2422 B. Davies et al 2 Experiments this temperature). Approximately 200 fibers of length 40 mm were coated individually with a thick A porous matrix composite was fabricated by layer of rhabdophane(hydrated LaPO4) powder infiltrating woven fabric of polycrystalline alumina and stacked together along a diameter of a 50 mm fibers(3M company, Nextel 610, 8-harness satin cylindrical graphite die. The remainder of the die weave) with a slurry of Al_O3/LaPO4. The slurry was filled with Al_O3 powder to prevent direct contained dispersed a-alumina powder(Sumitomo, contact between the graphite die and the LaPO4 AKP50)and solution precursor for LaPO4 Indivi The compact was hot pressed at 1400C for 1 h,to dual layers of fabric, 15 x 15 cm, were infiltrated d, form an alumina disk with a small embedded sap- stacked (10 layers), dried while vacuum bagging phire/LaPO4 composite, of cross-section dimen- and warm pressing, then sintered at 1100C in air sions m2 x 2 mm, along its diameter. for 1 h. The resulting composite plate was 2 mm The test specimen shown schematically in Fig. 2 hickness. Double-edge-notched tensile test speci- was cut from the hot pressed disc and loaded in mens, with dimensions 15 x I cm aligned parallel four-point bending with the embedded composite to the weave direction, were cut from the plate in tension. The hole drilled adjacent to the embed using a diamond saw(Fig. 1). The saw cuts that ded composite served to reduce the gradient of formed the notches were of 150 um width. The tensile bending stress across the composite and to specimens were loaded in tension using wedge grips allow cracking of the ligament of material between and tabs glued to the ends. The gauge section the hole and the edge of the specimen without extension was measured using a clip gauge attached fracturing the entire beam 12.5 mm above and below the mid plane, which contained the notche A composite with a fully dense LaPo 3 Results nd sapphire fiber reinforcement was fabricated by hot pressing. Since the sole purpose of this com- 3.1 Porous Al2O3/LaPO4 matrix composites posite was to test whether debonding and fiber The microstructure of the AlO3/LaPO4 matrix pullout occur in this system under tensile loading composite is shown in the scanning electron parallel to the fibers, the simplest possible fabrica- microscope(SEM) image of Fig. 3. The matrix tion route, making use of available hot pressing consists of a two-phase mixture of AlO3 and facilities, was used. Sapphire fibers were chosen to LaPO4 grains, both with dimensions smaller than allow hot pressing at sufficiently high temperature approximately 0.5 um, with fine-scale porosit to densify the matrix(1400oC)without damaging between the grains. The alumina grains in the the fibers(polycrystalline fibers would degrade at matrix are always separated from the fibers by a thin layer of LaPO4. The composites consisted of approximately 40 vol% of fibers(20% in each of the 0 and 90 orientations), 40 vol% matrix(AlO3 and LaPO4 in the ratio approximately 2: 1)and 20 vol% porosity(as estimated from weight measure- ments) The tensile of a test with notch depth greater than half of the test section (a/w=0.54)is shown in Fig. 4. The net section stress during testing (load divided by the remaining cross-sectional area between the ends of the not- gage ches)is plotted as a function of the extension mea- sured from the clip gauge. Also shown along the top border is the average strain calculated from the Fig. 1. Double-edge-notched tensile test specimen of compo- site consisting of woven AlO3 fibers and porous Al2O3- Fig. 2. Test specimen(Al2O3) with embedded composite of2 Experiments A porous matrix composite was fabricated by in®ltrating woven fabric of polycrystalline alumina ®bers (3M company, Nextel 610, 8-harness satin weave) with a slurry of Al2O3/LaPO4. The slurry contained dispersed -alumina powder (Sumitomo, AKP50) and solution precursor for LaPO4. Indivi￾dual layers of fabric, 15 15 cm2 , were in®ltrated, stacked (10 layers), dried while vacuum bagging and warm pressing, then sintered at 1100C in air for 1 h. The resulting composite plate was 2 mm thickness. Double-edge-notched tensile test speci￾mens, with dimensions 15 1 cm aligned parallel to the weave direction, were cut from the plate using a diamond saw (Fig. 1). The saw cuts that formed the notches were of 150 m width. The specimens were loaded in tension using wedge grips and tabs glued to the ends. The gauge section extension was measured using a clip gauge attached 12.5 mm above and below the mid plane, which contained the notches. A composite with a fully dense LaPO4 matrix and sapphire ®ber reinforcement was fabricated by hot pressing. Since the sole purpose of this com￾posite was to test whether debonding and ®ber pullout occur in this system under tensile loading parallel to the ®bers, the simplest possible fabrica￾tion route, making use of available hot pressing facilities, was used. Sapphire ®bers were chosen to allow hot pressing at suciently high temperature to densify the matrix (1400C) without damaging the ®bers (polycrystalline ®bers would degrade at this temperature). Approximately 200 ®bers of length 40 mm were coated individually with a thick layer of rhabdophane (hydrated LaPO4) powder and stacked together along a diameter of a 50 mm cylindrical graphite die. The remainder of the die was ®lled with Al2O3 powder to prevent direct contact between the graphite die and the LaPO4. The compact was hot pressed at 1400C for 1 h, to form an alumina disk with a small embedded sap￾phire/LaPO4 composite, of cross-section dimen￾sions 2 2 mm, along its diameter. The test specimen shown schematically in Fig. 2 was cut from the hot pressed disc and loaded in four-point bending with the embedded composite in tension. The hole drilled adjacent to the embed￾ded composite served to reduce the gradient of tensile bending stress across the composite and to allow cracking of the ligament of material between the hole and the edge of the specimen without fracturing the entire beam. 3 Results 3.1 Porous Al2O3/LaPO4 matrix composites The microstructure of the Al2O3/LaPO4 matrix composite is shown in the scanning electron microscope (SEM) image of Fig. 3. The matrix consists of a two-phase mixture of Al2O3 and LaPO4 grains, both with dimensions smaller than approximately 0.5 m, with ®ne-scale porosity between the grains. The alumina grains in the matrix are always separated from the ®bers by a thin layer of LaPO4. The composites consisted of approximately 40 vol% of ®bers (20% in each of the 0 and 90 orientations), 40 vol% matrix (Al2O3 and LaPO4 in the ratio approximately 2:1) and 20 vol% porosity (as estimated from weight measure￾ments). The tensile response of a test specimen with notch depth greater than half of the test section (a=w ˆ 0
54) is shown in Fig. 4. The net section stress during testing (load divided by the remaining cross-sectional area between the ends of the not￾ches) is plotted as a function of the extension mea￾sured from the clip gauge. Also shown along the top border is the average strain calculated from the Fig. 1. Double-edge-notched tensile test specimen of compo￾site consisting of woven Al2O3 ®bers and porous Al2O3± LaPO4 matrix. Fig. 2. Test specimen (Al2O3) with embedded composite of sapphire ®bers and LaPO4 matrix. 2422 J. B. Davies et al.