ICMCTF2017 Session B5-1: Hard and Multifunctional Nanostructured Coatings

High entropy alloys (HEAs) are multicomponent alloys with at least five elements in approximately equimolar concentrations. Due to the high entropy of mixing, solid solutions of e.g. a simple bcc or ccp phase are formed. Many HEAs exhibit unique properties such as extremely high strength, ductility, high thermal stability and corrosion resistance. Recently, we have also demonstrated that a HfNbTiVZr alloy with a bcc structure has exceptional hydrogen storage capability [1].

In this study we report the first successful deposition of highly textured HfNbTiVZr thin films by non-reactive unbalanced magnetron sputtering using five metal targets. The films were characterized with SEM, XPS, XRD and nanoindentation. Bragg-Brentano XRD analyses of these coatings show that the HfNbTiVZr films crystallize in a simple bcc structure with a strong (110) orientation. No additional phases were observed in the as-deposited films ensuring that the sample is a single phase HEA. The SEM cross-sections exhibit a dense microstructure and EDS mapping shows a random distribution of all five elements with no noticeable segregation.

The as-deposited films exhibited a hardness of 5.4 GPa. This is more two times higher than expected from rule of mixture. The high hardness can be explained by a significant lattice strain due to large differences in atomic radii. Upon annealing, XRD shows the formation of a second alloy phase which is unexpected for a true HEA. The precipitation of a second phase leads to a further hardness increase with at least 100%. The phase stability and the precipitation of secondary phases in the HfNbTiVZr films will be discussed based on lattice strain effects in the alloy. Finally, the electrochemical properties of the HfNbTiVZr films have been investigated. The films exhibited, compared to steel alloys, a very high corrosion resistance in chlorine-containing solutions.

Synthesis of nitride-based multilayered structures is prospective for enhancement of mechanical properties and wear resistance as well as for their resistance to harsh environments exposure, in particular, to high temperature oxidation and ion irradiation. Multilayered ZrN/SiN_x films (with bilayer thickness ranging from 6 to 40 nm) with a total thickness of about 300 nm were deposited at 300°C by reactive magnetron sputtering by sequentially alternating ZrN and SiN_x layers [1]. Annealing under air atmosphere was carried out for temperature intervals ranging from 400 to 950°С using in situ temperature XRD. Ion irradiation by Xe ions (180 keV, doses 5x10¹⁶ cm^-2 1x10¹⁷ cm^-2) was performed at room and high (800°C) temperatures.

According to TEM and XRD analysis the multilayered films consist of nanocrystalline ZrN and X-ray amorphous SiN_x layers. While pure ZrN films are characterized by [111] preferred orientation, the presence of SiN_x layers results in its change to [200]. Multilayered ZrN/SiN_x films show the improved oxidation resistance compared to ZrN reference layer. For pure ZrN film, the oxidation starts at 550°C. Higher oxidation resistance is observed for ZrN/SiN_x films and it strongly depends on ZrN fraction and number of layers. So, for ZrN/SiN_x (5 nm/10 nm) film the onset of oxide phase formation is delayed up to 950°C and nitride ZrN phase still remains. The presence of SiN_x layer favors the formation of tetragonal ZrO₂ phase during annealing.

There is no evident change of ZrN/SiN_x film structural state after Xe ion irradiation as it was proved by XRD investigations. However, the lattice parameter increase occurs that is connected with Xe incorporation. It was revealed by RBS analysis that the maximum concentration of Xe in the film bulk is about 4 at.% after irradiation at room temperature and about 5 at.% after high temperature irradiation. The shape of Xe concentration profile also changes. TEM studying indicates amorphization of ZrN layers and Xe bubbles formation in the film. There is a tendency of Xe depth penetration decrease with the SiN_x layer thickness increase. Thus multilayered ZrN/SiN_x films are considered to be promising for oxidation and irradiation resistance enhancement.

The present work focuses on the effect of nitrogen addition into hard and electrically conductive Hf–B–Si–C films [1] in order to significantly improve their thermal stability in air at very high temperatures (above 1200°C). Our motivation has been to develop new hard thin-film materials with a very low electrical and thermal conductivity, and high optical transparency or with a sufficiently high electrical conductivity for high-temperature protective coatings of electronic and optical elements, and for harsh-environment sensors.

