Y Bao, P.S. Nicholson /Journal of the European Ceramic Society 28(2008)3041-3048 (a) (a) (b) Fig. 10. Effect of AlPO4 coating on fracture surface of fiber/RBM composites Fig. 11. Porous AlPO4 coating after sintering at 1300C for 2h tested at R.T.(a) without AlPO4 coating and (b) with AlPO4 coating infiltration. In fact, a yellow layer was noted on the base of a green body after infiltration. A non-uniform distribution, or, less-MREO-present-than-designed will locally retard formation of mullite at1300°C Fig. 10 compares the effect of AlPO4-coating on fiber pullout, i.e., without AlPO4 on the fibers, strong bonds form between the fibers and the rBm matrix so that a planar fracture sur- face results. However, when AlPO4 is coated on the fibers significant fiber pullout is observed. The high covalent bond ing level in AlPO4 retards its sinterability so AlPO4 ceramics remain very porous even when sintered at 1550C. Thus the inherently-porous AlPO4 coating on Nextel 720 fiber surface, serves as a porous weak layer for crack deflection and fiber pu out. Fig. I l show porous AlPO4 coating attached to the matrix after the fiber pullout, indicating that the AlPO4/fiber bondin is also weak and cracks defect therefrom. Fig. 12 illustrates 200m the fiber bridging effect. Though the crack opening distance is M150 um, the fibers still bridge across it. Fiber pullout reaches 150um(10 times the fiber diameter). The pullout length Fig 12 Fiber bridging and pullout across a matrix crack in AlPO4-coated is shorter on fracture at 1100C (Fig. 13). Fig. 14 shows the fiber/RBM composite tested at room temperatureY. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 3045 Fig. 10. Effect of AlPO4 coating on fracture surface of fiber/RBM composites tested at R.T. (a) without AlPO4 coating and (b) with AlPO4 coating. infiltration. In fact, a yellow layer was noted on the base of a green body after infiltration. A non-uniform distribution, or, less-MREO-present-than-designed will locally retard formation of mullite at 1300 ◦C. Fig. 10 compares the effect of AlPO4-coating on fiber pullout, i.e., without AlPO4 on the fibers, strong bonds form between the fibers and the RBM matrix so that a planar fracture sur￾face results. However, when AlPO4 is coated on the fibers, significant fiber pullout is observed. The high covalent bond￾ing level in AlPO4 retards its sinterability so AlPO4 ceramics remain very porous even when sintered at 1550 ◦C.5 Thus the inherently-porous AlPO4 coating on Nextel 720 fiber surface, serves as a porous weak layer for crack deflection and fiber pull￾out. Fig. 11 show porous AlPO4 coating attached to the matrix after the fiber pullout, indicating that the AlPO4/fiber bonding is also weak and cracks deflect therefrom. Fig. 12 illustrates the fiber bridging effect. Though the crack opening distance is ∼150m, the fibers still bridge across it. Fiber pullout reaches ∼150m (>10 times the fiber diameter). The pullout length is shorter on fracture at 1100 ◦C (Fig. 13). Fig. 14 shows the Fig. 11. Porous AlPO4 coating after sintering at 1300 ◦C for 2 h. Fig. 12. Fiber bridging and pullout across a matrix crack in AlPO4-coated fiber/RBM composite tested at room temperature.