Contents lists available at ScienceDirect Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc Original Article Effect of high-energy ball-milling on the spark plasma sinterability of ZrB 2 with transition metal disilicides Victor Zamora, Fernando Guiberteau, Angel L. Ortiz* Departamento de Ingeniería Mecánica, Energética y de los Materiales, Universidad de Extremadura, 06006, Badajoz, Spain ARTICLEINFO Keywords: ZrB 2 Ultra-high-temperature ceramics Transition metal disilicides Spark plasma sintering High-energy ball-milling ABSTRACT A wide suite of powder mixtures of ZrB 2 with five different additions of transition metal disilicides (MeSi 2 ; 5, 17.5, or 30 vol% MoSi 2 ; 17.5 vol% TaSi 2 ; or 17.5 vol% ZrSi 2 ) were prepared by high-energy ball-milling (HEBM) for different times, and their optimal densification temperatures were evaluated by dilatometric spark-plasma- sintering (SPS) tests to compare their sinterability. SPS densification tests at the so-determined optimal densi- fication temperatures were also performed in selected cases. It was found that HEBM progressively enhanced the sinterability, especially once the ZrB 2 particle sizes were refined to the ultrafine range or below (< 250 nm). It was also observed that the sinterability was enhanced (i) with the greater addition of MeSi 2 , and (ii) with the lower refractoriness of the MeSi 2 . Nonetheless, under long-time HEBM, the addition of harder, more refractory MeSi 2 or of softer, less refractory MeSi 2 is equally effective in terms of sinterability, both enabling low-tem- perature densification (SPS at ∼1200 °C). 1. Introduction Composites based on ZrB 2 with addition of transition metal dis- ilicides (MeSi 2 ꞌs) are part of a wider family of fascinating materials, the so-called ultra-high-temperature ceramics (UHTCs) [1–4], for use in extreme environments (e.g., ultra-severe conditions of temperature, oxidation, heat flux, radiation, mechanical load, and chemical re- activity) like those encountered for example in hypersonic-flights and atmospheric re-entry vehicles, rocket propulsion, concentrated-sunlight thermal receivers, and nuclear fission reactors [4–6]. The appeal of these UHTCs is that they synergistically combine the excellent thermo- mechanical properties of ZrB 2 (high or ultra-high melting point, hard- ness, stiffness, strength, thermal conductivity, etc.) with the easier densification, enhanced oxidation resistance, and greater high-tem- perature ductility provided by MeSi 2 ꞌs [7,8]. The combination of ZrB 2 with MoSi 2 has received the most attention to date [7–15], but other relatively unexplored MeSi 2 ꞌs (TaSi 2 , ZrSi 2 , etc.) [16–22] are also in- teresting for tailoring the properties of these UHTCs, and merit further research effort. Nonetheless, it has been demonstrated that in terms of oxidation resistance MoSi 2 is the best MeSi 2 additive, that TaSi 2 is preferable over ZrSi 2 and WSi 2 in oxidizing atmospheres up to ∼1500 °C but unpractical above that temperature, and that ZrSi 2 and WSi 2 are equally resistant up to ∼1650 °C but with ZrSi 2 becoming unsuitable at higher temperatures [21]. An aspect that continues to be a challenge in the UHTCs field is to achieve their full densification at the mildest conditions of temperature and pressure. Besides the use of sintering aids, the poor sinterability of UHTCs and the need for fully-dense pieces have driven the search for alternatives to the conventional pressureless sintering or hot-pressing of the typical commercially-available micrometre-sized powders. The poor intrinsic sinterability of UHTCs derives from the strong covalent bonding and the low self-diffusion coefficients, and their poor extrinsic sinterability is due to the large particle sizes, the surface oxide im- purities, and the slow cycles possible in the furnaces with radiant heating elements. This has motivated growing interest in the powder processing by high-energy ball-milling (HEBM), a high impact force mechanical comminution technique [23], and in the powder densifi- cation by spark-plasma sintering (SPS), a pressure-assisted ultrafast sintering technique [24], both of which are scalable to industrial pro- duction and can be used separately or in combination. SPS produces only simple-shaped parts, but these are nonetheless amenable to elec- trical discharge machining. Consequently, the present study was undertaken to analyze for the first time how HEBM promotes the full densification by SPS of UHTCs based on ZrB 2 with additions of MeSi 2 ꞌs. The study involves the pro- cessing of 38 different powder mixtures with different compositions and times of HEBM, which were subjected to dilatometric SPS tests in the framework of two separate but interconnected sub-studies, one https://doi.org/10.1016/j.jeurceramsoc.2020.06.046 Received 6 April 2020; Received in revised form 11 June 2020; Accepted 15 June 2020 ⁎ Corresponding author. E-mail address: alortiz@unex.es (A.L. Ortiz). Journal of the European Ceramic Society 40 (2020) 5020–5028 Available online 20 June 2020 0955-2219/ © 2020 Elsevier Ltd. All rights reserved. T