K.L. More et al. /Composites: Part A 30 (1999)463-470 multiple layers of ceramic-grade Nicalon plain-weave fabric were stacked and rotated in a0 30 sequence in a graphite holder;(2)layers were compressed in the holder cavity to produce a preform having a nominal fiber loading of-40 vol %;()sizing was removed from fibers through multiple acetone washes; (4) preform was coated with BN deposited via chemical vapor deposition from gas mixtures (a) containing BCl3, NH,, and H2 at a temperature of 1100C and a reactor pressure of 5 kPa for 2 h, which resulted in an optimum coating thickness of -0.42 um [10]; and(5)the preforms were densified with SiC using the forced-flot thermal-gradient chemical vapor infiltration(FCVi) process which has been described in detail previously [11]. The final composite was 85-90% of theoretical density. Two slightly different Nicalon"/BN/SiC composites, both prepared as described above, were compared in this study; one compo- site had a BN interface coating with an oxygen content during oxidation >ll at. %(high-O BN) and the other composite had a BN Remove these interface coating with <2 at. oxygen(low-O BN). The composites with the high-O BN were made 2 years before the composites with the low-O BN. The high oxygen content in the bn was attributed to outgassing of moisture in the FCVI furnace. The furnace was completely cleaned and, shortly thereafter, BN having extremely low oxygen contents was successfully grown In order to determine the short- and long-term chemical (c)TEM preparation after oxidation and structural stability of composites with different BN interfacial coatings, both composite samples (low-O and Fig 1. Nicalon" BN SiC composite sample geometry used igh-O BN) were subjected to a series of identical, high- experiments.(a) Optical image showing the orientation of fib temperature oxidation exposures. Thin sections (sized center of 2x 2 mm sample.(b)Schematic of 200 um thick saI specifically to make TEM specimens after the exposures section illustrating preferred orientation of fibers.(c)Schem were used for this study instead of bulk pieces in order to sample as in(b) illustrating final preparation step for TEM maximize the effects of oxidation and to better simulate worst-case operating conditions at the surface of a real part stability/compatibility of any one particular BN coating, and during service in an oxidizing environment. Small pieces 2 X2 mm, were cut from each composite and mechanically the degradation mechanisms related to ground and polished to a final thickness of 200 um. Each the interface coating, it is necessary to characterize the BN coatings before and after testing/exposures using a combi specimen was also cut so that several fiber tows were nation of techniques oriented with the long axis of the fibers perpendicular to In an attempt to determine and separate various micro- the specimen surface, as shown in Fig. 1(a). In this way, structural parameters that contribute to the degradation of the ends/surfaces of both the fibers and bn coatings were fiber-reinforced composites containing BN interfacial coat- exposed during oxidation, as illustrated in the corresponding ings, two different BN interfacial coatings in Nicalon"/SiC schematic in Fig. 1(b). After oxidation, each exposed composites were evaluated for oxidative stability. The bn sample was dimpled in the center of one of the end-on coatings were structurally similar and differed only in their fiber tows and ion-milled to perforation for subsequent as-processed oxygen contents. The chemistry and micro- microstructural examination of the interface regions in structures of each BN coating were ully characterized before cross-section using TEN 1(c)). The final TEM and after oxidation using several analytical techniques specimen preparation pling and ion-milling Kinetics of the interface reactions have been calculated and also remove surface ox formed during the the microstructural results are being used to interpret the exposures The 200 um thick specimens were exposed to dry oxygen mechanical properties determined for each composite flowing at 100 ml/min in a compact reaction chamber specifically designed for reacting TEM specimens [121 2. Experimental procedure Reactions were conducted on two composite specimens simultaneously, one with the low-O BN and one with the Composites for this study were fabricated as follows: (1) high-O BN, at temperatures of 425, 600 and 950C for timestability/compatibility of any one particular BN