Fe-assisted low-temperature densification and mechanical property optimization of (Hf0.2Zr0.2Ti0.2Nb0.2Ta0.2)N composites

： High-entropy nitride ceramics (HENCs) require high sintering temperatures,and exhibit relatively low density and insufficient fracture toughness, greatly restricting their applicability in structural load-bearing scenarios under extreme service environments. To address these limitations, this study employed Fe as a sintering aid and prepared (Hf 0.2 Zr 0.2 Ti 0.2 Nb 0.2 Ta 0.2 )N-Fe composite ceramics with 0–20 wt% Fe via spark plasma sintering (SPS) in the temperature range of 1600–2100 ℃. We evaluated how Fe content and sintering temperature jointly affect microstructure and mechanical properties. The results show that pure HEN ceramics densifies only at 2100 °C, reaching 96.41% relative density, due to sluggish diffusion and lattice distortion. Although high temperature enhances diffusion, it also coarsens the grains to 5.40 ± 2.18 μm. In contrast, adding only 5 wt% Fe enables densification at a reduced temperature of 1700 °C, achieving 98.81% relative density and refining the grain size to 3.25 ± 1.01 μm. Regarding mechanical properties, pure HEN ceramics exhibits a hardness of 19.79 ± 0.49 GPa and a fracture toughness of 3.86 ± 0.32 MPa·m 1/2 . When 5 wt% Fe is added, both mechanical indices increase markedly, with hardness increasing to 21.09 ± 0.35 GPa and fracture toughness to 4.38 ± 0.31 MPa·m 1/2 . These results demonstrate a pronounced synergistic effect of Fe addition and reduced sintering temperature on the microstructure of HEN ceramics. During sintering, by forming Fe-rich interfacial phases, Fe accelerates low-temperature densification, while its segregation at grain boundaries and triple junctions regulates boundary migration, enabling fine grains and high density at reduced sintering temperatures.