Influence of substrate bias and temperature on the crystallization of metallic NbTaTiVZr high-entropy alloy thin films

Abstract

Metallic sputtered high-entropy alloy (HEA) thin films often result in amorphous structures, due to the film growth kinetics and the large atomic size mismatch of the constituent elements. In this paper, single-phase crystalline NbTaTiVZr HEA thin films were achieved by the appropriate choice of both the alloying elements and the synthesis conditions. Regarding the latter, substrate biasing prompts Ar+ ion irradiation during film growth, whereas substrate heating increases the adatom mobility, inducing specific structural modifications. The control of both variables eliminates traditional crystallization strategies, such as adding nitrogen to the gas mixture during film growth or post-thermal annealing of the as-deposited films. Therefore, we have investigated the relationship between the synthesis conditions, the structure, and the mechanical properties of a Nb20Ta26Ti22V16Zr16 HEA thin film, due to its potential application in the field of refractory coating materials. The as-deposited films prepared at 400 °C possessed a body-centered cubic (bcc) phase, and their preferential orientation changed according to the bias voltage value (Vb) chosen. Low energy ion irradiation (Vb ≈ –25 V) resulted in crystallite coarsening and surface roughening. On the other hand, higher negative bias voltages (Vb ≈ –75 V) led to lower growth rates, grain refining, and improved mechanical properties. In addition, the chemical states and composition were determined by X-ray photoelectron spectroscopy (XPS) and the HEA phase formation was predicted using empirical parameters and compared to the results obtained by the Calculation of Phase Diagrams (CALPHAD) approach.