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A Kirnbauer, C. Koller, P. Polcik, P.H. Mayrhofer:
"Structure, mechanical properties and thermal stability of magnetron sputtered HfTaVWZr high-entropy boride coatings";
Talk: 46th International Conference on Metallurgical Coatings and Thin Films, San Diego; 2019-05-19 - 2019-05-24.

In the field of materials research, a novel alloying concept of so-called high-entropy alloys (HEAs), has gained particular attention within the last decade. These alloys contain 5 or more elements in equiatomic or near-equiatomic composition. Properties, like hardness, strength, and toughness can be attributed to the specific elemental distribution and are often superior to those of conventional alloys. In parallel to HEAs also high-entropy ceramics (HECs) moved into in the focus of research. These consist of a solid solution of 5 or more binary borides, carbides, nitrides, or oxides. Within this work, we investigate the structure and mechanical properties of thin films based on the high-entropy materials concept with emphasis on their behaviour at elevated temperatures, as their structural integrity should be improved with increasing temperature according to the Gibbs free energy.
Therefore, HfTaVWZr boride coatings were synthesised in a lab-scale sputter deposition facility using a single powder-metallurgically produced composite target (nominal target composition 20 mole % of HfB2, TaB2, VB2, W2B5, and ZrB2, respectively). The coatings crystallise in a single-phased hexagonal α-structure (AlB2 prototype) with a fine columnar morphology. The hardness in as-deposited state is 45.6 ± 1.5 GPa and these films can thus be considered super hard. The structural evolution of free-standing powdered coating material upon annealing was investigated by DSC and X-ray diffraction, showing only marginal structural changes between 900 and 1200 °C, which can be interpreted by a stabilisation due to the high-entropy effect. Upon annealing up to 1400 °C slight indication for the initiation of decomposition processes is given by emergence of low intensity XRD peaks. Yet, the hardness after annealing at 1400 °C remains at least ~42 GPa.
Compared to their binary boride constituents a significant structural stability and mechanical enhancement at elevated temperatures could be achieved by applying the high-entropy concept to HfTaVWZr boride thin films.