Densification, microstructure and mechanical properties of SiO 2 –cBN composites by spark plasma sintering Jianfeng Zhang, Rong Tu * , Takashi Goto Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 28 April 2011; received in revised form 1 July 2011; accepted 5 July 2011 Available online 18th July 2011 Abstract SiO 2 –cBN composites were consolidated by spark plasma sintering at 1473–1973 K. The effects of cBN content and sintering temperature on the relative density, phase transformation, microstructure and mechanical properties of the SiO 2 –cBN composites were investigated. The relative density of the SiO 2 –cBN composites increased with increasing SiO 2 content. The phase transformation of cBN to hBN in SiO 2 –cBN composites was identified at 1973 K, showing the highest transformation temperature in cBN-containing composites. The SiO 2 –20 vol% cBN composites sintered at 1673 K showed the highest hardness and fracture toughness of 12.5 GPa and 1.5 MPa m 1/2 , respectively. # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Mechanical properties; Cubic boron nitride; Silicon oxide; Spark plasma sintering; Phase transformation 1. Introduction Since SiO 2 has excellent electrical-insulation, high chemical stability, a low thermal expansion coefficient and low thermal conductivity [1–3], it has been applied as a refractory material in crucibles for melting high-purity silicon and in circuit boards. However, the low hardness and high brittleness limits its applications as a structural material. The fabrication of SiO 2 - based composites with a hard phase is a promising way to improve the hardness and fracture toughness of SiO 2 . Like SiO 2 , cubic boron nitride (cBN) has low thermal expansion and electrical conductivity [4,5], but unlike SiO 2 , it is characterized by the high hardness and thermal conductivity [5–7]. Although cBN is hardly densified, SiO 2 is easily densified by hot pressing. Since cBN does not readily react with SiO 2 , SiO 2 –cBN composite should be a promising structural material. However, no report on SiO 2 –cBN composite has been published to date. Spark plasma sintering (SPS) has been widely used for the densification of materials due to its rapid heating, low sintering temperature and short sintering time. To date, many kinds of fully dense high-temperature structural ceramics [8,9], trans- parent ceramics [10,11], and intermetallics [12] have been successfully fabricated. SPS would inhibit the phase transfor- mation of cBN to hBN due to the short sintering time. In the present study, SiO 2 –cBN composites were prepared by SPS and the effects of cBN content and sintering temperature on the density, micorstructure and mechanical properties of SiO 2 – cBN composites were investigated. 2. Experimental details SiO 2 (SO-E1, Admatechs, mean diameter: 0.25 mm) and cBN (SBN-F, Showa Denko, mean diameter: 2.8 mm) powders were used as raw materials. cBN at 0–50 vol% was mixed with SiO 2 by a planetary ball mill at a rotary rate of 120 rpm for 48 h in ethanol. The mixed powder was dried at 333 K for 24 h and then passed through a 100-mm sieve three times. The powder was poured into a graphite die with an inner diameter of 10 mm and sintered by SPS (SPS-210LX, SPS Syntex Inc, Japan) at 1473–1973 K at a heating rate of 1.67 K s 1 , a holding time of 0.6 ks and a pressure of 100 MPa. The temperature was measured by an optical pyrometer focused on a hole (Ø 2 mm  5 mm) in the graphite die. The shrinkage of the specimens was continuously monitored by the displacement of a punch rod. The shrinkage was calibrated by deducting the displacement of the graphite die. www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 38 (2012) 351–356 * Corresponding author. Tel.: +81 22 215 2106. E-mail address: turong@imr.tohoku.ac.jp (R. Tu). 0272-8842/$36.00 # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2011.07.013