3314 Journal of the American Ceramic Society--Zok 0.8 between deflection and penetration. Pertinent experimental measurements and mode g 3), n a tudies matrix properties Tm and Em are presented in Section V combining with the Penetration corresponding fiber properties via Eq assessment is made of the efficacy of the porous-matrix for a specific mat rix/fiber combination(Section V(4) 0.4 1200°c IV. Evolution of porous- Matrix Materials 5 .LAging Early generations of porous-matrix CFCCs(produced by Gen- 1000h一 eral Electric(Cincinnati, OH)and later by CoI Ceramics(Sa 0.2 Diego, CA)) comprise alumina powder and a silica-forming 100h polymer precursor. Commercially, the composites are manu factured using procedures adapted from the polymer composites 2h industry. Prepregs are made by immersing woven fiber cloth into 0.2 a dispersed ceramic slurry. They are then stacked, warm molded Elastic mismatch parameter, A n an autoclave, and fired at an elevated temperature(typically 1000C)to remove organics and pyrolyze the polymer. This rocess yields a contiguous nanoporous silica phase within an alumina particle network. Although the early generation com- posites with these constituents exhibited attractive mechanical 10 properties after fabrication, significant degradation was ob- Low porosity tained following extended heat treatments at temperatures be- yond 1000.C: a consequence of matrix sintering. A variant of this concept uses a precursor-derived alumina as the binder (in place of silica), with the intent of enhancing morphological sta bility. However, the alumina particle network remains suscep tible to densification at yet higher temperatures(1100%-1200C) H typical of targeted service conditions, especially when the pa ticles are fine(<I um). More significant enhancements in stability have been (b) achieved through the use of mullite as the main matrix constitu ent and the binder. -In a common implementation, 0.1 mullite powder is dispersed in an aqueous slurry and infiltrated into a fiber preform via a vacuum-assisted technique. The alu- Modulus ratio, Em/Et mina is introduced in one of two ways: by mixing fine alumina particles into the mullite-containing slurry, or by subsequent Fig 12. (a)Conditions for crack deflection at a fiber-ma mpregnation and pyrolysis of an alumina precursor solution. (adapted from He and Hutchinson). Experimental dat lite-alumina particle mixtures, assuming a toughness rati The two processing routes lead to distinctly different matrix and fiber properties Tr= 15J/m-and Er= 260 GPa. (b)Ce topologies, shown schematically in Fig. 13(a). Compositional representation of crack deflection conditions, showing th maps of two prototypical systems are presented bination of matrix toughness and modulus as well as the Fig.13(b)40 The preceding processing routes and the resulting microstruc- tures are characterized by three attributes: o The mixed mullite/alumina slurry method allows both An estimate of the property combination that leads to de- matrix phases to be infiltrated simultaneously By contrast, the flection is obtained in the following way. For A>0, the energy precursor route requires additional steps, beyond that of slurry release rate ratio is well described by the empirical equation infiltration, and is thus more costly (i The presence of particulate alumina can compromise the stability of the mullite network, especially if its proportion ex- (2) ceeds the percolation threshold. Conversely, if the slurry is comprised of only mullite and the alumina is introduced subse- This formula has an error <4% over the range 0<A<0.95. quently via the precursor route, the contiguity of the pon setting Gd Gp=T Tr and combining the result with eq network is ensured 1), the deflection condition can be re-expressed as (iii) Because of tions on the allowable fraction of par- ticulate alumina(to inhibit densification), the slurry route results in matrices that are relatively weak. Although essential for crack ∑≡0.13 (3) deflection, this weakness compromises the off-axis properties, especially the resistance to delamination. In contrast, the pre- cursor route allows for filling of the void space between the where 2 is a non-dimensional parameter that characterizes the particles in the network(at least while the pores remain open), propensity for crack deflection resulting in increases in the mechanical integrity of the network. The requisite combinations of I m/Tr and Em Er are plotted in The latter route provides access to a broader range of matrix Fig 12(b)for three assumed values of ((0.3-1). As matrix sin- propertie tering/densification proceeds, the properties follow a trajectory The morphological stability of porous mullite-alumina from the lower left corner of the diagram (when the porosity ces at the targeted upper use temperatures of oxide CFCCs has is high) to the upper right, eventually crossing the boundar been demonstrated through experiments on neat (fiber-free)ma terials(Fig. 14(a)). Specifically, compacts of I um mullite par icles exhibit no detectable shrinkage after 1000 h of exposure at 200C. Mixtures containing <% alumina particles(0. 2 um diameter) are similarly stable, with porosity changing <0.5%An estimate of the property combination that leads to de- flection is obtained in the following way. For D  0, the energy release rate ratio is well described by the empirical equation Gd Gp ¼ 1 4 1ð Þ D 0:9 (2) This formula has an error r4% over the range 0rDr0.95. Upon setting Gd/Gp 5 Gi/Gf and combining the result with Eq. (1), the deflection condition can be re-expressed as1 S  0:13 Gf Gm
