Reactively sputtered high-entropy carbide thin films based on Al-Cr-Nb-Ta-Ti

Abstract

ith special characteristics is rising. To satisfy this demand, in the case of metals, the pure elements are often not sufficient. Thus, almost every component in modern engineering consists of alloyed materials with specially tailored properties for the intended field of use. Traditionally alloying is the combination of one primary element with only small amounts of one or two secondary elements. This concept has shown the capability to produce adequate compositions for a wide variety of applications. However, research on a new concept of “high-entropy alloys”, or in short, HEAs was started a few decades ago. The basic idea of this new approach, in contrast to traditional alloys, was that many contributing elements in equal or near equal amounts can be beneficial for the properties of the alloys.Besides the composition of the bulk material, the performance of a component is also determined by the surface. To alter mechanical, electrical, chemical, or optical properties of a part’s surface, the application of thin films with thicknesses in the micro or even nanometer range is a valuable approach. For such thin films as well as for the base materials HEAs are an interesting choice, promising potentially superior qualities. This thesis is focuses on thin films based on “high-entropy carbides”, which are a subcategory of “high entropy ceramics“, or in short, HECs. The specimens were synthesised by reactive magnetron sputtering of an equimolar AlCrNbTaTi composite-target with acetylene mixed into the argon working gas. Additionally, varying amounts of silicon were added to the composition to show its effects on the mechanical properties as well as oxidation behaviour. The produced films were investigated by X-ray diffraction, nanoindentation testing, scanning electron microscopy, and X-ray fluorescence spectroscopy. The reactively sputtered AlCrNbTaTi carbides as well as its silicon alloyed variants possess a clear cubic structure with a lattice parameter of 4.380 Å. Concerning the mechanical properties, hardness values of around 30 GPa were achieved while indentation modulus was between 380 GPa and 450 GPa. Both showed a slight tendency to increase with the silicon content. Vacuum annealing treatments up to 800 °C showed no measurable effect on the microstructure of the films while the annealing in ambient air led to the formation of a rutile-structured oxide scale. A clear impeding effect of the silicon content on the oxide growth was discovered. The sample without additional silicon formed an up to 7.8 μm thick oxide scale at a temperature of 800 °C while the sample with the highest Si content did not form a measurable scale at all.