December 1999 Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites 08 0 Energy(kev) Fig 3. Energy spectrum of BN coating after I h at 1400C even at 1400.C. The stability of the BN coating and the pres- ence of SiO,(and not silicates) at the BN/fiber interface sug gest that the observed changes resulted from oxidation of th Nicalon fiber surface. Ample experimental evidence showed the formation of an amorphous SiO, layer on the fiber surface 0.8 material(Fig. I(A). The edax results for this region also are 0.6 plotted in Fig. 4. Those results show that carbon disappeared and an SiO2 layer formed during high-temperature exposure in an inert environment. However, the layer was significantly thinner than that formed after I h of exposure at 1200C in Fig. 2(B)) The differences in the BN/fiber interlayer after I h at 1200oC in air and I h in argon manifest the critical contribution of oxygen from the environment to the modification of the inter- face microstructure. This relationship poses a question about Energy(kev) the diffusion pathway to the fiber/BN interface. Sun et al 12 suggested two possible diffusion pathways. Oxygen can diffuse TEM photogra to the interface through matrix microcracks that penetrate the here, energy spe SiC outer coating, or via pipeline diffusion along the fiber/BN with arrows interface, which starts from the cut ends of fiber exposed to the composite surface. Oxygen diffusion also is favored by th show that the Al O3 matrix in the as-received condition is and contained significant amounts of carbon and oxygen. The fractured, because of the thermal stresses generated during most significant difference from the previous composite was ooling, a result of the mismatch in the thermal expansion the absence of an interlayer region at the BN/fiber interface coefficients of the constituents. Those analyses also show that (Fig. 5(A). EDAX of the interfacial region revealed no carbon- the matrix cracks fail to penetrate the thick outer SiC coating, rich layer. which prevents oxygen access to the interface. Thus, pipeline The samples treated at 800 and 1200oC (Fig. 5(B))for I h diffusion along the BN/fiber interface is the most likely diffu- presented characteristics similar to those found in the Al2O3 ion pathway for the oxygen, in agreement with the results of matrix composite: Namely, the BN coating was unaffected by heat treatment, and the BN/SiC interface remained sharp and BN/Nicalon interfaces. I5 The carbon-rich interlayer may facili- free from any reaction products. The only noticeable difference tate this process, because the carbon is burned out rapidly at high temperature and oxygen access to the sample becomes after I h at 800oC and an interlayer developed only after Ihat easier. Changes in the interface microstructure as a function of 1200C(Fig. 5(B). However, the interlayer that developed the distance to the surface should be observable if pipeline was significantly thinner than that in the Al2O3-matrix com- diffusion is dominant but, unfortunately, all of the present TEM posite (20 nm versus 100 nm). The energy spectrum of this samples were taken from the center of the composites region(Fig. 5(B) showed an increase in the oxygen content, as compared to the same region in the as-received material. Nev- (2) Si-C-N-Matrir Composite ertheless, the interface oxidation was significantly lower in this The morphology of the Si-C-N te was siml- composite than in the Al2O3-matrix material to that of its Al2O3 counterpart. The Si-C-N matrix intro- Although the experimental evidence is limited, pipeline dif. duced by polymer infiltration and pyrolysis filled the gaps fusion along the BN/fiber interface again seems to be the most between the CVd SiC coatings, which surrounded the nica likely pathway for the access of oxygen to the interface in this fibers. The BN layer between the fiber and the external Sic composite, because the SiC outer coating prevents the penetra coating was thicker(300 nm) than that of the Al2O3-matrix tion of oxygen from pores and cracks in the matrix. Accordingeven at 1400°C. The stability of the BN coating and the pres￾ence of SiO2 (and not silicates) at the BN/fiber interface sug￾gest that the observed changes resulted from oxidation of the Nicalon fiber surface. Ample experimental evidence showed the formation of an amorphous SiO2 layer on the fiber surface when samples were exposed to high temperature in air.23–25 Finally, the appearance of the