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E驅≈3S ournal of the European Ceramic Society 21(2001)841-845 www.elsevier.com/locate/jeurc Creep behavior of SiC whisker-reinforced Si3 N4/BN fibrous monolithic ceramics Shuqin Li*, Yong Huang, Changan Wang, Yongming Luo, Linhua Zou, Cuiwei Li state Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084 People's Republic of China Received 15 June 2000: received in revised form 11 August 2000: accepted 21 August 2000 The flexural creep behavior of Sic-whisker reinforced hot-pressed Si3N4/BN fibrous monolithic ceramics in the temperature range 1000-1200@C and stress range 250-600 MPa were characterized. Creep curves generally showed extensive primary and lack of tertiary creep. The creep and fracture mechanism were mainly controlled by the BN interlayer Bn began to oxidise at 1200 C and decreased the creep resistance. Meanwhile SiC whisker pulling-out improved the creep resistance. Microscopic damage mechanisms were investigated by SEM. C 2001 Elsevier Science Ltd. All rights reserved. Keywords: BN; Composites; Creep: SiC whiskers; Si3N4/ BN fibrous monoliths 1. Introduction 2. Experimental methods Silicon nitride-based ceramics possess excellent prop 2. 1. Raw materials and fabrication process erties. 1,2 which make them ideal candidate materials for high temperature applications. However, the nature of Si3N4(Founder High-Tech ceramic Corp, China) brittleness is one of the most crucial problems in their powders with 8 wt %Y2O3(purity >99.9%), 4 wt. applications. Al,O3(>99.9%) were ball milled with 20 wt. SIC Enlightened from studying some natural composites, e whisker(TWS-400, Tokai Carbon Co., Japan)in ethanol bamboo, mollusk shell etc, ceramic matrix composites for 24 h to achieve a homogenous mixture. This was containing weak interfacial layers have been considered mixed with organic binders and then produced green to offer a very important approach to improve the filaments using a coextrusion process. The green fila- property of ceramics. 3.4 Whisker reinforcement is a well ments were subsequently coated with a slurry of 25 established method of enhancing the creep resistance of wt. BN and 75 wt. Al2O3, dried and parallel packed ceramic materials. 5,6 For this reason, the present inves- into a graphite die. After dewaxing, the green body was tigation was designed to characterize the creep and hot pressed in a graphite resistance furnace under N2 creep fracture properities of Si, N4/BN fibrous monolithic 1820 C for 1.5 h and under a pressure of 22 MPa. A ceramics. The creep responses at various temperatures detailed description of the fabrication process can be and stress levels were measured. From the data, the found in the literature. The test specimens were cut into kinetics of deformation and the stress dependence of 4x x36 mm rectangular bars, then polished with dia- flexural creep were determined. A numerical analysis mond pastes down to 7um was also employed to estimate the power-law creep parameters. The influence of microstructure on the 2. 2. Creep testing creep deformation mechanisms was also investigated Creep testing was conducted using a creep tester which consisted of a three-point bend test fixture of ax Co SiC with a span of 30 mm. Loads were applied to the upper ram via a lever arm having a 5: 1 leverage ratio 0955-2219/01/S. see front matter C 2001 Elsevier Science Ltd. All rights reserved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