Mechanical, electrical and thermal properties of HfC-HfB₂-SiC ternary eutectic composites prepared by arc melting

Highlights

• The ternary eutectic composition of 16HfC-17HfB₂-67SiC (mol%) and the eutectic temperature of 2760 K was first identified.

• The HfC-HfB₂-SiC ternary eutectic composite had higher microhardness and fracture toughness than the composites prepared by sintering.

• The HfC-HfB₂-SiC ternary eutectic composite showed excellent electrical and thermal conductivity, which is beneficial for its application.

Abstract

HfC-HfB₂-SiC composites were prepared by arc melting using HfC, HfB₂ and SiC powder as raw materials. The ternary eutectic composition of 16HfC-17HfB₂-67SiC (mol%) was first identified, showing a complicated maze microstructure of HfC, HfB₂ and SiC approximately 500 nm in thickness. The eutectic temperature of the HfC-HfB₂-SiC composite was nearly 2760 K. The Vickers hardness and fracture toughness of the HfC-HfB₂-SiC ternary eutectic composite were 20.8 GPa and 7.7 MPa m^1/2, respectively. With increasing temperature from 300 to 800 K, the electrical conductivity decreased from 8.8 × 10⁵ to 4.3 × 10⁵ Sm⁻¹, whereas the thermal conductivity increased from 28 to 32 W m⁻¹ K⁻¹.

Introduction

Hafnium diboride (HfB₂) and hafnium carbide (HfC), ultra-high temperature ceramics (UHTCs), have many excellent properties, e.g., high thermal conductivity and hardness and good chemical and thermal stability [[1], [2], [3]]. Among the UHTCs, IVB transition metal di-borides and carbides have been extensively studied due to their outstanding properties, and their basic properties are summarized in Table 1 [[4], [5], [6], [7], [8], [9], [10], [11], [12]]. However, all of them are difficult to sinter due to their ultra-high melting points and low diffusion coefficients [13]. Silicon carbide (SiC) is considered an effective sintering aid to improve the mechanical properties of HfC- and HfB₂-based composites [14]. HfC-HfB₂-SiC ternary composites are expected to possess the excellent combination of properties of each component. Binary composites of HfC, HfB₂ and SiC have been commonly prepared by pressureless sintering [15], hot pressing [16] (HP), reactive HP [17] and spark plasma sintering [18] (SPS), but a high temperature (over 2000 °C) and long sintering time (over 1 h) are indispensable. Yuan et al. [19] prepared HfC-HfB₂-SiC composites by hot pressing (HP). Licheri et al. [20] prepared HfB₂-SiC and HfC-HfB₂-SiC dense composites by self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) with a lower dwell temperature (1800 °C) and shorter time (10 min); however, the composites showed a low Vickers hardness of 18.3 GPa. Shahriari et al. [21] used HfO₂, Mg, H₃BO₃, C, and Si as raw materials to prepare HfC-HfB₂-SiC composites and found better mechanical properties for the composite with 15 vol% HfC. Furthermore, few reports on HfC-HfB₂-SiC composites are concerned with thermal and electrical conductivity, which may extend their potential applications.

Our research group has investigated many carbide-boride eutectic composites, such as TiB₂-TiC [22], ZrB₂-ZrC [23], TiC-TiB₂-SiC [24], and ZrC-ZrB₂-SiC [25]. There are limited investigations related to the synthesis, characterization and oxidation behavior of the HfC-HfB₂-SiC system. Some studies have shown that directionally solidified eutectic composites might improve the mechanical, thermal, and chemical stability compared to their monolithic or traditional composite counterparts [[26], [27], [28]]. Arc melting is a convenient approach for preliminary studies on new eutectic systems because it can provide temperatures exceeding 3000 °C, and an inert atmosphere is particularly suitable for nonoxide directionally solidified eutectics [29]. Because Ti, Zr and Hf belong to the same family in the elemental periodic table, their carbides and borides may have analogous improved behavior. In this study, HfC-HfB₂-SiC ternary composites were prepared by arc melting. This study is an extension of a previous investigation on a MeC-MeB₂-SiC (Me: Ti, Zr, Hf) system [[22], [23], [24], [25]] and focuses on the HfC-HfB₂-SiC system, exploring the consolidation of eutectic composites with a eutectic composition. The effect of the composition on their microstructure and mechanical, electrical and thermal properties was investigated.