Development of high-entropy carbide thin films via DC magnetron sputtering

Abstract

A thin film is a layer of material having a thickness of a few nanometers up to several micrometers. In materials science, thin films are grown on materials to improve certain properties such as mechanical properties, thermal stability, oxidation and corrosion resistance. Thin films are deposited via various methods of which physical vapor deposition (PVD) is most common. PVD includes several methods in which a solid target material is evaporated by physical means. The evaporated atoms then travel to the substrate where the thin film is formed. The properties of deposited thin films can be controlled by tuning deposition parameters such as voltage, current, temperature, time, and working gas pressure.This PhD research focuses on the development of high-entropy alloy (HEA) thin films, with particular emphasis on high-entropy carbides (HEC)s. HEAs are multicomponent systems, typically composed of five or more principal elements, in near-equiatomic proportions and are characterized by four fundamental effects: high configurational entropy, sluggish atomic diffusion, severe lattice distortion, and the cocktail effect. For materials to qualify as a high-entropy systems, the configurational entropy of mixing should exceed 1.5R, where Ris the universal gas constant.High-entropy carbides are a subclass of high-entropy ceramics in which a small anionic element, carbon, occupies interstitial sites and forms strong covalent bonds with the metallic cation sublattice. Transition metals from groups IV, V, and VI of the periodic table are commonly selected for the formation of HECs due to their strong carbide-forming tendencies.In this research work, HEC thin films are grown and their compositions, crystal structures, microstructure, mechanical properties, thermal and oxidation resistance are investigated through energy dispersive spectroscopy (EDS), X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning election microscope (SEM), transmission electron microscope (TEM), nano-indentation, and thermogravimetric analyses. Some HEC thin films were developed with Si addition to study the influence of Si on structure, morphology, hardness, elastic modulus, thermal and oxidation resistance of thin films. The results show that Si addition to HEC increases the deposition rate while preserving a dense, fibrous morphology. Si coatings form a continuous, adherent oxide scale that enhances the oxidation resistance of HEC thin films. During vacuum thermal annealing, Si promotes the formation of nanoscale, coherent SiC precipitates along grain boundaries, which impede grain-boundary motion and dislocation glide, thereby maintaining high hardness and elastic modulus. The outcomes of this PhD research work will be helpful in the design and development of functional thin films.