Carbothermal Synthesis of BN Coatings on SiC 1835 Sic 200nm BN 500nm Fig. 7. TEM image of a 200 nm BN-coated Tyranno ZMI SiC fiber(200 nm thickness CDC layer, nitridation at 1150.C for 80 min) to investigate the effect of nitridation on the mechanical properties with the reports from Shen. 2 The CDC layer not only participated of these fibers. SEM images of CDC-coated SiC fibers before and in the nitridation reactions, but also was sacrificially oxidized by after the nitridation are shown in Fig 9. It can be seen that the fiber the oxygen from the boria to protect the Sic fibers from degrada- retained smooth surface and cylindrical shape after chlorination tion as well as to control the thickness of BN coatings. Moreover, and nitridation. More important, no fiber-bridging occurred as can the mechanical strength and Young's modulus of the SiC fibers be seen in Fig. 9(b). The average breaking stress, ultimate strain, can also be slightly improved by thin carbon coatings formed and Young's modulus were calculated and are compared for these during chlorination.29 However, the mechanical properties of the fibers in Table Il. Although the high-temperature treatment car fibers with thick carbon layers(1.5 um) are much lower than those of the as-received fibers. a contributing factor is the 15% loss of from other synthesis attempts, 42 the BN-coated Sic fibers the fiber cross sections during chlorination at higher temperatures, prepared by this method have shown no significant change in mechanical strength as well as a slight increase in Young's moduli. compared with the thin carbon(0. 15 um)coated fibers with only which indicates the possibility of applications of such BN-coated 1 5% loss. The optimal condition to synthesize BN coatings on fibers in CMCs. Since the fibers for testing were taken from the thin-carbon-coated Sic fibers is nitridation in ammonia at 1150C for 60 min. More important, such BN-coated fibers show obvious evidence to exclude the possibility of fiber-bridging. Otherwise, enhancement in debonding and pullout from the polymer glue in the fibers would be inevitably damaged during their separation. single-fiber stress-strain mechanical tests(Fig. 10), which is an The absence of degradation in the SiC fibers during nitridation is important factor in the toughening of CMCs mainly attributed to the introduction of the carbon layer between The synthesis of Bn coatings by this method is not limited to the bn coating and Sic fiber in our method, which is consistent the Sic powders and fibers, but also can be explored to produce Surface of the fiber a-BN h-BN Fig 8. TEM images of BN crystals formed by the nitridation of Tyranno ZMI SiC fibers (1.5 um thickness CDC layer)at 1150C for 80 min.to investigate the effect of nitridation on the mechanical properties of these fibers. SEM images of CDC-coated SiC fibers before and after the nitridation are shown in Fig. 9. It can be seen that the fiber retained smooth surface and cylindrical shape after chlorination and nitridation. More important, no fiber-bridging occurred as can be seen in Fig. 9(b). The average breaking stress, ultimate strain, and Young’s modulus were calculated and are compared for these fibers in Table II. Although the high-temperature treatment can cause damage to the mechanical properties of SiC fibers as known from other synthesis attempts,41,42 the BN-coated SiC fibers prepared by this method have shown no significant change in mechanical strength as well as a slight increase in Young’s moduli, which indicates the possibility of applications of such BN-coated fibers in CMCs. Since the fibers for testing were taken from the fabric after nitridation, the high strength measured is more good evidence to exclude the possibility of fiber-bridging. Otherwise, the fibers would be inevitably damaged during their separation. The absence of degradation in the SiC fibers during nitridation is mainly attributed to the introduction of the carbon layer between the BN coating and SiC fiber in our method, which is consistent with the reports from Shen.42 The CDC layer not only participated in the nitridation reactions, but also was sacrificially oxidized by the oxygen from the boria to protect the SiC fibers from degrada￾tion as well as to control the thickness of BN coatings. Moreover, the mechanical strength and Young’s modulus of the SiC fibers can also be slightly improved by thin carbon coatings formed during chlorination.29 However, the mechanical properties of the fibers with thick carbon layers (1.5 m) are much lower than those of the as-received fibers. A contributing factor is the 15% loss of the fiber cross sections during chlorination at higher temperatures, compared with the thin carbon (0.15 m) coated fibers with only 1.5% loss. The optimal condition to synthesize BN coatings on thin-carbon-coated SiC fibers is nitridation in ammonia at 1150°C for 60 min. More important, such BN-coated fibers show obvious enhancement in debonding and pullout from the polymer glue in single-fiber stress–strain mechanical tests (Fig. 10), which is an important factor in the toughening of CMCs.47 The synthesis of BN coatings by this method is not limited to the SiC powders and fibers, but also can be explored to produce Fig. 7. TEM image of a 200 nm BN-coated Tyranno ZMI SiC fiber (200 nm thickness CDC layer, nitridation at 1150°C for 80 min). Fig. 8. TEM images of BN crystals formed by the nitridation of Tyranno ZMI SiC fibers (1.5 m thickness CDC layer) at 1150°C for 80 min. November 2003 Carbothermal Synthesis of BN Coatings on SiC 1835