Journal of the European Ceramic Society xxx (xxxx) xxx Please cite this article as: Arseniy Bokov, Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2021.05.007 Available online 7 May 2021 0955-2219/© 2021 Elsevier Ltd. All rights reserved. Interplay between decarburization, oxide segregation, and densifcation during sintering of nanocrystalline TaC and NbC Arseniy Bokov *, Anna Shelyug, Alexey Kurlov Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, Yekaterinburg, 91 Pervomaiskaya st., 620990, Russia A R T I C L E INFO Keywords: Sintering Tantalum Niobium Carbide ceramics Decarburization ABSTRACT The present study shows that ball-milled nanosized powders of TaC and NbC can be successfully sintered to high densities at 13001400 ◦ C for 30 min under vacuum. Fabricated ceramics demonstrate hardness of 2025 GPa due to decrease in carbon stoichiometry and submicron grain size. The main techniques to investigate the un- derlying phenomena during processing of initial powders were XRD, SEM and TGA-DSC. After milling, carbide particles demonstrate a signifcant amount of oxygen impurities but no signs of oxidation. During sintering, these impurities react with structural carbon and metal ions, which results in decarburization and segregation of oxides. Above 1000 ◦ C, the oxide phases undergo partial reduction by structural carbon, promoting decarburi- zation even further. Densifcation starts shortly after the reduction of oxides and provides dense microstructures. The effects of decarburization and oxide segregation can be compensated by carbon excess, however it can be diffcult to control densifcation curve in such case. 1. Introduction The group 5 transition metal carbides TaC and NbC are characterized by strong covalent bonding and high hardness [1], but slow diffusion makes it diffcult to sinter dense bodies without wetting by metallic phase. Therefore these compounds nowadays are used only as additives for improvement of mechanical properties in WC [2], Ti(C,N) [3], steel [4], or Ni-based [5] alloys. However, there are also some energy applications where the use of TaC and NbC ceramics might be benefcial. For example, tantalum carbide could be employed as solar receiver (absorber) in concentrated solar power applications [6,7], whereas niobium carbide could be used in fabrication of microencapsulated fuels [8]. Nevertheless, sinterability issues of TaC and NbC should be addressed in the frst place before introducing these materials to new application areas. The refnement of grain size has been known as an effective way to fabricate highly dense oxide ceramics at relatively low temperatures due to increase in the thermodynamic driving force for sintering [9]. However, literature data shows that this approach might not be suit- able for non-oxide materials. For instance, thermal treatment of nanocrystalline WC leads to formation of W 2 C phase [10,11], which causes embrittlement of tungsten-based hard alloys. Sintering of nanosized SiC particles often provides porous samples because of the abnormal grain growth [12,13]. Densifcation of nanocrystalline TiN results in a non-uniform microstructure due to segregation of the Ti 3 O 5 or Ti 4 O 7 phases [14,15]. In all these cases the observed phenomena were driven by oxygen impurities that existed in starting materials prior processing. Non-oxide particles always have an oxidized layer at the surface to be in equilibrium with the ambient air, therefore it can adversely affect properties of the product fabricated from nano- powders as the total amount of oxygen is directly proportional to the surface area [16]. In the context of nanomaterials, the TaC and NbC compounds are characterized by very useful features. These carbides remain as single phase under different carbon stoichiometry according to homogeneity range [1], whereas WC transforms into W 2 C upon carbon loss [10]. The segregation of oxide phases could be expected similarly to the TiN case [14], however these oxides might be reduced by structural carbon from the carbide lattice [17]. Additionally, the oxides of Ta and Nb have not been reported to trigger abnormal grain growth, whereas SiO 2 has been proven to promote this effect [12]. Therefore, tantalum and niobium carbides could be more suitable for grain refnement and sintering in comparison with other non-oxide materials. The goal of this study is to verify such hypothesis and investigate whether nano- crystalline TaC and NbC could provide dense ceramics with reasonable characteristics. * Corresponding author. E-mail address: bokov@ihim.uran.ru (A. Bokov). Contents lists available at ScienceDirect Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc https://doi.org/10.1016/j.jeurceramsoc.2021.05.007 Received 10 March 2021; Received in revised form 24 April 2021; Accepted 4 May 2021