Structure, mechanical properties, and thermal stability of (Gd,Hf,Sc,Ti,Zr)-nitride thin films

Abstract

In recent years the exploration of so-called high-entropy alloys (HEAs) and high-entropy metal-sublattice ceramics (HESCs) is in the focus of many research groups. Their unique properties, attributed to the high entropy (> 1.5R) on the metal sublattice, make them interesting for many applications. These materials combine high-hardness and toughness with increased thermal stability as, due to sluggish diffusion, softening processes are slowed down. In a previous study on (Hf,Ta,Ti,V,Zr) nitride we could show that thin films based on the high-entropy concept exhibit outstanding thermal stability as the coatings are single-phased and the elements are randomly distributed up to an annealing temperature of 1300 °C. At higher temperatures the coating decomposes due to nitrogen loss. To overcome the problem of nitrogen loss Ta and V which form a Me2N solid solution are exchanged by Gd and Sc which do not exhibit a Me2N phase. For the synthesis of (Gd,Hf,Sc,Ti,Zr) nitride thin films reactive magnetron sputtering was used. We investigated the influence of nitrogen partial pressure as well as substrate temperature on the phase formation and mechanical properties. Therefore, X-ray diffraction and nanoindentation measurements were carried out. The results show that with low nitrogen partial pressure the coatings are partly amorphous while with increasing nitrogen flow the coatings crystallise in a single-phase fcc structure. With increasing substrate temperature, the hardness increases from 23 to 32 GPa. To investigate the thermal stability, selected samples were vacuum annealed up to 1200 °C and investigated by XRD and nanoindentation. The results show that the coatings stay single-phased up to 1000 °C, whereas at higher temperatures phase separation occurs. The hardness thereby decreases from ~32 to 22 GPa.