CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 40 (2014) 4819–4826 Combined effect of SiC chopped fibers and SiC whiskers on the toughening of ZrB 2 Diletta Sciti n , Laura Pienti, Daniele Dalle Fabbriche, Stefano Guicciardi, Laura Silvestroni CNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza, Italy Received 3 May 2013; received in revised form 10 September 2013; accepted 10 September 2013 Available online 25 September 2013 Abstract ZrB 2 -based composites were toughened by the simultaneous additions of SiC chopped fibers and SiC whiskers. The fracture toughness measured by Chevron Notched Beam in flexure was of the order of 6 MPa m 1/2 and the 4-pt flexural strength around 500 MPa. The values of mechanical properties are compared to those of unreinforced ZrB 2 and composites reinforced by solely fibers or whiskers in order to understand whether a synergistic whiskers-fibers toughening action is present or not in these composites. The combination of two different reinforcing agents is a successful strategy to obtain a super reinforcement provided that densification, reinforcing agents chemical integrity and secondary phases composition and amount are properly controlled. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Whiskers; C. Mechanical properties; Fibers; ZrB 2 ; Microstructure 1. Introduction Zirconium diboride (ZrB 2 ) is currently considered among the most appealing materials for aerospace applications owing to its high melting temperature and better performances over metal components in high-temperature environments, including low creep rate and high strength at elevated temperatures. In the last decade, research has focused on fabricating high density composites possessing good strength (500–1000 MPa) [1–6]. One aspect that deserves investment of efforts regards the low fracture toughness of these compounds which ranges from 2.5 to 4.5 MPa m 1/2 . The possibility to produce transition metal borides with higher defect tolerance would open a wider scenario of uses and expand the market for this class of compounds. The simplest approach for increasing fracture toughness is the addition of discontinuous elongated reinforcements that are able to activate toughening mechanisms, such as crack pinning, crack deflection, crack bridging, microcracking and thermal residual stress. It has been reported that the addition of short fibers, nanotubes, whiskers or flakes gives promising results for the increase of the fracture toughness [7–12]. Addition of short carbon fibers led to an increase of toughness up to 6.6 MPa m 1/2 [7]. Fracture toughness values up to 6.7 MPa m 1/2 were reported by single edge notched beam (SENB) for SiC whiskers additions [8–12] or in the range of 5–6.2 MPa m 1/2 by chevron notched beam (CNB) in flexure for SiC chopped fibers [13–15]. The aim of the present study is to explore the effect of simultaneous addition of fibers and whiskers, thus exploiting a synergistic effect from both the reinforcing phases. In the past a few attempts were carried out on the basis of this approach. Becher et al. [16] suggested that coupling of two or more toughening mechanisms, such as addition of whiskers or platelets combined with microstructural tailoring, offers the potential to achieve fracture toughness values in excess of 10 MPa m 1/2 . He reported that the fracture toughness of a fine grained Si 3 N 4 matrix exhibits lower toughness than when elongated matrix grains are formed. Studies on mullite materials reinforced by 20 vol% SiC whiskers and 20 vol% ZrO 2 particles showed that the combined toughening effects were equal to, or greater than, the sum of individual toughening effects [17]. In the work of Kodama et al. [18] a notable increase of fracture toughness was obtained adding SiC particles and SiC whiskers to a silicon nitride matrix: when a 10 vol% of particles was combined with 20 vol% of SiC whiskers, the toughness changed from 7 MPa m 1/2 , for the addition of only whiskers, to 10.5 MPa m 1/2 . 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.2013.09.031 n Corresponding author. Tel.: þ39 546 699748; fax: þ 39 546 46381. E-mail address: diletta.sciti@istec.cnr.it (D. Sciti).