Journal of the American Ceramic Society-Carelli et al. Vol. 85. No. 3 matrix suggest an optimum state, dictated in part by the combina- air furnace for 1000 h at temperatures of either 1000, 1 100%,or tion of properties that are required in the application of interest. 1200C and subsequently tested in uniaxial tension at ambien In this study, comparisons are made between the tensile temperature, following the procedures outlined above. The furnace properties of a 2D woven CFCC both along the fiber direction was heated with resistance wire coated with a ceramic mixture of (0/90o) and at 45. to the fiber axes before and after high- aluminophosphate and alumina, and insulated with aluminosilicate temperature aging treatments. These orientations are selected to Either two or three tests were performed for most conditions elicit the fiber-dominated and matrix-dominated composite prop- representative fractured specimens were examined by bot rties. Examinations of the broken specimens by optical and agnification light microscopy and scanning electron microscopy. scanning electron microscopy are used to elucidate the role of The porosity both before and after aging was measured follow- aging in the fracture characteristics. Changes in the state of the ing ASTM Standard C20-92 Changes in the microstructure of the matrix are probed through two additional complementary methods: aged specimens were elucidated from SEM observations of pol ()measurement of matrix hardness using Vickers indentation, and ished samples. Matrix hardness measurements were also made on (ii) determination of the matrix Youngs mod using the these polished samples within the matrix-rich regions between th measured composite moduli coupled with classical laminate the- fiber tows, using Vickers indentation with a load of 300 g. This ory. Additionally, some comparisons are made with the retention load was selected to produce indentations that were no larger than in properties of a comparable porous-matrix composite with an half of the spacing between tows in all materials. Additionally, the aluminosilicate matrix indents were placed away from the processing-induced cracks (described below). At least 10 such measurements were made on IL. Materials and Test Procedures samples in each aged condition The matrix modulus of both pristine and aged specimens was The composite material consists of Nextel 720 fiber cloth in an inferred from the measured composite moduli in both the 0/90 8-harness satin weave and a porous matrix of mullite and alumi- and +45 orientations using laminate theory. Details of the theory na.5 The matrix was produced in two steps. In the first,an are described in the Appendix. queous slurry containing mullite and alumina particulates was vacuum-infiltrated into a stack of 12 fiber cloths. The matrix Ill. Experimental Results and Analysis particulates were "l um diameter MU-107 mullite(Showa Denko KK )and -0. 2 um diameter AKP-50 alumina(Sumitomo Chem ypical micrographs of polished cross sections of both cal). The slurry contained 80% mullite and 20% alumina, The rocessed specimen and one aged at 1200%C are shown in Fi packing density of the matrix powder foll infiltration was 1. A notable feature is the presence of a more-or-less regular 960%. The green panels were dried and sintered at 900oC for 2 h pattern of matrix cracks, caused by the constrained shrinkage of to promote the development of alumina bridges between the the matrix during drying of the green panels. In the as-processed mullite particles, thereby imparting some structural integrity to the composite, the cracks are concentrated in the matrix-rich regions matrix for subsequent processing. In the second step, the panel were impregnated with an alumina precursor solution(aluminum hydroxychloride) and pyrolyzed at 900C for 2 h. The volt yield of alumina on pyrolysis of this solution was #%. The(a)As-processed mpregnation and pyrolysis sequence was performed twice. The panels were given a final heat treatment at 1200.C for 2 h to stabilize the precursor-derived alumina and to enhance the integ ty of the alumina bridges. The panel dimensions were 200 mm X 130mm×3. I mm thic Panels were made with fibers oriented either parallel or at 4 to the panel edges, thus facilitating preparation of tensile speci mens in both the longitudinal(0°90°) and off-axis(±45°) orientations. The key physical properties of each of the tested summarized in Table I. The fiber content, , wa determined from knowledge of the cloth volume and the final plate ASTM Standard C20-92>P, was measured in accordance with A series of mechanical tests was performed to determine the tensile properties of the as-processed composite in both the 0%/900 and the +45 orientations. The properties were measured usin tandard dog-bone tensile specimens with a gauge length of 50 mm and a gauge width of 8 mm. The longitudinal strains were 1( b)1000 h at 1200C measured us d at room temperature at a displacement rate of an extensometer over a 25 mm gauge length. The 1.25 mm/min Either two or three tests were performed for each material the effects of thermal aging on the mechanical properties, tensile specimens in both orientations were heated in an Table L. Summary of Physical Properties of CFCC Panels b Matrix porosity (% Fiber volar designation orentation Initial Final 39.5 37.9 0.2mm ABC 0°/90° 38.3 40.2 37.7 After slurry infiltration and drying. After precursor impregnation and pyrolysis Fig. 1. SEM micrographs of as-processed and thermally aged composite backscatter imaging mode)matrix suggest an optimum state, dictated in part by the combina￾tion of properties that are required in the