Rapid liquid phase-assisted ultrahigh-temperature sintering of high-entropy ceramic composites

Abstract

High-entropy ceramics and their composites display high mechanical strength and attractive high-temperature stabilities. However, properties like strong covalent bond character and low self-diffusion coefficients make them difficult to get sintered, limiting their mass popularity. Here, we present a rapid liquid phase-assisted ultrahigh-temperature sintering strategy and use high-entropy metal diboride/boron carbide composite as a proof of concept. We use a carbon-based heater to fast-heat the composite to around 3000 K, and a small fraction of eutectic liquid was formed at the interface between high-entropy metal diborides and boron carbide. A crystalline dodecaboride intergranular phase was generated upon cooling to ameliorate the adhesion between the components. The as-sintered composite presents a high hardness of 36.4 GPa at a load of 0.49 N and 24.4 GPa at a load of 9.8 N. This liquid phase-assisted rapid ultrahigh-temperature strategy can be widely applicable for other ultrahigh-temperature ceramics as well.

Fig. 1.. Sintering schematic and comparison with conventional approaches.

(A) Schematic demonstrating the rapid liquid phase–assisted ultrahigh-temperature sintering process. The HEB/boron carbide composite is heated up to 3000 K for ~2 min in an atmospheric pressure environment, which results in the formation of a low–melting point ZrB₁₂-based intergranular phase that bonds with the HEB and boron carbide after cooling. (B) This sintering strategy has filled the temperature gap in conventional sintering approaches.

Fig. 2.. The influence of temperature and phases on the sintering results.

Phase diagrams of the (A) B-Zr, (B) B-C, and (C) ZrB₂-B₄C components (36, 38, 41). At 3000 K, the high-entropy metal diborides can maintain a solid state, while the boron carbide couples with the HEB to form the eutectic liquid phase. Scanning electron microscopy (SEM) images showing the morphologies of the composite samples at different sintering temperatures of (D) 2400 K, (E) 3000 K, and (F) above 3300 K. The optimal temperature here for the rapid sintering of the HEB composite is around 3000 K. at %, atomic %.

Fig. 3.. Composition analysis.

(A) The polished cross-sectional SEM image of the synthesized composite. (B) SEM image with false colors to delineate the three different phases. (C) XRD patterns of the synthesized composite. HEB dominates the peak signals due to the high Z values and electron density of the transition metal elements. (D) TEM image of the grains and EDS mapping of the different elements, including B, Mo, Zr, C, Ta, Ti, and W, respectively. The dominant phase is the HEB with Mo, Zr, Ta, Ti, and W uniformly distributed, and the small grains belong to boron carbide. a.u., arbitrary units.

Fig. 4.. EELS and electron diffraction analysis of the phases.

(A) STEM image with false colors (boron carbide, magenta; ZrB₁₂, blue; and HEB, yellow) and EELS mapping of the boron and carbon element distribution. (B) EELS B-K edges for the three different grains. (C) Carbon atomic ratio across a boron carbide grain, which indicates a composition of B₁₃C₂ here. (D to F) HRTEM of the grains from boron carbide, ZrB₁₂, and HEB, together with their corresponding SAED patterns, which further confirmed their crystal lattice structures: B₁₃C₂ (space group: R-3m, PDF number: 03-065-6874) (47), ZrB₁₂ (space group: Fm-3m, PDF number: 04-003-5571) (48), and HEB (space group: P6/mmm, PDF number: 04-001-2357 AlB₂ type) (42). Scale bars, 2 nm.

Fig. 5.. Mechanical properties and sintering demonstrations.

(A) TEM image showing the interface between the boron carbide and HEB grains. (B) One coherent interface between two grains of boron carbide and HEB. The zone axis [010] of the boron carbide is parallel to the $\left[21\overline{1}\right]$ zone axis of the HEB, while the (101) plane of the boron carbide is coherent with the $\left(\overline{1}20\right)$ plane of the HEB. The coherent interface can further improve the adhesion between the two components. (C) The Vickers hardness of the eutectic liquid–assisted ultrahigh-temperature sintered composite is substantially improved in comparison with that of the HEB without boron carbide and intergranular ZrB₁₂ phases. (D) Photos of a drill bit with and without HEB composite coating. (E) Cross-sectional SEM image showing the HEB composite coating on the drill bit. (F) The free-standing HEB composite membrane with a thickness of 50 μm. The inset is a SEM image showing the membrane with smaller magnification.