Wear and Corrosion Resistance of Chromium–Vanadium Carbide Coatings Produced Via Thermo-Reactive Deposition

coatings

Article Wear and Corrosion Resistance of Chromium–Vanadium Carbide Coatings Produced via Thermo-Reactive Deposition

Fabio Castillejo 1, Jhon Jairo Olaya 2,* and Jose Edgar Alfonso 3 1 Grupo de Ciencia e Ingeniería de Materiales, Universidad Santo Tomás, Carrera 9 No 51-11, Bogotá 110911, Colombia; [email protected] 2 Departamento de Ingeniería Mecánica y Mecatrónica, Universidad Nacional de Colombia, Carrera 45 No 26-85, Bogotá 110911, Colombia 3 Departamento de Física, Universidad Nacional de Colombia, Carrera 45 No 26-85, Bogotá 110911, Colombia; [email protected] * Correspondence: [email protected]

Received: 15 February 2019; Accepted: 20 March 2019; Published: 27 March 2019

Abstract: Chromium carbide, vanadium carbide, and chromium–vanadium mixture coatings were deposited on AISI D2 steel via the thermo-reactive deposition/diffusion (TRD) technique. The carbides were obtained from a salt bath composed of molten borax, ferro-chrome, ferro-vanadium, and aluminum at 1020 ◦C for 4 h. Analysis of the morphology and microstructure of the coatings was done via scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The hardness of the coatings was evaluated using nano-indentation, and the friction coefﬁcient was determined via pin-on-disk (POD) testing. The electrochemical behavior was studied through potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS). The XRD results show evidence of the presence of V8C7 in the vanadium carbide coating and Cr23C6 and Cr7C3 in the chromium carbide coating. The hardness value for the vanadium–chromium carbide coating was 23 GPa, which was higher than the 6.70 ± 0.28 GPa for the uncoated steel. The wear and corrosion resistance obtained was higher for the niobium–chromium carbide coating, due to the nature of the ceramic carbide produced.

Keywords: carbides; chromium; corrosion; TRD diffusion; vanadium; wear

1. Introduction AISI D2 steel is widely used in the manufacturing industry, especially in the development of dies, due to its high degree of hardness and wear resistance. However, the tribological and wear resistance performance of tool steels can be enhanced by the application of surface treatments that allow coatings of carbides or nitrides of transition metals to be obtained, which help to improve the tribological and chemical properties of materials that work under conditions of high wear or corrosion, such as the materials for forming dies and cutting tools, which function in aggressive environments such as coastal cities. The coatings are industrially produced using techniques such as physical vapor phase deposition (PVD) and chemical vapor deposition (CVD) [1,2]. In general, these processes are mainly limited to equipment to be used for operation in a high vacuum, making them very expensive to install and put into production at the industrial level. An alternative for producing hard coatings with good wear resistance is the thermo-reactive deposition/diffusion (TRD) process [3], which is applied over substrates containing a higher percentage of carbon, up to 0.3% by weight [4]. Coatings produced using this process have good adhesion to the substrate, a low friction coefﬁcient, and excellent thickness uniformity [4,5]. In this coating process, a bath of molten salts can be used, consisting of borax,

Coatings 2019, 9, 215; doi:10.3390/coatings9040215 www.mdpi.com/journal/coatings Coatings 2019, 9, 215 2 of 13 aluminum, or ferro-silicon as the reducing element and carbide-forming elements (FCEs) such as vanadium, chromium, titanium, and niobium [4]. The carbide layer growth is produced when the metal dissolved in the salt bath has a relatively low energy of formation of carbide and an energy of formation of the metal oxide greater than that of boron oxide (B2O3). If this condition is not met [6,7], the boron atoms do not rust and are free to diffuse into the matrix of the steel, combining with the iron to form layers of iron oxide (FeO and Fe2B) [8,9]. Papers published to date have studied binary metal carbides deposited via the TRD process and have characterized their microstructure, wear resistance, and mechanical behavior. Additionally, they have focused on the production of VC, CrC, and NbC coatings on substrates of AISI H13, AISI M2, and AISI D2 steel, and up to 2300 HV hardness has been reported [3,5]. Others researchers have studied the layer growth kinetics of niobium carbides, borides, and chromium [10–12]. With respect to the electrochemical behavior of the coatings, there are papers on chromium carbides in which the relation between the microstructure and the corrosion resistance was studied using potentiodynamic polarization, as well as impedance studies (EIS) [13]. Furthermore, studies have been conducted on the corrosion and mechanical properties of nano-vanadium carbide coatings deposited via sputtering on tool steels, showing better mechanical performance for the coating–substrate combination than that exhibited by the substrate [14]. Further investigations have been made on the electrochemical behavior of nano-coatings of chromium carbide alloys deposited on Ti6Al4V and Co–Cr–Mo [15] and on the resistance to the corrosion and wear of niobium–chromium carbides deposited on tool steels. However, there have been no reports of studies on the corrosion and wear resistance of vanadium carbide and chromium carbide systems. Therefore, the objective of this paper is to deposit these coatings on D2 steel substrates using the TRD technique with the aim of combining the properties of the high degree of hardness of vanadium carbide and the good corrosion resistance of chromium carbide. The coatings produced are characterized and compared with the performance of binary carbides and chromium carbides with respect to their mechanical and electrochemical behavior.

