ICMCTF 2025 Session MA4-2-WeM: High Entropy and Other Multi-principal-element Materials II

In the last decades, Refractory High Entropy Alloys (RHEAs) thin films have attracted more attention owing to their enhanced mechanical properties and better thermal stability at high temperatures, compared to conventional alloys. TiTaZrHfW-N and TiTaZrHfAl-N RHEAs thin films were deposited by reactive magnetron sputtering technology at different N2 flow rates. For both systems, nitrogen-free films are amorphous, and the nitrides are columnar, single-phased with an FCC-NaCl type structure. The strong Me-N bonds lead to hardness up to 29 GPa and a Young’s modulus up to 257 GPa, for the TiTaZrHfW-N system whereas the highest hardness and Young’s modulus for the TiTaZrHfAl-N system are 25.3 GPa and 201.3 GPa respectively. Unlike metallic films, TiTaZrHfW-N and TiTaZrHfAl-N nitride films are thermally stable at 800 °C under vacuum and have a much better oxidation resistance. Nanolayered architectures TiTaZrHfW-N/Si3N4 and TiTaZrHfAl-N/Si3N4 result in a significant improvement of the oxidation resistance at 800 °C due to the formation of amorphous Si-N barrier nanolayers, hindering the oxygen diffusion.

Although transition metal nitrides such as TiN or ZrN have been widely studied as plasmonic properties, the optical and electrical properties of multi-component nitride thin films are rather lacking in the literature for plasmonic applications, in spite of their well-known mechanical properties [1]. We attempt in this paper to alleviate this drawback by depositing the multi-component nitride (TiVZrNbTa)N_x films on silicon substrates by using reactive magnetron sputtering under different nitrogen flow rates (R_N2) and at different working pressures. The X-ray diffractograms of the as-deposited films have shown that they crystallize in a single-phase with a rocksalt-like structure. Moreover, the plasmonic potential of the films was investigated from their dielectric function determined by variable angle spectroscopic ellipsometry. The films prepared at low working pressures exhibit optimal metallic behaviour with the real part of their dielectric function displaying zero-crossover. In contrast, those fabricated at high working pressure show non-metallic behaviour without any zero-crossover and with low absorption in the near-infrared region. In particular, the real part of the dielectric function of the film produced at 0.5 Pa has a notable feature of double epsilon-near-zero (2ENZ) comparable to other transition metal nitrides such as TiN, ZrN and NbN [2-4]. The dielectric function of (TiVZrNbTa)N_x can be tuned by tailoring the deposition parameters such as working pressure to some desired plasmonic application. Specifically, the screened plasma energy (E_ps) is tuneable from near UV to visible ranges, from 3.4 to 2.1 eV. Their plasmonic performance were evaluated by calculating their intrinsic quality factor for surface plasmon polaritons (SPP). Additionally, the free-carrier density, the scattering time and the electrical resistivity of the films were also determined by means of Drude model for the free charge carrier contribution to the dielectric function [2]. Drude parameters were compared with additional electrical measurements performed by four-point probe method.

References

[1] Von Fieandt, K., Pilloud, D., Fritze, S., Osinger, B., Pierson, J. F., & Lewin, E. Vacuum, 2021, 193, 110517.

[2] Kassavetis, S., Hodroj, A., Metaxa, C., Logothetidis, S., Pierson, J. F., & Patsalas, P. Journal of Applied Physics, 2016, 120(22).

[3] Guo, Q., Wang, T., Ren, Y., Ran, Y., Gao, C., Lu, H., ... & Wang, Z., Physical Review Materials, 2021, 5(6), 065201.

[4] Ran, Y., Lu, H., Zhao, S., Guo, Q., Gao, C., Jiang, Z., & Wang, Z., Applied Surface Science, 2021,537, 147981.

High entropy alloys (HEAs) are multi-component materials composed of five or more principal elements, with each element's content ranging from 5 to 35 atomic percent. The properties of HEAs arise from four core effects: the high entropy effect, severe lattice distortion, sluggish diffusion, and the cocktail effect. This high entropy concept also extends to ceramics, including oxides, nitrides, borides, and carbides.

In this study, we are examining the temperature stability of high entropy nitrides from the Cr-Hf-Mo-Ta-W-N system. We utilized magnetron co-sputtering of segmented elemental targets for all depositions, which were performed on silicon and sapphire substrates. The first set of depositions was conducted at ambient temperature, while an elevated temperature of 700°C was used for the second set to enhance coating crystallization.

All the deposited coatings exhibited strong diffraction peaks corresponding to a face-centred cubic (fcc) lattice, which is anticipated for the formation of these high entropy ceramics. The coatings were annealed at temperatures of 1000°C, 1200°C, and 1400°C to observe changes in their chemical composition, phase, crystal structure, morphology, and mechanical properties.

High-entropy (AlCrTiMoVNi)N coatings were prepared using cathodic arc evaporation. The target composition was varied to investigate the effect of nickel concentration on the microstructure, mechanical properties and tribological performance of the coatings. All coatings assume a B1 face centered cubic structure, and contain many small droplet and large splat defects; and the amounts of those defects increase with the concentration of Ni. All coatings showed excellent high-temperature phase stability. The hardness and elastic modulus of the coatings reached maximum values at 2% Ni and then, decreased as the Ni content increases. In terms of performance, the coating with 2% Ni had the lowest wear rate while in erosion testing the coating free of Ni had the lowest erosion rates. Overall, the presence of droplets/splats had a significant influence on the tribological performance of the coating.

(AlNbSiTiZr)N thin films were deposited on silicon wafer and steel substrates using pulsed DC magnetron sputtering with four distinct targets (AlSi, Ti, Nb, Zr). The elemental concentration of each constituent was tuned by adjusting the discharge current applied to each target. The thickness of the deposited films was maintained at approximately 1.3 μm. Three different (AlNbSiTiZr)N thin films with slight variations in elemental concentrations were studied to assess the impact of compositional changes on their structure and properties. The films were characterized for surface and cross-sectional morphology, microstructure, roughness, and mechanical properties. A minor increase in the concentrations of Nb and Zr (5 at.% for each) led to a significant improvement in hardness, increasing from 12.7 ± 0.7 GPa to 20.8 ± 0.5 GPa. The tribological properties of the films were studied using a ball-on-plate tribometer under dry air conditions with a load of 0.5 N for 1000 sliding cycles. The results indicate that the wear resistance of the (AlNbSiTiZr)N thin films improved with the increased concentrations of Nb and Zr.