November 2002 Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composite 2709 below that critical level may have occurred and could be respon- References sible for the reduction in composite strength compared to that of composites without a fiber coating. Clearly, in minicomposite D E. Ryshkewitch, "Compression Strength of Porous Sintered Alumina and Zirco- the volume fraction of porosity in the coating has been reduced to nia”J.Am. Ceran.Soc,36|2]65-68(1953) less than the porosity present in the alumina matrix(Fig. 9) J. Am Ceram Soc., 39[11]377-85(1956). that th BL. A Simpson, "Effect of Microstructure on Measurements of Fracture Energy of orous Y AG coating gives a higher-strength composite, even if the Ai pna d Am Ceram soc 56mi-il( 1975) nce of Elastic Moduli on Porosity matrix is porous. Porosity alone in the matrix is not sufficient to JAm. Ceram.Soc,607-8]345-49(1977) give a high-strength composite at 1200oC. Whether the fine ngh, and R. B Poeppel, "Dependence of Ceramic Fracture porosity of the porous YAG fiber coating is needed to provide a Popertescon J. Mater.Sci,28,3589-93(1993) weaker interface than a porous matrix alone can provide, or C. Lam, F. F. Lange, and A. G. Evans, "Mechanical Properties of Partiall na Produced from Powder Compacts,J.Am. Cera. Soc., 7718 whether the fibers are damaged by the alumina matrix(including 2113-17(1994) its processing)in the absence of a porous Y AG fiber coating, is not S C Nanjangud, R. Brezny, and D J, Green,"Strength and Young's Modulu clear. The level of porosity present in the matrix (-40 vol%)(1996). Behavior of a Partially Sintered Porous Alumina,"JAm. Ceram Soc., 78[1 266-68 should be adequate to give composite-like behavior and high of Physical Property-Porosity Models strength In commercial porous oxide matrix composites, nearly Based on minimum solid fully retained fiber strengths are obtained after processing and 9A. P. Roberts and E. J Properties of Model Porous Ceramics, long-term heat treatment at 1200C. However, the commercial Soc,83[12 TOM. G. Harrison. M composites are produced with Nextel 720 diphasic alumina- Matrix Composite Member and Method of Making, "U.S. Pat. No. 5 306 554, Ap mullite fibers, not single-phase alumina, so a direct compariso cannot be made because of the differences in fiber moduli thermal Iw.-C. Tu, F. F. Lange, and A. G. Evans, "Concept for a Damage-Tolerant Ceramic Composite with Strong Interfaces,J.Am. Ceram. Soc., 79[2]417-24 expansion, and bonding/interaction with the matrix. Ideally, fibers would need to be extracted from the matrix after processing to test AC. G. Levi, J. Y. Yang, B J. Dalgleish, F. w. Zok, and A. G. Evans,"Processin for strength degradation; however, extraction of suitable lengths of and Performance of an All-Oxide Ceramic Composite, J. Am. Ceram Soc., 81 [81 alumina fibers from an alumina matrix for testing is difficult, if not 2077-86(1998). S. G. Steel, L. P. Zawada, and S. Mall, ""Fatigue Behavior of a Nextel impossible, without damaging the fibers In recent work, the presence of a YAG second phase in a porous Pro 2 i3 695-702 te o0 Room and Elevated lemperature, Ceram. Eng. alumina matrix composite without a separate fiber coating also M. K. Cinibulk, K. A. Keller, T. Mah, and T. A. Parthasarathy, "Nextel 610 and 650 Fiber Reinforced Porous Alumina-YAG Matrix Composites, Ceram. Eng. Sci. control composites were found to be of equal strength at 1100.C PasM 2 3 2000). M. H. Jaskowiak, S. I. Eldridge, J. B. Hurst, and