2. Materials and Methods Coatings of vanadium carbide, chromium carbide, and chromium–vanadium carbide were deposited on substrates of AISI D2 steel with dimensions of 15 mm in diameter and 4 mm in thickness, polished using emery paper of 220 to 1200, and subjected to ultrasonic cleaning with acetone. The chemical composition of annealed AISI D2 steel is 1.5 wt% C, 11.5–12.5 wt% Cr, from 0.15 to 0.45 wt% Mn, 0.6–0.9 wt% W, and Fe, which completes the chemical balance. The coatings were developed from a salt bath comprised of molten borax (Na2B4O7), ferro-chromium (Fe-Cr), ferro-vanadium (Fe-V), and aluminum (Al), varying the chemical| composition (see Table1). The aluminum was added to the salt bath as a reducing agent; i.e., it reduces boron oxide and rusts to prevent the added metal (chromium or vanadium) from rusting. It is thus available to combine with the carbon of the steel to form carbide. The TRD surface treatment was performed at 1020 ◦C for 4 h with the preheating of the samples at 600 ◦C.

Table 1. Chemical composition of the salt baths.

Sample Wt % Na2B4O7 Wt % Fe-Cr Wt % Fe-V Wt % Al M1 74 15 8 3 M2 66 26 5 3 M3 67 30 - 3 M4 81 - 16 3

The crystal structure of the coatings was determined by means of an analysis of the patterns of X-ray diffraction, obtained on a X-PertPro PANalytical device, working at 45 kV and 40 mA, emitting monochromatic radiation Ka of Cu (1.594 Å) in the configuration θ–2θ in a range of 35◦ to 85◦, with a step of 0.02◦. The chemical surface composition of the coating was determined through X-ray Coatings 2019, 9, 215 3 of 13 photoelectron spectroscopy (XPS). XPS spectra were recorded in a SPECS spectrometer in the constant pass energy mode at 50 eV, using Mg Kα radiation as the excitation source. Sample cleaning was performed with Ar+ ions of 3.5 keV for 5 min in a preparation chamber (base pressure 2 × 10−7 mbar) connected through a gate valve to the chamber. The calibration of the binding energy (BE) scale was checked using the C 1s signal (284.1 eV). X-ray spectroscopy (EDS) was carried out at a voltage of 20 kV and collection time of 120 s in a FEI Quanta 200 scanning electron microscope (Thermo Fisher Scientific, Waltham, MA, USA). The measurement of the hardness of the coatings was performed with a CSM Berkovich nanoindentation instrument (Peseux, Switzerland) with a speed of approach to the sample of 2000 nm/min, a rate of application of the load of 10 mN/min, and a maximum load of 30 mN, pausing for 15 s in the application of the load. The friction coefficient was evaluated through the pin-on-disk test using a C ETR-UMC-2 tribometer (San Jose, CA, USA) with steel balls of 6 mm in diameter, coated with a layer of Al2O3 and with an applied load of 4 N at a speed of 50 mm/s for 10 min. The wear track cross profile was measured for at least four points of the wear track with a Dektak 150 profilometer (Tucson, Arizona, USA) in order to obtain an average of the wear-track width. After the test, the wear tracks were examined using a Bruker contour GT optical profilometer (Tucson, Arizona, USA). The wear volume (Wv) of the films and the balls was calculated according to ASTM G99-17 [16]. The wear products were chemically analyzed using energy-dispersive X-ray spectroscopy (Thermo Fisher Scientific, Waltham, MA, USA) (EDS). The wearzrate (Ws) was calculated according to Archard’s equation, Equation (1), where F is the normal load (N) and L is sliding length (mm). The wear rate is reported in mm3/Nm [16].

