Sputter and arc deposition of new alloys (SPADONA)

Multicomponent materials are nowadays applied in a wide variety of applications, e.g. as hard coatings (TiAlN, CrAlN) in wear protection, as transparent conductive oxides (indium tin oxide (ITO), CuAlO2) in displays and touch panels or as photoactive layers (CuInGaSe (CIGS)) in photovoltaics. An interesting and challenging example of multicomponent materials are high entropy alloys (HEA) which are metallic compounds containing 5 or more metallic elements in approximately equimolar ratios, e.g. AlCoCrCuFeNi. The properties of HEAs depend not only on their complex composition, but also on their microstructure. This means, when they are synthesised as thin films, the deposition conditions, i.e. the deposition parameters and also the deposition technique, are of vital importance as they allow for controlling the thin film microstructure. Therefore, the present project will study the growth of HEA thin films using two different physical vapour deposition techniques: magnetron sputtering and cathodic arc deposition. A combination of high-level thin film and plasma characterisation will provide new insights in the compositional and processing influence on the structure and functional properties such as electrical resistance, hardness and tribological behaviour of HEA thin films. The expected results will contribute to setting-up a three dimensional structure zone model that describes the microstructure as a function of the available energy per arriving atom during thin film growth and the composition, with a link to the functional properties. In summary, the general objective of the proposed project is well reflected by the meaning of its acronym SPADONA (old Italian word for longsword): cutting edge research on a new type of alloys.

The outcome of the current project provides a comprehensive overview of the structure and properties of MoNbTaVW thin films. This thin film material belongs to the group of so-called refractory high entropy alloys (HEAs), a new material class that was introduced in the past two decades. HEAs are typically characterised by a chemical composition were all of the 5 or more elements are present in an equimolar ratio. Using target materials with 20 at.% of each of the 5 elements, MoNbTaVW thin films were synthesised by different physical vapour deposition techniques, namely dc magnetron sputtering, high power impulse magnetron sputtering and cathodic arc deposition, offering a variety in film growth conditions. Angular-resolved depositions revealed that higher energetic growth conditions are favourable for the synthesis of stoichiometric and dense MoNbTaVW films even at larger deposition angles with respect to the surface normal of the target. However, in all cases the formation of a solid solution phase with body-centred cubic structure was observed. Annealing experiments in vacuum and inert Ar atmosphere revealed that this structure is stable up to a temperature of 1500 C. The incorporation of N leads to the formation of MoNbTaVWNx nitride films and a change from body- to face-centred cubic solid solution structure with increasing N content in the film. This structural change is associated with a significant increase in hardness from about 18 to 30 GPa. However, as the hardness increases the films become more brittle as revealed by tensile tests of films deposited on polymer substrates. With increase in N content a decrease in the crack onset strain was observed. In general, the results of the project can serve as a basis for the further development of MoNbTaVW and similar refractory HEA thin films and their optimisation for intended applications.