S Li et al. Materials Letters 57(2003)1670-1674 8314832ek Fig. 2. The crack propagation in Si,N/BN fibrous monolithic Fig. 4. SEM of a typical fracture surface of a quenched specimen from825to25°C direction. The major features of the bn are extensive and positive, about 405X 10/C [14]. As the microcracks between the(0001) basal planes of BN posite is cooled from the hot-pressing temperature platelets. The microcrack structure in the Si,N/BN (1820C), the Bn contracts perpendicular to the basal fibrous monolithic ceramic has been described by plane (i.e, in the [000l] direction), while there is a Kovar et al A similar structure has been reported small expansion within the plane. If the surrounding by Mrozwski [14] in graphite that has a crystalline Si3N4 grains or glassy phase constrain the bn plate structure similar to bn. due to the microcrack struc lets, large tensile stresses are developed perpendicular ture, the intensive dissipation of energy absorbed by to the basal plane upon cooling. This acts to numerous microcracks is helpful to reinforce the the bn platelet into layers along the basal plane thermal damage resistance. Meanwhile, because the BN cell boundary is a weak interlayer and possesses fine-microstructure. crack deflection at the bn cell 811e8228KU49*2NM Fig. 3. SEM of Si3 N4 perpendicular to the hot-pressing direction. Fig. 5. SEM of the fracture surface of a normal specimen.and positive, about 40.5
10 6 /jC [14]. As the composite is cooled from the hot-pressing temperature (1820 jC), the BN contracts perpendicular to the basal plane (i.e., in the [0001] direction), while there is a small expansion within the plane. If the surrounding Si3N4 grains or glassy phase constrain the BN plate￾lets, large tensile stresses are developed perpendicular to the basal plane upon cooling. This acts to separate the BN platelet into layers along the basal plane direction. The major features of the BN are extensive microcracks between the (0001) basal planes of BN platelets. The microcrack structure in the Si3N4/BN fibrous monolithic ceramic has been described by Kovar et al. [13]. A similar structure has been reported by Mrozwski [14] in graphite that has a crystalline structure similar to BN. Due to the microcrack struc￾ture, the intensive dissipation of energy absorbed by numerous microcracks is helpful to reinforce the thermal damage resistance. Meanwhile, because the BN cell boundary is a weak interlayer and possesses fine-microstructure, crack deflection at the BN cell Fig. 2. The crack propagation in Si3N4/BN fibrous monolithic ceramic. Fig. 3. SEM of Si3N4 perpendicular to the hot-pressing direction. Fig. 4. SEM of a typical fracture surface of a quenched specimen from 825 to 25 jC. Fig. 5. SEM of the fracture surface of a normal specimen. S. Li et al. / Materials Letters 57 (2003) 1670–1674 1673