Synthesis of W1-xMxB2 based ternary diborides: Challenges and Possibilities

Abstract

In the sense of environmental sustainability, the application of ultra-stable materials featuring novel properties gets more and more into the focus of academia and industry. A rather new and highly promising class of thin film materials are borides. Especially, transition metal borides (TMBs) exhibit a tremendous potential to be applied in various applications ranging from wear and corrosion resistant coatings, to superconductive thin films, or as superhard and extremely stable protective layers in diverse fields of engineering. In contrast to classic diborides - such TiB2, ZrB2, WB2, or ReB2, which has recently predicted to be the most incompressible material - are ternary or even multinary diborides (e.g. W1-xMxB2) relatively unexplored. Atomistic modelling based studies [1,2], applying Density Functional Theory (DFT) calculations, highlighted the difficulties in phase stabilizing routes involving two hexagonal competing structure types - α AlB2-prototype (SG-191) vs. ω-W2B5-x-prototype (SG-194) - as well as structural defects (especially vacancies) for various ternary diborides. Within this study, we want to address the challenges in synthesizing ternary diborides in a prototype based on α-W1-xMxB2 solid solutions, applying non-reactive sputtering processes, whereas M represents different transition metals such as Ti [2]. Due to the strong tendency of WB2 to be stabilized through structural defects in the AlB2 structure type - exhibiting distinct advantages concerning the relatively low ductility of TMBs in general - it is an excellent origin for studying various alloying concepts utilizing physical vapour deposition (PVD) techniques. To gain an in-depth insight on the specific effects of selected transition metals on α structured W1-xMxB2 coatings, we correlated the synthesis parameter with structure property relationships applying a set of high-resolution characterization techniques as well as micro-mechanical testing methods - also after exposing to diverse aggressive¬ environments.