Isolating strengthening contributions in multiphase high entropy (Zr-Ta-W-Ti)C-SiC based carbide ceramics

Abstract

Fully dense (ZrC-TaC-WC-TiC)-SiC (named as ZTaWT-S) based high entropy carbide (HEC) composites were fabricated using spark plasma sintering (SPS) at 1750 °C. Herein, effect of carbonaceous reinforcements (multi-walled carbon nanotube (CNT), graphite (G) and graphene (GNP) on microstructure and mechanical properties of the composites is assessed. The formation of a cubic rock salt high entropy carbide (HEC) phase is indicated via phase and microstructural characterization. Further, local elemental distribution using energy dispersive spectroscopy, and interplanar spacing calculation using transmission electron microscopy (TEM) confirmed the HEC phase formation in the sintered samples. Refined grains (∼3.9 μm), and high densification (>99%), has elicited an increasing hardness from 23.1 GPa to 30.2 GPa (by 30.7%) and elastic modulus from 372 GPa to 419 GPa (by 12.6%), which is attributed to an effective load transfer between matrix/reinforcement in ZTaWT-SGNP composite. An attempt is made to delineate the strengthening contributions from the porosity, high entropy phase, and other complex oxy-carbide and/or unreacted phases in multiphasic (ZTaWT-S) based HEC. Instrumented indentations (micro-indentation and nanoindentation) have facilitated delineation of contribution from various phases (i.e. HEC, TaC, SiC, and oxide phases). A remarkable enhancement is observed in the flexural strength from 390 MPa for ZTaWT-S composite to 487 MPa (by 24.9%), 519 MPa (by 33.1%) and 576 MPa (by 47.7%) for CNT, graphite, and graphene reinforced ZTaWT-S composites, respectively. The graphene reinforced composite exhibited highest flexural strength due to wrapping of GNP along grain boundaries, enhanced matrix/reinforcement adhesion, grain shearing and crack deflection. In summary, ZTaWT-SGNP composite exhibit superior mechanical properties and, thus, have potential structural applications in extreme environments.