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)00141-3Accelerated 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 tem￾perature 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 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. # 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 com￾pared 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 volati￾lization 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 con￾trolled 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 volatili￾zation 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 spe￾cies 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)