une 1999 High-Temperature Oxidation of Boron Nitride: Boron Nitride Layers in Composites Table IV. Lowest Possible Rate Constants for Oxidation of This leads to borosilicate glass formation. As time progresses, the small (-20 ppm) amounts of water vapor remove boron B(um/h) /A(um/h) from the glass, leading to a substantially larger amount of Sio 4.5×10-6 85×10-4 uld be expected from simple SiC oxidation. Experi- 3.7×10 4.2×10-3 ments on SiC/BN/SiC layered structures allow examination of low-water-vapor oxidation effects deep within the composite In such cases, the supply of oxygen is limited, and SiC oxidizes preferentially to BN. As oxygen potential increases, borosil- Table v. Calculated Times to Seal Annular Region cate formation becomes more important The second mechanism involves volatilization of bn by Rate constant high-water-vapor gas streams. This is due to the high stabil- the HBO(g), H3BO3(g), and H3,O(g) molecules 4.5×I 5.57×104h 2.21×105h olatilization leads to recession of fiber coatings in a composite material. This process is described with gas- 2.21×103h diffusion through a channel and concurrent channel 4.5 55.7h closure (B)800°C Late constant 1.0um Acknowledgments: We wish to thank Dr. David Harding, formerly of the RBS work. We also wish to thank J Smith, NYMA, NASA Lewis Gr for the electron microprobe work. We appreciate Dr. M. K. Brun of Gener 3.7×10-3 227h Electric for providing BN-coated SiC, and Drs. A. Moore and H. Sayir of 3.7×10 ounds. Helpful dis- ussions with Drs. K. L. Luthra and P. Meschter of General Electric Company and Professor B. Sheldon of Brown University are appreciated Refe =2102m:m+01+1s最如R和mHA and SIMS in At(42+4B32 K N. Lee and N S. Jacobson, "Chemical Stability of the Fiber Coating/ This full expression is used, with B as an adjustable parameter, -Based Ceramic Matrix Composites, "J. Am. Ceram. to calculate the dashed lines in Figs. 10(aHc)and compare R. S. Roth, J. R. Dennis, and H. F. McMurdie, Fig 6455 in Phase Diagrams them to the measured distances for Ceramists, Vol. VI. Edited by M. A. Clevinger and H. M. Ondik. American In Figs. 10(aHc), the dashed lines are from Eq (16), and the olid circles are the experimental measurements. The calcula- tions reproduce many of the experimental observations in this data set-the decrease in recession with time, the weak Sic Composite, "J.Am. Ceram. Soc, 81(112777-84(1998) temperature dependence, and strong PH,o dependence. At 700C with 10% H2O/O2 and 0.5 um BN(Fig. 10(a)). ment on the Subcritical Crack the largest recession is observed due to the 10% H,O. A 500x posite, m Eng. Sci. Proc., 13 [ 7-8] N.S. Jacobson, S. Farmer, A Moore, and H. Sayir, "High-Temperature enhanced oxidation rate leads to the observed annulus closure ation Behavior of Boron Nitride: I. Monolithic Boron Nitride. " J. Am. At 700C with 1% H,O/O, and I um BN(Fig. 10(b),reces- m.Soc,8212393-98(1999 sion is less due to the lower-water-vapor content. Now a 5000x B. W. Sheldon, E. Y. Sun, S.R. Nutt, and JJ. Brennan, "Oxidation of enhanced oxidation rate constant approximates channel clo- N-Coated SiC Fibers in Ceramic-Matrix Composites, J Am Ceram Soc., 79 sure, very likely due to larger annulus width. Finally at 800C N. Morscher, D. Bryant, and R, E. Tressler, "Environmental Durability