Preparation of (Ti,Zr,Hf,Nb,Ta)C high-entropy carbide ceramics through carbosilicothermic reduction of oxides

Abstract

Fully dense high-entropy carbide (HEC) ceramic has been prepared from a mixture of the group IV and V transition metal oxides by a two-step technique, which involved the vacuum carbosilicothermic reduction (VCSTR) synthesis of a composite powder containing 75 vol.% HEC, 20 vol.% (Nb_1-xMe_x)Si₂ (where Me = Ti, Zr, Hf, Ta), and 5 vol.% SiC followed by hot pressing of the as-synthesized product. It was found that the reaction between (Nb_1-xMe_x)Si₂ and HEC took place during hot pressing, thereby allowing effective sintering to occur. The mechanical properties of the obtained nearly single-phase HEC ceramic were comparable to or even slightly better than those of HEC ceramics prepared by other methods. The use of VCSTR synthesis as a key step in the preparation of fully dense HEC ceramic was concluded to be effective both in lowering the sintering temperature and in improving the mechanical properties.

Introduction

The group IV and V transition metal binary carbides with the rock salt type structure (B1), including TiC, ZrC, HfC, NbC, and TaC, are well known as the most refractory compounds having melting temperatures above 3000 °C. At the same time they exhibit extremely high hardness and elastic moduli, providing a rationale for their use in high and ultra-high temperature structural applications [[1], [2], [3]]. Along with the growing interest in the binary carbides listed above, the group IV and V transition metal high-entropy carbides (HECs) have recently attracted great attention. These multi-principal element carbides are found to crystallize in the B1 structure in which the atoms of metals in near-equimolar ratios share a cation position, giving increase in thermodynamic stability due to mixing entropy factor [4]. They have much lower thermal conductivity and diffusivity and better mechanical properties in comparison to those of the binary carbides [[5], [6], [7], [8], [9], [10], [11]]. This makes HEC ceramics a very promising candidate for various industrial applications, e.g., for the components of high temperature nuclear reactors, jet engines and hypersonic vehicles operating under the most extreme environments, etc. Several approaches to fabricating HEC ceramics and ceramic powders are currently known, including pressure-assisted reactive sintering of binary carbides mixtures [[5], [6], [7], [8],[11], [12], [13], [14]], carbothermal reduction of metal oxides mixtures with either graphite or carbon black [[14], [15], [16], [17]], mechanosynthesis through reaction between metallic powders and graphite [[18], [19], [20]]. In addition, magnetron co-sputtering technique can be used to obtain HEC coatings [21,22].

The major difficulty with the preparation of fully dense HEC ceramics is that it requires very high temperature (1800–2200 °C) and pressure (20–60 MPa), because of poor sinterability of HECs. It is suggested that densification temperature can be lowered by using silicon-rich transition metal silicides as sintering aids, by analogy with the case of binary carbide ceramics where MoSi₂ and TaSi₂ sintering aids have been successfully utilized to improve densification and mechanical properties of the obtained ceramics [[23], [24], [25], [26], [27]]. It is also reasonable to consider the possibility of synthesizing both HEC and transition metal silicide at once in a single synthesis procedure as a convenient way to introduce sintering aid into the material. For this purpose, we suggest an approach based on vacuum carbosilicothermic reduction (VCSTR) of oxides, which has been recently developed for the synthesis of MAX phase T₃SiC₂ and Ti₄SiC₃ from TiO₂ [[28], [29], [30], [31], [32], [33]]. The essence of the VCSTR approach is that it utilizes a combination of silicon- and carbon-containing reductants, which necessarily includes SiC reductant. This brings together carbothermic and silicothermic reduction processes, allowing both carbide and silicide compounds to be formed simultaneously. In the present study, we aimed to apply the proposed approach for the preparation of HEC ceramics from a mixture of the group IV and V transition metal oxides by two-step technique, which involves the VCSTR synthesis of a composite powder containing both HEC and silicon-rich transition metal silicide sintering aid followed by the hot pressing of the as-synthesized product.