J. A. Setlock, "Interfacial Coatings for Sapphire/AL,O3, p. 84 in HITEMP Review. NASA Conference composites displayed greater strengths at 1200C for times of up to U J. T. Ogbuji, "A Porous Oxidation-Resistant Fiber Coating for CMC 100h. YAG was distributed as a dense phase bonding the alumina Interphase, "Ceram. Eng. Sci. Proc., 16[4]497-505(199 particles of the matrix and also separating the fiber from the matrix PL. U. J. T. Ogbuji, "Evaluation of a Porous Fiber Coating in SiC-Si3N4 alumina. These results, along with those of the present stud uggest that Y ag inhibits densification of the matrix at 1200 C. In IO Sudre, A G Razzell, L. Molliex, and M. Holmquist, "Alumina Single-Crystal Fiber Reinforced Alumina Matrix for Combustor Tiles, Ceram. Eng. Sci. Proc., 19 the absence of YAG in the matrix, however, a porous YAG fiber [4]273-80(1998) ating is sufficient to deflect nly until a critical coating density and/or matrix density is Lanthanum Phosphate Fiber Cou Proc,1753-60(19 Carbon-Aluminum Oxide between 5 and 100 h at 1200C later. Res. Soc. Symp. Proc., 432 E. Boakye, R. S. Hay, M. D. Petry, and T. A. Parthasarathy, "Sol-Gel Zircon-Carbon Precursors and Coating of Nextel 720 Fiber Tows, "Cera. Eng IV. Conclusions Sc.Poc,20B3165-72(1999 4-M. K. Cinibulk, T. A, Parthasarathy, K. A. Keller, and T. Mah, "Porous Polymeric solutions were used to apply a porous yttrium er Coatings for Oxide-Oxide Composites, "Ceram. Eng aluminum garnet(YAG,Y,Al, O12) coating to fiber tows at Sci, Proc, 21F41219-28(2000). temperatures that did not degrade fiber strength. The use of the Porous Zirconia-Silica and Monazite Coatings us Keller, and R S Hay,"Evaluation o xtel 720 Fiber-Reinforce polymeric solution also allowed for a fugitive carbon phase that lackglas Minicomposites, J. Am. Ceran. Soc., 84[7] 1526-32(2001) was intimately mixed with the oxide to provide homogeneously 243. D. Sibold, R. L. Cook, K. Bader, and I. Reimanis, "Porous Hexaluminate dispersed porosity. The coatings were initially amorphous, but, Advances in Ceramic Matrir Composites 1. Edited by N. P. Bansal, J.P. Singh, and ille, OH, 2001 crystallized to an intimate mixture of nanometer-sized YAG and 2M. Y. He and JW "Crack Deflection at the Interface between residual amorphous carbon. Further heat treatment in air resulted Dissimilar Materials, "Int J Solids Struct, 25, 1053-67(1989). T.A. Parthasarathy, T Mah, and KKeller, "Cree chanism of Polycrystalline n a porous YAG coating Strengths of coated tows were initially Y trium Aluminum Garnet,"J. Am. Ceram Soc, 7517)1756-59(1992) in air, the strengths of the coated tows were reduced but compa- Sci Lert, 12, 379-82(192 Yttrium Aluminium Garnet Single Crystals,".Mater. higher than those of as-received tows with longer heat treatments rable to those of uncoated tows heated under similar conditions for 2>S. Karato, Z. Wang, and K. Fujino, "High-Temperature Creep of Yttrium- short times. For times of 100 h, the coated tows had a strength of on of Aluminum for silicon 20% less than the uncoated tows the System 3MnO- A2O, SiOx-3Y2O3 5Al2O3, "Am. Mineral, 7, 519(1951). As-processed minicomposites containing porous YAG fiber coatings had strengths that were nearly twice as strong as mini- atrix Composites. Edited by w. Krenkel, R Naslain, and Schneider. Wiley-VCH, Weinheim, Germany, 200 composites prepared without a fiber coating, despite the presence 3M. K. Cinibulk, "Synthesis of Yttrium Aluminum Garnet from a Mixed-Metal