Structure formation and properties of sputter deposited Nbx-CoCrCuFeNi high entropy alloy thin films

Accepted Manuscript Structure formation and properties of sputter deposited Nbx-CoCrCuFeNi high entropy alloy thin films B.R. Braeckman, D. Depla PII:

S0925-8388(15)30212-7

DOI:

10.1016/j.jallcom.2015.06.097

Reference:

JALCOM 34450

To appear in:

Journal of Alloys and Compounds

Received Date: 30 April 2015 Revised Date:

11 June 2015

Accepted Date: 13 June 2015

Please cite this article as: B.R. Braeckman, D. Depla, Structure formation and properties of sputter deposited Nbx-CoCrCuFeNi high entropy alloy thin films, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.06.097. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structure formation and properties of sputter deposited Nbx-CoCrCuFeNi high entropy alloy thin films B. R. Braeckman*, D. Depla Research group DRAFT, Department of Solid State Sciences, Ghent University, Krijgslaan 281 (S1), 9000 Gent, Belgium

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*Corresponding author: [email protected]

1. Introduction

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Thin films of the high entropy alloy Nbx-CoCrCuFeNi with different niobium concentrations were deposited by magnetron sputtering. The film density and the residual stress of the niobium-free (x=0) thin films clearly decreases at higher pressure-distance products. This behaviour can only be explained by the momentum transfer of the sputtered atoms and the reflected Ar atoms on the growing film as the energy per arriving atom shows little variation. The addition of Nb, which is the heaviest atom of the alloy, amplifies this effect. Hence, thin films with a high Nb content still show a high density at large pressure-distance products. However, as Nb has the largest radius of all constituent elements, the crystallographic structure of the thin films changes from a crystalline face-centered cubic structure at x=0 to an amorphous (or nanocrystalline) structure for higher Nb fractions. Both trends, i.e. the changing deposition conditions and the niobium content, can be outlined by a study of the thin film microstrain. The trends observed in the intrinsic properties are correlated to a preliminary study of some functional properties (friction coefficient, thermal stability and contact resistance).

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In the last two decades, the role of complex materials in thin film research has shifted from marginal towards pivotal. Complex, multi-element coatings exhibit improved characteristics and new beneficial properties that simple materials cannot provide. Complex alloys, such as high entropy alloys (HEAs), first developed by Yeh et al. [1], are an interesting example of this trend. HEAs are metallic alloys composed of at least five principal elements in near-equimolar ratios. Despite the large number of different constituent elements, the high mixing entropy ensures the formation of simple solid solutions (FCC/BCC or amorphous) in contrast to the expected intermetallic phases. Bulk HEAs have been extensively studied for their excellent properties [2] such as high corrosion resistance [3, 4], good mechanical properties [5, 6, 7] and thermal stability [8, 9, 10, 11]. Thin film HEAs could be used as heat-or wear-resistant coatings, diffusion barriers [12] and hard coatings for cutting tools. Two of the most studied HEAs are the quinary CoCrCuFeNi alloy and the senary AlCoCrCuFeNi alloy [5, 13, 14, 15, 16, 17, 18, 19]. The first alloy exhibits a single face-centered cubic (FCC) structure without precipitates or intermetallic phases. Aluminium addition enhances the formation of the body-centered cubic (BCC) structure and alters the material properties. The FCC to BCC phase change is typically explained based on two arguments: (i) incorporation of Al atoms into the FCC lattice increases the lattice distortion whereas the BCC lattice has a lower packing fraction and is more open to accommodate the larger Al atoms, and (ii) the peculiar electronic structure of Al favours bonds between Al and transition metals with an incompletely filled d-shell. By replacing Al with Nb it is possible to investigate whether these two principles are also valid for other additive 1

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2. Experimental

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elements. Niobium has the same atomic radius as aluminium, but evidently another valence electron structure. Although the effect of Nb addition was already studied for thick coatings prepared by plasma transferred arc cladding [20] and for bulk alloys by copper-mould suction casting [21], it is not clear if some of the noticed trends are related to the deposition conditions and/or to the Nb content. In this study, the effect of Nb addition and process parameters is investigated for NbxCoCrCuFeNi thin films deposited by magnetron sputtering. The focus of this paper is on the intrinsic properties ((micro)structure, density and residual stress), but some preliminary results on functional properties (thermal stability, friction coefficient and contact resistance) are also discussed. Nbx-CoCrCuFeNi thin films were deposited by sputtering cold-pressed powder targets under various conditions. More details on the production and the use of powder targets can be found in the work of Boydens et al. [22, 23]. Five different targets were made and their compositions are summarized in Table 1.

