1832 Journal of the American Ceramic Sociery--Chen et al Vol. 86. No. 1I (C) Nitridation: Infiltrated CDC-coated Sic samples(pow- ders and fibers) were loaded in a quartz boat and put into a horizontal quartz tube furnace with inner diameter of 2.5 cm. 3500 Before each experimental run, the furnace was purged with argor for at least 30 min. Then the furnace was heated to the desired (110) operating temperature at a rate of 10C/min with ammonia(grade 4: purity 99.99%, BOC gases) flowing into the reaction tube at a reaction, and then cooled down in the fumace under the ammonia a 20on 1 Graphite a-C (0oz) flow rate of 10 sccm. The sample was held at the set temperature flow for protection. The specific treatment temperatures and times (100101004103)(104×110) for different SiC samples are shown in Table I. 1500 (3 Characterization The composition and structures of the samples nitrided under arious conditions were examined by X-ray diffraction(XRD B-SIc Siemens Model D500, CuKo radiation), Raman spectroscopy Renishaw 100, Ar ion laser at an excitation wavelength of 514.5 nm), scanning electron microscopy(SEM; AMRAY 1830), high resolution transmission electron microscopy(HRTEM; JEOL 2010F), and electron energy loss spectroscopy (EELs) Tensile strength and Youngs modulus of the Sic fibers 20( degree) before and after nitridation were measured according to ASTm 3379-75 using a SATEC Model T-5000 universal testing X-ray diffraction patterns for the powders: (a) as-received B-SiC (b) CDC powder, (c)H3 BOj-infiltrated CDC powder after machine equipped with a S/N U. K 327 load cell with permitted on at llc65°for60min. maximum load of 2.5 N. Single fibers extracted from a tow were fixed on paper frames using a hard acrylic resin(Super Glue, DURO). The 25 mm standard gauge length was used and the crosshead speed was set to 0.5 mm/min. The diameter of the products according to the XRD pattem. among the reaction fina sic fiber core has been used for the calculation of mechanical properties. For each treatment condition, at least 10 (B) TEM and EELS Analysis: Three distinct layers with fibers were mechanically tested. different compositions were detected by the EELs analysis from the nitrided CDC Sic powders as shown in Fig 3. The outermost layer of the powder is pure BN coating with an average thickness IV. Results and discussion of 50-70 nm, in which the carbon was totally consumed during the Uniform BN coatings were obtained by the nitridation of the with a thickness of 75-110 nm and the inner layer(core) of the I3BO3-infiltrated CDC under the conditions listed in Table I. The nitridation process was conducted in the quartz tube furnace at 1 coating is around 120-180 nm. Formation of bn and mixed BN/C atm at temperatures below 1200 C, which are low compared with layers is most probably attributed to the maximization of energet- the generally required temperatures for the production of ically favorable C-C and B-N bonds, rather than the B-C and N-C (1400-1700 C)at atmospheric pressure. 9 bonds.38-40 The introduction of CDC layer not only helps to facilitate the formation of bn by decreasing the gibbs energy of the reactions, but also helps to consume the excess B,O, on the (I) BN Coatings of sic Powders fiber. Therefore, no oxygen was detected in the Bn layers, which (A) x-ray Diffraction Analysis: X-ray diffraction analysis is an important advantage compared with other oxygen-containing was conducted for the Sic powders after each step of treatment, BN coatings synthesized from H, BO3.4.42 as shown in Fig. 2. B-SiC powders(Fig. 2(a))were completely Figure 4 shows the hRTEM images of BN-coated CDC converted into carbon by chlorination in pure Cl, at 1000C for powders. Hexagonal and amorphous bn were found in the 3 h(Fig. 2(b)). The broad peak in the XRD pattern shows that coatings. By the combination of HRTEM and EELS analysis, the carbon powders obtained are mainly amorphous, while the mechanism of formation of BN in this method could be small peaks of graphite are also visible. Bn coatings were understood. The inner layer of the coating at the BN/C interface obtained after the nitridation of CDC powders at 1165.C for 60 is mainly composed of amorphous BN. A certain amount of min in ammonia(Fig. 2(c)). The Bn obtained was mainly hexagonal BN crystals, with spacing dooz=0.336 nm as show composed of the amorphous phase, which was induced by the in Fig. 4(b), appeared in the middle of the BN coatings and amorphous carbon. Some hexagonal BN (JCPDS: 34-0421)wa also present in the coatings formed by this method. During the coatings. A small amount of cubic BN with spacing d,of nanocrystalline h-BN became dominant in the surface layer nitridation, boric acid powder first dehydrated to form boria 0.209 nm was also observed at the interface with the CDC layer. when the temperature was above 100 C, then part of the boria The mechanism of its formation is probably similar to that of sublimated at temperatures over 170 C, and the remainder nanocrystalline diamond growth on chlorination of SiC3I,43 completely reacted with ammonia at the synthesis temperature Detailed discussion of the structures of such BN coatings is Table L. Nitridation Conditions for CDC-Coated SiC Samples hickness of CDC coating Nitridation temperature Materials B-SiC Powder Complete transformation 1165 White Tyranno ZMI SiC Fibers 1150 G 0.15(C) Nitridation: Infiltrated CDC-coated SiC samples (pow￾ders and fibers) were loaded in a quartz boat and put into a horizontal quartz tube furnace with inner diameter of 