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Functional multi-nanolayer coatings of amorphous carbon/tungsten carbide with exceptional mechanical durability and corrosion resistance Narguess Nemati, Mansoor Bozorg, Oleksiy V Penkov, Dong-gap Shin, Asghar Sadighzadeh, and Dae-Eun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08565 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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ACS Applied Materials & Interfaces

Functional Multi-nanolayer Coatings of Amorphous Carbon/Tungsten Carbide with Exceptional Mechanical Durability and Corrosion Resistance Narguess Nemati,a Mansoor Bozorg, b Oleksiy V. Penkov, a,c Dong-Gap Shin, a,c Asghar Sadighzadeh d, and Dae-Eun Kim* a,c a

b

Center for Nano-Wear, Yonsei University, Seoul 03722, Republic of Korea

Department of Materials science and Engineering, Buein Zahra Technical University, 3451745346, Qazvin, Iran

c

Department of Mechanical Engineering, Yonsei University, Seoul 03722, Republic of Korea

d

Nuclear Science and Technology Research Institute (NSTR), A.E.O.I., Tehran 14155-1339, Iran *

Corresponding Authors email: [email protected]

Abstract

A novel functional multilayer coating with periodically stacked nanolayers of amorphous carbon (a:C)/ tungsten carbide (WC) and an adhesion layer of chromium (Cr) was deposited on 304 stainless steel using a dual magnetron sputtering technique. Through process optimization, highly densified coatings with high elasticity and shear modulus, excellent wear resistance, as well as minimal susceptibility to corrosive and caustic media could be acquired. The structural and mechanical properties of the optimized coatings were studied in detail using a variety of analytical techniques. Furthermore, finite element method simulations indicated that the stress generated due to contact against a steel ball was distributed well within the coating which

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allowed the stresses to be lower than the yield threshold of the coating. Thus, an ultra-low wear rate of ~E–12 mm3/N.mm could be achieved in dry sliding conditions under relatively high Hertzian contact pressures of ~0.4-0.9 GPa. The amorphous and pinhole-free structure of the individual layers, sufficient number of pairs, and the relatively dense stacked layers resulted in significant polarization resistance ( Z″= 5.5 × E6 Ω cm2) and increased the corrosion resistance of the coating by 10-fold compared to that of recently reported corrosion resistant coatings.

KEYWORDS: Multilayer coating, Durability, Wear, Corrosion, Sputtering, Nanostructure, Mechanical properties, Simulation

1. Introduction Harsh and extreme operating conditions imposed on a majority of the components used in new energy generating systems1–5 have been the main driving force for the development of more durable materials and coatings for these components.6,7 Particularly, extreme operating conditions present numerous challenges in the development of sustainable steel components and parts used for these systems.8,9 Future energy generating systems are expected to utilize nearly 75% of steel as structural components (in 2025 first commercial fusion reactor will produce electricity) which poses a great challenge for the development of advanced durable materials operating in extremely harsh conditions.3,10,11 A literature survey on the history of structural materials research and development for renewable energy and power plants indicated that most efforts have been focused on increasing the durability of the associated coatings to delay the degradation of the components.3,12–15 Several recently proposed multi-functional and wear resistant coatings are the latest generation of energy-related materials that require further optimization for improved performance.9,12,15 The


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properties of coatings used for extreme operating conditions are required to satisfy various requirements such as irradiation resistance, high thermal conductivity, good thermal resistivity, superb mechanical durability, enhanced toughness, wear, corrosion, and erosion resistance, high retention of properties at high temperatures, as well as the ease of manufacturing and cost effectiveness.5,7,9,16 Candidate materials with superior mechanical durability include carbon, tungsten, nitrides (TiN, Si3N4), and carbides (SiC, ZrC, WC).16,17 Considering the advantages and drawbacks of these materials, coatings based on diamond-like carbon (DLC) or amorphous structures of carbon (a:C)9,18 have been suggested to be favorable for applications in energy systems, owing to their exceptional thermal conductivity, tunable mechanical properties, and good thermomechanical load resistance.18–20 Extensive research has demonstrated the capability of various types of DLC coatings in providing excellent toughness as well as improved tribological properties.18,21–24 One of the main advantages of carbon-based coatings is that their structure and properties can be readily tuned to achieve characteristics desired for a given application. For instance, the corrosion resistance and oxidation wear properties of the amorphous carbon coatings could be significantly enhanced by optimizing the deposition methods.25–28 Furthermore, it has been shown that either as a periodic alternative structure with a suitable pair like Co29 or doped with an appropriate second phase like W, WC, Ag or diamond,

30–35

shortcomings of the amorphous

carbon coating could be adequately overcome. Few successful studies on the enhancement of wear, mechanical properties, and corrosion resistance of DLC/WC films have been published over the past years and these issues are still being investigated toward further improvement.35,36 Tungsten carbide is widely used in numerous harsh applications, owing to its very high hardness and exceptional inertness to chemical attack. Specifically, WC is one of the most favorable


