J Am Ceram Soc., 86[11] 1830-37(2003) ournal Carbothermal Synthesis of boron Nitride Coatings on Silicon Carbide Linlin Chen, Haihui Ye, and Yury Gogotsi* Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104 Michael J. mcnallan Department of Civil and Materials Engineering, University of Illinois, Chicago, Illinois 60607 Pure BN coatings have been synthesized on the surface of Sic interface layers in such SiC/SiC-based composites show low powders and fibers by a novel carbothermal nitridation oxidation resistance at elevated temperatures in air or water method. Three stages are involved in the process: first, forma vapor, 9-4 which leads to oxidation embrittlement of the CMC tion of a carbon layer on the Sic by the extraction of si with and degradation of their mechanical properties. BN has a better chlorine; second, infiltration of the resulting nanoporous oxidation resistance than carbon. At high temperatures, when carbide-derived carbon (CDC) coating by a saturated borie BN reacts with the oxygen ingressed through the interfaces acid solution; and finally, nitridation in ammonia at atmo- between fibers and the matrix, it forms B,O,, which could react spheric pressure to produce the pure BN coating. X-ray with SiO, to form a glassy protective layer to prevent the diffraction (XRD), Raman spectroscopy, scanning electron further reaction at the interface of the CMCs. Thus, synthesis of microscopy (SEM), high-resolution transmission electron mi- BN coatings on Sic fibers is one of the most promising croscopy (HRTEM), and electron energy loss spectroscopy techniques to improve performance of the fiber-reinforced (EELS) were used to characterize the phase, elemental com- CMCs at high temperatures position, and surface morphology of the coatings. The inter- Up to now, most of the bn coatings were prepared by ediate carbon layer acts as a template for BN growth, vapor deposition(CVD). 9-Also, plasma-assisted facilitates the formation of BN, and prevents the degradation vapor deposition(PACVD), magnetron sputtering, an of SiC fibers during nitridation. The whole process is simple, laser deposition(PLD) were used for the synthesis of cost-effective, and less toxic due to the use of H3 BO3 and NH3 BN coatings as precursors at atmospheric pressure compared with most Recently, the introduction of an intermediate carbon layer was commonly used chemical vapor deposition(CVD) methods. found to be helpful to reduce the Bn synthesis temperature at Uniform BN coatings obtained by this method prevent the ambient pressure. Carbothermal reduction-nitridation is widely bridging of fibers in the tow. The coating of powders is used to produce nitride powders.23,24 Direct nitridation of carbon possible, which cannot be achieved by conventional CVD nanotubes to obtain BN-coated nanotubes has also been demon strated by Bando. 5 However, for the dense graphitic carbon, such nitridation is a slow process and thick coating requires a temper- ature higher than 1500.C, which will damage the Sic fibers made L. Introduction from polymeric precursors. Relatively low temperature B RON NITRIDE (BN) has received considerable attention within Nextel 312TM in ammonia26 by reacting with the 14 wt%boria in optical, and chemical properties over a wide range of tempera the fiber, but it is only limited to boria-containing fibers and the thickness of the BN coating is less than 40 nm. Synthesis of BN uch excellent properties promote the broad applications coatings by transformation from carbon coatings produced by of BN, which include high-temperature insulators, self-lubricating dipping was also proposed, 728 but the coatings obtained by such and heat-dissipating coatings, passivation layers, diffusion masks, methods were not uniform enough for composite applications and wear-resistant coatings. Structurally and chemically well- defined films are required in many of these applications Furthermore, fiber bridging during those processes decreased the BN, like carbon, has four crystalline structural modifications efficiency of the coatings for the fiber tows or fabrics cubic(c-BN), wurzite(w-BN), hexagonal(h-BN), and rhombohe In this paper, we present a novel method to synthesize dral (r-BN), which correspond to diamond(zinc blende form), coatings by the nitridation of carbon layers on p-sic powders an hexagonal diamond(wurzite form), hexagonal, and rhombohedral Tyranno SiC fibers at relatively low temperatures(<1200%C)an graphite, respectively An important application of thin BN is as an interfacial layer for controlling the bonding ceramic-matrix composites(CMCs).7,The reinforced Il. Thermodynamic Modeling by Sic fibers or whiskers have attracted great attention due to their highly improved fracture toughness. However, carbon-rich Synthesis of Bn coatings on the Sic materials involves three stages. The first is the chlorination of Sic to produce a carbon layer with a desired thickness by the extraction of Si from SiC according to the following chemical reaction SiC(s)+ 2Cl(g)=SiCl(g)1 +C(s) The second step is to infiltrate the resulting nanoporous carbide Q This wor pt No. 186484 Received December 18, 2002; d July 7. 2003 derived carbon (CDC) coated Sic with saturated boric acid was supported by NASA via an SBIR grant to SSG Precision Optronics olution using a low vacuum(<I atm)at 100.C. The final step is American Ceramic Society nitridation in an ammonia atmosphere at ambient pressure to COrresponding autho produce the bn coating 830
