Characterization of ZrN coating low-temperature deposited on the preliminary Ar+ ions treated 2024 Al-alloy

Surface & Coatings Technology 361 (2019) 413–424

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Characterization of ZrN coating low-temperature deposited on the preliminary Ar+ ions treated 2024 Al-alloy

T

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M.A. Vasylyeva, B.N. Mordyuka, , S.I. Sidorenkob, S.M. Voloshkob, A.P. Burmakb, I.O. Kruhlovb, V.I. Zakievc a

G.V. Kurdyumov Institute for Metal Physics, National Academy of Science of Ukraine, 36, Academician Vernadsky Blvd., 03142 Kyiv-142, Ukraine National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Prospect Peremohy, 03056 Kyiv-56, Ukraine c National Aviation University, 1, Prospect Kosmonavta Komarova, 03058, Kyiv-58, Ukraine b

A R T I C LE I N FO

A B S T R A C T

Keywords: ZrN coating Vacuum-arc deposition Al-alloy Hardness Corrosion Friction/wear behaviors

The present paper considers the problem of the low-temperature deposition of the hard coatings on the alloys having a low melting point and a high affinity for oxygen, like aluminum or magnesium alloys. It is demonstrated that a hard ZrN coating can be produced on the 2024 aluminum alloy by the low-temperature vacuumarc deposition method. To achieve high adhesion strength between the coating and substrate, the substrate was sputtered by low-energy inert Ar+ ions before the deposition process in order to remove the natural oxide layer. The optimal technological regimes selected and used allowed obtaining the stoichiometric ZrN coating of extremely low roughness (0.061 μm), uniform thickness (~1 μm) with nano-scale columnar microstructure with the cross-sectional size of the columnar grains of ~20–50 nm. The layered microstructure of the obtained coating respectively consisted of the columnar and V-shaped grains in the lower and upper layers comes to be in the ‘transition zone’ on the ‘structure-zone diagram’. The lower layers consist of a number of AlxZry phases along with the Zr3N4 orthorhombic phase, while the outmost layer of the film contains the single ZrN fcc phase. In comparison with the substrate alloy, the produced ZrN coating is shown to possess the superior anti-corrosion properties in saline solution, a high hardness (~20 GPa) and elastic modulus (196 GPa), high adhesion strength both at the progressively increased load and at cyclic dry sliding of the conical diamond indenter with the 50 μm tip, low friction coefficients and high wear resistance at the reciprocating sliding both in the dry and wet (liquid paraffin) conditions against the conical diamond indenter with the 50 μm tip and 8 mm Si3N4 ball, respectively.

1. Introduction Deposition of the ceramic hard coatings is an effective technique to improve the physical, chemical and mechanical properties of the materials used in extreme conditions. By selecting suitable deposition technology and composition of the coatings, one may prolong the service life of the substrate material and increase the commercial efficiency of the parts and machines. In the last decades, the use of the coatings of the transition metal nitrides, such as titanium nitride (TiN) and zirconium nitride (ZrN), has been successfully applied for protective and decorative purposes, as the diffusion barriers in semiconductors technology, and for optical applications in the heat mirrors [1–3]. An increasing application of these coatings is facilitated by their excellent properties, such as high hardness, chemical stability, good biocompatibility, high melting point, low friction coefficient, corrosion resistance, and thermal stability [2–9].

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The ZrN coating has several advantages. This coating exhibits higher thermal stability and corrosion resistance at elevated temperatures in comparison with other hard coatings, such as TiN. More recently, ZrN began to be used as a protective and decorative coating, mostly due to its golden color. In addition, because of its low cross section capture for neutrons, it could be used as a protective coating for nuclear application [10,11]. ZrN is one of the ceramics considered as a candidate for inert matrix fuel host for fast reactors or acceleratordriven sub-critical systems. Recently, an excellent radiation tolerance of this material has been demonstrated after low energy bombardment by heavy and light ions [10,11]. A strong hydrophobic effect is an intrinsic property of the ZrN film owing to the low-electronegativity of Zr, and it does not depend on the mechanical properties and structure of ZrN. In comparison with the other widely used nitride coatings, the ZrN film possesses a higher hardness and elastic modulus, and thus it can be used as important structural coating materials in engineering. Additionally,

Corresponding author. E-mail address: [email protected] (B.N. Mordyuk).

https://doi.org/10.1016/j.surfcoat.2018.12.010 Received 8 September 2018; Received in revised form 22 November 2018; Accepted 4 December 2018 Available online 05 January 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 361 (2019) 413–424

M.A. Vasylyev et al.

