J. Haslam et al. Journal of the European Ceramic Society 20(2000)607-618 35 30 150 0/90°a/W=0.48 0/9D° WOF= 1373) 20 15 4+/-45°,a/W=049 飞wNOF=38/m2 0%0.05%0.1%0.15%0.2%0.25%0.3% 0.5 Flexure strain Displacement (mm) Fig. 6. Flexure stress versus Nominal Flexure Strain plot showing Fig. 7. Load versus displacement plots of notched composite sampl elastic modulus of composite with final sintering at 1250'C for 5 h in tested in the in-plane configuration. Final sintering of composite was HCL. The composite was tested in the in-plane fiber orientation. Tests at 1250C for 5 h in HCl. Composite samples with 0/90 and +/-45o for fiber weave orientations of 0/90 and +/-450 are shown. fiber orientations are shown. The normalized notch depth for these samples was afw=0.48 and 0.49, respectively. Table 2 Properties of in-plane notched composite 3-point bend tests Fiber WOF J/mm2 Strength ratio Net-section stress MPa KI at peak load MPam/2 Ks MPam/2 1250°C/5hHC11373 0.69 115 1250C/5 h/HCI 3.89 /-45°1250c/5h/HC1438 0.72 1250°C/5h/HCl 338 53.9 3.8 W≈0.49 3.3. Composite microstructure many fiber tows contained pairs of fibers that exhibited this frequent fracture origin. Fig. 9 illustrates the fracture region of a 0/90 speci- Fig 9(a)and(b)shows the planar fracture of the 90 men. Fig. 9(a)and(b) show that fibers in 0 tows in fiber tows in each cloth layer. Here, the crack top each cloth layer exhibit random failure to produce graphy can be characterized as nearly planar, and the brushes,, or what is known as ' fiber pull-out. Since no surface of this 'planar' region contains relatively long holes are observed in the matrix from where the fibers lengths of the 0 fibers extending from the fracture sur- could have pulled from, it must be concluded that por- face [see arrow in Fig 9(b). Fig. 10 shows the fracture ous matrix between the fibers disintegrated into smaller surface of the +/-45 oriented specimens. Large areas pieces, and that fibers did not slide out of matrix holes are seen where the crack path propagated along the as observed for composites produced with'weak'inter- cylindrical fiber surfaces(arrows); these cracks jog faces. Some of the matrix debris, and matrix still bon- across fiber tows. The surface produced when the crack ded to the fibers are seen in Fig 9(c). Fig. 9(d) shows propagates across the tow certainly does not form fib that some fracture regions in the 0 tows have a flatter, 'brushes, but as shown in the enlarged view, Fig. 10(b). more coordinated fracture topography. Close examina- it can be seen that the same crack front did not cause all tion of the fibers in this region shows that most of them fibers to fail fracture on different planes, indicating that one crack Generally, as shown in Figs. 9 and 10, the ZrO2 and front did not cause this fracture topography. One can mullite matrix fills all of the interstices between the see a few pairs of fibers(arrows )which do exhibit planar fibers. As detailed elsewhere, the HCl treatment at fracture; examination of these fiber pairs shows that temperatures between 1200 and 1300C is effective in they have a common fracture origin where they touch. producing a strong matrix without shrinkage via an As detailed elsewhere, 2I this common fracture origin evaporation/ condensation sintering and coarsening was produced during fiber processing. This occurs when phenomenon for the Zro, portion of the matrix. The adjacent fibers in the bundle (all floors are spun from lack of shrinkage of the powder matrix is evident in solution concurrently) stick to each other and sinter Figs. 9 and 10 by the lack of crack-like voids in the together along their cylindrical axis. It was shown that matrix. If the powder matrix were to densify its shrinkage,3.3. Composite microstructure Fig. 9 illustrates the fracture region of a 0/90 speci￾men. Fig. 9(a) and (b) show that ®bers in 0 tows in each cloth layer exhibit random failure to produce `brushes', or what is known as `®ber pull-out'. Since no holes are observed in the matrix from where the ®bers could have pulled from, it must be concluded that por￾ous matrix between the ®bers disintegrated into smaller pieces, and that ®bers did not slide out of matrix holes as observed for composites produced with 'weak' inter￾faces. Some of the matrix debris, and matrix still bon￾ded to the ®bers are seen in Fig. 9(c). Fig. 9(d) shows that some fracture regions in the 0 tows have a ¯atter, more coordinated fracture topography. Close examina￾tion of the ®bers in this region shows that most of them fracture on di€erent planes, indicating that one crack front did not cause this fracture topography. One can see a few pairs of ®bers (arrows) which do exhibit planar fracture; examination of these ®ber pairs shows that they have a common fracture origin where they touch. As detailed elsewhere,21 this common fracture origin was produced during ®ber processing. This occurs when adjacent ®bers in the bundle (all ¯oors are spun from solution concurrently) stick to each other and sinter together along their cylindrical axis. It was shown that many ®ber tows contained pairs of ®bers that exhibited this frequent fracture origin. Fig. 9(a) and (b) shows the planar fracture of the 90 ®ber tows in each cloth layer. Here, the crack topo￾graphy can be characterized as nearly planar, and the surface of this `planar' region contains relatively long lengths of the 0 ®bers extending from the fracture sur￾face [see arrow in Fig. 9(b)]. Fig. 10 shows the fracture surface of the +/ÿ45 oriented specimens. Large areas are seen where the crack path propagated along the cylindrical ®ber surfaces (arrows); these cracks jog across ®ber tows. The surface produced when the crack propagates across the tow certainly does not form ®ber `brushes', but as shown in the enlarged view, Fig. 10(b), it can be seen that the same crack front did not cause all ®bers to fail. Generally, as shown in Figs. 9 and 10, the ZrO2 and mullite matrix ®lls all of the interstices between the ®bers. As detailed elsewhere,6 the HCl treatment at temperatures between 1200 and 1300C is e€ective in producing a strong matrix without shrinkage via an evaporation/ condensation sintering and coarsening phenomenon for the ZrO2 portion of the matrix. The lack of shrinkage of the powder matrix is evident in Figs. 9 and 10 by the lack of crack-like voids in the matrix. If the powder matrix were to densify its shrinkage, Fig. 6. Flexure stress versus Nominal Flexure Strain plot showing elastic modulus of composite with ®nal sintering at 1250C for 5 h in HCl. The composite was tested in the in-plane ®ber orientation. Tests for ®ber weave orientations of 0/90 and +/ÿ45 are shown. Table 2 Properties of in-plane notched composite 3-point bend testsa Fiber Sintering WOF J/mm2 Strength ratio Net-section stress MPa KI at peak load MPa m1/2 Kss MPa m1=2 0/90 1250C/5 h/HCl 1373 0.69 115 4.46 9.3 0/90 1250C/5 h/HCl 1287 0.66 100 3.89 8.0 +/ÿ45 1250C/5 h/HCl 438 0.72 62.4 2.32 4.38 +/ÿ45 1250C/5 h/HCl 338 0.62 53.9 2.06 3.85 a a/W0.49. Fig. 7. Load versus displacement plots of notched composite samples tested in the in-plane con®guration. Final sintering of composite was at 1250C for 5 h in HCl. Composite samples with 0/90 and +/ÿ45 ®ber orientations are shown. The normalized notch depth for these samples was a=W ˆ 0:48 and 0.49, respectively. J.J. Haslam et al. / Journal of the European Ceramic Society 20 (2000) 607±618 613