Carbothermal Synthesis of Boron Nitride Coatings on Silicon Carbide Linlin Chen,* Haihui Ye,* and Yury Gogotsi* ,† Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104 Michael J. McNallan* Department of Civil and Materials Engineering, University of Illinois, Chicago, Illinois 60607 Pure BN coatings have been synthesized on the surface of SiC powders and fibers by a novel carbothermal nitridation method. Three stages are involved in the process: first, formation of a carbon layer on the SiC by the extraction of Si with chlorine; second, infiltration of the resulting nanoporous carbide-derived carbon (CDC) coating by a saturated boric acid solution; and finally, nitridation in ammonia at atmospheric pressure to produce the pure BN coating. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS) were used to characterize the phase, elemental composition, and surface morphology of the coatings. The intermediate carbon layer acts as a template for BN growth, facilitates the formation of BN, and prevents the degradation of SiC fibers during nitridation. The whole process is simple, cost-effective, and less toxic due to the use of H3BO3 and NH3 as precursors at atmospheric pressure compared with most commonly used chemical vapor deposition (CVD) methods. Uniform BN coatings obtained by this method prevent the bridging of fibers in the tow. The coating of powders is possible, which cannot be achieved by conventional CVD methods. I. Introduction BORON NITRIDE (BN) has received considerable attention within the last few years due to its favorable mechanical, electrical, optical, and chemical properties over a wide range of temperatures.1,2 Such excellent properties promote the broad applications of BN, which include high-temperature insulators, self-lubricating and heat-dissipating coatings, passivation layers, diffusion masks, and wear-resistant coatings. Structurally and chemically welldefined films are required3 in many of these applications. BN, like carbon, has four crystalline structural modifications: cubic (c-BN), wu¨rzite (w-BN), hexagonal (h-BN), and rhombohedral (r-BN), which correspond to diamond (zinc blende form), hexagonal diamond (wu¨rzite form), hexagonal, and rhombohedral graphite, respectively.4–6 An important application of thin BN coatings is as an interfacial layer for controlling the bonding in fiber-reinforced ceramic-matrix composites (CMCs).7,8 The CMCs reinforced by SiC fibers or whiskers have attracted great attention due to their highly improved fracture toughness. However, carbon-rich interface layers in such SiC/SiC-based composites show low oxidation resistance at elevated temperatures in air or water vapor,9–14 which leads to oxidation embrittlement of the CMC and degradation of their mechanical properties. BN has a better oxidation resistance than carbon. At high temperatures, when BN reacts with the oxygen ingressed through the interfaces between fibers and the matrix, it forms B2O3, which could react with SiO2 to form a glassy protective layer to prevent the further reaction at the interface of the CMCs. Thus, synthesis of BN coatings on SiC fibers is one of the most promising techniques to improve performance of the fiber-reinforced CMCs at high temperatures.15–18 Up to now, most of the BN coatings were prepared by chemical vapor deposition (CVD).19–21 Also, plasma-assisted chemical vapor deposition (PACVD),22 magnetron sputtering, and pulsed laser deposition (PLD)3 were used for the synthesis of different BN coatings. Recently, the introduction of an intermediate carbon layer was found to be helpful to reduce the BN synthesis temperature at ambient pressure. Carbothermal reduction–nitridation is widely used to produce nitride powders.23,24 Direct nitridation of carbon nanotubes to obtain BN-coated nanotubes has also been demonstrated by Bando.25 However, for the dense graphitic carbon, such nitridation is a slow process and thick coating requires a temperature higher than 1500°C., which will damage the SiC fibers made from polymeric precursors. Relatively low temperature (1200°C) in situ BN coating preparation has been conducted on Nextel 312TM in ammonia26 by reacting with the 14 wt% boria in the fiber, but it is only limited to boria-containing fibers and the thickness of the BN coating is less than 40 nm. Synthesis of BN coatings by transformation