Structure and properties of (TiZrHfNbTa)B2 films and first-principles models for high entropy diborides

The (TiZrHfNbTa)B2 2 films were prepared by direct current magnetron sputtering of the target manufactured of TiB2, 2 , ZrB2, 2 , HfB2, 2 , NbB2, 2 , and TaB2 2 powders at different substrate biases (0,-50,-100,-150,-200 V). The structure, surface morphology, chemical bonding, Knoop hardness, and tribological properties were studied by Xray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic-force microscopy, Raman spectroscopy, Rutherford backscattering, indentation and tribological tests. The crystalline phase was observed in the film's structure. The XPS results point to the formation of the following bonds in films: Ti-B, Ti-B-O, Zr-B, Nb-B, Hf-B, Hf-O, Ta-B. The XRD and XPS results allowed the conclusion on formation of a solid solution of constituting diborides. The film hardness increased from 34.8 GPa (Ub b = 0 V) to maximum value of 38.3 GPa (Ub b =-150 V) and then decreased to 33.2 GPa (Ub= b =-200 V). The friction coefficient mu decreased with increasing substrate bias from 0.85 to 0.49 exhibiting the minimal value for the film with the smallest hardness. A comparative first-principles investigation of the stability, electronic structures and mechanical properties of the random high entropy diborides (TiZrHfMTa)B2 2 (M=Nb, Sc, V, Mo) and the constituting binary diborides was performed to interpret the properties of deposited films. The calculated elastic moduli, Vickers hardness, fracture toughness, and Debye temperature of (TiZrHfMTa)B2 2 alloys are close to the average values of the corresponding characteristics of constituting diborides. First-principles calculations show that the formation of quinary high entropy diborides, in which TiB2 2 is a constituent compound does not lead to an improvement in their mechanical characteristics compared to those of TiB2. 2 .