Electrochemical Properties of Tiwn/Tiwc Multilayer Coatings Deposited by RF-Magnetron Sputtering on AISI 1060

coatings

Article Electrochemical Properties of TiWN/TiWC Multilayer Coatings Deposited by RF-Magnetron Sputtering on AISI 1060

Andrés González-Hernández 1,2,*, Ana Beatriz Morales-Cepeda 2, Martín Flores 3 , Julio C. Caicedo 4, William Aperador 5 and César Amaya 6

1 Faculty of Engineering, Universidad Autónoma de Tamaulipas, Centro Universitario Tampico-Madero Zona sur, Tampico 89109, Tamaulipas, Mexico 2 Petrochemical Research Center, Tecnológico Nacional de México/Instituto Tecnológico de Ciudad Madero, Altamira 89600, Tamaulipas, Mexico; [email protected] 3 Project Engineering Department, CUCEI, Universidad de Guadalajara, Jalisco 44430, Mexico; maﬂ[email protected] 4 Tribology, Polymers, Power Metallurgy and Processing of Recycled Solids Research Group, Universidad del Valle, Cali 76001, Colombia; [email protected] 5 Faculty of Engineering, Universidad Militar Nueva Granada, Bogotá 111111, Colombia; [email protected] 6 Technological Center, Laboratory of Hard Films, CDT-ASTIN SENA, Cali 760004, Colombia; [email protected] * Correspondence: [email protected]; Tel.: +52-(833)-2412000 (ext. 3335)

Abstract: Nitride and carbide ternary coatings improve the wear and corrosion resistance of carbon steel substrates. In this work, Ti-W-N and Ti-W-C coatings were deposited on AISI 1060 steel substrates using reactive radio frequency (RF) magnetron sputtering. The coatings were designed as monolayers, bilayers, and multilayers of 40 periods. The coatings were obtained with simultaneous sputtering of Ti and W targets. The microstructure, composition, and electrochemical properties were Citation: González-Hernández, A.; investigated by techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Morales-Cepeda, A.B.; Flores, M.; scanning electron microscopy (SEM), atomic force microscopy (AFM), electrochemical impedance Caicedo, J.C.; Aperador, W.; Amaya, C. Electrochemical Properties of spectroscopy (EIS), and potentiodynamic polarization. XRD results shower a mix of binary TiN TiWN/TiWC Multilayer Coatings and W2N structures in the Ti-W-N layer, a ternary phase in Ti-W-C layers, in addition of a quater- Deposited by RF-Magnetron nary phase of Ti-W-CN in the multilayers. The analysis of the XPS demonstrated that the atomic Sputtering on AISI 1060. Coatings concentration of Ti was more signiﬁcant than W in the Ti-W-N and Ti-W-C layers. The lowest corro- 2021, 11, 797. https://doi.org/ sion rate (0.19 mm/year−1) and highest impedance (~10 kΩ·cm2) out of all coatings were found in 10.3390/coatings11070797 n = 40 bilayers. In the simulation of equivalent electrical circuits, it was found that the Ti-W-N coating presented three processes of impedance (Pore resistance + Coating + Inductance). However, the Received: 23 April 2021 multilayer (n = 40) system presented a major dielectric constant through the electrolyte adsorption; Accepted: 28 June 2021 therefore, this caused an increase in the capacitance of the coating. Published: 1 July 2021

Keywords: hard coatings; monolayer; bilayer; forty-periods; corrosion Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction In the last few decades, transition-metal nitride and carbide coatings have been extensively used to enhance the life and performance of cutting tools and mechanical components. Conventional binary hard coating such as TiN, CrN, and TiC can improve Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the wear and corrosion resistance of metallic substrates, such as carbon steels [1]. These This article is an open access article coatings are applied to cutting, surface finishing, and other manufacturing processes to distributed under the terms and increase wear resistance, improve finishing [2], reduce friction, and improve chemical conditions of the Creative Commons resistance in components exposed to corrosive environments [3,4]. In tribological appli- Attribution (CC BY) license (https:// cations, the brittleness of some transition-metal nitrides is a drawback. Furthermore, the creativecommons.org/licenses/by/ porosity of coatings can weaken the corrosion resistance of the composite coating-substrate 4.0/). since the electrolyte forms a galvanic pair with the noble coating and active substrate [5].

Coatings 2021, 11, 797. https://doi.org/10.3390/coatings11070797 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 797 2 of 18

In order to increase the oxidation resistance at high temperatures, improve the tribological performance and increase the corrosion resistance of conventional binary coatings, ternary nitride and carbide coatings, such as TiAlN, Ti-W-C, and Ti-W-N, have been investigated, while Ghufran et al. [6]; and Akito et al. [7], registered a patent invention (#US20200156213A) denominated “Super-Abrasive Grain and Super-Abrasive Grinding Wheel”, grinding ternary nitride and carbide-based on Ti. The industrial sector of polymer petrochemistry has demanded improvement in the cutting of poly-vinyl chloride (PVC) profiles. The main problems presented in these systems are the deterioration and corrosion in the cutting devices. There are different methods to reduce these problems, such as modifying the tools’ surface [8,9], and through the use of hard carbide and nitride coatings deposited by reactive RF-magnetron sputtering. Tungsten-nitride (W-N) deposited on steel tool substrates improves their hardness and deterioration resistance [10]. Coatings of TiN and W2N have been successfully deposited on steel tools [11]. In order to improve adherence between the coating and the substrate, a metal interface has been included in the systems, for example, the deposition of a titanium (Ti) buffer layer in a titanium nitride (TiN/Ti) coating [12]. Another benefit of using pure Ti as an interlayer is the improvement of the corrosion behavior of the n = 40 nanostructured Ti/TiN [13]. With this, the corrosion rate decreases even further since the interface provides lower permeability to the coating, diminishing the defects and pores between the columns that reach the substrate [14,15]. The presence of many interfaces between individual layers of a multi-layered structure results in a drastic increase in hardness and strength [16]. Thus, multilayer coatings have also gained the interest of researcher due to their advantages over single coatings. Addi- tionally, multicomponent coatings based on different metallic and non-metallic elements combine the benefits of different individual components, leading to a further refinement of coating properties. Carbon steel 1060 is used in the cutting process of PVC, in which the steel is in contact with the chloride polyvinyl polymer matrix, which produces abrasive deterioration [17]. In addition to this, the high temperatures in the system ease the detachment of hydrogen chloride, promoting generalized corrosion of the metal due to the steel depassivation process [18]. The use of metal nitrides has been suitable for industrial applications, thus a comparison between nitrides has been established; for example, between Ti-W-N and Ti-W- C, being determined that the latter have better electrochemical properties [16]. Considering the above, Ti-W-N/Ti-W-C coatings are expected to present better mechanical properties, as well as other properties relevant to applications of elements subjected to high wear conditions [17]. Unfortunately, the literature presents little research focused on studying the electrochemical properties of Ti-W-N/Ti-W-C multilayers in aggressive environments. Although some authors [18,19] have studied the physical properties of steel substrates coated with single Ti-W-N layers, these studies do not relate to the Ti-W-N/Ti-W-C coating’s performance in aggressive environments. Therefore, the present work aimed to study the structural, morphological, and electrochemical behavior of Ti-W-N/Ti-W-C multilayer coatings on AISI steel substrates, in aggressive environments, with potential applications in the metal-mechanic industry. To this end, four different coating systems were deposited by the reactive RF- magnetron sputtering technique, specifically two monolayer coatings of ternary Ti-W-N and Ti-W-C, one bilayer coating of Ti-W-N / Ti-W-C (period n = 1), and a forty-bilayer coating of Ti-W-N/Ti-W-C (n = 40).

2. Materials and Methods 2.1. Materials For the deposition of the Ti-W-N and Ti-W-C coatings, high purity target discs of Ti (99.99%) and W (99.95%) with a 10 cm diameter and 5 mm thickness were used. For the plasma formation, argon (Ar) was used as the working gas for the carbides, and a mixture of Ar (40%) and high purity nitrogen gas (60%) was implemented for the nitrides. For the carbide compounds, methane (CH4) was used as a reactive gas. The dimensions of the carbon steel samples used as substrates were 28 mm in length, 28 mm in width, and 3 mm Coatings 2021, 11, 797 3 of 18

in height, consisting of 0.65% carbon content. For a cross-section analysis, silicon wafers (100) with an approximate area of 1 cm2 were also coated. The metallic substrates were sanded with 100, 220, 400, and 600 grit sandpapers and later washed with industrial soap and dried in a forced-air (Thermo Fischer Scientif, Waltham, MA, USA) stove at 100 ◦C for 30 min. All substrates were cleaned by immersion in an acetone/isopropyl alcohol ultrasonic bath for 10 min. Electrochemical analyses were performed using a 3.5 wt.% aqueous solution of sodium chloride (NaCl) in distilled water.

2.2. Coating’s Deposition The coatings were deposited via reactive RF-magnetron sputtering in a vacuum system using a power supply of 13.56 megahertz (MHz) of frequency and applying a negative bias of −10 V on the substrate. The substrates were placed on a rotatory system in the middle of both magnetrons with the Ti and W targets. The distance between the targets and the samples was 10 cm, with the substrate being heated to a temperature of 300 ◦C. The baseline pressure before the deposition process was 4 × 10−4 Pa. Samples were cleaned before the deposition process by plasma etching in the vacuum chamber for 20 min. The adhesion coating interlayers of pure Ti were deposited applying 450 watts of RF power and voltage bias of −10 V on the substrates for 10 min at 300 ◦C; and injecting Ar gas at a mass ﬂow of 50 standard cubic centimeters per minute (sccm). The work pressure of the sputtering chamber was kept constant at 1.46 Pa for all deposited layers. For the deposition of the coatings, a RF-power of 350 and 420 W were applied to the Ti and W targets during the three hours of deposition. The n = 1 and n = 40 multilayer coatings were deposited by alternating the nitride and carbide monolayers’ parameters, as shown in Table1.

Table 1. Deposition parameters and coating thicknesses for each coating: adhesion layer, monolayers, n = 1 bilayer and n = 40 bilayers.

Parameter Ti Ti-W-N Ti-W-C n = 1 n = 40 Power (W) Ti 450 350 350 350 350 W – 420 420 420 420 Flux (sccm) Ar 50 50 50 50/50 50/50 N2 0 12 0 12/0 12/0 CH4 0 0 16 0/16 0/16 Deposition time (min.) 20 180 180 90/90 (2.25/2.25) Total Thickness (µm) 0.21 2.81 3.54 1.57 + 1.20 = 2.77 3.57

2.3. Coatings’ Characterization Structure analysis of the coatings was performed by X-Ray Diffraction (XRD) using a Malvern Panalytical X-ray diffractometer (Almelo, The Netherlands) with a Cu kα (λ = 1.5406 Å), and the Bragg-Brentano conﬁguration with a 2θ scanning scale from 20◦ to 80◦. Chemical analysis was accomplished using the X-ray photoelectron spectroscopy (XPS) SAGE HR 100 (SPECS, Berlin, Germany) system. Samples were placed in an ultra-high vacuum (8 × 10−8 mbar.) previous to the XPS characterization. Following this, the binding energy was centered according to the location of the main peaks with 1.0 eV of full width at half maximum (FWHM). Coating thickness cross-section analyses were obtained by scanning electron microscopy (SEM, JEOL Ltd., Tokio, Japan), using the JEOL JSM 6490LV model equipment (Tokio, Japan) operating at 20 keV and on the secondary electron’s mode. Also, the sample preparation for the cross-section observations using transmission electron microscopy (TEM, JEOL Ltd., Tokio, Japan) was carried out using a focused ion beam (FIB) with a JEOL JEM-9320FIB equipment (JEOL Ltd., Tokio, Japan). The high-resolution (HR- TEM) images were acquired in scanning-TEM (STEM) mode with a JEOL JEM 2200Fs + Cs equipment operating at 200 keV. Coatings 2021, 11, 797 4 of 18

The roughness and topography analyses of the coatings were carried out using atomic force microscopy (AFM, Nanosurf, Liestal, Switzerland) in contact mode through an Asylum Research equipment model MFP-3D (MFP-3D-Stand Alone, Asylum Research, Abingdon-on-Thames, UK) on an area of 45 µm × 45 µm. A review of the images was executed using the Gwyddion 2017 version software (Czech Metrology Institute, Brno, Czech Republic). The roughness proﬁle analysis of the coatings was obtained with a Tesa-Rugosurf 90G Surface Proﬁlometer (Tesa, Bugnon, Switzerland).

2.4. Electrochemical Evaluation Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were used to evaluate the electrochemical behavior of the coatings. The dynamic corrosion evaluation was conducted using a Gamry potentiostat-galvanostat (Gamry Instruments, Warminster, PA, USA), Model PCI4. These tests were carried out in environmental conditions at a room temperature of 25 ◦C, using an acrylic rotatory cell that consists of a glass recipient where the solution (3.5% NaCl) that was used as the electrolyte was contained, and an acrylic covering which supported the electrodes. The test electrode (coating on steel) had an exposed area of 1 cm2 submerged under the sodium chloride solution; the reference electrode (silver /silver chloride (Ag/AgCl)) was submerged in a saturated potassium chloride (KCl) solution at 3.33 molar, using a platinum wire as a counter-electrode.

2.5. Considerations for Electrochemical Evaluation 2.5.1. Open Circuit Potential Before the electrochemical measurements were performed, the working electrode was subjected to a surface cleaning pretreatment with distilled water and then dried with air. Subsequently, it was subjected to an ultrasonic bath in acetone for 1 min followed by 1 min in ethanol to ensure complete cleaning of the surface. After the pretreatment, the metal was depolarized. The working electrode was positioned together with the reference electrode and the counter electrode, while the open circuit potential was monitored until it reached a stationary value. The potentiostatic polarization curves were performed by scanning discrete potential values. The evolution of the open circuit potential was carried out during 120 min, and in all cases, it presented a typical curve, exhibiting a rapid decrease in the initial period and then diminishing its slope until it stabilized. The evolution of the OPC towards more negative values was observed in each case, indicating a cathodic control in the metal dissolution process. This is observed in the polarization curves obtained by Tafel. The term Corrosion Potential, Ecorr, is related to the potential in an electrochemical experiment in which no current ﬂows. In the cases evaluated the values of Eoc and Ecorr were identical. The equations used to convert from one form of potential to the other is Equation (1): E vs. Eref = (E vs. Eoc) + Eoc (1) In the case of corrosion reactions, an electrochemical reaction under kinetic control obeys the Tafel equation, Equation (2):

(2.3(E−E0)/β) I = I0e (2)

where I = the current resulting from the reaction, I0 = a reaction-dependent constant called the Exchange Current, E = the electrode potential, E0 = the equilibrium potential (constant for a given reaction), β = the reaction’s Tafel Constant (constant for a given reaction). Beta has units of volts/decade. Coatings 2021, 11, 797 5 of 18

2.5.2. Corrosion Current Density According to ASTM G102-1994 [20], corrosion current values are obtained by Tafel Extrapolation through the conversion of the measured current value to current density, Equation (3): I i = cor (3) corr A where: 2 icorr = corrosion current density (µA/cm ) Icorr = total anodic current, µA A = exposed specimen area, cm2.

2.5.3. Corrosion Rate Calculate the corrosion rate (CR) in terms of penetration rate, Faraday’s law is used [20,21], Equation (4): i CR = K cor EW (4) 1 ρA where: CR = Given in mm·year−1 icorr = Corrosion current in amperes EW = Equivalent wight in grams/equivalent 3 K1 = 3.27 ×10 , mm g/µA cm·year A = Area of the sample in cm2.

2.5.4. Porosity Factor The permeable defects are detrimental to the substrate due to the propagation of the corrosion, caused by promoting direct ways that allow the corrosive electrolyte to reach the substrate. The evaluation of porosity by an electrochemical method is based on the correlation of the current density, but it takes into account the indirect potentiodynamic measurement of the electrochemical behavior and the current density, which is indirectly proportional to the resistance of polarization [22]. The polarization resistance is experimen- tally determined from the impedance measurements according to Tato’s equation [23]: Rp,u P (%) = (5) Rp,r−u

P is the porosity factor, if it’s near to 0%, it indicates that the coatings act as an inert barrier against the corrosive solution. If the factor of porosity is close to 100%, it indicates that the coating does not act as a barrier for corrosive diffused Cl− ions that are attributed to creating coating defects such as pores, cracks, delamination, material dissolution, etc. 2 Rp,u, means the total resistance (kΩ·cm ) of the bare substrate, and Rp,r–u, indicates the total resistance (kΩ·cm2) of the coating.

3. Results and Discussion 3.1. X-ray Diffraction Analysis Diffractograms of the monolayers, n = 1 bilayer and the multilayered coating are shown in Figure1. The XRD spectra of the Ti-W-N (green line) consisted of ﬁve peaks, corresponding with two crystalline phases of Ti2N and W2N respectively reported by different authors [10,11,19]. The peaks diffracted at 2θ angles 36.8◦, 43.9◦, and 76.5◦ by planes (111), (200), and (311) [24] corresponded to the typical simple cubic (SC) W2N, indexed by ICDD PDF No. 00-025-1257 database [25]. The Ti2N phase was identiﬁed as a tetragonal crystal structure (ICDD PDF No. 01-080-3438), corresponding to the peaks at 36.8◦, 41.5◦, 57.1◦, and 69.9◦ originated by planes (112), (004), (105), and (301) respectively. It is considered that the crystal structure is a mixed phase of W2N/Ti2N[11]. Furthermore, it was calculated that a pseudobinary mix of cubic TiN and hexagonal WN (as WC) phases Coatings 2021, 11, x FOR PEER REVIEW 6 of 18

Coatings 2021, 11, 797 57.1°, and 69.9° originated by planes (112), (004), (105), and (301) respectively. It is consid-6 of 18 ered that the crystal structure is a mixed phase of W2N/Ti2N [11]. Furthermore, it was calculated that a pseudobinary mix of cubic TiN and hexagonal WN (as WC) phases were werethermodynamically thermodynamically favored favored in Ti-W in-N Ti-W-N [26]. The [26 spectra]. The spectra of the Ti of-W the-C Ti-W-Cmonolayer monolayer coating coating(blue line) (blue had line) four had peaks, four where peaks, an where intense an intensepeak related peak to related the or toientation the orientation plane (111 plane) at ◦ 2 (111)49.8°, atcorresponding 49.8 , corresponding to the titanium to the titaniumdicarbide dicarbide (TiC ) cubic (TiC type2) cubic structure type [ structure27], according [27], accordingto chart PDF to chart 04-007 PDF-2539 04-007-2539.. While the While peaks the located peaks locatedat 43° and at 43 73°◦ and correspond 73◦ correspond to a W to2C a Whexagonal2C hexagonal structure structure (PDF (PDF 04-014 04-014-5679).-5679). The Theatomics atomics radius radius of tungsten of tungsten and andtitanium titanium are aresimilar similar,, thus thus are easily are easily formed formed into the into Ti the-W- Ti-W-CC ternary ternary system system [28]. T [he28 ].spectra The spectra of the n of = the1 bilayer n = 1 (orange bilayer (orangeline) show line)ed showedan intense an peak intense with peak a preferential with a preferential plane (111) plane is observed (111) is observedcorresponding corresponding to the TC2to structure. the TC2 structure.The Ti-W-C The-N Ti-W-C-N(red line) multilayer (red line) multilayer (n = 40) ha (dn peaks = 40) hadthat peaksbelong that to T belongi-W-N toand Ti-W-N Ti-W- andC, plus Ti-W-C, a quaternary plus a quaternary phase of Titanium phase of Titanium Tungsten Tungsten Carbide CarbideNitride (Ti Nitride0.75W0.25 (Ti)(0.75C0.75WN0.250.25)(C), which0.75N0.25 has), whicha cubic has structure a cubic structureaccording according to the ICDD to the 01 ICDD-081- 01-081-81228122 database database,, considering considering a phase stability a phase domain stability [29 domain]. The formation [29]. The of formation the quaternary of the quaternaryphase could phase possibly could be possiblydue to the be diffusion due to the between diffusion the between carbide theand carbide nitride andlayers, nitride as it layers,has been as found it has beenfor Ti found-N/Ti- forC multilayers, Ti-N/Ti-C multilayers,by means of byXRD means [30], of SEM XRD cross [30],-section SEM cross- [31], sectionand Glow [31 ],Discharge and Glow Optical Discharge Emission Optical Spectroscopy Emission Spectroscopy (GDOES) (GDOES)analysis [ analysis32,33]. XRD [32,33 re-]. XRDsults resultsindicate indicate the formation the formation of a quaternary of a quaternary compound, compound, meaning meaning that, that,when when taken taken into intoconsideration consideration with with the thepresence presence of mixed of mixed binary binary phases phases for for W W-Ti-N-Ti-N and and W W-Ti-C-Ti-C layers, and thethe evidenceevidence ofof thethe coexistencecoexistence ofof multiplemultiple crystallinecrystalline phasesphases inin thethe multilayermultilayer withwith 40 periods,periods, it can bebe supposedsupposed thatthat therethere isis aa gradedgraded zonezone alongalong withwith thethe interfacesinterfaces ofof multilayer. It wouldwould bebe interestinginteresting toto investigateinvestigate thethe phasephase distributiondistribution byby transmissiontransmission electron microscopy (TEM), as wellwell asas throughthrough compositionalcompositional depthdepth proﬁling,profiling, butbut thisthis isis beyond the reachreach ofof thisthis paper.paper.

