Microstructural origins of the high mechanical damage tolerance of NbTaMoW refractory high-entropy alloy thin films

Refractory NbTaMoW thin films in close to equiatomic composition were deposited by ion beam sputter-deposition at room temperature. Energy-filtered transmission electron microscopy shows uniform distribution of all elements and electron diffraction patterns reveals unvarying body-centred cubic crystalline structure. Transmission electron microscopy images show large grains with columnar morphology. Ar bubbles with diameters around of 1.3 ± 0.4 nm were witnessed. The film growth mechanisms are discussed based on high-entropy film nature, general nucleation and growth theory and the Movchan-Demchishin-Thornton structure-zone growth models. Nanoindentation showed that the films have hardness of 22.8 ± 0.7 GPa. Nanoscratching demonstrated that such high hardness is also connected with high crack and delamination resistances. This indicates high mechanical damage tolerance (e.g. toughness). The results show that the combination of refractory metals with the intrinsic characteristics of high-entropy alloy systems in the NbTaMoW case can be considered as a hard coating candidate for future application in extreme environments.