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V. Moraes, C. Fuger, L. Zauner, R Hahn, D. Primetzhofer, P. Polcik, H. Riedl, P.H. Mayrhofer:
"2020 ASED Awards Ceremony";
Talk: 2020 ASED Awards Ceremony, Online; 2020-12-14 - 2020-12-16.

Since many years, transition metal (TM) nitrides experience great success in thin film industry. Their outstanding properties like high hardness, good oxidation resistance, high thermal stability, and abrasion resistance make them widely suitable for various applications such as protective coatings for cutting and milling tools, or for microelectronic applications. The never-ending demand for increasing the efficiency of industrial processes, e.g., higher feedthroughs during machining - which implies the increase in cutting speed and therefore increased temperatures - still asks for further materials science based developments of protective coatings. Hence, industry calls for new material classes exceeding the possibilities of nitrides.

An interesting class of materials in this perspective are ternary TM-borides or ternary TM-diborides in particular, which are rather unexplored compared to their nitride-based counterparts. Therefore, I combine the fruitful approach of DFT calculations and experiments (Physical Vapor Deposition) to theoretically and experimentally study novel TM-diboride materials, with the quest of excellent mechanical and thermomechanical properties.

A huge drawback, when considering TM-diborides for hard coating applications is the pronounced brittle behavior of this material class. Hence, it is of great importance to enhance ductility and increase the toughness. Whereas early TM-diborides tend to crystallize in the α-type (AlB2-prototype, P6/mmm - with metal layers divided by planar hexagonal boron layers), late TM-diborides tend to be stabilized in the ω-type (W2B5-x-prototype, P63/mmc - the boron-layers are alternatingly puckered or flat).

By applying different criteria such as the Pugh, Frantsevich and Pettifor criteria on a screening of all the various TM-diborides, which classify them into ductile and brittle on behalf of their elastic constants, we could nicely show that WB2 in its metastable α-structure can be accounted to the ductile regime. An experimental study, analyzing the structure of this compound showed that contrary to bulk experiments, WB2 crystallizes in this metastable α-type when deposited via physical vapor deposition. By combining this polymorphism with well-established alloying concepts, we conducted target-driven experiments to increase the performance of this binary system.

Similar to the concept of yttrium stabilized zirconia (YSZ), where a specific alloying element (in this case Yttrium) is used to stabilize a metastable structure (achieving the cubic high temperature modification at room temperature), we suggested Ta as a possible candidate to stabilize the metastable α-type with lowest cost on ductility. Therefore, a home-built magnetron sputtering system, equipped with a 6-inch WB2- and 6-inch TaB2 compound target was used, to prepare ternary W1-xTaxB2 thin films (with x ranging from 0 to 100 at.%). The deposited films where analyzed with focus on their structure and mechanical properties using X-ray diffraction, nanoindentation and micromechanical bending experiments.

As point defects such as vacancies (which are highly present in PVD synthesized thin film materials) are strongly lowering the energy of formation and therefore the stabilization of WB2 in its metastable α-structure, we additionally investigate alloying concepts based on phase transformations (including an increase in volume) and precipitation hardening. Again combining results from theory and experiments suggests the V-W-B as an ideal candidate for allowing phase transformation related hardening and/or toughening effects. Hereby, the high degree of covalent bonds - suggesting a high hardness - and the excellent tribological properties (by the formation of V2O5-based phases in tribo contacts) are promising aspects of this material combination.

With respect to the high sensitivity of vacancies (and therefore minor changes in the stoichiometry), we not only relate our theoretical results to mechanical properties only, but also to detailed chemical anaylsis by elastic recoil detection analysis (ERDA). This provides us with the opportunity to connect and relate theoretical findings to experimental synthesis conditions (e.g., pressure, temperature, etc.) influencing structure and morphology and therefore the performance of the thin films in real-life applications.

As an outlook for further activities on this topic, I think the material class of multinary borides (or diborides specifically) for the use in thin film applications, is just about to be established in academia as well as in industry. Moreover, the results present show the enormous potential of combining theoretical and experimental results, when it comes to application-oriented design of thin film materials.

Magnetron Sputtering; Transition metal diborides; ERDA