S Guo, Y. Kagawa/Journal of the European Ceramic Society 22(2002)2349-2356 cracking usually initiates well below this stress 20.21 tested at room and elevated temperatures. The pulled- Thus, in the present study, the apparent matrix cracking out fibers are observed in the composite specimens stress obtained should be higher than the true matrix fractured at and below 800 K, however, this fiber pull- cracking stress. The matrix cracking behavior clearly out behavior is not seen in the composite tested at 1200 was observed in the polished longitudinal cross-section K, showing a brittle fracture fashion. Fig. 6 shows the of the composite specimens tested at room and elevated macroscopic fracture appearances of the Hi-Nicalon temperatures(Fig 4). The transverse matrix cracks are BN SiC tested at room and elevated temperatures. Dif- formed through the transverse fiber bundle, and some of fering from the Nicalon /C/SiC, the room temperature them are arrested at the interface between the long- fracture behavior of the Hi-Nicalon/BN/SiC shows a itudinal and transverse fiber bundles fracture path which is jagged and stepped across the The average values of Ec and ome that were calculated thickness, and there is extensive fiber and fiber tow pull from the duplicate tests at each temperature are shown out. Although the fracture surface became smoother in Table 2, together with the tensile strength, OTs, and with increasing test temperature, the pulled-out fibers le strain to failure, Ec. For the Nicalon/C/SiC, the ten- are observed over all test temperatures sile mechanical properties remain nearly constant from Fig. 7 shows SEM micrographs of the fracture sur- 298 to 800 K and that they degrade sharply by 1200 K, faces of both the composites. Although all fracture sur in particular, the tensile strength and strain to failure faces of the composites tested at room and hig dropped from 140 MPa and 0. 38% at 800 K to 41 MPa temperatures showed a fibrous fracture surface, only and 0.075% at 1200 K. For the Hi-Nicalon/ BN/SiC, on few and short pulled-out fibers are observed in the the other hand, the mechanical properties such as Ec, fracture surface of the Nicalon/C/SiC tested at 1200 K ome, Ec and ars remain nearly constant up to 1200 K This is probably attributed to the silica(Sio2) formation and begin to degrade at 1400 K; they are much higher at the interface by oxidation of both the matrix and than those of the Nicalon/C/SiC at all test temperatures fiber, because of air penetration into the gaps at the interface resulting form the elimination of the C-coating (ii) Fracture surface observations: Fig. 5 shows the layer by oxidation above 700 K.,3 The pulled-out macroscopic fracture appearance of the Nicalon/ C/Sic length of fiber was measured using the method reported elsewhere. 3 For the Nicalon/C/SiC, the average pulled ut fiber length is≈50mat298K,≈70pmat800K and a20 um at 1200 K. For the Hi-Nicalon/BN/SIC. the pulled-out fiber length is a300 um at 298 K, N250 mat1200Kand≈190μmatl400K. It is clear that he pulled-out fiber length of the Hi-Nicalon/ BN/SiC is much larger than that of the Nicalon/ C/Sic at room and elevated temperatures. Moreover, for the Nicalon/ C/Sic tested at and below 800 K there are regions in all the bundles in which the fiber -fracture locations are essentially coplanar with one another compared to the Hi-Nicalon/BN/SIC. 10m 3.2. In situ fiber strength Fig 4. An example of optical photographs of polished longitudinal The in situ fiber strength characteristics of the Nica- cross-section of the two composites after monotonic tension testing. lon TM Sic fiber and the hi-Nicalon TM Sic fiber were showing matrix cracking(T=298 K, Hi-Nicalon/BN/SiC obtained using Eq (I)and the results are summarized in Table 2 Tensile experimental results of both the Nicalon/C/SiC and Hi-Nicalon/ BN/SIC Composite temperature T(K) modulus E(GPa) cracking stress me(MPa)strength oTs(MPa) failure e(%)strength oTs Measured tensile Strain to Predicted tensi Nicalon/C/SiC 58±5 65±8 136±19 0.42士 55±4 5±5 0.38士 17: 1200 33±3 4l±5 0.075±0.01512 Hi-Nicalon/BN/SIC 298 80±5 75±9 226±l1 0.84±0.12223 76士4 0±7 237士6 0.9士0.06228 0±5 197±15 0.68±0.09209cracking usually initiates well below this stress.20,21 