at.% Nb 0 5 10 15 23

at.% Co 20 19 18 17 15.4

at.% Cr 20 19 18 17 15.4

at.% Cu 20 19 18 17 15.4

at.% Fe 20 19 18 17 15.4

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target Nb000 Nb005 Nb010 Nb015 Nb023

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Table 1: Composition of the powder targets

at.% Ni 20 19 18 17 15.4

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The maximum grain size of the powders was 5 μm for Nb, 44 μm for Co, Cr, Fe and Ni (Alfa Aesar), and 50 μm for Cu (Goodfellow). The powders were uniaxially cold-pressed (90 MPa) into targets of thickness 2 mm and diameter 46 mm. After pumping down the vacuum chamber to a base pressure of 10-4 Pa, 99.9 % pure argon gas was introduced at a flow rate of 30 sccm. The argon pressure (see further) was varied by a change of the pumping speed using a butterfly valve. The substrate holder was grounded and films were deposited on silicon, glass and stainless steel substrates. The 0.55 mm thick (111) oriented Si wafers (with native oxide) were cleaned in a Piranha solution prior to deposition. The glass substrates were 0.9 mm thick soda-lime glass microscope slides (Carl Roth) and were ultrasonically cleaned in methanol prior to deposition. The 0.9 mm thick mirror polished 316L stainless steel substrates were cleaned with acetone. For all five target compositions, six films were deposited by a combination of two different argon pressures (0.4 and 0.55 Pa) and three different target-to-substrate distances (7, 9 and 11 cm), or 6 so-called pressure-distance (p·d) products (2.8, 3.6, 3.85, 4.4, 4.95 and 6.0 Pa·cm). Sputtering was performed with an unbalanced magnetron. A constant discharge current of 0.09 A with a typical target power of 40 W was used for all the depositions. The film thickness was measured by contact profilometry (Taylor-Hobson Talystep) and varied between 300 and 800 nm. A passive thermal probe was used as a heat flux monitor to determine the temperature change during sputtering [24, 25]. This probe also enabled to measure the total energy flux reaching the substrate Etot. The energy per arriving particle (EPA) can be found by scaling Etot with the deposition rate. The chemical composition of the films was obtained with SEM/EDX (FEI Quanta 200F) with a beam current of 208 μA and a voltage of 20 kV. XRD θ/2θ measurements were performed with a Brüker D8 Discover equipped with a LynxEye silicon strip detector and Cu Kα radiation was used. An offset angle of 5° between the X-ray source and the detector was set to minimize the contribution of the Si substrate. The samples were scanned from 20° to 100° with a resolution of 0.01° per step and a step time of 4 s. The density of the films was measured with XRR. A Brüker D8 Advance equipped with a scintillation detector and Cu Kα radiation was used. The samples were scanned from 0.3° to 6° with a step size of 0.01°. Parratt’s method was used to determine the critical angle of total reflection [26, 27]. The glass samples were annealed in a quartz tube furnace at three different temperatures (200, 2

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450 and 600 °C) in an argon atmosphere. The contact resistance of the films was measured with a four-probe in-house setup [28]. A gold-plated CuBe probe with a spherical tip (diameter 1.5 mm) was pressed on the thin film surface with a force of 5 N. The contact resistance was monitored with a Schuetz Messtechnik MR300 Micro-Ohmmeter as function of the time. To calibrate the setup the contact resistance of a gold plate was measured. A sputter deposited Ag thin film was added as reference material. The resistivity of the films was measured with a four-point probe sheet resistance setup. Friction measurements were performed with a ball-on-disk setup. Stainless steel balls with a diameter of 6 mm were pressed against the film surface with a force of 1 N. The sliding speed was 0.05 m/s and the track radius 2.5 mm. Tests were conducted in ambient atmosphere with a relative humidity of 55 %. The wear track was investigated with SEM-EDX and optical profilometry.

3. Results

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3.1. SEM-EDX analysis: microstructure and composition The composition of the 30 thin films (5 target compositions (see table 1), deposited at six different p·d products) was measured with EDX. No influence of the p·d product on the sample composition was noticed (see Figure 1).

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Figure 1: (a) Composition of the CoCrCuFeNi films (target Nb000, see table 1) as a function of the p·d product. (b) The Nb atomic fraction of the Nbx-CoCrCuFeNi thin films (targets Nb005 up to Nb023, see table 1) as a function of the p·d product.

