Structure and properties of high-entropic vacuum-arc coatings.

1Andreev A., 2Gorban' V., 1Voevodin V., 1Stolbovoy V., 2Karpez M., 2Krapivka N,  2Kanzir A. 

1. National Science Center "Kharkov Institute of Physics and Technology", National Academy of Sciences of Ukraine, str. Academicheskaya, 1, Kharkov - 61108, 1Ukraine.
2. Institute for Problems in Materials Science, National Academy of Sciences of Ukraine,str. Krzhizhanovsky, 3, Kyiv-142, 03680,Ukraine. 

The structure, physico-mechanical properties and thermal stability of coatings obtained by vacuum-arc evaporation and sputtering in a plasma of a compressed arc discharge [1] of high-entropy alloys (HEA) are determined.
A characteristic feature of НEA-based coatings is a reduced lattice parameter. These changes are accompanied by an increase in hardness from 4.2 to 10 GPa and a reduced elastic modulus from 87 to 105 GPa. Technological factors significantly affect the structure and physico-mechanical properties of coatings of HEA. So the increase in negative bias voltage on the substrate contributes to the transition from a columnar structure to a uniform one, which is accompanied by an increase in hardness. When a bias potential is applied above -60 V, there is a decrease in the content of aluminum coatings. And at a bias potential of -200 V a sharp decrease in the content of such elements as copper and aluminum is recorded. The coating obtained by spraying in vacuum has a very high H / Er ratio, reaching values ​​of 0.094. 
A feature of high-entropic alloys is their cluster structure. In the presence of elements with different atomic radii, it is thermodynamically advantageous in each cluster to maintain a stable phase composition with small deviations in the lattice parameter.

Fig. 1. High resolution micrographs of the grain section of the coating, made of TiVZrNbHfТа alloy.   

The size of such nanograins is quite small and ranges from 2 to 5 nm. As shown by the analysis of the chemical composition with a probe spot of 0.5 nm, each nanograin is significantly different in composition. In the presence of elements with different atomic radii, it becomes thermodynamically advantageous due to varying chemical composition in each cluster to maintain a stable phase composition with small deviations in the lattice parameter. It is shown that vacuum-arc high-entropic coatings are thermally stable up to an annealing temperature of 1100 oC.
It was revealed that high-entropic single-phase nitride coatings are characterized by high values ​​of hardness (50-60 GPa) and modulus of elasticity. The formation of the lattice type of nitride coatings is most influenced by the heat of formation and the predominance of nitrides with one type of crystal lattice. Increasing the pressure to 0.6 Pa leads to the formation of textured coatings. At Uр = -50V and -100 V, a transition occurs from the partially textured state with the [111] axis, perpendicular to the growth plane, to the almost complete uniaxial texture state with the [111] axis. At Up = -200 V, a bit-texture state is formed with axes [111] and [110] perpendicular to the growth plane. The hardness of the coatings obtained at this pressure is rather high and amounts to 60 ... 70 GPa. It is established that high-entropic single-phase nitride coatings are thermostable up to annealing temperatures of 1200 ºС.

1.	Андреев А.А, Саблев Л.П., Григорьев С.Н. Вакуумно-дуговые покрытия.  – Харьков, ННЦ ХФТИ, 2010. – 318 с.