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Surface & Coatings Technology

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Microstructure and mechanical, electrical, and electrochemical properties ofsputter-deposited multicomponent (TiNbZrTa)Nx coatings

Rui Shua,⁎, Eirini-Maria Paschalidoub, Smita G. Raoa, Jun Lua, Grzegorz Greczynskia, Erik Lewinb,Leif Nyholmb, Arnaud le Febvriera, Per Eklunda

a Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping, SwedenbDepartment of Chemistry-Ångström, Uppsala University, Uppsala, Sweden

A R T I C L E I N F O

Keywords:Multicomponent nitridesTiNbZrTaNTextureCorrosion resistanceBipolar plates

A B S T R A C T

A series of (TiNbZrTa)Nx coatings with a thickness of ~1.1 μm were deposited using reactive magnetronsputtering with segmented targets. The deposition temperature was varied from room temperature to 700 °Cresulting in coatings with different microstructures. The coatings were characterized by electron microscopy,atomic force microscopy, compositional analysis, and X-ray diffraction. Effects of the deposition temperature onthe electrical, mechanical and corrosion properties were studied with four-point probe, nanoindentation andpotentiodynamic polarization measurements, respectively. X-ray photoelectron spectroscopy (XPS) analysesreveal a gradual change in the chemical state of all elements with increasing growth temperature from nitridic atroom temperature to metallic at 700 °C. A NaCl-type structure with (001) preferred orientation was observed inthe coating deposited at 400 °C, while an hcp structure was found for the coatings deposited above 400 °C. Theresistivities of the TiNbZrTa nitride coatings were found to be around 200 μΩcm. In 0.1 M H2SO4 aqueoussolution, a corrosion current density of 2.8 × 10−8 A/cm2 and a passive behaviour up to 1.5 V vs. Ag/AgCl werefound for the most corrosion resistant coating. The latter corrosion current is about two orders of magnitudelower than that found for a reference hyper-duplex stainless steel.

1. Introduction

Bipolar plates are vital components in fuel cell stacks, with the mainfunctions of separating the individual cells in the stack, distributing thefuel and oxidant, collecting the current, and facilitating the manage-ment of the heat and water. Different stainless steels are promisingindustrial candidates for bipolar plate materials, since they have goodmechanical stability, electrical and thermal conductivity and are easy toprocess into the desired shape compared to graphite and related com-posites. However, in polymer electrolyte membrane fuel cells(PEMFCs), the operating pH should be below three while the tem-perature is around 100 °C. Therefore, a corrosion-protective coating isusually necessary for bipolar plates made of metals. The main char-acteristics of these coatings should include [1–3]: a low interfacialcontact resistance (< 10 mΩcm−2); a good corrosion resistance in a cellenvironment (current density< 1 μA/cm2); good gas-tight properties,and thermal stability up to 100 °C.

Based on the above requirements, various binary and ternary tran-sition-metal-based coatings have been widely studied using growthmethods such as, physical vapor deposition, chemical vapor deposition

and electroplating. A promising class of coating candidates is based onthe concept of high-entropy alloys (HEAs, alternatively referred to asmultiprincipal element alloys), originally introduced [4,5] to refer to asingle-phase solid solution alloy made of five or more elements in equalor near-equal proportions. Furthermore, the approach to add lightelements, specifically nitrogen [6], carbon [7,8], boron [9] and oxygen[10] has led to the development of new-generation high-entropy ma-terials (HEMs) labelled high-entropy nitrides (HENs), carbides (HECs),borides (HEBs) and oxides (HEOs). Generally, HEMs tend to simulta-neously exhibit improved strength and ductility [11], thermal stability[12,13] and corrosion properties [14,15]. Most HENs are based on d-block transition metals (TM), and three classes can be identified ac-cording to the origin of their metallic component: (i) the refractorytransition metals [16] among groups 4 to 6, such as, Ti, Zr, Hf, Nb, Taand W; (ii) the TM HENs made up of elements from the late transitionmetal groups, for example, CoCrCuFeNiN [17]; and (iii) metals from thecross-groups, i.e., combining metals from one or both of the above-mentioned two categories with a main-group element (e.g., Al), such asin TiZrNbAlYN [18] or AlCrMoNiTiN [19].