a TiWCN ) b W N

2 200 (111)

c (

(311)

a a

W C (222)

2 a d TiC e 2 Ti N 2

n = 40

)

unit.

n = 1

arb

(

(102)

(220)

(111)

(220)

d d

Intensity TiWC

)

105

301

(311)

(

(200)

(112)

(111)

(004)

b e TiWN

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

2 a (Ti W )(C N ) 0,75 0,25 0.75 0.25 Figure 1. XRD patterns of of the the Ti Ti-W-N,-W-N, Ti Ti-W-C-W-C monolayer monolayer coatings coatings,, and and n n = = 1 1and and nn = =40 40 multilayer coatings depositedcoatings dep byosited RF magnetron by RF magnetron sputtering. sputtering.

3.2. Microstructural Analysis Cross-sectionCross-section micrographsmicrographs ofof Ti-W-N, Ti-W-N, Ti-W-C Ti-W-C monolayers, monolayers n, =n 1,= and1, and n =n 40 = 40 multilay- multi- erslayers are are shown shown in Figure in Figure2a–d. 2a As–d. seen As seen in Figure in Figure2a, the 2a thickness, the thickness of the of Ti-W-N the Ti- coatingW-N coating was 2.81was ±2.810.27 ± 0.27µm μm with with a buffer a buffer layer layer of 200 of 200 to 240 to 240 nm nm deposited deposited in all in coatings; all coating thes; thicknessthe thick- averageness average was similar was similar to the obtainedto the obtained by the proﬁlometryby the profilometry method, method which was, which 3.29 ±was0.06 3.29µm. ± The0.06 evolutionμm. The ofevolution the microstructure of the microstructure was the typical was columnar the typical growth columnar of PVD growth processes, of PVD sim- ilarprocesses, to those similar reported to inthose [34 ,35reported]. In the in case [34 of,35 the]. In Ti-W-C the case coating of the (Figure Ti-W-2Cb), coating the thickness (Figure was2b), the 3.54 thickness± 0.04 µ m,was however, 3.54 ± 0.04 when μm,measured however, when by the measured proﬁlometer by the it was profil2.95ometer± 0.08 it wasµm. The2.95 microstructure± 0.08 μm. The obtainedmicrostructure was of obtained columnar was growth of columnar typical ofgrowth WC and typical TiC monolayerof WC and coatings deposited at low temperatures between 200 to 500 ◦C by sputtering [36]. Figure2c evidenced that the thickness of the upper Ti-W-C coating was 1.56 ± 0.02 µm and the thickness of the lower Ti-W-N coating on the substrate was 1.20 ± 0.03 µm (total thickness of 2.76 µm); the thickness average obtained was of 2.06 ± 0.03 µm. The upper dark grey layer corresponded to the presence of carbon atoms in the formation of Ti-W-C, and the Coatings 2021, 11, x FOR PEER REVIEW 7 of 19

those reported in [34,35]. In the case of the Ti-W-C coating (Figure 2b), the thickness was 3.54 ± 0.04 μm, however, when measured by the profilometer it was 2.95 ± 0.08 μm. The microstructure obtained was of columnar growth typical of WC and TiC monolayer coatings deposited at low temperatures between 200 to 500 °C by sputtering [36]. Figure 2c Coatings 2021, 11, 797 7 of 18 evidenced that the thickness of the upper Ti-W-C coating was 1.56 ± 0.02 μm and the thickness of the lower Ti-W-N coating on the substrate was 1.20 ± 0.03 μm (total thickness of 2.76 μm); the thickness average obtained was of 2.06 ± 0.03 μm. The upper dark grey layer lightcorresponds grey layer to the is the presence Ti-W-N of [37 carbon]. Figure atoms2d exhibits in the formation the n = 40 of multilayers, Ti-W-C, and showing the a light greythickness layer ofis 3.57the Ti±-W0.06-N µ[3m;7].this Figure value 2d wasexhibit similars the ton that= 40, obtainedshowing bya thickness the proﬁlometer of 3.57 ±(3.29 0.06 µm± 0.12; thisµ m).value was similar to the obtained by the profilometer (3.29 ± 0.12 μm).

Figure 2. Cross-section SEM images of the coatings, (a) Ti-W-N monolayer, (b) Ti-W-C monolayer, and (c) n = 1. HR-TEM micrographs (d) n = 40 deposited on Si (001)/Ti by RF magnetron sputtering. Figure 2. Cross-section SEM images of the coatings, (a) Ti-W-N monolayer, (b) Ti-W-C monolayer, and (c) n = 1This. HR- thicknessTEM micrographs exhibited (d) n a = progressive 40 deposited increase on Si (001)/Ti in the by interfaceRF magnetron roughness sputtering with. the rise of the number of stacked layers; however, the thickness of each bilayer remained Thisunchanged thickness [34 exhibits]. This zig-zag-likea progressive interface increase roughnessin the interface is attributed roughness to thewith application the rise of of thenegative number substrate of stacked bias layers; during however, the deposition the thickness process, whichof each produces bilayer ionremains bombardment un- changedon the[34]. TiWC/TiWN This zig-zag- layerslike interface while theroughness add-atoms is attributed reach the to substrate the application surface of [38 neg-]. Järren- ative substratedahl et al. bi [as39 ]during found the that deposition sputtered process, layers display, which produces a cupola-like ion bombardment microstructure on with a the TiWC/TiWNworking pressure layers ofwhile 0.67 the Pa andadd- withoutatoms reach substrate the substrate bias. Conversely, surface [ Cancellieri38]. Järrendahl et al. [40], et al., reported[39] found waving that sputtered patterns layers due to display, the kinetic a cupola energy-like of microstructure the incident ionic with species a work- by the ing pressurerf-bias, of increasing 0.67 Pa and the velocitywithout andsubstrate surface bias. mobility Conversely, of the add-atoms,Cancellieri et thus, al. [ affecting40], re- the portedcrystalline waving patterns quality. due to the kinetic energy of the incident ionic species by the rf˗bias, increasing the velocity and surface mobility of the add-atoms, thus, affecting the crystalline3.3. XPS quality. Analysis The X-ray photoelectron broad spectra obtained in the range of 0–1000 eV for the

Ti-W-N coating are depicted in Figure3a–d. The quantiﬁcation of the atomic concentration (%) corresponding to Ti-W-N is shown in Table2. The photoelectron peaks of O (KLL), Ti 3.3. XPS Analysis (2s), O (1s), Ti (2p3), N (1s), Si (2p), and W (4f) are revealed (Figure3a–d). Figure3a puts Theon displayX-ray photoelectron the high-resolution broad Wspectra (4f) peak obtained of the in Ti-W-N the range coating. of 0– These1000 eV peaks for the were Ti- found W-N atcoating 38.31 andis depicted 34.42 eV, in corresponding Figure 3a–d. The to the quantification tungsten bonded of the with atomic N (W-N) concentration and Ti (W-Ti), (%) correspondingrespectively. Figureto Ti-W3b-N shows is shown thetwo in Table N (1s) 2. bondingThe photoelectron energies of peaks N-Ti withof O (KLL), 396.97 eVTi and (2s), Oof (1s), N-W-O Ti (2p3), with N 400.38 (1s), Si eV (2p), [41 ];and this W binding (4f) are energyrevealed N(1s) (Figure corresponds 3a–d). Figure to the 3a N-Tiputs bond on displayaccording the high to Restrepo-resolution et al.W [(424f)]. peak Figure of 3thec,shows Ti-W-N the coating photoelectron. These peaks peak were of Ti found (2p) was at 457.59 eV (Ti-N) and 462.79 eV (N-Ti-N) [43].

Coatings 2021, 11, x FOR PEER REVIEW 8 of 18 Coatings 2021, 11, 797 8 of 18

(a)

(b)

(c)

FigureFigure 3. 3.HighHigh resolution resolution XPS XPS spectra spectra of ofthe the Ti- Ti-W-NW-N coating coating deposited deposited by by RF RF magnetron magnetron sputtering. sputtering. (a)( aW) W (4f (4f),), (b ()b Ti) Ti (2p), (2p), (c ()c N) N(1s (1s).).

Coatings 2021, 11, 797 9 of 18

Table 2. Chemical composition in atomic concentration (%) using XPS.