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choices for modern cemented coatings.37 Oxidation of WC is known to initiate at a relatively high temperature of 500–600 °C in ambient atmosphere.38 Such high oxidation temperatures facilitate good hydrogen retention property under severe conditions, compared to pure W and carbon.39 WC also has a high melting point (2870 ͦ C) and exceptional thermal resistance and stability, with a hardness comparable to that of the hard DLC films. However, the inherent brittleness of WC sometimes restricts its use as a coating in certain applications.40 The main objective of the present work was to develop a functional coating by combining C and WC in the form of a multilayer to derive the synergistic benefits of each material. The functional multilayer coatings (FMCs) proposed in this work are comprised of consecutive a:C/WC pairs designed to enhance mechanical durability as well as chemical stability under harsh operating conditions.29 A few studies have previously shown that WC/C composite coatings provide better wear resistance compared to those of the individual materials.33,35,41–44 However, optimization of the hardness, modulus, density, durability, and wear resistance, for target applications still remains as a challenge. Owing to the benefits derived by the modified amorphous layer and a nanoscale periodic structure with gradually alternating properties (brittle-ductile and hard-soft), a:C/WC FMC is expected to demonstrate enhanced mechanical properties along with significantly improved resistance to oxidation and corrosion. Furthermore, the dual magnetron sputtering technique used to deposit the FMCs is a low-temperature deposition process that can reduce the probability of chemical alterations as well as the accumulation of residual stress within the coating. 2. Materials and methods 2.1. Fabrication


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The a:C/WC FMCs and single-layer coatings were prepared by a dual DC magnetron sputtering technique with a custom-built 3-target system in an argon atmosphere. Graphite (99.99%), tungsten carbide (99.999%), and chromium (99.999%) targets with diameters of 100 mm were used. The coatings were deposited onto various substrates, including polished Si (111) wafers with an area of 1 × 1 cm2 as well as smooth (Ra< 50 nm) 304 stainless steel (SUS) substrates with an area of 2 × 2 cm2. The base pressure before the deposition was about 10−3 Pa. The surfaces of the substrates were cleaned by a plasma of an Ar ion beam (W = 50 watt) for 20 min prior to deposition. The argon pressure during deposition was maintained at 0.3 Pa. The thickness of the individual layers was controlled by adjusting the exposure time that was calibrated through thickness measurement of depositions obtained by various radio frequency (RF) or direct current (DC) power supply. The substrate temperature was maintained below 50 °C during the sputtering process. Cr was used as an intermediate adhesion layer to sufficiently enhance the adhesion of the top consecutive pairs of layers on the substrate and to minimize the mismatch between the top stack and the surface structure of the substrate. The deposition rate was calibrated to 0.5 nm/s for chromium target with 150 watt RF power and 0.1 nm/s for WC with 100 watt RF power; and for carbon, the deposition rate was calibrated to 0.2 nm/s using DC assisted by bias voltages.

2.2. Coating structure and property evaluation The atomic and crystal structure of the coatings were evaluated by spherical aberration correction scanning transmission electron microscopy (Cs-corrected-STEM) (JEOL JEMARM200F) equipped with an energy dispersive X-ray spectroscopy (EDX; OXFORD INCA Energy) system. The surface roughness and wear scars were characterized by atomic force


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microscopy (AFM) (Park Systems NX 10) and high-resolution 3D laser microscopy (Keyence VK-X210). The mechanical properties of the coatings such as nanohardness (H), effective Young’s modulus (E*=E/(1-ν2)), and load-unload curve were evaluated using a high-precision ultra-nanohardness tester (UNHT, CSM).45 The diamond indenter used for the hardness measurements was a round-ended Berkovich pyramid with a diameter of 100 nm. A maximum load in the range of 600-700 mN was used to ensure that the indention depth remained less than 10% of the film thickness. The measurements were repeated 30 times for each specimen. The structures of the consecutive layers as well as wear scar characteristics were analyzed by Raman spectroscopy (JY Horiba Labram Aramis) and high-resolution X-ray diffractometry (HRXRD) (Rigaku, SmartLab). Focused ion beam (FIB) (JEOL, JIB-4601F) was used to obtain ultra-thin cross-sections of the films used as specimens for high-resolution transmission electron microscopy (HR-TEM) analysis.

2.3. Friction and wear behavior The mechanical durability of the coatings were investigated using a custom-built reciprocating tribotester.46 All experiments were performed under ambient conditions at a temperature of ∼25 °C and a relative humidity of 45−50% in a Class 100 clean room. For the reciprocating sliding tests, the speed was set to 2 mm/s with a stroke of 2 mm. The counter surface was chosen to be a stainless steel ball with a diameter of 1 mm. To ensure the repeatability of the experimental data, at least three sliding tests were performed for each experimental condition. The normal load applied during the sliding tests was in the range of 10–50 mN. The Hertzian contact pressure with respect to substrate, coating composition, and mechanical properties of various FMCs


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obtained by UNHT measurements were calculated using the commercial software COMSOL 5.1 in 2D mode (Multiphysics modules). The wear resistance of the coating was used as the primary indicator for durability assessment. The wear scars generated on the specimen after the sliding tests were measured by Raman spectroscopy (JY Horiba Labram Aramis) with 512 nm laser excitation, as well as a 3D laser microscopy. The wear resistance of FMCs was quantified by calculating the wear rate using the following equation: Wear rate (N.mm3/mm) = Wear volume (mm3)/ Normal load (N) × sliding distance(mm) (1).29

2.4. Finite Element Method (FEM) analysis FEM simulations were performed to assess the stress concentration and lateral shear stress distribution in the bilayered and multilayered FMCs. Commercial software COMSOL 5.1 in 2D mode was used for the analyses

29

. The actual structure of the coatings with respect to the total

thickness and mechanical properties of the multilayers obtained by UNHT were used as input parameters for the FEM simulation.

