CC BY
19iSHS 2019
Moscow, Russia
COMBINATION OF SHS AND SPS PROCESSING ROUTES FOR
ADVANCED CERAMICS
R. Orru*a, G. Tallarita", R. Lichen", and G. Cao"
aDipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Unita di Ricerca del
Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM),
Universita di Cagliari, Via Marengo 2, 09123 Cagliari, Italy *e-mail: roberto.orru@dimcm.unica.it
DOI: 10.24411/9999-0014A-2019-10115
The fabrication of fully dense components is well known to represent a difficult target to achieve for various refractory ceramic systems, particularly if the starting powders are produced by conventional (furnace or solution) methods while pressure-less or standard hot-pressing (HP) techniques are employed to consolidate them. For instance, when considering members of the materials family generally referred to as ultra-high temperature ceramics (UHTCs) based transition metal borides and carbides, powders have to be usually exposed for hours to temperature above 2000°C in order to reach reasonable densification levels. Therefore, it is apparent that more convenient synthesis/consolidation routes should be considered to fully exploit the unique combination of chemico-physical and mechanical properties of UHTCs (melting temperature exceeding 3000°C, high hardness, chemical inertness, good thermal and electrical conductivities, neutron and selective solar energy absorption capabilities, etc.) which makes them so attractive in the aerospace, nuclear, solar energy, and other industrial fields. In this context, the efficient spark plasma sintering (SPS), where the die/powders/punches ensemble is fastly heated by an electric current flowing through it, is often found to require less severe temperature/time conditions for powders consolidation with respect to standard HP [1]. Moreover, significant beneficial effects are reported in the literature when the self-propagating high-temperature synthesis (SHS) technique was adopted for the preparation of powders to be subsequently consolidated via SPS. The latter statement is supported by the several highly dense ceramics, including various UHTCs, as well as metallic, and composite systems, successfully fabricated through the SHS-SPS route. Alternatively, the synthesis reaction and the consolidation of the resulting product can be carried out concurrently, according to the reactive SPS (R-SPS) method. Various monophasic (ZrB2, HfB2, TaB2, TiB2, ZrC, HfC, TaC), binary (ZrB2-SiC, HfB2-SiC, TaB2-SiC) and ternary (ZrB2-ZrC-SiC, HfB2-HfC-SiC, TaB2-TaC-SiC) ultra-refractory ceramic systems have been produced either by R-SPS or SHS-SPS.
Because of the strong exothermic nature of the reactions typically involved for synthesizing UHTCs, the studies conducted so far evidenced that the intrinsic potential of the R-SPS method is often hampered by various negative features accompanying the sudden occurrence of combustion synthesis events, which make the process quite difficult to control, albeit the product densification could be correspondingly facilitated. It should be noted in this regard that the combustion synthesis regime could be suppressed during the R-SPS process, if the heating rate is adequately lowered [2].
The R-SPS and SHS-SPS procedures have been both recently considered for the fabrication of some high entropy borides (HEBs), an emerging class of UHTCs. HEBs belong to the more general family of high-entropy alloys, where metallic elements are combined in near equimolar ratios to provide a single crystalline solid-solution characterized by maximum configurational entropy. The method originally proposed in the literature to produce massive HEBs was based on the consolidation of the ceramic constituents (ZrB2, HfB2, TaB2, etc.) combined in equimolar
XV International Symposium on Self-Propagating High-Temperature Synthesis
ratio, after being subjected for 6 h to high energy ball milling [3]. Since such an intense mechanical treatment prolongs markedly the total processing time and the contamination of the resulting powders from milling media becomes unavoidable, more suitable fabrication strategies should be proposed.
In this context, the use of the R-SPS process for the preparation of HEBs, with the synthesis reaction evolving under the combustion regime, was attempted for the case of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2. Specifically, when the elemental reactants (Hf Mo, Ta, Nb, Ti, and B) were processed by SPS for 20 min at 1950°C, with a heating rate of 200°C/min and the mechanical pressure switched from 20 to 70 MPa immediately after the combustion synthesis event, a 91.5 % dense material was obtained. However, as evidenced in Figs. 1a, 1b, the resulting material display some large pores (approximately up to 15 |m sized) and the elemental distribution, particularly niobium, was not uniform across the sample. These negative features could be likely associated with the occurrence of the rapid combustion synthesis reaction, since the gases correspondingly liberated because of the presence of impurities or volatile species, are not able to freely escape from the die/punches tool, so that the resulting product microstructures become inhomogeneous with the presence of relatively large pores.
According to the SHS-SPS technique, the same elemental precursors were also reacted in few seconds by SHS using free standing pellets. The resulting product was composed of the desired (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 phase for approximately 96 wt %, with minor amounts of (Ta0.5Ti0.5)B2, (Hf0.5Ti0.5)B2, and HfB2, as well as traces of HfO2 [4]. Furthermore, when the corresponding powders were processed for 20 min at 1950°C by SPS under an applied pressure of 20 MPa, the full conversion to the single phase HEB was accomplished. In addition, the final product was about 92.5% dense, very similarly to consolidation levels achieved by Gild et al. for the same system [3]. From the SEM micrograph and the corresponding EDX elemental mapping shown in Figs. 1c and 1d, respectively, it can be deduced not only that a good densification level is reached but also that the species composition is very uniform across the material.
Similar results are also recently obtained for other HEB systems, to demonstrate that the use of the SHS-SPS approach could provide a useful contribution for the development of this new class of ultrarefractory ceramics.
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(a) (b) (c) (d)
Fig. 1. SEM micrographs and related niobium EDX maps of the HEB sample produced by (a, b) R-SPS and (c, d) SHS-SPS.
This research was supported by the Regione Autonoma della Sardegna (Italy), Fondo di Sviluppo e Coesione 2014-2020, (project ARCHIMEDES - A new family of reinforced ultrahigh-temperature metal diborides for advanced applications, Cod. RAS: RASSR88309, Cod. CUP: F76C18000980002). One of the authors (G.T.) performed her activity in the framework of the International PhD in Innovation Sciences and Technologies at the University of Cagliari, Italy.
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