In many studies on refractory-metal-based HEN coatings, the

https://doi.org/10.1016/j.surfcoat.2020.125651Received 3 February 2020; Received in revised form 16 March 2020; Accepted 17 March 2020

⁎ Corresponding author.E-mail address: rui.shu@liu.se (R. Shu).

Surface & Coatings Technology 389 (2020) 125651

Available online 19 March 20200257-8972/ © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

T

number of constituent metal elements typically exceeds four [20], andcan even be up to 11 in some cases [21]. This vast composition space,various nature of the metal elements and different process conditions,make it a great challenge to explore different HENs in depth, as well asto find the specific multicomponent coating system with the bestproperties for a given set of specifications. Although, quaternary nitridecoatings, such as (TiNbZr)N [22] and (TiZrHf)N [23], as well as mul-ticomponent nitrides, including (TiHfNbZrTa)N [24], (TiHfNbZrV)N[25] and (TiHfNbVZrTa)N [26,27] have been studied, there is still aneed for an improved understanding of the properties of multi-component nitrides.

In the present work, therefore, multicomponent refractory(TiNbZrTa)Nx (x = 0.25–0.59) coatings were deposited by reactivemagnetron sputtering with four segmented targets and a nitrogen flowratio [RN=N2/(Ar + N2)] of about 3%. The effect of the depositiontemperature on the structural, mechanical, electrical properties andcorrosion resistance of the above-mentioned system was investigated inorder to provide fundamental material-level understanding needed forthe future application of these coating materials, such as protectivecoatings on metallic bipolar plates, hard and conductive coatings inacid and humid environments. The purpose of the present work is toevaluate the coating material itself (on silicon substrates), while futurework will be directed towards assessing the system with the coatings onmetallic bipolar plates.

2. Experimental details

TiNbZrTa nitride coatings were deposited on Si(100) substrates byreactive magnetron sputtering in a high vacuum chamber (basepressure < 10−8 Pa) using two segmented targets of Nb/Zr and Ti/Tawith an area ratio of 50/50. The depositions were carried out withconstant DC power of 200 W for the two segmented targets. The Ar/N2

gas (97% Ar and 3% N2) flow rate was kept at 65 sccm corresponding toa total pressure of 0.48 Pa (3.6 mTorr). Silicon (100) substrates with asize of 10 × 10 mm2 were cleaned sequentially with acetone andethanol in an ultrasonic bath for 10 min, and finally blow-dried withnitrogen gas. The substrate holder, which was maintained at differenttemperatures from non-intentional heating (named “room tempera-ture”, RT) to 700 °C, was rotating, and the substrates were electricallyfloating. The substrate temperature was calibrated using an infraredthermometer (Cyclops 160B) in absence of any contribution from theplasma heating (the latter contribution is small in this lab-scale systembut can be substantial in industrial-scale systems). Prior to each de-position, the substrates were preheated for at least half an hour to ob-tain a stable temperature. Each deposition was maintained for 30 minyielding a coating thickness of ~1.1 μm.

The elemental compositions of the (TiNbZrTa)Nx coatings weredetermined by energy-dispersive X-ray spectrometry (EDS, OxfordInstruments X-Max) and XPS measurements. The XPS measurementswere performed in a Kratos Axis Ultra DLD instrument from KratosAnalytical (UK) employing monochromatic Al Kα radiation(hν= 1486.6 eV). The surface contamination due to the exposure of thesample to air was first removed by sputter-etching the films for 120 swith a 4 keV Ar+ ion beam incident at 70° with respect to the samplenormal; the Ar+ ion energy was then reduced to 0.5 keV for 600 s tominimize surface damage [28]. The size of the sputter-etch cleaned areawas 3 × 3 mm2 while the spectra were collected from the0.3 × 0.7 mm2 area centered in the middle of the etched crater andwith electrons emitted along the surface normal. The binding energy(BE) scale of the spectrometer was calibrated using the ISO-certifiedprocedure [29] to avoid problems related to the use of the C 1s peak ofadventitious carbon [30,31]. The analyser pass energy was set to 20 eVwhich resulted in the full width at half maximum of 0.55 eV for the Ag3d5/2 peak. Quantification of the elements in the samples was per-formed using Casa XPS software (version 2.3.16), based upon the peakareas obtained from narrow energy range scans and the elemental

sensitivity factors supplied by Kratos Analytical Ltd.The crystal structure of the coatings was determined by X-ray dif-

fraction (XRD) measurements performed with a PANalytical X'Pert PROdiffractometer with a Cu Kα radiation and a nickel filter with a Bragg-Bretano geometry. Pole figures were acquired using Philips X'Pert-MRDoperating with Cu Kα radiation with a configuration of crossed slits(2 × 2 mm2) as primary optics and a parallel plate collimator (0.27°) assecondary optics.

