D B. Marshall, J B. Davis/ Current Opinion in Solid State and Materials Science 5(2001)283-289 287 ponent oxide coatings by such methods is difficult. Solu- References tion precursor, sol-gel, or slurry methods potentially allow better control of stoichiometry, although it is more difficult Papers of particular interest, published within the annual to obtain uniform bridge-free coatings on individual fibers period of review, have been highlighted in tows or woven preforms s of special interest A technique that minimizes the bridging problem in- ** of outstanding interest volves passing the fiber tows through a coating solution or slurry covered with a layer of immiscible liquid that [1] Robinson RC, Smialek JL. SiC recession caused by SiO, scale displaces excess coating solution from between the fibers volatility under combustion conditions: I. Experimental results as they are withdrawn [58]. This method has proven nd empirical model. J Am Ceram Soc 1999: 82: 1817-25 effective for depositing thin coatings of monazite(=50- 2]Opila EJ, Smialek JL, Robinson RC, Fox DS, Jacobson NS. Sic 100 nm thickness ) on Nextel 610 and 720 fiber tows using caused by SiO, scale volatility under combustion conditions: Il. Thermodynamics and gaseous diffusion model. J a variety of different solution precursors and slurries Am Ceram Soc1999;82:1826-34 [42-44].It has also been used to deposit po 3]Cox BN, Zok Fw. Advances in ceramic composites reinforced by coatings using a fugitive carbon phase (La-hexaluminate, ontinuous fibers. Curr Opin Solid State Mater Sci 1996: 1: 666- YAG and zircon)[47, 48 14 Evans AG, Marshall DB, Zok F, Levi C. Recent advances in 7. Matrix processing methods 1999;8:17-23. [*5 Hay Rs, Boakeye E, Petry MD, Berta Y, Von Lehmden K, Welch J. Grain growth and tensile strength of 3-M Nextel 720TM after The composites described above were processed using a thermal exposure. Ceram Eng Sci Proc 1999, 20: 165-72, Tensile riety of liquid-based infiltration methods(slurry, sol-gel rength and grain growth of 3M Nextel 720 fibers measured recursor) to infiltrate the matrix material into fiber tows after heat-treatment from 1000 to 1500C for 20 min to 300 h woven fabrics at, or near room temperature, followed by Possible mechanisms for strength degradation are discussed sintering to strengthen the matrix. a challenge remains to [6]Petry MD, Mah T. Effect of thermal exposures on the strengths of extel 550 and 720 filaments. J Am Ceram Soc 1999: 82: 2801-7 develop more versatile methods capable of infiltrating high More strength and grain growth data for Nextel 720 and Nextel volume fractions of matrix into fiber preforms. An innova- 550 as a function of aging temperature and environment, with and tive method was described in Ref [23] for producing tapes ithout carbon coatings with very high particle packing and shear thinning charac u*7 Wilson DM, Visser LR. Nextel 650 ceramic oxide fiber. New teristics that allow infiltration by simple stacking of fabrics alumina-based fiber for high temperature composite rein- forcement. Ceram Eng Sci Proc 2000: 21: 363-73, Properties of and tapes followed by application of shear displacements new polycrystalline alumina fiber doped with Y2 O, and ZrO,to by vibration aprove microstructural stability and creep resistance. [8] Bruley J, Cho J, Chan HM, Harmer MP, Rickman JM. Scanning ansmission electron microscopy analysis of grain boundaries in 8. Conclusions creep-resistant yttrium- and lanthanum-doped alumina microstruc tures. J Am Ceram Soc 1999: 81: 2865-70. Several types of oxide composite now appear capable 9 Lewis MH, York S, Freeman C, Alexander IC, Al-Dawery 1 providing sustained use at temperatures as high as 1200C. butler EG, Doleman PA Oxide CMCs, novel fibres, coatings and abrication procedures. Ceram Eng Sci Proc 2000,21: 535-47 While considerable testing in combustion environments Laboratory scale polycrystalline mullite fibers with very