October 2003 Porous Oxide Matrix Composite Reinforced with Oxide Fibers 735 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 tor 2 h to promote the development of alumina bridges (a)h impregnated 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 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 1200.C for 2 h, which served to crystallize the precursor to the corundum(a) 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 were done ately by casting thin rods(~2×2×10 flon surface. Linear shrinkage was measure different heat treatments For each step the mens were treated in a way similar to the desired temperature, held for 2 h, and then cooled down at 5°Cmin) Test bar geometries used for(a) in-plane bend testing for flexural rk of fracture, and(c) interlaminar shear strength (2) Mechanical Testing Fiber dominated composite properties were evaluated using ar in-plane three-point flexural test shown in Fig. 1(a). The speci composites. A solution(which also was used in this work) was mens(58 mm long, 3 mm wide, 3.5 mm high) were processed as to place a rubber sheet between 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 ailure 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 mode of testing is not pure tension, and because the matrix of The tensile stress in the outer fibers in three-point bending is given the composites are known to continually fracture during loader which changes the modulus of the material during testing, the 3/P 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 study Thus, the midplane shear stress to maximum tensile stress(T /o)is The in-plane notch sensi given by otched specimens shown in Fig. 1(b), 58 mm long and 3 mm wide d 7 mm A notch with a length ao =3.5 mm(nominally alf of the test bar height, ao/w=0.50 0.002 mm)and a width of 0.65 mm was introduced by diamond machining. The net- section notch strength was compared with the unnotched strength To ensure failure by delamination(shear) rather than a tensile to assess the degree of notch sensitivity. Calculations of the energy ailure originating from the surface, the span(s)to thickness ratio uired to break the specimens were also conducted to further s/L, is kept small, Failure modes of the test bars were examined in characterize the work required for fracture. This was done by different microscopes. easuring the area under the load-displacement curve Fracture surfaces and the microstructure of the composites were esults and discussion studied by optical microscopy and scanning electron microscop Interlaminar shear strength (a matrix dominated property (I Composite Matrix and Ce Characteristics these composites) was determined using a short beam shear The results from the shown in Fig. I(c). The specimens were 30 mm long, 5 mm wide summarized in Fig. 2. During and 3 mm high, and the loading span was 15 mm. It has been 1.2% was observed. The change on sintering at bserved that local stress concentrations due to the loading pins temperatures up to 1200.C is -0.9%. Since the processin can give premature failure at low loads in porous oxide matrix temperature is of this order and the observed shrinkage is smalmatrix 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 70°C. An initial sintering treatment was done at 900°C for 2 h to promote the development of alumina bridges between the mullite network. The composites were subsequently impregnated 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.39,40 After 4 h the composites were removed, dried, and heated to 900°C to pyrolyze the precursor. This was repeated eight times and following the last cycle, the composites were given a final sintering treatment at 1200°C for 2 h, which served to crystallize the precursor to the corundum () 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 separately by casting thin rods (
2  2  10 mm3 ) of slurry on a Teflon surface. Linear shrinkage was measured after drying and different heat treatments. For each step, the specimens were treated in a way similar to the composites (heated at 5°C/min up to the desired temperature, held for 2 h, and then cooled down at 5°C/min). (2) Mechanical Testing Fiber dominated composite properties were evaluated using an in-plane three-point flexural test shown in Fig. 1(a). The speci￾mens (58 mm long, 3 mm wide, 3.5 mm high) were processed as described above and were tested using a loading span of 53 mm. This configuration and loading mode precluded interlaminar shear failure before failure via the tensile stresses on one surface. A servoelectric testing machine (Instron, Inc., Model 8562) with a 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 mode of testing is not pure tension, and because the matrix of the composites are known to continually fracture during loading30 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 study. The in-plane notch sensitivity was assessed using the edge￾notched specimens shown in Fig. 1(b), 58 mm long and 3 mm wide and 7 mm high. A notch with a length a0  3.5 mm (nominally half of the test bar height, a0/W  0.50  0.002 mm) and a width of 0.65 mm was introduced by diamond machining. The net￾section notch strength was compared with the unnotched strength to assess the degree of notch sensitivity. Calculations of the energy required to break the specimens were also conducted to further characterize the work required for fracture. This was done by measuring the area under the load–displacement curve. Fracture surfaces and the microstructure of the composites were studied by optical microscopy and scanning electron microscopy. Interlaminar shear strength (a matrix dominated property for these composites) was determined using a short beam shear test shown in Fig. 1(c). The specimens were 30 mm long, 5 mm wide, and 3 mm high, and the loading span was 15 mm. It has been observed that local stress concentrations due to the loading pins can give premature failure at low loads in porous oxide matrix composites.29 A solution (which also was used in this work) was to place a rubber sheet between the loading pins and the specimen surface to minimize stress concentrations.23,29 Interlaminar shear strength, i , was calculated from the maximum load, Pmax, and test bar dimensions using the equation for shear stress at the midplane of a flexural bar specimen described by beam theory: i 3 4
Pmax bt (1) The tensile stress in the outer fibers in three-point bending is given by 3 2
Ps bt2 (2) Thus, the midplane shear stress to maximum tensile stress ( i / ) is given by i 1 2
t s (3) To ensure failure by delamination (shear) rather than a tensile failure originating from the surface, the span (s) to thickness ratio, s/t, is kept small. Failure modes of the test bars were examined in different microscopes. III. Results and Discussion (1) Composite Matrix and Composite Characteristics The results from the sintering studies of pure matrix rods are summarized in Fig. 2. During drying, a mean linear shrinkage of
1.2% was observed. The additional change on sintering at temperatures up to 1200°C is
0.9%. Since the processing temperature is of this order and the observed shrinkage is small, Fig. 1. 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. October 2003 Porous Oxide Matrix Composite Reinforced with Oxide Fibers 1735