Top-view and cross-section surface morphologies of the coatingswere observed by a scanning electron microscope (SEM, LEO Gemini1550, Zeiss), with an acceleration voltage of 5.0 kV. A BrukerDimension 3100 atomic force microscopy (AFM) operated in the tap-ping mode was used to estimate the roughness of each coating and thedata were processed with the WS × M analysis software.

The mechanical properties (hardness and elastic modulus) of theTiNbZrTa nitride coatings were determined from nanoindentationmeasurements (Triboindenter TI 950) performed using a Berkovichdiamond tip with an apexradius of 100 nm where the tip area functionwas calibrated using a fused-silica reference sample. The indentationmeasurements were carried out in the load-controlled mode and thehardness and reduced elastic modulus were obtained from load–dis-placement curves following the Oliver and Pharr method [32].

The electrical resistivities of all samples were determined by mea-suring the sheet resistances of the films using a four-point-probe JandelRM3000 station. The resistivity was obtained by multiplying the sheetresistance with the sample thickness, which was obtained from cross-section SEM images.

Potentiodynamic polarization measurements were performed at RTto evaluate the corrosion resistances of selected TiNbZrTa nitridecoatings deposited at RT and at 400 °C. These curves were comparedwith the polarization curve obtained for a hyper-duplex stainless steelsample (SAF 3207HD, Sandvik AB) [15], used as a reference sample. APGSTAT302N potentiostat/galvanostat (Metrohm Instruments) wasused in conjunction with a typical three-electrode electrochemical cell,containing a 0.1 M H2SO4 aqueous solution. The thin film sample wasused as the working electrode while an Ag/AgCl (3.0 M NaCl) electrodeand a Pt wire served as reference electrode and counter electrode, re-spectively. Prior to the polarization curve experiments, the sampleswere kept in the electrolyte solution for 90 min under open circuitpotential (OCP) conditions. All polarization curves were recorded witha scan rate of 1 mV/s. The polarization curves were recorded between−0.7 V and 1.5 V vs. Ag/AgCl and the corrosion potential (Ecorr), andcorrosion current (icorr) were determined from the polarization curves.

3. Results and discussion

The elemental compositions of the coatings are presented in atomicpercent (Fig. 1) with a standard deviation of± 0.5 at. %, while theuncertainty in the nitrogen content is about 3–7 at.%, according to EDSand XPS measurements. The atomic percentages of the metals are es-timated from the EDS, while nitrogen percentage is calculated using theN/(Nb + Zr) ratio based on the XPS results. The inset shows that theZr/Nb ratios determined by EDS and XPS both are around 1.0, in-dicating that the two techniques are consistent with respect to the de-termination of these metal concentrations. The compositions regardingTi, Nb, Zr and Ta are kept close to the equimolar ratios. The N contentwas 37 at.%, i.e., MeN0.59, for the film deposited at room temperatureand decreased with temperature to 20 at.%, i.e., MeN0.25 for the filmdeposited at 700 °C. These results indicate that all the (TiNbZrTa)Nx

samples are more metallic and much less stoichiometric in nitrogen(with x = 0.25–0.59). The decrease in the N content at elevated de-position temperatures might be due to the higher nitrogen desorptionrates at higher substrate temperatures. The measured oxygen contentsin the series of samples are below 1.5 at.%.

