Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composite 2707 N610-Desized 1.6, 10 日N610-1200C2h11,6 2.1,19 1.6,9 b-1200c2h(A)1.3.12 1200c2h(Air)0.8,5 12000,100h(Air) 与4 44.555.56657758 Ln stress, MPa I 5. Weibull plot of tows 1-5 that were coated with 50 vol% YAG/C precursor and tows 12 and 13(desized, without coating) heat-treated under various here residual porosity is <10% in much thicker coatings; any flaws in the coating(or matrix) now penetrate the fiber readily in the absence of any crack-deflecting mech Incomp Table ll summarizes the st ns of minicomposites Minicom- osite A had an average strength that was nearly twice as high as the control minicomposite F. Minicomposites B and D, heat- treated for 100 h in air at 1200oC, had strengths that were equal to that of the control minicomposite F heated under the same conditions but without a fiber coating. Figure 6 contains sem images of fracture surfaces of minicomposites A and B, showing the difference in fracture behavior of the two composites. Long ber pullout lengths and holes left behind by fibers were clearly visible in the fracture surfaces of minicomposite A, as opposed to nly fibers exposed by matrix disintegration, as is often seen in (a) orous matrix composites. Minicomposite B displayed a brittle fracture surface with little, if any, fiber pullout A TEM image of the fiber/matrix interfacial region from minicomposite A is shown in Fig. 7. T and pore sizes in the YAG fiber coating increase to 40-50 nm, and particles begi to sinter following matrix processing at 1200%C for 2 h(F b), (c)). Because the matrix is porous as well and could pected to deflect cracks, the presence of the porous YAG coati er tensile strengths. Either rosity distribution in the matrix alone is not adequate to deflect acks, whereas that in the fiber coating is, or the coating protects the fiber from degradation during matrix processing. Sintering of matrix alumina particles to the alumina fiber is observed in the absence of a coating, which could lead to fiber degradation in the control composites by enhanced fiber/matrix bonding and stress concentration Figures 8 and 9 show TEM images of the fiber/ in minicomposites C and D, which contain an I matrix ite aes of Yag thatwhere residual porosity is 10% in much thicker coatings; any flaws in the coating (or matrix) now penetrate the fiber readily in the absence of any crack-deflecting mechanism.35 (2) Minicomposites Table II summarizes the strengths of minicomposites. Minicom￾posite A had an average strength that was nearly twice as high as the control minicomposite F. Minicomposites B and D, heat￾treated for 100 h in air at 1200°C, had strengths that were equal to that of the control minicomposite F heated under the same conditions but without a fiber coating. Figure 6 contains SEM images of fracture surfaces of minicomposites A and B, showing the difference in fracture behavior of the two composites. Long fiber pullout lengths and holes left behind by fibers were clearly visible in the fracture surfaces of minicomposite A, as opposed to only fibers exposed by matrix disintegration, as is often seen in porous matrix composites.41 Minicomposite B displayed a brittle fracture surface with little, if any, fiber pullout. A TEM image of the fiber/matrix interfacial region from minicomposite A is shown in Fig. 7. The grain and pore sizes in the YAG fiber coating increase to 40–50 nm, and particles begin to sinter following matrix processing at 1200°C for 2 h (Fig. 3(b),(c)). Because the matrix is porous as well and could be expected to deflect cracks, the presence of the porous YAG coating results in composites with higher tensile strengths. Either the porosity distribution in the matrix alone is not adequate to deflect cracks, whereas that in the fiber coating is, or the coating protects the fiber from degradation during matrix processing. Sintering of matrix alumina particles to the alumina fiber is observed in the absence of a coating, which could lead to fiber degradation in the control composites by enhanced fiber/matrix bonding and stress concentration. Figures 8 and 9 show TEM images of the fiber/matrix interfaces in minicomposites C and D, which contain an initial 30 vol% porosity coating and are heat-treated at 1100°C for 2 h in air and 1200°C for 100 h in air, respectively. Clearly evident is the grain growth, pore coarsening, and sintering of YAG that is occurring. In Fig. 5. Weibull plot of tows 1–5 that were coated with 50 vol% YAG/C precursor and tows 12 and 13 (desized, without coating) heat-treated under various conditions. Fig. 6. SEM images of (a) minicomposite A and (b) minicomposite B following heating at 1200°C for 2 and 100 h in air, respectively. Note fiber pullout and troughs left by fiber pulled out in Fig. 6(a). November 2002 Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites 2707