Orientation-dependent properties of silicon nitride with aligned reinforcing grains
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Orientation-dependent properties of silicon nitride with aligned reinforcing grains Dong-Soo Park,a) Myoung-Je Choi,b) Tae-Wook Roh,b) Hai-Doo Kim, and Byung-Dong Han Ceramic Materials Group, Korea Institute of Machinery and Materials, 66 Sang-Nam-Dong, Chang-Won City, Kyong-Nam, South Korea (Received 21 July 1999; accepted 25 September 1999)
Silicon nitride with the aligned reinforcing grains was prepared by tape casting with addition of the silicon nitride whiskers and gas-pressure sintering at 2148 K. The microstructure and the mechanical properties of the sintered sample, including the fracture toughness and the three-point flexural strength, were highly anisotropic. Both the fracture toughness and the flexural strength were the highest when the crack-propagation direction was normal to the alignment direction. This result was interpreted from the laminate composite materials’ point of view. Although the large elongated grains were as long as 44.4 ± 12 m and as wide as 5.1 ± 0.67 m, they were not the fracture origins.
I. INTRODUCTION
Silicon nitride has been studied due to the excellent properties including high strength and high fracture toughness among the ceramics. Even though its fracture toughness is the highest among the monolithic ceramics, silicon nitride is still brittle among the engineering materials. In order to find a way to improve the fracture toughness further, the toughening mechanism of silicon nitride ceramics has been investigated.1–3 Crack bridging by large elongated grains is assumed as an important toughening mechanism in self-reinforced silicon nitride ceramics.1 Since the fracture toughness increment due to the reinforcing grain is proportional to the square root of its width according to the crack bridging mechanism, efforts have been made for growing larger grains. However, the toughening effect due to randomly oriented reinforcing grains often accompanies the flexural strength decrease as shown in Becher’s report.1 Recently, Hirao et al. developed a highly anisotropic silicon nitride by using tape-casting seeded powders and then gas-pressure sintering.4 The reinforcing grains of the silicon nitride are highly oriented in the casting direction, and both the fracture toughness and the flexural strength are improved at the same time by developing the aligned reinforcing grains.5 This suggests that the presence of larger reinforcing grains does not necessarily mean a strength decrease. It also implies that the orientation of those grains might play an important role in
a) b)
Address all correspondence to this author. Graduate student, Department of Material Science and Engineering, Pukyong National University.
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http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 1, Jan 2000 Downloaded: 11 Mar 2015
determining the mechanical properties as reported on the fracture toughness anisotropy.6 In this study, the effect of orientation of those reinforcing grains on the mechanical properties of silicon nitride was investigated. II. E
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