E≈S ournal of the European Ceramic Society 20(2000)569-582 Hexaluminates as a cleavable fiber-matrix interphase: synthesis texture development, and phase compatibility Michael K, Cinibulk* Air Force Research laboratory Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433-7817, US.A Accepted 13 August 1999 Abstract The current state of research on hexaluminate as a potential cleavable oxide fiber-matrix interphase is reviewed. Calcium hex aluminate was used initially to produce highly textured fiber coatings and interphases in single-crystal alumina fiber-based matrix composites. Cracks were shown to deflect and propagate within the interphase by cleavage. Critical strain-energy release rates of 2.2 J/m- were measured for highly textured polycrystalline CaAl12O19 interphases. Subsequent work has focused on low- ering the temperature for synthesis and texturing of both calcium and lanthanum-based hexaluminate Doping of hexaluminate, primarily with transition-metal oxides, allows for their formation at temperatures as low as 1000C. Grain -growth rates are about an order of magnitude greater than for undoped powders. Textured coatings have been grown on single-crystal YAG plates at 1200oC. However, there does not seem to be an adequate driving force for grain growth and texturing of the coatings on poly- crystalline alumina fibers(NextelTM 610) at 1200 C, the maximum processing temperature for these fibers. The lack of a more refractory, commercially available fiber that is phase compatible with the hexaluminate currently limits further development of a hexaluminate fiber-matrix interphase. C 2000 Elsevier Science Ltd. All rights reserved Keywords: Aluminosilicate fibers; CaAl12O19: Coatings; Grain growth; Interfaces; Mechanical properties; YAG 1. Introduction than fully crystallized hexagonal BN, o but in another study cracks deflected very near the BN-glass interface Nonoxide fiber-reinforced ceramic-matrix composites where turbostratic BN was better aligned with the (CMCs) typically contain a layer of carbon or hex- interface. I These seemingly contradictory results make agonal boron nitride at the fiber-matrix interface to it difficult to rely on carbon or bn as models when encourage matrix cracks to deflect away from the fiber. designing alternate crack-deflecting interphases based Graphite and bn have essentially perfect cleavage, in on cleavage. However, obvious parameters to consider that they fracture readily along the basal(0001)plane. include fracture-energy anisotropy, degree of texture of However, it is not clear whether crack deflection in these cleavage planes, and coating thickness composites is solely the result of cleavage of grains An analogue to graphite or bn is needed that is stable within the interphase or debonding due to a weak fiber- in oxidizing conditions at elevated temperatures. A mat- coating interface. For example, carbon interphases in erial, with a sufficiently high anisotropy in fracture tough CMCs with graphitic basal planes parallel to the energy, for crack deflection to occur within the inter nterface, very weakly textured parallel to the interface, 3 layer, is necessary. The criterion usually employed to graphitic coatings with no texture, porous turbostratic specify a suitable candidate is that fracture-energy carbon, amorphous carbon,0-and multilayered coatings release-rate anisotropy must be at least a factor of with textured graphitic, untextured graphitic, and four, and the coating must be textured so that clea- amorphous carbon layers' have all been reported. Turbo- vage planes are parallel to the interface. However, the static hexagonal BN was reported to perform better required degree of texture that is necessary to deflect and contain the crack is unknown. Nevertheless if an Supported by the Air Force Research Laboratory under Contract interlayer material with poor cleavage is substituted for No.F33615-96-C-5258. one with perfect cleavage, it seems to follow that either Present address: UES Inc, Dayton, OH 45432-1894, USA better texture or a thicker coating may be required 0955-2219/00/S. see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(99)00255
Hexaluminates as a cleavable ®ber±matrix interphase: synthesis, texture development, and phase compatibility$ Michael K. Cinibulk* Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433-7817, USA Accepted 13 August 1999 Abstract The current state of research on hexaluminates as a potential cleavable oxide ®ber±matrix interphase is reviewed. Calcium hexaluminate was used initially to produce highly textured ®ber coatings and interphases in single-crystal alumina ®ber-based ceramic± matrix composites. Cracks were shown to de¯ect and propagate within the interphase by cleavage. Critical strain-energy release rates of 2.2 J/m2 were measured for highly textured polycrystalline CaAl12O19 interphases. Subsequent work has focused on lowering the temperature for synthesis and texturing of both calcium- and lanthanum-based hexaluminates. Doping of hexaluminates, primarily with transition-metal oxides, allows for their formation at temperatures as low as 1000C. Grain-growth rates are about an order of magnitude greater than for undoped powders. Textured coatings have been grown on single-crystal YAG plates at 1200C. However, there does not seem to be an adequate driving force for grain growth and texturing of the coatings on polycrystalline alumina ®bers (NextelTM 610) at 1200C, the maximum processing temperature for these ®bers. The lack of a more refractory, commercially available ®ber that is phase compatible with the hexaluminates currently limits further development of a hexaluminate ®ber±matrix interphase. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Aluminosilicate ®bers; CaAl12O19; Coatings; Grain growth; Interfaces; Mechanical properties; YAG 1. Introduction Nonoxide ®ber-reinforced ceramic±matrix composites (CMCs) typically contain a layer of carbon or hexagonal boron nitride at the ®ber±matrix interface to encourage matrix cracks to de¯ect away from the ®ber. Graphite and BN have essentially perfect cleavage, in that they fracture readily along the basal (0001) plane. However, it is not clear whether crack de¯ection in these composites is solely the result of cleavage of grains within the interphase or debonding due to a weak ®bercoating interface. For example, carbon interphases in tough CMCs with graphitic basal planes parallel to the interface,1 very weakly textured parallel to the interface,2,3 graphitic coatings with no texture,4 porous turbostratic carbon,5 amorphous carbon,1,6±9 and multilayered coatings with textured graphitic, untextured graphitic, and amorphous carbon layers1 have all been reported. Turbostratic hexagonal BN was reported to perform better than fully crystallized hexagonal BN,10 but in another study cracks de¯ected very near the BN±glass interface where turbostratic BN was better aligned with the interface.11 These seemingly contradictory results make it dicult to rely on carbon or BN as models when designing alternate crack-de¯ecting interphases based on cleavage. However, obvious parameters to consider include fracture-energy anisotropy, degree of texture of cleavage planes, and coating thickness. An analogue to graphite or BN is needed that is stable in oxidizing conditions at elevated temperatures. A material, with a suciently high anisotropy in fracture energy, for crack de¯ection to occur within the interlayer, is necessary. The criterion usually employed to specify a suitable candidate is that fracture-energy release-rate anisotropy must be at least a factor of four,12 and the coating must be textured so that cleavage planes are parallel to the interface. However, the required degree of texture that is necessary to de¯ect and contain the crack is unknown. Nevertheless, if an interlayer material with poor cleavage is substituted for one with perfect cleavage, it seems to follow that either better texture or a thicker coating may be required. 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(99)00255-1 Journal of the European Ceramic Society 20 (2000) 569±582 $ Supported by the Air Force Research Laboratory under Contract No. F33615-96-C-5258. * Present address: UES Inc., Dayton, OH 45432-1894, USA
