Fabrication and Characterization of Quinary High Entropy-Ultra-High Temperature Diborides” Ceramics

S. Barbarossa

First

;R. Orru'

Second

;V. Cannillo;A. Iacomini;S. Garroni;M. Murgia

Penultimate

;G. Cao

Last

2021-01-01

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Abstract

Due to their inherently chemical complexity and their refractory nature, the obtainment of highly dense and single-phase High Entropy (HE) diborides represents a very hard target to achieve. In this framework, homogeneous (Hf0.2Nb0.2Ta0.2Mo0.2Ti0.2)B2, (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2, and (Hf0.2Zr0.2Nb0.2Mo0.2Ti0.2)B2 ceramics with high relative densities (97.4, 96.5 and 98.2 %, respectively) are successfully produced by Spark Plasma Sintering (SPS) using powders prepared by Self-propagating High-temperature Synthesis (SHS). Although the latter technique does not lead to the complete conversion of initial precursors into the prescribed HE phases, such goal is fully reached after SPS (1950°C/20min/20 MPa). The three HE products show similar, even better in some cases, mechanical properties compared to ceramics with the same nominal composition attained using alternative processing methods. Superior Vickers hardness and elastic modulus values are found for the (Hf0.2Nb0.2Ta0.2Mo0.2Ti0.2)B2 and (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2)B2 systems, i.e. 28.1 GPa/538.5 GPa and 28.08 GPa/498.1 GPa, respectively, in spite of the correspondingly higher residual po-rosities (1.2 and 2.2 vol.%, respectively). In contrast, the third ceramic, not containing Tantalum, displays lower values of these two properties (25.1 GPa/404.5 GPa). However, the corresponding fracture toughness (8.84 MPa m1/2) is relatively higher. This fact can be likely ascribed to the smaller residual porosity (0.3 vol.%) of the sintered material.