2.5. Corrosion resistance evaluation Electrochemical impedance spectroscopy (EIS) was performed under atmospheric conditions using a conventional three-electrode electrochemical cell and an Autolab potentiostat/ galvanostat (Autolab 302N, Netherlands) in 3.5% NaCl solution. A saturated calomel electrode (SCE) separated from the cell by a glass frit was used as the reference electrode and a platinum electrode was used as the counter electrode. EIS measurements were conducted at the open circuit potential. The measurements were initiated about 120 min after the working electrode was


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immersed in the solution to stabilize the steady state potential. Experimental impedance spectra were obtained in the frequency range of 100 kHz to 10 mHz by a sine wave with a potential perturbation amplitude of 5 mV. EIS of each individual specimen was measured at least three times to ensure repeatability of the results.

3. Results and discussion In this section, the design concepts of the functional multilayer coatings for extended mechanical durability and anticorrosion characteristics are described. Then, structural properties as well as friction and wear characteristics of the coatings are assessed. The mechanism responsible for the high durability of the coatings are proposed based on the experimental results and FEM simulations. Finally, a comprehensive discussion on the corrosion properties including the mechanisms involved is presented.

3.1. Conceptual design of the functional multilayer coatings Figures 1a-1d provide the typical coating compositions as well as the schematic showing the detailed structures of the individual layers. Table 1 provides the details of the deposition conditions used to fabricate the coating specimens. Specimens with Cr-a:C and Cr-WC (Fig. 1a and 1b) coatings were fabricated to serve as references for comparing the behaviors of the multilayer coatings. Furthermore, FMCs comprising of 120-200 layers or 60-100 pairs of a:C/WC were prepared (Fig. 1c and 1d). The total thickness of the FMCs was limited by the residual stress generated during the deposition process. In this study, attempts were made to achieve a positive synergistic effect for enhanced mechanical durability and corrosion passivation through a combination of four basic concepts explained below.


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Figure 1. Schematic view of the design of functional multilayer coatings (FMCs): (a) Bi-layer Cr/a:C (FMC#1), (b) bi-layer Cr/WC (FMC#2), (c) 60 pairs a:C/WC and Cr sublayer (FMC#3) and (d) 70 (FMC#4) and 100 (FMC#5) pairs of a:C/WC and Cr sublayer.

3.1.1. Selection of constituent materials The first step was to select materials that can provide desirable properties with respect to stiffness, hardness, and corrosion resistance. Once these materials were identified, the synergistic benefits of these materials could be derived by stacking them in a strategic sequence. To fabricate the FMCs, Cr, WC and a:C were selected. Cr is a well-known conventional coating material frequently used in various grades of steels as either a monolayer or a sublayer for carbon-based coatings. In the FMC, Cr was used because of its good corrosion resistant property.27 It has also been extensively used as a successful agent for improving the adhesion of the top coating layers to the steel substrate.47 Furthermore, Cr exhibits high stiffness (elastic modulus of 300 GPa) which would be beneficial to resist the shear stress generated by the contact stress. Lastly, Cr shows no or low tendency to intermix with carbon and steel.9


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As a second layer, a relatively soft structure of a:C was selected to provide sufficient compliance that would be necessary to absorb the frictional energy generated during contact sliding. Furthermore, the resistance of certain a:C coatings to corrosive media have been welldocumented.25,32,48 WC was the third constituent selected for the multilayer coating due to its high hardness (~19 GPa), high stiffness (~600 GPa), and chemical inertness. Apart from the superior inherent physical and mechanical properties, WC can resist acids and is known to be attacked only by hydrofluoric acid/nitric acid (HF/HNO3) mixtures above room temperatures, and it is unreactive to dry H2 up to its melting point.40,43 More importantly, radiation and temperature resistance of WC makes it a favorable cemented coating for applications in harsh conditions.10,16,44

3.1.2. Stress, hardness, and elasticity control The second fundamental concept underlying the design of FMCs relates to the high load bearing capacity of the multilayer structure compared to that of a single layer. Such characteristics of a multilayer coating have been studied in some recent studies on Si/C bilayer49 and Co/C multilayer29 tribological coatings. In the former case, the combination of the two amorphous materials with different mechanical properties enabled a significant reduction in the macro-scale wear by means of bi-layered structures with thicknesses of 20−200 nm. In the latter case, a multilayer coating consisting of periodic structures of amorphous carbon and amorphous cobalt with a total thickness of few hundred nanometers was introduced to effectively decrease the coefficient of friction and microscale wear. The ability of the coatings to elastically deform under the applied load was the prominent underlying mechanism in both cases. However, the applicability of these coatings under harsh conditions would be limited because of the very high


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stiffness of silicon in the bilayered structure, as well as insufficient corrosion resistance properties of Co/C or Si/C, which could result in a rapid increase in wear and corrosion.