High-resolution XPS spectra recorded over the Nb 3d, Zr 3d, Ti 2p,Ta 4f and N 1s core-level regions, are displayed in Fig. 2(a–e) for films

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deposited in the temperature range from RT to 700 °C. There is a clearshift of all peaks towards lower binding energies (BE) with increasingdeposition temperature. Comparisons with reference values (see theblack vertical lines in Fig. 2a–e) reveal that the metals in the filmsdeposited at RT are close to their nitride state, while those deposited at700 °C are close to their metallic states (see the red vertical lines). Whilethe BE of the Nb 3d5/2 peak varies in the range from 203.8 eV to202.5 eV, a value of 204.0 eV was reported for stoichiometric NbN [33]whereas a value of 202.0 eV was found for pure Nb metal [34]. Simi-larly, the BEs of Zr 3d5/2, Ti 2p3/2, Ta 4f7/2 shift from 179.8 to 178.5 eV(the reference ZrN [33] and Zr [35] signals are located at 180.0 and178.5 eV), and from 454.9 to 454.3 eV (with the TiN [33] and Ti [36]reference signals located at 455.0 and 454.0 eV), and from 23.6 to22.3 eV (with the TaN [33] and Ta [37] reference signals positioned at23.9 and 21.9 eV), respectively. The abovementioned shift in the peakposition indicates a gradual change in the states of the elements from(mainly) nitridic (when deposited at RT) to almost metallic when de-posited at 700 °C. This shift is consistent with the decrease in the ni-trogen content (see Fig. 1).

Fig. 3 presents XRD patterns of the (TiNbZrTa)Nx coatings depositedat different temperatures. For temperatures between RT and 300 °C, thecoatings exhibit reflections corresponding to an fcc NaCl-type phase.The lattice parameter values of these coatings, determined by calcula-tion from all film peaks in the XRD patterns, are close to4.37 ± 0.01 Å. For the coatings sputter-deposited between 400 °C and600 °C, the structure is the same as for the samples deposited at lowertemperatures, but a (001) texture is apparent. With increasing tem-perature, there is also a shift of the (002) peak to lower 2θ angle,translating to an increasing lattice parameter, and the peak width isincreased, which may be due to the presence of different fcc phase orstress in the films. For the latter sample, a small peak appeared at2θ = 46.8°, and several additional peaks are also found for the coatingdeposited at 700 °C. All these peaks could be attributed to an hcp phasesimilar to the cases observed in HfNbTiVZr [38] and HfNbTaTiZr [39]high-entropy alloys annealed at 600 °C. The lower nitrogen content inthe coatings deposited above 600 °C, results in similar structures as forrelated refractory high-entropy alloys [38,39], which might be due tothe limited amount of nitrogen in the fcc crystals.

In order to further investigate how the texture of the TiNbZrTa ni-tride coatings depends on the deposition temperature, pole figures wereacquired. Fig. 4 shows pole figures of the (111) and (001) planes in thefcc phase for the coatings deposited on Si in the temperature range from

RT to 600 °C. The {111} pole figures of the coating deposited at RTexhibit a broad spot at the centre, while a broad ring around ψ ≈ 35.6°is seen for the coatings deposited at 200 °C and 300 °C. The {001} polefigures for the coatings deposited between RT to 300 °C exhibit thecorresponding features. These observations indicate a (111) preferredorientation for the RT sample, and a (110) preferred orientation for thesamples deposited at 200 °C and 300 °C. In contrast, for the coatingsdeposited at 400 to 600 °C, one sharp peak at the centre is observed in

Fig. 1. Element composition determined by EDS and XPS of TiNbZrTa nitridecoatings as a function of the substrate temperature.

Fig. 2. (a–e) High-resolution XPS spectra of the core levels: Nb 3d, Zr 3d, Ti 2p,Ta 4f and N 1s for all the series of MeNx (Me= TiNbZrTa) coatings. The verticallines indicate the reference binding energy values for binary nitrides (blacklines), and metals (red lines). (For interpretation of the references to colour inthis figure legend, the reader is referred to the web version of this article.)

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the 001 pole figure, while a distinct ring is seen at ψ ≈ 54.0° in the{111} pole figure, showing that the films deposited at 400 to 600 °Cexhibit a fibre-texture with (001) out-of-plane orientation. The changein texture from the (111) plane to the (001) plane in this work is likelydue to the enhanced surface mobility of adatoms on the (001) planesobtained when increasing the substrate temperature [40].

Fig. 5 shows top-view and cross-sectional SEM images of the(TiNbZrTa)Nx coatings deposited at various temperatures, while theinsets depict AFM images of each sample. The coatings deposited be-tween RT and 200 °C show a similar structure with a rough surface (6.2to 7.0 nm, root-mean-square roughness, Rrms) and a porous columnarstructure (Fig. 5a and b). At 300 °C, the microstructure of the coatingsbecame denser and with fewer micropores. For substrate temperaturesof 400, 500 and 600 °C, the surface roughness values were 0.9, 0.8 and1.2 nm, respectively. At 700 °C, grain coarsening occurred due to theelevated deposition temperature leading to an increased roughness(Rrms = 10.1 nm). In addition, Fig. 5h shows the deposition rate cal-culated from the measured thickness from the SEM images, yieldingvalues between 35 and 41 nm/min.

Fig. 6 shows cross-section TEM micrographs of the TiNbZrTa nitridecoatings deposited on Si substrates at RT and 400 °C. The rough surfaceof the coating deposited at RT can be seen in Fig. 6(a). A selected-areaelectron diffraction (SAED) pattern (Fig. 6b) was obtained near thesurface region. The observed diffraction spots can be assigned to thereflections of the NaCl-type phase, suggesting a polycrystalline struc-ture. High resolution TEM images (Fig. 6c), show a columnar micro-structure with fine columns, in addition to some microcracks (markedby arrows) and defects along the grain boundaries. In the cross-sectionTEM images of the coating deposited at 400 °C, a dense and uniformgranular structure is observed, without visible microcracks. Columnswith a diameter of around 15 to 20 nm are observed in Fig. 6d. Fig. 6eshows the corresponding SAED patterns, in which a broad arc appearsin the growth direction. The latter suggests a (001) texture of fcc phase,consistent with the broad 002 peak in the corresponding XRD pattern.Two local Fast Fourier Transform (FFT) patterns (A and B), which weretaken from the marked area of two neighbouring columns in theHRTEM image (Fig. 6f), provide further confirmation of the well-crys-talline fcc structure. At the same time, the ordered diffracted spots re-veal an improved crystalline structure along the growth direction(compared to Fig. 6c) in each column, which is identified by theHRTEM image (Fig. 6f).

Fig. 7 shows the nanoindentation hardness (H), the reduced elasticmodulus (Er) and the H/Er and H3/Er2 ratios for the TiNbZrTa nitride

coatings as a function of the deposition temperature. The hardness firstincreased from 17.4 ± 1.0 GPa to 26.0 ± 0.3 GPa when the de-position temperature was increased from RT to 400 °C, but then de-creased to about 17.8 ± 2.0 GPa at 700 °C. The elastic modulus only

Fig. 3. XRD patterns of the TiNbZrTa nitride coatings deposited at differenttemperatures from room temperature to 700 °C. The fcc reference values werebased on the theoretical power diffraction pattern of (TiNbZrTa)N while, thehcp reference values were obtained from ref. [39].

Fig. 4. The evolution of 111 and 002 pole figure of the fcc TiNbZrTa nitridecoatings deposited on Si (100) as a function of the substrate temperature.

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showed a slight increase up to 230.5 ± 10.4 GPa at 300 °C followed bya decrease to 199.0 ± 14.9 GPa at 700 °C. The error bars of H and Ercorrespond to the standard deviation of 25 measurement points and arerelated to the roughness of the coatings shown in Fig. 5h. The load-displacement curves of the 25 indentations for the TiNbZrTa nitridecoatings deposited at RT and 400 °C are displayed in Fig. 8. The blackRT curves exhibit large scatter due to the high film roughness, while theuniform green curves resulted from the small roughness of the coatingdeposited at 400 °C. A high H/Er ratio and a lower E value are indicativeof improved toughness and good wear resistance [41], while the H3/Er2

ratio is an indicator of the resistance to plastic deformation [42]. Thecoating deposited at 400 °C has the highest H/E value (i.e. 0.12) andH3/Er2 value (i.e., 0.34 GPa), which might be due to the strong (001)texture. Considering the change in N content due to the higher de-position temperature, the highest hardness in these multicomponent(AlCrSiTiZr)Nx coatings were observed for 22.4 at.% nitrogen [14],owing to their amorphous and dense structure combined with Me-Nbonding in the film.

Fig. 9 shows the room-temperature resistivities of the TiNbZrTanitride coatings deposited on Si as a function of the deposition tem-perature. The electrical resistivity first increases from 184 to 230 μΩcmwhen the deposition temperature is increased from RT to 500 °C, butthen decreases to 135 μΩcm for a deposition temperature of 700 °C. Thevariation in the resistivity values, which is much larger than those (i.e.a few tens of μΩcm) seen for stoichiometric TiN [43,44], NbN [45] andZrN [46], but comparable to those seen for stoichiometric TaN(~220 μΩcm [47]) and other multicomponent nitride coatings [48,49].The electrical properties of multicomponent nitride coating could begeneral affected by various parameters such as the defect [50] or ni-trogen contents [51].

