770 Journal of the American Ceramic Sociery-Lee and Riven Vol 84. No 4 0.2 Mullite 20 vol% Crack Matrix Mullite 5 vol% Only platelets 0.05 oUtset+Ky serum 0.15 Displacement (mm) Crack Matrix Mullite 3 vol% 星015F如m WOF=0.6 KJ/m2 0.05 Fig. 4. Failure-side view SEM micrographs of the laminate composites with mullite contents of (a)5 and (b)3 vol% in the interphases. In the latter image, crack deflection is evident along the porous platelet interphase 00.050.10.150.2025 Table l. Variation in Strength and Work of Fracture for Load-displacement curves of mullite-matrix laminate com- function of the mullite content in the weak interphases Laminates, According to the Thickness Ratio between the Mullite Matrix and the Alumina-Platelet (10-15 um) ase thickness ratios of (a)2: 1 and(b) 4: 1 and 6: 1 are Table D) Thickness llite content Flexural strength Work of fracture. WOF (vo9) 2:1 and 9: 1. Graceful-failure characteristics were observed in the corresponding load-displacement curve To increase the overall strength of the composite, the Al,O atrix was chosen in a bimodal sequence of alternating 12: 1 an Densified matrix interphase thickness ratio. "Bimodal"denotes an alternat 5: 1 ratios, where the alternating regions of 12: 1 and 5: I ratios qual thickness. When up to 3 vol% of 3Al,O3 2SiO, was added 4.0 mm x 3.0 mm .Mullite content in the alt the interphase, the WOF notably increased. The highest strength nd WOF values(112 MPa and 2.1 kJ/m", respectively )were bserved. Again, graceful-failure characteristics were observed in the load-displacement curves for Al2O,(see Fig. 5(a) and Table the matrix and the interphase, even though they consisted of the Ill). Figure 5(b) shows an SEM micrograph that illustrates the same material crack profile that corresponds to the optimized load-displacement curve shown in Fig. S(a). The deflected crack passed through the (4) Mechanical Behavior of the Laminates, Relative to the center of the porous interphase Size of the Alumina Platelets The Al,O3 platelets provided an easy crack-deflection rout The results of the flexural testing for the laminates, according to even in a thin platelet interphase with an Al,O,- matrix interphase the size of the alumina platelets in the pure-Al2O3 interphase, are ickness ratio of 15: 1. This result confirmed that the interphase listed in Tables IV and v. Correspondingly, the load-displacement was much weaker than the matrix. This finding was consistent with curves of the specimens that had platelets 5-10 um in size in the the expectation that no reaction or densification occurred between interphases are shown in Fig. 6. In the 3Al,O3 2SiO2-matrixand 9:1. Graceful-failure characteristics were observed in the corresponding load–displacement curves. To increase the overall strength of the composite, the Al2O3 matrix was chosen in a bimodal sequence of alternating 12:1 and 5:1 ratios, where the alternating regions of 12:1 and 5: 1 ratios had equal thickness. When up to 3 vol% of 3Al2O3z2SiO2 was added to the interphase, the WOF notably increased. The highest strength and WOF values (112 MPa and 2.1 kJ/m2 , respectively) were observed. Again, graceful-failure characteristics were observed in the load–displacement curves for Al2O3 (see Fig. 5(a) and Table III). Figure 5(b) shows an SEM micrograph that illustrates the crack profile that corresponds to the optimized load–displacement curve shown in Fig. 5(a). The deflected crack passed through the center of the porous interphase. The Al2O3 platelets provided an easy crack-deflection route, even in a thin platelet interphase with an Al2O3-matrix:interphase thickness ratio of 15:1. This result confirmed that the interphase was much weaker than the matrix. This finding was consistent with the expectation that no reaction or densification occurred between the matrix and the interphase, even though they consisted of the same material. (4) Mechanical Behavior of the Laminates, Relative to the Size of the Alumina Platelets The results of the flexural testing for the laminates, according to the size of the alumina platelets in the pure-Al2O3 interphase, are listed in Tables IV and V. Correspondingly, the load–displacement curves of the specimens that had platelets 5–10 mm in size in the interphases are shown in Fig. 6. In the 3Al2O3z2SiO2-matrix Fig. 3. (a) Load–displacement curves of mullite-matrix laminate com￾posites, as a function of the mullite content in the weak interphases. Alumina platelets 10–15 mm in size are present in the interphases, and matrix:interphase thickness ratios of (a) 2:1 and (b) 4:1 and 6:1 are observed (see Table I). Fig. 4. Failure-side view SEM micrographs of the laminate composites with mullite contents of (a) 5 and (b) 3 vol% in the interphases. In the latter image, crack deflection is evident along the porous platelet interphase. Table II. Variation in Strength and Work of Fracture for Laminates, According to the Thickness Ratio between the Mullite Matrix and the Alumina-Platelet (10–15 mm) Interphases Thickness ratio† Mullite content‡ (vol%) Flexural strength (MPa) Work of fracture, WOF (kJ/m2 ) 2:1 0 70 0.5 6:1 0 75 0.6 4:1 1 77 0.6 Bimodal 1 102 1.1 † Densified matrix:interphase thickness ratio. “Bimodal” denotes an alternative combination of thickness ratios of 3:1 and 9:1 and a specimen size of 30 mm (length) 3 4.5 mm (thickness) 3 3.0 mm (width), rather than the normal 30 mm 3 4.0 mm 3 3.0 mm. ‡ Mullite content in the alumina-platelet interphase. 770 Journal of the American Ceramic Society—Lee and Kriven Vol. 84, No. 4