April 2001 Toughened Oxide Composites Based on Porous Alumina-Platelet Interphases Table Ill. Variation in Strength and work of Fracture for Table Iv. variation in Strength and Work of Fracture for Laminates, According to the Thickness ratio between the Mullite-Matrix Laminates, According to Platelet Size in the Alumina Matrix and the Alumina-Platelet(10-15 pm Pure Alumina-Platelet Interphase Interphases Platelet size Thickness Flexural strength Work of fracture, wol Thickness Mullite content Flexural strength Work of fracture, WOF (vol% 0-15 Densified matrix: interphase thickness ratio th)x 4.5 mm( thickness)x 3.0 mm (width), rather than the normal 30 mm X Table V. Variation in Strength and work of fracture for 3.0 Mullite content in the alumina-platelet interphase. Alumina-Matrix Laminates, According to Platelet Size in the Pure Alumina-Platelet Interphase Platelet size Thickness Flexural strengt Work of fracture, WOF Thickness ratio 15: 1 Thickness ratio bimodal g=123 MPa 0=112 MPa 0.25WOF=1.5kJ/m WOF= 2.1 kJ/m 1.0 Densified matrix interphase thickness ratio. 0.2 1 0.25 0.15 =109 MPa 0.2 I Mullite matrix σ=84MI WOF=0.2 kJ/m2 0.15 0.3 Displacement (mm) Matrix L Crack () Mechanical Behavior of the Fibrous Ceramic Compo To make tough, flaw-tolerant, fibrous ceramic composites, the results of the mechanical testing of the laminate composites were g composite. The microstructures of the as-sintered, fibrous ceramic Fig.5.(a)Load-displacement curves of the alumina-matrix laminate composites are shown in Fig. 7. The discontinuous, Al2O3-platelet composites, as a function of the matrix: interphase thickness ratio(see cell boundaries, which defined the matrix and reinforcing regions Table III), the bimodal microstructure consisted of alternating layers of 2: 1 and 5: I matrix: interphase thickness ratios.(b) Failure-side view SEM of Al,O3, are clearly visible. The side view of the as-sintered micrographs of the alumina-matrix laminate composite; the laminate has a fibrous ceramic composite showed the degree of uniformity in the mullite content of 3 vol% in the interphase and a bimodal thickness ratio alignment of the as-extruded filaments(see Fig. 7(b)). The patterns are similar to the side view of the bimodally designed laminated composite. This feature is one of the main advantages that are associated with this forming technique, i.e., the ability to create a laminate, which had Al,O3 platelets 5-10 um in size, notable heterogeneous microstructure with uniform cell-boundary thick rack deflection did not occur at the interphase, and a lower WoF nan that in the 10-15 um platelet 3Al2O3 2SiO, composite was Plots of flexural load versus displacement are shown bserved(Table IV). In contrast, the Al,O,matrix laminate that The strength and woF increased slightly in comparison hat ad Al,O, platelets 5-10 um in size showed improved strength of the AlO3-matrix laminate that had a bimodal thickness ratio. In and woF, in comparison with the 10-15 um Al,O3-platelet ontrast to the fracture curves of the laminates. the fibrous com ceramics exhibited unusual plasticlike behavior, they retainedlaminate, which had Al2O3 platelets 5–10 mm in size, notable crack deflection did not occur at the interphase, and a lower WOF than that in the 10–15 mm platelet 3Al2O3z2SiO2 composite was observed (Table IV). In contrast, the Al2O3-matrix laminate that had Al2O3 platelets 5–10 mm in size showed improved strength and WOF, in comparison with the 10–15 mm Al2O3-platelet composite. (5) Mechanical Behavior of the Fibrous Ceramic Composite To make tough, flaw-tolerant, fibrous ceramic composites, the results of the mechanical testing of the laminate composites were applied to the fibrous design. An Al2O3 matrix and Al2O3 platelets 5–10 mm in size were used as the materials of the fibrous composite. The microstructures of the as-sintered, fibrous ceramic composites are shown in Fig. 7. The discontinuous, Al2O3-platelet cell boundaries, which defined the matrix and reinforcing regions of Al2O3, are clearly visible. The side view of the as-sintered fibrous ceramic composite showed the degree of uniformity in the alignment of the as-extruded filaments (see Fig. 7(b)). The patterns are similar to the side view of the bimodally designed laminated composite. This feature is one of the main advantages that are associated with this forming technique, i.e., the ability to create a heterogeneous microstructure with uniform cell-boundary thick￾nesses. Plots of flexural load versus displacement are shown in Fig. 8. The strength and WOF increased slightly in comparison with that of the Al2O3-matrix laminate that had a bimodal thickness ratio. In contrast to the fracture curves of the laminates, the fibrous ceramics exhibited unusual plasticlike behavior; they retained Table III. Variation in Strength and Work of Fracture for Laminates, According to the Thickness Ratio between the Alumina Matrix and the Alumina-Platelet (10–15 mm) Interphases Thickness ratio† Mullite content‡ (vol%) Flexural strength (MPa) Work of fracture, WOF (kJ/m2 ) 6:1 1 105 1.1 Bimodal 3 112 2.1 15:1 2 123 1.5 † Densified matrix:interphase thickness ratio. “Bimodal” denotes an alternative combination of thickness ratios of 5:1 and 12:1 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. Fig. 5. (a) Load–displacement curves of the alumina-matrix laminate composites, as a function of the matrix:interphase thickness ratio (see Table III); the bimodal microstructure consisted of alternating layers of 12:1 and 5:1 matrix:interphase thickness ratios. (b) Failure-side view SEM micrographs of the alumina-matrix laminate composite; the laminate has a mullite content of 3 vol% in the interphase and a bimodal thickness ratio. Table IV. Variation in Strength and Work of Fracture for Mullite-Matrix Laminates, According to Platelet Size in the Pure Alumina-Platelet Interphase Platelet size (mm) Thickness ratio† Flexural strength (MPa) Work of fracture, WOF (kJ/m2 ) 5–10 4:1 84 0.2 10–15 6:1 75 0.3 † Densified matrix:interphase thickness ratio. Table V. Variation in Strength and Work of Fracture for Alumina-Matrix Laminates, According to Platelet Size in the Pure Alumina-Platelet Interphase Platelet size (mm) Thickness ratio† Flexural strength (MPa) Work of fracture, WOF (kJ/m2 ) 5–10 6:1 109 1.2 10–15 6:1 105 1.0 † Densified matrix:interphase thickness ratio. Fig. 6. Load–displacement curves of (– – –) mullite-matrix and (—) alumina-matrix laminate composites with a platelet size of 5–10 mm in the interphases. 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