Are short Hi-Nicalon SiC fibers a secondary or a toughening phase for ultra-high temperature ceramics? D. Sciti ⇑ , S. Guicciardi, L. Silvestroni CNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza, Italy article info Article history: Received 30 July 2013 Accepted 6 October 2013 Available online 18 October 2013 Keywords: Ceramic Fibers Transmission electron microscopy Fracture toughness Ultra high temperature ceramics abstract This paper deals with the effect of the addition of Hi-Nicalon SiC fibers to Zr- and Hf-borides. The main scope is to understand the fiber/matrix chemical interaction and correlate it to the fracture toughness. Transmission electron microscopy (TEM) was used as key investigation tool to disclose the microstruc- tural features at nanoscale level. Several sintering additives were used to enable densification in the tem- perature range 1600–1850 °C. It was observed that the fiber strongly reacts with the matrix at the same temperature at which the sintering additive starts to be effective. At this point, the fibers themselves locally behave as sintering aid promoting a strong fiber/matrix bonding which prevents any possibility of fiber pullout. Fiber modification was correlated with the fracture toughness and it was at last deduced that these fibers exert a toughening action only when the sintering temperature is kept below 1700 °C. Above this temperature fibers start to significantly degrade and can be considered just as a secondary phase. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Zirconium and hafnium diboride (ZrB 2 and HfB 2 ) have been identified as two of the most promising materials for use as leading edges in hypersonic aerospace vehicles due to their potential per- formances over metal components in high-temperature environ- ments, including low creep rate, high strength at elevated temperatures and high melting temperature [1–4]. Despite the well-known positive engineering attributes, these borides are brittle, characteristic which dramatically limits their use in those applications where failure of a small part would result catastrophic [1,3]. A well-known method for increasing the fracture toughness of brittle ceramics is the addition of a higher aspect ratio reinforc- ing phase [5]. Recent studies report on the development of self-reinforced microstructures [6], controlled growth of SiC plate- lets [7–9], and the use of discontinuous SiC whiskers [10–14]. The authors of the present work have extensively studied the effect of the addition of Hi-Nicalon SiC short fiber to borides matrices and have obtained fracture toughness values above 6 MPa m 1/2 [13–17]. Hyuga et al. [18] added short C fibers to Si 3 N 4 ceramics and reported a toughness increase of about 50% accompanied by a 50% decrease of strength. Recently, chopped C fibers have been also introduced in a reaction bonded silicon carbide, again observing an increase in fracture toughness from 3 to 5 MPa m 1/2 [19], whilst other authors have used ZrO 2 fibers to toughen borides finding, however, a notable interaction of ZrO 2 with the matrix [20,21]. A major concern in the use of Hi-Nicalon SiC fibers in UHTC matrices is that they are not stable at temperatures higher than 1500 °C even in inert atmosphere [22], but at this temperature densification of ZrB 2 or HfB 2 is not complete. Therefore, sintering aids must be employed for lowering the sintering temperature as much as possible, and amongst all the possibilities, those permit- ting densification at relatively low temperature are ZrSi 2 (1550 °C) [23], Si 3 N 4 (1700 °C) [13], MoSi 2 (1750 °C) [24]. Recent published research [15–17] has outlined that the change of sinter- ing aid has consequences on mechanical properties both at room and high temperature and on stability in extreme environments [13–17]. As for fracture toughness, this property is affected by a number of additional factors including: fiber volumetric amount, matrix density, interface reaction, fiber chemical evolution. So far, only a few of these factors have been correlated with the exper- imental values. It was shown that fracture toughness has the ten- dency to increase with the increase of fiber content up to 20 vol% [15]. Secondly, it was found that in order to fully benefit from toughness increment, dense matrices should be obtained [15]. Nevertheless, even when observing data from composites contain- ing the same amount of fibers and the same level of density, significant differences are found. Microstructural observations of the SiC fiber in the final matrices showed that, during sintering, the fibers undergo a notable modification, developing a multilay- ered structure strongly bonded to the matrix. In a previous work, it was suggested that the type of sintering aid used could affect the fiber modification [15]. Sintering aid can have a direct role, 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.10.019 ⇑ Corresponding author. Tel.: +39 0546699748. E-mail address: diletta.sciti@istec.cnr.it (D. Sciti). Materials and Design 55 (2014) 821–829 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes