Structure evolution and mechanical properties of hard tantalum diboride films

Abstract

Tantalum diboride (TaB2) belonging to the ultrahigh temperature ceramics family is proving to be a promising material for hard protective films, thanks to its high thermal stability and excellent mechanical properties. However, growth of TaB2 ± x films prepared using physical vapor deposition techniques is strongly affected by Ar neutrals reflected from a stoichiometric TaB2 target due to a significant mass difference of heavy Ta and light B atoms leading to substantial changes in the final chemical composition and structure of films. In this work, TaB2 ± x films are experimentally prepared using high target utilization sputtering. Stopping and range of ions in matter simulations are used to investigate the behavior of Ar neutrals during deposition processes. A wide range of analytical methods is used to completely characterize the chemical composition, structure, and mechanical properties of TaB2 ± x films, and the explanation of the obtained results is supported by density functional theory calculations. TaB2 ± x films grow in a broad compositional range from TaB1.36 to TaB3.84 depending on the kinetic energy of Ar neutrals. The structure of overstoichiometric TaB2 + x films consists of 0001 preferentially oriented α-TaB2 nanocolumns surrounded by a boron-tissue phase. In the case of highly understoichiometric TaB2 − x films, the boron-tissue phase disappears and the structure consisting of 0001 and 101¯1 oriented α-TaB2 nanocolumns is formed. All TaB2 ± x films exhibit excellent mechanical properties with high hardness, ranging from 27 to 43 GPa and relatively low values of Young's modulus in the range of 304–488 GPa.