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Figure 2 shows the SEM cross-sections of a niobium-free thin film (0 at.% Nb) and a thin film with high Nb content (23 at.% Nb). Both films have a smooth surface and are dense.

Figure 2: SEM micrographs showing the cross-sections of the alloy thin films without Nb (left) and with 23 at.% Nb (right). Both films were deposited at 2.8 Pa·cm and are dense and smooth. 3

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3.2. X-ray analysis

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3.2.1. Crystallographic structure Figure 3 presents the XRD patterns of the Nbx-CoCrCuFeNi thin films deposited at p·d=2.8 Pa·cm, which are typical patterns, irrespective of the deposition conditions. Indeed, only small shifts in the peak positions (see paragraph 3.2.3 for the interpretation) were detected for other p·d values. For the Nb-free thin film (x=0), only a single FCC solid solution phase was detected, although there are five different constituent elements in agreement with the HEA concept. An increase of the niobium concentration results in less sharp diffraction peaks.

Figure 3: XRD patterns of the Nbx-CoCrCuFeNi thin films with increasing Nb fraction deposited at p·d = 2.8 Pa·cm. The peaks were normalized to the film thickness.

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3.2.2. Film density The film density was measured with XRR. To account for compositional differences, the measured density was normalized to the calculated bulk density. This density ratio is shown in Figure 4 as function of both the Nb atomic fraction and the p·d product. Some trends are clearly visible. Without Nb addition the film density decreases with increasing p·d product. At low p·d products the addition of Nb does not affect the film density, whereas the films that were deposited at high p·d products show a densification effect with increasing Nb fraction.

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Figure 4: Contour plot showing the density ratio of the Nbx-CoCrCuFeNi thin films as function of the Nb atomic fraction and p·d product.

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3.2.3. Residual stress The residual stress of the alloy thin films was measured with the sin2ψ-method. Under the assumption that the film is an isotropic elastic material, the relationship between the out-of-plane strain and the slope of the sin2ψ-plot can be used to calculate the residual in-plane stress: 2ν୤ 1 + ν୤ ଶ dந = d଴ ൬1 − σ +σ sin ψ൰ E୤ E୤ So: ∂dந E୤ σ= d଴ ሺ1 + ν୤ ሻ ∂ሺsinଶ ψሻ with Ef and νf the averaged elastic modulus and averaged Poisson’s ratio of the alloy [29]. After evaluation of the slopes of the Nb-free thin films (two examples are shown in Figure 5a), it appeared that the slope of the film deposited at p·d=4.95 Pa·cm was close to zero. Hence it was assumed that this film has no residual stress, and the measured interplanar spacing of this film was used to calculate the residual stress of the films deposited at other p·d products. As Figure 5b clearly shows there is a relationship between the residual stress and the film density of the CoCrCuFeNi thin films. The influence of the Nb addition on the film residual stress is presented in Figure 5c. An increase of the Nb fraction amplifies the compressive stress of the crystalline films (up to 10 at.% Nb). Further increasing the Nb content reduces the absolute value of the compressive stress, whereas the fully amorphous alloy (23 at.% Nb) exhibits a tensile stress. This trend coincides with the crystalline to amorphous transition as can be seen in figure 3.

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Figure 5: (a) d-sin2ψ plots of niobium-free thin films deposited at 2.8 Pa·cm (compressive stress) and at 6.05 Pa·cm (tensile stress). (b) Residual stress (left hand axis, round markers) and film density (right hand axis, triangle markers) as function of the p·d product for the niobium-free thin films. (c) Residual stress as function of the Nb content of the thin films deposited at low p·d product (3.6 Pa·cm) with comparable film thickness (420 to 520 nm).

3.3. Functional properties

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3.3.1. Thermal stability Films deposited at p·d=3.85 Pa·cm were annealed for one hour in an argon atmosphere in order to evaluate their thermal stability. After annealing, the interplanar spacing and the grain size (based on the Scherrer formula) were measured with XRD. Figure 6 shows the results. The interplanar spacing of the crystalline thin films (up to 10 at.% Nb) clearly shows a decrease up to 450 °C which can be explained by the relaxation of the compressive residual stress (see paragraph 3.2.3, Figure 5). The amorphous alloy (23 at.% Nb) exhibits a small tensile stress and annealing seems not to affect this stress state as the interplanar spacing remains more or less constant. At 600 °C the films show an increase of the interplanar spacing which is not expected based on the HEA concept. The XRD patterns (not shown) reveal small fractions of silicides and intermetallic phases after annealing at 600 °C. This results in a compositional change of the dominant phase. As silicides of chromium and cobalt are measured, it is likely that the relative Nb fraction of the dominant phase increases, and hence the interplanar spacing increases likewise. As expected the grain size increases with the annealing temperature. Indeed, a large interfacial energy is stored in nanocrystalline materials due to the high amount of grain boundaries which results in a high driving force for recrystallization and grain growth [30]. Grain boundary diffusion processes can become significant at temperatures in the order of 0.4Tm, with Tm the absolute melting temperature [31, 32, 33]. Based on the average melting temperature of the elements, the annealing temperature of 600°C just exceeds this threshold.