little less than that at 700C, possibly due to more rapid char vith 1% H,O/O, and I um BN (Fig. 10(c), recession seems of Different BN Interphases( for SiC/SiC)in H,O Containing Atmospheres at nel sealing at this temperature. A 1000x enhanced oxidation appears to account for channel sealing. These large enhance 加 Ceramic Society, Cincinnati. H. a gth Annual Meeting of the A ments in oxidation rates are expected because of the presence ites Symposium, Paper No. SIII-027-97) of boron and water vapor. 27 2G. N. Morscher, " Tensile Stress-Rupture of SiC /SiCm Mi Although this model describes many of the basic observa- Not, so ng 2029 t2 (19,. terphases at Elevated Temperatures in Air, J.A. Ceram. cies out of the glass are an important step. We know oxidation Melts at 1475 K, "J. Am. Ceram. Soc., 76[11] 2809-12(1993 ISL. U. J. T Ogbuji and E J. Opila, "A C of the Oxidation Kinetics kinetics differ for BN deposited at different temperatures, and 30(1995) these kinetics should be included N. P. Bansal and R H. Doremus, Handbook of Glass Properties, pp. 242- 43. Academic Press, New York, 1986. IV. Conclusions perature on Failure for Precracked Hi-Nicalon/BN/CVD SiC Minicomposites in M.wbP18|519 C A. Davies, J.R. Downey Jr, D J. Frurip,R. A Mac. Donald, and A N. Syverud, JANAF Thermochemical Tables, 3rd ed; pp 24 iber coating in SiC/SiC composites with a series of model 270,633, 6732 American Chemical Society and American Physical Society opposites. Two important reaction processes have been ex- PD. R. Bryant, "Oxidation and volatiliz plored: (1)oxidation in low-water-vapor streams, which pro- motes borosilicate glass formation, and(2)oxidation in high- versity, Ur water-vapor streams, where the bN is volatilized. Experiments on a BN-coated SiC coupon indicate the BN readily oxidizes to B2O3 simultaneously with enhanced SiC oxidation to SiO2 个队:M段Sm lex Chemical Equilibria," Metall. B,21B,1013-23 zzi and R. Naslain, "Oxidation Mechanisms and Kinetics of lD.yB 2 = 2VBN ~DHBO2 PHBO2 +DH2BO3 PH3BO3 +DH3B3O6 PH3B3O6 ! RT × St + At d − ~A2 + 4Bt! 3/2 12dB D (20) This full expression is used, with B as an adjustable parameter, to calculate the dashed lines in Figs. 10(a)–(c) and compare them to the measured distances. In Figs. 10(a)–(c), the dashed lines are from Eq. (16), and the solid circles are the experimental measurements. The calcula￾tions reproduce many of the experimental observations in this data set—the decrease in recession with time, the weak temperature dependence, and strong pH2O dependence. At 700°C with 10% H2O/O2 and 0.5 mm BN (Fig. 10(a)), the largest recession is observed due to the 10% H2O. A 500× enhanced oxidation rate leads to the observed annulus closure. At 700°C with 1% H2O/O2 and 1 mm BN (Fig. 10(b)), reces￾sion is less due to the lower-water-vapor content. Now a 5000× enhanced oxidation rate constant approximates channel clo￾sure, very likely due to larger annulus width. Finally at 800°C with 1% H2O/O2 and 1 mm BN (Fig. 10(c)), recession seems a little less than that at 700°C, possibly due to more rapid chan￾nel sealing at this temperature. A 1000× enhanced oxidation appears to account for channel sealing. These large enhance￾ments in oxidation rates are expected because of the presence of boron5 and water vapor.27 Although