Hf–B–Si–C–N films were deposited onto Si(100) and SiC substrates using pulsed magnetron co-sputtering of a single B₄C–Hf–Si target (at a fixed 15% Hf fraction and a 20% Si fraction in the target erosion area) in Ar + N₂ gas mixtures at the N₂ fraction ranging from 0% to 50%. A planar unbalanced magnetron (127×254 mm² target) was driven by a pulsed dc power supply operating at a repetition frequency of 10 kHz with an average target power of 500 W in a period and voltage pulse durations of 50 μs and 85 μs (duty cycles of 50% and 85%). The substrates were held at a floating potential and a temperature of 450 °C.

An increasing N content (from 0 to 52 at.%) in the films was compensated by decreasing contents of B (from 39 to 24 at.%), Si (from 24 to 15 at.%), Hf (from 25 to 4 at.%) and C (from 7 to 3 at.%). The structure of the Hf–B–Si–C film prepared in pure argon was nanocomposite, while the Hf–B–Si–C–N films were amorphous. An increase in the N₂ fraction in the gas mixture, resulting in an increasing N content in the films, led to a rapid rise in the optical transparency and the electrical resistivity of the films. All films exhibited a high hardness in the range of 17–21 GPa.

The as-deposited, optically non-transparent Hf₇B₂₃Si₂₂C₆N₄₀ film with 2 at.% of Ar possessing a hardness of 20 GPa and electrical resistivity of 4 Ωm, which was prepared with the 15% N₂ fraction in the gas mixture at the voltage pulse duration of 50 μs, and the as-deposited, highly optically transparent and electrically insulating Hf₆B₂₁Si₁₉C₄N₄₇ film with 3 at.% of Ar possessing a hardness of 20 GPa, which was prepared with the 25% N₂ fraction in the gas mixture and at the same voltage pulse duration, exhibited a very high oxidation resistance in air even up to 1600°C.

[1] J. Kohout, J. Vlcek, J. Houska, P. Mares, R. Cerstvy, P. Zeman, M. Zhang, J. Jiang, E.I. Meletis, S. Zuzjakova, Hard multifunctional Hf–B–Si–C films prepared by pulsed magnetron sputtering, Surf. Coat. Technol. 257 (2014) 301–307.

(TiZrHfVNb)N thin films, synthesised by reactive DC magnetron sputtering, demonstrate a large homogeneity range for a solid solution phase with the NaCl-type structure. The films have been deposited using elemental targets of the respective element and a gas mixture consisting of Ar and N₂ (15 % of total gas flow). In earlier studies, (TiZrHfVNb)N films have been synthesised by cathodic arc vapour deposition using a high entropy alloy of Ti-Zr-Hf-V-Nb as cathode material. These films have showed very interesting mechanical properties, such as high hardness and high wear resistance.¹ However, there are no previous studies on sputter deposited (TiZrHfVNb)N films. By using this deposition technique it could allow for more design possibilities, which is explored in this study by varying composition of the material. The films were characterised by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM).

Large variations in metallic ratio were obtained for the (TiZrHfVNb)N coatings, about 10-50 % for Ti, 20-35 % for Zr,10-50 % for Hf,10-30 % for V and 5-15 % for Nb. XPS spectra for each transition metal showed that the films are fully nitrided. The coatings were found to be of single NaCl type phase for all studied compositions. The lattice parameter varied from 4.32 Å for Ti rich coatings to 4.44 Å for Hf rich coatings, which is within the range of the corresponding binary nitrides (4.13 Å for VN and up to 4.58 for ZrN) and thus indicating a solid solution phase of all constituent elements. Cross sections of the coatings studied by SEM showed a columnar growth which varied in density depending on deposition conditions such as substrate temperature and bias.

Reference:

[1] A.D. Pogrebnjak et al., Mater. Chem. Phys., 147 (2014) 1079

This work intends to create a new surface acting as solid lubricant, gathering the outstanding self-cleaning and nearzero friction properties of fluor-doped WS₂-C coatings. These coatings can be deposited onto different porous anodized aluminum alloy surfaces in order to be able to control the water and oil wettability.

W-S-C-F coatings were produced by magnetron sputtering in reactive mode using an Ar/CF₄ gas mixture. The fluorine insertion in the produced coatings was controlled by varying the CF₄ gas flow aperture (0-20%).

Top view and cross-sectional morphologies, chemical composition/bonding, structure and wettability characterization of the coatings were respectively performed through FIB/SEM-EDS, XRD techniques and water/oil contact angle measurements. Mechanical properties such as hardness, elastic modulus and adhesion were also conducted under nanoindentation procedures and scratch testing.

The results revealed that the fluorine insertion on the W-S-C coatings did not change significantly the structure and morphology in relation to pure W-S-C coatings. However, a decrease on the mechanical properties was observed. Furthermore, the changes in the contact angles values show a potential for the control of the wettability behaviour in relation to water and oils.