BRENNAN: INTERFACIAL CHARACTERIZATION 4627 Dense Fiber Porous Fiber rv Fig. 12. AFM surface roughness analysis of dense versus porous Sylramic SiC fibers(porous fibers exhibit much rougher surfaces) cess. From tests conducted on minicomposites with Hi-Nicalon sic fibers and cvi SiC matrices with a number of different bn interfaces at 700-800oC in furnaces with water contents up to 90%, it has been found that accelerated oxidation and recession of the BN can occur [15]. Therefore, a series of furnace tests and tensile fatigue tests was conducted with the Syl ramic SiC fiber/BN interface MI SiC/SiC composites nder moist, high-temperature environments Figure 13 shows the fracture surface of an MI SiC/SiC composite after tensile fatigue testing at 760 C under a maximum stress of 1 17 MPa in a 90% 10. 0 ky 10um steam atmosphere. The sample failed after 44 h(22 cycles), which is significantly less than the thousands of hours to failure in a normal air environment. as was discussed earlier. From Fig. 13. it can be seen that the Bn fiber/matrix interfacial coating is entirel oxidized away, due to the volatile HBO phases that can form in high-temperature moist environments [15]. In order to prevent this, experiments have begun to investigate the moisture stability of Si-doped BN since this approach was found to result in the forma- nantly silica glass instead of boria, which tended to seal the interphase and resulted in minimal BN interphase recession [ 15]. Early results obtained on the Si-doped BN interface approach have Fig. 13. Fracture surface of an MI SiC/SiC indicated that the high-temperature moist environ- tensile fatigue testing at 760"C in a 90% steam atmosphere mental stability of the MI SiC/SiC composites under (BN interfacial layer oxidized ). fatigue test conditions is improved for atmospheres with relatively low moisture content(20%H]O), but not significantly improved for atmospheres with high 4. CONCLUSIONS moisture content(90% H_). Additional work on Under the HSCT/EPM ceramic composite combus environmentally stable interfaces for MI SiC/SiC tor program, a slurry-cast, Si-melt-infiltrated SiC opposites is recommended. fiber-reinforced Si/SiC matrix composite with a BNBRENNAN: INTERFACIAL CHARACTERIZATION 4627 Fig. 12. AFM surface roughness analysis of dense versus porous Sylramic SiC fibers (porous fibers exhibit much rougher surfaces). cess. From tests conducted on minicomposites with Hi-Nicalon SiC fibers and CVI SiC matrices with a number of different BN interfaces at 700–800°C in furnaces with water contents up to 90%, it has been found that accelerated oxidation and recession of the BN can occur [15]. Therefore, a series of furnace tests and tensile fatigue tests was conducted with the Syl￾ramic SiC fiber/BN interface MI SiC/SiC composites under moist, high-temperature environments. Figure 13 shows the fracture surface of an MI SiC/SiC composite after tensile fatigue testing at 760°C under a maximum stress of 117 MPa in a 90% steam atmosphere. The sample failed after 44 h (22 cycles), which is significantly less than the thousands of hours to failure in a normal air environment, as was discussed earlier. From Fig. 13, it can be seen that the BN fiber/matrix interfacial coating is entirely oxidized away, due to the volatile HBO phases that can form in high-temperature moist environments [15]. In order to prevent this, experiments have begun to investigate the moisture stability of Si-doped BN, since this approach was found to result in the forma￾tion of predominantly silica glass instead of boria, which tended to seal the interphase and resulted in minimal BN interphase recession [15]. Early results obtained on the Si-doped BN interface approach have indicated that the high-temperature moist environ￾mental stability of the MI SiC/SiC composites under fatigue test conditions is improved for atmospheres with relatively low moisture content (20% H2O), but not significantly improved for atmospheres with high moisture content (90% H2O). Additional work on environmentally stable interfaces for MI SiC/SiC composites is recommended. Fig. 13. Fracture surface of an MI SiC/SiC composite after tensile fatigue testing at 760°C in a 90% steam atmosphere (BN interfacial layer oxidized). 4. CONCLUSIONS Under the HSCT/EPM ceramic composite combus￾tor program, a slurry-cast, Si-melt-infiltrated SiC- fiber-reinforced Si/SiC matrix composite with a BN