Potentiodynamic polarization curves were recorded to evaluate thecorrosion resistance of the coatings deposited on Si substrates at RT and400 °C, as well as the hyper-duplex stainless steel SAF 3207HD used forcomparison (Fig. 10). The corrosion potential (Ecorr) for the hyper-du-plex stainless steel was found to be approximately −0.3 V vs. Ag/AgCl,while more positive corrosion potentials, i.e. −0.15 and −0.05 V vs.Ag/AgCl, were seen for the RT and 400 °C nitride coatings. The cor-rosion current (icorr) densities were 9.0 × 10−8 and 2.8 × 10−8 A/cm2

for the RT and 400 °C samples, respectively. As these values are abouttwo orders of magnitude lower than the value of 2.5 × 10−6 A/cm2

found for the hyper-duplex stainless steel it is immediately clear thatthe (TiNbZrTa)Nx coatings exhibited significantly higher corrosion re-sistances. Also, the fact that the coatings are more corrosion resistant isconfirmed by the fact that the values of their corrosion potentials aremore positive than that of the stainless steel (Fig. 10). The somewhathigher corrosion current seen for the RT coating than for the 400 °Ccoating can most likely be explained by its higher porosity, which canbe seen in the SEM images (Figs. 5, SI 1 and 2). It should also be noted,that the nitride samples exhibited a passive behaviour up to 1.5 V vs.Ag/AgCl but the true extent of the passivity region has not been eval-uated, whereas a transpassive behaviour featuring a dramatically in-creasing current could be seen above around 0.8 V vs. Ag/AgCl only forthe stainless steel. In Fig. 10, the passive regions are seen to start atabout 0.4 and 0.6 V for the 400 °C and RT samples, respectively. In bothcases the passive regions are characterized by stable plateaus withcurrent densities of about 0.5 to 1.0 × 10−6 A/cm2. The latter indicatethat the metal oxides formed upon the oxidation of the nitrides arehighly stable in this potential region, at least in 0.1 M H2SO4. The re-corded polarization curves of the pure elements, i.e. Ti, Ta, Nb and Zrsamples, in 0.1 M H2SO4 clearly demonstrate that all the elements

Fig. 5. (a–g) Top-view and cross-section SEM image of the (TiNbZrTa)Nx coatings deposited at different temperatures. AFM images are shown in the insets (the scalebars represent 200 nm); (h) deposition rate (nm/min) determined using the thickness and deposition time, and the root-mean-square roughness (Rrms) determinedfrom the AFM images.

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exhibit a passive behaviour up to 1.5 V vs. Ag/AgCl (Fig. SI 3). This isconsequently the main reason for the high corrosion resistance ex-hibited by the present (TiNbZrTa)Nx coatings, while previous resultsobtained for TiNbZrTa alloys tested in different electrolyte solutions[52] confirm the growth of the thickness of the passive layer whenpotential increases. Moreover, according to the Pourbaix diagrams[53], the Ti, Nb, Zr, and Ta elements should be present as TiO2, Nb2O5,ZrO++(zirconyl ion) and Ta2O5 at these potentials and pH, in goodagreement with the XPS results in Fig. SI 4. Therefore, it is reasonable toassume that the thickness of the passive oxide layer is increasing whenthe potential increases, giving this stable plateau.

The corrosion current densities obtained for the RT and 400 °Csamples, i.e. 9.0 × 10−8 and 2.8 × 10−8 A/cm2, respectively, areabout two orders of magnitude lower than the value maximum corro-sion current density accepted for application in a PEMFC. Future studiesare, however, needed to evaluate the corrosion resistance of the presentcoatings in more corrosive environments (e.g. in the presence ofchloride or fluoride) as well as the contact resistances and durability ofthe coatings in PEMFCs.

