Effect of graphene content on mechanical properties of (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)N high-entropy bulk ceramics synthesized via spark plasma sintering

A high-entropy composite bulk ceramic with (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2 )N and graphene was fabricated by a spark plasma sintering (SPS) method. The influence of graphene addition content on the microstructure and mechanical property (i.e., hardness, flexural strength, and fracture toughness) of (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2 )N bulk ceramic was investigated, and the toughening mechanism of (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2 )N composite bulk ceramic without and with graphene was analyzed. The results indicate that an appropriate addition of graphene (i.e., 0.60 wt%) enhances the compactness and the mechanical property of (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2 )N bulk ceramic. Compared to the (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2 )N bulk ceramic without graphene, the hardness, flexural strength, and fracture toughness of the composite bulk ceramic with a graphene content of 0.60 wt% can reach 18.36 GPa, 383 MPa, and 4.78 MPa m 1/2 , respectively, and the fracture toughness is increased by 63.1 %. It is indicated that the toughening mechanisms in (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2 )N bulk ceramics with graphene at an appropriate content are primarily attributed to the synergistic effects of graphene pull-out and crack bridging.