1522 Journal of the American Ceramic Society-Kriven and Lee Vol. 88. No. 6 the mixture was heated. The Pva solution wa MATRIX m 5 wt% PVa (degree of polymerization-1700, Air Products and CORE, Chemicals Inc, Allentown, PA)dissolved in water. The prop LAMINATE tions of the Pva to cation sources in the solution were adjusted TRANSFORMABLE n such a way that there were four times more positively charged OXIDE INTERPHASE valences from the cations than the negatively charged functiona ends of the organics. As the viscosity increased by evapo- ation of water, the mixture was vigorously stirred. The remain CRACK ing water was then dried, converting the gel into a solid. Finall the precursor was finely ground and calcined at 750C for I h The calcined powder was ball milled with zirconia media for 2 h. Iso-propyl alcohol was used as a solvent for milling. To Transformed grains observe the critical grain size, the ball-milled powder was uni- axially pressed at 20 MPa followed by iso-static pressing at 1 MPa for 10 min. The pellet-shaped green compacts were hot ressed by loading them in a graphite die and surrounding them with compatible oxides. Hot pressing was carried out at 28 MPa under an argon atmosphere, at a temperature of 1200C for I h After hot pressing, the samples were annealed in air at 1300C fiber reinforced. f tic diagram illustrating the mechanism of "transforma- for various times. Some stress-induced transformation was of ceramic interphases leading to overall toughening of fibrous monolithic or laminated ceramic matrix com- achieved by hand grinding the hot-pressed and annealed spec- nens on a # In this study, two powders with different particle sizes were prepared in order to observe the effect of initial powder particle size on phase stability. The first, transformation has been measured from single crystal studies. 4 designated as powder A, was calcined at 1100 C for 3 h and The fully expanded, high temperature B structure undergoes a furnace cooled. The second, designated as powder B, was as- reversible, displacive transformation to a collapsed a structure calcined powder, followed by I h of attrition milling Powders a n cooling at 265.C. This is accompanied by a volume decrease and B were annealed at 1300%C for 10 h of approximately 3. 2%. The temperature of the ae p in- Amorphous-type cordierite powder was also prepared by the version in cristobalite is variable and depends on the chemically ame method. The cordierite powder was synthesized from doped crystal structure of the starting material. In order Mg(NO3)2 6H2o(reagent grade, Aldrich Chemical Co. ) Al(- tabilize the B-cristobalite down to room temperature, it can be NO3) 9H,o(reagent grade, Aldrich Chemical Co. ) and Lu- chemically doped with"stuffing"cations.. In particular, in dox As-40 colloidal silica(40 wt% suspension in water, Du Pont the Cao-AlOxSio, system, the molar ratio of calcium oxide Chemicals). Commercial mullite powder(KM Mullite-101 Kyo. to alumina is one in which aluminum occupies a silicon tetra titsu, Nagoya, Japan), which had an average particle size of hedral site, while the calcium ion occupies all the interstitial non- 0.3 um and a specific surface area of 26 m/g, was used in the framework sites. The presence of foreign ions in the inter- mullite/cordierite mixture tices presumably inhibits the contraction of the structure during The mullite/cordierite mixtures having different cordierite thea→β cristobalite transformation content were characterized for relevant properties such as ther In this study, mixed mullite/cordierite laminates with B+a- mal expansion coefficient and flexural strength of hot-pressed cristobalite transformation weakened interphases were used for investigation of the phase transformation and fracture behavior. Laminates were fabricated by the tape casting process, ac- In the absence of commercially available, pure mullite fibers: a consisted of 25 vol% oxide powders, -63 vol% solvent, and the match of thermal expansion coefficients of mullite and 12 vol% organics. The 0.5 wt%(dry weight basis of oxide cristobalite, as well as to improve the sinterability of mullite, der) polyvinyl butyral(PVB, Monsanto, St. Louis, MO)was a mullite/cordierite mixture was chosen as the matrix phase. a added to the slurries as a dispersant. The solvent was composed tape casting technique was used to engineer a series of laminated of a mixture of trichloroethylene(CICH= CCl,, Aldrich Chem- composites in various sequences of stacked and hot pressed ical Co. )and ethanol (CH CH,OH, Aldrich Chemical Co tapes. The grain size of the polycrystalline B-cristobalite was while the organics included a binder(Pvb, monsanto)and plas- ontrolled by varying the annealing time at 1300C, and