recession important to consider only EBC candidates exhibiting of 270 H at 1200oC in 1000 h needs to be reduced to the ability to act as a barrier to steam along with phase probably no more than 25-50 H of either recession or stability and low volatility over the temperature range affected zone at the surface. The actual acceptable value of interest. With these goals in mind, at least three EBC eeds to be determined by engineering analysis of the systems have been discussed in the open literature to date. component based on either thermo-mechanical stress state and/ or airfoil performance for the particular com- 2.3. Calcium aluminosilicate ponent. Based on an acceptable 25-50 H effect, however, the ebc development goal is to reduce the accelerated The calcium aluminosilicate system8 was examined oxidation problem by 5-10x based on a chemistry of 40% by weight Al O3, 36% silica. and 24% Cao which has been referred to as non- 2.1. Baseline mullite/zirconia EBC stoichiometric calcium aluminosilicate (nS-CAS)and which is slightly deficient in silica compared with anorthite Considerable effort was undertaken in the 1980,s to (CaoAl2O3 2SiO2). This chemistry was chosen over the develop corrosion coatings for Si based ceramics to anorthite composition based on testing that indicated provide corrosion protection. The coating system con- instability of anorthite in steam Fig. 5 summarizes the sisting of a layer of mullite on SiC followed by a top work with ns-CAS. Thermal cycle testing of the ns- CAs layer of zirconia was developed to address the corrosion system in steam showed that it was at least an order of issue. 7 This system was also one of the first to be con- magnitude more resistant to steam than SiC after 250 sidered as an EBC candidate system. The advantages of cycles and 500 h at 1200C. The expansion of ns-CAs Mullite-ZrO(Y2O3) baseline EBC coating on thermal cycle exposure 1. ZrO2 cracks due to CtE mismatch propagating into existing cracks in mullite 2. exposing mullite which results in silica loss from mullite 3. and further cracking mullite resulting in SiC CMC oxidation ZrO, surface cracks ZrO2 Thermal and accelerated SiC CMC Fig 4. The mullite-zirconia EBC system problem(see Ref. 17)
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, efforts 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 suffi- cient protection to the surface of SiC such that surface recession or a surface affected 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 effect 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 affected 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 effect, however, the EBC development goal is to reduce the accelerated oxidation problem by 5–10 . 2.1. Baseline mullite/zirconia EBC Considerable effort 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 first to be considered as an EBC candidate system. The advantages of the system lie with the coefficient 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 coeffi- cient of the coating layers to the expansion of the substrate. In addition to expansion coefficient, 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 deficient 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). H.E. Eaton, G.D. Linsey/ Journal of the European Ceramic Society 22 (2002) 2741–2747 2743
HE.Eaton,GD. Linsey/ Journal of the European Ceran ciey22(2002)2741-2747 ns-CAS EBC system H;""° RT to 1200C CTE-59 ppm/'C Limited in temperatu Processing complex du scAS ns-CAS mullite SiC CMC 今购 Fig. 5. The ns-CAS EBC system(see Ref. 18) was measured to be 5.9 ppm/C between RT and 1200c ing to date. The barium aluminosilicate (BAs) which compares reasonably well with the expansion of composition, Bao. AlO3' 2SiO2, exhibits a high tem SiC which was measured to be approximately 4.9 ppm/ perature, hexagonal phase(hexacelsian) which transforms oC. The system was fabricated into an EBC on Sic to the monoclinic phase(celsian) at nominally 1550oC CMC by plasma spray technology and shown to provide The transformation, however, is quite sluggish and as a protection. The most significant problem with this system, result the hexacelsian phase is generally observed at room however,is the heat treatment(68 h) necessary after temperature23. Hexacelsian has an expansion of roughly thermal spraying. The coating was found to form pores 8-9 ppm/C over the range RT to 1200C. The celsian unless an elaborate heat treat was used (see Ref. 18). phase is desired in appplications with Sic since its Additionaly the maximum use temperature was prob- expansion is approximately 5. 4 ppm/c over the range ably limited to roughly 1300oC. Although the targeted RT to 1200oC. When strontium is substituted for bar- use temperature of 1200C is well below this maximum, ium at a 25% by mole level, the BSAs system now there is little room for growth with this system transforms much more readily. As a result the chemistry of 0.75BaO0.25SrO AlO3 2SiO, is used for the BSAs 2.4. Yttrium silicate based EBC system. The yttrium silicate system"was examined based on Fig. 7 shows the celsian phase field for the alumina, several yttria-silica ratios. The thermal expansion is is known that includes strontium) and the high tem ensitive to the amount of silica present. Higher silica perature steam exposure results for the BSAs system content results in lower expansion. This is shown in The BSAS system is shown to be roughly an order of Fig. 6. The steam behavior of yttrium silicate appears magnitude more stable in the steam environment after be more than an order of magnitude more stable than 250 thermal cycles and 500 h at 1200C than is SiC. Sic based on 250 thermal cycles and 500 h exposure Fig.& presents the thermal properties of BSAs.As time at 1200C. The system was fabricated into an eBc shown the rt to 1200C expansion is affected by the coating on SiC CMC by thermal spray processing and hexacelsian to celsian ratio. The celsian phase is shown to provide protection required to closely match the expansion behavior of the SiC substrate. The thermal conductivity of BSAS was 2.5. Barium strontium aluminosilicate measured over the range of 100。to1300° C and is approximately 1.6W/mK at 1200 oC. This is similar to The barium strontium aluminosilicate(BSAS) sys- the value of zirconia based thermal barrier coatings and tem720-2 has received the most development and test- as a result it is expected that the bSas based EBC will
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 significant 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 testing 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 field 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 affected 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 Fig. 5. The ns-CAS EBC system (see Ref. 18). 2744 H.E. Eaton, G.D. Linsey/ Journal of the European Ceramic Society 22 (2002) 2741–2747
H.E.Eaton,GD Linsey/ Journal of the European Ceran ciey22(2002)2741-2747 2745 Yttrium silicate EBC system Good high Temp. EBC candidate-but limited evaluation under HSCT program ""m灬" SiC CMC Fig. 6. The yttrium silicate system(see Ref. 19) BSAs weight change vs thermal cycles and SIC n90%H20-10%2at1 2 喜:,““ Fig. 7. The BSAS EBC system(see Ref. 20) BSAS EBC system-0.75BaO-0 25SrO.2SiO2-thermal properties (Sc CMC RTt12o· C CTE△ Thermal conductiv Ityhot pressed BSAS (75/2 5 Fig 8. Thermal properties of BSAS(see Refs. 6, 20, and 21)
Fig. 7. The BSAS EBC system (see Ref. 20). Fig. 8. Thermal properties of BSAS (see Refs. 6, 20, and 21). Fig. 6. The yttrium silicate system (see Ref. 19). H.E. Eaton, G.D. Linsey/ Journal of the European Ceramic Society 22 (2002) 2741–2747 2745