G. Hilmas et al. Materials Science and Engineering A195(1995)263-268 OUnn la)s bH b Fig. 1. Cell-cell boundary structure of a BS80 Sic-BN fibrous monolith, with boron nitride cell boundaries in bright contrast: Fig. 2. Cell-cell boundary structure of an(yttria-zirconia)S-Ni (a]end-on view of axially aligned SiC cells; (b)side view of cells. fibrous monolith, with nickel cell boundaries in bright contrast (a)end-on view of axially aligned zirconia cells; b side view of BN system has a quite similar cell-cell boundary structure. Within the bN cell boundaries there are flexure at room ter densely packed bN platelets 2-10 um wide and abor 2 um thick. The microstructure within the cells are designed with particularly low shear strengths, so characteristic of the matrix material. The silicon nitride they will fail by a combination of shear and tension. In consists of elongated 0.5 um B-silicon nitride grains. flexural loading, FM ceramics can display non-cata- The BS80 Sic consists primarily of elongated 6H Sic strophic or " graceful"failure because the specimen exhibits a shear-dominated failure mode. The shear The zirconia-Ni material is shown in Fig 2a)where cracking makes the specimen more compliant in the axially aligned zirconia cells are dark and the ni retaining a pility to cell boundaries are in bright contrast. The zirconia load. Fibrous monoliths can also display a graceful cells are about 300 um across, and the Ni cell bound failure mode which initiates in tension with subsequent aries are about 20 um wide. Fig. 2(b)is the side shear delamination. The relationships between flexural failure mode have been discussed elsewhere [4, 7.The longer than they appear, as the green fiber alignment ductile Ni-containing systems have quite different was irregular, so cells weave in and out of the plane of behavior, with tensile cracking The load-deflection curves for the Si3N2-BN syS 2. Mechanical behavior of fibrous monoliths in tem are shown in Fig 3 for three samples tested at fe room temperature,800℃C,and1000°C. The room temperature bar reached a peak load corresponding to Conventional ceramics have high shear strength, so an apparent flexural strength of 484 MPa, followed by they exhibit only tensile initiated failure when loaded in a sharp load drop due to shear delamination near theG. Hilmas et al. / Materials Science and Engineering A 195 (1995) 263-268 265 Fig. 1. Cell-cell boundary structure of a BS80 SiC-BN fibrous monolith, with boron nitride cell boundaries in bright contrast: (a) end-on view of axially aligned SiC cells; (b) side view of cells. BN system has a quite similar cell-cell boundary structure. Within the BN cell boundaries there are densely packed BN platelets 2-10/~m wide and about 2 ,urn thick. The microstructure within the cells is characteristic of the matrix material. The silicon nitride consists of elongated 0.5 /~m r-silicon nitride grains. The BS80 SiC consists primarily of elongated 6H SiC grains, about 4/~m wide. The zirconia-Ni material is shown in Fig. 2(a) where the axially aligned zirconia cells are dark and the Ni cell boundaries are in bright contrast. The zirconia cells are about 300/~m across, and the Ni cell bound￾aries are about 20/~m wide. Fig. 2(b) is the side view of the same specimen. The zirconia cells are actual much longer than they appear, as the green fiber alignment was irregular, so cells weave in and out of the plane of polish. 3.2. Mechanical behavior of fibrous monoliths in flexure Conventional ceramics have high shear strength, so they exhibit only tensile initiated failure when loaded in Fig. 2. Cell-cell boundary structure of an (yttria-zirconia)S-Ni fibrous monolith, with nickel cell boundaries in bright contrast: (a) end-on view of axially aligned zirconia cells; (b) side view of cells. flexure at room temperature. In contrast, FM ceramics are designed with particularly low shear strengths, so they will fail by a combination of shear and tension. In flexural loading, FM ceramics can display non-cata￾strophic or "graceful" failure because the specimen exhibits a shear-dominated failure mode. The shear cracking makes the specimen more compliant in flexure, while still retaining ability to bear significant load. Fibrous monoliths can also display a graceful failure mode which initiates in tension with subsequent shear delamination. The relationships between flexural loading conditions, specimen span:depth ratio, and failure mode have been discussed elsewhere [4,7]. The ductile Ni-containing systems have quite different behavior, with tensile cracking [6]. The load-deflection curves for the Si3N3-BN sys￾tem are shown in Fig. 3 for three samples tested at room temperature, 800 °C, and 1000 °C. The room temperature bar reached a peak load corresponding to an apparent flexural strength of 484 MPa, followed by a sharp load drop due to shear delamination near the