2. Experimental details

ZrN coatings also have potential as a biocompatible material [3,9,12]. Different methods, such as physical vapor deposition (PVD) processes, including reactive sputtering [13–15], ion plating [16] and cathodic arc evaporation [2,17], were successfully used to produce the ZrN coatings. In those works, the ZrN coatings were deposited on various substrates, such as AISI 304, AISI 410, 316 and 17–4 pH stainless steel, Ti-6A1-4 V, Inconel 718 and high-speed steel. Most of these conventional deposition techniques involve a hightemperature processing during the film growth or in the post-deposition period, and there is a need for subsequent heating of the substrates (300–600 °C). However, a low-temperature processing (< 100 °C) is necessary to preserve the deterioration of the properties of the Al alloy substrate. In addition, such heating can lead to the cracks' formation in the coating due to a significant difference in the thermal expansion coefficients of the coatings and substrate alloy. The works devoted to the formation ZrN coating on the AA 7075-T6 aluminum alloy are limited [18]. The main difficulty in the production of the ZrN/Al alloy system may be related to the strong natural affinity of Al to oxygen resulting in the oxide layer (Al2O3) immediately formed on the surface of aluminum alloys. This surface oxide would worsen the adhesion in the substrate/film system if the film (coating) is produced by using the conventional methods of CVD or PVD deposition operating with the low energy plasma, which cannot sputter the surface oxide off. Currently, a coating methodology allowing the production of a thick hard coating deposited on the low melting temperature substrates, such as Al-based alloys, is lacking. At the same time, the large-scale manufacture of light-weight engine parts is demanded in the automotive and aerospace industries. Therefore, an extensive research and further development of the coating technologies suitable for convenient modification of the surface properties of the engine components made of Al alloys are so important. With the aim of increasing the deposition rate and having the possibility to deposit thick coatings (2–5 μm) in commercial production, the vacuum-arc deposition (VAD) technique has been developed [19–23]. The VAD systems employ the vacuum arc for the creation of highly ionized (50–100%) and energetic (up to 100 eV) metal plasmas for thick coating deposition. At present time, a lot of the arc-deposition equipment (employing the CVD and PVD deposition processes) is created especially for protection of tools and machine elements by hard coatings or in the decorative purposes. Typically, to secure high adhesion strength on the coating/substrate contact surface the substrate should be heated to a high temperature (up to 600–900 °C). However, this operation cannot be acceptable in the cases of coating the quenched and tempered hard steels and/or to the materials having a low melting temperature such as Al- or Mg-based alloys. Currently, no appropriate technology is available, which would allow the room-temperature deposition of the high-quality hard coatings onto the substrates with a low melting temperature. Owing to a large strength-to-density ratio and good machinability the 2024 Al alloy is one of the important and widely used engineering materials, and it can be a good candidate for the automotive and aerospace applications. Unfortunately, the susceptibility of this alloy to corrosion and wear greatly diminish its applications, especially in some adverse service circumstances. Therefore, the improvement of the surface properties of this alloy is of special practical interest. The surface coated by protective ceramic layers can be successfully applied to improve the surface properties of metallic materials, such as hardness, friction, resistance to abrasion, fatigue durability and anti-corrosion properties. In this paper, we discuss the production peculiarities of the ZrN coating deposited on 2024 Al-alloy by filtered vacuum-arc deposition at room temperature and analyze its microstructure and properties.

2.1. Materials The ZrN coating was deposited on the 2024 Al-alloy substrate (composition in wt%: Al-93.6%, Cu-3.97%, Mg-1.43%, Mn-0.625%, Si0.5%) of cylindrical shape (10 mm diameter, 5 mm height). The specimens were cut perpendicularly to the rolled bar axis. A standard thermal treatment forms α-Al solid solution with an average grain size of 3–5 μm and uniform precipitation of the rod-like particles of the orthorhombic T-phase (Al20Cu2Mn3) of ~200 nm in length and particles of tetragonal θ-phase (Al2Cu) of ~100 nm in size. The microstructure leads to high enough hardness (HV = 1.1 GPa), yield stress (σy ≈ 320 MPa) and plasticity (Ψ ≈ 16%). Before the deposition of ZrN coating, the samples of 2024 Al-alloy were carefully ground with SiC abrasive papers and polished with diamond paste to grit size of 0.3 μm. The polished samples were surface mirroring by diamond liquid and the substrate was ultrasonically cleaned in acetone for 25 min at room temperature, thoroughly rinsed with distilled water and dried using nitrogen gas to avoid contamination. 2.2. Deposition procedure With the aim of increasing the deposition rate and having the possibility to deposit thick coatings in commercial production the vacuumarc plasma deposition machine with magnetic filter has been used [24]. A Zr cathode of 50 mm in diameter was placed in the cylindrical vacuum chamber (an anode), which was grounded. The arc was ignited by a mechanical triggering system with a W trigger electrode and powered by DC power supply in constant current mode. A permanent magnet was placed behind the cathode to drive the cathode spot on the cathode surface and reduce the generation of macrodroplets. The radial component of the magnetic flux density at the edge of the cathode surface was approximately 6 mT. The chamber was evacuated down to < 10−3 Pa, and then N2 gas was introduced via a mass flow controller. The vacuum arc was ignited by contacting a W trigger electrode rod. The following parameters were applied to the deposition of the ZrN coating: an arc current was set to be of 80 A, the bias voltage was of 100 V, the nitrogen gas pressure in the chamber was sustained to be of 0.95 Pa. The substrates were positioned at the center of the substrate water-cooled table, located at a distance of 200 mm from the cathode surface with no bias voltage and no additional heating. The substrate temperature was maintained to be within the range 30–60 °C. The overall sample temperature was continuously measured by thermocouple during the deposition process, and it was adjusted to be within the requested temperature range. The substrate temperature after the deposition was < 50 °C. To solve the problem of low-temperature deposition, a new approach for modification of the arc-cathode and substrate surfaces by using low energy Ar gas ion bombardment prior to the deposition process is suggested [25]. Additionally, a new high-power large aperture ion source was mounted inside the industrial deposition installation for ion treatment of the Al alloy surface. To remove a thin oxide layer and other adherent impurities from the substrate surface, it was cleaned for 10 min by a normally incident Ar + beam generated by the installed ion source. The 600 eV Ar + beam of 15 mm in diameter operated at a current density of 5 μA/cm2 was used. During the cleaning process, the substrate bias voltage was of −200 V, and the absorbed radiation dose was of 2×1017. This approach improves the adhesion in a wide range of the coating-substrate systems and provides the formation of the dense, well-bonded hard coatings with advanced mechanical properties at relatively low substrate temperature. 2.3. Microstructure and properties examination methods The produced thin ZrN coating was examined both by a grazing 414

