Current Opinion in Materials science PERGAMON Current Opinion in Solid State and Materials Science 5(2001)283-289 Ceramics for future power generation technology: fiber reinforced oxide composites DB all*.JB. Day Rockwell Science Center. 1049 Camino Dos Rios. Thousand Oaks. CA 91360. USA Major advances have been made during the past 2 years in the development of fiber-reinforced oxide composites for long-life combustion components. These include demonstration of long-term stability of mullite-based porous-matrix composites at 1200C, development of fiber coating and slurry infiltration methods to produce composites with weakly bonded La-monazite interphases, and development of new alumina-based fibers. o 2001 Elsevier Science Ltd. All rights reserved eywords: Ceramics, Power generation technology, Fiber-reinforced oxide composites; Long-life combustion components; Alumina-based fibers, Porus matrix: Weak interface: La-monazite 1. Introduction temperatures, processing r infiltration of the matrix into fiber preforms been developed to the The need for high temperature ceramic composites for degree of sophistication for chemical vapor he power generation industry(both land-based and trans- infiltration, melt infiltration and polymer precursor infiltra- portation) to achieve significant gains in efficiency and tion of SiC; and until recently, weakly bonded interphases, reductions in NO, emissions is well established. Typical a key ingredient needed for damage tolerance(cf. carbon components include combustors, exhaust components, high or BN in SiC-based composites) were not available or even temperature ducts, thermal insulation, heat exchangers and identified. During the period covered by this review, 1999 hot gas filters. Engine and burner rig testing during the past and 2000, key advances have been made towards overcom- few years have demonstrated encouraging progress, the ing each of these limitations. See Refs. 3, 4,59] for reviews result of 20 years of world-wide research. However, the of testing has also shown that the long-term survival of Three approaches have been taken to design micro- Sic-containing composites, the materials which have re- structures of oxide composites that are damage tolerant ceived the vast majority of attention to date, can be limited All aim to prevent damage(cracking) in the matrix from by recession of the matrix by volatility of Sio, scale in crossing to the fibers, thereby allowing the dissipation of combustion environments containing water vapor [1, 21, stress concentrations while retaining the high strengths of contrary to previous expectations that oxidation embrittle- the fibers. Two of these are analogous to the conventional ment would be life-limiting. The undesirable need for approach taken with SiC-based composites, based on fiber environmental barrier coatings to protect these composites coatings that bond weakly, if at all, to the surface of the adds motivation to develop oxide composite systems that fibers, or coatings that are inherently weak(e.g, layered or are inherently stable in oxidizing environments porous). The third relies on the weakness and low stiffness However, oxide composites introduce a new set of of a porous matrix itself to prevent damage from extending imitations:their development is relatively immature, into the fibers, even though the interface between the fibers having received widespread attention only during the past and matrix might be strongly bonded. Tough composites years,the available fibers are not as microstructurally that show wood-like fracture have now been demonstrated able or creep resistant as non-oxide fibers at high using all three approaches. The key remaining challenges are to optimize high temperature properties, demonstrate long-term stability in high-temperature combustion en ding author. Tel. +1-805-373-4170. vironments, and develop robust economical manufacturing ress:dbmarsha@rsc.rockwell.com(.B.Marshall) methods 1359-0286/01/S- see front matter 2001 Elsevier Science Ltd. All rights reserved PII:S1359-0286(01)00017-1