Thus, in the present study, the apparent matrix cracking stress obtained should be higher than the true matrix cracking stress. The matrix cracking behavior clearly was observed in the polished longitudinal cross-section of the composite specimens tested at room and elevated temperatures (Fig. 4). The transverse matrix cracks are formed through the transverse fiber bundle, and some of them are arrested at the interface between the long￾itudinal and transverse fiber bundles. The average values of Ec and mc that were calculated from the duplicate tests at each temperature are shown in Table 2, together with the tensile strength, TS, and the strain to failure, c. For the Nicalon/C/SiC, the ten￾sile mechanical properties remain nearly constant from 298 to 800 Kand that they degrade sharply by 1200 K, in particular, the tensile strength and strain to failure dropped from 140 MPa and 0.38% at 800 Kto 41 MPa and 0.075% at 1200 K. For the Hi-Nicalon/BN/SiC, on the other hand, the mechanical properties such as Ec, mc, c and TS remain nearly constant up to 1200 K and begin to degrade at 1400 K; they are much higher than those of the Nicalon/C/SiC at all test temperatures. (ii) Fracture surface observations: Fig. 5 shows the macroscopic fracture appearance of the Nicalon/C/SiC tested at room and elevated temperatures. The pulled￾out fibers are observed in the composite specimens fractured at and below 800 K, however, this fiber pull￾out behavior is not seen in the composite tested at 1200 K, showing a brittle fracture fashion. Fig. 6 shows the macroscopic fracture appearances of the Hi-Nicalon/ BN/SiC tested at room and elevated temperatures. Dif￾fering from the Nicalon/C/SiC, the room temperature fracture behavior of the Hi-Nicalon/BN/SiC shows a fracture path which is jagged and stepped across the thickness, and there is extensive fiber and fiber tow pull￾out. Although the fracture surface became smoother with increasing test temperature, the pulled-out fibers are observed over all test temperatures. Fig. 7 shows SEM micrographs of the fracture sur￾faces of both the composites. Although all fracture sur￾faces of the composites tested at room and high temperatures showed a fibrous fracture surface, only few and short pulled-out fibers are observed in the fracture surface of the Nicalon/C/SiC tested at 1200 K. This is probably attributed to the silica (SiO2) formation at the interface by oxidation of both the matrix and fiber, because of air penetration into the gaps at the interface resulting form the elimination of the C-coating layer by oxidation above 700 K.22,23 The pulled-out length of fiber was measured using the method reported elsewhere.3 For the Nicalon/C/SiC, the average pulled￾out fiber length is 50 mm at 298 K, 70 mm at 800 K and 20 mm at 1200 K. For the Hi-Nicalon/BN/SiC, the pulled-out fiber length is 300 mm at 298 K , 250 mm at 1200 Kand 190 mm at 1400 K. It is clear that the pulled-out fiber length of the Hi-Nicalon/BN/SiC is much larger than that of the Nicalon/C/SiC at room and elevated temperatures. Moreover, for the Nicalon/ C/SiC tested at and below 800 Kthere are regions in all the bundles in which the fiber-fracture locations are essentially coplanar with one another compared to the Hi-Nicalon/BN/SiC. 3.2. In situ fiber strength The in situ fiber strength characteristics of the Nica￾lonTM SiC fiber and the Hi-NicalonTM SiC fiber were obtained using Eq. (1) and the results are summarized in Fig. 4. An example of optical photographs of polished longitudinal cross-section of the two composites after monotonic tension testing, showing matrix cracking (T=298 K, Hi-Nicalon/BN/SiC). Table 2 Tensile experimental results of both the Nicalon/C/SiC and Hi-Nicalon/BN/SiC Composite materials Test temperature T (K) Young’s modulus Ec (GPa) Apparent matrix cracking stress mc (MPa) Measured tensile strength TS (MPa) Strain to failure c (%) Predicted tensile strength TS Nicalon/C/SiC 298 58 5 65 8 136 19 0.42 0.1 177 800 55 4 55 5 140 12 0.38 0.08 175 1200 49 4 33 3 41 5 0.075 0.015 127 Hi-Nicalon/BN/SiC 298 80 5 75 9 226 11 0.84 0.12 223 1200 76 4 70 7 237 6 0.9 0.06 228 1400 60 3 50 5 197 15 0.68 0.09 209 2352 S. Guo, Y. Kagawa / Journal of the European Ceramic Society 22 (2002) 2349–2356