816 M. Takeda et al./ Composites Science and Technology 59(1999)813-819 and retains it even after exposure at 1873 K. Fig. 6 retains a high strength of 1. 4 GPa and modulus of shows seM photographs of the fibers after thermal GPa even after 1873 K exposure exposure testing. NicalonTM forms small particles and The silicon oxycarbide-based fibers, NicalonM and changes to a rough fiber surface after the exposure at Tyranno TM, mainly decompose according to the following 1873 K. TyrannoM and HPZ fibers exhibit obvious reaction at elevated temperature smooth filament surface, showing no change in appea: 9 structural damage. However, Hi-Nicalon'M has SixC1O2→xSiC+zCO+( ance Fig. 7 shows SEM photographs of Nicalon and Hi-Nicalon'M after exposure at 2273 K for I h in argon. The weight loss of about 20% in Nicalon and Tyr Hi-NicalonTM has a smooth fiber surface, but, by con- anno M fibers will be explained by co gas evolution, trast, Nicalon M has changed into clusters of large Sic and some SiO evaporation would occur simultaneously crystals. The NicalonTM, TyrannoTM, and HPZ fibers In the case of Si-N-C-O fiber, the HPZ shows a more decreased substantially in their weight and strength complicated phenomenon. The weight loss of HPZ after the thermal exposure test, whereas, Hi-Nicalon M would be caused not only by Co or Sio evaporation As received 1873K 1ohours In argon Hi-Nicalon Nicalon (NL202) 10 um Tyranno LOX-M) HPZ 10 Fig. 6. SEM photographs of as received ceramic fibers and those after the exposure for 10 h in argon at 1873 K.and retains it even after exposure at 1873 K. Fig. 6 shows SEM photographs of the ®bers after thermal exposure testing. NicalonTM forms small particles and changes to a rough ®ber surface after the exposure at 1873 K. TyrannoTM and HPZ ®bers exhibit obvious structural damage. However, Hi-NicalonTM has a smooth ®lament surface, showing no change in appear￾ance. Fig. 7 shows SEM photographs of NicalonTM and Hi-NicalonTM after exposure at 2273 K for 1 h in argon. Hi-NicalonTM has a smooth ®ber surface, but, by con￾trast, NicalonTM has changed into clusters of large SiC crystals. The NicalonTM, TyrannoTM, and HPZ ®bers decreased substantially in their weight and strength after the thermal exposure test, whereas, Hi-NicalonTM retains a high strength of 1.4 GPa and modulus of 250 GPa even after 1873 K exposure. The silicon oxycarbide-based ®bers, NicalonTM and TyrannoTM, mainly decompose according to the following reaction at elevated temperature: SixCyOz ! xSiC ‡ zCO ‡ …y ÿ x ÿ z†C The weight loss of about 20% in NicalonTM and Tyr￾annoTM ®bers will be explained by CO gas evolution, and some SiO evaporation would occur simultaneously. In the case of Si±N±C±O ®ber, the HPZ shows a more complicated phenomenon. The weight loss of HPZ would be caused not only by CO or SiO evaporation, Fig. 6. SEM photographs of as received ceramic ®bers and those after the exposure for 10 h in argon at 1873 K. 816 M. Takeda et al. / Composites Science and Technology 59 (1999) 813±819