Metallic boro-carbides of A 2 BC (A = Ti, Zr, Hf and W): a comprehensive theoretical study for thermo-mechanical and optoelectronic applications.

High-hardness materials with ductile deformation behavior have recently piqued interest due to their prospective applications, particularly as hard and protective coatings. The crack formation, especially in metal and ceramic materials, is one of the biggest problems of the surface hard coatings on heavy-duty tools. In this regard, mechanical properties (Vickers hardness, fracture toughness, machinability index, index of brittleness, as well as Pugh's ratio) have been studied for the metallic boro-carbides of A 2 BC (A = Ti, Zr, Hf, and W) compounds using the state-of-the-art density functional theory in detail. The compounds under investigation are both thermodynamically and mechanically stable. The value of Vickers hardness (in GPa) for A 2 BC (A = Ti, Zr, Hf, and W) compounds are 28.20, 23.12, 12.44, and 35.70, respectively, which indicates the W 2 BC could be a member of the hard family ( H v > 30 GPa). Pugh's ratio suggests ductile deformation for the W 2 BC compound, whereas the other three (Ti 2 BC, Zr 2 BC, and Hf 2 BC) compounds exhibit brittle deformation behavior. The W 2 BC compounds have the highest ductility among the other metallic boro-carbides (M 2 BC; M = V, Nb, Mo and Ta) and some other benchmark coating materials (TiN, TiAlN, C-BN, and Cr 0.5 Al 0.5 N). The fracture toughness ( K IC ) values are in the following sequence: Zr 2 BC < Ti 2 BC < Hf 2 BC < W 2 BC, which indicates that, the highest resistance ( K IC = 4.96 MPam 1/2 ) found for W 2 BC is suitable to prevent the crack propagation within the solid. In addition, the structural, electronic, optical, and thermal properties are also investigated for the A 2 BC (A = Ti, Zr, Hf, and W) compounds. The Ti 2 BC (W 2 BC) reflectivity spectra never fall below 53 (45)% in the 0 to 10.3 eV (0 to 16.70 eV) photon energy range, suggesting that these compounds have promise for usage as coating materials to reduce solar heating. Hf 2 BC and W 2 BC compounds could also be exploited as promising thermal barrier coating materials, while Ti 2 BC could be used as heat sink material based on the results of Debye temperature, melting temperature, thermal conductivity, and thermal expansion coefficient. The electronic properties reveal the metallic behavior of these compounds. The results obtained here are compared with those of some commercially known compounds, where available.