Current Opinion in Solid State and Materials Science 5 (2001) 283–289 Ceramics for future power generation technology: fiber reinforced oxide composites D.B. Marshall , J.B. Davis * Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360, USA Abstract Major advances have been made during the past 2 years in the development of fiber-reinforced oxide composites for long-life combustion components. These include demonstration of long-term stability of mullite-based porous-matrix composites at 12008C, development of fiber coating and slurry infiltration methods to produce composites with weakly bonded La-monazite interphases, and development of new alumina-based fibers. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ceramics; Power generation technology; Fiber-reinforced oxide composites; Long-life combustion components; Alumina-based fibers; Porus matrix; Weak interface; La-monazite 1. Introduction temperatures; processing methods for infiltration of the matrix into fiber preforms have not been developed to the The need for high temperature ceramic composites for degree of sophistication available for chemical vapor the power generation industry (both land-based and trans- infiltration, melt infiltration and polymer precursor infiltraportation) to achieve significant gains in efficiency and tion of SiC; and until recently, weakly bonded interphases, reductions in NO emissions is well established. Typical a key ingredient needed for damage tolerance (cf. carbon x components include combustors, exhaust components, high or BN in SiC-based composites) were not available or even temperature ducts, thermal insulation, heat exchangers and identified. During the period covered by this review, 1999 hot gas filters. Engine and burner rig testing during the past and 2000, key advances have been made towards overcomfew years have demonstrated encouraging progress, the ing each of these limitations. See Refs. [3,4,59] for reviews result of 20 years of world-wide research. However, the of developments before 1999. testing has also shown that the long-term survival of Three approaches have been taken to design microSiC-containing composites, the materials which have re- structures of oxide composites that are damage tolerant. ceived the vast majority of attention to date, can be limited All aim to prevent damage (cracking) in the matrix from by recession of the matrix by volatility of SiO scale in crossing to the fibers, thereby allowing the dissipation of 2 combustion environments containing water vapor [1,2], stress concentrations while retaining the high strengths of contrary to previous expectations that oxidation embrittle- the fibers. Two of these are analogous to the conventional ment would be life-limiting. The undesirable need for approach taken with SiC-based composites, based on fiber environmental barrier coatings to protect these composites coatings that bond weakly, if at all, to the surface of the adds motivation to develop oxide composite systems that fibers, or coatings that are inherently weak (e.g., layered or are inherently stable in oxidizing environments. porous). The third relies on the weakness and low stiffness However, oxide composites introduce a new set of of a porous matrix itself to prevent damage from extending limitations: their development is relatively immature, into the fibers, even though the interface between the fibers having received widespread attention only during the past and matrix might be strongly bonded. Tough composites 5 years; the available fibers are not as microstructurally that show wood-like fracture have now been demonstrated stable or creep resistant as non-oxide fibers at high using all three approaches. The key remaining challenges are to optimize high temperature properties, demonstrate long-term stability in high-temperature combustion en- *Corresponding author. Tel.: 11-805-373-4170. vironments, and develop robust economical manufacturing E-mail address: dbmarsha@rsc.rockwell.com (D.B. Marshall). methods. 1359-0286/01/$ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S1359-0286(01)00017-1
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, compresadequate 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-temtemperature 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 sinterallowing 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 sufup-to-date accounts of the design and mechanical prop- fered strength loss of more than 60%
D B. Marshall, J B. Davis /Current Opinion in Solid State and Materials Science 5(2001)28. all the proposed compounds, has attracted the most atten- 39% Nextel 720 fiber Further studies of debonding and sliding at the LaPo Al, O3 interface have confirmed the weak bondi 0, * 31], but have also indicated a possibly important role of plastic deformation in the monazite itself to accommo- date interfacial roughness [31. Both La-monazite and CaWO,(scheelite) have been shown to deform relatively 48% Nextel 720 fiber easily at room temperature by twinning and dislocation motion[ 32]. Connections may exist between unique resistance of monazite to amorphizaton from o00 hour exposure radiation damage, which makes it the prime candidate for long-term storage of actinide wastes 33] 1100 The effectiveness of monazite interface coatings in Heat Treatment Temperature (C) providing a phase-compatible debond layer has now been us matrix composites after high temperature confirmed from tensile strength testing in a variety aging in air(from Ref.