November 2003 Carbothermal synthesis of BN Coatings on Sic During the nitridation process, the ammonia is decomposed to below 1200 4, as well as to reduce the production cost using nitrogen and hydrogen when the temperature is higher than 227C. a quartz tube furnace. As seen from the calculations, the H3 BO The boric acid dehydrates to form boria, which reacts with can be completely converted to Bn at temperatures above 1000C nitrogen to form BN. The presence of carbon facilitates this with proper control of the amount of incoming ammonia. How- process by reaction with hydrogen. The reactions associated with ever, it is necessary to mention that this cannot truly represent the this procedure are the following case of reaction due to kinetic limitations. The practical synthesis +3H temperature may be higher to achieve full transformation from carbon to BN and produce coatings with the required thickness of 2H3BO3=B2O3+3H2Og↑ 100-300 nm. In Fig. I(b), I mol of H3B03, 3 mol of carbon, and 2B2O3+ 9C 4NH3(g)= 4BN 3CH4(g)+ 6CO(g 1 3 mol of NHy are used for calculating the equilibrium comRs. g over the same temperature range. This analysis shows that synthesized Bn coating is thermodynamically stable and coexis Thermodynamic simulation was conducted in a closed system with the gas mixture of the products at high temperatures by using ChemSage 4.1 Gibbs energy minimization software. It helps to optimize the BN synthesis temperature and gives a clear description of how the reactions proceed during processing. Figure Ill. Materials and Experiments 1(a) shows the thermodynamic calculation results for the synthesis (1 f BN at temperatures from 800 to 1200C in a closed system at 1 atm. The initial reactants for this reaction include 1 mol of B-SiC powders, supplied by Superior Graphite Co, USA, with H3BO3, 3 mol of carbon, which is sufficient for the reaction, and purity of 99. 8% and I um particle size, and Tyranno ZMI SIC 0 to 5 mol of NH, with a 0.5 mol increase for each step. The bers(56% silicon, 34% carbon, 9% oxygen, and 1% zirconium) produced by UBE Industries of Japan, were used as the raw temperature range selected was to prevent the possible degradation materials for the synthesis of Bn coatings by the proposed method of polymer-derived SiC fibers by holding the process temperature B-sic powder was used as a model system to understand the process mechanism due to its easier sample preparation for XRD and TEM studies. Tyranno ZMI SiC fibers were used in our work to show the feasibility of BN synthesis because of their higher 口T=850° T=1150°c(a tensile strength compared with other types of Sic fibers(Tyrann I=1200°C SA, Hi-Nicalon, Sylramic, etc) and well-studied formation of T=1100°C ntermediate carbon coatings on this fiber -However, the method is applicable to virtually any SiC material that can be coated with a uniform CDC film (2) Experimental Procedure The whole synthesis process includes three main steps: chlori- nation, infiltration, and nitridation. The detailed process for each 120c//100c1050c step is presented below with respect to the different types of Sic (Ay Chlorination: The process for synthesis of intermediate carbon films on Sic uses a chlorine extraction method. 30-33 The carbon coatings obtained are conformal and do not change the shape or surface quality of the samples. They have nanoscale porosity, low friction coefficient, and excellent adherence to SiC. (i) Powders: B-SiC powder hlorine flow rate of 10 standard cubic centimeters per minute(sccm)at 1000C for 3 h in a quartz tube furnace with diameter of 2.5 cm Amount of NH3血mo XRD and Raman spectroscopy analysis confirmed that the si was totally converted into carbon (ii) Fibers: Tyranno ZMI SiC fibers(-10 um in diameter) were treated in pure Cl, with gas flow of 10 sccm for 3 h at 550 fibers with thicknesses of -0.15 and.5 um, respective/y. on and 650C in atmospheric pressure to form carbon coatings (B) Infiltration: Boric acid was used as the most economical recursor for the synthesis of BN. Unlike other precusors, 728it does not contaminate the samples by metal impurities. A vacuum H2令 chamber was used to infiltrate saturated H, BO(99.99% purity solution into the carbon layer formed on the sic. The reduced pressure in the chamber induced the release of the gases absorbed n the surface of the nanoporous CDC layer, so that better infiltration could be achieved co (i)Powders: The CDC powders obtained in the first step of process were ther with hbo stoichiometric ratio of 9: 4. Then the mixture was groun mortar to achieve a grain size of -l um. Infiltration powders by H3B03 occurred during the heating for nitridation because B,o, melts at 450%C i)Fibers: The CDC-coated fibers were placed in the vac- uum infiltration chamber and pumped down for about 30 min Then they were infiltrated with a saturated H3 BO, solution at 100 C. Cold distilled water was used to wash out the excess of Fig. 1.(a) Thermodynamic calculation for the yield of BN as a function H3 BO3 from the fiber surface at room temperature. To achieve of incoming NH, amount at various synthesis temperatures. (b) Equilib- good infiltration effect for the nanoporous carbon coating, this step rium phase compositions as a function of temperature. was repeated two or three timesDuring the nitridation process, the ammonia is decomposed to nitrogen and hydrogen when the temperature is higher than 227°C. The boric acid dehydrates to form boria, which reacts with nitrogen to form BN. The presence of carbon facilitates this process by reaction with hydrogen. The reactions associated with this procedure are the following: 2NH3 g N2 g