interface after 1 h at 1200°C under an argon atmosphere is shown in Fig. 4. The contrast of the BN/fiber interlayer is more marked than in the as-received material (Fig. 1(A)). The EDAX results for this region also are plotted in Fig. 4. Those results show that carbon disappeared and an SiO2 layer formed during high-temperature exposure in an inert environment. However, the layer was significantly thinner than that formed after 1 h of exposure at 1200°C in air (Fig. 2(B)). The differences in the BN/fiber interlayer after 1 h at 1200°C in air and 1 h in argon manifest the critical contribution of oxygen from the environment to the modification of the inter￾face microstructure. This relationship poses a question about the diffusion pathway to the fiber/BN interface. Sun et al.12 suggested two possible diffusion pathways. Oxygen can diffuse to the interface through matrix microcracks that penetrate the SiC outer coating, or via pipeline diffusion along the fiber/BN interface, which starts from the cut ends of fiber exposed to the composite surface. Oxygen diffusion also is favored by the submicrometer-scale porosity of the BN coatings deposited by CVD. Previous microstructural analyses of this composite13,26 show that the Al2O3 matrix in the as-received condition is fractured, because of the thermal stresses generated during cooling, a result of the mismatch in the thermal expansion coefficients of the constituents. Those analyses also show that the matrix cracks fail to penetrate the thick outer SiC coating, which prevents oxygen access to the interface. Thus, pipeline diffusion along the BN/fiber interface is the most likely diffu￾sion pathway for the oxygen, in agreement with the results of other investigations on the oxidation of carbon/Nicalon9 and BN/Nicalon interfaces.15 The carbon-rich interlayer may facili￾tate this process, because the carbon is burned out rapidly at high temperature and oxygen access to the sample becomes easier. Changes in the interface microstructure as a function of the distance to the surface should be observable if pipeline diffusion is dominant but, unfortunately, all of the present TEM samples were taken from the center of the composites. (2) Si–C–N-Matrix Composite The morphology of the Si–C–N-matrix composite was simi￾lar to that of its Al2O3 counterpart. The Si–C–N matrix intro￾duced by polymer infiltration and pyrolysis filled the gaps between the CVD SiC coatings, which surrounded the Nicalon fibers. The BN layer between the fiber and the external SiC coating was thicker (∼300 nm) than that of the Al2O3-matrix composite, and its energy spectrum was analogous to that plot￾ted in Fig. 1(B). The BN also exhibited a turbostratic structure and contained significant amounts of carbon and oxygen. The most significant difference from the previous composite was the absence of an interlayer region at the BN/fiber interface (Fig. 5(A)). EDAX of the interfacial region revealed no carbon￾rich layer. The samples treated at 800° and 1200°C (Fig. 5(B)) for 1 h presented characteristics similar to those found in the Al2O3- matrix composite: Namely, the BN coating was unaffected by heat treatment, and the BN/SiC interface remained sharp and free from any reaction products. The only noticeable difference was at the BN/fiber interface. No distinct interlayer was seen after 1 h at 800°C and an interlayer developed only after 1 h at 1200°C (Fig. 5(B)). However, the interlayer that developed was significantly thinner than that in the Al2O3-matrix com￾posite (20 nm versus 100 nm). The energy spectrum of this region (Fig. 5(B)) showed an increase in the oxygen content, as compared to the same region in the as-received material. Nev￾ertheless, the interface oxidation was significantly lower in this composite than in the Al2O3-matrix material. Although the experimental evidence is limited, pipeline dif￾fusion along the BN/fiber interface again seems to be the most likely pathway for the access of oxygen to the interface in this composite, because the SiC outer coating prevents the penetra￾tion of oxygen from pores and cracks in the matrix. According Fig. 3. Energy spectrum of BN coating after 1 h at 1400°C. Fig. 4. TEM photograph of BN coating after 1 h at 1200°C in argon atmosphere; energy spectrum corresponding to the BN/fiber interlayer (marked with arrows in the micrograph) also is plotted. December 1999 Oxidation of BN/Nicalon Fiber Interfaces in Ceramic-Matrix Composites 3497