CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 39 (2013) 6637–6646 Spark plasma sintering of graphene reinforced zirconium diboride ultra-high temperature ceramic composites Govindaraajan B. Yadhukulakrishnan, Sriharsha Karumuri, Arif Rahman, Raman P. Singh, A. Kaan Kalkan, Sandip P. Harimkar n School of Mechanical and Aerospace Engineering Oklahoma State University, Stillwater, OK 74078, USA Received 8 January 2013; received in revised form 14 January 2013; accepted 15 January 2013 Available online 6 February 2013 Abstract Spark plasma sintering (SPS) of monolithic ZrB 2 ultra-high temperature ceramic and 2–6 vol% graphene nanoplates (GNPs) reinforced ZrB 2 matrix composites is reported. The SPS at 1900 1C with a uni-axial pressure of 70 MPa and soaking time of 15 min resulted in near-full densification in ZrB 2 –GNP composites. Systematic investigations on the effect of GNP reinforcement on densification behavior, microstructure, and mechanical properties (microhardness, biaxial flexural strength, and indentation fracture toughness) of the composites are presented. Densification mechanisms, initiated by interfacial reactions, are also proposed based on detailed thermodynamic analysis of possible reactions at the sintering temperature and the analysis of in-process punch displacement profiles. The results show that GNPs can be retained in the ZrB 2 matrix composites even with high SPS temperature of 1900 1C and cause toughening of the composites through a range of toughening mechanisms, including GNP pull-out, crack deflection, and crack bridging. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Sintering; B. Composites; B. Microstructure-final; C. Toughness and toughening; D. Borides 1. Introduction Ceramics with melting point greater than 3200 1C are generally classified as ultra-high temperature ceramics (UHTC). The most important application of UHTC is in the leading edges and nose caps of space vehicles re-entering the earth’s atmosphere. These materials also find applications in high temperature crucibles and parts of electrical heaters and igniters [1–4]. Among the several UHTC systems (TaC, HfC, NbC, ZrC, HfB 2 , ZrB 2 , and TiB 2 ), ZrB 2 based ceramics are the most promising for thermal protection systems due to their good combination of elevated temperature mechanical properties and oxida- tion resistance. However, there is a critical need to further improve their oxidation resistance, thermal conductivity, and fracture toughness to realize the full potential of ZrB 2 - based ceramics as next generation materials for sharp leading edge re-entry space vehicles [5–8]. In general, reinforcements are added to ZrB 2 ceramic matrix to improve sinterability and mechanical, thermal, and oxida- tion properties of the composites. Several investigations have reported improved oxidation resistance and/or frac- ture toughness of SiC reinforced ZrB 2 ceramic composites [9–14]. Recently, carbon nanotubes (CNTs) have also been used as nano-scale filler in the ZrB 2 ceramics [14–15]. The CNT reinforcement resulted in improved densification particularly in the early (particle rearrangement) and final stages (diffusion) of densification. The CNT reinforcement also resulted in improvement in room temperature fracture toughness [14]. While the effectiveness of CNT reinforce- ment in improving densification and fracture toughness was demonstrated, exact mechanisms of densification and toughening are still not fully understood. Furthermore, uniform dispersion of the CNTs in the UHTC composites is often difficult [16]. Recently, graphene nanoplates (GNPs) are attracting significant attention as potential nano-scale reinforcement www.elsevier.com/locate/ceramint 0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.01.101 n Corresponding author. Tel.: þ 1 405 744 5830. E-mail address: sandip.harimkar@okstate.edu (S.P. Harimkar).