4. Conclusions

Coatings in the multicomponent nitride system TiNbZrTa-N weredeposited using reactive magnetron sputtering. This series of coatingsdeposited with different temperature and a constant nitrogen flow ratioof 3% were much less stoichiometric from MeN0.59 at RT to MeN0.25 at700 °C. XPS data indicates a gradual change in the chemical state of thetransition metals with increasing growth temperature Ts, from nitridicwith Ts = RT to metallic with Ts = 700 °C. For low-temperature de-position, the coatings exhibited fcc solid-solution polycrystallinestructures, with a rough surface (i.e. 6.2 to 7.0 nm). Upon increasing thetemperature in the range of 400–600 °C, the coatings developed a(001)-texture, a roughness of 0.9 to 1.2 nm and dense structures

without visible grain features. An hcp structure was observed togetherwith the fcc phase in the coating deposited at 700 °C which had aroughness of 10.1 nm due to a grain coarsening effect caused by theelevated temperature. The maximum hardness value of 26 GPa as wellas the highest H/Er and H3/Er2 values (0.12 and 0.34 GPa) were ob-tained for the coating deposited at 400 °C. The RT electrical resistivitiesof the TiNbZrTaN nitride coatings were found to be around 200 μΩcm.The corrosion behaviour of the RT and 400 °C coatings seen in 0.1 MH2SO4 aqueous solutions demonstrate that these coatings were morecorrosion resistant than the hyper-duplex stainless-steel referencesample. This makes the present type of TiNbZrTaN nitride coatingspotentially well-suited for use as corrosion resistant coatings on me-tallic bipolar plates in PEMFCs.

CRediT authorship contribution statement

Rui Shu: Conceptualization, Investigation, Data curation, Formalanalysis, Writing - original draft. Eirini-Maria Paschalidou:Conceptualization, Investigation, Formal analysis, Writing - review& editing. Smita G. Rao: Investigation, Formal analysis. Jun Lu:Investigation, Formal analysis. Grzegorz Greczynski: Investigation,Formal analysis, Writing - review & editing, Funding acquisition.Erik Lewin: Writing - review & editing. Leif Nyholm: Supervision,Writing - review & editing. Arnaud le Febvrier: Conceptualization,Investigation, Supervision, Formal analysis, Writing - review &editing. Per Eklund: Project administration, Conceptualization,Supervision, Writing - review & editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Fig. 6. Cross-sectional TEM micrograph of the (TiNbZrTa)Nx coating deposited on Si (100) substrate at RT (a–c) and 400 °C (d–e). (a) and (d), low-magnification TEMimages of the film and surface; (b) and (e) SAED patterns; (c) and (f) HRTEM images of nearby columnar crystal. The insets in (f) show local FFT patterns for the zonesA and B indicated in image (c).

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Acknowledgments

This work is supported by the VINNOVA Competence CenterFunMat-II (grant no. 2016-05156), the Swedish Government StrategicResearch Area in Materials Science on Functional Materials atLinköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971),and M – ERA.net (project MC2 grant no. 2013-02355). The Knut andAlice Wallenberg Foundation is acknowledged for support through the

Wallenberg Academy Fellows program (P.E.) and the ElectronMicroscopy Laboratory at Linköping University. GG acknowledges fi-nancial support from the Swedish Research Council VR Grant 2018-03957, the VINNOVA Grant 2018-04290, the Åforsk Foundation Grant16-359, and Carl Tryggers Stiftelse för Vetenskaplig Forskning contractCTS 17:166.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.surfcoat.2020.125651.

References

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[2] Pre-coated strip steel for bipolar plates for PEM FC—Sandvik materials technology,(n.d.). https://www.materials.sandvik/en/products/strip-steel/strip-products/

Fig. 7. (a) Hardness and reduced elastic modulus, (b) H/Er and H3/Er2 ratios forthe (TiNbZrTa)Nx (x = 0.25–0.59) coatings as a function of the depositiontemperature.

Fig. 8. Nanoindentation load–displacement curves for 25 indentations recordedwith the TiNbZrTa nitride coatings deposited at RT and 400 °C.

Fig. 9. Room-temperature electrical resistivity for the (TiNbZrTa)Nx

(x = 0.25–0.59) coatings deposited at different temperatures.

Fig. 10. Polarization curves for the TiNbZrTa nitride coatings deposited on Sisubstrates at RT (black) and 400 °C (green) as well as for a hyper-duplexstainless steel SAF 3207HD reference (red). The curves were recorded in 0.1 MH2SO4 from−0.7 to 1.5 V using a scan rate of 1 mV/s. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the webversion of this article.)

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