Effect of zirconia addition on pressureless sintering of boron carbide C. Subramanian a, * , T.K. Roy b , T.S.R.Ch. Murthy a , P. Sengupta a , G.B. Kale a , M.V. Krishnaiah c , A.K. Suri a a Materials Group, Bhabha Atomic Research Centre, Mumbai, India b Variable Energy Cyclotron Centre, Kolkata, India c Materials Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India Received 11 September 2006; received in revised form 11 March 2007; accepted 28 April 2007 Available online 2 June 2007 Abstract This paper presents the results of experiments on pressureless sintering of boron carbide with varying addition of zirconia (ZrO 2 : 0–30 wt.%). Green pellets were densified by sintering at 2275 8C in vacuum for 60 min and characterized by measurement of density, hardness, thermal conductivity and microstructure. Samples prepared with the addition of 5 wt.% ZrO 2 showed higher densities in the range of 93–96% rth, compared to 86.63% rth for boron carbide only. Addition of ZrO 2 was found to increase the hardness of sintered samples and regardless of ZrO 2 content, the hardness values ranged between 30 and 31.5 GPa. XRD of the sintered pellets showed the presence of ZrB 2 . Optical microscope as well as electron probe microanalysis (EPMA) showed the presence of two phases, grey matrix with white precipitates. EPMA analysis of second phase revealed the presence of Zirconium in this phase. Fractography of boron carbide with 25% ZrO 2 showed the failure to be by mixed fracture (transgranular and intergranular). Thermal conductivity values of the samples measured in the temperature range of 400–1000 8C were marginally higher with the addition of ZrO 2 . # 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Thermal conductivity; Boron carbide; Pressureless sintering; Additive ZrO 2 1. Introduction: Boron carbide is an important ceramic material with high melting point (2450 8C), outstanding hardness and low specific gravity (2.52 g/cc). Knoop hardness of pressureless sintered boron carbide and hot pressed material are reported as 25.5  2.4 GPa and 29.0  1.5 GPa, respectively [1]. Boron carbide is used as neutron absorber owing to its high boron content, chemical stability and refractory character [2]. Other industrial uses of boron carbide are as abrasives for lapping, grinding and polishing media for hard materials and as wear-resistant components. B 4 C possesses strong bonding, low plasticity, high resistance to grain boundary sliding and low superficial tension in the solid state; all these factors make densification of powders difficult by sintering [3]. In addition, the presence of B 2 O 3 on B 4 C surface slows down the densification process [4]. A few of the recent publications that appeared on the sintering of B 4 C with oxide additives are given below. Goldstein et al. reported that heating of B 4 C–YTZP (Yttria stabilized zirconia polycrystals) mixtures to temperature of 2000 8C yielded B 4 C–ZrB 2 composites with better densifica- tion than monolithic B 4 C [5]. Kim et al. have reported on the reaction sintering and mechanical properties of B 4 C with addition of ZrO 2 by hot pressing and pressureless sintering [6]. Levin et al. have studied the effect of TiO 2 and Ti additions on the sintering behavior of B 4 C in the temperature range of 1800–2190 8C [7]. TiO 2 reacts with B 4 C to yield a two phase composite consisting of TiB 2 and sub-stiochometric B 4 C 1x [7,8]. Skorokhod et al. studied the mechanical properties of pressureless sintered boron carbide with in situ reaction of TiO 2 and carbon [9]. Frage studied the effect of addition of chromium oxide on sintering of boron carbide and identified the formation of reaction product CrB 2 in the sintered composite [10]. Golstein et al. reported the formation of B 4 C + metal boride (TiB 2 , ZrB 2 , VB 2 , VB, CrB 2 , YB 2 , YB 4 , LaB 6 , LaB 6 + ZrB 2 ) composites (>95% rth) by in situ reaction of B 4 C and metal oxide (TiO 2 , ZrO 2 ,V 2 O 5 , Cr 2 O 3 ,Y 2 O 3 , La 2 O 3 , www.elsevier.com/locate/ceramint Ceramics International 34 (2008) 1543–1549 * Corresponding author. Tel.: +91 22 25593937; fax: +91 22 25505151. E-mail address: csubra@barc.gov.in (C. Subramanian). 0272-8842/$34.00 # 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2007.04.017