Bulk (ZrxNb1-x)B-2 solid solution systems with x ranging from 0 to 1 with intervals of 0.25, were consolidated by spark plasma sintering (SPS) at 1850 degrees 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)B-2 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)B-2 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)B-2 solid solution. Namely, the addition of GNPs increased the relative density by similar to 2% by removing oxide impurities. The highest Vickers hardness was obtained for h-BN-modified Zr0.25Nb0.75B2, 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)B-2 exhibited the best oxidation performance. The optical absorbance of the (Zr,Nb)B-2 solid solution was higher than that of the GNP- and h-BN-modified samples. (C) 2021 Elsevier B.V. All rights reserved.