Modulating hardness in Sc2(Ru5−xTMx)B4 through empirical considerations and computational analysis

Abstract

Ternary and higher-order borides remain an underexplored area in the search for hard materials. The difficulties associated with purely systematic experimental investigations have largely hindered the consideration of such complex phases. Here, traditional design rules are merged with computation-based methods to direct synthetic efforts, addressing this challenge. The compound Sc₂Ru₅B₄ was selected to demonstrate this approach. The phase was first prepared using arc melting, and the crystal structure was resolved with single-crystal X-ray diffraction. Vickers microhardness indentation revealed that Sc₂Ru₅B₄ is a hard metal, and the mechanical properties can be enhanced by substituting Ru for the isoelectronic but more electron-dense Os following an empirical understanding of hardness in borides. Analyzing the ensuing density of states (DOS) of Sc₂(Ru₄Os)B₄ indicates the Fermi level nevertheless falls in an unfavorable position requiring a reduction in the electron count to optimize the electronic structure. The subsequent synthesis of Sc₂(Ru₄TM)B₄ (TM = Ta, W, Re, and Ir) alters the electron count moving the Fermi level from the peak. At the same time, Vickers indentation measurements, combined with DFT-level stress–strain calculations and a bonding analysis, show that shifting the Fermi level reduces the occupation of antibonding interactions, which also increases the hardness. These data suggest that electron density, Fermi level position, and chemical bonding are essential markers when developing high hardness materials.