[#16 specimen configurations(using uncoated control specimens or comparisons): coatings and coating/matrix combina- tions on individual sapphire fibers [31, 341: individual tows of nextel 610 alumina fibers coated with monazite Detailed measurements are now available for the room and infiltrated with alumina matrix 35]; and unidirectional mperature in-plane tensile properties of a 2-D composi composites of Nextel fibers(610, 650 and 720)with consisting of Nextel 610 fibers(stacked layers of woven monazite coatings and alumina matrix [**36]. In the last fabric)in a porous mullite-alumina matrix(both 0/90 and case the matrix, although porous, was sufficiently strong 45)[*16, 25]. Measurements of notch sensitivity using that very little fragmentation of the matrix took place center-hole tension tests indicate some loss of net -sectio during fracture. Clear evidence was found on the fracture strength in the presence of notches, but much less than for surfaces for the role of the weak interface rather than a a fully notch-sensitive material. Effects of hole size on weak matrix in allowing extensive debonding and pullout strength were rationalized by comparing with stress dis- of fibers. After heat treatment at 1100 and 1200.C, the ributions predicted using non-linear continuum and shear strengths of composites with monazite coatings were all band models: a size-scale dependence in the failur higher than those of the corresponding control specimens, condition is implied. The trends are similar to those the largest differences being seen after 1200.C treatment observed in conventional weak-interface composites (e.g, strength of 200 MPa for coated Nextel 610 composite Systematic studies have now been done to vary the after 5 h at 1200'C compared with 45 MPa for uncrate strength and density of the matrix of the mullite-based control composite composite and measure the effect on the mechanical Although an effective method has been developed for properties [17, *26]. These involved further processing producing thin coatings of monazite on fiber tows(see with multiple cycles of reinfiltration and heat treatment Section 6), a method for coating fibers in a woven fabric using an alumina precursor(Al,(OH)S CI). Bonding of the has not been demonstrated. An approach that circumvents particle network by evaporation-condensation in reactive the need for a separate coating step has been developed on atmosphere has also been used to strengthen the matrix the basis of observations that monazite deposits pref- without decreasing the porosity [23]. As the matrix was erentially between adjacent fibers in tows or fabrics after strengthened and densified there was a trend to increase the infiltration with solution precursors and heat treatment tensile strength in the +45 direction, decrease the strength [ 37]. Infiltration of a slurry consisting of alumina powder in the 0/90 direction, and decrease the degree of uncorre- in a solution precursor for monazite results in a two-phase lated fiber fracture and pullout lengths on the fracture (porous) matrix of alumina and monazite, with a continu- surface ous layer of monazite around all of the fibers. Composites formed by infiltrating fabrics of Nextel 610 and 720 in this manner, followed by stacking, pressing and sintering, have 4. Weak interface developments shown damage-tolerant behavior and absence of notch sensitivity when tested at room temperature and at 1000C Since the discovery that rare-earth phosphates (most (Fig. 2)[37-39]. Although this behavior may be partly notably La-monazite)bond sufficiently weakly with other due to the porous nature of the mati significant oxides to allow debonding of fibers [27], other mixed difference in notch sensitivity is seen between this and the oxide compounds have been proposed and tested for the porous matrix composites described in the previous sec same purpose(tungstates, vanad dates, niobates [9, 28, 29]). tion, thus implying an additional effect of the Nevertheless, La-monazite, which is the most refractory of interface
D.B. Marshall, J.B. Davis / Current Opinion in Solid State and Materials Science 5 (2001) 283–289 285 all the proposed compounds, has attracted the most attention. Further studies of debonding and sliding at the LaPO –4 Al O interface have confirmed the weak bonding 2 3 [30,*31], but have also indicated a possibly important role of plastic deformation in the monazite itself to accommodate interfacial roughness [*31]. Both La-monazite and CaWO (scheelite) have been shown to deform relatively 4 easily at room temperature by twinning and dislocation motion [*32]. Connections may exist between this and the unique resistance of monazite to amorphizaton from radiation damage, which makes it the prime candidate for long-term