application of interest. In this study, comparisons are made between the tensile properties of a 2D woven CFCC both along the fiber direction (0°/90°) and at 45° to the fiber axes before and after high￾temperature aging treatments. These orientations are selected to elicit the fiber-dominated and matrix-dominated composite prop￾erties. Examinations of the broken specimens by optical and scanning electron microscopy are used to elucidate the role of aging in the fracture characteristics. Changes in the state of the matrix are probed through two additional complementary methods: (i) measurement of matrix hardness using Vickers indentation, and (ii) determination of the matrix Young’s modulus, using the measured composite moduli coupled with classical laminate the￾ory.8 Additionally, some comparisons are made with the retention in properties of a comparable porous-matrix composite with an aluminosilicate matrix.7 II. Materials and Test Procedures The composite material consists of Nextel 720 fiber cloth in an 8-harness satin weave and a porous matrix of mullite and alumi￾na.3,5 The matrix was produced in two steps. In the first, an aqueous slurry containing mullite and alumina particulates was vacuum-infiltrated into a stack of 12 fiber cloths. The matrix particulates were
1 m diameter MU-107 mullite (Showa Denko K.K.) and
0.2 m diameter AKP-50 alumina (Sumitomo Chem￾ical). The slurry contained 80% mullite and 20% alumina. The packing density of the matrix powder following infiltration was 60%. The green panels were dried and sintered at 900°C for 2 h to promote the development of alumina bridges between the mullite particles, thereby imparting some structural integrity to the matrix for subsequent processing. In the second step, the panels were impregnated with an alumina precursor solution (aluminum hydroxychloride) and pyrolyzed at 900°C for 2 h. The volumetric yield of alumina on pyrolysis of this solution was 3%. The impregnation and pyrolysis sequence was performed twice. The panels were given a final heat treatment at 1200°C for 2 h to stabilize the precursor-derived alumina and to enhance the integ￾rity of the alumina bridges. The panel dimensions were 200 mm  130 mm  3.1 mm thick. Panels were made with fibers oriented either parallel or at 45° to the panel edges, thus facilitating preparation of tensile speci￾mens in both the longitudinal (0°/90°) and off-axis (45°) orientations. The key physical properties of each of the tested panels are summarized in Table I. The fiber content, f, was determined from knowledge of the cloth volume and the final plate dimensions. The porosity, p, was measured in accordance with ASTM Standard C20-92. A series of mechanical tests was performed to determine the tensile properties of the as-processed composite in both the 0°/90° and the 45° orientations. The properties were measured using standard dog-bone tensile specimens with a gauge length of 50 mm and a gauge width of 8 mm. The longitudinal strains were measured using an extensometer over a 25 mm gauge length. The tests were performed at room temperature at a displacement rate of 1.25 mm/min. Either two or three tests were performed for each material. To assess the effects of thermal aging on the mechanical properties, tensile specimens in both orientations were heated in an air furnace for 1000 h at temperatures of either 1000°, 1100°, or 1200°C and subsequently tested in uniaxial tension at ambient temperature, following the procedures outlined above. The furnace was heated with resistance wire, coated with a ceramic mixture of aluminophosphate and alumina, and insulated with aluminosilicate fiber. Either two or three tests were performed for most conditions. Representative fractured specimens were examined by both low￾magnification light microscopy and scanning electron microscopy. The porosity both before and after aging was measured follow￾ing ASTM Standard C20-92. Changes in the microstructure of the aged specimens were elucidated from SEM observations of pol￾ished samples. Matrix hardness measurements were also made on these polished samples within the matrix-rich regions between the fiber tows, using Vickers indentation with a load of 300 g. This load was selected to produce indentations that were no larger than half of the spacing between tows in all materials. Additionally, the indents were placed away from the processing-induced cracks (described below). At least 10 such measurements were made on samples in each aged condition. The matrix modulus of both pristine and aged specimens was inferred from the measured composite moduli in both the 0°/90° and 45° orientations using laminate theory. Details of the theory are described in the Appendix. III. Experimental Results and Analysis Typical micrographs of polished cross sections of both an as-processed specimen and one aged at 1200°C are shown in Fig. 1. A notable feature is the presence of a more-or-less regular pattern of matrix cracks, caused by the constrained shrinkage of the matrix during drying of the green panels. In the as-processed composite, the cracks are concentrated in the matrix-rich regions Table I. Summary of Physical Properties of CFCC Panels Panel designation Fiber orientation Matrix porosity (%) Fiber volume Initial fraction (%) † Final‡ A 0°/90° 39.5 37.9 38.6 B 0°/90° 40.1 38.3 38.9 C 45° 40.2 37.7 38.5 † After slurry infiltration and drying. ‡ After precursor impregnation and pyrolysis and final sintering treatment. Fig. 1. SEM micrographs of as-processed and thermally aged composite specimens (viewed in backscatter imaging mode). 596 Journal of the American Ceramic Society—Carelli et al. Vol. 85, No. 3