Carbothermal synthesis of BN Coatings on Sic 33 (b) 2018K 20.00B-K .C-K 6* N-K N-K 2000 C-K (c) ocvo59o ergy Loss (ev) Fig. 3. Typical EELS spectra from the CDC powders nitrided in NH, at 1165.C for 60 min: (a)outermost layer of pure BN with average thickness of 50-70 nm,(b) intermediate layer of BN and C mixture with average thickness of 75-110 nm, (c) interior layer of pure C. The backgrounds of all spectra are subtracted. The blunt carbon edge in(c) is due to the increasing thickness toward the center of the analyzed particle published elsewhere. It can be assumed that the amorphous (A) X-ray Diffraction Analysis: The XRD analysis of the and diamond-structured BN formed by the reaction with am nitrided fibers is shown in Fig. 5. Like the powders, BN nonia at the c/bn interface transformed to the more stable gs synthesized on the SiC fibers with thin CDC layers hexagonal modification as the reaction front propagated toward mainly composed of amorphous BN ), while onal BN in the intermediate layer of BN coatings suggests this thick CDC layers(Fig. 5(d)). The pure on the SiC fibers with the particle core during the nitridation. The presence of hexag ble reason is that the mechanism. The increased temperature during nitridation thick carbon coating allows production of a relatively thick BN makes the conversion from a-bn to h-Bn more kinetically layer, from which the amorphous BN formed at the beginning favorable.The small amount of boron oxides generated from of the reaction tends to convert to the hexagonal modification the reactants at the beginning of the nitridation also helps to Also longer nitridation time helps to increase the content of form the hexagonal-structured boron nitride. The same phenom- h-BN crystals in the coatings. This is consistent with the results enon has been reported during the synthesis of Bn by CVD. 45 obtained on powders. In addition, the increase of the diffraction Sic in the pattern can be attributed to the (2) BN Coatings of Sic Fibers lization of Sic in the fiber from its original amorphous SiC fibers coated with 150-250 nm(thin coating) and 1.5 um during the high-temperature treatment. The longer the (thick coating) CDC layers were nitrided at 1150.C in ammonia nation and nitridation times used the more obvious SiC appear. This is not desirable for SiC fibers, so optimization ofpublished elsewhere.44 It can be assumed that the amorphous and diamond-structured BN formed by the reaction with am￾monia at the C/BN interface transformed to the more stable hexagonal modification as the reaction front propagated toward the particle core during the nitridation. The presence of hexag￾onal BN in the intermediate layer of BN coatings suggests this mechanism. The increased temperature during nitridation makes the conversion from a-BN to h-BN more kinetically favorable.3 The small amount of boron oxides generated from the reactants at the beginning of the nitridation also helps to form the hexagonal-structured boron nitride. The same phenom￾enon has been reported during the synthesis of BN by CVD.45 (2) BN Coatings of SiC Fibers SiC fibers coated with 150–250 nm (thin coating) and 1.5 m (thick coating) CDC layers were nitrided at 1150°C in ammonia for various periods of time, respectively. (A) X-ray Diffraction Analysis: The XRD analysis of the nitrided fibers is shown in Fig. 5. Like the powders, BN coatings synthesized on the SiC fibers with thin CDC layers are mainly composed of amorphous BN (Figs. 5(b) and (c)), while hexagonal BN was the dominant structure on the SiC fibers with thick CDC layers (Fig. 5(d)). The probable reason is that the thick carbon coating allows production of a relatively thick BN layer, from which the amorphous BN formed at the beginning of the reaction tends to convert to the hexagonal modification. Also longer nitridation time helps to increase the content of h-BN crystals in the coatings. This is consistent with the results obtained on powders. In addition, the increase of the diffraction peaks from SiC in the pattern can be attributed to the crystal￾lization of SiC in the fiber from its original amorphous phase during the high-temperature treatment. The longer the chlori￾nation and nitridation times used, the more obvious SiC peaks appear. This is not desirable for SiC fibers, so optimization of Fig. 3. Typical EELS spectra from the CDC powders nitrided in NH3 at 1165°C for 60 min: (a) outermost layer of pure BN with average thickness of 50–70 nm, (b) intermediate layer of BN and C mixture with average thickness of 75–110 nm, (c) interior layer of pure C. The backgrounds of all spectra are subtracted. The blunt carbon edge in (c) is due to the increasing thickness toward the center of the analyzed particle. November 2003 Carbothermal Synthesis of BN Coatings on SiC 1833