3.1.3.

Layout design and orders of deposited layers

The third fundamental concept of the coating design relates to constructing the multilayer coating in a specified order so that the periodic pairs of a:C and WC function effectively. The strategy of coating a hard and high-shear-strength material with a thin layer of a softer, lowshear-strength material was presumed to be effective in reducing friction and wear. A previous study has shown that by stacking layers of soft and hard materials, a functional coating with extremely low wear could be obtained.29 Based on this concept, a multilayer coating structure consisting of stiff Cr sublayer, soft a:C, and hard and high strength WC was designed.

3.1.4.

Amorphous structure of individual layer

Lastly, the amorphous structure was preferred for both constituents of the FMCs. WC and a:C layers were deposited to be extremely thin, with a thickness corresponding to the critical threshold of forming an amorphous structure. The thickness of the individual layers could be controlled by the deposition time, which was calibrated by the deposition rate. As high elasticity29 and grain boundary free structure27 were desirable to achieve good mechanical properties and high corrosion resistance, respectively, crystalline coating structures were not preferred. Furthermore, amorphous materials not only show higher elasticity compared to their polycrystalline counterparts,50 but also have much lower probability of corrosion because there are no grain boundaries that can serve as potential defect sites. In particular, pinhole-free coating, lack of grain boundaries and sufficient number of pairs are the main requirements for the


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passivation of corrosion.27,51 The threshold thickness of the crystallite formation of a:C and WC was precisely calibrated by means of HR-XRD and TEM analyses. As a consequence, sufficiently thin layers of a:C and WC could be fabricated. Based on the concepts mentioned above, a multilayer coating structure was designed as shown in Fig. 1. The basic construction of FMCs consisted of a Cr sublayer and stacks of consecutive pairs of a:C and WC.

Table 1. Deposition parameters and characteristics of the functional multilayer coatings.

Coating conditions Coating Name

Design of Multilayers Working pressure (mTorr)

Power

Thickness (nm) Sub layer

Cr

WC

FMC#1

Cr(sublayer) – a:C 2 layers

3

RF: 150 watt

FMC#2

Cr (sublayer) – WC 2 layers

3

RF : 150 watt

RF : 100 watt

FMC#3

Cr (sublayer) – a:C- WC 60 pairs

3.8-4

RF : 150 watt

RF : 100 watt

FMC#4

Cr (sublayer) – a:C- WC 70 pairs

3.8-4

RF : 150 watt

FMC#5

Cr (sublayer) – a:C-WC 100 pairs

3.8-4

RF : 150 watt

WC

C

C

DC: 250mA 14 kHz Bias voltage: 40 volt

80

200

80

200

DC : 250mA 14 kHz-Bias voltage : 40 volt

100

3.5

1.5

RF : 100 watt


200

3.5

1.5

RF : 100 watt


200

3.5

1.5


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The top deposited layer was a:C with a couple of nanometers in thickness (~2 nm). This layer was intended to protect the bottom stack layers from oxidation and to serve as a relatively soft, viscoelastic layer to provide ease of shear at the contact interface.52

3.2. Structure of the functional multilayer coatings The high-resolution cross-sectional transmission electron microscopy (Cs-corrected-STEM) images of a:C/WC FMCs, the corresponding EDX, and selected area electron diffraction pattern (SADP) of a typical FMCs with 60 and 100 pairs of a:C and WC layers are presented in Fig. 2. The magnified HR-TEM image in Fig. 2a, 2b, and 2c show the periodic stacks of dark (WC) and light (a:C) stripes. The thickness of each individual layer precisely matched the values calibrated by the deposition time, which was 3.5-4 nm and 1.5-2 nm for WC and a:C, respectively. The corresponding SADP of the Chromium sublayer showed (magnified image in the inset of Fig. 2b) a body centered cubic (BCC) crystal structure which consisted of disordered atoms in the crystal plains. Blurred and wavy rings and dots of the SAD pattern observed for Cr indicated a disordered structure. On the other hand, the multilayer region, which exhibited continuous rings, indicated an amorphous structure (Fig. 2c). The compatibility of the amorphous structure of stacks of a:C/WC with highly ordered FCC structure of 304 steel substrate was enhanced more by the intermediate disordered structure of the Cr sublayer. Each individual layer was shown to be solely consisted of its composition only. HR-EDX analysis of the 30 nm line selected within the multilayer in Fig. 2d and 2e clearly demonstrated that there was no intermixing of WC and a:C layers. Particularly, Fig. 2e showed that the overall atomic ratio of W and C within the WC


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layer was close to 50:50 as expected. This could be achieved by using a 99.999% pure WC target during the sputtering process to deposit the WC layer.

