(Hf, Zr)B₂ solid solutions are promising ultra-high temperature ceramics (UHTCs) for hypersonic applications based on the high-temperature stability of HfB₂ and the low cost of ZrB₂, exhibiting superior properties to single-component counterparts. However, a covalent bonding leads to a poor sinterability, making the pressureless densification of pure (Hf, Zr)B₂ a challenge. While B₄C is known as an effective sintering aid for monolithic borides, its role in the densification and microstructural evolution of (Hf, Zr)B₂ solid solutions under pressureless sintering conditions remains unexplored. This study was to investigate the effects of B₄C content (i.e., 0-5%) and sintering temperature (i.e., 1900-2200 ℃) on the densification behavior and microstructure of (Hf, Zr)B₂, to provide theoretical guidance and process references for the pressureless sintering preparation and microstructural control of (Hf, Zr)B₂-B₄C composites.

(Hf, Zr)B₂ powders were synthesized via boro/carbothermal reduction at 1700 ℃ with HfO₂, ZrO₂, B₄C, and graphite as raw materials. The obtained powders were then mixed with 0, 2% and 5% B₄C by milling in ethanol for 12 h, followed by rotary evaporation and drying at 60 ℃. The mixed powders were uniaxially pressed into compacts and subsequently cold isostatically pressed at 200 MPa. The pressureless sintering was conducted in an ultra-high temperature furnace in argon atmosphere at a heating rate of 10 ℃/min. A holding step at 1600 ℃ for 30 min was employed before reaching target temperatures (i.e., 1900-2200 ℃) for 2 h.

The density was measured by the Archimedes method. The phase composition was analyzed by X-ray diffraction (XRD), and the microstructure was characterized by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS). The grain size was determined based on the SEM images, and the electron backscatter diffraction was employed to evaluate the crystallographic orientation and geometrically necessary dislocation density.

(Hf, Zr)B₂ without B₄C addition exhibits a poor sinterability, achieving the relative density of only 57.2% at 1900 ℃. and gradually increasing to merely 88.6% even at 2200 ℃. The addition of B₄C transforms a densification behavior. At 2000 ℃, the sample containing 2% and 5% B₄C achieves the relative densities of 97.8% and 98.1%, respectively, representing an increase of >30% compared to the sample without B₄C (i.e., 74.8% at 2000 ℃). This enhancement is attributed to the effective "deoxidizing" function of B₄C, reacting with and removing surface oxide impurities, thereby activating mass transport mechanisms essential for pore elimination and grain boundary migration. At > 2000 ℃, the relative density decreases slightly for the samples with B₄C. At 2100 ℃, accelerated grain boundary mobility begins to outpace pore migration, leading to the formation of some intragranular pores. At 2200 ℃, the B₄C-(Hf, Zr)B₂ system reaches its eutectic point, triggering liquid-phase sintering as evidenced by B₄C exhibiting melt-like morphology along grain boundaries with good wetting characteristics. The samples with B₄C addition exhibite a significant grain growth at elevated temperatures. For the sample with 2% B₄C, the grain size increases from (4.81 ± 0.99) μm at 2000 ℃ to (45.07 ± 16.20) μm at 2200 ℃, while the sample with 5% B₄C increases from (3.35 ± 0.84) μm to (26.21 ± 7.50) μm. The finer grain in the sample with the more addition of B₄C is attributed to an enhanced pinning effect due to a higher content of secondary phase particles, which effectively inhibits a grain boundary migration. This grain coarsening, accompanied by pore coalescence, contributes to a slight decrease in relative density to 94.5% for the sample with 2% B₄C and 94.6% for the sample with 5% B₄C at 2200 ℃. The EBSD analysis reveals that the geometrically necessary dislocation density decreases at 2200 ℃, compared to the cases at lower sintering temperatures. The Kernel average misorientation mapping shows that the strain concentrations near grain boundaries and B₄C particles are somewhat relieved in liquid-phase sintered samples. This phenomenon is related to the dissolution-reprecipitation mechanism and the presence of the liquid phase during sintering. Although B₄C participates in liquid formation at 2200 ℃, it recrystallizes upon cooling and retains its structure in the final ceramic, indicating a chemical compatibility with the matrix throughout the thermal cycle.

The introduction of B₄C significantly promoted pressureless sintering densification of (Hf, Zr)B₂ ceramics. The samples containing 2% and 5% B₄C achieved the relative densities of 97.8% and 98.1% at 2000 ℃, respectively, which exceeded the sample without B₄C. Sintering temperature critically controlled the microstructural evolution. The dense microstructures with fine grains were obtained at 2000 ℃. At 2100 ℃, accelerated mass transport led to intragranular pores and rapid grain growth during solid-state sintering. At 2200 ℃, liquid-phase formation caused a significant grain coarsening and a slight density decrease, while effectively relieving internal strain, with B₄C recrystallizing upon cooling and retaining its structure. Considering both density and microstructure, the optimal pressureless sintering temperature for (Hf, Zr)B₂ ceramics with B₄C additions could be approximately 2000 ℃, providing a viable process window for (Hf, Zr)B₂-based ultra-high temperature ceramic composites.