of high levels of matrix porosity. After heat-treating for 100 h in air, the strengths of minicomposites with and without fiber 32R. S. Hay, "Sol-Gel Coating of Fiber Tows, "Ceram. Eng. Sci. Proc., 12 [7-8] coatings were the same. This was attributed to densification of the 064-74(1991) 3K. A. Keller, T Mah, E. E. Boakye, and T. A Parthasarathy, "Gel-Casting and YAG coating, whic Reaction Bonding of Oxide-Oxide Minicomposites with Monazite Interphase vol% that no longe ned to deflect matrix cracks. hence Ceram. Eng. Sci Proc., 21 [3]525-34(2000) after long times at without 34K.A. Keller, T. A. Parthasarathy, T. Mah, M. K. Cinibulk, and E.E. boakye valuation of mona ber Coatings in a Dense Matrix Composite, Ceram. Eng coatings behaved similarly Sc.Proc,2013]451-61(1999below that critical level may have occurred and could be respon￾sible for the reduction in composite strength compared to that of composites without a fiber coating. Clearly, in minicomposite D, the volume fraction of porosity in the coating has been reduced to less than the porosity present in the alumina matrix (Fig. 9). One particularly interesting finding is that the presence of a porous YAG coating gives a higher-strength composite, even if the matrix is porous. Porosity alone in the matrix is not sufficient to give a high-strength composite at 1200°C. Whether the fine porosity of the porous YAG fiber coating is needed to provide a weaker interface than a porous matrix alone can provide, or whether the fibers are damaged by the alumina matrix (including its processing) in the absence of a porous YAG fiber coating, is not clear. The level of porosity present in the matrix (
40 vol%) should be adequate to give composite-like behavior and high strength. In commercial porous oxide matrix composites, nearly fully retained fiber strengths are obtained after processing and long-term heat treatment at 1200°C.13 However, the commercial composites are produced with Nextel 720 diphasic alumina– mullite fibers, not single-phase alumina, so a direct comparison cannot be made because of the differences in fiber moduli, thermal expansion, and bonding/interaction with the matrix. Ideally, fibers would need to be extracted from the matrix after processing to test for strength degradation; however, extraction of suitable lengths of alumina fibers from an alumina matrix for testing is difficult, if not impossible, without damaging the fibers. In recent work, the presence of a YAG second phase in a porous alumina matrix composite without a separate fiber coating also gave increased strengths over control composites at 1200°C.42 The control composites were found to be of equal strength at 1100°C but decreased rapidly at 1200°C, whereas the YAG-containing composites displayed greater strengths at 1200°C for times of up to 100 h. YAG was distributed as a dense phase bonding the alumina particles of the matrix and also separating the fiber from the matrix alumina. These results, along with those of the present study, suggest that YAG inhibits densification of the matrix at 1200°C. In the absence of YAG in the matrix, however, a porous YAG fiber coating is sufficient to deflect cracks only until a critical coating density and/or matrix density is reached, which occurs sometime between 5 and 100 h at 1200°C. IV. Conclusions Polymeric solutions were used to apply a porous yttrium aluminum garnet (YAG, Y3Al5O12) coating to fiber tows at temperatures that did not degrade fiber strength. The