Material Type Element Coatings 2021, 11, x FOR PEER REVIEW 9 of 18 C N W Ti Ti-W-N – 24.16 35.72 40.12 Ti-W-C 20.30 – 36.97 42.72 On the other hand, Figure 4a–d discloses the X-ray photoelectron wide spectra obtained for the coatingOn Ti the-W- otherC. The hand, quantification Figure4a–d of atomic discloses concentration the X-ray photoelectron (%) correspond- wide spectra ob- ing to the Ti-Wtained-C coating for the is shown coating in Ti-W-C. Table 2 The. The quantiﬁcation photoelectron of peaks atomic of concentration O (KLL), Ti (2s), (%) corresponding O (1s), Ti (2p3),to N the (1s), Ti-W-C Si (2p) coating, and W is (4f) shown have in been Table shown2. The. photoelectronFigure 4a, shows peaks the of line O (KLL),of Ti (2s), O the photoelectron(1s), of Ti W (2p3), 4f, identifying N (1s), Si (2p),two bond and W energies (4f) have corresponding been shown. to Figure W-C 4(34.37a, shows eV) the line of the and W-Ti (38.85photoelectron eV). In Figure of W 4b, 4f, the identifying peak of twoC 1s bond is due energies to the correspondingformation of carbide to W-C (34.37 eV) and titanium C-Ti withW-Ti a (38.85 bond eV).energy In Figureof 284.734b, eV. the Moreover peak of C, 1sit presents is due to a the bond formation energy of W carbide- titanium C-O (289.06 eV)C-Ti [44]. with Finally, a bond Figure energy 4c indicate of 284.73s the eV. peak Moreover, of photoelectron it presents Ti a 2p, bond which energy of W-C-O presents two bond(289.06 energies eV)[44 of]. Finally,Ti-N (457.31 Figure eV)4c indicatesand W-Ti the-O peak(463.26 of photoelectroneV). The presence Ti 2p, of which presents residual oxygentwo on bondcoating energiess is due of to Ti-N the typical (457.31 desorption eV) and W-Ti-O in PVD (463.26 deposition eV). The processes presence of residual [45]. oxygen on coatings is due to the typical desorption in PVD deposition processes [45].

(a) (b)

(c) Figure 4. High resolution XPS spectra of Ti-W-C coating deposited by RF magnetron Figure 4. High resolution XPS spectra of Ti-W-C coating deposited by RF magnetron sputtering. (a) W (4f), (b) Ti (2p), sputtering. (a) W (4f), (b) Ti (2p), (c) N (1s). (c) N (1s).

Table 2. Chemical3.4. composition Roughness in Analysis atomic concentration (%) using XPS. Material TypeRoughness by AFM Element AFM imagesC of the Ti-W-N,N Ti-W-C monolayers,W and n = 1,Ti n = 40 multilayers are Ti-W-Nshown in Figure5–a–d. In Figure24.165a, the Ti-W-N coating35.72 shows the40.12 topography of regular Ti-W-Ctexture, exhibiting20.30 certain homogeneous– formations36.97 with similar42.72 dimensions, in which not more than three or four isolated cone-type crests were observed on the surface. These 3.4. Roughness Analysis Roughness by AFM AFM images of the Ti-W-N, Ti-W-C monolayers, and n = 1, n = 40 multilayers are shown in Figure 5a–d. In Figure 5a, the Ti-W-N coating shows the topography of regular texture, exhibiting certain homogeneous formations with similar dimensions, in which

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not more than three or four isolated cone-type crests were observed on the surface. These cone-typescone-types crestscrests areare producedproduced byby anan intermixingintermixing processprocess ofof columnscolumns thatthat occursoccurs duringduring thethe coating’scoating’s growth at the the interf interfacesaces with with the the substrate substrate [4 [646]. ].The The surface surface roughness roughness Ra Ra(average (average)) was was 15.5 15.5 ± 0.1± nm0.1 and nm the and top the of topthe columns of the columns were 132.3 were ± 3.9 132.3 nm.± In3.9 Figure nm. 5b In, Figurethe monolayer5b , the monolayer coating of coatingTi-W-C of presents Ti-W-C similar presents topography similar topography in the surface; in the it surface; also pre- it alsosents presents some isolated some isolated crests, a crests, few small a few sharp small pyramid sharp pyramid-type-type crests with crests an with increase an increase in their inheight their profile, height with profile, an withaverage an averageroughness roughness of 12.6 ± of0.9 12.6 nm,± and0.9 formations nm, and formations of 128.5 ± 3.8 of 128.5nm. Figure± 3.8 nm.5c reveFigureals5 thec reveals most thesignificant most significant texture, texture,the roughness the roughness of the material of the material,, which whichwas 8.1 was ± 0.18.1 nm± 0.1 with nm domeswith domesof 115.6 of ± 115.6 3.5 nm.± 3.5 Figure nm. 5d Figure shows5d showsan uniformly an uniformly smooth smoothtexture texturewith bulk with grains; bulk grains;it also indicates it also indicates a decrease a decrease in roughness in roughness of 4.7 of± 0.34.7 nm,± 0.3 and nm a, and96.4 a± 96.42.9 nm± 2.9grain nm size.grain These size. roughness These roughness values are values in agreement are in agreement with thosewith previously those previouslyreported by reported Petrović by et Petrovićetal. [47]. The al. n = [47 40]. multilayer’s The n = 40 multilayer’ssmoother surface smoother can be surface explained can beby explainedthe reduced by width the reduced of the column width ofgrowth the column (see Figure growth 2d), (see evidenced Figure 2ind), the evidenced morphology in theof the morphology cross-section of the observed cross-section by SEM. observed The n = by 40 SEM. multilayer’s The n = 40repeated multilayer’s nucleation repeated pro- nucleationcesses impeded processes the columns impeded’ widening the columns’ [48,49 widening]. [48,49].

a) b) Cone

c) d)

FigureFigure 5.5. AFMAFM imagesimages inin 2D2D of of the the coatings, coatings, ( a()a) Ti-W-N Ti-W-N monolayer, monolayer (b, ()b Ti-W-C) Ti-W- monolayer,C monolayer, (c) ( nc) = n 1, = and1, and (d) ( nd) = n 40 = 40 coatings. multilayer coatings.

3.5.3.5. ElectrochemicalElectrochemical BehaviorBehavior 3.5.1.3.5.1. PotentiodynamicPotentiodynamic CurvesCurves CorrosionCorrosion tends to to happen happen in in the the most most reactive reactive points points of ofthe the surface, surface, considering considering fac- factorstors as asemerging emerging dislocations, dislocations, high high index index face face grains, grains, inclusions, inclusions, grain grain limits, particles,particles, secondsecond phases,phases, andand crackscracks [[5050].]. TheThe behaviourbehaviour ofof thethe potentiodynamicpotentiodynamic curvescurves usingusing thethe 3.5%3.5% NaClNaCl solution solution as as the the electrolyte electrolyte is revealed is revealed in Figure in Figure6. All 6 polarization. All polarization tests evidenced tests evi- typicaldenced zonestypical of zones anodic-cathodic of anodic-cathodic activation. activation. However, However, the monolayer the monolayer of Ti-W-N of Ti (green-W-N curve) indicated a primary passive potential (E ) and critical current density [51]. These (green curve) indicated a primary passive potentialpp (Epp) and critical current density [51]. areThese results are results of the kineticof the kinetic reactions reaction satisfyings satisfying the equation the equation for i (total) for i =(total) 0 [52 ],= promoting0 [52], pro- themoting formation the formation and development and development of a passive of coatinga passive on coating the surface. on the The surface. passive The protective passive coatingprotective may coating act as a may barrier act layeras a barrier to prevent layer the to metalprevent of thethe metalmetal ion of the [53 ].metal The currention [53]. limit The (i ) is increased in the anodic region. The intersection curve for the cathodic reaction of currentlim limit (ilim) is increased in the anodic region. The intersection curve for the cathodic the anodic polarization is the passivation region, which is due to the spontaneous coating

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the anodic polarization is the passivation region, which is due to the spontaneous coating passivation [54]. For the case of the coating with carbides (blue curve), the corrosion resistance decreased when the carbon contained in the material increased [55]. The n=1 (orange line) coating presented spontaneous symmetry in the anodic and cathodic regions. The Ecorr value of −0.55 V tends to be positive, and the icorr of 2.0 × 10−6 A·cm2 decreased compared with the bare substrate. For the n=40 coating (red line), the Ecorr was −0.26 V and the icorr was 9.2 × 10−6 A·cm2. Thus, it can be considered that the material tends to become noble (electropositive potential), improving the decrease of the anodic current density around two scales of magnitude compared with the bare substrate. This behavior was similar to that obtained by Alves et al. [56]; however, it was better than the one obtained by Caicedo et al. [25]. The potential and current increased due to the dissolution of active metal generated by the galvanic coupling between the substrate and the coating [57]. Coating passivation (active-passive) can be attributed to the presence of Ti, considering the chemical property of this transition metal which inhibits corrosion. The Coatings 2021, 11, 797 11 of 18 increase of corrosion resistance in the materials was attributed to the compact microstructure, and density of the coating [58]. The low corrosion current was influenced by the chemical composition close to the surface, due to the formation of the passive layer of oxide (TiO2) onpassivation the surface [ [5954].]. For the case of the coating with carbides (blue curve), the corrosion resistance decreased when the carbon contained in the material increased [55].

0.0 [TiWC/TiWN]n

-0.3

-0.6

Passivation potential

-0.9 n =40 n =1 -1.2 TiWC

Potential V vs Ag/AgCl TiWN AISI 1060 -1.5 10-8 10-7 10-6 10-5 10-4 10-3 10-2 -2 Log I / A· cm

Figure 6. PotentiodynamicFigure 6. Potentiodynamic curves of the substrate curves and of the coated substrate samples and in coated 3.5 wt.% samples NaClin solution. 3.5 wt.% NaCl solution.