M K. Cinibulk/Journal of the European Ceramic Society 20(2000)569-582 Besides fracture anisotropy and texture, intrinsic high- lanthanide cation a nonstoichiometric, highly defective temperature stability in air and thermochemical stability structure is produced. 0 The ideal structure of with potential CMC fiber and matrix phases is needed. LaAl1O18 can be represented as Lao2[Al1O16]. By Oxides that have been investigated include the micas, substituting a divalent cation for one Al+ local charge hexaluminate, and some layered perovskites. Evidence balance is obtained. 9 The structure now becomes LaAl for crack deflection at room temperature was found in O3[MAl1oO16], with the divalent cation substituting for laminates with mica interphases. 3-15 However, micas an aluminum cation in the spinel block. This mechanism of dehydrate and decompose to alumina, feldspars or charge balance leads to the ideal stoichiometric magneto- feldspathoids, and/or other oxides at 500-900 C 3 plumbite compounds LnMAlO19, for Ln=La+Gd Fluoromicas such as potassium-fluorophlogopite and M=Mg, Mn+Zn, which have been used as laser [KMg3 (AISi3)Oo F2] are stable above 1200oC in a dry or and luminescent materials(see for example Ref. 19 and closed environment at ambient pressure. However, in references therein) an open environment with water vapor, potassium- The incongruent vaporization of alkali oxides decom fluorophlogopite starts to breakdown to forsterite, poses alkali-stabilized B-aluminas above M1000oC in open potassium-aluminosilicates, and HF at 1000C 13 Similarly, systems; 21, 22 whereas incongruent vaporization is negli oxides with sufficiently good cleavage to be lubricious, such gible in magnetoplumbite stabilized with alkaline-earth as talc(Mg3 Si4O12H2)and pyrophyllite(Al Si4O12H2) are or rare-earth cations. 22,23 The magnetoplumbite are well known. Unfortunately, all are hydrous and stable at temperatures well in excess of their proposed decompose at temperatures of interest for CMCs. 6 use in oxide CMCs. Therefore, the magnetoplumbite Layered perovskites, such as KCa? 010, were have received the most attention and are considered investigated for suitable crack deflection behavior and exclusively in this paper were found to have some potential for this applica The mineral hibonite(CaAl12O1g)occurs naturally in tion.KCa2 O10 is reportedly stable up to 1500C alluvial deposits and in metamorphosed limestones rich with respect to melting or incongruent vaporization of in calcic plagioclase, containing 3.2 wt% Mgo, 8.5 wt% alkali oxide, and is compatible with alumina to at least T1O2, 2.3 wt% Feo, 0.45 wt% Fe2O3, and 1. 5 wt% 1200.C, but is not compatible with silica. Recent work7 SiO2.24 Dayal and Glasser 5 showed that synthetic suggests that interlayers texture properly and are cap- CaAl12O19 formed solid solutions with the hypothetical able of crack deflection. However, they are chemically end member CaFe12O19, extending to N70 mol% of the complex and as such are compatible with a very limited latter Meteoritic hibonite typically contains substantial number of potential CMC constituent phases amounts of Mg. Ti. V, and Si. 26,27 The substitution A review of the work on hexaluminate, primarily mechanism is the replacement of Al with an isovalent calcium hexaluminate(CaAl12O19, the mineral hibonite) cation or the replacement of 2AF+ with two charge- and lanthanum hexaluminate(LaAl1O18)synthesis and compensating aliovalent cations such that charge neu- fiber coatings for CMCs is the focus of this paper trality is maintained. For example, binary and ternary solid-solution hibonites have been produced in the CaAl12O19-CaAljoMgSiO1g and CaAl12o19-CaAlIoMg- 2. Structure and chemistry of the hexaluminate Sio1g-CaAlloMgT1O19 systems, respectively, with up to 15 mol% CaAloMgSiO19 and 34 mol% CaAlIoMg Hexagonal aluminates, having the B-alumina or mag- TiO19(up to 1.7 wt% SiO2, 3.6 wt% MgO, and 5.2 wt% netoplumbite structures, are commonly referred to as TiO2)with the most Si-rich hibonites restricted to the hexaluminate; both structures are composed of binary system. The effect of impurities as controlled layered spinel blocks [Al1O16 separated by mirror dopants on the synthesis and grain growth of calcium the stabilizing cations(M+ or M2+)reside(Fig. I). The Sections 4 and,f hexaluminate is discussed further in planes [M+O]-and [M*AlO3]-, respectively, in which and lanthanum structure that is preferred depends on the radius and Hexaluminates have basal plane cleavage that is qua alence of the stabilizing cation. In general, the alkali litatively inferior to that of micas, graphite and BN. 29 oxides react with alumina to form B-aluminas, while Preferred cleavage occurs at the weakly bonded inter- alkaline-earth and rare-earth oxides form magneto- spinel layers(see Fig. 1). It is not known if there are plumbite. The mineral magnetoplumbite, Pb(Fe, Mn, significant differences in cleavage between the different D)12O19, is prototypical of a larger class of compounds hexaluminate. However, high fracture-energy aniso- with the general composition A+B3*019. 19 The iso- tropy, with values which differ by a factor of 100, has structural hexaluminate are formed by replacing Fe+ been demonstrated in the B-aluminas 30, 3 and there is with Al+, and Pb2+ with either alkaline-earth or rare- evidence that magnetoplumbite fiber-matrix interlayers earth cations of similar radii. Only when Pb2+ is cleave sufficiently well to deflect matrix cracks in CMCs replaced by Ca2+ or Sr+ is the stoichiometric compo- with single-crystal alumina and YAG fibers, but evi- sition obtained. When Pb2+ is replaced by a trivalent dence for fiber pullout after crack deflection is still
Besides fracture anisotropy and texture, intrinsic hightemperature stability in air and thermochemical stability with potential CMC ®ber and matrix phases is needed. Oxides that have been investigated include the micas, hexaluminates, and some layered perovskites. Evidence for crack de¯ection at room temperature was found in laminates with mica interphases.13±15 However, micas dehydrate and decompose to alumina, feldspars or feldspathoids, and/or other oxides at 500±900C.13 Fluoromicas such as potassium-¯uorophlogopite [KMg3(AlSi3)O10F2] are stable above 1200C in a dry or closed environment at ambient pressure.13 However, in an open environment with water vapor, potassium- ¯uorophlogopite starts to breakdown to forsterite, potassium-aluminosilicates, and HF at 1000C.13 Similarly, oxides with suciently good cleavage to be lubricious, such as talc (Mg3Si4O12H2) and pyrophyllite (Al2Si4O12H2) are well known. Unfortunately, all are hydrous and decompose at temperatures of interest for CMCs.16 Layered perovskites, such as KCa2Nb3O10, were investigated for suitable crack de¯ection behavior and were found to have some potential for this application.17 KCa2Nb3O10 is reportedly stable up to 1500C with respect to melting or incongruent vaporization of alkali oxide, and is compatible with alumina to at least 1200C, but is not compatible with silica. Recent work17 suggests that interlayers texture properly and are capable of crack de¯ection. However, they are chemically complex and as such are compatible with a very limited number of potential CMC constituent phases. A review of the work on hexaluminates, primarily calcium hexaluminate (CaAl12O19, the mineral hibonite) and lanthanum hexaluminate (LaAl11O18) synthesis and ®ber coatings for CMCs is the focus of this paper. 2. Structure and chemistry of the hexaluminates Hexagonal aluminates, having the b-alumina or magnetoplumbite structures, are commonly referred to as hexaluminates;18 both structures are composed of layered spinel blocks [Al11O16] + separated by mirror planes [M+O]ÿ and [M2+AlO3] ÿ, respectively, in which the stabilizing cations (M+ or M2+) reside (Fig. 1). The structure that is preferred depends on the radius and valence of the stabilizing cation. In general, the alkali oxides react with alumina to form b-aluminas, while alkaline-earth and rare-earth oxides form magnetoplumbites. The mineral magnetoplumbite, Pb(Fe, Mn, Al)12O19, is prototypical of a larger class of compounds with the general composition A2+B3+ 12 O19. 19 The isostructural hexaluminates are formed by replacing Fe3+ with Al3+, and Pb2+ with either alkaline-earth or rareearth cations of similar radii. Only when Pb2+ is replaced by Ca2+ or Sr2+ is the stoichiometric composition obtained. When Pb2+ is replaced by a trivalent lanthanide cation a nonstoichiometric, highly defective structure is produced.20 The ideal structure of LaAl11O18 can be represented as LaO2[Al11O16]. By substituting a divalent cation for one Al3+ local charge balance is obtained.19 The structure now becomes LaAl O3[MAl10O16], with the divalent cation substituting for an aluminum cation in the spinel block. This mechanism of charge balance leads to the ideal stoichiometric magnetoplumbite compounds LnMAl11O19, for Ln=La$Gd, and M=Mg, Mn$Zn, which have been used as laser and luminescent materials (see for example Ref. 19 and references therein). The incongruent vaporization of alkali oxides decomposes alkali-stabilized b-aluminas above 1000C in open systems;21,22 whereas incongruent vaporization is negligible in magnetoplumbites stabilized with alkaline-earth or rare-earth cations.22,23 The magnetoplumbites are stable at temperatures well in excess of their proposed use in oxide CMCs. Therefore, the magnetoplumbites have received the most attention and are considered exclusively in this paper. The mineral hibonite (CaAl12O19) occurs naturally in alluvial deposits and in metamorphosed limestones rich in calcic plagioclase, containing 3.2 wt% MgO, 8.5 wt% TiO2, 2.3 wt% FeO, 0.45 wt% Fe2O3, and 1.5 wt% SiO2. 