G H. Min et al./ Ceramics International 29(2003)323-326 pressed into a perform at 523 K under a pressure of 25 3. 2. Mechanical properties MPa, and followed by hot pressing at temperature of 1773 K for I h under the same pressure, yielding the The mechanical properties of Al_O3 /SiC prismatic prismatic fib rous ceramIc opposites with various boundary thicknesses are listed The bending strength was tested by three-point bend- in Table 1. On the basis of the bending data, the tough ng with 0.5 mm/min crosshead speed. The used samples ness energy appears to be improved greatly with little had a polished tensile surface with 1.5 mm thick (in hot- increased toughness but decreased bending strength. It pressing direction), 4 mm wide and 40 mm long(in fiber can be considered that the bending strength is decreased direction). The four-point Single Edge Notched Beam due to the reduction of effective load-carrying area by method(SENB) was used for evaluation of fracture interphase SiC addition, which also results in little toughness and fracture work. The used samples were increase of fracture toughness. However, the fracture prepared 3.0 mm thick, 4 mm wide and 40 mm long, work is improved greatly because the weak interfaces per- and a notch was cut with 1. 5 mm in depth and 0. 1 mm mit the crack propagation with multi-directional routes In case of the interphase Sic thickness of 9.3-15.6 Microstructures and crack propagation of the pris- um, the fracture work of 1221.4-1481.6 J/m- is achieved matic ceramic were observed by optical microscope with good combination of bending strength and tough ness. When the thickness is higher than 15.6 um, the prismatic ceramic is easy to peel/ cleave due to weak 3. Results and discussion bonding between the layers, and shear stress along the axial direction is very low, which results from poor sin 3.. Microstructure tenability of Sic Fig. 2 shows the load-displacement curves of the Fig. I shows the microstructure in the cross section of prismatic ceramics with various SiC interphase. The prismatic composites with various boundary thick composites display a non-catastrophic and graceful nesses Flattened hexagonal Al2O3 cells about 250 um in failure with reasonable load-carrying capability, as thickness and 550 um in width are separated by a thin described in detail elsewhere [171 cell boundary (dark), this uniform structure being The work-of-fracture of the prismatic ceramic is attributed to the deformation of the green fibers during composed of two parts, earlier stage and later stage, warm-pressing and hot-pressing. With the increasing of corresponding to the displacement of lower or larger boundary thickness, the cross sections of Al2O3 cells than about 0.5 mm, respectively. During the initial stage showed an ellipse shape. It can be assumed that cells with relatively high load-carrying capacity, the deflect were permitted to move under pressure in the case of ing, delaminating and tensile fracturing occur alter thicker cell boundaries natively, yielding a saw-teeth curve. Crack deflection In practice, the fibrous monolithic ceramic is a special and delamination are two main contributions to the xample of laminated or multilayered ceramics. A con- improved fracture energy. During the later stage of trolled three-dimensional structure exists in fibrous bending test, the load-carrying capacity becomes very ceramics, and each fibrous cell is separated completely low, which means the ending of tensile fracturing and by a thin cell boundary, although the weaker interphase the starting of fiber sliding. The sliding friction among mics, uniform monolith existed within each layer(sepa- work-of-fracture in the later stage of bending te o,ng was introduced just in inter-layers in multilayered cera- fibers becomes a dominant factor for the improved rated as two-dimensional). Thus, the difference in the The total displacements are up to 1.0 mm and similar structure results in the variations of mechanical proper- to those of fiber reinforced ceramic composites [101 ties and fracture behavior [16] However, relatively short displacement occurs for the I Fig. 1. Cross-section observation of the alumina-based prismatic ceramics with SiC boundaries in various thickness of (A)4.4 um, (B)9.3 um, andpressed into a perform at 523 K under a pressure of 25 MPa, and followed by hot pressing at temperature of 1773 K for 1 h under the same pressure, yielding the prismatic fibrous ceramic. The bending strength was tested by three-point bend￾ing with 0.5 mm/min crosshead speed. The used samples had a polished tensile surface with 1.5 mm thick (in hot￾pressing direction), 4 mm wide and 40 mm long (in fiber direction). The four-point Single Edge Notched Beam method (SENB) was used for evaluation of fracture toughness and fracture work. The used samples were prepared 3.0 mm thick, 4 mm wide and 40 mm long, and a notch was cut with 1.5 mm in depth and 0.1 mm in tip diameter. Microstructures and crack propagation of the pris￾matic ceramic were observed by optical microscope. 3. Results and discussion 3.1. Microstructure Fig. 1 shows the microstructure in the cross section of prismatic composites with various boundary thick￾nesses. Flattened hexagonal Al2O3 cells about 250 mm in thickness and 550 mm in width are separated by a thin cell boundary (dark), this uniform structure being attributed to the deformation of the green fibers during warm-pressing and hot-pressing. With the increasing of boundary thickness, the cross sections of Al2O3 cells showed an ellipse shape. It can be assumed that cells were permitted to move under pressure in the case of thicker cell boundaries. In practice, the fibrous monolithic ceramic is a special example of laminated or multilayered ceramics. A con￾trolled three-dimensional structure exists in fibrous ceramics, and each fibrous cell is separated completely by a thin cell boundary, although the weaker interphase was introduced just in inter-layers in multilayered cera￾mics, uniform monolith existed within each layer (sepa￾rated as two-dimensional). Thus, the difference in the structure results in the variations of mechanical proper￾ties and fracture behavior [16]. 3.2. Mechanical properties The mechanical properties of Al2O3/SiC prismatic composites with various boundary thicknesses are listed in Table 1. On the basis of the bending data, the tough￾ness energy appears to be improved greatly with little increased toughness but decreased bending strength. It can be considered that the bending strength is decreased due to the reduction of effective load-carrying area by interphase SiC addition, which also results in little increase of fracture toughness. However, the fracture work is improved greatly because the weak interfaces per￾mit the crack propagation with multi-directional routes. In case of the interphase SiC thickness of 9.3–15.6 mm, the fracture work of 1221.4–1481.6 J/m2 is achieved with good combination of bending strength and tough￾ness. When the thickness is higher than 15.6 mm, the prismatic ceramic is easy to peel/cleave due to weak bonding between the layers, and shear stress along the axial direction is very low, which results from poor sin￾terability of SiC. Fig. 2 shows the load–displacement curves of the prismatic ceramics with various SiC interphase. The composites display a non-catastrophic and graceful failure with reasonable load-carrying capability, as described in detail elsewhere [17]. The work-of-fracture of the prismatic ceramic is composed of two parts, earlier stage and later stage, corresponding to the displacement of lower or larger than about 0.5 mm, respectively. During the initial stage with relatively high load-carrying capacity, the deflect￾ing, delaminating and tensile fracturing occur alter￾natively, yielding a saw-teeth curve. Crack deflection and delamination are two main contributions to the improved fracture energy. During the later stage of bending test, the load-carrying capacity becomes very low, which means the ending of tensile fracturing and the starting of fiber sliding. The sliding friction among fibers becomes a dominant factor for the improved work-of-fracture in the later stage of bending test. The total displacements are up to 1.0 mm and similar to those of fiber reinforced ceramic composites [10]. However, relatively short displacement occurs for the Fig. 1. Cross-section observation of the alumina-based prismatic ceramics with SiC boundaries in various thickness of (A) 4.4 mm, (B) 9.3 mm, and (C) 15.6 mm. 324 G.H. Min et al. / Ceramics International 29 (2003) 323–326