Figure 2. (a) HR-TEM of (a) a typical cross-section of FMC with 60 pairs, (b) magnified image of the Cr sublayer with the corresponding SADP and few consecutive a:C/WC nanolayers. (c) Magnified image of the consecutive a:C/WC layers with the corresponding SADP showing the


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amorphous structure of the layers. (d) Nanoscale line HR-EDX measurement area of FMC and (e) the corresponding atomic composition measurement by the HR-EDX line scan. Furthermore, no intermixing was observed between Cr and the first a:C layer of the FMCs. As shown in Fig. 2a, an ultra-thin oxide layer (< 5 nm) was formed in the initial stage of sputtering from the Cr target on the steel substrate. Amorphous structures of both C and WC were also confirmed by Raman shifts and high-resolution, in-plane (Grazing Incidence (GI-mode)) XRD pattern (HR-XRD) in Fig. 3a-d. Basically, broad peaks of the HR-XRD for all three FMCs (Fig. 3a) roughly confirmed poor or lack of crystallinity of the coating structure. It also revealed that as the number of layers increased (100 pairs) the cumulative peaks (both (111) and (200) planes of WC as well as the C peak) became broader. It should be noted that C and WC typically show overlapped characteristic peaks in the range of 2θ = 22–30°.34 Increasing the number of layers induced a more amorphous structure and resulted in less intense and broader XRD peaks (magnified image in the inset of Fig. 3a). Raman peaks corresponding to FMC#3 with 60 pairs (Fig. 3c) had an approximate intensity 10 folds lower compared to the absolute intensity of the single layer of amorphous carbon (Fig. 3b). On the other hand, the Raman peaks of FMC#5 with 100 pairs (Fig. 3d) showed much lower characteristic carbon bond intensities. The fixed positions of the D and G peaks in the single layer and 60 pairs of FMCs (Fig. 3a and 3b), as well as the fixed positions of the characteristic WC vibrations in 60 and 100 pairs of FMCs (Fig. 3c and 3d) revealed that the a:C and WC structures remain identical in different FMCs. However, in the case of 100 pairs (FMC#5) the WC vibration bond dominated the spectrum.


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Figure 3. (a) High-resolution, in-plane, GI-mode X-ray diffraction pattern for FMCs #3, #4, and #5. Inset shows a magnified pattern of the selected section. Raman spectra showing the D, G, and W-C band peaks obtained with the excitation wavelength of 532 nm for (b) FMC#1, (c) FMC#3, and (d) FMC#5.

3.3. Mechanical properties of functional multilayer coatings 3.3.1. Hardness and elastic modulus The mechanical properties of the coatings were assessed by the UNHT method. Different indentation loads were used with respect to the thickness of the coatings to maintain the


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indentation depth to less than 10 percent of the coating thickness and thereby avoid the substrate effect. Elastic moduli and hardness values of the coatings with respect to the coating types are shown in Fig. 4a. The intrinsic elastic modulus and hardness of the deposited monolayer of WC were higher than those of the monolayer of a:C, satisfying the third concept of FMC layout design described earlier. The effect of various bias voltages on the hardness and modulus of FMC#5 coating is shown in Fig. 4b. The maximum hardness with a relatively large H/E (~0.1) ratio was achieved for FMC#5 with a bias voltage of 40 V (Fig. 4b). This outcome was thought to be due to the increased ratio of IG/ID (graphite to disordered peak ratio) and strong WC bonds generated with a 40-volt bias voltage (Fig. 3 b and Fig. 3d). With respect to the bias voltage for the deposition of a:C and WC, the WC peaks were more pronounced in case of FMC#5 (Fig. 3d). However, Fig. 4a shows that, while other deposition parameters were kept constant, the elastic modulus of the coatings did not change significantly because of the increase in the number of coating pairs (FMC#4 and #5). This result was in agreement with similar trends observed in a previous study on the multilayer coatings of Co/C reported by Penkov et al.29 The effective modulus (E*) of a composite coating generally depends on the modulus and ratio of the individual components. Consequently, as long as the ratio of the individual components in a single a:C/WC layer is fixed, the effective modulus of the multilayer does not change remarkably. Average load-unload curves for the FMCs tested with the pyramid indenter are presented in Fig. 4c. The results showed that by modifying the design of the coating (from FMC#3 to FMC#5), as expected, the maximum indentation depth (ultimate Pd) of the diamond indenter into the coating decreased with increasing hardness and stiffness of the coating. From the measured values of H and E, the resistance of the coating to plastic deformation indicated by H3/E*2 ratio, was obtained. It is generally known that the higher the resistance to


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plastic deformation, the higher is the H3/E*2 ratio (Fig. 4d).53 FEM simulations were performed to validate the experimental results as explained in the following sections.

Figure 4. Elastic modulus and hardness values with respect to (a) Multilayer coatings. (b) Bias voltage for the deposition of FMC#5. (c) Load-unload curves of UNHT for FMCs. (d) Plastic deformation resistance (H3/E*2) values with respect to the multilayer coatings.

