Metallic boro-carbides of A2BC (A = Ti, Zr, Hf and W): a comprehensive theoretical study for thermo-mechanical and optoelectronic applications

Abstract

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 A₂BC (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 A₂BC (A = Ti, Zr, Hf, and W) compounds are 28.20, 23.12, 12.44, and 35.70, respectively, which indicates the W₂BC could be a member of the hard family (H_v > 30 GPa). Pugh's ratio suggests ductile deformation for the W₂BC compound, whereas the other three (Ti₂BC, Zr₂BC, and Hf₂BC) compounds exhibit brittle deformation behavior. The W₂BC compounds have the highest ductility among the other metallic boro-carbides (M₂BC; M = V, Nb, Mo and Ta) and some other benchmark coating materials (TiN, TiAlN, C-BN, and Cr_0.5Al_0.5N). The fracture toughness (K_IC) values are in the following sequence: Zr₂BC < Ti₂BC < Hf₂BC < W₂BC, which indicates that, the highest resistance (K_IC = 4.96 MPam^1/2) found for W₂BC is suitable to prevent the crack propagation within the solid. In addition, the structural, electronic, optical, and thermal properties are also investigated for the A₂BC (A = Ti, Zr, Hf, and W) compounds. The Ti₂BC (W₂BC) 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. Hf₂BC and W₂BC compounds could also be exploited as promising thermal barrier coating materials, while Ti₂BC 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.