Abstract
Multicomponent (TiNbZrTa)Nx films were deposited on Si(100) substrates at room temperature using magnetron sputtering with a nitrogen flow ratio fN [fN = N2/(Ar + N2)], which was varied from 0 to 30.8%. The nitrogen content in the films varied between 0 and 45.2 at.%, i.e., x = 0 to 0.83. The microstructure was characterized by X-ray diffraction and electron microscopy. The metallic TiNbZrTa film comprised a dominant bcc solid-solution phase, whereas a single NaCl-type face-centred cubic structure was observed in all nitrogen-containing films (TiNbZrTa)Nx. The mechanical, electrical, and electrochemical properties of these films varied with nitrogen content. The maximum hardness was achieved at 22.1 ± 0.3 GPa when N = 43.0 at.%. The resistivities increased from 95 to 424 μΩcm with increasing nitrogen content. A detailed study of the variation of morphology and chemical bonding with nitrogen content was performed and the corrosion resistance of the TiNbZrTa nitride films was explored in 0.1 M H2SO4. While all the films had excellent corrosion resistances at potentials up to 2.0 V vs. Ag/AgCl, the metallic film and the films with low nitrogen contents (x < 0.60) exhibited an almost stable current plateau up to 4.0 V vs. Ag/AgCl. For the films with higher nitrogen contents (x ≥ 0.68), the current plateau was retained up to 2.0 V vs. Ag/AgCl, above which a higher nitrogen content resulted in a higher current. The decrease in the corrosion resistance at these high potentials indicate the presence of a potential-dependent activation effect resulting in an increased oxidation rate of the nitrides (present under the passive oxide film) yielding a release of nitrogen from the films. TEM results indicate that the oxide layer formed after this corrosion measurement was thick and porous for the film with x = 0.76, in very good agreement with the increased corrosion rate for this film. The results demonstrate that an increased nitrogen content in (TiNbZrTa)Nx system improves their mechanical properties with retained high corrosion resistance at potentials up to 2.0 V vs. Ag/AgCl in 0.1 M H2SO4. At even higher potentials, however, the corrosion resistance decreases with increasing nitrogen concentration for films with sufficiently high nitrogen contents (i.e. x ≥ 0.68).
Highlights
The introduction of the concept of high-entropy metal alloys in 2004 [1,2] has been rapidly extended to high-entropy ceramics [3,4,5,6,7]
These stu dies, which were conducted with multicomponent nitrides containing Al and Cr, e.g. (AlCrSiTiV)Nx (0.48 ≤ x ≤ 1.14) films in 3.5% NaCl electrolyte [10], (AlCrSiTiZr)Nx (0 ≤ x ≤ 1.13) films in 0.1 M H2SO4 [24], suggest that the corrosion resistance increases with increasing nitrogen concentration in the films
The effect of nitrogen content on the microstructure, mechanical and electrical properties, and the corrosion resistance has been studied for multicomponent (TiNbZrTa)Nx films (0 ≤ x ≤ 0.83), covering the entire composition range from metal to near-stoichiometric nitride MeN0.83
Summary
The introduction of the concept of high-entropy metal alloys in 2004 [1,2] has been rapidly extended to high-entropy ceramics (ni trides, carbides, borides, and oxides) [3,4,5,6,7] These materials have re markable properties leading to a wide range of applications as thin films, including diffusion barrier layers [8], hard wear-resistant coat ings [9], and corrosion-resistant coatings [10]. Though promising, they still face barriers that limit the fundamental scientific understanding as well as practical applications, such as choices of metal elements, lattice distortion, and unexpected properties emerging by adding species. The outcome of this research is intended to provide funda mental material-level understanding of the corrosion resistance and corrosion mechanisms for this class of multicomponent coatings, which is of potential importance for applications in the field of batteries and fuel cells
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