800 Journal of the American Ceramic Sociery Kim and Kriven Vol. 87. No. 5 The SEM micrographs of the sintered two-layer composite are presented in Fig 9. The dense mullite matrix, as well as the porous nd weak AlPO interphase, sintered to a uniform microstructur The higher-magnification SEM micrograph shows the microstruc ture of the porous AlPO4 interphase layer. The arrows in Fig. %(a) indicate microcracks formed along the mullite-AlPOa interfaces after sintering. Figures 10(a) and(b) are SEM micrographs of the three-layer mullite-AIPO4 fibrous monolithic composite. The three-layer structure of the mullite inner rod-AlPO4 interphase layer-mullite outer layer can clearly be seen. There is no formation of interface microcracks in the three-layer composite The difference in the sintering behaviors of the two-layer and three-layer fibrous monolithic composites is schematically ex- plained in Fig. 11. In the two-layer fibrous monolithic composite (Fig. Il(a), the isolated, highly sinterable, inner mullite matrix rods underwent sintering shrinkage, but the interconnected, poorly sinterable interphase layers did not densify significantly. Because Fig. 12. SEM micrograph of the sintered, mixed 50% two-layer: 50% of this sinterability difference, circumferential shrinkage cracks three-layer, mullite-AIPOA fibrous monolithic composite. were formed around the mullite inner rods, as seen in Fig 9(a), and the two-layer composite had little sintering shrinkage. In the case of the three-layer structure(Fig. 11(b)), the isolated, mullite inner rods shrunk, but the AlPOa interphase layers did not shrink as composites. These materials showed apparent nonbrittle, interme- much. However, the interconnected, mullite outer layer shrunk considerably and produced compressive stresses in the AlPO nterphase layers. Thus, the overall sample size was reduced composite are seen in Fig. 14. The side view shows that the crack during sintering and the circumferential cracks did not form followed a tortuous path due to deflection. The cross-sectional around the mullite inner rods as can be seen in Fig. 10. The view confirms that the crack propagates along the weak AlPO4 SEM micrograph of the sintered, mixed 50% two-layer: 50% nterphase. The SEM micrographs of Fig. 15 display the fracture three-layer fibrous monolithic composite is presented in Fig. surfaces of the two-layer fibrous monolithic composite, and give 12. The mullite matrix and AlPO4 interphase layer formed an evidence for somewhat extensive fiber pullout and hence a rough terlocking microstructure. fracture surface. This kind of extensive fiber pullout, apparent The three-point bending strengths for the two-layer, mixed 50% nonbrittle fracture, and low strength behavior in the two-layer corresponding works of fracture of 0.45 +0.02, 0.30+0.05, and 9(a)and as discussed using a xpected from the circumferential shrinkage cracks around the mullite center rods as depicted in Fig 0.25 0.01 kJ/m, respectively. Figure 13 shows load versus three-layer fibrous monolithic composite is seen in Fig. 16. The two-layer: 50% three-layer, and three-layer fibrous monolithic crack propagates not only along the weak APOS surfaces of the displacement curves corresponding to the two-layer, mixed 50% across the strong mullite matrix phase. Fracture three-layer fibrous monolithic composites are seen in Fig. 17. The three-layer structure was maintained throughout the composite, but there was very little fiber pullout, at room temperature, leading to a relatively smooth fracture surface. These poor fiber pullout, brittle fracture, and higher strength behaviors of the three-layer fibrous monolithic composite could be predicted from the com- pressional stresses acting on the interphase layer during sintering. as can be seen from Fig. 10 and as illustrated in Fig. 11(b) OOo O0O 0.15 (a)Two-layer fibrous monolithic composite (Mullite- AIPO4) 50% 2-layer and 50%3-layer 3·laye 005 000000 (b) Three-layer fibrous monolithic composite(Mullite- AIPO4-mullite) E::::: Matrix(Mullite )2: Interphase(AIPO):Crack Displacement (mm) Fig. 13. Load-displacement curves for the two-layer, mixed 50% two- Fig. 11. Schematic of the sintering behavior of the two-layer and layer: 50% three-layer, and three-layer mullite-AlPO4 fibrous monolithic three-layer fibrous monolithic composite.The SEM micrographs of the sintered two-layer composite are