from carbon coatings produced by dipping was also proposed,27,28 but the coatings obtained by such methods were not uniform enough for composite applications. Furthermore, fiber bridging during those processes decreased the efficiency of the coatings for the fiber tows or fabrics. In this paper, we present a novel method to synthesize BN coatings by the nitridation of carbon layers on -SiC powders and Tyranno SiC fibers at relatively low temperatures (1200°C) and atmospheric pressure. II. Thermodynamic Modeling Synthesis of BN coatings on the SiC materials involves three stages. The first is the chlorination of SiC to produce a carbon layer with a desired thickness by the extraction of Si from SiC according to the following chemical reaction: SiCs 2Cl2g SiCl4g1 Cs The second step is to infiltrate the resulting nanoporous carbidederived carbon (CDC) coated SiC with saturated boric acid solution using a low vacuum (1 atm) at 100°C. The final step is nitridation in an ammonia atmosphere at ambient pressure to produce the BN coating. J. L. Smialek—contributing editor Manuscript No. 186484. Received December 18, 2002; approved July 7, 2003. This work was supported by NASA via an SBIR grant to SSG Precision Optronics Corp. *Member, American Ceramic Society. † Corresponding author. J. Am. Ceram. Soc., 86 [11] 1830–37 (2003) 1830 journal
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 times
During 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. However, 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: chlorination, 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 vacuum 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) Equilibrium phase compositions as a function of temperature. November 2003 Carbothermal Synthesis of BN Coatings on SiC 1831
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 (powders 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), highresolution 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 energetically 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
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 of
published elsewhere.44 It can be assumed that the amorphous and diamond-structured BN formed by the reaction with ammonia 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 hexagonal 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 phenomenon 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 crystallization of SiC in the fiber from its original amorphous phase during the high-temperature treatment. The longer the chlorination 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
1834 Journal of the American Ceramic Sociery--Chen et al Vol. 86. No. 1I {a) (b) a-BN d002 - BNA m am Fig 4. Typical HRTEM images of BN structures formed after the nitridation of CDC Sic powders at 1165C for 60 min:(a) amorphous BN and(b) the nitridation condition is required to suppress the fiber for 80 min. Judging from EELS and TEM analysis, the layer crystallization formed on the surface of the cDc-coated sic fibers is similar to B) Raman Spectra Analysis: Figure 6 shows the coating on the CDC SiC powder. It is composed of pure BN petra of the SiC fibers after nitridation under various c (80-100 nm) at the surface and a mixture of Bn with some All of the fibers had the same cdc lavers of ab carbon, which have not been totally consumed during the reaction thickness The d and g bands originate from the free carbon in the adjacent to the interface of the SiC/CDC coating. A uniform and raw fibers, while the amorphous Sic does not show any peaks in smooth BN coating with good adherence to the fiber core is clearly Raman spectra. With increasing nitridation time and temperatures seen in the hrTEM image in Fig. 7. However, the thickness of the the peak in the d band position increases in intensity, shifts BN coating observed in Fig. 7 must be calculated accounting for adually to higher wavenumbers, and eventually reaches the the geometric factors in TEM analysis. The diameter of su position of 1367 cm, which is the characteristic peak of h-BN BN-coated fiber is 10.8 Hm as known by sEM, so the real It gives additional confirmation that BN can be successfully thickness of the bn coating on the sic fiber is around 200 nm by produced on SiC fibers by this method. Moreover, for the same the geometric calculation thickness of carbon coating on the Sic fibers, longer reaction time Moreover, the crystal structures of the BN formed in the coatin or higher temperatures help to form the relatively thick BN layer were also detected by the hrTEM analysis as shown in Fig coatings 8. Coatings are primarily composed of am rphous and hexagon (c) EELS and HRTEM Analysis. The fibers used for pr BN. Also longer nitridation time and higher treatment temperature aration of the TEM samples were nitrided in ammonia at 115 help to increase the relative amount of hexagonal BN crystals. The results obtained from fibers are consistent with those from the powders 5000 D) Mechanical Properties of the Coated Fibers: Tensile tests were performed on the sic fibers before and after nitridation 4500 U-BN 3500 于hN"1 g里 25000 票目 a 20( degree) 0014001600180020002200 Fig. 5. X-ra received Tyranno ZMI SiC fibers,(b) CDC (0. 