Comparison of mechanical and tribological performance of the ZrB₂ to (Ta, Zr, Ti, V, Hf)B₂ borides system

Borides are promising wear-resistant materials. The mechanical and tribological properties of ZrB₂, (Ta, Zr)B₂, (Ta, Zr, Ti)B₂, (Ta, Zr, Ti, V)B₂ and (Ta, Zr, Ti, V, Hf)B₂ synthesized by spark plasma sintering were compared to examine the elemental dependence. The results indicated that the hardness and fracture toughness of borides increased with the increase of entropy. (Ta, Zr, Ti, V, Hf)B₂ obtained the highest hardness and fracture toughness of 23.14 GPa and 5.13 Mpa·m^1/2 due to the pores and second phases that change the path of crack propagation. In terms of wear, all the Ta-contained borides have a lower wear rate than that of ZrB_2. It is almost one order of magnitude lower for (Ta, Zr, Ti, V, Hf)B₂ than that of ZrB_2. This is mainly due to the formation and action of tribofilm rich of amorphous Ta₂O₅. Thermodynamic calculation indicates that the Ta atoms in borides are more prone to being oxidized resulting in the formation of a layer of Ta₂O₅ on the surface, which is beneficial to wear resistance improvement. This work provides guidelines for the composition-design of high-entropy borides for wear-resistant material applications.