Journal of the american Ce via chemical vapor deposition, onto a pitch-based carbon-fiber Oxidation of SiC!Sic composites with carbon interphases can sult in the formation of SiO2 temperature. Tortorelli and More observed an initial weight loss, followed by a weight gain, in a Nicalon- fiber-reinforced Sic sic composite with a 0.3 um thick carbon layer exposed to dry air (Po, =2X 10 Pa)at 1223 K. Following the initial weight loss from oxidation of the carbon, they found that Sio, formation ccurred within the interfacial region previously occupied by the carbon. For the Nicalon-reinforced composite material, complete 0E+001E+042E+04 carbon depletion occurred within 900 s at 1223 K, followed by weight gain from SiO, formation. Unal et al. observed their largest Time, s weight loss(5%)at 1223 K, for an exposure of 1.8 X 10 s in dry xygen, and a decreasing weight loss with increasing temperature Fig.1.TGA mass loss, as a function of time, for Sic/sic with carbon up to 1673 K (2%). Their material was a Nicalon-reinforced interfaces exposed to various Po, values at 1373 K(O)2.4 x 10, (o) SiC/SiC with a 0.5 um thick fiber-matrix carbon interphase Kleykamp et al. observed the following reactions of air with Sic-fiber-reinforced SiC composites: (1)oxidation of free carbon at 800-965K, (2)a rapid exothermic reaction and weight gain 0.20 beginning at 1073 K and continuing up to 1773 K, and for times up to 3.6X 10 s, followed by (3)the diffusion-controlled 0.15 oxidation of bulk SiC. Sebire-Lhermitte et al. 7 identified the presence and location of SiO, formation in SiC/SiC composites using transmission electron microscopy(TEM). They noted the g0.10 presence of 15 nm thick SiO, layers at both the fiber-carbon and the matrix-carbon interfaces following exposure for 3.6 X 10sat 1123 K in air That Windisch et al. o observed a weight loss alone. whereas others observed a weight gain following the weight loss, could be the result of lower oxygen pressures, shorter exposure time, and, perhaps, greater carbon-layer thickness. The lower oxygen IE+042E+04 pressure would decrease the SiO, formation rate and, therefore, the chance for a measurable weight gain during the 1. 8X 10-s Time. s xposures. Tortorelli et al.5,6 used exposures of up to 5.4X 105 s ig. 2. TGA mass loss, as a function of time for SiC/SiC with carbon but observed weight loss alone for times <1. X 10 s, with temperatures((D)1373, (O)1273, (O)1173, and(4)l0? e at various exposure at 1223 K, at Po2=2 X 10 Pa, for a composite with a interfaces exposed to 2.5 x 10 Pa of oxygen press 0.3 um thick carbon interphase. Unal et al. used 1.8X 105 s exposures. Measurable SiO, formation at Po. <2 X 10 Pa would require a much greater exposure time than that used by Windisch identical, suggesting that the same chemical reaction was control- et al. and even greater than the time used by Tortorelli and More ng. Conceivably, some glass formation occurred during these The existence of subcritical crack growth, as described in the next measurements, although for the weight-loss measurements to giv section, which coincides with weight loss alone or interphase greater recession rates than those derived by optical microscop removal without the embrittling effect of SiO, or other solid- would have required that the weight gain from glass formation roduct formation, is the primary difference between the IRM and I. Weight loss alone also was observed over the temperature the IRM only. 1073-1373 K, which borders on the temperature range 873-1073 K suggested by Evans et al. for the OEM The material studied by windisch et al. u was reinforced with Ill. Interphase Removal Mechanism ceramic-grade Nicalon fibers, coated with a 1.0 um thick carbon nterphase. Lewinsohn et al. measured the rate of interphase 1) Subcritical Crack Growth behavior oxidation for up to 7.2 X 10 s, at 1073 K in air, for effect of oxygen on the subcritical crack growth velocity of reinforced by Hi-Nicalon' fibers and with a I um thick SiC/ SiC is clearly demonstrated by the data given in Fig. 3 The interphase recession distance increased linearly Oxygen has little effect on the midpoint displacement (i.e, crack velocity) for -2 X 10- s, but a marked increase in the crack under these conditions. Based on Eq. (1) by Windisch et al. the velocity is noted for longer times. These tests were performed in carbon interphase RR was predicted to be 0. 