15 um) coated fibers Fig. 6. Raman analysis of Tyranno ZMI SiC fibers(with 0. 15-um-thick nitrided at 1150.C for 60 min, (c)CDC (0. 15 um) coated fibers nitrided at CDC layer) before and after nitridation under various treatment conditions l150°Cfor80min,(d)CDC(1.5μm) coated fibers nitrided at 11150°C (a) as-received SiC fibers, (b) nitridation at 1.C for 60 min, (c) 60 min nitridation at 1150%C for 80 min, (d) nitridation at 1165.C for 65 mi
the nitridation condition is required to suppress the fiber crystallization. (B) Raman Spectra Analysis: Figure 6 shows the Raman spectra of the SiC fibers after nitridation under various conditions. All of the fibers had the same CDC layers of about 0.15 m thickness. The D and G bands originate from the free carbon in the raw fibers, while the amorphous SiC does not show any peaks in Raman spectra. With increasing nitridation time and temperatures, the peak in the D band position increases in intensity, shifts gradually to higher wavenumbers, and eventually reaches the position of 1367 cm 1 , which is the characteristic peak of h-BN.46 It gives additional confirmation that BN can be successfully produced on SiC fibers by this method. Moreover, for the same thickness of carbon coating on the SiC fibers, longer reaction time or higher temperatures help to form the relatively thick BN coatings. (C) EELS and HRTEM Analysis: The fibers used for preparation of the TEM samples were nitrided in ammonia at 1150°C for 80 min. Judging from EELS and TEM analysis, the layer formed on the surface of the CDC-coated SiC fibers is similar to the coating on the CDC SiC powder. It is composed of pure BN (80–100 nm) at the surface and a mixture of BN with some carbon, which have not been totally consumed during the reaction, adjacent to the interface of the SiC/CDC coating. A uniform and smooth BN coating with good adherence to the fiber core is clearly seen in the HRTEM image in Fig. 7. However, the thickness of the BN coating observed in Fig. 7 must be calculated accounting for the geometric factors in TEM analysis. The diameter of such BN-coated fiber is 10.8 m as known by SEM, so the real thickness of the BN coating on the SiC fiber is around 200 nm by the geometric calculation. Moreover, the crystal structures of the BN formed in the coating layer were also detected by the HRTEM analysis as shown in Fig. 8. Coatings are primarily composed of amorphous and hexagonal BN. Also longer nitridation time and higher treatment temperature help to increase the relative amount of hexagonal BN crystals. The results obtained from fibers are consistent with those from the powders. (D) Mechanical Properties of the Coated Fibers: Tensile tests were performed on the SiC fibers before and after nitridation Fig. 4. Typical HRTEM images of BN structures formed after the nitridation of CDC SiC powders at 1165°C for 60 min: (a) amorphous BN and (b) hexagonal BN. Fig. 5. X-ray diffraction patterns for the synthesis of BN coating on Tyranno ZMI SiC fabrics under various nitridation conditions: (a) asreceived Tyranno ZMI SiC fibers, (b) CDC (0.15 m) coated fibers nitrided at 1150°C for 60 min, (c) CDC (0.15 m) coated fibers nitrided at 1150°C for 80 min, (d) CDC (1.5 m) coated fibers nitrided at 1150°C for 60 min. Fig. 6. Raman analysis of Tyranno ZMI SiC fibers (with 0.15-m-thick CDC layer) before and after nitridation under various treatment conditions: (a) as-received SiC fibers, (b) nitridation at 1150°C for 60 min, (c) nitridation at 1150°C for 80 min, (d) nitridation at 1165°C for 65 min. 1834 Journal of the American Ceramic Society—Chen et al. Vol. 86, No. 11
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 degradation 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
1836 Journal of the American Ceramic Sociery--Chen et al Vol. 86. No. 1I c coating 0.15 5 um 中。=11um 10um Fig 9. SEM images of the cross section of CDC-coated Tyranno ZMI SiC fibers before and after nitridation: (a) secondary electron image of Sic fiber with 0. 15 um thickness carbon coating, (b) BN-coated SiC fiber bundles after nitridation at 1 150.C for 80 min to show no fiber-bridging between each other Table Il. Mechanical Properties of the Tyranno ZMI SiC Fibers after Various Nitridation Treatments Thickness of CDC coating Percentage of fibers showing Ultimate stress Breaking strain Youngs modulus Material eived Tyranno ZMI fibers 0 3.33±1.281.71±0.57193± 364±0.901.56±0.41 247±48 650°C.3h 0.73±0.341.07±0.31 70±29 1165C, 15±0.05 60 2.76±0.781.37±0.28199 0.15±0.05 2.51±0.851.30±048 195 1150°C 0.15±0.05 3.25±0.931.57±0.25 203 (2) Thermodynamic analysis predicts the possibility of carbo- thermal synthesis of Bn on the surface of Sic at and above 1000.C. However, kinetic limitations do not allow coatings of the 2500 Pul|。