3.5.2. Corrosion RatesThe n = 1(orange line) coating presented spontaneous symmetry in the anodic and ca- −6 2 thodic regions. The Ecorr value of −0.55 V tends to be positive, and the icorr of 2.0 × 10 A·cm In Table 3, the Icorr of each system was taken, and the corrosion rates (CR) were cal- decreased compared to the bare substrate. For the n = 40 coating (red line), the Ecorr was culated. The n=40 coating presented a significant−6 decrease2 in the CR, specifically 0.19 −0.26 V and the icorr was 9.2 × 10 A·cm . Thus, it can be considered that the material −1 mm·year ; thistends is significantly to become more noble than (electropositive 16 times less potential),than the bare improving steel substrate. the decrease This of the anodic behavior is characteristiccurrent density of the around n=40 layered two scales structure of magnitude due to the compared increased with number the bare of substrate. This multilayers (n),behavior density, and was the similar number to that of interfaces obtained bywhile Alves maintaining et al. [56]; thougha uniform was thick- better than the one ness in the multilayeredobtained bysystem Caicedo [60]. et al. [25]. The potential and current increased due to the dissolution of active metal generated by the galvanic coupling between the substrate and the coating [57]. Table 3. ElectrochemicalCoating parameters passivation of the (active-passive) potentiodynamic can curves be attributed of the monolayer to thes,presence n=1 and n=40 ofTi, considering coatings. the chemical property of this transition metal which inhibits corrosion. The increase of

Material Typecorrosion resistanceEcorr (V vs. Ag/AgC in the materialsl) icorr was (A·c attributedm2) toCR the (mm compact·year−1) microstructure, and AISI 1060density of the coating–1.13 [58]. The low4.2 corrosion × 10−5 current was3.12 influenced by the chemical Ti-W-Ncomposition close–0.86 to the surface, due5.7 to × the 10− formation6 of the0.58 passive layer of oxide (TiO2) Ti-W-C on the surface [59–].0.69 2.2 × 10−6 0.42 n = 1 –0.55 2.0 × 10−6 0.35 3.5.2. Corrosion Rates n = 40 –0.26 1.08 × 10−6 0.19 In Table3, the I corr of each system was taken, and the corrosion rates (CR) were calculated. The n = 40 coating presented a significant decrease in the CR, specifically 0.19 mm·year−1; this is significantly more than 16 times less than the bare steel substrate. This behavior is characteristic of the n = 40 layered structure due to the increased number of multilayers (n), density, and the number of interfaces while maintaining a uniform thickness in the multilayered system [60].

Table 3. Electrochemical parameters of the potentiodynamic curves of the monolayers, n = 1 and n = 40 coatings.

2 −1 Material Type Ecorr (V vs. Ag/AgCl) icorr (A·cm ) CR (mm·year ) AISI 1060 −1.13 4.2 × 10−5 3.12 Ti-W-N −0.86 5.7 × 10−6 0.58 Ti-W-C −0.69 2.2 × 10−6 0.42 n = 1 −0.55 2.0 × 10−6 0.35 n = 40 −0.26 1.08 × 10−6 0.19 Coatings 2021, 11, x FOR PEER REVIEW 12 of 18 Coatings 2021, 11, 797 12 of 18

3.5.3. Electrochemical Impedance Spectroscopy 3.5.3. Electrochemical Impedance Spectroscopy Nyquist Diagram Nyquist Diagram Figure7 7 shows shows thethe NyquistNyquist diagramdiagram forfor all all the the systems systems evaluated, evaluated, including including both both monolayers,monolayers, n n = = 1 1,, and and n = 40 multilayers.multilayers. The bare substratesubstrate presentedpresented lessless impedanceimpedance valuevalue due to itsits highhigh reactivity reactivity compared compared with with the the coatings; coatings; this this is because is because the measurementthe measure- mentof impedances of impedances is between is between 0.001 0.001 Hz and Hz 100 and kHz. 100 ThekHz real. The axis real (Z axis´) presented (Z´) presented a decrease a de- 2 2 creasein the in electrical the electrical resistance resistance (kΩ·m (kΩ·m) of the) of electrolyte the electrolyte (Rs), (R ands), and greater greater resistance resistance of the of thetransference transference of the of chargethe charge (R1), (R the1), double the double layer (Rlayer2), the(R2 polarization), the polarization of capacitance of capacitance (CPE1) (CPEand the1) and double the double layer (CPE layer2). (CPE Thus,2). theThus, real the impedance real impedance Rsol. + RR1sol+. + R R2 1presented + R2 presented the same the samebehavior behavior in the in imaginary the imaginary axis (Zaxis´´ ),(Z´´) that, that is, it is, presented it presented high high resistance resistance of polarization of polariza- tioncompared compared with with the otherthe other systems systems (see (see results results in Table in Table4). 4).

AISI 1060 TiWN 15 TiWC n=1

n=40 2 10

· cm

 Z´´/ k Z´´/ 5

0 2 4 6 8 10 12 14 16 2 Z´/ k cm Figure 7. Nyquist diagram of all the coatingscoatings after AC impedance testing in 3.5 wt.% NaCl solution.

TableThe 4. Electrochemical dissolution resistance behaviors by obtain EIS ofed the at monolayers, 100 kHz are n =considered 1 and n = 40 high coatings. frequencies and the data is acquired in the range between 100–0.0001 Hz, while low frequencies provide 2 n n informationMaterial Typeabout reactionRp (k Ωkinetics.·cm ) AllCPE proposed1 S × s systemsn1 had CPEa better2 S × resistances n2 compared AISIwith 1060 the bare steel substrate. 0.32 The1.87 response× 10−9 to impedance0.72 26.09 in the× 10 semi−9 -circle0.92 is at- tributedTi-W-N to the controlled process 12.15 of energy8.70 × activation.10−7 0.84 However,119.7 the× repressed10−6 semi0.85-cir- − − cles canTi-W-C be found in these 16.48coatings since0.74 the× 10system6 s are0.94 not ideal,133.6 and× 10therefore,6 0.86 the ap- n = 1 32.02 6.66 × 10−6 0.82 177.9 × 10−6 0.86 plication of a resistor-capacitor pair (R1-CPE1) would not be enough to model that imped- × −6 × −6 ance responsen = 40 [61]. The n = 56.8240 multilayer8.70 system10 formed0.92 the symmetrical20.97 10 semi-circle0.92 (loop) n Swhere is siemens the (=1/ohm),limit of slowis second-to-the-power-n. frequency (ω→0) was without any defect in the semi-circumference. The element of constant phase CPE is accompanied by a parameter denominated as n, whichThe dissolutionis independent resistances of the frequency obtained atwhen 100 kHzits value are consideredis near to 1, high When frequencies this occurs and, it the data is acquired in the range between 100–0.0001 Hz, while low frequencies provide behaves like an ideal capacitor and represents the capacity of the interface; when it takes information about reaction kinetics. All proposed systems had a better resistance compared a value close to 0, it associates it to an element resistor. Additionally, the system describes with the bare steel substrate. The response to impedance in the semi-circle is attributed the behaviour of the surface heterogeneity; hence, the values close to n = 1 are associated to the controlled process of energy activation. However, the repressed semi-circles can with a decrease in surface roughness or coating homogeneity. be found in these coatings since the systems are not ideal, and therefore, the application Table 4 shows how Rp changes as a function of the coated materials and the different of a resistor-capacitor pair (R -CPE ) would not be enough to model that impedance multilayers. With respect to the1 AISI 10601 steel substrate, as the multilayers increase, there response [61]. The n = 40 multilayer system formed the symmetrical semi-circle (loop) was an increase in Rp, possibly attributed to the fact that the corrosion products had where the limit of low frequency (ω→0) was without any defect in the semi-circumference. blocked the pores, increasing the resistive path. But a better indicator of the corrosion re- The element of constant phase CPE is accompanied by a parameter denominated as n, sistance of coatings is the polarization resistance. At higher Rp values, increased corrosion which is independent of the frequency when its value is near to 1, When this occurs, it behavesprotection like is anachieved, ideal capacitor and fewer and defects represents are present the capacity in thin of films; the interface; it is also when inversely it takes pro- a valueportional close to tothe 0, corrosion it associates rate. it toThis an parameter element resistor. is observed Additionally, to take higher the system values describes in multi- thelayers behaviour than in ofbare the AISI surface 1060 heterogeneity; steel, confirm hence,ing that the these values coatings close tooffer n = better 1 are associatedprotection withagainst a decrease corrosion. in surfaceIn relation roughness to the change or coating experienced homogeneity. by Rp through time, it can be appreciated that the multilayers were evaluated at different times. After the second hour of