use of the polymeric solution also allowed for a fugitive carbon phase that was intimately mixed with the oxide to provide homogeneously dispersed porosity. The coatings were initially amorphous, but, when heated at 1000°C for 1 h in inert atmospheres, they crystallized to an intimate mixture of nanometer-sized YAG and residual amorphous carbon. Further heat treatment in air resulted in a porous YAG coating. Strengths of coated tows were initially higher than those of as-received tows. With longer heat treatments in air, the strengths of the coated tows were reduced but compa￾rable to those of uncoated tows heated under similar conditions for short times. For times of 100 h, the coated tows had a strength of
20% less than the uncoated tows. As-processed minicomposites containing porous YAG fiber coatings had strengths that were nearly twice as strong as mini￾composites prepared without a fiber coating, despite the presence of high levels of matrix porosity. After heat-treating for 100 h in air, the strengths of minicomposites with and without fiber coatings were the same. This was attributed to densification of the YAG coating, which resulted in residual porosity levels of 10 vol% that no longer functioned to deflect matrix cracks; hence, after long times at 1200°C, the composites with and without coatings behaved similarly. References 1 E. Ryshkewitch, “Compression Strength of Porous Sintered Alumina and Zirco￾nia,” J. Am. Ceram. Soc., 36 [2] 65–68 (1953). 2 R. L. Coble and W. D. Kingery, “Effect of Porosity on Physical Properties of Sintered Alumina,” J. Am. Ceram. Soc., 39 [11] 377–85 (1956). 3 L. A. Simpson, “Effect of Microstructure on Measurements of Fracture Energy of Al2O3,” J. Am. Ceram. Soc., 56 [1] 7–11 (1973). 4 E. A. Dean and J. A. Lopez, “Empirical Dependence of Elastic Moduli on Porosity for Ceram Materials,” J. Am. Ceram. Soc., 60 [7–8] 345–49 (1977). 5 A. S. Wagh, J. P. Singh, and R. B. Poeppel, “Dependence of Ceramic Fracture Properties on Porosity,” J. Mater. Sci., 28, 3589–93 (1993). 6 D. C. C. Lam, F. F. Lange, and A. G. Evans, “Mechanical Properties of Partially Dense Alumina Produced from Powder Compacts,” J. Am. Ceram. Soc., 77 [8] 2113–17 (1994). 7 S. C. Nanjangud, R. Brezny, and D. J. Green, “Strength and Young’s Modulus Behavior of a Partially Sintered Porous Alumina,” J. Am. Ceram. Soc., 78 [1] 266–68 (1996). 8 R. W. Rice, “Evaluation and Extension of Physical Property–Porosity Models Based on Minimum Solid Area,” J. Mater. Sci., 31, 102–18 (1996). 9 A. P. Roberts and E. J. Garboczi, “Elastic Properties of Model Porous Ceramics,” J. Am. Ceram. Soc., 83 [12] 3041–48 (2000). 10M. G. Harrison, M. L. Millard, and A. Szweda, “Fiber Reinforced Ceramic Matrix Composite Member and Method of Making,” U.S. Pat. No. 5 306 554, Apr. 26, 1994. 11W.-C. Tu, F. F. Lange, and A. G. Evans, “Concept for a Damage-Tolerant Ceramic Composite with ’Strong’ Interfaces,” J. Am. Ceram. Soc., 79 [2] 417–24 (1996). 12C. G. Levi, J. Y. Yang, B. J. Dalgleish, F. W. Zok, and A. G. Evans, “Processing and Performance of an All-Oxide Ceramic Composite,” J. Am. Ceram. Soc., 81 [8] 2077–86 (1998). 13S. G. Steel, L. P. Zawada, and S. Mall, “Fatigue Behavior of a Nextel 720/Alumina Composite at Room and Elevated Temperature,” Ceram. Eng. Sci. Proc., 22 [3] 695–702 (2001). 14M. K. Cinibulk, K. A. Keller, T. Mah, and T. A. Parthasarathy, “Nextel 610 and 650 Fiber Reinforced Porous Alumina–YAG Matrix Composites,” Ceram. Eng. Sci. Proc., 22 [3] 677–86 (2001). 