prelimin- with realistic gas flow rates and compositions is still ry property measurement. Compatibility tests of LaPO, NdPO needed to prove their viability, and further improvements and Lavo, interphases in properties and processing methods are desirable, there is [10 Mah Tl, Parthasarathy TA, Kerans R, Processing, microstructure now an opportunity to explore innovative combustion and strength of Alumina-YAG eutectic polycrystals. J Am Ceram Soc2000;83:2088-90 component designs that avoid limitations of metallic [11] Sayir A editor, Directional Solidification of eutectic ceramics. In: ystems. Applications of monazite-based composites Pechenik A, Kalia RK, Vashita P, editors. Computer Aided Desig thermal protection systems for space re-entry vehicles are of High-Temperature Materials, New York: Oxford University already progressing rapidly in this direction [**41]. Sub- Press. 1999 component testing in wind tunnel, arc jet, and impact [12] Yoshikawa A, Epelbaum BM, Fukuda T, Suzuki K, Waku Y. environments at temperatures up to 1300.C, have shown rowth of Al203/Y3A15o12 eutectic fiber by micro-pulling down method and its high temperature strength and thermal large gains compared with conventional silica-based sys- bility. Jpn J Appl Phys 1999, 38: 55-8 [13 Yoshikawa A, Haseg Fukuda T. Suzuki K. Waku Growth and diameter control of Al2031Y3A15012 eutectic fiber by micro-pulling-down method and its high temperature strength Acknowledgements d thermal stability. Ceram Eng Sci Proc 1999: 20:275-82. [14 Lange FF, Tu w-C, Evans AG. Processing of damage-tolerant, oxidation-resistant ceramic-matrix composites by a precur Financial support was supplied by the Us Air Force infiltration and pyrolysis method. Mater Sci Eng 1995; A195: 145- Office of Scientific Research. contract No. F49620-00-C [15] Levi CG, Yang JY, Dalgleish BJ, Zok FW, Evans AG. ProcessingD.B. Marshall, J.B. Davis / Current Opinion in Solid State and Materials Science 5 (2001) 283–289 287 ponent oxide coatings by such methods is difficult. Solu- References tion precursor, sol–gel, or slurry methods potentially allow better control of stoichiometry, although it is more difficult Papers of particular interest, published within the annual to obtain uniform bridge-free coatings on individual fibers period of review, have been highlighted as: in tows or woven preforms. * of special interest; A technique that minimizes the bridging problem in- ** of outstanding interest. volves passing the fiber tows through a coating solution or slurry covered with a layer of immiscible liquid that [1] Robinson RC, Smialek JL. SiC recession caused by SiO scale 2 displaces excess coating solution from between the fibers volatility under combustion conditions: I. Experimental results and empirical model. J Am Ceram Soc 1999;82:1817–25. as they are withdrawn [58]. This method has proven [2] Opila EJ, Smialek JL, Robinson RC, Fox DS, Jacobson NS. SiC effective for depositing thin coatings of monazite (|50– recession caused by SiO scale volatility under combustion 2 100 nm thickness) on Nextel 610 and 720 fiber tows using conditions: II. Thermodynamics and gaseous diffusion model. J a variety of different solution precursors and slurries Am Ceram Soc 1999;82:1826–34. [**42–**44]. It has also been used to deposit porous [3] Cox BN, Zok FW. Advances in ceramic composites reinforced by coatings using a fugitive carbon phase (La-hexaluminate, continuous fibers. Curr Opin Solid State Mater Sci 1996;1:666– 73. YAG and zircon) [47,48]. [4] Evans AG, Marshall DB, Zok F, Levi C. Recent advances in oxide-oxide composite technology. Adv Composite Mater 1999;8:17–23. 