24 Dayal and Glasser25 showed that synthetic CaAl12O19 formed solid solutions with the hypothetical end member CaFe12O19, extending to 70 mol% of the latter. Meteoritic hibonite typically contains substantial amounts of Mg, Ti, V, and Si.26,27 The substitution mechanism is the replacement of Al3+ with an isovalent cation or the replacement of 2Al3+ with two chargecompensating aliovalent cations such that charge neutrality is maintained. For example, binary and ternary solid-solution hibonites have been produced in the CaAl12O19±CaAl10MgSiO19 and CaAl12O19±CaAl10MgSiO19-CaAl10MgTiO19 systems, respectively, with up to 15 mol% CaAl10MgSiO19 and 34 mol% CaAl10MgTiO19 (up to 1.7 wt% SiO2, 3.6 wt% MgO, and 5.2 wt% TiO2) with the most Si-rich hibonites restricted to the binary system.28 The eect of impurities as controlled dopants on the synthesis and grain growth of calcium and lanthanum hexaluminates is discussed further in Sections 4 and 5. Hexaluminates have basal plane cleavage that is qualitatively inferior to that of micas, graphite, and BN.29 Preferred cleavage occurs at the weakly bonded interspinel layers (see Fig. 1). It is not known if there are signi®cant dierences in cleavage between the dierent hexaluminates. However, high fracture-energy anisotropy, with values which dier by a factor of 100, has been demonstrated in the b-aluminas30,31 and there is evidence that magnetoplumbite ®ber±matrix interlayers cleave suciently well to de¯ect matrix cracks in CMCs with single-crystal alumina and YAG ®bers, but evidence for ®ber pullout after crack de¯ection is still 570 M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582
M K. Cinibulk/Journal of the European Ceramic Society 20(2000)569-582 β- ALUMINA MAGNETOPLUMBITE mirror plane→ -mirror plane ● mirror plane Fig. 1. B-Alumina and magnetoplumbite structures. Mirror planes viewed along the c-axes are given below each structure insubstantial32-36 Strain-energy release rates of 2.2 J/m2 fiber. Most commercially available oxide fibers(Nextel were measured of a highly textured polycrystalline 610 and 720, 3M) begin to lose strength at these tem- CaAl12O1g layer between two alumina sheets. 3SEM, peratures due to grain growth. Also, during the coating EDS, and X-ray diffraction indicated that the process, compounds volitalize as byproducts and may propagated transgranularly by cleavage of the hibonite act to weaken the fibers via a stress-corrosion mechan grains, rather than at the hibonite alumina interface. ism. Fibers can also be weakened if they react with the pecific examples are given in Section 5.3 precursor or the intermediate products during the coat ing process. If the coating has a significantly lower coefficient of thermal expansion than the fiber and 3. Issues in coating polycrystalline fiber tows remains strongly bonded, tensile stresses at the fiber- coating interface can lead to lower strengths. The pres The application of coatings to fibers, more often th ence of imperfections in the coating, such as excess ot, compromises their strength.3738 Often this is a material bridging individual filaments can also lead to result of the coating process and often this is due to the problems. Liquid-phase precursors often form bridges presence of the coating itself. Most oxide coatings are of coating between filaments in a tow that later break applied at temperatures greater than 1000 C to ensure during handling. The influence of bridges on composite the plete reaction and conversion of the precursor to processing and properties has yet to be fully explored desired phase and to fully sinter the coating onto the In the worst case, where bridges are extensively linked
insubstantial.32±36 Strain-energy release rates of 2.2 J/m2 were measured of a highly textured polycrystalline CaAl12O19 layer between two alumina sheets.33 SEM, EDS, and X-ray diraction indicated that the crack propagated transgranularly by cleavage of the hibonite grains, rather than at the hibonite alumina interface. Speci®c examples are given in Section 5.3. 3. Issues in coating polycrystalline ®ber tows The application of coatings to ®bers, more often than not, compromises their strength.37,38 Often this is a result of the coating process and often this is due to the presence of the coating itself. Most oxide coatings are applied at temperatures greater than 1000C to ensure complete reaction and conversion of the precursor to the desired phase and to fully sinter the coating onto the ®ber. Most commercially available oxide ®bers (Nextel 610 and 720, 3M) begin to lose strength at these temperatures due to grain growth. Also, during the coating process, compounds volitalize as byproducts and may act to weaken the ®bers via a stress-corrosion mechanism. Fibers can also be weakened if they react with the precursor or the intermediate products during the coating process. If the coating has a signi®cantly lower coecient of thermal expansion than the ®ber and remains strongly bonded, tensile stresses at the ®ber± coating interface can lead to lower strengths. The presence of imperfections in the coating, such as excess material bridging individual ®laments can also lead to problems. Liquid-phase precursors often form bridges of coating between ®laments in a tow that later break during handling. The in¯uence of bridges on composite processing and properties has yet to be fully explored. In the worst case, where bridges are extensively linked, Fig. 1. b-Alumina and magnetoplumbite structures. Mirror planes viewed along the c-axes are given below each structure. M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582 571
M K. Cinibulk/Journal of the European Ceramic Society 20(2000)569-582 they can lead to failure of the tow during handling, Cao+Al2O3-CaAl2O4 impede infiltration of matrix, greatly increasing void space and severely reducing matrix strength. Linked CaO+2Al2O3→CaA4O bridges may also be a source of low-energy crack pro- pagation through a bundle of fibers CaAlO4+ AlO3-CaAl4O Issues specific to hexaluminate interphases include synthesis, microstructure and texture, and the process of CaAl407+4Al2O3-CaAl12O19 (3) coating fibers and texturing the interphase. To coat fine diameter polycrystalline alumina-based fibers with a Residual CaAl4O, and Al2O3 often accompany textured hexaluminate coating, a precursor must have CaAl12O19 in the final product. In cases where <5 wt% the following properties. First, the precursor must wet Cao was added to M500-nm particle size a-Al2O3 pow and completely infiltrate the fiber tow. The first ders, reactions(1)and(2)preceded reaction (3).48 requirement makes the use of nonaqueous precursors Whereas, in the case where the appropriate amount of a attractive with respect to obtaining sols with good wet- Ca-salt was added to 40-nm particle size (5-nm crystal ting characteristics, although viscosities can be higher. lites) boehmite to form a stoichiometric CaO: 6Al2O3 Second, the viscosity of the precursor should remain colloidal sol, or the precursor was a polymeric solution low at the desired concentrations for infiltration and only reaction(2a)was found to precede reaction(3).34. 50 deposition of coatings of adequate thickness and allow In general, the completion of such reactions depends on for ready displacement of the excess sol. Third, the pre- the diffusion distances, i. e. particle size and degree of cursor should yield the desired hexaluminate at tem- mixing of the reactant powders peratures below those at which the fibers begin to The precursor sol originally used to apply coatings to degrade. Commercially available polycrystalline alu- single-crystal alumina and YAG fibers and plates was a mina fibers(Nextel 610, 3M)are limited to processing boehmite colloid, doped with a stoichiometric amount temperatures of 1200c to avoid significant grain of calcium acetate, which required temperatures of over growth that can lead to lower strengths. Precursor con- 1400C for complete reaction to hibonite and to fully stituents can also diffuse into the fibers and accelerate texture the coating 32-35 Coating polycrystalline alu degradation. In the case of hibonite, where Cao is mina-based fibers with this same colloidal sol resulted in present in the precursor, Cao can readily segregate severe embrittlement of the fiber and the formation of to the grain boundaries of the fiber and lead to an a-Al,O3 coating. Prior to the formation of calcium strength reduction, since Cao is known to embrittle hexaluminate, Cao diffuses out of the coating and to alumina. 