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Coatings 2021, 11, 797 13 of 18 immersion, the polarization resistance presented an average value of 3 kΩ higher; however, at 72 h a constant value of Rp was appreciated, which is recorded in Table 4. These results can be explained by the relationship between Rp and the evolution of porosity throughTable time4 shows; an increa how Rsep inchanges polarization as a function resistance of the and coated consequently materials anda decrease the different in porosity,multilayers. which can With be related respect to the AISIaccumulation 1060 steel substrate,of corrosion as theproducts multilayers in the increase, defects. thereAnother was an increase in R , possibly attributed to the fact that the corrosion products had blocked parameter obtainedp by modeling the electrical circuits and used to analyze the corrosion the pores, increasing the resistive path. But a better indicator of the corrosion resistance of resistance is the capacitance of the coatings (CPE2). The decrease of this parameter indi- coatings is the polarization resistance. At higher Rp values, increased corrosion protection catesis achieved, that the and coating fewer’s defectsporosity are increased present in, and thin this films; greater it is also number inversely of pores proportional allowed the electrolyteto the corrosion to penetrate rate. This and parameter affect the substrate. is observed The to value take higher of the valuesAISI 1060 in multilayers steel substrate correspondsthan in bare AISIto the 1060 passive steel, layer confirming on its thatsurface these, specifically coatings offer the better iron oxide protection film. against This layer hascorrosion. the lowest In relation capacitive to the properties change experienced compared by to R thep through multilayers time, itand can it be is appreciated due to the fact thatthat this the multilayers layer dissolves were. evaluated at different times. After the second hour of immersion, the polarization resistance presented an average value of 3 kΩ higher; however, at 72 h Tablea constant 4. Electrochemical value of Rp wasbehavior appreciated, by EIS of whichthe monolayer is recordeds, n = in1 and Table n =4 .40 These multilayer results coatings can . be explained by the relationship between Rp and the evolution of porosity through time; CPE2 an increaseMaterial inType polarization Rp resistance (kΩ·cm2) and consequentlyCPE1 S × sn a decreasen1 in porosity, which cann2 S × sn be related to the accumulation of corrosion products in the defects. Another parameter −9 −9 obtainedAISI by 1060 modeling the electrical0.32 circuits and1.87 used× 10 to analyze0.72 the corrosion26.09 × resistance10 0.92 is Ti-W-N 12.15 8.70 × 10−7 0.84 119.7 × 10−6 0.85 the capacitance of the coatings (CPE2). The decrease of this parameter indicates that the coating’sTi-W porosity-C increased,16.48 and this greater0.74 number × 10−6 of pores0.94 allowed 133.6 the electrolyte× 10−6 0.86 to penetraten = and 1 affect the substrate.32.02 The value6.66 of the× 10 AISI−6 10600.82 steel substrate177.9 × corresponds 10−6 0.86 ton the = 40 passive multilayer layer on its surface,56.82 specifically8.70 × the 10−6 iron oxide0.92film. This20.97 layer× 10−6 has the0.92 Slowest is siemens capacitive (=1/ohm) properties, sn is second compared-to-the-power to the- multilayersn. and it is due to the fact that this layer dissolves. 3.5.4. Determination of Porosity 3.5.4. Determination of Porosity The percentage of porosity calculated from Equation (3) indicates that the coatings The percentage of porosity calculated from Equation (3) indicates that the coatings had had a high homogeneity, which is due to their low percentage of porosity value. The coat- a high homogeneity, which is due to their low percentage of porosity value. The coatings ings obtained by PVD associate this effect to the continuous bombardment of energetic obtained by PVD associate this effect to the continuous bombardment of energetic ions of ionsargon of gas argon in the gas deposition in the deposition of the coatings, of the coatings, increasing increasing the movement the mov of absorbedement of atomsabsorbed atomson the on steel the substrates. steel substrates. This generates This generates a coated a surfacecoated thatsurface is more that compactis more compact compared com- paredto the bareto the substrate. bare substrate. In Figure In 8Figure shows 8 the shows best the system best wassystem the nwas = 40 the multilayer, n = 40 multilayer that , thatmultilayers multilayers because because of its of low its low porosity porosity value, value which, which is related is related to the to grainthe grain size, size whose, whose measurementmeasurement waswas obtainedobtained byby AFM; AFM showing; showing a significanta significant quantity quantity of grainof grain boundary, boundary, andand hencehence aa more more homogeneous homogeneous surface. surface. The The effect effect of increasing of increasing the number the number of multilayers of multi- layersalso allowed also allow the relaxationed the relaxation of residual of residual stress; thus, stress; the symmetrythus, the symmetry in the interfaces in the relatesinterfaces relateswith the with sinkhole the sinkhole of energy of thatenergy prevents that prevents the propagation the propagation of cracksinside of cracks the coatinginside the and coat- ingthe and reduction the reduction in porosity. in porosity.

3.0

2.5

2.0

1.5

percentage of porosity porosity of percentage 1.0

0.5

TiWN TiWC n=1 n=40 Material Type FigureFigure 8. Percentage ofof porosityporosity as as a a function function of of the the coatings. coatings.

3.5.5.3.5.5. Equivalent ElectricalElectrical Circuits Circuits Randle’s equivalent electrical circuits, which are the physical model’s representation to Randle’s equivalent electrical circuits, which are the physical model’s representation understand the EIS mechanism of the monolayers, n = 1, and n = 40 multilayer coatings are to understand the EIS mechanism of the monolayers, n = 1, and n = 40 multilayer coatings

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displayedare displayed in Figure in Figure9a,b. Figure 9a,b. 9Figurea, reveals 9a, thatreveal thes circuitthat the consisted circuit consisted of a double of capacitancea double ca- (CPEpacitance1) which (CPE was1) which connected was inconnected parallel within parallel the charge with resistancethe charge (R resistance2), also known (R2), also as resistanceknown as polarization;resistance polarization; both connected both inconnected series with in series the solution with the resistance solution resistance (RS) between (RS) thebetween working the electrode working (WE)electrode and reference(WE) and electrodereference (RE). electrode This circuit(RE). This model circuit represents model rep- the simpleresents corrosion the simple system, corrosion which system, is completely which is completely in control ofin thecontrol charge of the transference. charge transfer- The double-layerence. The double capacitance-layer capacitance provides information provides information on the polarity on the and polarity the quantity and th ofe quantity charge ofof thecharge AISI of 1060/electrolyte the AISI 1060/electrolyte interface. Wheninterface. the When coating the was coating in direct wascontact in direct with contact the electrolyte,with the electrolyte, it provided it aprovide direct diffusiond a direct path diffusion to the corrosivepath to the medium. corrosive In thismedium. process, In thethis galvanicprocess, cellthe corrosiongalvanic wascell corrosion formed and was its formed domain and was its localized domain as wa well.s localized In such aas case, well. the In interfacesuch a case, was the divided interface into twowas sub-interfaces:divided into two electrolyte/coatings sub-interfaces: electrolyte/ and electrolyte/AISIcoatings and 1060.electrolyte/AISI The equivalent 1060. The circuits equivalent R1 and circuits CPE1 areR1 and related CPE1 to are the related coating’s to the properties coating’s prop- and reactionserties and between reactions the between electrolyte the electrolyte and the coating. and the R1 coatingand CPE. R11 andare relatedCPE1 are to related the reaction to the ofreaction the charge-transference of the charge-transference in the electrolytes/AISI in the electrolyte 1060s/AISI interface 1060 [62interface]. When [6 an2]. alternateWhen an potentialalternate waspotential applied was to applied the system to the (electrolyte system (electrolyte + coating ++ coatin substrate),g + substrate), a condenser a conden- was formed,ser was knownformed, as known the capacitance as the capacitance of the coating of the coating (CPE1). (CPE In addition,1). In addition, the capacitor the capacitor was formedwas formed when when the coating the coating began began to be to deﬁned,be defined, and and the the electrolyte electrolyte penetrated penetrated the the space space betweenbetween the the layer layer and and the the metallic metallic substrate. substrate. The The electrolyte electrolyte and and the the substrate substrate formed formed two two condensercondenser plates, plates, while while only only one one layer layer of waterof water molecules molecules (Helmholtz (Helmholtz plane) plane) separated separated the twothe two plates plates that that formed formed the the dielectric dielectric [61 ].[61 This]. This capacitance capacitance is is well well known known as as the the double double layerlayer capacitance capacitance (CPE (CPE22).). ForFor thethe casecase studystudy ofof the the coatings, coatings, the the n n = = 40 40 multilayer multilayer system system presentedpresented a majora major dielectric dielectric constant constant through through the electrolyte the electrolyte adsorption; adsorption; therefore, therefore, it caused it thecaused increase the increase in the coating’s in the coating capacitance.’s capacitance In the. In Ti-W-N the Ti- coating,W-N coating the dielectric, the dielectric constant con- decreased,stant decreased, which which indicates indicates that the that electrolyte the electrolyte penetrated penetrated to the to metalthe metal through through pores pores or cracksor cracks (as Figure(as Figure9b) [963b)]. [63].

(a)

Electrolyte Coating AISI 1060 (NaCl al 3.5%)

Coating capacitance Porous resistance Electrolyte resistance Charge transference resistance Porous

(b) Layer double capacitance

FigureFigure 9.9. Schematic of of a a s standardtandard equivalent equivalent circuit circuit model model (Randles (Randles circuit) circuit) (a), ( anda), and impedance impedance sim- simulatedulated of the of the coating coating system system (b). (b ).

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4. Conclusions

In this study, Ti-W-N and Ti-W-C monolayers as well as (TiWC/TiWN)n = 1 and (TiWC/TiWN)n = 40 multilayers were successfully synthesized by RF magnetron sputtering in order to examine the phase formation, microstructure, roughness, and electrochemical behavior in a NaCl solution.

• The Ti-W-N film structure consisted of a mix of two phases corresponding to Ti2N (tetragonal crystal structure) and W2N (typical simple cubic). For the n = 1 the diffraction peaks were identified as a sum of their respective phases for each layer. In the case of n = 40, the appearance of new intense peaks was observed, which were attributed to a quaternary compound Ti-W (CN) that could have been formed by the diffusion between nitride and carbide layers or the co-deposition of the used reactive gases: N2 and CH4. This quaternary compound, with the presence of mixed binary phases of WTiN and WTiC, could indicate that there is a graded zone along with the interfaces, which would be interesting to investigate by transmission electron microcopy and by small angle x-ray diffraction in future research. • The microstructure was of the columnar growth type in all films, and the multilayers showed the smoothest surface because of the repeated nucleation processes that occurred in their growth. The same defects as cone-dome were observed in the monolayers. • In the quantification of atomic concentration by XPS, a significantly larger presence of Ti in comparison to W was found in all the films. −6 2 • The results of the potentiodynamic prove were −0.26 V for Ecorr, and 9.2 × 10 (A·cm ) −1 for icorr. The corrosion rate (CR) was 0.19 mm·year . Also, n = 40 exhibited the best protection and excellent dielectric resistance (~52.86 KΩ·cm2). This behavior can be correlated to the interruption of the pores and defects that reach the substrate by the multilayer interfaces, making the coatings less permeable. • In the equivalent electrical circuits, the n = 40 system presented a major dielectric constant throughout the adsorption of the electrolyte; hence, they have a greater capacitance. However, the simulation through the equivalent electrical circuits showed that in the TiWN system, the material presented three processes of impedance (pore resistance + film + inductance).