7. Matrix processing methods [**5] Hay RS, Boakeye E, Petry MD, Berta Y, Von Lehmden K, Welch J. Grain growth and tensile strength of 3-M Nextel 720TM after The composites described above were processed using a thermal exposure. Ceram Eng Sci Proc 1999;20:165–72, Tensile variety of liquid-based infiltration methods (slurry, sol–gel, strength and grain growth of 3M Nextel 720E fibers measured after heat-treatment from 1000 to 15008C for 20 min to 300 h. precursor) to infiltrate the matrix material into fiber tows or Possible mechanisms for strength degradation are discussed. woven fabrics at, or near room temperature, followed by [6] Petry MD, Mah T. Effect of thermal exposures on the strengths of sintering to strengthen the matrix. A challenge remains to Nextel 550 and 720 filaments. J Am Ceram Soc 1999;82:2801–7, develop more versatile methods capable of infiltrating high More strength and grain growth data for Nextel 720 and Nextel volume fractions of matrix into fiber preforms. An innova- 550 as a function of aging temperature and environment, with and without carbon coatings. tive method was described in Ref. [23] for producing tapes [**7] Wilson DM, Visser LR. Nextel 650 ceramic oxide fiber. New with very high particle packing and shear thinning charac- alumina-based fiber for high temperature composite rein- teristics that allow infiltration by simple stacking of fabrics forcement. Ceram Eng Sci Proc 2000;21:363–73, Properties of and tapes followed by application of shear displacements new polycrystalline alumina fiber doped with Y O and ZrO to 23 2 by vibration. improve microstructural stability and creep resistance. [8] Bruley J, Cho J, Chan HM, Harmer MP, Rickman JM. Scanning transmission electron microscopy analysis of grain boundaries in 8. Conclusions creep-resistant yttrium- and lanthanum-doped alumina microstruc￾tures. J Am Ceram Soc 1999;81:2865–70. [9] Lewis MH, York S, Freeman C, Alexander IC, Al-Dawery I, Several types of oxide composite now appear capable of butler EG, Doleman PA. Oxide CMCs, novel fibres, coatings and providing sustained use at temperatures as high as 12008C. fabrication procedures. Ceram Eng Sci Proc 2000;21:535–47, While considerable testing in combustion environments Laboratory scale polycrystalline mullite fibers with very prelimin￾with realistic gas flow rates and compositions is still ary property measurement. Compatibility tests of LaPO , NdPO 4 4 and LaVO interphases. needed to prove their viability, and further improvements 4 [10] Mah TI, Parthasarathy TA, Kerans RJ. Processing, microstructure, in properties and processing methods are desirable, there is and strength of Alumina-YAG eutectic polycrystals. J Am Ceram now an opportunity to explore innovative combustion Soc 2000;83:2088–90. component designs that avoid limitations of metallic [11] Sayir A. editor, Directional Solidification of eutectic ceramics. In: systems. Applications of monazite-based composites in Pechenik A, Kalia RK, Vashita P, editors. Computer Aided Design thermal protection systems for space re-entry vehicles are of High-Temperature Materials, New York: Oxford University Press, 1999: 197–211. already progressing rapidly in this direction [**41]. Sub- [12] Yoshikawa A, Epelbaum BM, Fukuda T, Suzuki K, Waku Y. component testing in wind tunnel, arc jet, and impact Growth of Al2O3/Y3Al5O12 eutectic fiber by micro-pulling- environments at temperatures up to 13008C, have shown down method and its high temperature strength and thermal large gains compared with conventional silica-based sys- stability. Jpn J Appl Phys 1999;38:L55–8. tems. [13] Yoshikawa A, Hasegawa K, Fukuda T, Suzuki K, Waku Y. Growth and diameter control of Al2O31Y3Al5012 eutectic fiber by micro-pulling-down method and its high temperature strength Acknowledgements and thermal stability. Ceram Eng Sci Proc 1999;20:275–82. [14] Lange FF, Tu W-C, Evans AG. Processing of damage-tolerant, oxidation-resistant ceramic-matrix composites by a precursor Financial support was supplied by the US Air Force infiltration and pyrolysis method. Mater Sci Eng 1995;A195:145– Office of Scientific Research, contract No. F49620-00-C- 50. 0010. [15] Levi CG, Yang JY, Dalgleish BJ, Zok FW, Evans AG. Processing