3.3.2. Mechanical durability The mechanical durability of the FMCs was examined by conducting wear experiments using a custom-built, ball-on-flat reciprocating tribotester.46 First, friction and wear behaviors were assessed over 1.1 × 104 cycles at applied normal loads of 10 and 50 mN. Corresponding Hertzian


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contact pressures with respect to the mechanical properties and thickness of each individual coating were calculated by COMSOL 5.1 in 2D mode as shown in Table 2. Fig. 5a, presents the coefficient of friction with respect to the sliding cycles for FMC#5. As shown, at the relatively high Hertzian contact pressure of 0.88 GPa (50 mN), the coefficient of friction was reproducible (average of 0.12) and remained stable for more than 104 sliding cycles (Fig. 5a). AFM analysis of the wear track revealed that the depth of the wear area was found to be slightly lower than 3 nm for this relatively large number of sliding cycles. This resulted in a very low wear rate of 2.7 × E–12 mm3/N.mm.

Table 2. Hertzian contact pressure for functional multilayer coatings and 304 SUS.

Hertzian contact pressure

Hertzian contact pressure

under 50 mN

under 10 mN

(GPa)

(GPa)

Bare 304 SUS

0.88

0.41

FMC#1

0.81

0.37

FMC#2

0.83

0.32

FMC#3

0.84

0.36

FMC#4

0.85

0.37

FMC#5

0.87

0.35

Specimen No.


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Figs. 5b, 5c, and 5d present the 3D topography and 2D surface profile of the wear track of FMC#5 after 1.1 × E4 sliding cycles. As shown in Fig. 5d wear occurred only in few top layers of the multilayer structure (~3 nm, one pair of a:C/WC). The inset of Fig. 5a schematically shows the degree of wear with respect to the thickness of the multilayer coating. To determine the mechanical durability of the coatings, prolonged sliding tests up to 1.5 × E5 cycles with the same load and the same sliding speed were conducted (corresponding to 60 h of dry sliding in reciprocating motion). Fig. 6 shows the coefficient of friction and wear behavior of the FMCs obtained from the prolonged durability tests.


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Figure 5. (a) Coefficient of friction with respect to the number of sliding cycles for FMC#5 under 50 mN, inset scheme of (a) shows the wear occurring only in few top layers of the multilayer coating after 11000 cycles. (b) 3D and (c) 2D AFM images of the corresponding wear track after 11000 sliding cycles under 50 mN load, and (d) 2D profile of the wear showing the corresponding wear depth.


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The coefficient of friction remained stable between 0.11~0.2 even after 1.5 × E5 cycles, for both FMC#4 and #5 coatings as shown in Fig. 6a. This range of coefficient of friction was quite low considering the fact that the sliding tests were conducted in dry condition. As for the wear characteristics, Fig. 6b and 6c, the 3D topography and 2D profile of the wear track of FMC#5 indicated that the wear depth was extremely shallow even after the large number of sliding cycles under 0.88 GPa contact pressure, with some indication of burr formation.

Figure 6. (a) Coefficient of friction with respect to the number of prolonged sliding cycles for FMC#4 and FMC#5 under 50 mN. (b) Corresponding 3D wear profile of FMC#5, (c) Corresponding 2D wear profile showing the wear depth in (b) and (d) High resolution 2D AFM images and 2D profile of the corresponding wear track after 1.5 × E5 sliding cycles. To quantify the depth of the wear track, AFM image was taken over an area of 40x40 µm2 on the wear track of FMC#5 as shown in Fig. 6d. As can be seen from the 2 D profile of the wear track,


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the depth of the wear track with respect to the horizontal plane to the left and right of the wear track was ~7 nm and ~12 nm, respectively. The superb wear resistance and stability of coefficient of friction of FMC#4 and #5 indicated that these coatings had a high mechanical durability. The mechanism of wear reduction of FMCs was due to synergistic effects of hardness and elastic deformation which was described earlier in design concepts and confirmed by enhanced H3/E*2 and H/E values described in the previous section.

3.4.

FEM simulation results

In order to understand the stress distribution state within the coating during contact, FEM simulations were performed. For this purpose, three models (A, B and C) were constructed as follows: (A) 2-layer model comprised of substrate/Cr (80 nm)/a:C (200 nm); (B) 2-layer model comprised of substrate/Cr (80 nm)/WC (200 nm); (C) 4-layer model comprised of substrate/Cr (80 nm)/a:C (50 nm)/WC (100 nm)/a:C (50 nm). Essentially, models A and B represented FMC#1 and FMC#2, respectively. As for model C with 4 layers, it was constructed to compare the stress distribution behavior with the single layer coatings with the same total thickness. Distribution of the shear stress within the coating under 50 mN (0.88 GPa) normal load was analyzed. The actual mechanical properties determined by UNHT for a:C and WC layers were applied to all FEM models. The shear stress distribution with respect to the depth from the contact point for the 3 models under 50 mN normal load, after 2 s of loading are shown in Fig. 7. In the multilayer model (Fig. 7c), the maximum shear stress was located near the contact point of the top layer, whereas in the single-layer models, the shear stress was distributed over a greater depth. Furthermore, the extent of high shear stress distribution for the single layer WC model (Fig. 7b) was less than that of the single layer a:C model (Fig. 7a).


