Microstructure and fracture behavior of b-Si 3 N 4 based nanoceramics Ching-Huan Lee a , Horng-Hwa Lu b , Chang-An Wang c , Wen-Tse Lo a , Pramoda K. Nayak a , Jow-Lay Huang a, * a Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan, ROC b Department of Mechanical Engineering, National Chin-Yi University of Technology, Taiping, Taichung 411, Taiwan, ROC c 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 30 May 2010; received in revised form 2 September 2010; accepted 4 October 2010 Available online 4 November 2010 Abstract Fully dense b-Si 3 N 4 based nanoceramics were consolidated by spark plasma sintering (SPS). A commercially available b-Si 3 N 4 nano-powder was used as starting material. The sintering bulks were fabricated with a heating rate of 90 8C/min by varying SPS temperature from 1550 8C to 1700 8C, and the linear shrinkage was used to evaluate the sintering behavior of the nano-powder. The microstructures of the developed Si 3 N 4 based ceramics were achieved, and the grain length, grain width, and aspect ratio for these sintering bulks were found to increase gradually with elevating sintering temperature. The hardness and fracture toughness are associated with the microstructural characteristics. The crack propagation and fracture behavior for these b-Si 3 N 4 based nanoceramics have been observed for the specimens sintered at different sintering temperatures. Crack deflection is found to be one of the toughening mechanisms for these b-Si 3 N 4 based nanoceramics. # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Microstructure; C. Fracture; Silicon nitride; Nanoceramic; Spark plasma sintering 1. Introduction Spark plasma sintering technique has provided efficient strategies to activate sintering kinetics for poorly sinterable materials, including nanostructured ones, and results in an overall improvement in materials performance [1]. Among them, Si 3 N 4 based ceramics with ultrafined or nanosized grain has shown much potential applications in technical and engineering fields. Only a few studies have demonstrated to fabricate Si 3 N 4 based ceramics with finer grain size by SPS. Nishimura et al. [2] got ultrafine-grained Si 3 N 4 from finer b- Si 3 N 4 powder; Xu et al. [3–5] have fabricated a series of Si 3 N 4 based nanoceramics through high-energy mechanical milling and SPS. These finer grained materials are expected to exhibit noticeable high-temperature ductility, improved wear resis- tance, and elevated hardness [6–9]. However, the use of Si 3 N 4 nanoceramics in engineering applications has been limited by the lack of Si 3 N 4 nanoprecursor powder and a commercially available sintering process. Utilization of b-Si 3 N 4 based nanopowder prevents the formation of a Si 2 N 2 O phase, which degrades mechanical, thermal, and tribological properties due to the large agglomeration of nanocrystalline grains and its high temperature instability [10]. Therefore, b-Si 3 N 4 nanopowder doped with sintering additives can be used to obtain Si 3 N 4 based nanoceramics without complex processing steps, such as high energy milling [3] or carbothermal reduction [9]. In the present study, the b-Si 3 N 4 based nanoceramics can be obtained in a proper sintering condition by a large-scale SPS. The aim of this work is to investigate the densification behavior at a fixed heating rate during sintering. The microstructural characteristics and mechanical properties of dense b-Si 3 N 4 based nanoceramics were then studied. Owing to the lack of information on the relationship between microstructure and fracture behavior so far, the crack propagation and fracture topography for these nanoceramics with different grain sizes and aspect ratios were discussed in the present study. www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 37 (2011) 641–645 * Corresponding author. Tel.: +886 6 2348188; fax: +886 6 2763586. E-mail address: jlh888@mail.ncku.edu.tw (J.-L. Huang). 0272-8842/$36.00 # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2010.10.009