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incidence X-ray diffraction (GIXRD) analysis and standard ‘Θ–2Θ’ analysis by means of a Rigaku MiniFlex600 XRD apparatus with a Cu Kα radiation (λ = 1.5406 Å). The X-ray scanning step length was 0.02° and the residence time was 2 s. When the diffractometer worked in the α–2θ mode the angle between the incident X-rays and the sample was fixed at γ = 0.3°. In our case, a geometrical X-ray penetration analysis by the equation [26,27]:

h = −(1/μk) ln(1 − R).

three-electrode cell with a platinum counter electrode and saturated calomel reference electrode [35]. The corrosion potential Ecorr and corrosion current icorr were estimated by analyzing the polarization curves using standard techniques. Wear and friction behaviors of the ZrN coating were assessed both in the dry and wet conditions. The dry wear was studied at reciprocating sliding of a conical diamond indenter with the tip of small curvature (50 μm) loaded by 0.39–0.59 N by means of micro-nanohardness/scratch tester “Micron-gamma” [32,33]. The reciprocating sliding of a Si3N4 ball (8 mm in diameter) loaded with 20 N in inactive liquid paraffin along a track of 4 mm long was also analyzed using the computer-assisted tribological complex described elsewhere [36,37]. Accounting for the wear test conditions for both studied cases, the magnitudes of the contact pressure were respectively estimated to differ on the order of magnitude, i. e. ~1 GPa and ~10 GPa at the sliding processes of the Si3N4 ball and the conical diamond indenter, respectively.

(1)

Here k = sin(γ) sin(2θ − γ) / (sin(γ) + sin(2θ − γ)), γ is the X-ray incident angle, 2θ is the diffraction angle for the particular measurement, R = Ih/I0 is the ratio of intensities diffracted by the surface layer Ih and the bulk I0, and μ = 805.88 cm−1 is the line absorption coefficient for ZrN, which was calculated using Braggs' additive law and specific mass absorption coefficients for Zr and N [26]. The penetration depth of X-rays in ZrN film h was evaluated to be of ~90 nm that is in line with that reported for the incident angle of 0.2° (h = 40 nm) [28]. Using ‘Θ–2Θ’ XRD scan the crystal structure and the lattice parameter of the ZrN coating were identified and measured. The texture coefficient, crystallite size, and lattice microstrains were also evaluated. To describe the texture the (111) pole figures for the ZrN film and aluminum substrate were registered and analyzed. The full widths at half maximum (FWHM) of the (111), (200), (220) and (311) reflections were used to estimate the crystallite size using Scherrer's equation [22,26,29]:

β = (K λ/ Dcosθ) + η tgθ,

3. Results and discussion 3.1. Microstructure observations Analysis of the surface morphology of the as-deposited ZrN coating (Fig. 1) shows that it is characterized by extremely low surface roughness (Ra = 0.061 μm, Rz = 0.365 μm). The arithmetic mean roughness Ra is only slightly higher than the size of individual grains, which can be distinguished on the film surface by SEM (Fig. 1d). It is also comparable to the Ra magnitude reported for the ZrN coated 1075 Al alloy, and it is significantly lower than the upper limit of 0.2 μm established by the ASTM E 606 standard [18]. The low roughness is known to be beneficial with regard to the prolonged fatigue life of materials owing to the almost complete absence of the surface stress raisers. Fig. 2a illustrates X-ray diffraction pattern collected using GIXRD analysis over the 2θ range of 20–80°. As seen, a single ZrN phase is observed in the top surface layer of the produced coating, and the registered reflections corresponding to the (111), (200), (220), and (311) planes indicate that a typical B1 NaCl FCC structure is formed. According to the Bragg's law, the lattice constant of the ZrN film was calculated to be equal to 4.622 ± 0.002 Å. This value agrees with the results reported in the literature [4,38]. The XRD pattern recorded using the ‘Θ–2Θ’ focusing scheme demonstrates that the phase composition of the deposited film is not uniform (Fig. 2b). Along with the fcc ZrN phase observed by GIXRD analysis in the top surface layer of the deposited film, the peaks corresponded to the other phases (other than aluminium substrate) are present in the lower layers of the film. The angular positions of these peaks indicated that a number of ZrxAly phases were formed on the 2024 Al substrate under the bombardment by the Zr+ ions. Further, some portion of additional orthorhombic Zr3N4 phase might also be formed on the beginning stages of the deposition process. Then, the fcc ZrN phase of stoichiometric composition is formed with the ongoing deposition process. The orthorhombic Zr3N4 phase was also observed in the zirconium nitride films deposited by reactive magnetron sputtering, and the intensity of its XRD peaks correlated to the volume fraction was shown to be dependent on the flow rate of nitrogen [15]. In our case, the phase compositions and microstructural morphologies of different film layers are affected by the change in the reactive components involved and thermal conditions during the ongoing deposition process. X-ray stress analysis shows the formation of the compressive stresses of 2.9 GPa in the deposited ZrN coating, which is known to be a beneficial factor in the sense of the enhanced operational properties (fatigue and wear behaviors) of the coated materials [4,18]. The texture is analyzed based on the registered (111) pole figures for the ZrN film and 2024 substrate (Fig. 3). The results show that the ZrN film texture (Fig. 3a) is inherent with that of the substrate (Fig. 3b)

