Dctober 2003 Porous Oxide Matrix Composite Reinforced with Oxide Fibers 1735 matrix composite(CMC) body is very flexible and could be shaped much like an epoxy/fiber prepreg After shaping, the water was removed from the powder matrix by drying at 70C. An initial sintering treatment was done at 900C for 2 h to promote the development of alumina bridges between the mullite network. The composites were subsequently npregnated with the alumina precursor solution under vacuum The impregnation step was performed in a dry nitrogen atmo sphere to prevent premature gelation of the precursor due to atmospheric water vapor. The composites were left in the precur sor solution for 2 h at atmospheric pressure and transferred into ammoniated water(pH 10) to gel the precursor throughout the body and prevent it from redistributing to the surface during the evaporation of the solvent. After 4 h the composites were removed, dried, and heated to 900C to pyrolyze the precursor This was repeated eight times and following the last cycle, the composites were given a final sintering treatment at 1200C for (b)W 2 h, which served to crystallize the precursor to the corundum(a) structure. In this way, the strength of the connection between mullite particles could be increased without any shrinkage of the mullite network taking place Knowing the volume fraction of fibers per unit area of cloth (data obtained from the manufacturer). the volume fraction of fibers in the composite was calculated by measuring the volume of the composite and counting the number of fiber layers in each specimen. The porosity of the composites was measured using the Archimedes technique Studies of the shrinkage of the matrix slurry were done rately by casting thin rods (-2 2X 10 mm )of slurry on (c) h different heat treatments. For each step, the specimens were treate similar to the composites (heated at 5C/min up to the mperature, held for 2 h, and then cooled down at Fig. I. Test bar geometries used for(a) in-plane bend testing for flexural strength and elastic modulus, (b) in-plane bend testing for notch sensitivity and work of fracture, and (c) interlaminar shear strength. (2) Mechanical Testing Fiber dominated composite properties were evaluated using an composites. A solution which also was used in this work)was mens(58 mm iong. 3 mm wide, 3.5 mm high) were processed as to place a rubber tween the loading pins and the specimen described above and were tested using a loading span of 53 mm urface to minimize stress concentrations 23.29 Interlaminar shear This configuration and loading mode precluded interlaminar shear strength, T, was calculated from the maximum load, Pmax, and test failure before failure via the tensile stresses on one surface. A bar dimensions using the equation for shear stress at the midplane servoelectric testing machine (Instron, Inc. Model 8562) with a of a flexural bar specimen described by beam theory high stiffness loading frame was used. A crosshead speed of 0.1 mm/min was used. Nylon rods were used as loading pins to reduce contact stress. Strain was calculated based on crosshead displace- ment and by correcting for the compliance in the load train. Since The tensile stress in the outer fibers in three-point bending is given the mode of testing is not pure tension, and because the matrix of b the composites are known to continually fracture during loading" which changes the modulus of the material during testing, the results of these tests should be considered as qualitative rather than quantitative. Although qualitative, the results will serve to com pare the different specimens in this stud sessed using the edge. Thus, the midplane shear stress to maximum tensile stress(T/o)is The in-plane notch sensitivity was as given by notched specimens shown in Fig. I(b), 58 mm long and 3 mm wide and 7 mm high. A notch with a length ao =3.5 mm(nominally half of the test bar height, a/w=0.50 + 0.002 mm)and a width section notch strength was compared with the unnotched strength To ensure failure by delamination rather than a tensile to assess the degree of notch sensitivity. Calculations of the energy failure originating from the surface, t (s)to thickness ratio, equired to break the specimens were also conducted to further s/t, is kept small. Failure modes of the bars were examined in characterize the work required for fracture. This was done by difterent microscope Fracture surfaces and the microstructure of the composites were tudied by optical microscopy and scanning electron microscopy I. Results and Discussion Interlaminar shear strength(a matrix dominated property fo 1) Composite Matrix and Composite Characteristics composites) was determined using a short beam shear tes The results from the sintering of pure matrix rods are n in Fig. I(c). The specimens were 30 mm long, 5 mm wide, ummarized in Fig. 2. During dry mean linear shrinkage of 3 mm high. and the loading span was 15 mm. It has been -1. 2% was observed. The ad change on sintering at erved that local stress concentrations due to the loading pins temperatures up to 1200C is-0.9%. Since the processing can give premature failure at low loads in porous oxide matrix temperature is of this order and the observed shrinkage is small