exam- ticizers(polyethylene glycol(PEG)2000 and dioctyl phthalate ination by scanning electron microscopy (SEM). In addition, the (DP), Aldrich Chemical Co. ) After pulverization, dispersion effects of the laminate design and heating conditions on the and mixing by ball-milling two times, the slurries were stirred in trength and toughness of the laminated composites were stud- vacuum. This helped in removing bubbles and adjusting the ied, both qualitatively by optical microscopy and indentation working viscosity. After aging for 2 days, the slurries were tape techniques, as well as quantitatively by measuring flexure cast using a doctor blade opening of 150-300 um, to obtain strengths and the works of fracture tape cast green sheets of 60-150 um thickness. Drying of the cast tapes was carried out under a saturated solvent atmosphere for 1 II. Experimental Procedure The green laminated composites had area dimensions of 25 mm x 51 mm after stacking of green sheets. Thermocompression (1) Preparation of Powder and laminate was performed at 10 MPa load for 30 min at 80.C, which was Chemically stabilized, amorphous-type silica powder was pre- the softening point of the organics. The organic additives were pared by the organic steric entrapment technique empl polyvinyl alcohol(PVA)solution as a polymeric carrier i-g removed by heating to 550C in an air atmosphere, using a two- step heating process. After these additives were burned out, the a clear sol was prepared from Ludox As-40 colloidal silica green laminates were iso-statically cold pressed at 170 MPa for 40 wt% suspension in water, Du Pont Chemicals, Wilmington, 10 min The laminated green bodies were hot pressed in the same DE): Al(NO3)3.9H20(reagent grade, Aldrich Chemical Co way as mentioned earlier. Milwaukee, Wn) and Ca(NO3)2. 4H2o(reagent grade, Aldrich All laminated composites after densification had a 30-layer Chemical Co. )in proportions to form a final composition of repetitive sequence of matrix and interphase. These were made Cao. 2Al2O3.80Sio2. After dissolving these reagents inin a separately optimized 5: 1 thickness ratio of matrix to inter DI water, the organic carrier, PVA solution, was added and phase, by stacking each green sheet. After hot pressing, the lam-transformation has been measured from single crystal studies.34 The fully expanded, high temperature b structure undergoes a reversible, displacive transformation to a collapsed a structure on cooling at 2651C. This is accompanied by a volume decrease of approximately 3.2%.35,36 The temperature of the a3b in￾version in cristobalite is variable and depends on the chemically doped crystal structure of the starting material.36,37 In order to stabilize the b-cristobalite down to room temperature, it can be chemically doped with ‘‘stuffing’’ cations.38,39 In particular, in the CaO–Al2O3–SiO2 system, the molar ratio of calcium oxide to alumina is one in which aluminum occupies a silicon tetra￾hedral site, while the calcium ion occupies all the interstitial non￾framework sites.40–42 The presence of foreign ions in the inter￾stices presumably inhibits the contraction of the structure during the a3b cristobalite transformation. In this study, mixed mullite/cordierite laminates with b-a￾cristobalite transformation weakened interphases were used for investigation of the phase transformation and fracture behavior. In the absence of commercially available, pure mullite fibers, a model laminate configuration was chosen. In order to optimize the match of thermal expansion coefficients of mullite and cristobalite, as well as to improve the sinterability of mullite, a mullite/cordierite mixture was chosen as the matrix phase. A tape casting technique was used to engineer a series of laminated composites in various sequences of stacked and hot pressed tapes. The grain size of the polycrystalline b-cristobalite was controlled by varying the annealing time at 13001C, and exam￾ination by scanning electron microscopy (SEM). In addition, the effects of the laminate design and heating conditions on the strength and toughness of the laminated composites were stud￾ied, both qualitatively by optical microscopy and indentation techniques, as well as quantitatively by measuring flexure strengths and the works of fracture. II. Experimental Procedure (1) Preparation of Powder and Laminate Chemically stabilized, amorphous-type silica powder was pre￾pared by the organic steric entrapment technique employing polyvinyl alcohol (PVA) solution as a polymeric carrier.43–45 A clear sol was prepared from Ludox AS-40 colloidal silica (40 wt% suspension in water, Du Pont Chemicals, Wilmington, DE); Al(NO3)3 9H2O (reagent grade, Aldrich Chemical Co., Milwaukee, WI) and Ca(NO3)2.4H2O (reagent grade, Aldrich Chemical Co.) in proportions to form a final composition of CaO. 2Al2O3 . 