28 Hum/s at 1073 K in air. the O2 pressure, temperature, and time regime where weight loss The experimental results agree alone was observed during oxidation studies. Therefore, the recession rate, considering the po of slight differences in embrittling effect of a solid reaction product should not have been the carbon interphase materials car y differences in process- ing. Furthermore, the ion rate and linear time should have contributed to the crack growth rate. However, even if nce also are lent with the values measured b SiO or other glassy phases had been present, they would have had Eckel et al. for the of the carbon core of an SCs-6 low viscosity at this high temperature and would not likely have fiber (SCS-6 fibers ar ated by depositing silicon carbide, affected the crack growth behavior or caused brittle fracture The dependence of the crack velocity on oxygen partial pressure up to Po, =2 X 10 Pa is given in Fig. 4 for tests at 1373K.A sharp increase in the crack velocity occurred at low pressure and a ippon Carbon Co., Tokyo, Japan. slower increase at pressures of -0.25 x 10- to 2.5X 10- Pa.identical, suggesting that the same chemical reaction was control￾ling. Conceivably, some glass formation occurred during these measurements, although for the weight-loss measurements to give greater recession rates than those derived by optical microscopy would have required that the weight gain from glass formation be compensated for by weight loss from the fibers. The thermody￾namic and kinetic results of the study by Windisch et al. are in agreement with those reported by Filipuzzi et al. 12,13 and Eckel et al.14 Weight loss alone also was observed over the temperature range 1073–1373 K, which borders on the temperature range 873–1073 K suggested by Evans et al. 1 for the OEM. The material studied by Windisch et al. 10 was reinforced with ceramic-grade Nicalon† fibers, coated with a 1.0 mm thick carbon interphase. Lewinsohn et al. 15 measured the rate of interphase oxidation for up to 7.2 3 103 s, at 1073 K in air, for materials reinforced by Hi-Nicalon† fibers and with a 1 mm thick interphase. The interphase recession distance increased linearly with time, at a rate of 0.19 mm/s. There was no evidence of SiO2 formation under these conditions. Based on Eq. (1) by Windisch et al., the carbon interphase RR was predicted to be 0.28 mm/s at 1073 K in air. The experimental results agree quite well with the predicted recession rate, considering the possibility of slight differences in the carbon interphase materials caused by differences in process￾ing. Furthermore, the measured recession rate and linear time dependence also are in agreement with the values measured by Eckel et al. 14 for the recession of the carbon core of an SCS-6 fiber. (SCS-6 fibers are fabricated by depositing silicon carbide, via chemical vapor deposition, onto a pitch-based carbon-fiber core.) Oxidation of SiCf /SiC composites with carbon interphases can also result in the formation of SiO2 and a weight gain, following an initial weight loss,5,6 or a decreased weight loss, with increasing temperature.7 Tortorelli and More5 observed an initial weight loss, followed by a weight gain, in a Nicalon-fiber-reinforced SiCf /SiC composite with a 0.3 mm thick carbon layer exposed to dry air (pO2 5 2 3 104 Pa) at 1223 K. Following the initial weight loss from oxidation of the carbon, they found that SiO2 formation occurred within the interfacial region previously occupied by the carbon. For the Nicalon-reinforced composite material, complete carbon depletion