Effects of graphene nanoplatelets and hexagonal boron nitride on spark plasma sintered (Zr,Nb)B2 solid solutions

Highlights: • NbB₂ completely entered ZrB₂ structure for all compositions of (Zr,Nb)B₂. • GNPs facilitated sintering by removing oxide impurities and enhanced densification of Zr_0.25Nb_0.75B₂. • The addition of h-BN to Zr_0.25Nb_0.75B₂ decreased the grain size; thus, the maximum hardness was achieved. • h-BN-modified Zr_0.25Nb_0.75B₂ exhibited the best oxidation performance. -- Abstract: Bulk (Zr_xNb_1−x)B₂ solid solution systems with x ranging from 0 to 1 with intervals of 0.25, were consolidated by spark plasma sintering (SPS) at 1850 °C under 50 MPa with a holding time of 5 min. In addition, 1 vol% graphene nanoplatelets (GNPs) and 1 vol% hexagonal boron nitride (h-BN) were added to (Zr,Nb)B₂ solid solutions to investigate their effects on densification, mechanical properties, and optical properties. Transmission electron microscopy analysis, phase analysis, and lattice parameters confirmed the formation of a complete (Zr,Nb)B₂ solid solution. Raman analysis confirmed the structural integrity of both the GNPs and h-BN in the solid solution. The additives modified different properties of the (Zr,Nb)B₂ solid solution. Namely, the addition of GNPs increased the relative density by ~2% by removing oxide impurities. The highest Vickers hardness was obtained for h-BN-modified Zr_0.25Nb_0.75B₂, which had the lowest grain size. The addition of GNPs and h-BN contributed to the improvement in toughness by activating toughening mechanisms. The h-BN-modified (Zr,Nb)B₂ exhibited the best oxidation performance. The optical absorbance of the (Zr,Nb)B₂ solid solution was higher than that of the GNP- and h-BN-modified samples.