2.5 cm. Before each experimental run, the furnace was purged with argon for at least 30 min. Then the furnace was heated to the desired operating temperature at a rate of 10°C/min with ammonia (grade 4: purity 99.99%, BOC gases) flowing into the reaction tube at a flow rate of 10 sccm. The sample was held at the set temperature for a certain period of time to secure the completion of the reaction, and then cooled down in the furnace under the ammonia flow for protection. The specific treatment temperatures and times for different SiC samples are shown in Table I. (3) Characterization The composition and structures of the samples nitrided under various conditions were examined by X-ray diffraction (XRD; Siemens Model D500, CuK radiation), Raman spectroscopy (Renishaw 100, Ar ion laser at an excitation wavelength of 514.5 nm), scanning electron microscopy (SEM; AMRAY 1830), high￾resolution transmission electron microscopy (HRTEM; JEOL 2010F), and electron energy loss spectroscopy (EELS). Tensile strength and Young’s modulus of the SiC fibers before and after nitridation were measured according to ASTM 3379-75 using a SATEC Model T-5000 universal testing machine equipped with a S/N U.K. 327 load cell with permitted maximum load of 2.5 N. Single fibers extracted from a tow were fixed on paper frames using a hard acrylic resin (Super Glue, DURO). The 25 mm standard gauge length was used and the crosshead speed was set to 0.5 mm/min. The diameter of the final SiC fiber core has been used for the calculation of mechanical properties. For each treatment condition, at least 10 fibers were mechanically tested. IV. Results and Discussion Uniform BN coatings were obtained by the nitridation of the H3BO3-infiltrated CDC under the conditions listed in Table I. The nitridation process was conducted in the quartz tube furnace at 1 atm at temperatures below 1200°C, which are low compared with the generally required temperatures for the production of BN (1400°–1700°C) at atmospheric pressure.19–22,35–37 (1) BN Coatings of SiC Powders (A) X-ray Diffraction Analysis: X-ray diffraction analysis was conducted for the SiC powders after each step of treatment, as shown in Fig. 2. -SiC powders (Fig. 2(a)) were completely converted into carbon by chlorination in pure Cl2 at 1000°C for 3 h (Fig. 2(b)). The broad peak in the XRD pattern shows that the carbon powders obtained are mainly amorphous, while small peaks of graphite are also visible. BN coatings were obtained after the nitridation of CDC powders at 1165°C for 60 min in ammonia (Fig. 2(c)). The BN obtained was mainly composed of the amorphous phase, which was induced by the amorphous carbon. Some hexagonal BN (JCPDS: 34-0421) was also present in the coatings formed by this method. During nitridation, boric acid powder first dehydrated to form boria when the temperature was above 100°C, then part of the boria sublimated at temperatures over 170°C,3 and the remainder completely reacted with ammonia at the synthesis temperature. There is no crystalline boria remaining among the reaction products according to the XRD pattern. (B) TEM and EELS Analysis: Three distinct layers with different compositions were detected by the EELS analysis from the nitrided CDC SiC powders as shown in Fig. 3. The outermost layer of the powder is pure BN coating with an average thickness of 50–70 nm, in which the carbon was totally consumed during the reaction. The intermediate layer is a mixture of BN and carbon with a thickness of 75–110 nm and the inner layer (core) of the particle is unreacted pure carbon. The total thickness of the BN coating is around 120–180 nm. Formation of BN and mixed BN/C layers is most probably attributed to the maximization of energet￾ically favorable C–C and B–N bonds, rather than the B–C and N–C bonds.38–40 The introduction of CDC layer not only helps to facilitate the formation of BN by decreasing the Gibbs energy of the reactions, but also helps to consume the excess B2O3 on the fiber. Therefore, no oxygen was detected in the BN layers, which is an important advantage compared with other oxygen-containing BN coatings synthesized from H3BO3. 41,42 Figure 4 shows the HRTEM images of BN-coated CDC powders. Hexagonal and amorphous BN were found in the coatings. By the combination of HRTEM and EELS analysis, the mechanism of formation of BN in this method could be understood. The inner layer of the coating at the BN/C interface is mainly composed of amorphous BN. A certain amount of hexagonal BN crystals, with spacing d002  0.336 nm as shown in Fig. 4(b), appeared in the middle of the BN coatings and nanocrystalline h-BN became dominant in the surface layer of the coatings. A small amount of cubic BN with spacing d111  0.209 nm was also observed at the interface with the CDC layer. The mechanism of its formation is probably similar to that of nanocrystalline diamond growth on chlorination of SiC.31,43 Detailed discussion of the structures of such BN coatings is Fig. 2. X-ray diffraction patterns for the powders: (a) as-received -SiC powder, (b) CDC powder, (c) H3BO3-infiltrated CDC powder after nitridation at 1165°C for 60 min. Table I. Nitridation Conditions for CDC-Coated SiC Samples Materials Thickness of CDC coating (m) Nitridation temperature (°C) Nitridation time (min) Color description after nitridation -SiC Powder Complete transformation 1165 65 White Tyranno ZMI SiC Fibers 1.5 1150 60 Gray 0.15 1150 60 Brown 0.15 1150 80 Violet 0.15 1165 65 Blue 1832 Journal of the American Ceramic Society—Chen et al. Vol. 86, No. 11