J. Haslam et al. Journal of the European Ceramic Society 20(2000)607-618 much smaller than the fiber diameter to ensure good tows produced by the 3M Corporation( St Paul, MN) particle packing. 7 To avoid large cracklike voids from Each tow nominally contains 420 fibers. The Nextel 720 developing within the matrix, the powder should not fiber is an experimental fiber composed of a mixture of densify during subsequent heat treatments and at applica- submicron alumina and mullite grains. The two inter tion temperatures. 8.9 For this reason, in our previous work penetrating phases ensure a small grain size during pro- we used a mullite powder that did not begin to shrink until cessing. The mullite in the fiber contributes to high A1300oC, the maximum fiber application temperature. creep resistance compared to a similar all-alumina fiber After removing the liquid via evaporation, the powder (Nextel 610). The strength of the Nextel 720 fiber matrix was strengthened by infiltrating the composite with about 30% less than the 610 for single filament proper a solution containing precursor molecules. After evapor- ties, but it was selected because of its greater creep ating the liquid, heating causes the precursor molecules to resistance. I decompose to form an inorganic material that bonds the as detailed below laminated ceramic cloth was infil particles together. The inorganic phase that bonds and trated with a previously consolidated mixture of 70 strengthens the powder matrix also bonds the particles vol% cubic zirconia(solid solution with 8 mol%Y203 (matrix)to the fibers. Cyclic solution precursor infiltration, TZ8YS, Toso Ceramics, average particle diameter of evaporation, and decomposition further strengthens the(0.4 um) and 30 vol% mullite(MU-107, Showa Denko) matrix phase. Care must be taken to avoid precursor As detailed below and elsewhere, zirconia was used as molecules from migrating to the surface during evapora- the matrix because it can be sintered, without shrinkage, tion. This condition produces surface cracking during when heat treated in HCI at temperatures as low as drying due to a thin layer of precursor molecules that 1 C. 3 Mullite was introduced because previous form on the surface. 0 An all-oxide. fiber reinforced cera- work has shown that mullite does not allow the sintered mic composite can be processed in this method. Extensive and coarsened zirconia to shrink(densify)after expo- mechanical testing by Levi et al. has shown that this sure to air at 1200 C for 100 h.6 The zirconia was com- type of composite can exhibit a significant notch insen- osed of agglomerated particles which contained sitive strength in tensile loading. It also has all the primary particles of 50-100 nm in diameter. Infrared attributes found for fiber reinforced ceramics fabricated spectroscopy indicated that the mullite contained an with dense matrixes and weak fiber/matrix interfaces organic contaminant that had to be removed from the Here we report a much simpler and less time consuming powder before it was formulated as a slurry. A 10 h heat fabrication method for processing these new CMCs with treatment in air at 800C was sufficient to remove the porous matrices. In the new method, the powder is treated contaminant. The particle size(average=0.7um) did not to produce a special interparticle pair potential which change during the heat treatment. llows the powder compact(previously consolidated by Dispersed, aqueous slurries containing 20 vol% of the pressure filtration)to be fluidized. It can then be formed two powders were formed by adding 1.3 vol% poly nto a thin sheet by vibrating between plastic sheets. The ethylene oxide urethane silane(PEg-silane, Gelest, Inc) plastic sheets help to avoid evaporation and the con- at pH 10.5. This was found to be sufficient to coat the sequent drying of the thin particle layers. The ceramic particle surfaces. As detailed elsewhere, the PEG-silane sheet is then frozen to enable removal from between the molecules chem-adsorb to the particles by reacting with plastic sheets. The frozen ceramic sheet of powder is the -M-OH(M=metal atom) surface sites. 14, 15The then sandwiched between sheets of ceramic fibers(e.g. zirconia slurry was attrition milled for 15 min after the woven cloth). After thawing, the powder sheet is flui- powder was added. The mullite slurry was sonicated dized by vibrating. It then flows to surround all fibers in with an ultrasonic horn for 5 min prior to the final the adjacent fiber sheets After evaporation, the powder adjustment of the pH. Tetraethylammonium chloride urrounding the fibers can be made strong either by the TEACI)salt(0. 1 molar) was added to form weakly use of precursors described above or by an HCl eva- attractive pair potentials between the particles. As poration/condensation treatment described below. Pre- detailed elsewhere, TMA+ counter ions aid in produ- liminary mechanical measurements show that this new cing a weakly attractive particle network which can be route can result in similar properties as the previous packed to a high density via pressure filtration and route to manufacture CMCs with porous matrices allow the consolidated body to be fluidized via vibra- tion 16-18 TEA+ counter ions were used in this work these counter ions are slightly larger than TMA. Other 2. Experimental methods can be used to produce weakly attractive net- works such as surfactants or chemi-sorption of alco- 2.1. Composite processing hols. 9 The PEG-silane plus TEACI approach was appropriate here due to the two different powders used Composites were formed from layers of two dimen- to form a composite slurry The two slurries were mixed sional, 8 harness woven cloth of Nextel M 720 fiber in appropriate portions described above. The