Hardness and toughness of hot pressed ZrB 2 –SiC composites consolidated under relatively low pressure Mehdi Shahedi Asl a , Mahdi Ghassemi Kakroudi a,⇑ , Seyedreza Noori b a Department of Materials Science and Engineering, University of Tabriz, Tabriz, Iran b Department of Chemistry, Materials and Chemical Engineering, Polytechnic University of Milan, Milan, Italy article info Article history: Received 24 June 2014 Received in revised form 29 August 2014 Accepted 1 September 2014 Available online 16 September 2014 Keywords: Composite materials Sintering Mechanical properties Scanning electron microscopy Metallography Microstructure abstract ZrB 2 -based composites, containing 15, 20, 25, and 30 vol% SiC, have been prepared by hot pressing at temperatures of 1700, 1850 and 2000 °C for 30 min under a relatively low pressure of 10 MPa. Densifica- tion and mechanical properties of ZrB 2 –SiC composites have been investigated. Fully dense ZrB 2 –30 vol% SiC composite with a relative density of 99.8% is obtained at 2000 °C. The highest values of Vickers hard- ness (21.3 GPa) and fracture toughness (4.7 MPa m 1/2 ) belong to this sample. Vickers hardness increases exponentially as the relative density of composite increases. A simplified equation was developed for the Vickers hardness of the investigated ZrB 2 –SiC composites as a function of relative density and SiC content. Microstructural investigation by means of optical and scanning electron microscopy shows that by addi- tion of SiC particles, some toughening mechanisms such as crack deflection, crack branching, microcrack- ing, crack bridging, break of large SiC grains, and crack arresting by porosity are appeared. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Zirconium diboride is remarkable for its ultra-high melting temperature (3245 °C) as well as its hardness and elastic modulus, low electrical resistivity, and resistance to chemical attack. As a result, this material has been proposed for a variety of structural applications at room and elevated temperature including armor, cutting tools, molten metal containment, steel processing, and electrodes. Zirconium diboride is also considered to be an ultra- high-temperature ceramic and is a candidate for use as leading edges and propulsion components in hypersonic aerospace vehi- cles and advanced reusable atmospheric reentry vehicles [1,2]. SiC reinforcement of ZrB 2 is known to increase flexural strength, fracture toughness and oxidation resistance [3,4]. Some physical and mechanical properties of monolithic ZrB 2 and SiC are summa- rized in Table 1. Hardness of a ZrB 2 –SiC composite scales with the volume fraction of SiC and generally follows the composites’ rule of mixtures with respect to the amount of SiC included in the com- posite. Reported hardness values are 15–23 GPa for ZrB 2 . Therefore, addition of SiC (HV  28 GPa) to ZrB 2 results in a slight increase in hardness [1]. ZrB 2 , containing 27 vol% SiC, with fracture toughness of 3.5 MPa m 1/2 and Vickers hardness of 22.8 GPa was fabricated by reactive hot pressing of ZrH 2 ,B 4 C and Si between 1600 and 1900 °C [5]. In a systematic study of the effect of additive content, it was reported that fracture toughness increased from 3.5 MPa m 1/2 for monolithic ZrB 2 ceramic to 5.3 MPa m 1/2 for ZrB 2 –30 vol% SiC composite. Hardness did not vary significantly with respect to SiC content variation. It was 24 GPa for all compositions. Speci- mens exhibited crack deflection and crack bridging [6,7]. Com- pared to monolithic ZrB 2 , the increase in fracture toughness of ZrB 2 , containing 10 vol% submicron SiC (0.8 lm), was of relative significance (4.8 MPa m 1/2 ) [8]. Structure–property relations have been investigated for ZrB 2 composites (with starting particle sizes of 2 and 6 lm), containing 30 vol% SiC (with particle size of 0.7 lm), at different hot pressing times and temperatures. Hard- ness was not affected by processing conditions, with average value of 22 GPa. In contrast, fracture toughness varied with processing conditions. For specimens prepared from larger-particle-size ZrB 2 , fracture toughness increased from 3.9 to 4.3 MPa m 1/2 , as hot pressing temperature increased from 1850 to 2050 °C. For specimens prepared from smaller-particle-size ZrB 2 , fracture toughness decreased from 5.5 to 4.3 MPa m 1/2 , as hot pressing tem- perature increased [9]. The effect of SiC particle size, ranging from 0.45 to 10 lm, on the microstructure of ZrB 2 -based composites, containing 30 vol% SiC, was also studied. Investigations showed that smaller starting SiC particles led to improved densification, finer grain sizes, and higher hardness and strength [10]. A ZrB 2 - based composite, containing 20 vol% Nano-sized (30 nm) b-SiC http://dx.doi.org/10.1016/j.jallcom.2014.09.006 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: 29 Bahman Blvd., Department of Materials Science and Engineering, Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran. Tel.: +98 411 339 2470. E-mail address: mg_kakroudi@tabrizu.ac.ir (M.G. Kakroudi). Journal of Alloys and Compounds 619 (2015) 481–487 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom