M. Takeda et al. Composites Science and Technology 59(1999)813-819 Hi-Nicalon 3 15 0.5 0 140016001800 1200140016001800 Received Temperature( K) Received Temperature(K) Fig. 11. Tensile strength of SiC fibers after 10 h exposure at elevated Fig. 12. Oxide layer thickness of Sic fibers after 10 h exposure at ele- temperature in vated temperature in air(% H2O) would inhibit crystal growth. Nicalon'M contains less 1673 K exposure in argon for 10 h. Fig. 10 shows the excess carbon after thermal exposure because it decom- crystallite sizes of ceramic fibers, Hi-Nicalon, Nica- poses into near-stoichiometric composition. The Si-C lonM, TyrannoTM, and HPZ, after the thermal exposure O ceramic material prepared by oxidation-curing of test. The crystallite sizes of any fibers grow with increasing PCS loses carbon and oxygen as CO evolution by the temperature. The crystallite size of the oxidation-cured carbothermal reduction and changes into a nearly stoi- fibers, Nicalon M and Tyranno TM, significantly increases chiometric composition of SiC. This near-stoichiometric above 1673 K. On the other hand, the irradiation-cured SiC material tends to grow into larger crystals at a fiber, Hi-Nicalon TM has smaller crystal growth above higher temperature than a Sic containing excess carbon 1773 K compared to the oxidation-cured fiber, because of prepared by the irradiation-curing process Fig 9 shows the presence of excess carbon in the fiber. HPZ would KRD patterns for the HPZ fiber. The HPZ fiber shows decompose to form free silicon after thermal exposure. amorphous XRD characteristics in the as-received state Excess silicon in SiC accelerates crystal growth extensively and after exposure at 1573 K for 10 h, although exten The oxidation-cured SiC fibers, NicalonTM and Tyr sive crystallization of SiC and Si, N4 was observed after annoTM, decreased in their weight. The Si-C-N-O βsicB-S|c Hi-Nicalon 673 K 10 h treated 1673 K 1 h treated As Fabricated 10 20 30 40 Fig. 13. XRD patterns of SiC fibers before and after 1673 K exposure for 10 h in air(2% H2O)would inhibit crystal growth. NicalonTM contains less excess carbon after thermal exposure because it decom￾poses into near-stoichiometric composition. The Si±C± O ceramic material prepared by oxidation-curing of PCS loses carbon and oxygen as CO evolution by the carbothermal reduction and changes into a nearly stoi￾chiometric composition of SiC. This near-stoichiometric SiC material tends to grow into larger crystals at a higher temperature than a SiC containing excess carbon prepared by the irradiation-curing process. Fig. 9 shows XRD patterns for the HPZ ®ber. The HPZ ®ber shows amorphous XRD characteristics in the as-received state and after exposure at 1573 K for 10 h, although exten￾sive crystallization of SiC and Si3N4 was observed after 1673 K exposure in argon for 10 h. Fig. 10 shows the crystallite sizes of ceramic ®bers, Hi-NicalonTM, Nica￾lonTM, TyrannoTM, and HPZ, after the thermal exposure test. The crystallite sizes of any ®bers grow with increasing temperature. The crystallite size of the oxidation-cured ®bers, NicalonTM and TyrannoTM, signi®cantly increases above 1673 K. On the other hand, the irradiation-cured ®ber, Hi-NicalonTM has smaller crystal growth above 1773 K compared to the oxidation-cured ®ber, because of the presence of excess carbon in the ®ber. HPZ would decompose to form free silicon after thermal exposure. Excess silicon in SiC accelerates crystal growth extensively. The oxidation-cured SiC ®bers, NicalonTM and Tyr￾annoTM, decreased in their weight. The Si±C±N±O Fig. 11. Tensile strength of SiC ®bers after 10 h exposure at elevated temperature in air (2% H2O). Fig. 12. Oxide layer thickness of SiC ®bers after 10 h exposure at ele￾vated temperature in air (2% H2O). Fig. 13. XRD patterns of SiC ®bers before and after 1673 K exposure for 10 h in air (2% H2O). 818 M. Takeda et al. / Composites Science and Technology 59 (1999) 813±819