Available online at www.sciencedirect.com Journal of the European Ceramic Society 33 (2013) 603–614 Synthesis, consolidation and characterization of monolithic and SiC whiskers reinforced HfB 2 ceramics Clara Musa a , Roberto Orrù a,∗ , Diletta Sciti b , Laura Silvestroni b , Giacomo Cao a a Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Centro Studi sulle Reazioni Autopropaganti (CESRA), Unità di Ricerca del Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Università degli Studi di Cagliari, Piazza D’Armi, 09123 Cagliari, Italy b CNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza, Italy Received 9 March 2012; received in revised form 28 September 2012; accepted 8 October 2012 Available online 2 November 2012 Abstract Spark Plasma Sintering is used for the fabrication of highly dense HfB 2 monolithic and HfB 2 –26 vol.% SiC w composite. Reactive SPS from elemental reactants is preferred for the preparation of bulk HfB 2 instead of classical sintering. The desired phase is rapidly formed through a solid–solid combustion synthesis mechanism, while full densification is achieved in 30 min at 1350 A when the applied pressure is switched from 20 to 50 MPa after the synthesis reaction. A 99.4% dense whiskers-reinforced HfB 2 ceramic matrix composite is also obtained in 30 min by SPS (I = 1350 A, P = 20 MPa) using SHSed HfB 2 powders and SiC w . Nevertheless, whiskers degradation into SiC p resulted under such conditions (temperature up to 1830 ◦ C). On the other hand, the presence of whiskers is clearly evidenced in 96% dense products obtained when the applied current was decreased down to 1200 A (1700 ◦ C) while P was increased to 60 MPa. © 2012 Elsevier Ltd. All rights reserved. Keywords: Composites; Borides; Whiskers; Self-propagating High-temperature Synthesis; Spark Plasma Sintering 1. Introduction Due to their well-known peculiar characteristics, i.e. high melting point, high hardness, high electrical and thermal con- ductivity, good chemical inertness and resistance in oxidizing environments, ZrB 2 - and HfB 2 -based ultra-high-temperature- ceramics (UHTCs) have attracted considerable attention in several traditional and innovative application fields where severe conditions have to be withstood. 1 In particular, these materials have recently found interest in the aerospace industry for the fabrication of thermal protection systems of components to be exposed to high-flow conditions like leading edges in hypersonic vehicles. 1 With the aim of making these promising ceramics widely employed, a noticeable effort has been made over the past decades for the fabrication of such refractory ceramics in bulk form and to improve their properties, particularly fracture ∗ Corresponding author at: Università degli Studi di Cagliari, Piazza D’Armi, 09123 Cagliari, Italy. Tel.: +39 070 6755076; fax: +39 070 6755057. E-mail addresses: roberto.orru@dimcm.unica.it, orru@dicm.unica.it (R. Orrù). toughness. 1 Specifically, the first aspect relates to the severe densification conditions (sintering temperature and applied pres- sure) generally needed to overcome the low intrinsic sinterability of HfB 2 and ZrB 2 . In this regard, the use of suitable techniques for the efficient preparation of massive UHTCs, such as the rela- tively novel Spark Plasma Sintering (SPS) technique, where the processing powders and/or the die containing them are crossed by an electric pulsed current, 2 is considered very crucial. Indeed, while relatively long processing times (on the order of hours) are required in conventional Hot Pressing (HP), heating processes, and consequently sintering phenomena, are strongly acceler- ated during SPS (few minutes). Correspondingly, materials with relatively more uniform and finer microstructure are typically obtained. 2 These considerations hold also true when, starting from proper reactants, the reaction synthesis and densification is accomplished in one step, by the so-called reactive SPS (RSPS). Another complimentary approach for overcoming the intrin- sic low sinterability of HfB 2 and ZrB 2 compounds is represented by the use of certain sintering aids, such as MoSi 2 , 3 HfN and Si 3 N 4 , 4 and, especially, SiC, 5–8 which also improve oxidation resistance of the resulting ceramic matrix composites at high temperatures. 0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.10.004