ut required thickness(100 nm) below 1150C (3) An intermediate CDC layer allows the synthesis of form Bn coatings with no quality degradation and good bonding te 2000 (4) Amorphous and hexagonal graphitic bn have been 1500 formed in the coatings with a gradient of these phases from the BN/C interface to the surface. a small amount of cubic BN nanocrystals was also detected at the very outmost surface layer of (5) Thickness and structure of bn coatings can be controlled atures. However, excessive heat treatment may lead to the crys- tallization of metastable SiC-based fibers produced from poly- 2 de(6) BN coatings can be produced on SiC fibers with no gradation in the mechanical strength. a certain increase in Strain (% Young's modulus and significant enhancement in debonding and pullout have been achieved for the BN-coated fibers Fig. 10. Typical stress-strain curve of a ck BN-coated Tyr- SiC fiber showing pull-out in the me ile test ally and mechanically stable bn coatings on a variety of e materials in a simple and cost-effective way We gratefully acknowledge Dr. Alexei Nikitin and Ms, Beth Carroll for experi- the SEM characterizations. We also thank Dr. J. Schwarz of SSG Precision Optronics V. Conclusions Corp. for the supply of SiC fibers and Dr. I, Barsukov of Superior Graphite Corp. for providing SiC powders. I)bn coatings of uniform thickness can be synthesized the nitridation of H3 BO,-infiltrated CDC coatings on SiC powders d fibers. Unlike CVD, the proposed method allows homoge- References neous coatings on SiC particles and whiskers, and does not bridge ' S.P. S Arya and A D Amico, "Preparation, Properties and Applications of Boron SiC fibers Nitride Thin Film, Thin Solid Films, 157, 267-82(1988)
thermally and mechanically stable BN coatings on a variety of carbide materials in a simple and cost-effective way. V. Conclusions (1) BN coatings of uniform thickness can be synthesized by the nitridation of H3BO3-infiltrated CDC coatings on SiC powders and fibers. Unlike CVD, the proposed method allows homogeneous coatings on SiC particles and whiskers, and does not bridge SiC fibers. (2) Thermodynamic analysis predicts the possibility of carbothermal synthesis of BN on the surface of SiC at and above 1000°C. However, kinetic limitations do not allow coatings of the required thickness ( 100 nm) below 1150°C. (3) An intermediate CDC layer allows the synthesis of uniform BN coatings with no quality degradation and good bonding to the fiber core. (4) Amorphous and hexagonal graphitic BN have been formed in the coatings with a gradient of these phases from the BN/C interface to the surface. A small amount of cubic BN nanocrystals was also detected at the very outmost surface layer of the coating. (5) Thickness and structure of BN coatings can be controlled by the thickness of the CDC layers, nitridation time, and temperatures. However, excessive heat treatment may lead to the crystallization of metastable SiC-based fibers produced from polymeric precursors. (6) BN coatings can be produced on SiC fibers with no degradation in the mechanical strength. A certain increase in Young’s modulus and significant enhancement in debonding and pullout have been achieved for the BN-coated fibers. Acknowledgments We gratefully acknowledge Dr. Alexei Nikitin and Ms. Beth Carroll for experimental assistance and useful discussions and Mr. David Von Rohr (all in the Department of Materials Science and Engineering, Drexel University) for his help in the SEM characterizations. We also thank Dr. J. Schwarz of SSG Precision Optronics Corp. for the supply of SiC fibers and Dr. I. Barsukov of Superior Graphite Corp. for providing SiC powders. References 1 S. P. S. Arya and A. D. Amico, “Preparation, Properties and Applications of Boron Nitride Thin Film,” Thin Solid Films, 157, 267–82 (1988). Fig. 9. SEM images of the cross section of CDC-coated Tyranno ZMI SiC fibers before and after nitridation: (a) secondary electron image of SiC fiber with 0.15 m thickness carbon coating, (b) BN-coated SiC fiber bundles after nitridation at 1150°C for 80 min to show no fiber-bridging between each other. Fig. 10. Typical stress–strain curve of a 200-nm-thick BN-coated Tyranno SiC fiber showing pull-out in the mechanical tensile test. Table II. Mechanical Properties of the Tyranno ZMI SiC Fibers after Various Nitridation Treatments Materials Thickness of CDC coating (m) Percentage of fibers showing pullout (%) Ultimate stress (GPa) Breaking strain (%) Young’s modulus (GPa) As-received Tyranno ZMI fibers — 0 3.33 1.28 1.71 0.57 193 29 Chlorination 550°C, 3 h 0.15 0.05 10 3.64 0.90 1.56 0.41 247 48 650°C, 3 h 1.50 0.10 1 0.73 0.34 1.07 0.31 70 29 Nitridation 1165°C, 65 min 0.15 0.05 60 2.76 0.78 1.37 0.28 199 20 1150°C, 80 min 0.15 0.05 60 2.51 0.85 1.30 0.48 195 26 1150°C, 60 min 0.15 0.05 65 3.25 0.93 1.57 0.25 203 35 1836 Journal of the American Ceramic Society—Chen et al. Vol. 86, No. 11
