2632 J. Martinez.Fernandez, G N Morscher Journal of the European Ceramic Society 20(2000)2627-2636 No Sio.o atrix surfac pR matrix face Fig. 7. Fracture surface of a C-HN minicomposite for stress rupture at 1200.C (439 MPa, 217 h). Hn versus C-NIC are clearly related with the better strength to 2100 MPa was used for comparison thermal stability of the HN fibers. The stress-rupture as-produced fiber rupture properties with the resistance of the C-Hn minicomposites, however, is composite stress-rupture data. If the fiber degradation is inferior to BN-HN(Fig 3b) merely due to the fiber degradation observed for as- In order to compare minicomposite data at different produced fibers, the minicomposite rupture data should temperatures and fiber fast fracture data, the Larson- decrease the same as the fiber data if the starting Miller(LM)approach, Io was used. In this empirical strength of the fibers were 2100 MPa approach the effect of temperature and time are com- The C-HN and BN-HN minicomposite properties bined in one parameter: are closer to the individual fiber rupture behavior at low and high temperatures. At intermediate temperatures q= T(logar +O ( there is degradation of the minicomposite behavior compared to the individual fiber because of the embrit tlement associated with fiber-matrix bonding due to where T is the temperature(K), fr the time(h), and C SiO2 formation. Fig 8 shows the improvement in rup- the LM constant, which was found to be 22 for both ture properties of BN interphase over the C interphase NIC and Hn fibers. In Fig. 8, data obtained in this It is also clear from Fig 8 that C-NIC suffered more and previous studies are plotted as the applied rupture serious fiber degradation due to oxidation than C-HN stress versus the LM parameter(). The room tempera At 700C, the loss of carbon by oxidation results in ture(rT) failure stress was 2020, 2300, and 2100 MPa long load-bearing gage lengths for the Hn fibers. The for C-HN, C-NIC, and BN-HN, respectively. Also oxidation kinetics of a continuous carbon interphase plotted in Fig. 8 is the data for stress-rupture of as- a composite has been published in the literature. 9,20 If produced HN fibers. The room temperature strength we assume that the oxidation is controlled by the diffu of the HN fibers studied in Ref. 17 was 2800 MPa. It sion of oxygen, the recession length(5)is was shown in Ref 18 that HN-BN minicomposites with different RT strengths had the same rupture behavior at low and high temperatures as the fibers [i.e. same slope 5=klr/2 n In(o)vs q. However, the absolute rupture strengths were lower for the lower RT strength minicomposites in proportion to the absolute strength of the as-produced where kp is the parabolic rate constant and t the time fibers. Therefore, normalizing the rt as-produced fiber For an opening of 0. 4 um(carbon interface thickness ),HN versus C±NIC are clearly related with the better thermal stability of the HN ®bers. The stress-rupture resistance of the C-HN minicomposites, however, is inferior to BN±HN (Fig. 3b). In order to compare minicomposite data at di€erent temperatures and ®ber fast fracture data, the Larson± Miller (LM) approach15,16 was used. In this empirical approach the e€ect of temperature and time are com￾bined in one parameter: q ˆ T…† … logtR ‡ C 3† where T is the temperature (K), tR the time (h), and C the LM constant, which was found to be 22 for both NIC and HN ®bers.17 In Fig. 8, data obtained in this and previous studies2 are plotted as the applied rupture stress versus the LM parameter (q). The room tempera￾ture (RT) failure stress was 2020, 2300, and 2100 MPa for C±HN, C±NIC, and BN±HN, respectively. Also plotted in Fig. 8 is the data for stress-rupture of as￾produced HN ®bers.17 The room temperature strength of the HN ®bers studied in Ref. 17 was 2800 MPa. It was shown in Ref. 18 that HN±BN minicomposites with di€erent RT strengths had the same rupture behavior at low and high temperatures as the ®bers [i.e. same slope on ln() vs. q]. However, the absolute rupture strengths were lower for the lower RT strength minicomposites in proportion to the absolute strength of the as-produced ®bers. Therefore, normalizing the RT as-produced ®ber strength to 2100 MPa was used for comparison of the as-produced ®ber rupture properties with the mini￾composite stress-rupture data. If the ®ber degradation is merely due to the ®ber degradation observed for as￾produced ®bers, the minicomposite rupture data should decrease the same as the ®ber data if the starting strength of the ®bers were 2100 MPa. The C±HN and BN±HN minicomposite properties are closer to the individual ®ber rupture behavior at low and high temperatures. At intermediate temperatures there is degradation of the minicomposite behavior compared to the individual ®ber because of the embrit￾tlement associated with ®ber±matrix bonding due to SiO2 formation. Fig. 8 shows the improvement in rup￾ture properties of BN interphase over the C interphase. It is also clear from Fig. 8 that C±NIC su€ered more serious ®ber degradation due to oxidation than C±HN. At 700C, the loss of carbon by oxidation results in long load-bearing gage lengths for the HN ®bers. The oxidation kinetics of a continuous carbon interphase in a composite has been published in the literature.19,20 If we assume that the oxidation is controlled by the di€u￾sion of oxygen, the recession length () is:  ˆ k1=2 p t 1=2 …4† where kp is the parabolic rate constant and t the time. For an opening of 0.4 mm (carbon interface thickness), Fig. 7. Fracture surface of a C±HN minicomposite for stress rupture at 1200C (439 MPa, 217 h). 2632 J. Marti nez-FernaÂndez, G.N. Morscher / Journal of the European Ceramic Society 20 (2000) 2627±2636