BRENNAN: INTERFACIAL CHARACTERIZATION Environmeatal degradation dimcult 2 Fig. I Microstructural differences between CVI and MI SiC/SiC composites porosity also leads to better environmental stability SiC fiber from Dow Corning Corp, Midland, MI and a higher proportional limit (matrix Both of these fibers have very low oxygen content, microcracking)stress. Figure 2 shows a thin-foil with the Hi-Nicalon fiber being carbon-rich while the transmission electron micrograph of an MI composite Sylramic fiber is very close to stoichiometic SiC ith Sylramic SiC fibers. The crystalline nature of Since the Sylramic fiber consists of 100-500 nm crys the fibers and the feathery CvI SiC layer can be seen tals of B-SiC, while the Hi-Nicalon fiber is a mixture clearly, in contrast to the more amorphous to turbos- of microcrystalline(2-20 nm) B-Sic plus turbostratic tratic nature of the bn interface coating domains of carbon, the Sylramic fiber has properties Once the fabrication method of the composite was much like those of sintered SiC; i.e., an elastic modu hosen, developmental efforts concentrated on the lus of -385 GPa and a thermal expansion coefficient type of SiC fiber to be utilized, the thickness and of -54x10-o/C [2]. The elastic modulus, therma chemistry of the BN fiber/matrix interface, and the expansion coefficient and thermal conductivity of the thermo-mechanical properties of the composite. Two Hi-Nicalon fiber are much lower than those of the different SiC fibers that were commercially available Sylramic fiber [2]. The following discussion covers were selected as the fiber candidates: Hi-Nicalon Sic the experimental results obtained for composites with fiber from Nippon Carbon Co., Japan, and Sylramic these two fibers, the rationale for the selection of th Sylramic fiber as the primary candidate, and the results of environmentally sensitive tests relating to the bn fiber/matrix interf 2. EXPERIMENTAl 2. 1. MI SiC/SiC composite fabrication The MI SiC/SiC composites utilized in the EPM program were initially fabricated by Carborundum nc, Niagara Falls, NY, and later in the program by REaver Allied Signal Composites, Inc, now called Ho Iss:ic Matris i. well Advanced Composites, Inc (HACD), Newark DE. The fabrication of these composites consisted of oating a two-dimensional(2D)woven SiC-fiber pre- form, usually five- or eight-harness satin, with an nterface coating of CVI BN, followed by an over- coating of CVI SiC. The rigidized preform was then subjected to infiltration by an SiC particulate slurry in order to fill the large residual porosity with SiC, followed by a melt infiltration of silicon metal which Fig. 2. TEM thin-foil analysis of the microstructure of a Syl. would fill the fine porosity left between the SiC grain particles. The fabrication and properties of similar MI4620 BRENNAN: INTERFACIAL CHARACTERIZATION Fig. 1. Microstructural differences between CVI and MI SiC/SiC composites. porosity also leads to better environmental stability and a higher proportional limit (matrix microcracking) stress. Figure 2 shows a thin-foil transmission electron micrograph of an MI composite with Sylramic SiC fibers. The crystalline nature of the fibers and the feathery CVI SiC layer can be seen clearly, in contrast to the more amorphous to turbos￾tratic nature of the BN interface coating. Once the fabrication method of the composite was chosen, developmental efforts concentrated on the type of SiC fiber to be utilized, the thickness and chemistry of the BN fiber/matrix interface, and the thermo-mechanical properties of the composite. Two different SiC fibers that were commercially available were selected as the fiber candidates: Hi-Nicalon SiC fiber from Nippon Carbon Co., Japan, and Sylramic Fig. 2. TEM thin-foil analysis of the microstructure of a Syl￾ramic SiC fiber MI SiC/SiC composite. SiC fiber from Dow Corning Corp., Midland, MI. Both of these fibers have very low oxygen content, with the Hi-Nicalon fiber being carbon-rich while the Sylramic fiber is very close to stoichiometic SiC. Since the Sylramic fiber consists of 100–500 nm crys￾tals of β-SiC, while the Hi-Nicalon fiber is a mixture of microcrystalline (2-20 nm) β-SiC plus turbostratic domains of carbon, the Sylramic fiber has properties much like those of sintered SiC; i.e., an elastic modu￾lus of |385 GPa and a thermal expansion coefficient of |5.4×1026 /°C [2]. The elastic modulus, thermal expansion coefficient and thermal conductivity of the Hi-Nicalon fiber are much lower than those of the Sylramic fiber [2]. The following discussion covers the experimental results obtained for composites with these two fibers, the rationale for the selection of the Sylramic fiber as the primary candidate, and the results of environmentally sensitive tests relating to the BN fiber/matrix interface. 2. EXPERIMENTAL 2.1. MI SiC/SiC composite fabrication The MI SiC/SiC composites utilized in the EPM program were initially fabricated by Carborundum, Inc., Niagara Falls, NY, and later in the program by Allied Signal Composites, Inc., now called Hone￾ywell Advanced Composites, Inc. (HACI), Newark, DE. The fabrication of these composites consisted of coating a two-dimensional (2D) woven SiC-fiber pre￾form, usually five- or eight-harness satin, with an interface coating of CVI BN, followed by an over￾coating of CVI SiC. The rigidized preform was then subjected to infiltration by an SiC particulate slurry in order to fill the large residual porosity with SiC, followed by a melt infiltration of silicon metal which would fill the fine porosity left between the SiC grain particles. The fabrication and properties of similar MI