Microstructure and properties of ZrB₂–SiC composites prepared by spark plasma sintering using TaSi₂ as sintering additive

Abstract

ZrB₂–SiC composites were fabricated by spark plasma sintering (SPS) using TaSi₂ as sintering additive. The volume content of SiC was in a range of 10–30% and that of TaSi₂ was 10–20% in the initial compositions. The composites could be densified at 1600 °C and the core–shell structure with the core being ZrB₂ and the shell containing both Ta and Zr as (Zr,Ta)B₂ appeared in the samples. When the sintering temperature was increased up to 1800 °C, only (Zr,Ta)B₂ and SiC phases could be detected in the samples and the core–shell structure disappeared. Generally, the composites with core–shell structure and fine-grained microstructure showed the higher electrical conductivity and Vickers hardness. The completely solid soluted composites with coarse-grained microstructure had the higher thermal conductivity and Young's modulus.

Introduction

Recently, ZrB₂ has been considered as the promising thermal protective materials for reentry spacecrafts due to its low density (6.11 g/cm³), high melting point (3250 °C), high elastic modulus (491 GPa), high hardness (23 GPa), high thermal conductivity (56 W/m K), good high temperature oxidation resistance, and excellent thermal shock resistance.1, 2, 3, 4 During the dropping in the atmosphere of earth, the nose cap and leading edge of spacecrafts need to endure the high temperature up to 2200 °C and high speed ions ablation.⁵ According to the entry simulation (arc-jet) testing, SiC reinforced ZrB₂ composites possess the remarkable anti-ablation capability and oxidation resistance.6, 7 Also, the introduction of SiC particles into ZrB₂ matrix enhances the fracture toughness of composites from 2 to 4 MPa m^1/2, which upgrades the use reliability.⁸ In previous reports, many works have focused on investigating the in situ synthesis, low temperature sintering, particle or whisker toughening, as well as the relationship between microstructure and properties of ZrB₂–SiC composites.3, 9, 10, 11 For the investigation branch about low temperature sintering, there are mainly two ways. The first way is using the deoxidized carbides, such as B₄C, WC, or VC, to remove the surface oxides of ZrB₂ for promoting the sintering.12, 13, 14 The second way is selecting the low melting point additives, such as MoSi₂ (melting point, 2020 °C) or TaSi₂ (melting point, 2040 °C) to form the plastic interface phases.15, 16, 17 For using TaSi₂ as sintering aid, Opila et al.¹⁶ and Peng and Speyer¹⁸ found that the introduction of TaSi₂ into ZrB₂–SiC could further enhance the oxidation resistance of composites. Additionally, Talmy et al.¹⁹ reported that solid solution appeared in ZrB₂–10, 20, and 30 vol.% TaSi₂ composites sintered at 2000 °C by hot pressing, which formed the core–shell structure with the core being ZrB₂ and the shell containing both Ta and Zr as (Zr,Ta)B₂. This phenomenon was associated with the decomposition of TaSi₂ and solid solution between Ta–Si phase and ZrB₂, which was proved by Sciti et al.²⁰ who found that Ta_xSi_y phase containing a certain amount of Zr existed in ZrB₂–15 vol.% TaSi₂ composite prepared by hot pressing at 1850 °C. The core–shell structure indicates the heterogeneous microstructure characteristic of composites. It is a sub-stable structure which is prone to become stable by atoms diffusion. For eliminating the core–shell structure of crystals, Talmy et al.¹⁹ considered that an increase in holding time probably contributed to complete the formation of the solid solutions.

In present work, dense bulk ZrB₂–SiC composites were fabricated at 1600–1800 °C by spark plasma sintering using TaSi₂ as additive.21, 22, 23 It was found that increasing the sintering temperature was an effective way to remove the core–shell structure. The microstructure, physical, and mechanical properties of as-prepared ZrB₂–SiC composites with and without core–shell structure were investigated and compared.