storage of actinide wastes [33]. The effectiveness of monazite interface coatings in providing a phase-compatible debond layer has now been confirmed from tensile strength testing in a variety of Fig. 1. Strengths of porous matrix composites after high temperature aging in air (from Ref. [**16]). specimen configurations (using uncoated control specimens for comparisons): coatings and coating/matrix combinations on individual sapphire fibers [*31,34]; individual tows of Nextel 610 alumina fibers coated with monazite Detailed measurements are now available for the room and infiltrated with alumina matrix [35]; and unidirectional temperature in-plane tensile properties of a 2-D composite composites of NextelE fibers (610, 650 and 720) with consisting of Nextel 610 fibers (stacked layers of woven monazite coatings and alumina matrix [**36]. In the last fabric) in a porous mullite-alumina matrix (both 0/90 and case the matrix, although porous, was sufficiently strong 458) [**16,*25]. Measurements of notch sensitivity using that very little fragmentation of the matrix took place center-hole tension tests indicate some loss of net-section during fracture. Clear evidence was found on the fracture strength in the presence of notches, but much less than for surfaces for the role of the weak interface rather than a a fully notch-sensitive material. Effects of hole size on weak matrix in allowing extensive debonding and pullout strength were rationalized by comparing with stress dis- of fibers. After heat treatment at 1100 and 12008C, the tributions predicted using non-linear continuum and shear strengths of composites with monazite coatings were all band models: a size-scale dependence in the failure higher than those of the corresponding control specimens, condition is implied. The trends are similar to those the largest differences being seen after 12008C treatment observed in conventional weak-interface composites. (e.g., strength of 200 MPa for coated Nextel 610 composite Systematic studies have now been done to vary the after 5 h at 12008C compared with 45 MPa for uncoated strength and density of the matrix of the mullite-based control composite). composite and measure the effect on the mechanical Although an effective method has been developed for properties [*17,*26]. These involved further processing producing thin coatings of monazite on fiber tows (see with multiple cycles of reinfiltration and heat treatment Section 6), a method for coating fibers in a woven fabric using an alumina precursor (Al (OH) Cl). Bonding of the has not been demonstrated. An approach that circumvents 2 5 particle network by evaporation–condensation in reactive the need for a separate coating step has been developed on atmosphere has also been used to strengthen the matrix the basis of observations that monazite deposits prefwithout decreasing the porosity [23]. As the matrix was erentially between adjacent fibers in tows or fabrics after strengthened and densified there was a trend to increase the infiltration with solution precursors and heat treatment tensile strength in the 6458 direction, decrease the strength [*37]. Infiltration of a slurry consisting of alumina powder in the 0/908 direction, and decrease the degree of uncorre- in a solution precursor for monazite results in a two-phase lated fiber fracture and pullout lengths on the fracture (porous) matrix of alumina and monazite, with a continusurface. ous layer of monazite around all of the fibers. Composites formed by infiltrating fabrics of Nextel 610 and 720 in this manner, followed by stacking, pressing and sintering, have 4. Weak interface developments shown damage-tolerant behavior and absence of notch sensitivity when tested at room temperature and at 10008C Since the discovery that rare-earth phosphates (most (Fig. 2) [*37–39]. Although this behavior may be partly notably La-monazite) bond sufficiently weakly with other due to the porous nature of the matrix, a significant oxides to allow debonding of fibers [27], other mixed difference in notch sensitivity is seen between this and the oxide compounds have been proposed and tested for the porous matrix composites described in the previous secsame purpose (tungstates, vanadates, niobates [9,28,29]). tion, thus implying an additional effect of the weak Nevertheless, La-monazite, which is the most refractory of interface