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Figure 6: The interplanar spacing (a) and the grain size (b) as a function of the annealing temperature for the NbxCoCrCuNiFe thin films

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3.3.2. Friction coefficient Nbx-CoCrCuFeNi thin films were deposited on mirror-polished stainless steel substrates. Figure 7 shows the friction coefficients of the CoCrCuFeNi thin films deposited with increasing p·d product. For the films deposited at low p·d product, the friction coefficient remains approximately constant for at least 40 m sliding distance which was the end point of the measurement.

Figure 7: Friction measurements of the CoCrCuFeNi thin films as function of the deposition conditions. The stainless steel-on-stainless steel reference was added. The inset shows the stable friction coefficient as function of the Nb concentration.

The film deposited at 6.05 Pa·cm breaks down after 30 m sliding distance as can be seen from the increase in friction coefficient. The inset of figure 7 indicates that the amount of niobium does not have an impact on the friction coefficient. Regardless of the relatively high values, the Nbx-CoCrCuFeNi alloys are relatively robust since the friction coefficients remain stable over a large sliding distance (at least 40 m). 7

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3.3.3. Contact resistance The contact resistance of the Nbx-CoCrCuFeNi thin films was measured as function of the contact time and the stable values are summarized in table 2. Although only three data points were measured, it is clear that the contact resistance decreases with increasing p·d. In contrast, no clear trend is noticed for the contact resistance as a function of the Nb concentration. Table 2: (a) Contact resistance of the CoCrCuFeNi thin films as function of the deposition conditions. (b) The stable contact resistance of the Nbx-CoCrCuFeNi thin films deposited at 2.8 Pa·cm. The sputter deposited Ag thin film reference was added.

Ag

4. Discussion

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contact resistance (mΩ) 395 253 71

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744 619 425 863

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alloy (a) CoCrCuFeNi (p·d=2.80 Pa·cm) CoCrCuFeNi (p·d=3.60 Pa·cm) CoCrCuFeNi (p·d=6.05 Pa·cm) (b) 5 at.% Nb-CoCrCuFeNi 10 at.% Nb-CoCrCuFeNi 15 at.% Nb-CoCrCuFeNi 23 at.% Nb-CoCrCuFeNi

2.8

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The XRD patterns (see Figure 3) show a crystalline to amorphous transition. To make this phase transition more clear, the XRD patterns were fitted with Voigt profiles. The width of the fitted profile is a combination of the sample-related width, and the instrumental broadening. All XRD patterns were measured on the same apparatus and under the same measuring conditions, so the same instrumental broadening is expected for all samples. The Voigt profile can be described as a convolution of a Gaussian and a Lorentzian component. The width of the Lorentzian component is related to the grain size, while the Gaussian width is related to the microstrain of the sample. Figure 8 shows the calculated microstrain as function of the p·d product and the niobium atomic fraction. It is clear that the microstrain, which is related to lattice defects and dislocations, substantially increases if the Nb content increases. This is understandable when the radius of the constituent elements is considered (Co: 125 pm, Cr: 125 pm, Cu: 128 pm, Fe: 124 pm, Ni: 125 pm and Nb: 143 pm). The 5-element, niobium-free, high entropy alloy can be topologically regarded as a packing of quasi-equal spheres where the atoms form a dense, but distorted FCC solid solution phase. In contrast, the 6-element alloy can be topologically regarded as a binary alloy constituted of small atoms and large atoms. According to the Hume-Rothery rules for binary alloys, solid solutions are stable if the atomic size mismatch is lower than 15 % [34, 35]. Binary mixtures with larger atomic size ratios are thermodynamically not favoured and only rapid-quenching methods, such as magnetron sputter deposition, are able to produce these metastable solid solutions. The maximal atomic size ratio in the Nbx-CoCrCuFeNi alloy, rNb/rFe=1.153 is just above the Hume-Rothery threshold. The atomic-scale elasticity theory of Egami and Waseda describes the topological instability of binary alloys [36]. The required solute concentration to transform a disordered crystalline solid solution to an amorphous phase is inversely proportional to the atomic size ratio. Based on Egami’s theory, the critical Nb concentration for amorphization was calculated and has a value of 19 at.% Nb. In the present study, the amorphization is already noticed at lower Nb fraction (15 at.%). This could be traced back to the high effective quenching rate during sputter deposition that favours the formation of amorphous phases already at lower Nb contents. 8