(2)

where D is the crystallite size, K is the constant (K = 0.9, assuming that all the grain sections are spherical in shape), λ is the wavelength of Xray radiation, θ is the angle of diffraction, η relates to the lattice microstrains, β = (B2 − b2)1/2 is the physical broadening (FWHM) of peak, B is the FWHM obtained from the coatings, and b is the FWHM of instrumental broadening. The instrumental broadening was preliminarily determined using the annealed ZrN powder. The residual stress in the ZrN coating was determined using the cos2αsin2ψ method, i.e. based on the slope of the linear fitting of the ‘strain – cos2αsin2ψ’ plot [30]. The surface morphologies and chemical compositions of as-deposited, corroded and worn surfaces of the coatings were observed by Scanning Electron Microscope (SEM) TESCAN Mira 3 LMU equipped with an EDS microanalyzer OXFORD X-MAX 80 mm2 and a 3D interferential profilometer “Micron-alpha” [31]. To measure mechanical properties and adhesion of the deposited ZrN thin film the nanoindentation and scratch tests were respectively carried out by means of micro-nanohardness/scratch tester “Microngamma” [32,33]. The nanoindentation tests were used to characterize the hardness, H, and elastic modulus, E, by analyzing the load-displacement data obtained at the penetration of a Berkovich indenter into the ZrN film. To account for the bluntness of the indenter tip affecting the H magnitudes at low loads the calculation procedure reported in [34] was used. For the adhesion assessment, the scratch test was carried out by moving of a conical diamond indenter (a tip curvature of 50 μm) along the specimen surface under progressively increasing and then decreasing normal load (with a maximum value of 0.5 N). The friction force was registered simultaneously during the scratch test, and it was used as a measure of the critical load values corresponding to the cracks' initiation/opening processes. Measurements of the Vickers microhardness were performed using a PMT-3 microhardness tester at different normal loads (0.098, 0.147, 0.245 N) applied to the pyramidal indenter for 15 s. The diagonal lengths of the indentations were measured in high magnification optical micrographs and then converted to the Vickers' microhardness magnitudes using a standard procedure. At least 10 measurements were taken for each load and the average is reported. The corrosion behaviors of the original 2024 alloy and ZrN/2024 alloy specimens in a 3.5% NaCl solution were evaluated by registering the polarization curves by using a potentiostat P5827-M connected to a 415

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Fig. 1. Light microscopic image (a), 2D (b) and 3D (c) surface topography, and SEM image (d) of the surface of the deposited ZrN film.

energy. The experimental parameters of the ZrN coating deposition process used in the present work facilitate preferential grains' orientation during their growth, and in good consistency with the literature data the (111) texture becomes pronounced. It was suggested [39] that for the transition-metal nitride with NaCl structure, (200) plane has the lowest surface energy while (111) plane has the lowest strain energy. And the larger the thickness of the film the higher the fraction of the strain energy in the overall energy, and, in its turn, the higher the probability of the (111) texture. Additionally, according to a competitive growth theory by Gall et al. [40] explaining the texture transition in TiN, the diffusion length for the Ti adatoms on (200) plane is much longer than that on (111) one. Thus, the growth of grain with (111) orientation is thereby enhanced because the Ti adatoms are more likely to be trapped on (111) plane. Estimations of the crystallite size using Scherrer's equation (Eq. (2)) show that the grown ZrN film contains the nano-scale grains/crystallites although it appears slightly difficult due to some ambiguity related to the possible contribution of additional phases into the overall width of the peaks analyzed. Notice additionally that the grain size assessed by the Scherrer equation can be considered only as estimation but not a quantitatively precise value [44], and the microstrains η formed in the coating may also contribute to the line broadening. Considering the slope of the function β = f (tgθ), these microstrains were assessed to be 0.2–0.25%. Fortunately, the crystallite size assessed by the function β = f (tgθ) falling into the nanoscale range of 21–38 nm is in a close agreement with the results of the SEM plane-view and cross-sectional observations (Fig. 4) and with the data reported by other authors [41–43]. Fig. 4 shows the typical SEM images of the plane-view of the ZrN coating (Fig.4a) and its cross-sectional microstructure (Fig. 4b). As can be observed, the coating is quite homogeneous and has an overall thickness of about 1 μm. The cross-section of the coating is free from macroparticles, which could deteriorate the coating properties [4,6,43]. At the same time, some quantity of submicronic (0.4–0.6 μm) droplets can be found on the ZrN coating surface (Fig. 4b). Additionally, the produced ZrN coating has a dense columnar microstructure consisted of fine fibrous grains elongated in the growth direction (especially, in the lower layers). The size of the fibrous grains (~25–50 nm) observed by

Fig. 2. GIXRD (a) and ‘θ–2θ’ X-ray diffraction (b) patterns of the ZrN coating deposited on the 2024 Al alloy substrate.

owing to only slight mismatch between the lattice parameters of the ZrN film and 2024 Al alloy substrate. It is well known that the preferential orientation is determined by the competition between two thermodynamic parameters: the free surface energy and the strain 416

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Fig. 3. Partial (111) pole figures of the ZrN thin film (a) and the 2024 Al alloy substrate (b).

ZrN coating (say, of ~0.5 μm thick) are formed. Then, the deposition of the upper layers already occurs at slightly higher temperatures, and it thus results in the formation of the V-shaped grains. It is important that the temperature of the Al substrate remains low enough to avoid its melting and to provide the formation of the grains of nano-scale range. In addition, the deposition energies and temperatures are enough to avoid porous structure, which is usually caused by limited ad-atom diffusivities in the Zone I of the SZD [47].

