S Guo, Y. Kagawa/Journal of the European Ceramic Society 22(2002)2349-2356 (b 38s 100um 100um 产100m 100m Fig. 7. Scanning electron micrographs of fracture surfaces of the NicalonM SiC/C/SiC tested at(a)298 K, and(b)1200 K and the Hi-Nicalon/BN/ Sic tested at(c)298 K, and (d)1400 K Table 3 In situ fiber tensile strength characteristics and interface shear stress of both the Nicalon/C/SiC and Hi-Nicalon/BN/SiC at room and elevated temperatures In situ fibre Weibull Fibre pullout Interfacial shear Number of materials odulus m length L (um) stress Ti(MPa) measurements 0±35 59±7 ±10 126±15 Hi-Nicalon/BN/SiC 2199 8.7 300±140 13±5 2187 250±110 15±5 9.6 90±90 17±7 growth. 26 Nearly the same temperature dependence was values at room and elevated temperatures are much observed as in the present study lower than those of the Nicalon/ C/SiC. This should be attributed to a better oxidation resistance of BN-coating 3.3. Interface shear stress on the Hi-Nicalon M fiber surface than C-coating on Nicalon fiber surface The Interface shear stresses of the Nicalon/C/SiC and the Hi-Nicalon/ BN/SiC composites are obtained using Eq(2), and the results are also summarized in Table 3. 4. Discussion For the Nicalon/ C/SiC, the interfacial shear stress is 59 MPa at 298K. 44 MPa at 800 K and 126 MPa at 1200 The experimental results indicated that the K. The interfacial shear stress decreases at 800 K from fracture behavior and properties of both the nica that of 298 K and then increases again at 1200 K. For SiC and the Hi-Nicalon/ BN/SiC depended on test tem- the Hi-Nicalon/BN/SiC, the interface shear stress tends perature. The temperature dependence essentially origi- to increase with increasing test temperature, and the nates from the change of in situ constituent propertiesgrowth.26 Nearly the same temperature dependence was observed as in the present study. 3.3. Interface shear stress The Interface shear stresses of the Nicalon/C/SiC and the Hi-Nicalon/BN/SiC composites are obtained using Eq. (2), and the results are also summarized in Table 3. For the Nicalon/C/SiC, the interfacial shear stress is 59 MPa at 298 K, 44 MPa at 800 K and 126 MPa at 1200 K. The interfacial shear stress decreases at 800 K from that of 298 Kand then increases again at 1200 K. For the Hi-Nicalon/BN/SiC, the interface shear stress tends to increase with increasing test temperature, and the values at room and elevated temperatures are much lower than those of the Nicalon/C/SiC. This should be attributed to a better oxidation resistance of BN-coating on the Hi-NicalonTM fiber surface than C-coating on NicalonTM fiber surface.22,23 4. Discussion The experimental results indicated that the tensile fracture behavior and properties of both the Nicalon/C/ SiC and the Hi-Nicalon/BN/SiC depended on test tem￾perature. The temperature dependence essentially origi￾nates from the change of in situ constituent properties Table 3 In situ fiber tensile strength characteristics and interface shear stress of both the Nicalon/C/SiC and Hi-Nicalon/BN/SiC at room and elevated temperatures Composite materials Test temperature T (K) In situ fibre strength fu (MPa) Weibull modulus m Fibre pullout length L (mm) Interfacial shear stress i (MPa) Number of measurements Nicalon/C/SiC 298 1765 6.2 50 35 59 7 70 800 1705 6.7 70 40 44 10 70 1200 1235 6.9 20 10 126 15 70 Hi-Nicalon/BN/SiC 298 2199 8.7 300 140 13 5 70 1200 2187 9.1 250 110 15 5 70 1400 1961 9.6 190 90 17 7 70 Fig. 7. Scanning electron micrographs of fracture surfaces of the NicalonTM SiC/C/SiC tested at (a) 298 K, and (b) 1200 K and the Hi-Nicalon/BN/ SiC tested at (c) 298 K, and (d) 1400 K. 2354 S. Guo, Y. Kagawa / Journal of the European Ceramic Society 22 (2002) 2349–2356