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Figure 8. Contour plot showing the microstrain of the Nbx-CoCrCuFeNi thin films as a function of the Nb atomic fraction and the p·d product.

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When studying the influence of the p·d product on the microstrain, it is clear that an increase of the microstrain is noticed for increasing p·d products when the film is crystalline (up to 10 at% Nb). In paragraphs 3.2.3 and 3.3.3, it was shown that the film density and the residual stress are also connected to the p·d product. The film density increases with increasing Nb content and decreasing p·d product. Sputter deposition at high p·d typically results in underdense film whereby attractive forces at the grain boundaries induce tensile stresses [37, 38, 39, 40]. A densification effect accompanied by a change to compressive stress is noticed for films deposited at low p·d. As shown in the overview paper of Windischmann [41], the compressive stress was ascribed to the atomic peening effect of the backscattered neutral gas atoms. Sputtering at lower p·d results in less gas scattering events and the momentum transfer to the growing film is more effective [42]. The influence of the niobium addition on the film density and residual stress is based on the same effect. Indeed, an increase in the amount of niobium in the target leads to an increase in the fraction of the backscattered Ar atoms. SRIM simulations were performed and show that both the Ar atom reflection probability, and the reflected atom energy are larger for the heavier niobium as compared to the other elements, as is presented in Figure 9.

Figure 9: (a) Average energy of the backscattered Ar atoms and the sputtered atoms leaving the target, as calculated with SRIM (400 eV Ar+, perpendicular incidence). (b) The probability that an Ar ion reflects on the target is determined by the mass ratio of the target material and the inert gas. 9

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The increased probability and energy of the backscattered Ar atoms increase the transferred momentum and influences the film growth. Furthermore the persistence of the sputtered Nb atoms is larger than the base elements, and the incoming Nb atoms serve as an extra source of momentum [43]. From Figure 8 we can therefore conclude that this enhanced momentum flux seems to improve the crystallinity of the thin films by lowering the microstrain. The trends noticed in the residual stress, density and microstrain help to understand the behaviour of the contact resistance and the friction coefficient. The contact resistance of the CoCrCuFeNi films decreases if the p·d product increases. As shown by Mahieu et al. [44] the momentum flux not only influences the film density but also the hardness of the thin films. Although not measured, it is reasonable to assume that the film hardness decreases with increasing p·d product (decreasing density). According to Holm’s theory of electrical contacts [45], the contact resistance for monometallic contact surfaces without insulating contaminants is given by R=ρ(πH/4F)0.5 with F the contact force, ρ the electrical resistivity and H the Vickers’ hardness of the contacting materials. Hence as the film density and the hardness decrease with increasing p·d product, the contact resistance of the CoCrCuFeNi films should decrease as was experimentally found. This is also consistent with the lack of trend between the contact resistance and the Nb concentration of the Nbx-CoCrCuFeNi films. These films were deposited at low p·d product (2.8 Pa·cm), and in this case the density (and implicitly assumed the hardness) barely changes. However, one could argue that the resistivity can be dependent on the Nb concentration. Nonetheless, the four point probe sheet resistance measurements (not shown) displayed no trend with the Nb content. The obtained trends in the intrinsic properties can provide a similar reasoning to explain the behaviour of the friction coefficient. An additional point which assists in understanding the negligible influence of the Nb content on the friction coefficient, are the similar surface properties (see Figure 2) for thin films, irrespective of the Nb content.

5. Conclusions

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Nbx-CoCrCuFeNi thin films were deposited by magnetron sputtering. The influence of the Nb concentration and the deposition conditions on the intrinsic properties such as texture, density, residual stress and microstrain were studied. The understanding of the behaviour of these intrinsic properties as function of the Nb concentration and the deposition conditions allows to explain the functional properties such as the friction coefficient and the contact resistance. The study elucidates that for complex materials such as high entropy alloys both the composition and the deposition conditions influence the film properties. The paper also reveals that, besides the evident relationship between target and substrate composition, the film growth conditions can be affected by the target processes such as backscattering of argon atoms.