this model describes many of the basic observa￾tions, many refinements could be made. Channel sealing is a critical issue and could be treated in more detail. The liquid borosilicate products are important, and diffusion of boron spe￾cies out of the glass are an important step. We know oxidation kinetics differ for BN deposited at different temperatures,8 and these kinetics should be included. IV. Conclusions We have examined the high-temperature oxidation of the BN fiber coating in SiC/SiC composites with a series of model composites. Two important reaction processes have been ex￾plored: (1) oxidation in low-water-vapor streams, which pro￾motes borosilicate glass formation, and (2) oxidation in high￾water-vapor streams, where the BN is volatilized. Experiments on a BN-coated SiC coupon indicate the BN readily oxidizes to B2O3 simultaneously with enhanced SiC oxidation to SiO2. This leads to borosilicate glass formation. As time progresses, the small (∼20 ppm) amounts of water vapor remove boron from the glass, leading to a substantially larger amount of SiO2 than would be expected from simple SiC oxidation. Experi￾ments on SiC/BN/SiC layered structures allow examination of low-water-vapor oxidation effects deep within the composite. In such cases, the supply of oxygen is limited, and SiC oxidizes preferentially to BN. As oxygen potential increases, borosili￾cate formation becomes more important. The second mechanism involves volatilization of BN by high-water-vapor gas streams. This is due to the high stabil￾ity of the HBO2(g), H3BO3(g), and H3B3O6(g) molecules. This volatilization leads to recession of fiber coatings in a model composite material. This process is described with gas￾phase diffusion through a channel and concurrent channel closure. Acknowledgments: We wish to thank Dr. David Harding, formerly of NYMA, NASA Lewis Group, and currently with SUNY Buffalo, for facilitating the RBS work. We also wish to thank J. Smith, NYMA, NASA Lewis Group, for the electron microprobe work. We appreciate Dr. M. K. Brun of General Electric for providing BN-coated SiC, and Drs. A. Moore and H. Sayir of Advanced Ceramic Corporation for providing model compounds. Helpful dis￾cussions with Drs. K. L. Luthra and P. Meschter of General Electric Company and Professor B. Sheldon of Brown University are appreciated. References 1 O. Dugne, S. Pouhet, A. Guette, R. Naslain, R. Fourmeaux, Y. Khin, J. Sevely, J. P. Rocher, and J. Cotteret, “Interface Characterization by TEM, AES, and SIMS in Tough SiC (ex-PCS) Fibre-SiC (CVI) Matrix Composites with a BN Interphase,” J. Mater. Sci., 28, 3409–22 (1993). 2 R. Naslain, “Fibre–Matrix Interphases and Interfaces in Ceramic Matrix Composites Processed by CVI,” Compos. Interfaces, 1 [3] 253–86 (1993). 3 K. N. Lee and N. S. Jacobson, “Chemical Stability of the Fiber Coating/ Matrix Interface in Silicon-Based Ceramic Matrix Composites,” J. Am. Ceram. Soc., 78 [3] 711–15 (1994). 4 R. S. Roth, J. R. Dennis, and H. F. McMurdie; Fig. 6455 in Phase Diagrams for Ceramists, Vol. VI. Edited by M. A. Clevinger and H. M. Ondik. American Ceramic Society, Columbus, OH, 1987. 5 J. Schlicting, “Oxygen Transport Through Glass Layers Formed by a Gel Process,” J. Non-Cryst. Solids, 63, 173–81 (1984). 