80SiO2. 42,46 After dissolving these reagents in DI water, the organic carrier, PVA solution, was added and the mixture was heated. The PVA solution was prepared from 5 wt% PVA (degree of polymerization-1700, Air Products and Chemicals Inc., Allentown, PA) dissolved in water. The propor￾tions of the PVA to cation sources in the solution were adjusted in such a way that there were four times more positively charged valences from the cations than the negatively charged functional ends of the organics.43–45 As the viscosity increased by evapo￾ration of water, the mixture was vigorously stirred. The remain￾ing water was then dried, converting the gel into a solid. Finally, the precursor was finely ground and calcined at 7501C for 1 h. The calcined powder was ball milled with zirconia media for 12 h. Iso-propyl alcohol was used as a solvent for milling. To observe the critical grain size, the ball-milled powder was uni￾axially pressed at 20 MPa followed by iso-static pressing at 170 MPa for 10 min. The pellet-shaped green compacts were hot pressed by loading them in a graphite die and surrounding them with compatible oxides. Hot pressing was carried out at 28 MPa under an argon atmosphere, at a temperature of 12001C for 1 h. After hot pressing, the samples were annealed in air at 13001C for various times. Some stress-induced transformation was achieved by hand grinding the hot-pressed and annealed spec￾imens on a #800 mesh SiC paper.27,47 In this study, two powders with different particle sizes were prepared in order to observe the effect of initial powder particle size on phase stability. The first, designated as powder A, was calcined at 11001C for 3 h and furnace cooled. The second, designated as powder B, was as￾calcined powder, followed by 1 h of attrition milling. Powders A and B were annealed at 13001C for 10 h. Amorphous-type cordierite powder was also prepared by the same method.48 The cordierite powder was synthesized from Mg(NO3)2 6H2O (reagent grade, Aldrich Chemical Co.); Al(- NO3)3 9H2O (reagent grade, Aldrich Chemical Co.), and Lu￾dox AS-40 colloidal silica (40 wt% suspension in water, Du Pont Chemicals). Commercial mullite powder (KM Mullite-101 Kyo￾titsu, Nagoya, Japan), which had an average particle size of 0.3 mm and a specific surface area of 26 m2 /g, was used in the mullite/cordierite mixture. The mullite/cordierite mixtures having different cordierite content were characterized for relevant properties such as ther￾mal expansion coefficient and flexural strength of hot-pressed samples. Laminates were fabricated by the tape casting process, ac￾cording to the procedure summarized in Fig. 2. The slurries consisted of B25 vol% oxide powders, B63 vol% solvent, and 12 vol% organics. The 0.5 wt% (dry weight basis of oxide pow￾der) polyvinyl butyral (PVB, Monsanto, St. Louis, MO) was added to the slurries as a dispersant. The solvent was composed of a mixture of trichloroethylene (ClCH 5 CCl2, Aldrich Chem￾ical Co.) and ethanol (CH3CH2OH, Aldrich Chemical Co.) while the organics included a binder (PVB, Monsanto) and plas￾ticizers (polyethylene glycol (PEG) 2000 and dioctyl phthalate (DP), Aldrich Chemical Co.). After pulverization, dispersion and mixing by ball-milling two times, the slurries were stirred in vacuum. This helped in removing bubbles and adjusting the working viscosity. After aging for 2 days, the slurries were tape cast using a doctor blade opening of 150–300 mm, to obtain tape cast green sheets of 60–150 mm thickness. Drying of the cast tapes was carried out under a saturated solvent atmosphere for 1 day. The green laminated composites had area dimensions of 25 mm 51 mm after stacking of green sheets. Thermocompression was performed at 10 MPa load for 30 min at 801C, which was the softening point of the organics. The organic additives were removed by heating to 5501C in an air atmosphere, using a two￾step heating process. After these additives were burned out, the green laminates were iso-statically cold pressed at 170 MPa for 10 min. The laminated green bodies were hot pressed in the same way as mentioned earlier. All laminated composites after densification had a 30-layer repetitive sequence of matrix and interphase. These were made in a separately optimized 5:1 thickness ratio of matrix to inter￾phase, by stacking each green sheet. After hot pressing, the lam￾Fig. 1. Schematic diagram illustrating the mechanism of ‘‘transforma￾tion weakening’’ of ceramic interphases leading to overall toughening of fiber reinforced, fibrous monolithic or laminated ceramic matrix com￾posites. 1522 Journal of the American Ceramic Society—Kriven and Lee Vol. 88, No. 6