N P Bansal /Journal of the European Ceramic Sociery 29(2009)525-535 1mm|[ 100 uml(d) 100pm Fig. 6. SEM micrographs of polished cross-sections of unidirectional Hi-Nicalon/BN/SIC/BSAS composites annealed at 1200'C for 100 h in ai up to 850C, probably due to the loss of absorbed moisture. exposed surfaces. Pores were present in the surface layer. Signs This is followed by a large weight increase, possibly due to the of partial melting and gas bubble formation during heat treatment oxidation of BN into B2O3 and also SiC to SiO2, particularly at were also observed. From XRD analysis, both amorphous and higher temperatures. The total weight gain is found to be 3%0. celsian phases were detected in the surface layer. Since such a behavior was not observed23 in monolithic bSas material even 3. 4. Thermal ageing in air after heat treatment for 20 h in air at 1500oc. it is assumed to be caused by the presence of Hi-Nicalon fibers and the BN/SiC CMC bars were heat treated in ambient air for 100h at var- coating. In the presence of air, BN is probably oxidized to B2O3 lous temperatures. Optical photographs showing the physical which reacts with the matrix and/or silica formed from the oxi- appearance of the CMc bars before and after annealing at var 1000 ious temperatures are shown in Fig. 4. No changes in physical appearance were observed in samples annealed at 1000C or lower. However, the specimen annealed at 1100C was covered with a thin porous white layer that could be easily removed by polishing with a fine emery paper. The samples aged at 1200C were deformed and developed a thick shiny white layer on the MC+ HC(trace) 550°C Strain. 0, deg coated Hi-Nicalon fiber-reinforced celsian matrix composites annealed in air for Fig. 7. X-ray diffraction pattern taken from the surface of as hot-pressed. Hi- 100 h at various temperatures.( For interpretation of the references to color in Nicalon/BN/SiC/BSAS composite MC: monoclinic celsian: HC: hexacelsian. the artwork, the reader is referred to the web version of the article.N.P. Bansal / Journal of the European Ceramic Society 29 (2009) 525–535 529 Fig. 6. SEM micrographs of polished cross-sections of unidirectional Hi-Nicalon/BN/SiC/BSAS composites annealed at 1200 ◦C for 100 h in air. up to ∼850 ◦C, probably due to the loss of absorbed moisture. This is followed by a large weight increase, possibly due to the oxidation of BN into B2O3 and also SiC to SiO2, particularly at higher temperatures. The total weight gain is found to be ∼3%. 3.4. Thermal ageing in air CMC bars were heat treated in ambient air for 100 h at var￾ious temperatures. Optical photographs showing the physical appearance of the CMC bars before and after annealing at var￾ious temperatures are shown in Fig. 4. No changes in physical appearance were observed in samples annealed at 1000 ◦C or lower. However, the specimen annealed at 1100 ◦C was covered with a thin porous white layer that could be easily removed by polishing with a fine emery paper. The samples aged at 1200 ◦C were deformed and developed a thick shiny white layer on the Fig. 7. X-ray diffraction pattern taken from the surface of as hot-pressed. Hi￾Nicalon/BN/SiC/BSAS composite. MC: monoclinic celsian; HC: hexacelsian. exposed surfaces. Pores were present in the surface layer. Signs of partial melting and gas bubble formation during heat treatment were also observed. From XRD analysis, both amorphous and celsian phases were detected in the surface layer. Since such a behavior was not observed23 in monolithic BSAS material even after heat treatment for 20 h in air at 1500 ◦C, it is assumed to be caused by the presence of Hi-Nicalon fibers and the BN/SiC coating. In the presence of air, BN is probably oxidized to B2O3 which reacts with the matrix and/or silica formed from the oxi￾Fig. 8. Apparent stress–strain curves recorded in three-point flexure for BN/SiC￾coated Hi-Nicalon fiber-reinforced celsian matrix composites annealed in air for 100 h at various temperatures. (For interpretation of the references to color in the artwork, the reader is referred to the web version of the article.)