404 Finally, significant abnormal grain- the grain boundaries in the fiber, leaving an alumina growth of calcium hexaluminate must occur to obtain rich precursor behind. The viscous nature of the sol, a coating with basal plane texture. The two most cri- which results from doping aqueous boehmite sols with tical issues with respect to the application of hex- even low levels of metal salts leads to extensive bridging luminate coatings onto polycrystalline fibers need to be of the filaments in the tow, despite the use of an immis- ddressed to further research of these materials as cible liquid to remove excess sol during the coating potential fiber-matrix interphases and to validate the process concept. The first is the lowering of the temperature of Many alternate precursors have been investigated hexaluminate formation. The second is increasing grain including other colloidal based sols, organometallics, growth to achieve basal texture at these lowered tem- and polymeric solutions. 0 However, only one sol, a peratures mixed-metal citric acid complex, was found to sig nificantly lower the reaction temperature to yield hibo- nite, free of any intermediate phases, at 1300C(Fig. 2) 4. Synthesis from sol and solution presursors The resin that is formed during the condensation reac- tion with ethylene glycol is amorphous and contains the 4. Calcium hexaluminate Ca2+ and Al+ cations uniformly distributed on a very fine scale. The short diffusion distances between con- The reaction of Cao and Al2O3 generally proceeds stituents and their uniform, stoichiometric dispersion namIc n a nonequilibrium manner with the gives rise to enhanced reactivity. The formation of the formation of calcia-rich aluminates, followed by the kinetically favored intermediate CaAl,O,, which must formation of the relatively calcia-poor aluminates, until then react with AlO3 to form CaAl12O19, seems to be eventually the stoichiometric calcium aluminate is suppressed during heating of the citrate-based precursor formed 45-47 For the case of CaO and Al2O3 in a 1: 6 However, even with a reaction temperature of 1300 C ratio, monocalcium aluminate and/or calcium dia- significant degradation of polycrystalline fibers occurs luminate usually forms before the hexaluminate as It is well known that dopants in the form of solid-solu- tion and liquid-phase formers can enhance the sintering
they can lead to failure of the tow during handling, impede in®ltration of matrix, greatly increasing void space and severely reducing matrix strength. Linked bridges may also be a source of low-energy crack propagation through a bundle of ®bers. Issues speci®c to hexaluminate interphases include synthesis, microstructure and texture, and the process of coating ®bers and texturing the interphase. To coat ®ne diameter polycrystalline alumina-based ®bers with a textured hexaluminate coating, a precursor must have the following properties. First, the precursor must wet and completely in®ltrate the ®ber tow. The ®rst requirement makes the use of nonaqueous precursors attractive with respect to obtaining sols with good wetting characteristics, although viscosities can be higher.39 Second, the viscosity of the precursor should remain low at the desired concentrations for in®ltration and deposition of coatings of adequate thickness and allow for ready displacement of the excess sol. Third, the precursor should yield the desired hexaluminate at temperatures below those at which the ®bers begin to degrade. Commercially available polycrystalline alumina ®bers (Nextel 610, 3M) are limited to processing temperatures of 1200C to avoid signi®cant grain growth that can lead to lower strengths. Precursor constituents can also diuse into the ®bers and accelerate degradation. In the case of hibonite, where CaO is present in the precursor, CaO can readily segregate to the grain boundaries of the ®ber and lead to strength reduction, since CaO is known to embrittle alumina.40±44 Finally, signi®cant abnormal graingrowth of calcium hexaluminate must occur to obtain a coating with basal plane texture. The two most critical issues with respect to the application of hexaluminate coatings onto polycrystalline ®bers need to be addressed to further research of these materials as potential ®ber±matrix interphases and to validate the concept. The ®rst is the lowering of the temperature of hexaluminate formation. The second is increasing grain growth to achieve basal texture at these lowered temperatures. 4. Synthesis from sol and solution presursors 4.1. Calcium hexaluminate The reaction of CaO and Al2O3 generally proceeds dynamically in a nonequilibrium manner with the formation of calcia-rich aluminates, followed by the formation of the relatively calcia-poor aluminates, until eventually the stoichiometric calcium aluminate is formed.45±47 For the case of CaO and Al2O3 in a 1:6 ratio, monocalcium aluminate and/or calcium dialuminate usually forms before the hexaluminate as follows34,48±50 CaO Al2O3 ! CaAl2O4 1 CaO 2Al2O3 ! CaAl4O7 2a CaAl2O4 Al2O3 ! CaAl4O7 2b CaAl4O7 4Al2O3 ! CaAl12O19 3 Residual CaAl4O7 and Al2O3 often accompany CaAl12O19 in the ®nal product. In cases where 45 wt% CaO was added to 500-nm particle size a-Al2O3 powders, reactions (1) and (2) preceded reaction (3).48,49 Whereas, in the case where the appropriate amount of a Ca-salt was added to 40-nm particle size (5-nm crystallites) boehmite to form a stoichiometric CaO:6Al2O3 colloidal sol, or the precursor was a polymeric solution, only reaction (2a) was found to precede reaction (3).34,50 In general, the completion of such reactions depends on the diusion distances, i.e. particle size and degree of mixing of the reactant powders. The precursor sol originally used to apply coatings to single-crystal alumina and YAG ®bers and plates was a boehmite colloid, doped with a stoichiometric amount of calcium acetate, which required temperatures of over 1400C for complete reaction to hibonite and to fully texture the coating.32±35 Coating polycrystalline alumina-based ®bers with this same colloidal sol resulted in severe embrittlement of the ®ber and the formation of an a-Al2O3 coating. Prior to the formation of calcium hexaluminate, CaO diuses out of the coating and to the grain boundaries in the ®ber, leaving an aluminarich precursor behind. The viscous nature of the sol, which results from doping aqueous boehmite sols with even low levels of metal salts leads to extensive bridging of the ®laments in the tow, despite the use of an immiscible liquid to remove excess sol during the coating process. Many alternate precursors have been investigated including other colloidal based sols, organometallics, and polymeric solutions.50 However, only one sol, a mixed-metal citric acid complex, was found to signi®cantly lower the reaction temperature to yield hibonite, free of any intermediate phases, at 1300C (Fig. 2). The resin that is formed during the condensation reaction with ethylene glycol is amorphous and contains the Ca2+ and Al3+ cations uniformly distributed on a very ®ne scale. The short diusion distances between constituents and their uniform, stoichiometric dispersion gives rise to enhanced reactivity. The formation of the kinetically favored intermediate CaAl4O7, which must then react with Al2O3 to form CaAl12O19, seems to be suppressed during heating of the citrate-based precursor. However, even with a reaction temperature of 1300C, signi®cant degradation of polycrystalline ®bers occurs. It is well known that dopants in the form of solid-solution and liquid-phase formers can enhance the sintering 572 M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582
M K. Cinibulk/Journal of the European Ceramic Society 20(2000)569-582 (a)100 aAL o ■x= Undoped Mg Si Ti V Cr Mn Fe Co Ni 1200°c 1100°C 1000°C 200°C 900 52025303540455055 Amount of Fe Substituted for Al [at. % Fig. 2. XRD patterns of citrate-gel CaAl12019 precursor after heating 0.4 at%-doped citrate-based precursors) after I h at 1100.C.(b) hibonite are labeled: [-AlyO3: a transitional alumina, CA2: calcium Minimum temperature for obtaining phase-pure (>95 mol%) a(Al, Fe)l2Ojg within I h as a function of amount of Fe,+ substituted dialuminate. a corundum for Al3+ and grain growth, -oo creep, -o and the r-a phase aluminate formation. Ironically, most of the dopants transformation of alumina 57.59 Dopants were incorpo- that enhanced the formation of CaAl12O19 also are rated into the citrate-based CaAl12O19 sol to investigate reported to lower the y-o phase transformation ter their effects on hibonite formation at 1100%C 50 Incor perature of alumina, with the exception of SiO2.59,65-68 poration of dopants into the citrate-based precursors The positive influence of the dopants on calcium hex significantly affected formation of calcium hexaluminate; aluminate formation is believed to be the enhanced in some cases it was suppressed, while in most of the reactivity of Cao with Al2O, to form CaAl12O1g rather others the extent of formation was increased dramati- than CaALO7, prior to destabilization of the y-Al2O3 cally under identical heat treatments [Fig 3(a)]. Whether spinel structure. The cations that resulted in the greatest or not this was due to a solid-solution effect is not clear. hexaluminate yield at 1100 C are all known to stabilize While a second phase was detected by XRD only in the the spinel structure, MAl2O4 CoO-, NiO-, CuO-, or ZnO-doped powders, the other dopants could have either segregated to the grain 4.2. Lanthanum hexaluminate boundaries or formed a lower - volume-fraction amor. phous second phase, not readily detectable by XRD Similar to the case of CaAl12O19, La,O3 reacts with Fe was added at levels of up to 25 at% to exploit the Al2O3 to form an intermediate, in this case a perovskite, large solubility of Fe2O3 in CaAl12O19, resulting in a first: 39.69, 70 continuous decrease in formation temperature with increasing Fe substitution for Al [Fig. 3(b) LaO3+AlO3→2 LaAlo With the doped-citrate powders, the y-o phase transformati ion of alumina was enhanced in every case LaAlO3+ 5Al203* LaAln O18 at 1100C with the exception of the MgO-doped pow der. For the powders doped at 4 at% with Ti, V, Mn, The formation of LaLu Oi8 can be enhanced by the Fe, Co, and Cu the transformation to a-Al2O3 appeared use of precursors with improved chemical homogeneity to be complete. The suppression of a-Al2O3 formation, and decreased diffusion distances, compared with con except in the case of MgO-doped powder, favors calcium ventional mixing of elemental powders. 70 Ropp and