3H2 g 2H3BO3 B2O3
3H2O g1 2B2O3
9C
4NH3 g 4BN
3CH4 g
6CO g1 Thermodynamic simulation was conducted in a closed system by using ChemSage 4.1 Gibbs energy minimization software. It helps to optimize the BN synthesis temperature and gives a clear description of how the reactions proceed during processing. Figure 1(a) shows the thermodynamic calculation results for the synthesis of BN at temperatures from 800° to 1200°C in a closed system at 1 atm. The initial reactants for this reaction include 1 mol of H3BO3, 3 mol of carbon, which is sufficient for the reaction, and 0 to 5 mol of NH3 with a 0.5 mol increase for each step. The temperature range selected was to prevent the possible degradation of polymer-derived SiC fibers by holding the process temperature below 1200°C14,18 as well as to reduce the production cost using a quartz tube furnace. As seen from the calculations, the H3BO3 can be completely converted to BN at temperatures above 1000°C with proper control of the amount of incoming ammonia. How￾ever, it is necessary to mention that this cannot truly represent the case of reaction due to kinetic limitations. The practical synthesis temperature may be higher to achieve full transformation from carbon to BN and produce coatings with the required thickness of 100–300 nm. In Fig. 1(b), 1 mol of H3BO3, 3 mol of carbon, and 3 mol of NH3 are used for calculating the equilibrium composition over the same temperature range. This analysis shows that the synthesized BN coating is thermodynamically stable and coexists with the gas mixture of the products at high temperatures. III. Materials and Experiments (1) Materials -SiC powders, supplied by Superior Graphite Co., USA, with purity of 99.8% and 1 m particle size, and Tyranno ZMI SiC fibers (56% silicon, 34% carbon, 9% oxygen, and 1% zirconium), produced by UBE Industries of Japan, were used as the raw materials for the synthesis of BN coatings by the proposed method. -SiC powder was used as a model system to understand the process mechanism due to its easier sample preparation for XRD and TEM studies. Tyranno ZMI SiC fibers were used in our work to show the feasibility of BN synthesis because of their higher tensile strength compared with other types of SiC fibers (Tyranno SA, Hi-Nicalon, Sylramic, etc) and well-studied formation of intermediate carbon coatings on this fiber.29 However, the method is applicable to virtually any SiC material that can be coated with a uniform CDC film. (2) Experimental Procedure The whole synthesis process includes three main steps: chlori￾nation, infiltration, and nitridation. The detailed process for each step is presented below with respect to the different types of SiC. (A) Chlorination: The process for synthesis of intermediate carbon films on SiC uses a chlorine extraction method.30–33 The carbon coatings obtained are conformal and do not change the shape or surface quality of the samples. They have nanoscale porosity, low friction coefficient, and excellent adherence to SiC.34 (i) Powders: -SiC powder was treated in pure chlorine at a flow rate of 10 standard cubic centimeters per minute (sccm) at 1000°C for 3 h in a quartz tube furnace with diameter of 2.5 cm. XRD and Raman spectroscopy analysis confirmed that the SiC was totally converted into carbon. (ii) Fibers: Tyranno ZMI SiC fibers (
10 m in diameter) were treated in pure Cl2 with gas flow of 10 sccm for 3 h at 550° and 650°C in atmospheric pressure to form carbon coatings on fibers with thicknesses of
0.15 and
1.5 m, respectively. (B) Infiltration: Boric acid was used as the most economical precursor for the synthesis of BN. Unlike other precusors,27,28 it does not contaminate the samples by metal impurities. A vacuum chamber was used to infiltrate saturated H3BO3 (99.99% purity) solution into the carbon layer formed on the SiC. The reduced pressure in the chamber induced the release of the gases absorbed on the surface of the nanoporous CDC layer, so that better infiltration could be achieved. (i) Powders: The CDC powders obtained in the first step of the process were mixed together with H3BO3 powders with a stoichiometric ratio of 9:4. Then the mixture was ground in a mortar to achieve a grain size of
1 m. Infiltration of the powders by H3BO3 occurred during the heating for nitridation because B2O3 melts at 450°C. (ii) Fibers: The CDC-coated fibers were placed in the vac￾uum infiltration chamber and pumped down for about 30 min. Then they were infiltrated with a saturated H3BO3 solution at 100°C. Cold distilled water was used to wash out the excess of H3BO3 from the fiber surface at room temperature. To achieve good infiltration effect for the nanoporous carbon coating, this step was repeated two or three times. Fig. 1. (a) Thermodynamic calculation for the yield of BN as a function of incoming NH3 amount at various synthesis temperatures. (b) Equilib￾rium phase compositions as a function of temperature. November 2003 Carbothermal Synthesis of BN Coatings on SiC 1831