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Figure 7. FEM simulation of the contact between FMC and a counter surface (ball): The shear stress distribution in the FMCs after the application of normal force on the counter surface under the 50 mN normal load: (a) Model A: 2-layer model comprised of: 80 nm Cr/200 nm a:C, (b) Model B: 2-layer model comprised of: 80 nm Cr/200 nm WC, (c) Model C: 4-layer model comprised of: 80 nm Cr/50 nm a:C/100 nm WC/50 nm a:C. The FEM simulation results revealed that the state of shear stress distribution could be effectively controlled by adjusting the layout of the coating structure as explained in a previous section. Particularly, the benefit of the 4-layer coating structure compared to the 2-layer coating structure in minimizing the shear stress within the coating under a given load was verified. This outcome was consistent with the design strategy of the multilayer coating, which was further


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confirmed by the low wear rate of FMC#4 and FMC#5 specimens that consisted of a multilayer structure.

3.5. Corrosion behavior of functional multilayer coatings EIS was performed to assess the corrosion resistance and electrochemical properties of the FMCs.54 Steady state conditions prior to performing the impedance measurements was achieved within two hours of immersion time.55,56 Fig. 8a shows the open-circuit potential (Eocp) obtained in the steady state condition for various FMCs and bare 304 SUS substrate. The Eocp values for the bare steel substrate, FMC#1, and FMC#2 (single layer coatings) after immersion for 2h were in the range of –200 to –250 mV while that of the FMCs showed remarkably higher Eocp values. Significantly improved values of Eocp for FMCs with higher number of pairs (FMC#4 and #5), was due to the multilayer nature and dense structure of the coating with a pinhole-free microstructure (Figs. 2a and 2b), which impeded the penetration of the corrosive solution compared to single layer coatings of WC and a:C.51 Furthermore, the higher Eocp of FMCs indicated that the multilayer coating was thermodynamically more stable compared to the single layer coating of the parent constituents. Fig. 8b depicts the corresponding Nyquist plot of various FMCs. The magnified Nyquist plot is shown in the inset to illustrate the large differences in the corrosion resistances of bare 304 SUS, single layers, and multilayers. As shown in Fig. 8b, the shape of the plots and hence, the involved corrosion mechanisms were not the same for 304 SUS and FMCs. Moreover, considering their relatively large diameters, FMCs apparently provided superb protection compared to the uncoated steel and those coated with a single layer. Given the fact that coated steel was locally corroded when the corrosive electrolyte found a path to penetrate through and


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reach the substrate, the coating could act as a leaky capacitor. The equivalent circuit shown in Figure 8c was chosen to analyze the EIS curves. This circuit includes two time constants attributed to the capacitive behavior of the coating layer. RS, Rf, and Rct elements represent the solution, film, and charge transfer resistances, respectively. The heterogeneity of the solution system is compensated by the constant phase element (CPE).57,58 CPE (dl) represents the constant phase element, which acts as the double layer capacitance of the interface between the substrate and solution.

Figure 8. (a) Open-circuit potential (Eocp) obtained in the steady state condition for various FMCs and bare 304 SUS substrate, (b), The Nyquist impedance curves for various FMCs in 3.5% NaCl solution under ambient conditions and (c) the equivalent circuit model used to fit the obtained impedance spectra. The electrochemical parameters calculated by fitting the theoretical impedance plots of the equivalent circuits to the experimentally plotted ones for the coating and bare 304 SUS are listed


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in Table 3. The Rp values reported in Table 3 denote the polarization resistant-corrosion protection criteria and are the summation of Rct and Rf determined from the impedance curves.59 According to the significant increase in Rp values for FMC#4, #3, and #5 and given the chemically homogenous and uniform defect-free structure of the barrier film (amorphous structure without grain boundaries), it can be concluded that the application of a dense, inert, and amorphous WC and carbon coating protected the 304 SUS specimens by restricting the penetration of corrosive species through the substrate.60 It should be mentioned that a principal indicator for proper identification of the corrosion resistance of coating materials is through measurement of Icorr (corrosion current density) and Ecorr (corrosion potential). However, the polarization curves for all specimens used in this study were initially tested. It was found that due to the very high corrosion resistance of the coating material as indicated by the EIS measurements, the magnitude of the obtained corrosion current density was not detectable even by the high quality potentiostat instrument. Therefore, the EIS measurement was used as the main criterion for evaluating the corrosion resistance of all the specimens.

Table 3. EIS parameters obtained for the functional multilayer coatings and bare 304 SUS in 3.5% NaCl solution.

Specimen No.

