D B. Marshall, J.B. Davis Current Opinion in Solid State and Materials Science 5(2001)283-20 2. Fiber developments erties of such composites see Refs. [*16, 17]. The optimum strength for the matrix in such composites The major constraint on further development of oxide involves a trade-off of properties: the matrix strength must during processing of the matrix and during use. The between a broken fiber and its neighbors, whereas a low highest performance fibers that are now available in matrix strength leads to poor off-axis properties, compres- adequate quantities and at reasonable cost are fine-grained sive strength, and erosion/abrasion resistance. The major high-purity alumina(Nextel 610) and alumina/mullite challenge is to ensure the high temperature stability of the (Nextel 720")fibers produced by the 3M company. The optimum microstructure creep, grain growth and strength characteristics of these The earliest work on porous matrix composites used fibers are now well documented [*5, 6]. The Nextel 610 silica-based matrices [18], which place a restrictive limit fibers have the higher strengths(3.3 GPa, cf. 2. 1 GPa) on high temperature stability. Recent measurements from a and are expected to be more corrosion resistant in certain composite produced by geae (Gen IV ), consisting of environments, whereas the Nextel 720 fibers are more stacked layers of woven alumina fibers(Nextel 610)in an creep resistant and stable to higher temperatures. Based on alumino-silicate matrix, indicate a significant loss in data presently available, the anticipated maximum tem- properties and embrittlement at 950C [19]. The high peratures for extended use are about 1 100C for the Nextel temperature tensile strengths were reduced compared with 610 and 1200 C for the Nextel 720(but dependent on room temperature values by 15 and 50% for unnotched and tress levels and lifetimes) notched test specimens, while observations of fracture a promising approach for improving the creep resist- morphologies indicated a large reduction in the degree of Ice of the alumina fibers has been exploited in a new uncorrelated fiber fracture and pullout. Significant notch fiber (Nextel 650")from 3M[+7. These fibers contain sensitivity was observed even at room temperature(30% 1%Y2O3 as well as -10% ZrO2, based on studies reduction in net-section strength). Large improvements showing that doping of grain boundaries in bulk poly- were seen in another alumino-silicate matrix composite crystalline alumina with Y or La leads to large reductions with more stable mullite fibers(Nextel 720), with no effect in creep and grain growth [8]. Improvements were of notches on net section strength at temperatures up to achieved in the creep resistance compared with Nextel 1100.C, and small effect at 1200.C(20% reduction)[20 610, although the creep rates remain higher than those of However, under creep conditions the effect of notches was the mullite-containing Nextel 720 fibers. Most of the stronger (-40% reduction in net section strength at properties of the new fibers fall between those of Nextel 1100.C)[21], while long-term aging at temperatures of 1 100C and above caused significant reductions in ro main on the role of the ZrO, in changing the properties temperature strengths(60% reduction at 1200C)/222om 610 and Nextel 720. Some interesting questions re and in the relative amounts of Y,O3 in the grain A major advance for porous matrix composites came boundaries and in solid solution in the zro with the development of a mullite-based matrix consisting Further increases in temperature capabilities of poly- of relatively large(l um)mullite powder particles, which crystalline oxide fibers are possible with multi-phase are resistant to sintering, forming a continuous network microstructures designed to resist creep and grain growth bonded together with smaller alumina particles (-0.2 um) or with other systems such as YAG. However, large which sinter more readily [14, 15]. Zirconia has also beer increases will be difficult, for the fine grain sizes required used as the more readily sintered phase [23]. With a to achieve high strengths lead to relatively rapid creep and processing temperature(1200C) close to the limit of the grain growth. Laboratory-scale fabrication of polycrystal- most refractory oxide fibers presently available(Nextel line mullite fibers with promising, but very preliminary 720), these or related matrix compositions appear to offer property data have been reported [9]. Several groups are the most promising prospects for long-term stability developing directionally solidified alumina/YAG eutectic Recent high temperature aging experiments with a 2-D fibers, which if they could be produced in small sizes and woven mullite-alumina composite(formed from multiple large quantities would provide very large increases in layers of fabric)[24] have demonstrated that room-tem- temperature capabilities and expand the options for matrix perature tensile properties were not degraded after aging for 1000 h at temperatures up to 1200C in air, as shown in Fig. 1. The 0/90 strengths and failure strains were unchanged(Fig. 1), while the +45 strength, Youngs 3. Weak(porous)matrix composites modulus, and matrix hardness all increased after aging at the highest temperature. The increases indicate that the effectiveness of the porous matrix concept in mullite/alumina matrix underwent some degree of sinter- allowing damage-tolerant composites without the presence ing during the aging period. Under the same conditions, of fiber coatings was previously demonstrated [14, 15]. Fo the previously mentioned alumino-silicate composites suf- up-to-date accounts of the design and mechanical prop- fered strength loss of more than 60%284 D.B. Marshall, J.B. Davis / Current Opinion in Solid State and Materials Science 5 (2001) 283–289 2. Fiber developments erties of such composites see