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 homoge￾neous coatings on SiC particles and whiskers, and does not bridge SiC fibers. (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 required thickness ( 100 nm) below 1150°C. (3) An intermediate CDC layer allows the synthesis of uni￾form 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 temper￾atures. However, excessive heat treatment may lead to the crys￾tallization of metastable SiC-based fibers produced from poly￾meric 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 experi￾mental 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 Tyr￾anno 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