15M. H. Jaskowiak, S. I. Eldridge, J. B. Hurst, and J. A. Setlock, “Interfacial Coatings for Sapphire/Al2O3”; p. 84 in HITEMP Review. NASA Conference Publication 10082, 1991. 16L. U. J. T. Ogbuji, “A Porous Oxidation-Resistant Fiber Coating for CMC Interphase,” Ceram. Eng. Sci. Proc., 16 [4] 497–505 (1995). 17L. U. J. T. Ogbuji, “Evaluation of a Porous Fiber Coating in SiC–Si3N4 Minicomposites,” J. Mater. Res., 12, 1287–96 (1997). 18O. Sudre, A. G. Razzell, L. Molliex, and M. Holmquist, “Alumina Single-Crystal Fiber Reinforced Alumina Matrix for Combustor Tiles,” Ceram. Eng. Sci. Proc., 19 [4] 273–80 (1998). 19E. Boakye, M. D. Petry, and R. S. Hay, “Porous Aluminum Oxide and Lanthanum Phosphate Fiber Coatings,” Ceram. Eng. Sci. Proc., 17 [4] 53–60 (1996). 20E. Boakye, R. S. Hay, and M. D. Petry, “Mixed Carbon-Aluminum Oxide Coatings from Aqueous Sols and Solutions,” Mater. Res. Soc. Symp. Proc., 432, 363–68 (1997). 21E. Boakye, R. S. Hay, M. D. Petry, and T. A. Parthasarathy, “Sol–Gel Synthesis of Zircon–Carbon Precursors and Coating of Nextel 720 Fiber Tows,” Ceram. Eng. Sci. Proc., 20 [3] 165–72 (1999). 22M. K. Cinibulk, T. A. Parthasarathy, K. A. Keller, and T. Mah, “Porous Rare-Earth Aluminate Fiber Coatings for Oxide–Oxide Composites,” Ceram. Eng. Sci. Proc., 21 [4] 219–28 (2000). 23T. A. Parthasarathy, E. E. Boakye, K. A. Keller, and R. S. Hay, “Evaluation of Porous Zirconia–Silica and Monazite Coatings using Nextel 720 Fiber-Reinforced Blackglas Minicomposites,” J. Am. Ceram. Soc., 84 [7] 1526–32 (2001). 24J. D. Sibold, R. L. Cook, K. Bader, and I. Reimanis, “Porous Hexaluminate Coatings for Oxide/Oxide Composites”; pp. 3–13 in Ceramic Transactions, Vol. 103, Advances in Ceramic Matrix Composites V. Edited by N. P. Bansal, J. P. Singh, and E. Ustundag. American Ceramic Society, Westerville, OH, 2001. 25M. Y. He and J. W. Hutchinson, “Crack Deflection at the Interface between Dissimilar Materials,” Int. J. Solids Struct., 25, 1053–67 (1989). 26T. A. Parthasarathy, T. Mah, and K. Keller, “Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet,” J. Am. Ceram. Soc., 75 [7] 1756–59 (1992). 27G. S. Corman, “Creep of Yttrium Aluminium Garnet Single Crystals,” J. Mater. Sci. Lett., 12, 379–82 (1993). 28S. Karato, Z. Wang, and K. Fujino, “High-Temperature Creep of Yttrium￾Aluminium Garnet Single Crystals,” J. Mater. Sci., 29, 6458–62 (1994). 29H. S. Yoder and M. L. Keith, “Complete Substitution of Aluminum for Silicon: the System 3MnO
Al2O3
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5Al2O3,” Am. Mineral., 7, 519 (1951). 30D. M. Wilson, “New High Temperature Oxide Fibers”; pp. 3–12 in High Temperature Ceramic Matrix Composites. Edited by W. Krenkel, R. Naslain, and H. Schneider. Wiley-VCH, Weinheim, Germany, 2001. 31M. K. Cinibulk, “Synthesis of Yttrium Aluminum Garnet from a Mixed-Metal Citrate Precursor,” J. Am. Ceram. Soc., 83 [5] 1276–78 (2000). 32R. S. Hay, “Sol–Gel Coating of Fiber Tows,” Ceram. Eng. Sci. Proc., 12 [7–8] 1064–74 (1991). 33K. A. Keller, T. Mah, E. E. Boakye, and T. A. Parthasarathy, “Gel-Casting and Reaction Bonding of Oxide–Oxide Minicomposites with Monazite Interphase,” Ceram. Eng. Sci. Proc., 21 [3] 525–34 (2000). 34K. A. Keller, T. A. Parthasarathy, T. Mah, M. K. Cinibulk, and E. E. Boakye, “Evaluation of Monazite Fiber Coatings in a Dense Matrix Composite,” Ceram. Eng. Sci. Proc., 20 [3] 451–61 (1999). November 2002 Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites 2709