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 830Carbothermal 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, forma￾tion 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 atmo￾spheric pressure to produce the pure BN coating. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron mi￾croscopy (HRTEM), and electron energy loss spectroscopy (EELS) were used to characterize the phase, elemental com￾position, and surface morphology of the coatings. The inter￾mediate 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 tempera￾tures.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 well￾defined 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 rhombohe￾dral (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 demon￾strated by Bando.25 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 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 carbide￾derived 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