SEM analysis both in the plane view images (Fig. 4a) and in the fracture surface of the ZrN coating (Fig. 4b) well correlates to the crystallite size assessed by XRD analysis. At the very beginning of the deposition process the fibers (columns) grew nearly perpendicular to the substrate surface (see the lower layer of the coating below the arrows), and then some V-shaped fibers start to form (in the upper layer of the coating above the arrows). The observed microstructure of the ZrN coating obtained at the middle deposition energies and temperatures to avoid undesirable melting of 2024 alloy substrate is very similar to that obtained in [45]. This microstructure is in good agreement with the so-called structure–zone diagram (SZD) developed by Movchan and Demchishin back to 1969 [46] and recently extended by Anders [47] on the base of a comprehensive overview of the influence of the various deposition parameters and the resultant microstructure. According to the cross-sectional view of the ZrN coating (Fig. 4b), the used deposition parameters allowed producing a fine dense columnar microstructure with some trend to the formation of a V-shaped faceted dense columnar structure in the upper layers of the coating. Thus, the so-called transition zone (Zone T) seems can be matched in the SZD, where the surface diffusion is active but not the grain boundary diffusion. It can be explained by the change in the thermal conductivity of the substrate with on-going ZrN deposition. Indeed, the high thermal conductivity of intact 2024 alloy provides quick heat transfer inside the substrate leading to relatively constant and low deposition temperature. Further, it becomes lower when the first layers of

3.2. Properties assessments Corrosion behavior of the ZrN coating was studied by analyzing the anodic and cathodic polarizations in saline solution. The polarization tests in 3.5%NaCl medium showed that the ZrN coasting is characterized by lower corrosion rates and more positive corrosion potentials (Ecorr) than those of the original alloy (Fig. 5 and Table 1). The corrosion current density (icorr) is an important parameter, which allows evaluating the kinetics of corrosion reactions. Normally, it is proportional to the corrosion current density measured via polarization. The lower the icorr, the lower the corrosion rate of the sample is. In the present test, the icorr value obtained for 2 μm ZrN was always lower than that of the bare 2024 alloy substrate. The registered potentiodynamic polarization curves (Fig. 5) provide the values of anodic and cathodic slopes necessary to obtain the correct estimations of the corrosion potential (Ecorr) and corrosion current (icorr) [7,35]. The corrosion potential of the ZrN-coated 2024 alloy is more electropositive than that of

Fig. 4. The SEM observations of the surface (a) and cross-section (b) of the ZrN coating. 417

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corrosion potential than those of the substrate 2024 alloy. In addition to the anticorrosion properties, the strength of any coating is also determinative in the sense of the protection efficiency of the coated material. Accounting for the low thickness of the ZrN film to be studied, the nanoindentation tests were adopted to characterize the hardness, H, and elastic modulus, E (Fig.7) to avoid the substrate effect. Analysis of the load-displacement data obtained at the penetration of a Berkovich indenter into the ZrN film shows that the loading curve begins to deviate from the theoretical parabolic curve already starting from the load magnitude of ~8 mN, which corresponds to the indentation depth of ~0.12 μm. To calculate the true hardness magnitude we used the correction methodology accounting the bluntness of the indenter tip [34]. The obtained results listed in Table3 (H = 20.2 GPa and E = 196 GPa) correlates well to the hardness and elastic modulus values reported for the ZrN films in the literature [5,15,18,38]. Note that the calculations performed on the base of the curve 2 in Fig.7, which was registered at a higher load, already give significantly underrated hardness/modulus magnitudes (11.4 GPa and 103 GPa, respectively) (Table 3) owing to excessive influence of the soft substrate on the shape of the recorded loading curve 2. Additionally, when the well-known requirement regarding the admissible ratio of the indentation depth the coating thickness is satisfied, then the hardness of the ZrN films can also be assessed on the base of the measured hardness of the ZrN/substrate system and using the model proposed by Puchi-Cabrera [18,49,50]. According to such a model, the composite hardness can be considered as a combination of the substrate hardness (HVS) and film (coating) hardness (HVF) as follows [18,49,50]:

Fig. 5. Polarization curves of 2024 alloy (0, 1 [35]) and the ZrN arc coatings (2,3) on the 2024 alloy (2) and 304 stainless steel (3) [6].

Table 1 Corrosion parameters. Material 2024-T351 alloy 2024 alloy ZrN arc coatings/2024 alloy ZrN arc coatings/304 steel

Ecorr vs SCE, V

log(icorr), μA/cm2

Reference

−0.991 −1.03 −0.528 −0.539

−5.34 −5.25 −5.97 −5.28

[35] This study [6]

the blank substrate (Table 1). As seen, the electrochemical data obtained in this study perfectly coincide with those found in the literature both for ZrN [6] and 2024 alloy [48]. It was also reported that the corrosion behavior can be enhanced by the deposition of nitride CrN/ ZrN [3], TiN/TiAlN [8] or ZrN/Zr/C [9] multilayer coatings. Fig. 6 shows the morphological features of the ZrN costing surface after potentiodynamic tests. In comparison with the SEM images of the original ZrN surface (Fig. 1d), the corroded area remains almost unchanged. Still, it is homogeneous and almost free from macrovoids and pinholes. The slightly corroded circular areas are rarely observed, and often small droplets of submicronic size (0.4–0.6 μm) remain uncorroded. EDX analysis did not recognize any variations in chemical compositions of the microdroplets and other surface areas (Table 2). Similar corroded microdroplets were also observed on the surface of the ZrN, TiN and CrN coatings deposited by arc evaporation method [6]. Those droplets were embedded in the coating surfaces and were accompanied by the small craters, which were often found as a result of weak droplet-coating bonding. Anyway, the ZrN coating deposited in this study demonstrates much lower corrosion rate and more positive

HVZrN/2024 = HVS + (HVF − HVS) exp [−(β/β0 ) n].