and grain growth,51±60 creep,61±64 and the g!a phase transformation of alumina.57,59 Dopants were incorporated into the citrate-based CaAl12O19 sol to investigate their eects on hibonite formation at 1100C.50 Incorporation of dopants into the citrate-based precursors signi®cantly aected formation of calcium hexaluminate; in some cases it was suppressed, while in most of the others the extent of formation was increased dramatically under identical heat treatments [Fig. 3(a)]. Whether or not this was due to a solid-solution eect is not clear. While a second phase was detected by XRD only in the CoO-, NiO-, CuO-, or ZnO-doped powders, the other dopants could have either segregated to the grain boundaries or formed a lower-volume-fraction amorphous second phase, not readily detectable by XRD. Fe was added at levels of up to 25 at% to exploit the large solubility of Fe2O3 in CaAl12O19, resulting in a continuous decrease in formation temperature with increasing Fe substitution for Al [Fig. 3(b)]. With the doped-citrate powders, the g!a phase transformation of alumina was enhanced in every case at 1100C with the exception of the MgO-doped powder. For the powders doped at 4 at% with Ti, V, Mn, Fe, Co, and Cu the transformation to a-Al2O3 appeared to be complete. The suppression of a-Al2O3 formation, except in the case of MgO-doped powder, favors calcium aluminate formation. Ironically, most of the dopants that enhanced the formation of CaAl12O19 also are reported to lower the g!a phase transformation temperature of alumina, with the exception of SiO2. 59,65±68 The positive in¯uence of the dopants on calcium hexaluminate formation is believed to be the enhanced reactivity of CaO with Al2O3 to form CaAl12O19 rather than CaAl4O7, prior to destabilization of the g-Al2O3 spinel structure. The cations that resulted in the greatest hexaluminate yield at 1100C are all known to stabilize the spinel structure, MAl2O4. 4.2. Lanthanum hexaluminate Similar to the case of CaAl12O19, La2O3 reacts with Al2O3 to form an intermediate, in this case a perovskite, ®rst:39,69,70 La2O3 Al2O3 ! 2LaAlO3 4 LaAlO3 5Al2O3 ! LaAl11O18 5 The formation of LaAl11O18 can be enhanced by the use of precursors with improved chemical homogeneity and decreased diusion distances, compared with conventional mixing of elemental powders.70 Ropp and Fig. 2. XRD patterns of citrate-gel CaAl12O19 precursor after heating for 1 h in air at the indicated temperatures. Peaks not attributed to hibonite are labeled: t-Al2O3: a transitional alumina, CA2: calcium dialuminate, a: corundum. Fig. 3. (a) CaMxAl12ÿxO19 yield for x=0.5 and 0.05 (4.0 at%- and 0.4 at%-doped citrate-based precursors) after 1 h at 1100C. (b) Minimum temperature for obtaining phase-pure (>95 mol%) Ca(Al,Fe)12O19 within 1 h as a function of amount of Fe3+ substituted for Al3+. M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582 573
M.K. Cinibulk/Journal of the European Ceramic Society 20(2000)569-582 Carroll have proposed that the rate limiting step in the reaction to LaAlyO18 is the conversion of O to 202 that accompanies La'+ diffusion. 1. 72 Jero et al. have found that the formation of laLu Ois from metallo- 1100°C ganic precursors is enhanced in oxygen and hindered in argon, compared with air. Saruhan and co-workers74, 75 have recently reported the synthesis of LaAl1O18 at 1300C in air by seeding a metallorganic sol with LaAl1O1s particles. While improved reactivity has been a1000c shown, phase-pure LaAl1 O1s has not been obtained at temperatures below 1300C As discussed earlier, substituting a divalent cation for one Al+ in LnAluOjg establishes charge balance and stabilizes the magnetoplumbite structure, producing lanthanide hexaluminate of the type LnMAl1O19, for Ln=La→Gd,andM=Mg,Mn→Zn. The easier prep aration of the doped materials is attributed to the enhanced stabilization of the spinel blocks by the divalent cations. which reduce the number of vacancies in the unit cell, resulting in a stoichiometric magneto- 100°C plumbite. All of the divalent cations that stabilize the lanthanide-hexaluminate magnetoplumbite are also those whose oxides(MO) form spinel-type(MAl2O4 phases with alumina. LaMnAllO1g and lacuAllo19 1000°C have been found to form completely at temperatures as low 1000C from citrate-gel precursors, without the appearance of the intermediate perovskite, while La MgAl1O19, LaFelsAl1o. 5O19, LaCoAluO19, and ZnAluO1 form directly at 1100 C within 1 h with Fig. 4. XRD patterns of (a)LaMnAlnO19 and(b) LaFelsAllosO out residual LaAlO3(Fig. 4). Unlike its effect on cal- powders heated at 1000 or 1100C for I h cium hexaluminate, where Fe2O3 was found to be most effective at enhancing crystallization of the magneto- constrain grain orientation when its formation is pre- plumbite, greater concentrations were necessary to pro- ceded by epitaxial seeding of alumina; and (iii) abnormal duce phase-pure Fe-doped lanthanum hexaluminate at grain growth driven by surface /interface-energy aniso- 1 100C. One reason for this is likely to be that the tropy. In the case where the film was either free-standing favored oxidation state of iron at these temperatures is or the substrate was not involved in the reactions, the low 3+, rather than the required 2+ surface-energy(0001) planes of the hexaluminate were favored and abnormal grain-growth proceeded to form 0001-textured films without the aid of preferred reaction 5. Films and interphases direction. Enhanced basal texture can be achieved by increasing abnormal grain growth by enhancing grain 5.1. Development of texture in films and interphases boundary mobility, thinning the film, increasing surface/ interface-energy anisotropy, and increasing the relative Near perfect texture of calcium and lanthanum hex- grain size of basal oriented grains to those in other aluminate films and interphases has been obtained on orientations. single-crystal substrates, as determined from XRD of Grain growth can be significantly affected by the pre- films on plates(Fig. 5)and TEM of fiber coatings and sence of impurities. Dopants may modify the lattice interphases in composites(Fig. 6). Several mechanisms defect structure of the host, segregate to grain bound- have been presented that could explain the 0001-tex- aries, or form a second phase(amorphous or crystalline tured CaAl12O1g free-standing films and coatings on at the grain boundaries. The mobility of grain bound single-crystal plates and fibers. 34 These include: (i) faster aries can be either reduced by solutes due to an interac reaction along basal plane directions, geometric con- tion energy between grain boundaries and the solutes straint of those grains not oriented for rapid growth, (solute drag), b or increased by an enhancement of diffu ind subsequent seeding of abnormal grain growth by sive mass-transport of the rate-limiting species. 77 Sig- the initially larger size of the basal-oriented grains; (ii) nificant amounts of insoluble dopants can also give rise to rapid lattice diffusion along basal planes that partially grain boundaries that are either partially or completely