mixedmuch smaller than the ®ber diameter to ensure good particle packing.7 To avoid large, cracklike voids from developing within the matrix, the powder should not densify during subsequent heat treatments and at applica￾tion temperatures.8,9 For this reason, in our previous work we used a mullite powder that did not begin to shrink until 1300C, the maximum ®ber application temperature. After removing the liquid via evaporation, the powder matrix was strengthened by in®ltrating the composite with a solution containing precursor molecules. After evapor￾ating the liquid, heating causes the precursor molecules to decompose to form an inorganic material that bonds the particles together. The inorganic phase that bonds and strengthens the powder matrix also bonds the particles (matrix) to the ®bers. Cyclic solution precursor in®ltration, evaporation, and decomposition further strengthens the matrix phase. Care must be taken to avoid precursor molecules from migrating to the surface during evapora￾tion. This condition produces surface cracking during drying due to a thin layer of precursor molecules that form on the surface.10 An all-oxide, ®ber reinforced cera￾mic composite can be processed in this method. Extensive mechanical testing by Levi et al.5 has shown that this type of composite can exhibit a signi®cant notch insen￾sitive strength in tensile loading. It also has all the attributes found for ®ber reinforced ceramics fabricated with dense matrixes and weak ®ber/matrix interfaces. Here we report a much simpler and less time consuming fabrication method for processing these new CMCs with porous matrices. In the new method, the powder is treated to produce a special interparticle pair potential which allows the powder compact (previously consolidated by pressure ®ltration) to be ¯uidized. It can then be formed into a thin sheet by vibrating between plastic sheets. The plastic sheets help to avoid evaporation and the con￾sequent drying of the thin particle layers. The ceramic sheet is then frozen to enable removal from between the plastic sheets. The frozen ceramic sheet of powder is then sandwiched between sheets of ceramic ®bers (e.g. woven cloth). After thawing, the powder sheet is ¯ui￾dized by vibrating. It then ¯ows to surround all ®bers in the adjacent ®ber sheets After evaporation, the powder surrounding the ®bers can be made strong either by the use of precursors described above or by an HCl eva￾poration/condensation treatment described below. Pre￾liminary mechanical measurements show that this new route can result in similar properties as the previous route to manufacture CMCs with porous matrices. 2. Experimental 2.1. Composite processing Composites were formed from layers of two dimen￾sional, 8 harness woven cloth of NextelTM 720 ®ber tows produced by the 3M Corporation (St. Paul, MN). Each tow nominally contains 420 ®bers. The Nextel 720 ®ber is an experimental ®ber composed of a mixture of submicron alumina and mullite grains. The two inter￾penetrating phases ensure a small grain size during pro￾cessing. The mullite in the ®ber contributes to high creep resistance compared to a similar all-alumina ®ber (Nextel 610). The strength of the Nextel 720 ®ber is about 30% less than the 610 for single ®lament proper￾ties,11 but it was selected because of its greater creep resistance.12 As detailed below, laminated ceramic cloth was in®l￾trated with a previously consolidated mixture of 70 vol% cubic zirconia (solid solution with 8 mol% Y2O3, TZ8YS, Toso Ceramics, average particle diameter of (0.4 mm) and 30 vol% mullite (MU-107, Showa Denko). As detailed below and elsewhere, zirconia was used as the matrix because it can be sintered, without shrinkage, when heat treated in HCl at temperatures as low as 1100C.13 Mullite was introduced because previous work has shown that mullite does not allow the sintered and coarsened zirconia to shrink (densify) after expo￾sure to air at 1200C for 100 h.6 The zirconia was com￾posed of agglomerated particles which contained primary particles of 50±100 nm in diameter. Infrared spectroscopy indicated that the mullite contained an organic contaminant that had to be removed from the powder before it was formulated as a slurry. A 10 h heat treatment in air at 800C was sucient to remove the contaminant. The particle size (average=0.7mm) did not change during the heat treatment. Dispersed, aqueous slurries containing 20 vol% of the two powders were formed by adding 1.3 vol% poly￾ethylene oxide urethane silane (PEG-silane, Gelest, Inc.) at pH 10.5. This was found to be sucient to coat the particle surfaces. As detailed elsewhere, the PEG-silane molecules chem-adsorb to the particles by reacting with the -M±OH (M=metal atom) surface sites.14,15 The zirconia slurry was attrition milled for 15 min after the powder was added. The mullite slurry was sonnicated with an ultrasonic horn for 5 min prior to the ®nal adjustment of the pH. Tetraethylammonium chloride TEACl) salt (0.1 molar) was added to form weakly attractive pair potentials between the particles. As detailed elsewhere, TMA+ counter ions aid in produ￾cing a weakly attractive particle network which can be packed to a high density via pressure ®ltration and allow the consolidated body to be ¯uidized via vibra￾tion.16±18 TEA+ counter ions were used in this work; these counter ions are slightly larger than TMA+. Other methods can be used to produce weakly attractive net￾works such as surfactants or chemi-sorption of alco￾hols.19 The PEG-silane plus TEACI approach was appropriate here due to the two di€erent powders used to form a composite slurry The two slurries were mixed in appropriate portions described above. The mixed J.J. Haslam et al. / Journal of the European Ceramic Society 20 (2000) 607±618 609