(3)

Considering the film/substrate system analyzed in this study Eq. (3) should contain the following parameters: the hardness of the 2024 alloy substrate HVS = 1.1 GPa, β is defined as the ratio of the indentation depth δ = d/7 [18] and the film thickness h, d is the indent diagonal, n = 0.4, and β0 = 0.27 is taken from work [18] describing the ZrNcoated aluminum alloy. Table 3 summarizes the values of δ, β, HVZrN/ 2024, and HVF obtained for different indentation modes (nanoindentation and microindentation with the Berkovich and Vickers tips, respectively). The calculation showed that the hardness HVZrN/2024 of the ZrN/ 2024 Al-alloy composite system measured at the lowest used load (0.098 N) reached the value of ~3.5 GPa. The HVZrN/2024 remains almost double higher (~2.2 GPa) than that of the substrate alloy (1.1 GPa) even in the case of the higher loads (0.245 N) resulted in numerous cracking of the ZrN film (Fig.8) and the indentation depth value of the Vickers indenter higher than the double film thickness. Nevertheless, the hardness magnitudes of the ZrN film HVF

Fig. 6. Plane-view SEM image of the corroded arc ZrN coating (a) with the magnified view of the rectangular area (b). Black and white arrows indicate the corroded and uncorroded droplets, respectively. 418

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Table 2 Chemical compositions (at.%) in the marked areas of the wear tracks on the Zr coating and 2024 Al alloy substrate. Spectrum Fig. 6 (ZrN) Fig. 13 (ZrN) Fig. 14 (2024)

1 2 1 2 1 2

C

O

Mg

Mn

Si

Fe

Zn

Cu

Al

N

Zr

2.70 3.7 3.46 3.94 18.87 19.59

3.74 4.2 2.41 3.08 18.60 10.79

– – – – 1.42 1.32

– – – – 0.56 0.66

– – – – 0.58 0.65

– – – – 0.55 0.76

– – – – 0.64 0.73

– – – – 1.56 1.75

– – – – 57.20 63.73

48.26 47.3 48.59 47.93 – –

45.27 44.78 45.53 45.05 – –

Fig. 7. Nanoindentation loading/unloading curves registered using the ‘Micron-gamma’ nano-microhardness tester. Fig. 9. Dependencies of the penetration depth (δ) and friction force (F) on the increasing normal load (L) at the scratch of the conical diamond indenter with the 50 μm tip.

Table 3 Parameters involved in Eq. (3) and estimated hardness of the ZrN film. Load (N) 0.008 0.019 0.098 0.147 0.245

δ = d/7 (μm)

β = δ/h

HVZrN/2024 (GPa)

HF (GPa)

0.12 0.35 1.036 1.36 2.025

0.118 0.346 1.025 1.345 2.003

18.1 6.3 3.466 3.056 2.263

20.2a 11.4 19.2 20.6 21.2

cracks are. Additionally, starting from the load of 0.147 N two or more perimeter cracks are visible on each side of the indent forming something like downstairs. The diagonal cracks initiated from the indentation center are almost invisible at the loads lower than 0.245 N due to the indenter geometry. The cracks of this type become pronounced and long enough when the higher loads are applied. Nonetheless, the cracks' opening occurs very weakly seemingly owing to a high adhesion of the deposited film and substrate. The adhesion between the coating and substrate is known to be one of the important factors affecting the lifetime of hard coatings. Fig. 9 shows the curves recorded for the penetration depth and friction force dependently on the progressively increasing normal load on the conical diamond indenter moving along the specimen surface. As seen, the penetration depth begins to deviate from the linear dependence at δ = 0.7–0.8 μm. The simultaneously recorded curve of the friction force, F, demonstrates a two-fold behavior: the low-amplitude F oscillations noted during the first stage of the scratching test occurred owing to the surface roughness are replaced by the increasing F variations after some load value (~0.3 N) is achieved. This replacement occurs simultaneously with the deviation of the penetration depth δ from the

β0 = 0.27 [18], n = 0.4. a H values assessed based on the nanoindentation data corrected using the methodology [34] accounting the tip bluntness.

(19.2–21.2 GPa) evaluated using Eq. (3) appears well comparable with the appropriate literature data [18,49,50]. Note, that the exponent n was taken to be n = 0.4 (which is four times lower than that used in [18]) to account for the soft substrate providing the deep indentation supplemented by the film multiple cracking without cracks' opening (Fig. 8). Thus, the value of the exponent n in the Puchi-Cabrera's model can be used as a fitting parameter. The SEM images of the Vickers indents made of different loads on the ZrN films are shown in Fig. 8. It is seen that regardless the load value the perimeter cracks are visible around the produced indents. Naturally, the larger the load, the more pronounced the perimeter

Fig. 8. SEM images of the Vickers indents produced on the ZrN surface by different loads of 0.098 N (a), 0.147 N (b) and 0.245 N (c). Arrows indicate the perimeter and diagonal cracks. 419

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Fig. 10. Light microscopic (a), 2D/3D surface topography (c, d, e, f) of the sliding grooves and the appropriate friction force records (b) produced by dry reciprocating sliding (130 cycles) of the conical diamond indenter with the tip radius of 50 μm at the normal loads of 0.39 N (1, c, d), 0.49 N (2, e, f), and 0.59 N (3).

