Preparation and characterization of high entropy alloy nitride thin films

Abstract

High-entropy alloy (HEA) thin films are new in the field of research for surface engineering, garnering great interest due to their unique properties. HEA is a material group consisting of five or more constituent elements with equal or nearly equal atomic ratios. High-entropy nitride (HEN) thin films represent a novel ceramic thin film pioneered upon the foundation of the HEA concepts, exhibiting a series of outstanding properties, such as exceptional strength, high thermal stability, and excellent electrochemical properties. AlCrFeCoNiCu0.5 nitride thin films are one of the surface layer materials of interest, which was reported to be extraordinarily hard. However, limited work has been done to deposit AlCrFeCoNiCu0.5 nitride thin films by physical vapour deposition (PVD). In this thesis, AlCrFeCoNiCu0.5 nitride thin films were fabricated on Si (100) substrates using a filtered cathodic arc deposition (FCAD). The nitrogen concentration of the HEN thin films was adjusted by changing the nitrogen partial pressure ratio (RN) from 5% to 100% in the FCAD system during the deposition. The arc discharge in different nitrogen partial pressure conditions was studied, and the variations in deposition rate were analysed based on the discharge energy. No AlN signal was observed at low RN, showing that a portion of nitrogen occupied the interstitial sites without forming nitrides. Moreover, XPS after etching showed that CrN tends to appear only on the surface due to chemical vapour deposition. XRD demonstrated a transition in HEN thin film's crystallinity from a single cubic FCC phase to an amorphous structure with increasing RN. AFM revealed a reduction of surface roughness from 0.51 to 0.29 nm at maximum RN. Based on the nanoindentation results, a 23% enhancement in hardness was achieved, reaching a maximum of 9.6 ± 0.1 GPa at an RN of 50%. The hardness of HEN thin films is superior to the HEA thin films with similar compositions deposited by FCAD.