Carroll69 have proposed that the rate limiting step in the reaction to LaAl11O18 is the conversion of O2 to 2O2+, that accompanies La3+ diusion.71,72 Jero et al.73 have found that the formation of LaAl11O18 from metallorganic precursors is enhanced in oxygen and hindered in argon, compared with air. Saruhan and co-workers74,75 have recently reported the synthesis of LaAl11O18 at 1300C in air by seeding a metallorganic sol with LaAl11O18 particles. While improved reactivity has been shown, phase-pure LaAl11O18 has not been obtained at temperatures below 1300C. As discussed earlier, substituting a divalent cation for one Al3+ in LnAl11O19 establishes charge balance and stabilizes the magnetoplumbite structure, producing lanthanide hexaluminate of the type LnMAl11O19, for Ln=La!Gd, and M=Mg, Mn!Zn. The easier preparation of the doped materials is attributed to the enhanced stabilization of the spinel blocks by the divalent cations, which reduce the number of vacancies in the unit cell, resulting in a stoichiometric magnetoplumbite. All of the divalent cations that stabilize the lanthanide-hexaluminate magnetoplumbites are also those whose oxides (MO) form spinel-type (MAl2O4) phases with alumina. LaMnAl11O19 and LaCuAl11O19 have been found to form completely at temperatures as low 1000C from citrate-gel precursors, without the appearance of the intermediate perovskite, while La MgAl11O19, LaFe1.5Al10.5O19, LaCoAl11O19, and La ZnAl11O19 form directly at 1100C within 1 h without residual LaAlO3 (Fig. 4).39 Unlike its eect on calcium hexaluminate, where Fe2O3 was found to be most eective at enhancing crystallization of the magnetoplumbite, greater concentrations were necessary to produce phase-pure Fe-doped lanthanum hexaluminate at 1100C. One reason for this is likely to be that the favored oxidation state of iron at these temperatures is 3+, rather than the required 2+. 5. Films and interphases 5.1. Development of texture in ®lms and interphases Near perfect texture of calcium and lanthanum hexaluminate ®lms and interphases has been obtained on single-crystal substrates, as determined from XRD of ®lms on plates (Fig. 5) and TEM of ®ber coatings and interphases in composites (Fig. 6). Several mechanisms have been presented that could explain the 0001-textured CaAl12O19 free-standing ®lms and coatings on single-crystal plates and ®bers.34 These include: (i) faster reaction along basal plane directions, geometric constraint of those grains not oriented for rapid growth, and subsequent seeding of abnormal grain growth by the initially larger size of the basal-oriented grains; (ii) rapid lattice diusion along basal planes that partially constrain grain orientation when its formation is preceded by epitaxial seeding of alumina; and (iii) abnormal grain growth driven by surface/interface-energy anisotropy. In the case where the ®lm was either free-standing or the substrate was not involved in the reactions, the low surface-energy (0001) planes of the hexaluminate were favored and abnormal grain-growth proceeded to form 0001-textured ®lms without the aid of preferred reaction direction. Enhanced basal texture can be achieved by increasing abnormal grain growth by enhancing grainboundary mobility, thinning the ®lm, increasing surface/ interface-energy anisotropy, and increasing the relative grain size of basal oriented grains to those in other orientations. Grain growth can be signi®cantly aected by the presence of impurities. Dopants may modify the lattice defect structure of the host, segregate to grain boundaries, or form a second phase (amorphous or crystalline) at the grain boundaries. The mobility of grain boundaries can be either reduced by solutes due to an interaction energy between grain boundaries and the solutes (solute drag),76 or increased by an enhancement of diusive mass-transport of the rate-limiting species.77 Signi®cant amounts of insoluble dopants can also give rise to grain boundaries that are either partially or completely Fig. 4. XRD patterns of (a) LaMnAl11O19 and (b) LaFe1.5Al10.5O19 powders heated at 1000 or 1100C for 1 h. 574 M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582
M K. Cinibulk/Journal of the European Ceramic Society 20(2000)569-582 1300°c/10h 21200c/1 1100c/1h 1300c/10h 200c!10h 15202530354 Fig. 7. XRD LaFelsAl1o5O Fig.5.(a)SEM image of textured CaAl12Oug on single-crystal alu- 1200.C, with 做m of spin-coated (a) CaFeo.5Al11.5O19 and (b nto(Ill)YAG. The films begin to texture at 1) peaks present. mina plate after heating to 1400 C for 2 h:(b)XRD pattern of hibo- only at temperatures above 1400 C. Textured films have been obtained by annealing doped spin-coated films on single-crystal YAG at 1200.C. ,0 In the latter case the use of an inert substrate which does not interact with the film by seeding grain growth, results in enhanced reaction and texture. Similarly, self-supporting films can be highly textured at 1200.C when they are extremel 5. 2. Coatings on polycrystalline fibers 略 In Section 3, issues germane to the application of a textured hexaluminate interphase were presented. Many of these issues do not pertain to the coating of single crystal and monofilament fibers, such as the Saphikon single-crystal alumina fibers that were used in earlier Fig. 6. TEM image of textured hibonite interphase between a single. ork 32-35 In this section some of these issues are crystal alumina fiber and polycrystalline YAG matrix, oriented along addressed as a result of work on solution -derived cal the fiber axis cium and lanthanum hexaluminate, and their applica tion to alumina polycrystalline fibers. wetted, which can lead to increased abnormal grain To date basal-plane textured coatings have not been growth. 56,58 Spin coated films of doped calcium and obtained on polycrystalline fibers. Early work on coat lanthanum hexaluminate on YAG have shown ing polycrystalline fil rs wI h boehmite-based pre enhanced texture at temperatures below those obtain- cursors to CaAl12O19 showed a severe degradation of able with undoped films(Fig. 7). 3950 fiber strength after coating. While even dilute sols were In the above examples highly textured films and inter- used to minimize fiber bridging, the apparent tow phases have been observed on single-crystal alumina strength was still greatly reduced. Fig. 8 shows the
wetted, which can lead to increased abnormal grain growth.56,58 Spin coated ®lms of doped calcium and lanthanum hexaluminates on YAG have shown enhanced texture at temperatures below those obtainable with undoped ®lms (Fig. 7).39,50 In the above examples highly textured ®lms and interphases have been observed on single-crystal alumina only at temperatures above 1400C. Textured ®lms have been obtained by annealing doped spin-coated ®lms on single-crystal YAG at 1200C.39,50 In the latter case the use of an inert substrate, which does not interact with the ®lm by seeding grain growth, results in enhanced reaction and texture. Similarly, self-supporting ®lms can be highly textured at 1200C when they are extremely thin.39,50 5.2. Coatings on polycrystalline ®bers In Section 3, issues germane to the application of a textured hexaluminate interphase were presented. Many of these issues do not pertain to the coating of singlecrystal and mono®lament ®bers, such as the Saphikon single-crystal alumina ®bers that were used in earlier work.32±35 In this section some of these issues are addressed as a result of work on solution-derived calcium and lanthanum hexaluminates, and their application to alumina polycrystalline ®bers. To date basal-plane textured coatings have not been obtained on polycrystalline ®bers. Early work on coating polycrystalline ®bers with boehmite-based precursors to CaAl12O19 showed a severe degradation of ®ber strength after coating. While even dilute sols were used to minimize ®ber bridging, the apparent tow strength was still greatly reduced. Fig. 8 shows the Fig. 6. TEM image of textured hibonite interphase between a singlecrystal alumina ®ber and polycrystalline YAG matrix, oriented along the ®ber axis. Fig. 5. (a) SEM image of textured CaAl12O19 on single-crystal alumina plate after heating to 1400C for 2 h; (b) XRD pattern of hibonite coating. Fig. 7. XRD patterns of spin-coated (a) CaFe0.5Al11.5O19 and (b) LaFe1.5Al10.5O19 ®lms onto (111) YAG. The ®lms begin to texture at 1200C, with only (0001) peaks present. M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582 575
M.K. Cinibulk /Journal of the European Ceramic Society 20(2000)569-582 esults of single-filament tensile tests from Nextel 610 hows the results of varying citric acid concentration on tows after soaking in various dilute salt solutions and aqueous- and ethanol-based lanthanum hexaluminate then heating at 1000C for 10 h. While the strength of precursor solutions. While not quantified, viscosities of fibers soaked in the alkaline-earth nitrates were much 2 cP seem to provide the best coatings with the least lower than a control sample(heated but not exposed to amount of bridging. However, precursors using ethanol solutions). the fibers exposed to lanthanum nitrate were seem to wet the fibers better than those using water as a not degraded. More recent work indicates that at solvent. The following coatings were applied with etha 1 100C lanthanum nitrate-treated Nextel 610 also nol-based citric-acid polymeric solutions shows some degradation. This agrees with the effects of Nextel 610 and 720 fibers have been coated with 30-g/ some of these cations on polycrystalline alumina reported CaFeo5Al115O19(CAF)and LaFel5Al105O19(LAF) in the literature. 