November 200 Carbothermal Synthesis of BN Coatings on SiC 1837 -P B Mirkarimi, K. F. McCarty, and D. L. Medlin, "Review of Advances in Cubic 2D. Golberg, Y. Bando, L. Bourgeois, R. Ma, K. Ogawa, K. Kurashima, and T. Boron Nitride Film Synthesis, Mater. Sci. Eng, R21, 47-100(1997) tube, Nanocable and Nanocone, Mater. Res. Soc. Symp otzbucher, w, Pfleging, M. Me A. Wesner, and E. w. Kreutz, Proc,706,Z23.1-Z2.3.10(2002) Structure and Chemical Composition of BN Thin Films Grown by Pulsed-Laser S. Ulrich, T BlackglasM Ceramic Matrix Composite Properties, "Ceram. Eng. Sci. Proc., 15, M. B. Mekki, M, A Djouadi, E. Guiot, V. Mortet, J. Pascallon, V. Stambouli, D Bouchier, N. Mestres, and G Nouet, "Structure Investigation of BN Films Grown by L. S. Brown and R. E. Partch,"" Boron Nitride Coatings on Silicon Carbide Means of Polarised IR and Raman Spectroscopy, 2M. Avella, E. Martuscelli, M. Raimo, R. E. Partch, S. G. Gangolli, and B Coat. Technol,116-1993-99(1999 Pascucci,"Polypropylene Reinforced with Silicone Carbide W R. D. Veltri and F. S. Galasso, "Chemical-Vapor-Ir itride,and Silicon Carbide Matrix Composites,J.An 二) Behlau, D. Koch, G. Grathwohl, S. Welz, M. J. McNallan, L. Chen, and Y Gogotsi,""Mechanical Properties of Carbon Coated Ceramic Fibers, 27th Ann ocoa Beach Conference on Advanced Ceramics and Composites, Ceram. Eng. Sci. R N. Singh and M. K. Brun, "Effect of Boron Nitride Coating on Fiber-Matrix Interactions, Ceram. Eng. Sci. Proc., 8[7-8]636-43(1987). Y.G. Gogotsi, "Formation of Carbon Coatings on Carbide Fibers and particles Filipuzzi, G. Camus, R. Naslain, and T. Imamura,"( by Dispropor ns. NATO AS/ Ser.43.217- m.Soc,77[2]459-66(1994) patinas or IJ. D. Cawley, "Effect of Interphase Carbon Thickness on Environmental in Halogens and Supercritical Water,Ceram. Eng. Sci. Proc., 19, 87-94(1998) 377-84 in Ceramic Transactions, Vol 58, High-Temperature Ceramic-Matrir Com- IC F. Windisch Jr, C. H. Henager Jr, G. D. Springer, and R H. Jones, "Oxidation The Minerals, Metals an of the Carbon Interface in Nicalon-Fiber-Reinforced Silicon Carbide Composite, 33Y. Gogotsi, I. D Jeon, and M. J. McNallan, "Carbon Coatings Produced by High J. Am. Ceram. Soc., 80 [3] 569-74(19 Temperature Chlorination of Silicon Carbide Ceramics, J. Mater. Chen,7 T. E. Steyer, F. W. Zok, and D. P. Walls, "Stress Rupture of an Enhanced Nicalon/Silicon Carbide Composite at Intermediate Temperature, "J. An. Ceram D. A Ersoy, M. MeNallan, Y Gogotsi, and A. Erdemir, "Tribological Properties Soc,8I8]2140-46(19989 of Carbon Coatings Produced by High Temperature Chlorination of Silicon Carbide, IG. N. Morscher, "Tensile Stress Rupture of SiC/Sicm Minicomposite Tribol.Tas.,43,809-15(2000) Carbon and Bo phase at Elevated Temperatures in Air,J.Am. 3R. Ma. Y. Bando, T. Sato, and K.Kurashima, th Morphology, and IY. G. Gogotsi, and M. Yoshimura, "Low-Temperature Oxidation, Hydrothermal 36Y. Bando, D. Golberg, M. Mitome, K. Kurashima, and T. Sato, " C to BN Corrosion, and Their Effects on properties of SiC(Tyranno)Fibers," J. Am. Ceram. Electron Microscopy, "Chem. Plys. Let, 346, 29-34(2001). J. E. Lane and A. R. Pebler"Failure Characteristics of Low dielectric Constant Z F. Zhou, I. Bello, V Kremnica, M. K. Fung, K. H. Lai, K. Y Li, C S. Le Ceramic Composites Reinforced with BN-Coated Fibers, "Ceram. Eng. Sci. Proc., 10 and S. T. Lee, "Formation of Cubie Boron Nitride Films on Nickel Substrates, Th id films,368,292-96(2000 I6J. Llorca and R. N. Singh, "Influence of Fiber and Interfacial Properties H Sachdev, R. Haubner, H. Noth, and B Lux, "Investigation of the c-BN/h-BN Fracture Behavior of Fiber-Reinforced Ceramic Composites, " J. Am. Ceram Soc., 74 Phase Transformation at Normal Pressure, Diamond Relat. Mater. 6, 286-92 12882-90(1991 (1997) R. Naslain, O. Dugne, A. Guette, J. Sevely, C. R. Brosse, J. P. Rocher, and J R Ma, Y. Bando, and T. Sato, "Coaxial Nanocables: Fe Nanowires Encapsu Cotteret, "Boron Nitride Interphase in Ceramic-Matrix Composites, J. Am. Ce in BN Nanotubes with Intermediate C Layers, "Chem. Phys. Lett., 350, 1-5(2001) Soc,74[102482-88(1991) 4H Nozaki and S Itoh, "Structural Stability of BC2N, ".. Phys. Chem. Solids, 5 e141-4 Shen, B J. Tan, w.S. Willis, F. S. Galasso, and S L, Suib, "Characterizatio ZMI Fiber/Silicon-Titanium-Carbon-Oxygen Matrix Composites, J. Am. Ceram. of Dip-Coated Boron Nitride on Silicon Carbide Fibers,.Am. Ceram Soc., 77 4 Soc,85[2]393-400(2002) 011-16(1994 Y G. Andreev, T. Lundstrom, R. K Harris, S. w. Oh, D. C. Apperley, and D.P. 42L. Shen, W.S. willis, F. S. Galasso, and S. L. Suib, "Coating of Boron Nitride interfaces on Ceramic Y arms from Boric Acid and Ammonia, "J Am Ceram Soc., 14 L. Andujar,"Micromechanical 43Y. Gogotsi, S. Welz, D. A. Ersoy, and M. J. McNallan, "Conversion of Silicon Properties of BN and B-CN Coatings Obtained by RF Plasma-Assisted CVD, Ca ystalline Diamond-Structured Carbon at Ambient Pressure, "Natwre Diamond Relat. Mater, 8, 423-27(1999). London,411,2838702001) opL. Phys. Lett., 74[20]2948- Ceramic Transactions, Vol. 148, Ceramic Nanomaterials and Nanotechnology I1. 