Author Contributions: Conceptualization, A.B.M.-C. and A.G.-H.; methodology J.C.C.; Software, W.A.; validation, J.C.C., A.B.M.-C., and A.G.-H.; formal analysis, M.F., A.B.M.-C. and A.G.-H.; re- sources, J.C.C., C.A. and A.B.M.-C.; data curation, A.G.-H., M.F. and J.C.C.; writing-original draft preparation, A.G.-H., M.F. and J.C.C.; writing-review and editing, A.G.-H., M.F. and W.A.; visualiza- tion, A.G.-H., A.B.M.-C. and M.F.; supervision, A.B.M.-C., M.F. and J.C.C.; project administration, A.G.-H. and A.B.M.-C.; funding acquisition, A.B.M.-C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Consejo Nacional de Ciencia y Tecnología (CONACYT), Frontier Science Project 1103 and fellowship to study PhD in materials science at Tecnológico Na- cional de México/Instituto Tecnológico de Ciudad Madero; in addition, postgraduate scholarship denominated as “Beca Mixta Movilidad al Extranjero” from the 291212 announcements from July to December 2017. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Coatings 2021, 11, 797 16 of 18

Acknowledgments: This research was possible thanks to the TPMR research group for the provided facilities and the support in their laboratory and equipment use. Thanks to the laboratory personnel of hard films and the SEM resource support of the University of Valle, Cali, Colombia. W. Aperador thanks to Universidad Militar Nueva Granada, project IMP ING 3123, Bogotá, Colombia. Addition- ally, high appreciation to the public workers and GIDEMP group of SENA-ASTIN Cali Colombia, for their attention towards me. Thanks, are also due to J. A. Andraca-Adame of Centro de Nanociencias y Micro-Nanotecnologías, Instituto Politécnico Nacional for his technical provision. Also, we want to thank to CONACYT for the economic support. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Vera, E.; Vite, M.; Lewis, R.; Gallardo, E.; Laguna-Camacho, J. A study of the wear performance of TiN, CrN and WC/C coatings on different steel substrates. Wear 2011, 271, 2116–2124. [CrossRef] 2. Montgomery, S.; Kennedy, D.; O’Dowd, N. PVD and CVD Coatings for the Metal Forming Industry. Conference Papers; Technological University Dublin: Dublin, Ireland, 2010; Available online: https://arrow.tudublin.ie/engschmeccon/?utm_source=arrow. tudublin.ie%2Fengschmeccon%2F37&utm_medium=PDF&utm_campaign=PDFCoverPages (accessed on 29 March 2021). 3. Çalı¸skan,H.; Panjan, P.; Kurbanoglu, C. Comprehensive Materials Finishing. In Hard Coatings on Cutting Tools and Surface Finish; Chapter 3; Hashmi, M.S.J., Ed.; Elsevier: Oxford, UK, 2017; Volume 3, pp. 230–242. [CrossRef] 4. Paiva, J.M.; Fox-Rabinovich, G.; Junior, E.L.; Stolf, P.; Ahmed, Y.S.; Martins, M.M.; Bork, C.; Veldhuis, S. Tribological and wear performance of nanocomposite PVD hard coatings deposited on aluminum die casting tool. Materials 2018, 11, 358. [CrossRef] 5. Patel, N.S.; Menghani, J.; Pai, K.B.; Totlani, M.K. Corrosion behavior of Ti2N thin films in various corrosive environments. J. Mater. Environ. Sci. 2010, 1, 134–143. Available online: https://www.researchgate.net/publication/279585673_Corrosion_behavior_of_ Ti2N_thin_films_in_various_corrosive_environments (accessed on 29 March 2021). 6. Ghufran, M.; Uddin, G.M.; Arafat, S.M.; Jawad, M.; Rehman, A. Development and tribo-mechanical properties of functional ternary nitride coatings: Applications-based comprehensive review. Proc. Inst. Mech. Eng. Part J J. Eng. Tribol. 2021, 235, 196–232. [CrossRef] 7. Akito, I.; Nobuhide, N.; Tomokazu, N.; Katsumi, O. Super-Abrasive Grain and Super-Abrasive Grinding Wheel. U.S. Patent Application Number 20200156213 16/635329, 12 May 2020. Available online: https://uspto.report/patent/app/20200156213 (accessed on 29 March 2021). 8. Fernandes, F.A.P.; Heck, S.C.; Picon, C.A.; Totten, G.E.; Casteletti, L.C. Wear and corrosion resistance of pack chromised carbon steel. Surf. Eng. 2012, 28, 313–317. [CrossRef] 9. Fernandes, F.A.P.; Heck, S.C.; Totten, G.; Casteletti, L.C. Wear evaluation of pack boronized AISI 1060 steel. Mater. Perform. Charact. 2013, 2, 58–66. [CrossRef] 10. Addonizio, M.L.; Castaldo, A.; Antonaia, A.; Gambale, E.; Iemmo, L. Influence of process parameters on properties of reactively sputtered tungsten nitride thin films. J. Vac. Sci. Technol. A 2012, 30, 31506. [CrossRef] 11. Suetin, D.V.; Shein, I.R.; Ivanovskii, A.L. Electronic structure of cubic tungsten subnitride W2N in comparison to hexagonal and cubic tungsten mononitrides WN. J. Struct. Chem. 2010, 51, 199–203. [CrossRef] 12. Bemporad, E.; Sebastiani, M.; Pecchio, C.; De Rossi, S. High thickness Ti/TiN multilayer thin coatings for wear resistant applications. Surf. Coat. Technol. 2006, 201, 2155–2165. [CrossRef] 13. Shanaghi, A.; Ghasemi, S.; Chu, P.K.; Ahangarani, S.; Zhao, Y. Effect of Ti interlayer on corrosion behavior of nanostructured Ti/TiN multilayer coating deposited on TiAl6V4. Mater. Corros. 2019, 70, 2113–2127. [CrossRef] 14. Jia-Hong, H.; Chi-Hsin, H.; Ge-Ping, Y. Effect of nitrogen flow rate on the structure and mechanical properties of ZrN thin films on Si (100) and stainless-steel substrates. Mater. Chem. Phys. 2007, 102, 31–38. [CrossRef] 15. Panjan, P.; Drnovšek, A.; Gselman, P.; Cekada,ˇ M.; Panjan, M.; Bonˇcina,T.; Merl, D.K. Merl influence of growth defects on the corrosion resistance of sputter-deposited TiAlN hard coatings. Coatings 2019, 9, 511. [CrossRef] 16. PalDey, S.; Deevi, S. Single layer and multilayer wear resistant coatings of (Ti,Al)N: A review. Mater. Sci. Eng. A 2003, 342, 58–79. [CrossRef] 17. Davis, J.R. Alloying: Understanding the basic. In Carbon and Alloys Steels, 1st ed.; The Materials Information Society Editorial; ASM International®: Materials Park, OH, USA, 2001; pp. 123–125. Available online: https://www.asminternational.org/ documents/10192/1849770/ACFAAA3.pdf (accessed on 29 March 2021). 18. Rorrer, R.A.L.; Mabie, H.H.; Eiss, N.S., Jr.; Furey, M.J. The wear and friction of polyvinyl chloride coatings under fretting Conditions. Tribol. Trans. 1988, 31, 98–104. [CrossRef] 19. Zhou, S.; Liu, W.; Liu, H.; Cai, C. Structural and electrical properties of Ti-W-N thin films deposited by reactive RF sputtering. Phys. Procedia 2011, 18, 66–72. [CrossRef] Coatings 2021, 11, 797 17 of 18