40-44 These results suggest that lanthanum solutions using a continuous fiber coater and passed hexaluminate is preferred to calcium hexaluminate as a through an 1 100 C furnace, positioned in series with the fiber coating to minimize strength loss of the fiber. coater, at l cm/s Hexadecane, immiscible with ethanol, Furthermore, La2O3 segregated to the grain boundaries was floated on top of the sol to aid in removal of excess of polycrystalline alumina has been reported to increase sol from between filaments, as discussed previously its creep resistance. 78 Surfaces of coated fibers were characterized by SEM solutions has a significant effect on viscosity. Fig. 9 by TEM. b mpregnated coated fibers were characterized Nextel 610 and 720 fibers coated with CaF at 1100.C resulted in very uniform coatings on selected fibers, but 24GPa,6.1 were often not present on all filaments; coatings ranged from 0 to 200 nm in thickness Selected-area electron a diffraction of coatings on fibers gave diffuse 1.8GPa,4.4 were indexed as nanocrystalline 8-Al2O3 EDS of the coatings indicated easd e was pre- esence of ca in some of the coatings but not all, wher ATAs-Received sent in all coatings. Heat treating the coated fibers for 2.7GPa,5.6 h at 1 C resulted in a well crystallized a(Al, Fe)O coating [Fig. 10(b). No Ca could be detected within the coating. After heating at 1200C for 10 h, large elon 24GPa,58 gated grains of(Al, Fe)2O3(due to Cao in amounts below the eds detection limit)could be seen oriented with the basal planes growing radially outward from the In o [GPal fiber, as though seeded by the fiber surface or simply due to a radial reaction direction or grain growth within Fig 8. Weibull plot of single-filament tensile tests of Nextel 610 fibers the coating after heating tows exposed to various dilute nitrate solutions at Nextel 610 fiber tows were also coated with laf at 1000C for 10 h(courtesy T.A. Parthasarathy). 1100%C. tEM of the as-coated fibers indicated that the coating was a nanocrystalline magnetoplumbite. EDS confirmed the presence of La and Fe. After heating at 1100C for I h the grains in the coating had significantly coarsened but the composition had remained the same After heating at 1200C for 10 h, some texturing was 2 mo! CA-ethanol evident with grains in a similar orientation as those observed in the CAF-coated fibers. Fig. 11 shows the coatings on the fibers as coated and after heating for 10 5 mol CA-ethar While basal-textured coatings were not obtained with either compound on Nextel 610 fibers, the lanthanum hexaluminate phase was formed. Texture development is believed to require temperatures in excess of those Solids Concentration [g/L used in this work due to the interaction of the fiber with reaction and grain growth in the coating, which ca Fig. 9. Plot of viscosity as a function of solids concentration ot initiate growth directions outward from the fiber surface either 0.5 or 2 mol citric acid(CA) per mol of aluminum in water or as shown in Figs. 10(b)and 1l(b). Unfortunately,at greater temperatures, the reduction in fiber strength is
results of single-®lament tensile tests from Nextel 610 tows after soaking in various dilute salt solutions and then heating at 1000C for 10 h. While the strength of ®bers soaked in the alkaline-earth nitrates were much lower than a control sample (heated but not exposed to solutions), the ®bers exposed to lanthanum nitrate were not degraded. More recent work indicates that at 1100C lanthanum nitrate-treated Nextel 610 also shows some degradation. This agrees with the eects of some of these cations on polycrystalline alumina reported in the literature.40±44 These results suggest that lanthanum hexaluminate is preferred to calcium hexaluminate as a ®ber coating to minimize strength loss of the ®ber. Furthermore, La2O3 segregated to the grain boundaries of polycrystalline alumina has been reported to increase its creep resistance.78 The medium used for citric acid based polymeric solutions has a signi®cant eect on viscosity. Fig. 9 shows the results of varying citric acid concentration on aqueous- and ethanol-based lanthanum hexaluminate precursor solutions. While not quanti®ed, viscosities of 2 cP seem to provide the best coatings with the least amount of bridging. However, precursors using ethanol seem to wet the ®bers better than those using water as a solvent. The following coatings were applied with ethanol-based citric-acid polymeric solutions. Nextel 610 and 720 ®bers have been coated with 30-g/l CaFe0.5Al11.5O19 (CAF) and LaFe1.5Al10.5O19 (LAF) solutions using a continuous ®ber coater79 and passed through an 1100C furnace, positioned in series with the coater, at 1 cm/s. Hexadecane, immiscible with ethanol, was ¯oated on top of the sol to aid in removal of excess sol from between ®laments, as discussed previously. Surfaces of coated ®bers were characterized by SEM and epoxy-impregnated coated ®bers were characterized by TEM.80 Nextel 610 and 720 ®bers coated with CAF at 1100C resulted in very uniform coatings on selected ®bers, but were often not present on all ®laments; coatings ranged from 0 to 200 nm in thickness. Selected-area electron diraction of coatings on ®bers gave diuse rings that were indexed as nanocrystalline y-Al2O3 [Fig. 10(a)]. EDS of the coatings indicated the presence of Ca in some of the coatings, but not all, whereas, Fe was present in all coatings. Heat treating the coated ®bers for 1 h at 1100C resulted in a well crystallized a-(Al,Fe)2O3 coating [Fig. 10(b)]. No Ca could be detected within the coating. After heating at 1200C for 10 h, large elongated grains of (Al,Fe)2O3 (due to CaO in amounts below the EDS detection limit) could be seen oriented with the basal planes growing radially outward from the ®ber, as though seeded by the ®ber surface or simply due to a radial reaction direction or grain growth within the coating. Nextel 610 ®ber tows were also coated with LAF at 1100C. TEM of the as-coated ®bers indicated that the coating was a nanocrystalline magnetoplumbite. EDS con®rmed the presence of La and Fe. After heating at 1100C for 1 h the grains in the coating had signi®cantly coarsened but the composition had remained the same. After heating at 1200C for 10 h, some texturing was evident with grains in a similar orientation as those observed in the CAF-coated ®bers. Fig. 11 shows the coatings on the ®bers as coated and after heating for 10 h at 1200C. While basal-textured coatings were not obtained with either compound on Nextel 610 ®bers, the lanthanum hexaluminate phase was formed. Texture development is believed to require temperatures in excess of those used in this work due to the interaction of the ®ber with reaction and grain growth in the coating, which can initiate growth directions outward from the ®ber surface as shown in Figs. 10(b) and 11(b). Unfortunately, at greater temperatures, the reduction in ®ber strength is Fig. 9. Plot of viscosity as a function of solids concentration of LaFe1.5Al10.5O19 solutions at 23C. Solutions were synthesized with either 0.5 or 2 mol citric acid (CA) per mol of aluminum in water or ethanol as indicated. Fig. 8. Weibull plot of single-®lament tensile tests of Nextel 610 ®bers after heating tows exposed to various dilute nitrate solutions at 1000C for 10 h (courtesy T.A. Parthasarathy). 576 M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582
M.K. Cinibulk/Journal of the European Ceramic Society 20(2000)569-582 heta-alumina 簧 LAF N20 200mm Fig. 10. TEM images of Nextel 720 fibers containing coatings Fig. l1. TEM images of Nextel 610 fibers containing coatings applied vith the iron-doped calcium hexaluminate citrate precursor: ( with the iron-doped lanthanum hexaluminate citrate coated at 1100C(15 s residence time). EDS spectrum of the fiber coated at 1100C(15 s residence time). EDS spectrum of the indicates presence of Ca and Fe(peak at 8.05 keV is Cu from ion coating indicates presence of La and Fe. SAD pattern identifies the milling). SAD pattern identifies the coating as being composed of q- coating as LaFel.sAl1o50:(b)fiber coating after heating for 10 h at (Al, Fe),O3:(b) fiber coating after heating for I h at 1100C, with 1200C, with magnetoplumbite grains growing radially outward from grains of a-(AL, Fe)2O3 growing radially outward from the fiber surface fiber surface, with basal planes approximately normal to the plane of with basal planes normal to the plane of the interface. Similar results the interface. Large peak at 8.05 keV in spectrum is Cu sputtered were obtained with Nextel 610 fibers much greater still and would not result in a useful fiber, tively rough debond surface that results and is believed let alone a useful coated fiber to give rise to extremely high radial compressive stresses up which results in m 53. Crack-interphase interaction in model composites out. 81, 82, 83 Fig. 12 illustrates these features rather well The coating can be seen bonding to both the fiber and To date, coatings on model composites using single- matrix in the region surrounding the partially debonded crystals as one or more of the constituents are the only fiber. Flat cleavage fracture can be seen, yet due to the evidence that has been presented in support of hex- presence of grain boundaries where the crack can also aluminates as a viable cleavable oxide fiber coating. The propagate intergranularly the surface is rather rough earliest encouraging results came from the observation This is also believed to be the reason why fibers coated of cracks that had been deflected within a CaAl12O19 with textured hibonite cannot be pushed out of a interphase and propagated around a fiber via transgra- matrix. 33 Simple geometric arguments suggest that the nular cleavage in a TEM foil 32-35 Subsequent work minimum roughness amplitude, A, for intragranular imaging the fracture surfaces of various model compo- cleavage fracture must depend on the hexaluminate sites containing single-crystal alumina with textured grain size, d, and the fiber diameter, rs,83 CaAl12O19 coating clearly indicated crack deflection within the coating. However, it also indicated the rela A=G2+d2)12