Acknowledgements

The authors are grateful for the support provided by the U4 network between the universities of Ghent, Göttingen, Uppsala and Groningen. Furthermore, the assistance with the friction coefficient and contact resistance measurements by Prof. U. Jansson and P. Berastegui is highly appreciated.

References [1] J. W. Yeh et al., Adv. Eng. Mater. 6 (5) (2004) 299 [2] Y. Zhang et al., Prog. Mater. Sci. 61 (2014) 1 [3] Y. L. Chou et al., Corros. Sci. 52 (2010) 3481 [4] C. P. Lee et al., Thin Solid Films 517 (2008) 1301 [5] J. M. Wu et al., Wear 261 (2006) 513 10

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[6] C. Li et al., J. Alloys Compd. 475 (2009) 752 [7] J. Y. He et al., Acta Mater. 62 (2014) 105

[8] J. W. Yeh et al., Mater. Chem. Phys. 103 (2007) 41 [9] J. Appl. Crystallogr. 46 (2013) 736 [10] C. W. Tsai et al., J. Alloys Compd. 490 (2010) 160 [11] O. N. Senkov et al., Intermetallics 19 (2011) 698 [13] C. C. Tung et al., Mater. Lett. 51 (2007) 1 [14] S. Singh et al., Acta Mater 59 (2011) 182 [15] C. Lin et al., Intermetallics 18 (2010) 1244 [16] C. Lin, H. Tsai, Intermetallics 19 (3) (2011) 288

SC

[17] C. J. Tong et al., Metall. Mater. Trans. A 36 (4) (2005) 881

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[12] S. Y. Chang et al., Sci. Rep. 4 (2014) 4162

[18] C. J. Tong et al., Metall. Mater. Trans. A 36 (2005) 1263

M AN U

[19] B. R. Braeckman et al., Thin Solid Films 580 (2015) 71 [20] J. B. Cheng et al., Surf. Coat. Tech. 240 (2014) 184

[21] S. G. Ma, Y. Zhang, Mater. Sci. Eng., A 532 (2012) 480

[22] F. Boydens et al., Physica Status Solidi A 209 (2012) 524 [23] F. Boydens et al., Thin Solid Films 531 (2013) 32

[24] H. Kersten et al., Thin Solid Films 332 (1998) 282

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[25] H. Kersten et al., J. Appl. Phys. 87 (8) (2000) 3637 [26] L. G. Parratt, Phys. Rev. 95 (1954) 359

[27] B. L. Henke et al., Atom. Data Nucl. Data Tables 54 (1993) 181 [28] N. Nedfors et al., J. Vac. Sci. Technol. A 32 (4) (2014) 041503

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[29] W. H. Wang, Prog. Mater. Sci. 57 (3) (2012) 487 [30] A. Cavaleiro, Nanostructured Coatings, Springer (2006)

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[31] A. Gleitner, Acta Mater. 48 (2000) 1 [32] R. Dannenberg et al., Thin Solid Films 370 (2000) 54 [33] Y. Kuru et al., Appl. Phys. Lett. 95 (2009) 163112 [34] W. Hume-Rothery, J. Inst. Metals. 35 (1926) 295 [35] W. Hume-Rothery, Phase Stability in Metals and Alloys, Series in Materials Science and Engineering, McGraw-Hill, New York (1967) [36] T. Egami, Y. Waseda, J. Non-Cryst. Solids 64 (1984) 113 [37] R. W. Hoffman, Thin Solid Films 34 (1976) 185 [38] G. C. A. M. Janssen et al., Appl. Phys Lett. 83 (2003) 3287 [39] D. Magnfält et al., Appl. Phys. Lett. 103 (2013) 051910 [40] E. Chason, Thin Solid Films 526 (2012) 1 [41] H. Windischmann, J. Vac. Sci. Technol. A 9 (4), (1991) 2431 11

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[42] H. Windischmann, J. Appl. Phys. 62 (1987) 1800

[43] W. D. Westwood, J. Vac. Sci Technol. 15 (1) (1978) 1 [44] S. Mahieu, D. Depla, Surf. Coat. Technol. 204 (2010) 2085

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[45] R. Holm, Electric Contacts-Theory and Application, Springer-Verlag, 4th ed. (1967)

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Highlights:

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Nbx-CoCrCuFeNi thin films were deposited by sputtering pressed powder targets. The Nb fraction and deposition conditions influence the intrinsic film properties. The functional film properties are explained by the momentum transfer concept.

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