Structure and mechanical properties of (Al,B,Cr,Si,Ti)-based thin films

Abstract

High-entropy alloys (HEAs) and high-entropy metal-sublattice ceramics (HESCs) have recently gained particular attraction in the field of materials research due to their promising properties, such as high hardness, high strength, and thermal stability. Ceramics based on the high-entropy concept mostly consist of refractory metals such as Ta, Hf, Zr, W, V etc. These metals are good nitride and carbide formers which is why they are mainly used especially for PVD coatings. Nevertheless, the production of these elements needs a lot of energy input due to their very high melting points. Furthermore, these elements are very heavy which in consequence makes them hard to process and rather expensive. In this study we want to focus on a material system consisting of Al, B, Cr, Si, and Ti which are comparably light and cheap elements and the production of a corresponding compound target consumes less energy. To get an idea of the properties of coatings based on this material system we investigated “metallic” coatings as well as nitrides and oxides. The coatings were synthesised by magnetron sputtering using a single composite target with an equiatomic composition and different gas mixtures. All the coatings produced show XRD amorphous diffraction patterns without any indication of crystalline phases. Also, SEM images of fracture cross-sections do not show the, usually characteristic, columnar growth which further underpins the results obtained by XRD measurements. The hardness and indentation modulus of the coatings range from ~10 to 22 GPa and from ~170 to 260 GPa, respectively, depending on the character of the coating. To get information of the bonding state, XPS measurements were carried out. Furthermore, in-situ cantilever bending tests were done to investigate the fracture toughness of the coating depending on their either “metallic”, nitride, or oxide character.