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Shuji Sakaguchi Synergy Materials Research Center, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, 463-8560, Japan

Koichi Niihara Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan (Received 3 October 2003; accepted 27 January 2004)

Hexagonal graphitic BN (h-BN) is interesting as a second phase for high-temperature structural ceramics because it has the same crystal structure as graphite, for which fracture strength and Young’s modulus increase with increased temperature. In this study, high-temperature mechanical properties of Si3N4/BN nanocomposite were evaluated to clarify the effect of fine h-BN particles at elevated temperatures. As a result, we found that high-temperature strength and hardness of the nanocomposite were maintained up to high temperatures; also, its Young’s modulus increased gradually, concomitant with elevated temperatures up to 1400 °C. Finally, these properties were compared with those of monolithic Si3N4 and Si3N4/BN microcomposite. I. INTRODUCTION

Si3N4 ceramics are one 1of the most promising engineering materials1 because of their excellent thermomechanical properties: high strength at elevated temperatures, low coefficient of thermal expansion, and relatively high resistance to thermal shock and chemical attack. In particular, their applications have been anticipated in the field of high-temperature structural ceramics for applications in gas-turbine engines, rocket and laser nozzles, radomes, and antenna windows. Although they have good fracture strength and fracture toughness at room temperature, Si3N4 ceramics have somewhat decreased fracture strength at high temperatures due to softening of grain boundary glassy phases and consequent grain boundary sliding and cavity formation. Generally, two methods have reportedly improved high temperature mechanical properties of Si3N4 ceramics. One is to enhance refractoriness of the grain boundary phase2–6; another is to add the second phase with superior mechanical properties at high temperatures. Incorporation of a higher refractory second phase such as SiC, TiC, and TiN was expected to be effective in yielding Si3N4 ceramics with excellent high-temperature mechanical properties;

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0192 1432

http://journals.cambridge.org

J. Mater. Res., Vol. 19, No. 5, May 2004 Downloaded: 02 Apr 2015

therefore, it has been studied frequently.7–10 Among these highly refractory materials, h-BN is especially interesting as a second phase because it has the same crystal structure as graphite.11–14 this fact indicates increasing fracture strength and Young’s modulus with increased temperature. There have been many investigations of the effect of SiC particle addition on mechanical properties of Si3N4 ceramics at elevated temperatures.6–8,15,16 On the other hand, studies on Si3N4/BN composites have rarely been reported bec