D B. Marshall, J.B. Davis/ Current Opinion in Solid State and Materials Science 5(2001)283-28 Al2O3 fiber /AL2O3-LaPO4 matrix plumbite with intrinsically weak cleavage planes, analo- gous to mica. These approaches have not proven as effective as either the porous matrix approach or the weakly bonded interface materials described in the previ ous sections [34]. Nevertheless, some useful progress has been made in demonstrating composite properties, refining coating methods, and in defining the relative merits of porosity being concentrated at an interface or distributed 50英z 瞿 throughout the matrix Porous interphases are most commonly produced by 100 mixing fugitive carbon with the fiber coating material ●1100°c/1h; Room Temp Zirconia. rare-earth aluminates. and zircon have been 1100°c/24h; Room Temp investigated recently [45-48]. Composites have been ■1100c/1h;1025 C Test produced by hot pressing sapphire fibers with porous zirconia coatings in an alumina matrix [45, 46]. Althor the composites had relatively low strengths (-100-130 MPa for 0/90 cross-ply ) they were not degraded after ycling at 1200C (1300 cycles)and aging at tempera- Fig. 2. Effect of notch depth and heat-treatment conditions on net-section rengths measured at room temperature and at 1025C. Monazite/ tures that would severely degrade composites with poly umina matrix, Nextel"610 fibers. Specimen width: w=13 mm. crystalline oxide fibers (100 h at 1400C Several groups have produced composites with a thin continuous gap between the fibers and matrix by using a Several studies have shown that infiltration of Nextel fugitive carbon coating on the fibers [49-52]. Useful fiber tows and fabrics with solution precursors, followed insight into the merit of the approach is provided in Ref. by heat treatment to form monazite, often leads to strength [491, where comparisons with uncoated control specimens degradation [40-**44]. Many combinations of precu indicated that the gap was effective in enabling fiber chemistry, fiber composition, temperature, and time have pullout in composites of Nextel 720 fibers in a dense been assessed, with strength measurements being obtained calcium alumino-silicate(CAS)matrix, whereas the gap both from individual fibers with thin monazite coatings of had no effect composite with a porous matrix of 100 nm thickness(see Section 6 for coating method) mullite-alumina. Although the starting strengths were low [*42-**44 and from infiltrated fiber tows that form in this study (130 MPa), there was no degradation in the mini-composites [40, **41]. Although the detailed mecha- porous matrix composite after 500 h at 1150C and a smal nisms of degradation in most cases are not known, it is loss(-20%)in the CAs matrix composite after aging for clear that the mechanisms involve the precursor chemistry 500 h at 1000C rather than the monazite itself, since degradation does not The use of layered crystal structures presents several occur with coatings formed from aqueous slurries of challenges:(1) to form the desired phase at a rhabdophane particles (hydrated monazite) and in low temperature that the fibers are not degraded during minicomposites formed by infiltration with slurries of processing; (ii) to orient the weak basal planes of the monazite particles. Indeed, in these cases the strengths crystals parallel to the fiber surface; and (iii) to control the after heat treatment are often higher than strengths of roughness of the fracture in the coating to allow con- uncoated control specimens. Strength loss is also avoided strained sliding of the fibers. Progress has been made in in some cases when fiber tows are infiltrated with slurries lowering the formation temperature for refractory hexa- consisting of alumina particles in solution precursors described above, possibly a result of the alumina acting as techniques [53-57. However, although textured coatings internal buffer. Many observations are consistent with a have been grown on single-crystal YAG plates at 1200C, hypothesis that trapped gases remaining from the pre- there is a suggestion that on polycrystalline alumina fibers (Nextel 610) there may not be an adequate driving forc during heat treatment and react with the fiber surface for grain growth and texturing at this temperature(which the limit to which these fibers may be exposed during 5. Layered and porous interphases 6. Fiber coating methods Some of the earliest attempts to produce damage-toler nt oxide composites were based on forming weak fiber Although chemical and physical vapor methods have coatings,either by the introduction of porosity or by use of been used to deposit coatings of monazites and hexalumi- layered crystal structures such as B-alumina/magneto- nates [29], the deposition of stoichiometric multicom