much greater still and would not result in a useful ®ber, let alone a useful coated ®ber. 5.3. Crack-interphase interaction in model composites To date, coatings on model composites using singlecrystals as one or more of the constituents are the only evidence that has been presented in support of hexaluminates as a viable cleavable oxide ®ber coating. The earliest encouraging results came from the observation of cracks that had been de¯ected within a CaAl12O19 interphase and propagated around a ®ber via transgranular cleavage in a TEM foil.32±35 Subsequent work imaging the fracture surfaces of various model composites containing single-crystal alumina with textured CaAl12O19 coating clearly indicated crack de¯ection within the coating. However, it also indicated the relatively rough debond surface that results and is believed to give rise to extremely high radial compressive stresses upon sliding, which results in minimal ®ber pullout.81,82,83 Fig. 12 illustrates these features rather well. The coating can be seen bonding to both the ®ber and matrix in the region surrounding the partially debonded ®ber. Flat cleavage fracture can be seen, yet due to the presence of grain boundaries where the crack can also propagate intergranularly the surface is rather rough. This is also believed to be the reason why ®bers coated with textured hibonite cannot be pushed out of a matrix.33 Simple geometric arguments suggest that the minimum roughness amplitude, A, for intragranular cleavage fracture must depend on the hexaluminate grain size, d, and the ®ber diameter, rf, 83 A r 2 f d2 1=2 ÿ rf Fig. 10. TEM images of Nextel 720 ®bers containing coatings applied with the iron-doped calcium hexaluminate citrate precursor: (a) ®ber coated at 1100C (15 s residence time). EDS spectrum of the coating indicates presence of Ca and Fe (peak at 8.05 keV is Cu from ion milling). SAD pattern identi®es the coating as being composed of q- (Al,Fe)2O3; (b) ®ber coating after heating for 1 h at 1100C, with grains of a-(Al,Fe)2O3 growing radially outward from the ®ber surface with basal planes normal to the plane of the interface. Similar results were obtained with Nextel 610 ®bers. Fig. 11. TEM images of Nextel 610 ®bers containing coatings applied with the iron-doped lanthanum hexaluminate citrate precursor: (a) ®ber coated at 1100C (15 s residence time). EDS spectrum of the coating indicates presence of La and Fe. SAD pattern identi®es the coating as LaFe1.5Al10.5O19; (b) ®ber coating after heating for 10 h at 1200C, with magnetoplumbite grains growing radially outward from ®ber surface, with basal planes approximately normal to the plane of the interface. Large peak at 8.05 keV in spectrum is Cu sputtered during ion milling. M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582 577
M K. Cinibulk/Journal of the European Ceramic Society 20(2000)569-582 ibonite Bonded to Fiber d Fig 12. SEM images of two mating fracture surfaces from a tape-cast composite consisting of textured-hibonite-coated sapphire fibers in a potas- (b) and (d) show interphase bonding to fiber and matrix. respectively, indicating crack deflection and propagation within the fiber coating. Note fat cleavage features and rough surface due to intergranular stepping of crack at grain boundaries Low roughness amplitudes that allow fiber pullout 6. Compatability with potential CMC phases after debonding require d < rf, but may also require thin interlayers that force d to be small, and limit large Hexaluminate are the most alumina-rich aluminate inter granular jumps in the fracture path between grains that form and, therefore, are phase compatible with that may cause much larger roughness amplitudes. alumina. Incongruent melting points for the magneto- Coating thicknesses less than 0. 2 mm and grain sizes plumbite-structured hexaluminate have been reported larger than 10 um are suggested for significant debone to range from 183384 to 1883C85 for CaAl12O1g and lengths on 140-um diameter fibers. This dictates from 184886 to 1930 C87 for LaAl,Oi8. Calcium hex- monolayer of plate-shaped grains with an aspect ratio aluminate has been shown to be stable with YAG up to of <0.02, which is approximately an order of magni- at least 1650 C: 32-35 lanthanum hexaluminate is als tude less than that currently observed in hibonite coat- phase compatible with YAG. While the various doped ngs on sapphire( see Fig. 6) calcium and lanthanum hexaaluminates have not beer Tensile tests of microcomposites, which contain a single 135-Hm diameter sapphire fiber coated with a 2 textured hibonite interlayer and an outer layer of poly crystalline alumina as the matrix indicated that there was no perceptible nonlinearity in the load-diplacement 0F 1.18 GPa plot(Fig. 13).36 However, the compliance of the micro- composites was lower than that of bare sapphire fibers ∮Ftel450C2h indicating significant load transfer occurred. While the edian strength of hibonite -containing micro- Fiber-Hibonite composites was the same as that of microcomposites without an hibonite interphase, the Weibull modulus was nearly three times greater, which suggests that the 1-0.50 .5 hibonite was protecting the fibers from matrix cracks seM of the fracture surfaces revealed crack deflection g [GPa within the hibonite fiber coating however. debond Icroa lengths were limited to a few micrometers hibonite fiber coat
Low roughness amplitudes that allow ®ber pullout after debonding require d << rf, but may also require thin interlayers that force d to be small, and limit large intergranular jumps in the fracture path between grains that may cause much larger roughness amplitudes. Coating thicknesses less than 0.2 mm and grain sizes larger than 10 mm are suggested for signi®cant debond lengths on 140-mm diameter ®bers. This dictates a monolayer of plate-shaped grains with an aspect ratio of <0.02, which is approximately an order of magnitude less than that currently observed in hibonite coatings on sapphire (see Fig. 6). Tensile tests of microcomposites, which contain a single 135-mm diameter sapphire ®ber coated with a textured hibonite interlayer and an outer layer of polycrystalline alumina as the matrix indicated that there was no perceptible nonlinearity in the load±diplacement plot (Fig. 13).36 However, the compliance of the microcomposites was lower than that of bare sapphire ®bers, indicating signi®cant load transfer occurred. While the median strength of the hibonite-containing microcomposites was the same as that of microcomposites without an hibonite interphase, the Weibull modulus was nearly three times greater, which suggests that the hibonite was protecting the ®bers from matrix cracks. SEM of the fracture surfaces revealed crack de¯ection within the hibonite ®ber coating; however, debond lengths were limited to a few micrometers. 6. Compatability with potential CMC phases Hexaluminates are the most alumina-rich aluminates that form and, therefore, are phase compatible with alumina. Incongruent melting points for the magnetoplumbite-structured hexaluminates have been reported to range from 183384 to 1883C85 for CaAl12O19 and from 184886 to 1930C87 for LaAl11O18. Calcium hexaluminate has been shown to be stable with YAG up to at least 1650C;32±35 lanthanum hexaluminate is also phase compatible with YAG.39 While the various doped calcium and lanthanum hexaluminates have not been Fig. 13. Weibull plot comparing tensile test results of microcomposites and ®bers with and without a hibonite ®ber coating. Fig. 12. SEM images of two mating fracture surfaces from a tape-cast composite consisting of textured-hibonite-coated sapphire ®bers in a potassium±borosilicate glass matrix. (a) and (c) show two halves of the ®ber, which is partially debonded from the matrix. Higher magni®cation images in (b) and (d) show interphase bonding to ®ber and matrix, respectively, indicating crack de¯ection and propagation within the ®ber coating. Note ¯at cleavage features and rough surface due to intergranular stepping of crack at grain boundaries. 578 M.K. Cinibulk / Journal of the European Ceramic Society 20 (2000) 569±582