2T. H. Tsai, T S. Yang, C L Cheng, and M. S. Wo of Boron Nit C)Films by Pulsed-DC Magnetron Sputtering, Chem Y Matsui, Y Sekikawa, T. Sato, T. Ishii, S. Isakawa, and K. Shii, "Formations Plys.,72,264-68(2001) of Rhombohedral Boron Nitride, as Revealed by TEM-Electron Energy Loss Cho and J. A. Charles,"Synthesis of Nitrogen Ceramic Powders by on and Nitridation, Part 3: Aluminum Nitride, " Mater. Sci. P. B. Mirkarimi, K. F. McCarty, and D. L. Medlin, "Review of Advances Cubic Boron Nitride Film Synthesis, "Mater. Sci. Eng, R21, 47-100(199 Un, "Carbothermal Reduction and Nitridation of Titanium 473. S. Ha and K. K Chawla, "Effect of SiC/BN Double Coating on Fiber Pullout in and Products, " J. Chem. Eng. Jpn., 31 [5]804-807(1998). Mullite Fibre/Mullite Matrix Composites, "J. Mater. Sci. Leff, 12, 84-86(1993) D
2 P. B. Mirkarimi, K. F. McCarty, and D. L. Medlin, “Review of Advances in Cubic Boron Nitride Film Synthesis,” Mater. Sci. Eng., R21, 47–100 (1997). 3 T. Klotzbu¨cher, W. Pfleging, M. Mertin, D. A. Wesner, and E. W. Kreutz, “Structure and Chemical Composition of BN Thin Films Grown by Pulsed-Laser Deposition (PLD),” Appl. Surf. Sci., 86, 165–69 (1995). 4 S. Ulrich, T. Theel, J. Schwan, and H. Ehrhardt, “Magnetron-Sputtered Superhard Materials,” Surf. Coat. Technol., 97, 45–59 (1997). 5 M. B. Mekki, M. A. Djouadi, E. Guiot, V. Mortet, J. Pascallon, V. Stambouli, D. Bouchier, N. Mestres, and G. Nouet, “Structure Investigation of BN Films Grown by Ion-Beam-Assisted Deposition by Means of Polarised IR and Raman Spectroscopy,” Surf. Coat. Technol., 116–119, 93–99 (1999). 6 B. T. Kelly, Physics of Graphite. Applied Science, London, U.K., 1981. 7 R. D. Veltri and F. S. Galasso, “Chemical-Vapor-Infiltrated Silicon Nitride, Boron Nitride, and Silicon Carbide Matrix Composites,” J. Am. Ceram. Soc., 73 [7] 2137–40 (1990). 8 R. N. Singh and M. K. Brun, “Effect of Boron Nitride Coating on Fiber–Matrix Interactions,” Ceram. Eng. Sci. Proc., 8 [7–8] 636–43 (1987). 9 L. Filipuzzi, G. Camus, R. Naslain, and T. Ymamura, “Oxidation Mechanisms and Kinetics of 1D SiC/SiC Composite Materials: I, An Experimental Approach,” J. Am. Ceram. Soc., 77 [2] 459–66 (1994). 10J. D. Cawley, “Effect of Interphase Carbon Thickness on Environmental Resistance of Continuous Fiber-Reinforced Ceramic Matrix Composites”; pp. 377–84 in Ceramic Transactions, Vol. 58, High-Temperature Ceramic-Matrix Composites II. American Ceramic Society, Westerville, OH, 1995. 11C. F. Windisch Jr., C. H. Henager Jr., G. D. Springer, and R. H. Jones, “Oxidation of the Carbon Interface in Nicalon-Fiber-Reinforced Silicon Carbide Composite,” J. Am. Ceram. Soc., 80 [3] 569–74 (1997). 12T. E. Steyer, F. W. Zok, and D. P. Walls, “Stress Rupture of an Enhanced Nicalon/Silicon Carbide Composite at Intermediate Temperature,” J. Am. Ceram. Soc., 81 [8] 2140–46 (1998). 13G. N. Morscher, “Tensile Stress Rupture of SiCf/SiCm Minicomposites with Carbon and Boron Nitride Interphase at Elevated Temperatures in Air,” J. Am. Ceram. Soc., 80 [8] 2029–42 (1997). 14Y. G. Gogotsi, and M. Yoshimura, “Low-Temperature Oxidation, Hydrothermal Corrosion, and Their Effects on properties of SiC (Tyranno) Fibers,” J. Am. Ceram. Soc., 78 [6] 1439–50 (1995). 15J. E. Lane and A. R. Pebler, “Failure Characteristics of Low Dielectric Constant Ceramic Composites Reinforced with BN-Coated Fibers,” Ceram. Eng. Sci. Proc., 10 [9–10] 1213–22 (1989). 16J. Llorca and R. N. Singh, “Influence of Fiber and Interfacial Properties on Fracture Behavior of Fiber-Reinforced Ceramic Composites,” J. Am. Ceram. Soc., 74 [11] 2882–90 (1991). 17R. Naslain, O. Dugne, A. Guette, J. Sevely, C. R. Brosse, J. P. Rocher, and J. Cotteret, “Boron Nitride Interphase in Ceramic-Matrix Composites,” J. Am. Ceram. Soc., 74 [10] 2482–88 (1991). 18T. Ogasawara, T. Ishikawa, Y. Ohsawa, Y. Ochi, and S. Zhu, “Tensile Creep Behavior and Thermal Stability of Orthogonal Three-Dimensional Woven TyrannoTM ZMI Fiber/Silicon–Titanium–Carbon–Oxygen Matrix Composites,” J. Am. Ceram. Soc., 85 [2] 393–400 (2002). 19Y. G. Andreev, T. Lundstrom, R. K. Harris, S. W. Oh, D. C. Apperley, and D. P. Thompson, “On the Nature of Boron–Carbon–Nitrogen Compounds Synthesized from Organic Precursors,” J. Alloys Compd., 227, 102–108 (1995). 20M. C. Polo, E. Martinez, J. Esteve, and J. L. Andujar, “Micromechanical Properties of BN and B–CN Coatings Obtained by RF Plasma-Assisted CVD,” Diamond Relat. Mater., 8, 423–27 (1999). 21J. Yu and E. G. Wang “Turbostratic Boron Carbonitride Film and Its FieldEmitting Behavior,” Appl. Phys. Lett., 74 [20] 2948–50 (1999). 