Ab initio inspired design of ternary boride thin films

Vincent Moraes,Hamid Bolvardi,P H Mayrhofer,David Holec,Christoph Fuger,Helmut Riedl,Peter Polcik

doi:10.1038/s41598-018-27426-w

Vincent Moraes, Hamid Bolvardi + Show 5 more

Open Access

https://doi.org/10.1038/s41598-018-27426-w

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Abstract

The demand to discover new materials is scientifically as well as industrially a continuously present topic, covering all different fields of application. The recent scientific work on thin film materials has shown, that especially for nitride-based protective coatings, computationally-driven understanding and modelling serves as a reliable trend-giver and can be used for target-oriented experiments. In this study, semi-automated density functional theory (DFT) calculations were used, to sweep across transition metal diborides in order to characterize their structure, phase stability and mechanical properties. We show that early transition metal diborides (TiB2, VB2, etc.) tend to be chemically more stable in the AlB2 structure type, whereas late transition metal diborides (WB2, ReB2, etc.) are preferably stabilized in the W2B5−x structure type. Closely related, we could prove that point defects such as vacancies significantly influence the phase stability and even can reverse the preference for the AlB2 or W2B5−x structure. Furthermore, investigations on the brittle-ductile behavior of the various diborides reveal, that the metastable structures are more ductile than their stable counterparts (WB2, TcB2, etc.). To design thin film materials, e.g. ternary or layered systems, this study is important for application oriented coating development to focus experimental studies on the most perspective systems.

Highlights

Designing new materials is a highly relevant topic covering many different aspects, like discovering materials with entirely new properties, improving existing materials in use, or increasing biocompatibility or environmental sustainability
Regarding the synthesis via physical vapor deposition, it is a well known fact, that metastable structures can be captured via this synthesis route[21,22], which is often achieved by the implementation of point defects such as vacancies[23,24]
Summarizing the results, we have confirmed the tendency of early TMB2 compounds preferring the α-structure due to their lower Ef values compared to the ω-structure

Summary

Introduction

Designing new materials is a highly relevant topic covering many different aspects, like discovering materials with entirely new properties (e.g., carbon-nanotubes), improving existing materials in use, (e.g., reduce costs or weight), or increasing biocompatibility or environmental sustainability. Materials science allows for at least two helpful scenarios: 1) decreasing the difference in Gibbs free energy between metastable and stable state (which decreases the driving force for the phase transformation) and 2) increasing the Gibbs free energy of the peak separating the metastable and stable state (which increases the necessary activation energy to reach the stable state.) Picking up the concept of ZrO2 based ceramics, where alloying of certain elements (e.g., Mg, Y, Ce)[25] leads to stabilization of high temperature modifications at room temperature, the two hexagonal diboride phases draw a great starting point for the exploration of ternary diboride materials Since mechanical properties are of central importance for protective coatings, we calculated the elastic constants (bulk and shear modulus, poisson ratio, and the cauchy pressure) of all these transition metal diborides (3 to 5d elements excluding Hg) in their α and ω crystal structure

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