occurred within 900 s at 1223 K, followed by weight gain from SiO2 formation. Unal et al. 7 observed their largest weight loss (5%) at 1223 K, for an exposure of 1.8 3 105 s in dry oxygen, and a decreasing weight loss with increasing temperature, up to 1673 K (2%). Their material was a Nicalon-reinforced SiCf /SiC with a 0.5 mm thick fiber–matrix carbon interphase. Kleykamp et al. 16 observed the following reactions of air with SiC-fiber-reinforced SiC composites: (1) oxidation of free carbon at 800–965 K, (2) a rapid exothermic reaction and weight gain, beginning at 1073 K and continuing up to 1773 K, and for times up to 3.6 3 103 s, followed by (3) the diffusion-controlled oxidation of bulk SiC. Sebire-Lhermitte et al. 17 identified the presence and location of SiO2 formation in SiCf /SiC composites using transmission electron microscopy (TEM). They noted the presence of 15 nm thick SiO2 layers at both the fiber–carbon and the matrix–carbon interfaces following exposure for 3.6 3 103 s at 1123 K in air. That Windisch et al. 10 observed a weight loss alone, whereas others5–7 observed a weight gain following the weight loss, could be the result of lower oxygen pressures, shorter exposure time, and, perhaps, greater carbon-layer thickness. The lower oxygen pressure would decrease the SiO2 formation rate and, therefore, the chance for a measurable weight gain during the 1.8 3 104 s exposures. Tortorelli et al. 5,6 used exposures of up to 5.4 3 105 s but observed weight loss alone for times ,1.4 3 104 s, with exposure at 1223 K, at pO2 5 2 3 104 Pa, for a composite with a 0.3 mm thick carbon interphase. Unal et al. 7 used 1.8 3 105 s exposures. Measurable SiO2 formation at pO2 , 2 3 103 Pa would require a much greater exposure time than that used by Windisch et al. and even greater than the time used by Tortorelli and More. The existence of subcritical crack growth, as described in the next section, which coincides with weight loss alone or interphase removal without the embrittling effect of SiO2 or other solid￾product formation, is the primary difference between the IRM and the OEM. The oxidation results of Windisch et al. demonstrate that the results of Henager and Jones8 and Jones et al. 9 at temperatures ranging from 1073 to 1473 K and pO2 , 2 3 103 Pa occurred by the IRM only. III. Interphase Removal Mechanism (1) Subcritical Crack Growth Behavior The effect of oxygen on the subcritical crack growth velocity of SiCf /SiC is clearly demonstrated by the data given in Fig. 3. Oxygen has little effect on the midpoint displacement (i.e., crack velocity) for ;2 3 104 s, but a marked increase in the crack velocity is noted for longer times. These tests were performed in the O2 pressure, temperature, and time regime where weight loss alone was observed during oxidation studies.10 Therefore, the embrittling effect of a solid reaction product should not have been a factor; only the effect of fiber creep and interfacial removal should have contributed to the crack growth rate. However, even if SiO2 or other glassy phases had been present, they would have had low viscosity at this high temperature and would not likely have affected the crack growth behavior or caused brittle fracture. The dependence of the crack velocity on oxygen partial pressure up to pO2 5 2 3 103 Pa is given in Fig. 4 for tests at 1373 K. A sharp increase in the crack velocity occurred at low pressure and a slower increase at pressures of ;0.25 3 102 to 2.5 3 103 Pa. † Nippon Carbon Co., Tokyo, Japan. Fig. 1. TGA mass loss, as a function of time, for SiC/SiC with carbon interfaces exposed to various pO2 values at 1373 K ((M) 2.4 3 104 , (e) 2.5 3 103 , (E) 1.2 3 103 , (‚) 6.3 3 102 and (ƒ) 3.1 3 102 Pa). Fig. 2. TGA mass loss, as a function of time, for SiC/SiC with carbon interfaces exposed to 2.5 3 103 Pa of oxygen pressure at various temperatures ((M) 1373, (e) 1273, (E) 1173, and (‚) 1073 K). 2000 Journal of the American Ceramic Society—Jones et al. Vol. 83, No. 8