286 D.B. Marshall, J.B. Davis / Current Opinion in Solid State and Materials Science 5 (2001) 283–289 plumbites with intrinsically weak cleavage planes, analogous to mica. These approaches have not proven as effective as either the porous matrix approach or the weakly bonded interface materials described in the previous sections [34]. Nevertheless, some useful progress has been made in demonstrating composite properties, refining coating methods, and in defining the relative merits of porosity being concentrated at an interface or distributed throughout the matrix. Porous interphases are most commonly produced by mixing fugitive carbon with the fiber coating material. Zirconia, rare-earth aluminates, and zircon have been investigated recently [*45–48]. Composites have been produced by hot pressing sapphire fibers with porous zirconia coatings in an alumina matrix [*45,46]. Although the composites had relatively low strengths (|100–130 MPa for 0/90 cross-ply), they were not degraded after cycling at 12008C (.1300 cycles) and aging at temperaFig. 2. Effect of notch depth and heat-treatment conditions on net-section tures that would severely degrade composites with poly- strengths measured at room temperature and at 10258C. Monazite/ crystalline oxide fibers (100 h at 14008C). alumina matrix, NextelE 610 fibers. Specimen width: w513 mm. Several groups have produced composites with a thin continuous gap between the fibers and matrix by using a Several studies have shown that infiltration of Nextel fugitive carbon coating on the fibers [*49–52]. Useful fiber tows and fabrics with solution precursors, followed insight into the merit of the approach is provided in Ref. by heat treatment to form monazite, often leads to strength [*49], where comparisons with uncoated control specimens degradation [40–**44]. Many combinations of precursor indicated that the gap was effective in enabling fiber chemistry, fiber composition, temperature, and time have pullout in composites of Nextel 720 fibers in a dense been assessed, with strength measurements being obtained calcium alumino-silicate (CAS) matrix, whereas the gap both from individual fibers with thin monazite coatings of had no effect in a composite with a porous matrix of |100 nm thickness (see Section 6 for coating method) mullite-alumina. Although the starting strengths were low [**42–**44] and from infiltrated fiber tows that form in this study (|130 MPa), there was no degradation in the mini-composites [40,**41]. Although the detailed mecha- porous matrix composite after 500 h at 11508C and a small nisms of degradation in most cases are not known, it is loss (|20%) in the CAS matrix composite after aging for clear that the mechanisms involve the precursor chemistry 500 h at 10008C. rather than the monazite itself, since degradation does not The use of layered crystal structures presents several occur with coatings formed from aqueous slurries of challenges: (i) to form the desired phase at a sufficiently rhabdophane particles (hydrated monazite) and in low temperature that the fibers are not degraded during minicomposites formed by infiltration with slurries of processing; (ii) to orient the weak basal planes of the monazite particles. Indeed, in these cases the strengths crystals parallel to the fiber surface; and (iii) to control the after heat treatment are often higher than strengths of roughness of the fracture in the coating to allow conuncoated control specimens. Strength loss is also avoided strained sliding of the fibers. Progress has been made in in some cases when fiber tows are infiltrated with slurries lowering the formation temperature for refractory hexaconsisting of alumina particles in solution precursors as luminates as low as 10008C using sol–gel and doping described above, possibly a result of the alumina acting as techniques [*53–57]. However, although textured coatings an internal buffer. Many observations are consistent with a have been grown on single-crystal YAG plates at 12008C, hypothesis that trapped gases remaining from the pre- there is a suggestion that on polycrystalline alumina fibers cursors can build up high pressures in dense coatings (Nextel 610) there may not be an adequate driving force during heat treatment and react with the fiber surface for grain growth and texturing at this temperature (which is [**44]. the limit to which these fibers may be exposed during processing) [*53]. 5. Layered and porous interphases 6. Fiber coating methods Some of the earliest attempts to produce damage-tolerant oxide composites were based on forming weak fiber Although chemical and physical vapor methods have coatings, either by the introduction of porosity or by use of been used to deposit coatings of monazites and hexalumilayered crystal structures such as b-alumina/magneto- nates [29], the deposition of stoichiometric multicom-
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. Processing
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 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 microstructures. 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 preliminwith 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