tribological properties of 2024 Al-alloy are found to be significantly enhanced by the deposition of the ZrN coating of high hardness. Better tribological properties of ZrN manifest themselves by significant lowering in the coefficient of friction (CoF) evaluated as a ratio of the registered friction force to the applied load (Fig. 12a). Notice that two types of testing conditions were employed in our experiments, i.e. usual quasi-static load (20 N) applied to the sliding Si3N4 ball in a normal direction to the tested surface during sliding and the so-called dynamic tests, which consisted in applying both the quasi-static (P) and alternating components (0.1P) of load [36]. In the quasi-static conditions, the CoF of the ZrN coating (~0.1) is about three times lower than that of the 2024 alloy (0.285) (Fig. 12a). Relatively low CoF (0.1–0.4) was also reported for the ZrN coating deposited on 316 stainless substrates by cathode arc evaporation in a reactive nitrogen atmosphere [45]. In the dynamic sliding conditions, the CoF magnitude of the ZrN coating (~0.04) is more than four times lower than that registered for the 2024 alloy (~0.18). A couple of reasons can be considered for the explanation of such difference in CoFs of the ZrN coating and 2024 alloy. There is both the significantly lower surface roughness of the ZrN coating and the absence of any mechano-chemical reactions. On the contrary, the wear induced oxidation and/or oil carburization can be expected in the case of 2024 alloy due to the friction-induced heating. The grooves' depths experimentally measured after sliding tests are presented in Fig. 12b. A quasi-static load applied to the sliding Si3N4 ball resulted in the formation of a very small groove on the coated surface, which depth was five times lower than that of the groove worn on the surface of the intact 2024 alloy. It is naturally connected to a relatively low contact pressure (~1 GPa) arisen at sliding of 8 mm ball tip against the specimen surface. In the case of the dynamic test conditions, the depth and the thickness of the formed groove became much larger. Moreover, the grooves' depths of the 2024 alloy and ZrN coating appear to be almost equal after dynamic tests. Nonetheless, the registered depths of the grooves formed after the sliding test can hardly be

linearity and can be considered as a signal of the material response to the deformation action, i.e. a transition from the sliding to the multiple cracking. At the same time, the film seems to remain firmly attached to the substrate. Only the load of ~0.45–0.5 N results in the high amplitude oscillations of the friction force, which can be related to the transition from multiple cracking to some delaminating of the film. The film delaminating is seen more pronounced at the reciprocating sliding of a conical diamond indenter against the specimen surface carried out for 130 cycles (Fig.10). Fig.10a shows an optical image of three tracks formed after sliding at 0.39, 0.49 and 0.59 N, respectively. The registered curves of the friction force F indicate that only the highest load leads to the intensive delaminating of the film (that manifests itself by significant F increase) and to intensive wear. Conversely, two lower loads (0.39 and 0.49 N) do not result in any wear signs at the bottom of the formed grooves, and the amplitude of the friction force variations remains almost unchanged throughout the tests. It allows concluding that even so high contact pressure (~9.6–10.4 GPa) results in only the edge cracking along the sliding grooves, which are formed mainly due to the soft substrate and are not owing to the wear of the ZrN film itself. It is clearly seen in the 3D images of the formed grooves, and naturally, the higher the normal load applied the deeper the formed groove is (Fig. 10c,d,e,f). The scratch tests by the Vickers indenter were also performed at different normal loads. The results indicate that the critical load for the film fracture/delamination is two times lower (~0.25 N) in comparison with the case of the cyclic sliding of the conical indenter with the 50 μm semispherical tip, which is due to more tough sliding conditions related to the much higher tangential friction of the pyramidal indenter. The SEM images of these scratches show that the lowest applied load of 0.098 N does not result in essential cracking (Fig. 11b), only a few cracks/spallation are visible at the normal load of 0.147 N (Fig. 11c), and significant frequent spallation occurs when the heaviest load (0.245 N) was applied (Fig. 11d). Considering wet sliding conditions, 420

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Fig. 11. SEM observations of scratch grooves produced by Vickers indenter at normal loads of 0.098 N (b), 0.147 N (c), and 0.196 N (d).

inclined lighting (Fig. 13a, Fig. 14a), but they are worse seen in SEM. It can be concluded that the ZrN coating and 2024 alloy have different wear mechanisms. The ZrN coating deposited on 2024 Al-alloy substrate has a light yellow gold color. It is important that the worn surface of the ZrN coating is almost free from any cracks. This result is in good accordance with the data reported for the mild wear regimes of the ZrN coating of about 2 μm thick deposited by magnetron sputtering [43] and against the Si3N4 counter-body [51], but it contradicts to the severely worn ZrN coating deposited on the tungsten carbide [4,52]. The 2024 alloy is conversely fractured during the sliding process, and the elongated cracks parallel to the sliding direction near the worn track are visible in the SEM images (arrows in Fig. 14c). It is also of interest that in the case

directly interpreted as a wear loss, especially for the dynamic tests. On the one hand, considering the relatively small thickness of the hard ZrN coating and a high enough ductility of the 2024 substrate results in its easy upsetting under the applied load without any visible worn cracks or scars (Fig. 13b). On the other hand, the propensity of the 2024 alloy to mechanically induced oxidation/carburization (the formation of the oxide/oxicarbide scales diminishing the real worn depth) should be considered at the interpretation of the registered wear loss (especially in the dynamic conditions) (Fig. 14b). The analysis of the morphologies of the surfaces of the formed grooves allows clarifying the sliding and/or wear processes and their mechanisms and outcomes. The formed grooves are well visible on the specimen surface in the light microscopy images registered in the

Fig. 12. Friction force (coefficient of friction – CoF) (a) and wear loss (b) of the ZrN coating and 2024 Al alloy substrate registered at quasi-static (s) and dynamic (d) sliding conditions. 421

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Fig. 13. Optical (a) and SEM (b,c) images of the wear surface of the ZrN coating registered after dynamic (on the left in (a)) and quasi-static sliding conditions.

