12 D. Rodeghiero et al / Materials Science and Engineering 4244(1998)11-21 increase in both fracture strength and fracture tough ness for the ZrO -reinforced Al,O3 system was re- ported. Rice attributed this beneficial behavior to the extreme homogeneity of the sol-gel derived com osites. In 1984. Hoffman et al. also demonstrated the extensive composite homogeneity and dispersion that could be achieved through sol-gel approaches by syn and CdS/SiO, (where the AgCl and Cds phases were in s fine-scale photosensitive composites such as AgCl/SiO crystalline form), to phase-separated phosphate and mixed oxide glasses such as CrPO4/SiO2, CePO/Sio and Nd, SIO2(where both the matrix and minor phases were amorphous)[10, 1l]. At the same time, Roy et al. were also using these same synthesis procedures to produce the first sol-gel derived metal-ceramic com- osites(including Cu/Al,O3, Ni/Al,O3, Cu/ZrO2 and 2-theta( degrees) Cu/SiO2)[12]. In fact, Hoffman et al. and Roy et al. Fi of XRD patterns for the processing of a 20/80 vol% collectively performed the most extensive work on the Ni al-ceramic composite: (a) dried, unreduced powder, sol-gel synthesis of ceramic matrix composites to the blet drogen reduced powder, and(c)1400C hot-pressed present day, thoroughly investigating over 30 different hemical systems. However, in their efforts no attention was given to high temperature consolidation, mechani- stiffer and tougher than pure alumina, these composites cal properties or structural applications. Finally, sol were consolidated at lower temperatures and pressures gel techniques have even been successfully used to than would have been required had conventional pow- roduce multilayer ceramic-ceramic composites [13]. In der processing techniques been used this work, the phenomenon of Liesegang band forma tion was used to produce two-dimensional precipitated CuCrOa layers in silica gels. The thickness and spacing 2. Experimental of these layers were shown to be tailorable, and the bands were also shown to survive sintering tempera- 2. 1. Metal-ceramic composite synthesis tures as high as 1100C, indicating that high tempera ure anisotropic composites could be produced To produce the Ni/alumina and Ni/zirconia metal In this paper, we review our synthesizing a ceramic composites, first a 0.15 M absolute ethanol range of different high temperature ceramic matrix composites using sol-gel techniques. These composites vary from being metal-ceramic in nature(e.g. Ni/ a Al2O3, Fe/a-Al2O3, etc. ) to ceramic-ceramic (e.g. SiC articulate reinforced a-AL, O3). Throughout this work, it is the physical and mechanical properties of the g Sic composites, the composite microstructures and the property: microstructure: synthesis relationships which are the elements of primary interest. The advantages 3 gained from using the sol-gel type syntheses in place of conventional powder mixing and processing are numer ous. For instance. in the case of the metal-ceramic composites, extremely fine (often nanoscale)mi- crostructure with a high degree of dispersion between the metal and ceramic phases have been produced. As a result, these composites exhibit enhanced toughness and durability as well as a simultaneous potential for catal- ysis applications. Likewise, the efforts at producing 2-theta(degree ol-gel derived, Sic-reinforced alumina composites have resulted in materials with highly uniform and Fig. 2 ce of XRD patterns for th Alo posite:(a) dried, uncal homogeneous morphologies without the presence of cined (b)900C air calcined powder, and(c)1750.C hot- Sic agglomerates. Furthermore, while being lighter, pressed pellet12 E.D. Rodeghiero et al. / Materials Science and Engineering A244 (1998) 11–21 increase in both fracture strength and fracture tough￾ness for the ZrO2-reinforced Al2O3 system was re￾ported. Rice attributed this beneficial behavior to the extreme homogeneity of the sol–gel derived com￾posites. In 1984, Hoffman et al. also demonstrated the extensive composite homogeneity and dispersion that could be achieved through sol–gel approaches by syn￾thesizing various types of di-phasic gels ranging from fine-scale photosensitive composites such as AgCl/SiO2 and CdS/SiO2 (where the AgCl and CdS phases were in crystalline form), to phase-separated phosphate and mixed oxide glasses such as CrPO4/SiO2, CePO4/SiO2 and Nd2O3/SiO2 (where both the matrix and minor phases were amorphous) [10,11]. At the same time, Roy et al. were also using these same synthesis procedures to produce the first sol–gel derived metal–ceramic com￾posites (including Cu/Al2O3, Ni/Al2O3, Cu/ZrO2 and Cu/SiO2) [12]. In fact, Hoffman et al. and Roy et al. collectively performed the most extensive work on the sol–gel synthesis of ceramic matrix composites to the present day, thoroughly investigating over 30 different chemical systems. However, in their efforts no attention was given to high temperature consolidation, mechani￾cal properties or structural applications. Finally, sol– gel techniques have even been successfully used to produce multilayer ceramic–ceramic composites [13]. In this work, the phenomenon of Liesegang band forma￾tion was used to produce two-dimensional precipitated CuCrO4 layers in silica gels. The thickness and spacing of these layers were shown to be tailorable, and the bands were also shown to survive sintering tempera￾tures as high as 1100°C, indicating that high tempera￾ture anisotropic composites could be produced. In this paper, we review our work in synthesizing a range of different high temperature ceramic matrix composites using sol–gel techniques. These composites vary from being metal–ceramic in nature (e.g. Ni/a￾Al2O3, Fe/a-Al2O3, etc.) to ceramic–ceramic (e.g. SiC particulate reinforced a-Al2O3). Throughout this work, it is the physical and mechanical properties of the composites, the composite microstructures and the property:microstructure:synthesis relationships which are the elements of primary interest. The advantages gained from using the sol–gel type syntheses in place of conventional powder mixing and processing are numer￾ous. For instance, in the case of the metal–ceramic composites, extremely fine (often nanoscale) mi￾crostructures with a high degree of dispersion between the metal and ceramic phases have been produced. As a result, these composites exhibit enhanced toughness and durability as well as a simultaneous potential for catal￾ysis applications. Likewise, the efforts at producing sol–gel derived, SiC-reinforced alumina composites have resulted in materials with highly uniform and homogeneous morphologies without the presence of SiC agglomerates. Furthermore, while being lighter, Fig. 1. Sequence of XRD patterns for the processing of a 20/80 vol.% Ni/a-Al2O3 metal–ceramic composite; (a) dried, unreduced powder, (b) 1000°C hydrogen reduced powder, and (c) 1400°C hot-pressed pellet. stiffer and tougher than pure alumina, these composites were consolidated at lower temperatures and pressures than would have been required had conventional pow￾der processing techniques been used. 2. Experimental 2.1. Metal–ceramic composite synthesis To produce the Ni/alumina and Ni/zirconia metal– ceramic composites, first a 0.15 M absolute ethanol Fig. 2. Sequence of XRD patterns for the processing of a 20/80 vol.% SiC(whisker)/a-Al2O3 ceramic–ceramic composite; (a) dried, uncal￾cined powder, (b) 900°C air calcined powder, and (c) 1750°C hot￾pressed pellet