1 þ Ef Em
0:9 > o (3) where S is a non-dimensional parameter that characterizes the propensity for crack deflection. The requisite combinations of Gm/Gf and Em/Ef are plotted in Fig. 12(b) for three assumed values of o (0.3–1). As matrix sin￾tering/densification proceeds, the properties follow a trajectory from the lower left corner of the diagram (when the porosity is high) to the upper right, eventually crossing the boundary between deflection and penetration. Pertinent experimental measurements and modeling studies on the matrix properties Gm and Em are presented in Section V. Upon combining with the corresponding fiber properties via Eq. (3), an assessment is made of the efficacy of the porous-matrix concept for a specific mat￾rix/fiber combination (Section V(4)). IV. Evolution of Porous-Matrix Materials Early generations of porous-matrix CFCCs (produced by Gen￾eral Electric (Cincinnati, OH) and later by COI Ceramics (San Diego, CA)) comprise alumina powder and a silica-forming polymer precursor.6 Commercially, the composites are manu￾factured using procedures adapted from the polymer composites industry. Prepregs are made by immersing woven fiber cloth into a dispersed ceramic slurry. They are then stacked, warm molded in an autoclave, and fired at an elevated temperature (typically 10001C) to remove organics and pyrolyze the polymer. This process yields a contiguous nanoporous silica phase within an alumina particle network. Although the early generation com￾posites with these constituents exhibited attractive mechanical properties after fabrication, significant degradation was ob￾tained following extended heat treatments at temperatures be￾yond 10001C: a consequence of matrix sintering. A variant of this concept uses a precursor-derived alumina as the binder (in place of silica), with the intent of enhancing morphological sta￾bility. However, the alumina particle network remains suscep￾tible to densification at yet higher temperatures (11001–12001C), typical of targeted service conditions, especially when the par￾ticles are fine (o1 mm). More significant enhancements in stability have been achieved through the use of mullite as the main matrix constitu￾ent and alumina as the binder.8,12 In a common implementation, mullite powder is dispersed in an aqueous slurry and infiltrated into a fiber preform via a vacuum-assisted technique. The alu￾mina is introduced in one of two ways: by mixing fine alumina particles into the mullite-containing slurry, or by subsequent impregnation and pyrolysis of an alumina precursor solution.39 The two processing routes lead to distinctly different matrix topologies, shown schematically in Fig. 13(a). Compositional maps of two prototypical systems are presented in Fig. 13(b).40,41 The preceding processing routes and the resulting microstruc￾tures are characterized by three attributes: (i) The mixed mullite/alumina slurry method allows both matrix phases to be infiltrated simultaneously. By contrast, the precursor route requires additional steps, beyond that of slurry infiltration, and is thus more costly. (ii) The presence of particulate alumina can compromise the stability of the mullite network, especially if its proportion ex￾ceeds the percolation threshold.42 Conversely, if the slurry is comprised of only mullite and the alumina is introduced subse￾quently via the precursor route, the contiguity of the mullite network is ensured. (iii) Because of limitations on the allowable fraction of par￾ticulate alumina (to inhibit densification), the slurry route results in matrices that are relatively weak. Although essential for crack deflection, this weakness compromises the off-axis properties, especially the resistance to delamination. In contrast, the pre￾cursor route allows for filling of the void space between the particles in the network (at least while the pores remain open), resulting in increases in the mechanical integrity of the network. The latter route provides access to a broader range of matrix properties. The morphological stability of porous mullite–alumina matri￾ces at the targeted upper use temperatures of oxide CFCCs has been demonstrated through experiments on neat (fiber-free) ma￾terials (Fig. 14(a)).42 Specifically, compacts of 1 mm mullite par￾ticles exhibit no detectable shrinkage after 1000 h of exposure at 12001C. Mixtures containing r20% alumina particles (0.2 mm diameter) are similarly stable, with porosity changing o0.5% Fig. 12. (a) Conditions for crack deflection at a fiber–matrix interface (adapted from He and Hutchinson37). Experimental data are for mul￾lite–alumina particle mixtures, assuming a toughness ratio oGi/Gm 5 1 and fiber properties Gf 5 15 J/m2 and Ef 5 260 GPa. (b) Complementary representation of crack deflection conditions, showing the critical com￾bination of matrix toughness and modulus as well as the effects of o. 1 Here, Poisson’s ratios of the fiber and the matrix are assumed to be the same. Con￾sequently, the plane strain modulus ratio E
f =E
m can be replaced with the Young’s modulus ratio Ef =Em. 3314 Journal of the American Ceramic Society—Zok Vol. 89, No. 11