CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 41 (2015) 5843–5851 Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB 2 –SiC composite Mehdi Shahedi Asl a,n , Mahdi Ghassemi Kakroudi a , Rouhollah Abedi Kondolaji a , Hadi Nasiri b a Department of Materials Science and Engineering, Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran b Department of Chemical Engineering, Amirkabir University of Technology, Tehran, Iran Received 7 November 2014; received in revised form 13 December 2014; accepted 3 January 2015 Available online 9 January 2015 Abstract Hot pressed monolithic ZrB 2 ceramic (Z), ZrB 2 –20 vol% SiC composite (ZS 20 ) and ZrB 2 –20 vol% SiC–10 vol% nano-graphite composite (ZS 20 G n10 ) were investigated to determine the influence of graphite nano-flakes on the sintering process, microstructure, and mechanical properties (Vickers hardness and fracture toughness) of ZrB 2 –SiC composites. Hot pressing at 1850 1C for 60 min under 20 MPa resulted in a fully dense ZS 20 G n10 composite (relative density: 99.6%). The results disclosed that the grain growth of ZrB 2 matrix was efficiently hindered by SiC particles as well as graphite nano-flakes. The fracture toughness of ZS 20 G n10 composite (7.1 MPa m 1/2 ) was essentially improved by incorporating the reinforcements into the ZrB 2 matrix, which was greater than that of Z ceramic (1.8 MPa m 1/2 ) and ZS 20 composite (3.8 MPa m 1/2 ). The fractographical observations revealed that some graphite nano-flakes were kept in the ZS 20 G n10 microstructure, besides SiC grains, which led to toughening of the composite through graphite nano-flakes pull out. Other toughening mechanisms such as crack deflection and branching as well as crack bridging, due to the thermal residual stresses in the interfaces, were also observed in the polished surface. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Hot pressing; B. Composites; C. Toughness and toughening; ZrB 2 –SiC; Graphite nano-flakes 1. Introduction Zirconium diboride (ZrB 2 ), among ultrahigh temperature ceramic matrix composites (UHTCs), is a candidate of wonderful interest for employing in thermal protection systems for leading edge sections of hypersonic re-entry aerospace vehicles, hypersonic aircraft, rocket engines or other industrial applications, such as plasma arc electrodes and furnace elements. ZrB 2 has a superior combination of high melting point, high strength and hardness, high electrical and thermal conductivities and chemical and physical stability at ultrahigh temperatures. However, its weak intrinsic sinterability, due to the strong covalent bonds, the low bulk and grain boundary diffusivities and the presence of surface oxide impurities, as well as poor fracture toughness and thermal shock resistance are the prime impediments to solve [1–4] . Ordinarily, appropriate amounts of reinforcements (e.g. SiC and C) are introduced in the ZrB 2 matrix to encourage the sintering process, improve the mechanical and thermal properties, and also enhance the oxidation resistance of ZrB 2 -based composites by promoting the formation of silicate glasses as oxidation inhibitors at elevated temperatures. Many researchers have reported increased mechanical properties (espe- cially the fracture toughness) and oxidation resistance of SiC reinforced ZrB 2 -based composites [5–8]. In addition, carbon reinforcements, with different sources and morphologies, caused improved consolidation and fracture toughness of ZrB 2 [3, 9–11] . A fully dense zirconium diboride, coated with at least 1 wt% carbon, pressurelessly sintered at 1900 1C for 120 min [3]. By coating the ZrB 2 powder with polycarbosilane, which converts to carbon and silicon carbide by pyrolysis, the improved consolida- tion and mechanical properties were obtained [12–13]. Carbon reacts with and removes the surface oxide impurities, ZrO 2 and B 2 O 3 as well as SiO 2 in SiC containing composites, which assists densification of ZrB 2 by minimizing grain growth [3,14]. www.elsevier.com/locate/ceramint http://dx.doi.org/10.1016/j.ceramint.2015.01.014 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Correspondence to: Department of Materials Science and Engineering, Faculty of Mechanical Engineering, University of Tabriz, 29 Bahman Blvd., Tabriz, Iran Tel.: þ 98 912 3277186. E-mail address: shahedi@tabrizu.ac.ir (M. Shahedi Asl).