MIATERIAL TENGE ENGMEERIM ELSEVIER Materials Science and Engineering A244(1998)11-21 Sol-gel synthesis of ceramic matrix composites E D. Rodeghiero, B C. Moore, B.S. Wolkenberg, M. Wuthenow, O. K. Tse, EP Giannelis Department of Materials Science and Engineering, Cornell Uninersity, Ithaca, NY 14853-1501, US.A Abstract Sol-gel techniques have been used to produce various high temperature ceramic matrix composites including Ni/a-Al,O3, e/a-Al,O3, Ni/ZrO2, SiC(whisker)/a-Al,O3, and SiC(platelet)/a-Al,O,, as well as chemically modified versions of some of these ystems. In all cases, the composites have displayed uniform microstructures with a high degree of dispersion between the matrix nd reinforcement phases, a goal often not achieved when utilizing conventional powder mixing and processing techniques. The metal-ceramic composites investigated exhibit enhanced toughness and machinability as well as the potential for catalytic applications due to their novel fine-scale microstructure. Likewise, the Sic-reinforced alumina materials have been shown to be lighter, stiffer and tougher than pure alumina, without the use of the extreme hot-pressing temperatures and pressures needed by conventional powder processing approaches to produce the same results. o 1998 Elsevier Science S.A. All rights reserved Keywords: Sol-gel; Ceramic matrix composites; Microstructure 1. ntroduction processes, and as a result, virtually all sol-gel research since the late 1980s has been carried out in the thin Sol-gel processing received extensive attention in the film/coating area. However, this ignores sol-gels po- 1980s as literally hundreds of re- tential to play a supporting role in the synthesis of searchers sought after novel, low temperature methods monolithic structural ceramics and ceramic composites of producing common oxide ceramics such as silica, In other words, while the production of structural alumina, zirconia and titania in fully dense monolithic ceramics will most likely never be accomplished solely form [1]. Much of this excitement resulted from the by low temperature sol-gel techniques, the incorpora roduction of the first large-scale xerogels by Yoldas in tion of some sol-gel aspects into a broader synthesis 975 [2-5]. These self-supporting monolithic alumina scheme can nevertheless be highly beneficial. In fact, gels were highly porous (60-70%), but nevertheless many pioneers of the sol-gel community have felt this suggested the potential for producing fully dense ce- way from the very beginning. For instance, Roy et al. ramic components at reduced temperatures in a near- described their original goal in the sol-gel field as net-shape fashion. Due to the inherent fracture achievement of homogeneity on the finest possible associated with the drying and consolidation of bulk scale he production of mono-phasic glasses and gels, however, it later became accepted that sol-gel mono-phasic ceramic powders and precursors [7]. In- would instead be limited to a much smaller realm of deed, it was Roy who first brought sol-gel science to applications, namely the production of thin films, wh broad attention in the ceramics industry in the 1950s because of their planar geometry were not susceptible and 1960s for exactly this reason [2] to the formidable cracking problem of monoliths More recently, it is the incorporation of sol-gel Fortunately for the sol-gel community, the great techniques into the synthesis of ceramic matrix com- explosion in the microelectronics industry came during posites which seems especially appealing. In 1981, Rice the same time period. This translated into a lar and Becher demonstrated that ZrO,Al,O3 ceramic-ce demand for both thin film materials and thin film ramic composites produced through sol-gel approaches were superior to their ball milled, powder-derived coun- Corresponding author. Tel: +1 607 2556684: fax: +1 607 terparts in overall fracture properties [8,9]. In fact, in 2552365 this work the first demonstration of a simultaneous 0921-5093/98/S19.00 0 1998 Elsevier Science S.A. All rights reserved PIS0921-5093(9700821-6Materials Science and Engineering A244 (1998) 11–21 Sol–gel synthesis of ceramic matrix composites E.D. Rodeghiero, B.C. Moore, B.S. Wolkenberg, M. Wuthenow, O.K. Tse, E.P. Giannelis * Department of Materials Science and Engineering, Cornell Uni6ersity, Ithaca, NY 14853-1501, USA Abstract Sol–gel techniques have been used to produce various high temperature ceramic matrix composites including Ni/a-Al2O3, Fe/a-Al2O3, Ni/ZrO2, SiC(whisker)/a-Al2O3, and SiC(platelet)/a-Al2O3, as well as chemically modified versions of some of these systems. In all cases, the composites have displayed uniform microstructures with a high degree of dispersion between the matrix and reinforcement phases, a goal often not achieved when utilizing conventional powder mixing and processing techniques. The metal–ceramic composites investigated exhibit enhanced toughness and machinability as well as the potential for catalytic applications due to their novel fine-scale microstructure. Likewise, the SiC-reinforced alumina materials have been shown to be lighter, stiffer and tougher than pure alumina, without the use of the extreme hot-pressing temperatures and pressures needed by conventional powder processing approaches to produce the same results. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Sol–gel; Ceramic matrix composites; Microstructure 1. Introduction Sol–gel processing received extensive attention in the 1970s and early 1980s as literally hundreds of re￾searchers sought after novel, low temperature methods of producing common oxide ceramics such as silica, alumina, zirconia and titania in fully dense monolithic form [1]. Much of this excitement resulted from the production of the first large-scale xerogels by Yoldas in 1975 [2–5]. These self-supporting monolithic alumina gels were highly porous (60–70%), but nevertheless suggested the potential for producing fully dense ce￾ramic components at reduced temperatures in a near￾net-shape fashion. Due to the inherent fracture associated with the drying and consolidation of bulk gels, however, it later became accepted that sol–gel would instead be limited to a much smaller realm of applications, namely the production of thin films, which because of their planar geometry were not susceptible to the formidable cracking problem of monoliths [6]. Fortunately for the sol–gel community, the great explosion in the microelectronics industry came during the same time period. This translated into a large demand for both thin film materials and thin film processes, and as a result, virtually all sol–gel research since the late 1980s has been carried out in the thin film/coating area. However, this ignores sol–gel’s po￾tential to play a supporting role in the synthesis of monolithic structural ceramics and ceramic composites. In other words, while the production of structural ceramics will most likely never be accomplished solely by low temperature sol–gel techniques, the incorpora￾tion of some sol–gel aspects into a broader synthesis scheme can nevertheless be highly beneficial. In fact, many pioneers of the sol–gel community have felt this way from the very beginning. For instance, Roy et al. described their original goal in the sol–gel field as achievement of ‘homogeneity on the finest possible scale’ in the production of mono-phasic glasses and mono-phasic ceramic powders and precursors [7]. In￾deed, it was Roy who first brought sol–gel science to broad attention in the ceramics industry in the 1950s and 1960s for exactly this reason [2]. More recently, it is the incorporation of sol–gel techniques into the synthesis of ceramic matrix com￾posites which seems especially appealing. In 1981, Rice and Becher demonstrated that ZrO2/Al2O3 ceramic–ce￾ramic composites produced through sol–gel approaches were superior to their ball milled, powder-derived coun￾terparts in overall fracture properties [8,9]. In fact, in this work the first demonstration of a simultaneous * Corresponding author. Tel.: +1 607 2556684; fax: +1 607 2552365. 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(97)008 21-6