# Modeling of metastable phase formation for sputtered $Ti_{1-x}Al_{x}N$ and $V_{1-x}Al_{x}N$ thin films

Liu, Sida; Schneider, Jochen M. (Thesis advisor); Chang, Keke (Thesis advisor)

Aachen (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

The metastable transition metal aluminum nitride (TMAlN) coatings are widely applied in cutting and forming applications. In this thesis, the metastable phase formation of sputtered TM1−xAlxN (TM = Ti, V) thin films was studied by thermodynamic calculations, ab initio calculations, and experiments. The addition of Al to TMN, resulting in the formation of metastable TMAlN improves the oxidation resistance compared to TMN. The modeling of the effect of energetic and kinetic factors on phase formation allows for quantum-mechanically guided design of face-centered cubic (fcc) TMAlN thin films with increased Al concentration. In the first part, the metastable phase formation of TiAlN is predicted based on one combinatorial magnetron sputtering experiment, the activation energy for surface diffusion, the critical diffusion distance, as well as thermodynamic calculations. Although it is generally accepted that the phase formation of metastable TiAlN is governed by kinetic factors, modeling attempts today are based solely on energetics. The phase formation data obtained from further combinatorial growth experiments varying chemical composition, deposition temperature, and deposition rate are in good agreement with the model. Furthermore, it is demonstrated that a signiﬁcant extension of the predicted critical solubility range is enabled by taking kinetic factors into account. Explicit consideration of kinetics extends the Al solubility limit to lower values, previously unobtainable by energetics, but accessible experimentally. TMAlN (TM = Ti, V) thin films are today deposited utilizing ionized vapor phase condensation techniques where variations in ion flux and ion energy cause compressive film stress, in turn affecting Al solubility. While the metastable phase formation of TiAlN has been modeled, the influence of film stresses on phase formation has so far been overlooked. In the second part, using combinatorial deposition via magnetron sputtering, thermodynamic modeling, and density functional theory calculations, the phase formation of V1−xAlxN and Ti1−xAlxN thin films at various substrate temperatures and deposition rates is investigated. Ab initio calculations indicate that the maximum solid solubility of Al in fcc-V1−xAlxN or fcc-Ti1−xAlxN shows a linear trend as a function of the magnitude of compressive stress. Here, the influence of film stresses on the metastable phase formation of fcc-V1−xAlxN and fcc-Ti1−xAlxN is considered for the first time. Specifically, experimental data from a single combinatorial deposition is utilized to predict the stress-dependent formation of metastable phases based on thermodynamic and ab initio data. Explicit consideration of stress extends the Al solubility limit to higher values for both Ti1-xAlxN and V1-xAlxN thin films, previously unobtainable by energetics, but accessible experimentally. These predictions are experimentally verified and thus provide guidance for experimental efforts with the goal of increasing the Al concentration in fcc-TMAlN thin films. The present work shows that CALPHAD modeling and ab initio calculations can be used successfully to predict the metastable phase formation of TMAlN thin films. This enables future knowledge-based design of face-centered cubic TMAlN thin films with increased Al concentration.