Bare 304

Rs

CPE-T1

Rf

CPE-T2

CPE-P1 ( Ω.cm2)

(µF/cm2)

25.3

-

-

Rct

Rp

(kΩ.cm2)

(kΩ.cm2)

90.2

70.2

CPE-P2 (kΩ.cm2)

(µF/cm2)

-

12.34

0.86


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SUS FMC#1

21.2

6.39

0.89

27.9

4.02

0.80

447.8

475.8

FMC#2

31.0

1.03

0.94

33.0

3.74

0.78

1544.0

1577.2

FMC#3

19.8

9.91

0.92

44.7

3.45

0.88

2815.4

2860.1

FMC#4

21.9

5.7

0.92

50.1

3.28

0.91

3874.7

3924.8

FMC#5

20.8

6.2

0.91

66.9

3.11

0.86

5508.5

5575.4

Figs. 9a and 9b schematically show the corrosion process of the multilayer coating compared to that of the single layer. Essentially, the enhanced corrosion resistance of the multilayer coating could be attributed to introduction of new interfaces and deterrence of diffusion of the corrosive species. As a result, the polarization resistance increased up to 5.5 × 106 Ω cm2 for FMC#5, which indicated the superior corrosion protection ability of the a:C/WC multilayer coating. Closer inspection of an individual layer indicated that the amorphous structure of the layers and minimized intermixing of the parent materials at the interfaces of the counterparts in multilayer design played crucial roles in the anticorrosion behavior of the coating. As there were no crystallite and no grain boundary in the deposited layers, there was no potential site or defect to allow seepage of the corrosive agent. It has been previously reported that amorphous thin film layers of Nb2O5 and Ta2O5 served as remarkable barriers for aluminum in caustic environments.21 Furthermore, the corrosion resistance of coatings could be increased significantly by the multilayer coating structure.51,61 Compared to the best results of the most recent similar corrosion test reports (2 × 105 Ω cm2 resistivity) for carbon based multilayer coatings on 304 SUS,48 results of the present study showed an enormous increase in the corrosion resistance by approximately an order of magnitude.


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Figure 9. Anti-corrosion nature of the (a) multilayer coatings compared to (b) single layer counterparts through the formation of new interfaces, providing the lateral spreading of the corrosive agent rather than directly penetrating into the substrate.

4.

Conclusions

Novel functional multilayer coatings comprised of periodically stacked nanolayers of amorphous carbon and WC that are highly durable and extremely resistant to corrosion were developed. Experimental results showed that FMC with 100 pairs of a:C/WC with a Cr sublayer had an extremely low wear rate of 2.7 × E–12 mm3/N·mm under a relatively high contact pressure of 0.88 GPa. Also, a low coefficient of friction of ~0.15 could be maintained for a prolonged period up to 1.5 x E5 cycles. Furthermore, the corrosion resistance (polarization resistance of 5.5 × E6 Ω cm2) of the FMC was far superior than the most recently developed anticorrosion coatings for steel. The effectiveness of the multilayer coating structure in lowering the shear stress within the coating under contact was demonstrated through FEM simulation. It is expected that the FMCs developed in this work may be applied to prolong the life of mechanical components used in harsh conditions.

AUTHOR INFORMATION


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Corresponding Author * Email:[email protected] (D.-E.K.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2010-0018289) and Brain Korea 21 Plus Project in 2017. ABBREVIATIONS FMCs, Functional Multilayer Coatings; TEM, Transitions electron Microscope; FEM, Finite Element Method; Atomic Force Microscope (AFM). REFERENCES (1)

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(50) Fan, C.; Li, C.; Inoue, A.; Haas, V. Deformation Behavior of Zr- Based Bulk Nanocrystalline Amorphous Alloys. Phys. Rev. B: Condens Matter Mater. Phys. 2000, 61, R3761−R3763. (51) Rashmi, S.; Elias, L.; Hegde, A. C. Multilayered Zn-Ni Alloy Coatings for Better Corrosion Protection of Mild Steel Eng. Sci. Technol. an Int. J. 2016, 22, 1–11. (52) Martinez, D. M.; Hosson, J. T. M. De. On the Deposition and Properties of DLC Protective Coatings on Elastomers : A Critical Review Surf. Coat. Technol. 2014, 258, 677–690. (53) Musil, J. Hard and Superhard Nanocomposite Coatings Surf. Coat. Technol. 2000, 125, 322–330. (54) Ehsani, A.; Mahjani, M. G.; Hosseini, M.; Safari, R.; Moshrefi, R.; Shiri, H. M. Evaluation of Thymus Vulgaris Plant Extract as an Eco-Friendly Corrosion Inhibitor for Stainless Steel 304 in Acidic Solution by Means of Electrochemical Impedance Spectroscopy , Electrochemical Noise Analysis and Density Functional Theory J. Colloid Interface Sci. 2017, 490, 444–451. (55) Zhang, T. F.; Deng, Q. Y.; Liu, B.; Wu, B. J.; Jing, F. J.; Leng, Y. X.; Huang, N. Wear and Corrosion Properties of Diamond like Carbon ( DLC ) Coating on Stainless Steel, CoCrMo and Ti6Al4V Substrates Surf. Coat. Technol. 2015, 32, 652–863. (56) Tuominen, J.; Vuoristo, P.; T. Ma¨ntyla¨, Ahmaniemi, S.; Vihinen, J.; and Andersson , P.H. Corrosion Behavior of HVOF-Sprayed and Nickel-Chromium Coatings J. Therm. Spray Technol. 2002, 11, 233–243.


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