coating, and in order to elucidate the degradation mechanisms related to the interface coating, it is necessary to characterize the BN coatings before and after testing/exposures using a combi￾nation of techniques. In an attempt to determine and separate various micro￾structural parameters that contribute to the degradation of fiber-reinforced composites containing BN interfacial coat￾ings, two different BN interfacial coatings in Nicalone/SiC composites were evaluated for oxidative stability. The BN coatings were structurally similar and differed only in their as-processed oxygen contents. The chemistry and micro￾structures of each BN coating were fully characterized before and after oxidation using several analytical techniques. Kinetics of the interface reactions have been calculated and the microstructural results are being used to interpret the mechanical properties determined for each composite. 2. Experimental procedure Composites for this study were fabricated as follows: (1) multiple layers of ceramic-grade Nicalone plain-weave fabric were stacked and rotated in a 0 ^ 308 sequence in a graphite holder; (2) layers were compressed in the holder cavity to produce a preform having a nominal fiber loading of ,40 vol.%; (3) sizing was removed from fibers through multiple acetone washes; (4) preform was coated with BN deposited via chemical vapor deposition from gas mixtures containing BCl3, NH3, and H2 at a temperature of 11008C and a reactor pressure of 5 kPa for 2 h, which resulted in an optimum coating thickness of ,0.42 mm [10]; and (5) the preforms were densified with SiC using the forced-flow, thermal-gradient chemical vapor infiltration (FCVI) process which has been described in detail previously [11]. The final composite was 85–90% of theoretical density. Two slightly different Nicalone/BN/SiC composites, both prepared as described above, were compared in this study; one compo￾site had a BN interface coating with an oxygen content .11 at.% (high-O BN) and the other composite had a BN interface coating with ,2 at.% oxygen (low-O BN). The composites with the high-O BN were made 2 years before the composites with the low-O BN. The high oxygen content in the BN was attributed to outgassing of moisture in the FCVI furnace. The furnace was completely cleaned and, shortly thereafter, BN having extremely low oxygen contents was successfully grown. In order to determine the short- and long-term chemical and structural stability of composites with different BN interfacial coatings, both composite samples (low-O and high-O BN) were subjected to a series of identical, high￾temperature oxidation exposures. Thin sections (sized specifically to make TEM specimens after the exposures) were used for this study instead of bulk pieces in order to ‘maximize’ the effects of oxidation and to better simulate worst-case operating conditions at the surface of a real part during service in an oxidizing environment. Small pieces, 2 × 2 mm, were cut from each composite and mechanically ground and polished to a final thickness of , 200 mm. Each specimen was also cut so that several fiber tows were oriented with the long axis of the fibers perpendicular to the specimen surface, as shown in Fig. 1(a). In this way, the ends/surfaces of both the fibers and BN coatings were exposed during oxidation, as illustrated in the corresponding schematic in Fig. 1(b). After oxidation, each exposed sample was dimpled in the center of one of the end-on fiber tows and ion-milled to perforation for subsequent microstructural examination of the interface regions in cross-section using TEM (see Fig. 1(c)). The final TEM specimen preparation steps (dimpling and ion-milling) also remove surface oxides that formed during the exposures. The 200 mm thick specimens were exposed to dry oxygen flowing at 100 ml/min in a compact ‘reaction chamber’ specifically designed for reacting TEM specimens [12]. Reactions were conducted on two composite specimens simultaneously, one with the low-O BN and one with the high-O BN, at temperatures of 425, 600 and 9508C for time 464 K.L. More et al. / Composites: Part A 30 (1999) 463–470 Fig. 1. Nicalone BN SiC composite sample geometry used for oxidation experiments. (a) Optical image showing the orientation of fiber tows in center of 2 × 2 mm sample. (b) Schematic of 200 mm thick sample cross￾section illustrating preferred orientation of fibers. (c) Schematic of same sample as in (b) illustrating final preparation step for TEM examination after oxidation