Refs. [**16,*17]. The optimum strength for the matrix in such composites The major constraint on further development of oxide involves a trade-off of properties: the matrix strength must composites in the near future will be the temperature limit be low enough to prevent damage in the matrix from imposed by the microstructural stability of the fibers, both extending into the fibers and to prevent transfer of stress during processing of the matrix and during use. The between a broken fiber and its neighbors, whereas a low highest performance fibers that are now available in matrix strength leads to poor off-axis properties, compres￾adequate quantities and at reasonable cost are fine-grained sive strength, and erosion/abrasion resistance. The major high-purity alumina (Nextel 610E) and alumina/mullite challenge is to ensure the high temperature stability of the (Nextel 720E) fibers produced by the 3M company. The optimum microstructure. creep, grain growth and strength characteristics of these The earliest work on porous matrix composites used fibers are now well documented [**5,6]. The Nextel 610 silica-based matrices [18], which place a restrictive limit fibers have the higher strengths (|3.3 GPa, cf. 2.1 GPa) on high temperature stability. Recent measurements from a and are expected to be more corrosion resistant in certain composite produced by GEAE (Gen IVE), consisting of environments, whereas the Nextel 720 fibers are more stacked layers of woven alumina fibers (Nextel 610) in an creep resistant and stable to higher temperatures. Based on alumino-silicate matrix, indicate a significant loss in data presently available, the anticipated maximum tem- properties and embrittlement at 9508C [19]. The high peratures for extended use are about 11008C for the Nextel temperature tensile strengths were reduced compared with 610 and 12008C for the Nextel 720 (but dependent on room temperature values by 15 and 50% for unnotched and stress levels and lifetimes). notched test specimens, while observations of fracture A promising approach for improving the creep resist- morphologies indicated a large reduction in the degree of ance of the alumina fibers has been exploited in a new uncorrelated fiber fracture and pullout. Significant notch fiber (Nextel 650E) from 3M [**7]. These fibers contain sensitivity was observed even at room temperature (30% |1% Y O as well as |10% ZrO , based on studies reduction in net-section strength). Large improvements 23 2 showing that doping of grain boundaries in bulk poly- were seen in another alumino-silicate matrix composite crystalline alumina with Y or La leads to large reductions with more stable mullite fibers (Nextel 720), with no effect in creep and grain growth [8]. Improvements were of notches on net section strength at temperatures up to achieved in the creep resistance compared with Nextel 11008C, and small effect at 12008C (|20% reduction) [20]. 610E, although the creep rates remain higher than those of However, under creep conditions the effect of notches was the mullite-containing Nextel 720E fibers. Most of the stronger (|40% reduction in net section strength at properties of the new fibers fall between those of Nextel 11008C) [21], while long-term aging at temperatures of 610E and Nextel 720E. Some interesting questions re- 11008C and above caused significant reductions in room main on the role of the ZrO in changing the properties temperature strengths (60% reduction at 12008C) [22]. 2 and in the relative amounts of Y O in the grain A major advance for porous matrix composites came 2 3 boundaries and in solid solution in the ZrO . with the development of a mullite-based matrix consisting 2 Further increases in temperature capabilities of poly- of relatively large (|1 mm) mullite powder particles, which crystalline oxide fibers are possible with multi-phase are resistant to sintering, forming a continuous network microstructures designed to resist creep and grain growth bonded together with smaller alumina particles (|0.2 mm), or with other systems such as YAG. However, large which sinter more readily [14,15]. Zirconia has also been increases will be difficult, for the fine grain sizes required used as the more readily sintered phase [23]. With a to achieve high strengths lead to relatively rapid creep and processing temperature (12008C) close to the limit of the grain growth. Laboratory-scale fabrication of polycrystal- most refractory oxide fibers presently available (Nextel line mullite fibers with promising, but very preliminary 720), these or related matrix compositions appear to offer property data have been reported [9]. Several groups are the most promising prospects for long-term stability. developing directionally solidified alumina/YAG eutectic Recent high temperature aging experiments with a 2-D fibers, which if they could be produced in small sizes and woven mullite-alumina composite (formed from multiple large quantities would provide very large increases in layers of fabric) [*24] have demonstrated that room-tem￾temperature capabilities and expand the options for matrix perature tensile properties were not degraded after aging processing [10–13]. for 1000 h at temperatures up to 12008C in air, as shown in Fig. 1. The 0/908 strengths and failure strains were unchanged (Fig. 1), while the 6458 strength, Young’s 3. Weak (porous) matrix composites modulus, and matrix hardness all increased after aging at the highest temperature. The increases indicate that the The effectiveness of the porous matrix concept in mullite/alumina matrix underwent some degree of sinter￾allowing damage-tolerant composites without the presence ing during the aging period. Under the same conditions, of fiber coatings was previously demonstrated [14,15]. For the previously mentioned alumino-silicate composites suf￾up-to-date accounts of the design and mechanical prop- fered strength loss of more than 60%