20. ASTM G102. Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. 1994. Available online: https://www.google.com.mx/search?q=Standard+practice+for+calculation+of+corrosion+rates+ and+related+information+from+electrochemical+measurements&source=hp&ei=-KJwYLihDcLSsAWkjazoDA&iflsig= AINFCbYAAAAAYHCxCJqDW4vQOw_q2oNGeUq6L0_C_zjx&oq=Standard+practice+for+calculation+of+corrosion+ rates+and+related+information+from+electrochemical+measurements&gs_lcp=Cgdnd3Mtd2l6EANQxRNYxRNg4 BpoAXAAeACAAQCIAQCSAQCYAQCgAQKgAQGqAQdnd3Mtd2l6sAEA&sclient=gws-wiz&ved=0ahUKEwi4_d- V6_HvAhVCKawKHaQGC80Q4dUDCAc&uact=5 (accessed on 29 March 2021). 21. Calculation of Corrosion Rate. Available online: https://www.gamry.com/Framework%20Help/HTML5%20-%20Tripane%20-% 20Audience%20A/Content/EFM/Introduction/Calculation%20of%20Corrosion%20Rate.htm (accessed on 1 January 2021). 22. Liu, C.; Bi, Q.; Leyland, A.; Matthews, A. An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5N NaCl aqueous solution: Part II. EIS interpretation of corrosion behaviour. Corr. Sci. 2003, 45, 1257–1273. [CrossRef] 23. Tato, W.; Landolt, D. Electrochemical determination of the porosity of single and duplex PVD coatings of titanium and titanium nitride on brass. J. Electrochem. Soc. 1998, 145, 4173–4181. [CrossRef] 24. Alegre, D.; Acsente, T.; Martin-Rojo, A.B.; Oyarzabal, E.; Tabarés, F.L.; Dinescu, G.; De Temmerman, G.; Birjega, R.; Logofatu, C.; Kovac, J.; et al. Characterization of tungsten nitride layers and their erosion under plasma exposure in nano-psi. Rom. Rep. Phys. 2015, 67, 532–546. Available online: https://www.researchgate.net/publication/278385220_Characterisation_of_tungsten_ nitride_layers_and_their_erosion_under_plasma_exposure_in_NANO-PSI (accessed on 7 April 2021). 25. Caicedo, J.; Yate, L.; Montes, J. Improving the physicochemical surface properties on AISI D3 steel coated with Ti-W-N. Surf. Coat. Technol. 2011, 205, 2947–2953. [CrossRef] 26. Sangiovanni, D.; Hultman, L.; Chirita, V.; Petrov, I.; Greene, J. Effects of phase stability, lattice ordering, and electron density on plastic deformation in cubic TiWN pseudobinary transition-metal nitride alloys. Acta Mater. 2016, 103, 823–835. [CrossRef] 27. Gusev, A.I. Phase equilibria, phases and chemical compounds in the T-C system. Russ. Chem. Rev. 2002, 71, 439–463. [CrossRef] 28. Jung, W.; Lee, H.; Nam, K.; Han, J. The synthesis of W–Ti–C films with a control of element composition by hybrid system. Surf. Coat. Technol. 2005, 200, 721–725. [CrossRef] 29. Kwon, H.; Moon, A.; Kim, W.; Kim, J. Investigation of the conditions required for the formation of (Ti,W)(CN) during carburization of titanium under nitrogen. Mater. Lett. 2017, 209, 287–290. [CrossRef] 30. Holleck, H.; Schulz, H. Preparation and behaviour of wear-resistant TiC/TiB2, TiN/TiB2 and TiC/TiN coatings with high amounts of phase boundaries. Surf. Coat. Technol. 1988, 36, 707–714. [CrossRef] 31. Azadi, M.; Rouhaghdam, A.S.; Ahangarani, S.; Mofidi, H. Mechanical behavior of TiN/TiC multilayer coatings fabricated by plasma assisted chemical vapor deposition on AISI H13 hot work tool steel. Surf. Coat. Technol. 2014, 245, 156–166. [CrossRef] 32. Iwai, Y.; Miyajima, T.; Mizuno, A.; Honda, T.; Itou, T.; Hogmark, S. Micro-slurry-jet erosion (MSE) testing of CVD TiC/TiN and TiC coatings. Wear 2009, 267, 264–269. [CrossRef] 33. Hurkmans, T.; Trinh, T.; Lewis, D.; Brooks, J.; Münz, W.-D. Multilayered titanium tungsten nitride coatings with a superlattice structure grown by unbalanced magnetron sputtering. Surf. Coat. Technol. 1995, 76–77, 159–166. [CrossRef] 34. Ramarotafika, H.; Lemperiere, G. Influence of a d.c. substrate bias on the resistivity, composition, crystallite size and microstrain of WTi and WTi-N films. Thin Solid Films 1995, 266, 267–273. [CrossRef] 35. Jalali, R.; Parhizkar, M.; Bidadi, H.; Naghshara, H.; Eshraghi, M.J. Characterization of nano-crystalline Ti–W–N thin films for diffusion barrier application: A structural, microstructural, morphological and mechanical study. Appl. Phys. A 2018, 124, 810. [CrossRef] 36. Djafer, Z.A.A.; Saoula, N.; Madaoui, N.; Zerizer, A. Deposition and characterization of titanium carbide thin films by magnetron sputtering using Ti and TiC targets. Appl. Surf. Sci. 2014, 312, 57–62. [CrossRef] 37. Vernon, S.P.; Stearns, D.G.; Rosen, R.S. Ion-assisted sputter deposition of molybdenum-silicon multilayers. Appl. Opt. 1993, 32, 6969–6974. [CrossRef] 38. Xu, J.; He, T.; Chai, L.; Qiao, L.; Wang, P.; Liu, W. Growth and characteristics of self-assembled MoS2/Mo-S-C nanoperiod multilayers for enhanced tribological performance. Sci. Rep. 2016, 6, 25378. [CrossRef] 39. Järrendahl, K.; Ivanov, I.; Sundgren, J.-E.; Radnóczi, G.; Czigany, Z.; Greene, J.E. Microstructure evolution in amorphous Ge/Si multilayers grown by magnetron sputter deposition. J. Mater. Res. 1997, 12, 1806–1815. [CrossRef] 40. Cancellieri, C.; Klyatskina, E.; Chiodi, M.; Janczak-Rusch, J.; Jeurgens, L.P.H. The Effect of interfacial Ge and RF-bias on the microstructure and stress evolution upon annealing of Ag/AlN multilayers. Appl. Sci. 2018, 8, 2403. [CrossRef] 41. Soto, G.; De La Cruz, W.; Farıas, M. XPS, AES, and EELS characterization of nitrogen-containing thin films. J. Electron Spectrosc. Relat. Phenom. 2004, 135, 27–39. [CrossRef] 42. Restrepo, P.E.; Arango, A.P.J.; Benavides, P.V.J. XPS structure analysis of tin/tic bilayers produced by pulsed vacuum arc discharge. Dyna 2010, 77, 64–74. Available online: http://www.scielo.org.co/scielo.php?script=sci_arttext&pid=s0012-73532010000300007& lng=en&nrm=iso\T1\textgreater{} (accessed on 1 April 2021). 43. Jiang, N.; Zhang, H.; Bao, S.; Shen, Y.; Zhou, Z. XPS study for reactively sputtered titanium nitride thin films deposited under different substrate bias. Phys. B Condens. Matter 2004, 352, 118–126. [CrossRef] 44. Grant, J.T. Analysis of surfaces and thin films by using auger electron spectroscopy and X-ray photoelectron spectroscopy. J. Korean Phys. Soc. 2007, 51, 295–932. [CrossRef] Coatings 2021, 11, 797 18 of 18

45. Castillo, H.; Restrepo-Parra, E.; Arango-Arango, P. Chemical and morphological difference between TiN/DLC and a-C:H/DLC grown by pulsed vacuum arc techniques. Appl. Surf. Sci. 2011, 257, 2665–2668. [CrossRef] 46. Del Pino, A.P.; György, E.; Marcus, I.C.; Roqueta, J.; Alonso, M.I. Effects of pulsed laser radiation on epitaxial self-assembled Ge quantum dots grown on Si substrates. Nanotechnology 2011, 22, 295304. [CrossRef] 47. Petrović,S.; Bundaleski, N.; Perusko, D.; Radović,M.; Kovaˇc,J.; Mitrić,M.; Gaković,B.; Rakoˇcević,Z. Surface analysis of the nanostructured W–Ti thin film deposited on silicon. Appl. Surf. Sci. 2007, 253, 5196–5202. [CrossRef] 48. Suseendran, J.; Halder, N.; Chakrabarti, S.; Mishima, T.; Stanley, C. Stacking of multilayer InAs quantum dots with combination capping of InAlGaAs and high temperature grown GaAs. Superlattices Microstruct. 2009, 46, 900–906. [CrossRef] 49. Voronov, D.L.; Gawlitza, P.; Cambie, R.; Dhuey, S.; Gullikson, E.M.; Warwick, T.; Braun, S.; Yashchuk, V.V.; Padmore, H.A. Conformal growth of Mo/Si multilayers on grating substrates using collimated ion beam sputtering. J. Appl. Phys. 2012, 111, 093521. [CrossRef] 50. Moreno, H.; Caicedo, J.; Amaya, C.; Saldaña, J.M.; Yate, L.; Esteve, J.; Prieto, P. Enhancement of surface mechanical properties by using TiN[BCN/BN]n/c-BN multilayer system. Appl. Surf. Sci. 2010, 257, 1098–1104. [CrossRef] 51. Perez, N. Chapter in 6: Corrosivity and Passivity. In Electrochemistry and Corrosion Science; Springer: Cham, Switzerland, 2016; pp. 199–264. [CrossRef] 52. Ricker Richard, E. Difference between Corrosion Potential and Corrosion Current. Which is More Detrimental for Corrosion? 2017. Available online: https://www.researchgate.net/post/Difference_between_corrosion_potential_and_corrosion_current_ Which_is_more_detrimental_for_corrosion/59b7e6e5217e20269876a2bc/citation/download (accessed on 1 March 2021). 53. Zha, L.; Li, H.; Wang, N. In situ electrochemical study of the growth kinetics of passive film on TC11 alloy in sulfate solution at 300 ◦C/10 MPa. Materials 2020, 13, 1135. [CrossRef] 54. Anthony, E.H.; Johannes, M.C.M.; Zheludkevich, L.; Rudolph, G.B. Book charter in: G.S. Frankel, Part I Fundamentals. Fundamen- tals of corrosion kinetics. In Active Protective Coatings: New-Generation Coatings for Metals; Springer: Berlin/Heidelberg, Germany, 2016; pp. 17–57. Available online: http://www.daryatamin.com/uploads/Books%20File/Active%20Protective%20Coatings.pdf (accessed on 1 March 2021). 55. Senna, L.F.; Achete, C.A.; Hirsch, T.; Freire, F.L., Jr. Structural, chemical and corrosion resistance characterization of TiCN coatings prepared by magnetron sputtering. Surf. Coat. Technol. 1997, 94–95, 390–397. [CrossRef] 56. Alves, V.A.; Brett, C.M.; Cavaleiro, A. Electrochemical corrosion of magnetron sputtered WTiN-coated mild steels in a chloride medium. Surf. Coat. Technol. 2002, 161, 257–266. [CrossRef] 57. Madaoui, N.; Saoula, N.; Zaid, B.; Saidi, D.; Ahmed, A.S. Structural, mechanical and electrochemical comparison of TiN and TiCN coatings on XC48 steel substrates in NaCl 3.5% water solution. Appl. Surf. Sci. 2014, 312, 134–138. [CrossRef] 58. Rahmati, B.; Sarhan, A.A.D.; Zalnezhad, E.; Kamiab, Z.; Dabbagh, A.; Choudhury, D.; Abas, W. Development of tantalum oxide (Ta-O) thin film coating on biomedical Ti-6Al-4V alloy to enhance mechanical properties and biocompatibility. Ceram. Int. 2016, 42, 466–480. [CrossRef] 59. Massiani, Y.; Medjahed, A.; Gravier, P.; Argème, L.; Fedrizzi, L. Electrochemical study of titanium nitride films obtained by reactive sputtering. Thin Solid Films 1990, 191, 305–316. [CrossRef] 60. Jehn, H. Improvement of the corrosion resistance of PVD hard coating–substrate systems. Surf. Coat. Technol. 2000, 125, 212–217. [CrossRef] 61. Grips, V.W.; Selvi, V.E.; Barshilia, H.; Rajam, K. Effect of electroless nickel interlayer on the electrochemical behavior of single layer CrN, TiN, TiAlN coatings and nanolayered TiAlN/CrN multilayer coatings prepared by reactive dc magnetron sputtering. Electrochim. Acta 2006, 51, 3461–3468. [CrossRef] 62. Barsoukov, E.; Macdonald, J.R. Book chapter in: 1.3 Elementary analysis of impedance spectra. In Impedance Spectroscopy Theory, Experiment, and Applications, 2nd ed.; Wiley-Interscience, Jonn Wiley & Sons, Inc.: Hoboken, NJ, USA, 2005; pp. 13–20. ISBN 0-471-64749-7. 63. Waters, N.; Connolly, R.; Brown, D.; Laskowski, B. Electrochemical impedance spectroscopy for coating evaluation using a micro sensor. In Proceedings of the Annual Conference of the Prognostics and Health Management Society, Fort Worth, TX, USA, 29 September–2 October 2014; Volume 5, pp. 1–6. Available online: https://www.semanticscholar.org/paper/ Electrochemical-Impedance-Spectroscopy-for-Coating-Waters-Connolly/7c58d6b539466073b319333db9b10c8a2374f7ad (accessed on 1 March 2021).