presented in Fig. 9. The dense mullite matrix, as well as the porous and weak AlPO4 interphase, sintered to a uniform microstructure. The higher-magnification SEM micrograph shows the microstruc￾ture of the porous AlPO4 interphase layer. The arrows in Fig. 9(a) indicate microcracks formed along the mullite–AlPO4 interfaces after sintering. Figures 10(a) and (b) are SEM micrographs of the three-layer mullite–AlPO4 fibrous monolithic composite. The three-layer structure of the mullite inner rod–AlPO4 interphase layer–mullite outer layer can clearly be seen. There is no formation of interface microcracks in the three-layer composite. The difference in the sintering behaviors of the two-layer and three-layer fibrous monolithic composites is schematically ex￾plained in Fig. 11. In the two-layer fibrous monolithic composite (Fig. 11(a)), the isolated, highly sinterable, inner mullite matrix rods underwent sintering shrinkage, but the interconnected, poorly sinterable interphase layers did not densify significantly. Because of this sinterability difference, circumferential shrinkage cracks were formed around the mullite inner rods, as seen in Fig. 9(a), and the two-layer composite had little sintering shrinkage. In the case of the three-layer structure (Fig. 11(b)), the isolated, mullite inner rods shrunk, but the AlPO4 interphase layers did not shrink as much. However, the interconnected, mullite outer layer shrunk considerably and produced compressive stresses in the AlPO4 interphase layers. Thus, the overall sample size was reduced during sintering and the circumferential cracks did not form around the mullite inner rods as can be seen in Fig. 10. The SEM micrograph of the sintered, mixed 50% two-layer:50% three-layer fibrous monolithic composite is presented in Fig. 12. The mullite matrix and AlPO4 interphase layer formed an interlocking microstructure. The three-point bending strengths for the two-layer, mixed 50% two-layer:50% three-layer, and three-layer structures were mea￾sured as 76  5, 123  12, and 176  7 MPa, respectively, having corresponding works of fracture of 0.45  0.02, 0.30  0.05, and 0.25  0.01 kJ/m2 , respectively. Figure 13 shows load versus displacement curves corresponding to the two-layer, mixed 50% two-layer:50% three-layer, and three-layer fibrous monolithic composites. These materials showed apparent nonbrittle, interme￾diate, and brittle fracture behavior, respectively. The side and cross- sectional views of a fractured two-layer fibrous monolithic composite are seen in Fig. 14. The side view shows that the crack followed a tortuous path due to deflection. The cross-sectional view confirms that the crack propagates along the weak AlPO4 interphase. The SEM micrographs of Fig. 15 display the fracture surfaces of the two-layer fibrous monolithic composite, and give evidence for somewhat extensive fiber pullout and hence a rough fracture surface. This kind of extensive fiber pullout, apparent nonbrittle fracture, and low strength behavior in the two-layer fibrous monolithic composite is expected from the circumferential shrinkage cracks around the mullite center rods as depicted in Fig. 9(a) and as discussed using Fig. 11(a). Crack propagation in the three-layer fibrous monolithic composite is seen in Fig. 16. The crack propagates not only along the weak AlPO4 interphase, but also across the strong mullite matrix phase. Fracture surfaces of the three-layer fibrous monolithic composites are seen in Fig. 17. The three-layer structure was maintained throughout the composite, but there was very little fiber pullout, at room temperature, leading to a relatively smooth fracture surface. These poor fiber pullout, brittle fracture, and higher strength behaviors of the three-layer fibrous monolithic composite could be predicted from the com￾pressional stresses acting on the interphase layer during sintering, as can be seen from Fig. 10 and as illustrated in Fig. 11(b). Fig. 11. Schematic of the sintering behavior of the two-layer and three-layer fibrous monolithic composite. Fig. 12. SEM micrograph of the sintered, mixed 50% two-layer:50% three-layer, mullite–AlPO4 fibrous monolithic composite. Fig. 13. Load–displacement curves for the two-layer, mixed 50% two￾layer:50% three-layer, and three-layer mullite–AlPO4 fibrous monolithic composite. 800 Journal of the American Ceramic Society—Kim and Kriven Vol. 87, No. 5