CERAMIC COMPOSITE INTERFACES 511 rate of formation of the carbon layer depends upon the initial oxygen activity n the glass. In pure SiO,, for example, oxygen activity is low and no carbon layer is observed (4). The multicomponent glasses having variable valence ions like Nb2O5 show enhanced oxygen activity and carbon formation. The Qi et al model(47) predicts carbon layer thickness and growth times for a Las glass The drawback of carbon resides in its lack of stability at elevated temper- atures in oxidizing environments. The interphase coating vanishes and is re- placed by an oxidation product that strongly bonds the fiber to the matrix, as shown in Figure 6b. The work of Lu& Evans (48)on strength degradation after high-temperature exposure highlighted the seriousness of the problem Mechanical property measurements coupled with electron microscopy demon strate unequivocally that the carbon interlayer in SiC-LAS composites evapo- rates, leaving a strong SiO2 interlayer. Eckel et al (49)examined the oxidation kinetics of a continuous carbon phase through analytical modeling and the ex perimental measurement of recession rates of carbon and found them to be best described by a linear-parabolic law. In the temperature regime 800 to 1200oC rapid oxidation of the carbon interphase is found to be nearly independent of crack opening (in the matrix or the debonded region), the partial pressure of oxygen, and the total system pressure. This easily explains the severe reduction in strength observed after high-temperature exposure. A method for determin- ing the"pest temperature, i.e. the lowest temperature at which embrittlement is observed, has been designed by Heredia et al (50) In an effort to preclude the damaging oxidation effects in composites with arbon interphases, boron nitride has been explored as a logical alternative (51-53). Boron nitride has a crystal structure similar to that of carbon and better oxidation resistance. Such layers are mechanically equivalent and pro- vide effective deflection paths. Sun et al (54-56)have examined dual phase SiC/BN coatings in glass-ceramic matrices to preclude oxidation of the carbon nterlayer. The boron nitride layer provides the necessary low-fracture energy provided it is amorphous or partially turbostatic and contains excess carbon the sic acts as a diffusion barrier to boron diffusion into the matrix and matrix elements into the boron nitride. This combination provides stability to tem- peratures of 1100oC in fast fracture and long-term loading(creep). Oxidation of near-surface fibers and along interfaces when the matrix cracking stress exceeded was observed under cyclic loading, which may limit application in thermal fatigue situations. Long-term stability of the boron nitride in the pres- ence of Sic has been studied by Lee Jacobson(57). At temperatures in excess of 1400oC, boron nitride layers deplete measurably the boron nitride coating have bee ed by moore et al (58)who added silicon to pyrolytic boron nitride for SiC fiber coa atingsP1: ARK/MBL/rkc P2: MBL/vks QC: MBL/agr T1: MBL May 16, 1997 13:47 Annual Reviews AR034-16 CERAMIC COMPOSITE INTERFACES 511 rate of formation of the carbon layer depends upon the initial oxygen activity in the glass. In pure SiO2, for example, oxygen activity is low and no carbon layer is observed (4). The multicomponent glasses having variable valence ions like Nb2O5 show enhanced oxygen activity and carbon formation. The Qi et al model (47) predicts carbon layer thickness and growth times for a LAS glass containing As2O3+x. The drawback of carbon resides in its lack of stability at elevated temper￾atures in oxidizing environments. The interphase coating vanishes and is re￾placed by an oxidation product that strongly bonds the fiber to the matrix, as shown in Figure 6b. The work of Lu & Evans (48) on strength degradation after high-temperature exposure highlighted the seriousness of the problem. Mechanical property measurements coupled with electron microscopy demon￾strate unequivocally that the carbon interlayer in SiC-LAS composites evapo￾rates, leaving a strong SiO2 interlayer. Eckel et al (49) examined the oxidation kinetics of a continuous carbon phase through analytical modeling and the ex￾perimental measurement of recession rates of carbon and found them to be best described by a linear-parabolic law. In the temperature regime 800 to 1200◦C, rapid oxidation of the carbon interphase is found to be nearly independent of crack opening (in the matrix or the debonded region), the partial pressure of oxygen, and the total system pressure. This easily explains the severe reduction in strength observed after high-temperature exposure. A method for determin￾ing the “pest” temperature, i.e. the lowest temperature at which embrittlement is observed, has been designed by Heredia et al (50). In an effort to preclude the damaging oxidation effects in composites with carbon interphases, boron nitride has been explored as a logical alternative (51–53). Boron nitride has a crystal structure similar to that of carbon and better oxidation resistance. Such layers are mechanically equivalent and pro￾vide effective deflection paths. Sun et al (54–56) have examined dual phase SiC/BN coatings in glass-ceramic matrices to preclude oxidation of the carbon interlayer. The boron nitride layer provides the necessary low-fracture energy provided it is amorphous or partially turbostatic and contains excess carbon; the SiC acts as a diffusion barrier to boron diffusion into the matrix and matrix elements into the boron nitride. This combination provides stability to tem￾peratures of 1100◦C in fast fracture and long-term loading (creep). Oxidation of near-surface fibers and along interfaces when the matrix cracking stress is exceeded was observed under cyclic loading, which may limit application in thermal fatigue situations. Long-term stability of the boron nitride in the pres￾ence of SiC has been studied by Lee & Jacobson (57). At temperatures in excess of 1400◦C, boron nitride layers deplete measurably. Some improvements in the boron nitride coating have been reported by Moore et al (58) who added silicon to pyrolytic boron nitride for SiC fiber coatings