Fig. 14. Optical (a) and SEM (b,c) images of the wear surface of the 2024 al alloy substrate registered after dynamic (on the left in (a)) and quasi-static sliding conditions. Arrows in (a, b) and (c) point on the oxide scales and longitudinal cracks, respectively.

together with an aluminum oxide phase formed owing to mechanicalchemical reactions. The mechanically mixed particles (but predominantly aluminum) might be deposited on the contact surfaces and form the dark, compacted layers on the surface of aluminum, like those seen in the worn track on the 2024 alloy surface (Fig. 14a). The transition in mild wear regime mechanisms of Al alloys from mechanical mixing and oxidation to delamination of metal flakes was shown to be directly related to friction heating effects [54]. Especially, it is important in the case of the wear tests conducted against counter materials having different thermal conductivities as in our experiments. It should be mentioned that it is difficult to compare the tribological characteristics of the ZrN film observed here with the literature data because there the ZrN coatings deposited on hard substrates like steels [7,45,51] or WC-Co hard materials [4,52] were mostly studied. However, regarding the wear/friction properties of the 2024 alloy, the

of the ZrN coating the chemical composition of the sliding track did not change with respect to the original surface (see rectangles in Fig. 13b,c indicated the examined surface areas and Table 2). On the contrary, the sliding process of the Si3N4 ball against the 2024 alloy surface presumably results in the intensive friction induced heating of the contacted surfaces (especially in the case of static conditions) and promotes the formation of scales of some thickness (visible in Fig. 14a). The latter may mask (diminish) the real wear loss. The chemical composition of this area points on the increased contents of oxygen and carbon (presumably from the environment air and liquid paraffin used during the tests) (Table 2). Moreover, there is no a single wear controlling mechanism of Al alloys throughout the mild wear regime [53]. At low loads and velocities, submicroscopic aluminum and second phase particulates can be detached from the contact surfaces and can form a mechanical mixture 422

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results obtained in this study show that the ZrN coating provides their essential improvement owing to much higher hardness and lower friction coefficient and the intimate adherence with the 2024 alloy substrate.

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4. Conclusion 1. The chosen coating method of vacuum-arc deposition with preliminary Ar+ ion sputtering substrate pre-treatment allows producing the homogeneous and dense nanocrystalline ZrN film (coating), which is free from accidental macroparticles, has a thickness of ~1 μm, and is intimately adhered to the 2024 Al alloy surface. 2. The produced ZrN coating is shown to have a crystal structure typical B1 NaCl FCC lattice with the lattice constant equal to 4.622 ± 0.002 Å, the (111) texture, and compressive residual stresses of 2.9 GPa. 3. Microstructural studies by means of XRD and SEM analysis show that the produced ZrN film (coating) respectively contains the columnar/fiber grains and V-shaped grains in the lower and the upper coating layers grew nearly perpendicular to the substrate surface. The grain size is found to be about 20–50 nm. 4. The polarization tests in 3.5%NaCl medium showed that the ZrN coating is characterized by lower corrosion rates and more positive corrosion potential than those of the original 2024 alloy indicating higher corrosion resistance of the ZrN coating. 5. Nanoindentation experiments and calculations show that the hardness and elastic modulus of the ZrN film are respectively as high as 20.2 GPa and 196 GPa. The registered hardness of the ZrN film can be also evaluated on the base of the analysis of the microhardness of composite ZrN/2024 Al-alloy system using the Puchi-Cabrera's model and applying β0 = 0.27 and n = 0.4 as fitting parameters. 6. The adhesion between the coating and substrate is characterized by the critical load, which was shown to be of 0.5 N at progressively increased normal load on the sliding conical diamond indenter with the 50 μm tip and its reciprocating movement. 7. The dry sliding wear analyzed on the base of reciprocating sliding (130 cycles) of the conical diamond indenter with the 50 μm tip show that intensive wear supplemented with the film delaminating on the groove edge occurs only at the load of 0.59 N, but the lower loads (0.39 N and 0.49 N) do not initiate any wear although leading to the grooves' edge cracking. 8. The wet sliding wear tests in inactive liquid paraffin against Si3N4 ball showed that the ZrN coating is characterized by much lower friction coefficient and wear loss than those of the original 2024 alloy. Acknowledgments This study is financially supported by the Ministry of Education and Science of Ukraine [grant numbers 0118U000220, 0118U000221] and National Academy of Sciences of Ukraine [grant number 0114U001127]. The authors are grateful to Dr. M.A. Skoryk for his help in SEM/EDX experiments. References [1] P.J. Matin, R.P. Netterfield, W.G. Sainty, Optical properties of TiNx produced by reactive evaporation and reactive ion-beam sputtering, Vacuum 32 (1982) 359, https://doi.org/10.1016/0042-207X(82)63829-5. [2] J. Musil, I. Stepanek, J. Musil Jr., M. Kolega, O. Blaihova, J. Vyskoci, J. Kasl, Properties of TiN, ZrN and ZrTiN coatings prepared by cathodic arc evaporation, Mater. Sci. Eng. A 163 (1993) 211–214, https://doi.org/10.1016/0921-5093(93) 90792-D. [3] O.V. Maksakova, O.D. Pogrebnjak, V.M. Beresnev, Features of investigations of multilayer nitride coatings based on Cr and Zr, Prog. Phys. Met. 19 (1) (2018) 25–48, https://doi.org/10.15407/ufm.19.01.025. [4] D. Valerini, M.A. Signore, L. Tapfer, E. Piscopiello, U. Galietti, A. Rizzo, Adhesion and wear of ZrN films sputtered on tungsten carbide substrates, Thin Solid Films

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