22T. H. Tsai, T. S. Yang, C. L. Cheng, and M. S. Wong, “Synthesis and Properties of Boron Nitride (BN:C) Films by Pulsed-DC Magnetron Sputtering,” Mater. Chem. Phys., 72, 264–68 (2001). 23Y. W. Cho and J. A. Charles, “Synthesis of Nitrogen Ceramic Powders by Carbothermal Reduction and Nitridation, Part 3: Aluminum Nitride,” Mater. Sci. Technol., 7 [6] 495–504 (1991). 24C. K. Yu and C. I. Un, “Carbothermal Reduction and Nitridation of Titanium Dioxide—Intermediates and Products,” J. Chem. Eng. Jpn., 31 [5] 804–807 (1998). 25D. Golberg, Y. Bando, L. Bourgeois, R. Ma, K. Ogawa, K. Kurashima, and T. Sato, “Boron Nitride Nanotube, Nanocable and Nanocone,” Mater. Res. Soc. Symp. Proc., 706, Z2.3.1-Z2.3.10 (2002). 26S. S. Campbell and S. T. Gonczy, “In Situ Formation of Boron Nitride Interfaces on NextelTM 312 Continuous Ceramic Fiber: I, Nitriding Process and BlackglasTM Ceramic Matrix Composite Properties,” Ceram. Eng. Sci. Proc., 15, 327–36 (1994). 27I. S. Brown and R. E. Partch, “Boron Nitride Coatings on Silicon Carbide Whiskers,” Mater. Res. Soc. Symp. Proc., 458, 421 (1996). 28M. Avella, E. Martuscelli, M. Raimo, R. E. Partch, S. G. Gangolli, and B. Pascucci, “Polypropylene Reinforced with Silicone Carbide Whiskers,” J. Mater. Sci., 32 [9] 2411–16 (1997). 29G. Behlau, D. Koch, G. Grathwohl, S. Welz, M. J. McNallan, L. Chen, and Y. Gogotsi, “Mechanical Properties of Carbon Coated Ceramic Fibers,” 27th Annual Cocoa Beach Conference on Advanced Ceramics and Composites, Ceram. Eng. Sci. Proc., in press. 30Y. G. Gogotsi, “Formation of Carbon Coatings on Carbide Fibers and Particles by Disproportionation Reactions,” NATO ASI Ser., 43, 217–30 (1997). 31Y. G. Gogotsi, S. Welz, J. Daghfal, M. J. McNallan, I. D. Jeon, K. G. Nickel, and T. Kraft, “Formation of Carbon Coatings on SiC fibers by Selective Etching in Halogens and Supercritical Water,” Ceram. Eng. Sci. Proc., 19, 87–94 (1998). 32M. McNallan, Y. Gogotsi, and D. Erosy, “Reaction Formed Coatings for SiC Fibers in Ceramic Matrix Composites”; pp. 351–59 in Elevated Temperature Coatings: Science and Technology III. Edited by J. M. Hampikian and N. B. Dahorte. The Minerals, Metals and Materials Society (TMS), Warrendale, PA, 1999. 33Y. Gogotsi, I. D. Jeon, and M. J. McNallan, “Carbon Coatings Produced by High Temperature Chlorination of Silicon Carbide Ceramics,” J. Mater. Chem., 7 [9] 1841–48 (1997). 34D. A. Ersoy, M. McNallan, Y. Gogotsi, and A. Erdemir, “Tribological Properties of Carbon Coatings Produced by High Temperature Chlorination of Silicon Carbide,” Tribol. Trans., 43, 809–15 (2000). 35R. Ma, Y. Bando, T. Sato, and K. Kurashima, “Growth Morphology, and Structure of Boron Nitride Nanotubes,” Chem. Mater., 13, 2965–71 (2001). 36Y. Bando, D. Golberg, M. Mitome, K. Kurashima, and T. Sato, “C to BN Conversion in Multi-walled Nanotubes as Revealed by Energy-filtering Transmission Electron Microscopy,” Chem. Phys. Lett., 346, 29–34 (2001). 37Z. F. Zhou, I. Bello, V. Kremnican, M. K. Fung, K. H. Lai, K. Y. Li, C. S. Lee, and S. T. Lee, “Formation of Cubic Boron Nitride Films on Nickel Substrates,” Thin Solid Films, 368, 292–96 (2000). 38H. Sachdev, R. Haubner, H. No¨th, and B. Lux, “Investigation of the c-BN/h-BN Phase Transformation at Normal Pressure,” Diamond Relat. Mater., 6, 286–92 (1997). 39R. Ma, Y. Bando, and T. Sato, “Coaxial Nanocables: Fe Nanowires Encapsulated in BN Nanotubes with Intermediate C Layers,” Chem. Phys. Lett., 350, 1–5 (2001). 40H. Nozaki and S. Itoh, “Structural Stability of BC2N,” J. Phys. Chem. Solids, 57 [1] 41–49 (1996). 41L. Shen, B. J. Tan, W. S. Willis, F. S. Galasso, and S. L. Suib, “Characterization of Dip-Coated Boron Nitride on Silicon Carbide Fibers,” J. Am. Ceram. Soc., 77 [4] 1011–16 (1994). 42L. Shen, W. S. Willis, F. S. Galasso, and S. L. Suib, “Coating of Boron Nitride interfaces on Ceramic Yarns from Boric Acid and Ammonia,” J. Am. Ceram. Soc., 14 [7–8] 556–62 (1993). 43Y. Gogotsi, S. Welz, D. A. Ersoy, and M. J. McNallan, “Conversion of Silicon Carbide to Crystalline Diamond-Structured Carbon at Ambient Pressure,” Nature (London), 411, 283–87 (2001). 44L. Chen, H. Ye, and Y. Gogotsi, “Nanocrystalline BN/C Coating on SiC,” in Ceramic Transactions, Vol. 148, Ceramic Nanomaterials and Nanotechnology II. American Ceramic Society, Westerville, OH, in press. 45Y. Matsui, Y. Sekikawa, T. Sato, T. Ishii, S. Isakawa, and K. Shii, “Formations of Rhombohedral Boron Nitride, as Revealed by TEM—Electron Energy Loss Spectroscopy,” J. Mater. Sci., 16, 1114–16 (1981). 46P. B. Mirkarimi, K. F. McCarty, and D. L. Medlin, “Review of Advances in Cubic Boron Nitride Film Synthesis,” Mater. Sci. Eng., R21, 47–100 (1997). 47J. S. Ha and K. K. Chawla, “Effect of SiC/BN Double Coating on Fiber Pullout in Mullite Fibre/Mullite Matrix Composites,” J. Mater. Sci. Lett., 12, 84–86 (1993). November 2003 Carbothermal Synthesis of BN Coatings on SiC 1837