6 L. U. J. T. Ogbuji, “A Pervasive Mode of Oxidative Degradation in a SiC– SiC Composite,” J. Am. Ceram. Soc., 81 [11] 2777–84 (1998). 7 C. H. Henager Jr. and R. H. Jones, “The Effects of an Aggressive Environ￾ment on the Subcritical Crack Growth of a Continuous-Fiber Ceramic Com￾posite,” Ceram. Eng. Sci. Proc., 13 [7–8] 411–19 (1992). 8 N. S. Jacobson, S. Farmer, A. Moore, and H. Sayir, “High-Temperature Oxidation Behavior of Boron Nitride: I, Monolithic Boron Nitride,” J. Am. Ceram. Soc., 82 [2] 393–98 (1999). 9 B. W. Sheldon, E. Y. Sun, S. R. Nutt, and J. J. Brennan, “Oxidation of BN-Coated SiC Fibers in Ceramic-Matrix Composites,” J. Am. Ceram. Soc., 79 [2] 539–43 (1996). 10G. N. Morscher, D. Bryant, and R. E. Tressler, “Environmental Durability of Different BN Interphases (for SiC/SiC) in H2O Containing Atmospheres at Intermediate Temperatures,” Ceram. Eng. Sci. Proc., 18 [3] 525–34 (1997). 11M. K. Brun and K. L. Luthra, “Ends-on Oxidation of BN Fiber Coatings in SiC–Si/SiC Composites”; presented at the 99th Annual Meeting of the Ameri￾can Ceramic Society, Cincinnati, OH, May 6, 1997 (Ceramic Matrix Compos￾ites Symposium, Paper No. SIII-027-97). 12G. N. Morscher, “Tensile Stress–Rupture of SiCf /SiCm Minicomposites with C and BN Interphases at Elevated Temperatures in Air,” J. Am. Ceram. Soc., 80 [8] 2029–42 (1997). 13L. R. Doolittle, “Algorithms for the Rapid Simulation of Rutherford Back￾scattering Spectra,” Nucl. Instrum. Methods Phys. Res., B9 [3] 334–51 (1985). 14M. Boike, K. Hilpert, and F. Muller, “Chemical Activities in B2O3–SiO2 Melts at 1475 K,” J. Am. Ceram. Soc., 76 [11] 2809–12 (1993). 15L. U. J. T. Ogbuji and E. J. Opila, “A Comparison of the Oxidation Kinetics of SiC and Si3N4,” J. Electrochem. Soc., 142 [3] 925–30 (1995). 16N. P. Bansal and R. H. Doremus, Handbook of Glass Properties; pp. 242– 43. Academic Press, New York, 1986. 17G. N. Morscher, “The Effect of Static and Cyclic Tensile Stress and Tem￾perature on Failure for Precracked Hi-Nicalon/BN/CVD SiC Minicomposites in Air,” Ceram. Eng. Sci. Proc., 18 [3] 737–45 (1997). 18M. W. Chase Jr., C. A. Davies, J. R. Downey Jr., D. J. Frurip, R. A. Mac￾Donald, and A. N. Syverud, JANAF Thermochemical Tables, 3rd ed; pp. 246, 270, 633, 1673. American Chemical Society and American Physical Society, New York, 1986. 19D. R. Bryant, “Oxidation and Volatilization of BN Interphases in SiC Fi￾ber-Reinforced SiC Matrix Composites”; M.S. Thesis. Pennsylvania State Uni￾versity, University Park, PA, 1997. 20G. Eriksson and K. Hack, “ChemSage—A Computer Program for the Cal￾culation of Complex Chemical Equilibria,” Metall. Trans. B, 21B, 1013–23 (1990). 21L. Filipuzzi and R. Naslain, “Oxidation Mechanisms and Kinetics of 1D￾Table IV. Lowest Possible Rate Constants for Oxidation of Pore Walls Temperature (°C) B (mm2 /h) B/A (mm/h) 700 4.5 × 10−6 8.5 × 10−4 800 3.7 × 10−5 4.2 × 10−3 Table V. Calculated Times to Seal Annular Region (A) 700°C Rate constant (mm2 /h) 0.5 mm 1.0 mm 4.5 × 10−6 5.57 × 104 h 2.21 × 105 h 4.5 × 10−5 5.57 × 103 h 2.21 × 104 h 4.5 × 10−4 557 h 2.21 × 103 h 4.5 × 10−3 55.7 h 221 h (B) 800°C Rate constant (mm2 /h) 1.0 mm 3.7 × 10−5 2.27 × 104 h 3.7 × 10−4 2.27 × 103 h 3.7 × 10−3 227 h 3.7 × 10−2 22.7 h June 1999 High-Temperature Oxidation of Boron Nitride: Boron Nitride Layers in Composites 1481