W W = v (1) s F·L

The maximum contact pressure was 1.5 Pa with a contact radius of 1.5 × 10−3 m, assuming an elastic modulus of 380 GPa and a Poisson ratio of 0.3 for Al2O3, and for the ternary carbide coating, an elastic modulus of 308 GPa and a Poisson ratio of 0.24. The contact radius and the contact pressure were useful for calculating the shear stress distribution along the x, y, and z directions for the two surfaces in contact, using the following Equations [17]:

−1 n h z i h a io 1 z2 σ = σ = −P (1 + v) 1 − tan−1 + 1 + , (2) x y 0 a z 2 a2

−1 z2 σ = −P 1 + , (3) z 0 a2

σx − σz τ = , (4) max 2 where σx, σy and σz are the shear stress along the x, y, and z directions, respectively, z is the depth of the shear stress, and τmax is the maximum shear stress. Figure1 shows the determined maximum shear stress and its distribution. The maximum shear stress was 239 MPa, located at a depth of 0.91 µm below the coating surface. The electrochemical behavior of the coatings was analyzed by means of potentiodynamic polarization and electrochemical impedance (EIS) tests. These measurements were performed on a Gamry 600 brand potentiostat instrument using an electrochemical cell with a volume of 100 mL in a solution of 3% NaCl at 23 ◦C. The analysis area was 0.196 cm2, using a bar of platinum and a calomel electrode as the auxiliary and reference electrodes, respectively. Polarization assays were performed with a scanning rate of 0.5 mV/s, supplying a bias from −0.3 to 0.4 V with respect to the resting potential. EIS tests were conducted while varying the frequency from 0.01 KHz to 100 MHz, applying a voltage of 10 mV. CoatingsCoatings2019 2019, ,9 8,, 215 x FOR PEER REVIEW 44 of of 13 12

200 0 -200 -400 -600 max -800 x y -1000 z

-1200 stress(MPa) Contact -1400 0,000 0,001 0,002 0,003 0,004 Depth(mm)

Figure 1. Shear stress distribution for the carbide coating of vanadium–chromium deposited on AISI Figure 1. Shear stress distribution for the carbide coating of vanadium–chromium deposited on AISI D2 steel. D2 steel.

3.3. ResultsResults FigureFigure2 2shows shows the the SEM SEM micrographs micrographs of of the the cross cross section section of of three three of of the the coatings coatings produced produced on on AISIAISI D2.D2. UsingUsing thethe measurementmeasurement scale,scale, itit cancan be be establishedestablished thatthat the the coatingscoatings have have thethe followingfollowing thicknesses:thicknesses: M4: 12.4 12.4 ±± 0.20.2 µm,µm, M3: M3: 13.1Cr7C3(150) 13.1 ±Cr7C3(201) 0.1± µm0.1Cr7C3(112) ,µ andm,Cr7C3(060) andM2: M2:12.7 Cr7C3(202) ± 12.7 0.5 Cr7C3(331) ±µm.0.5Cr7C3(222) TheCr7C3(161) µm. Cr7C3(260) M1 The sample M1 sample (not shown) (not shown)had a thickness had a thickness of 15.9 of± 0.5 15.9 um.± 0.5 In um.general, In general, the cross the-section cross-section of the ofcoatings the coatings is compact is compact,, since sincethere therewere wereno pores no pores and the and deposited the deposited material material was completely was completely melted melted.. Cr23C6(420) Cr23C6(422) Cr23C6(511) Cr23C6(440) Cr23C6(531) V8C7(222) V8C7(400)

Fe(111) M3

M1 Intensity (a.u) M2

M4 Figure 2. SEM micrographs of the (a) M4; (b) M3; (c) M2; and (d) M1 samples. Figure0 2. SEM micrographs of the (a) M4; (b) M3; (c) M2; and (d) M1 samples. Figure3 shows the35 XRD patterns 40 of each of the coatings 45 (M1, M2, 50 M3, and M4).55 The M4 sample Figure 3 shows the XRD patterns of each of the coatings (M1, M2, M3, and M4). The M4 sample exhibits the cubic V8C7 crystalline phase structure of the NaCl type [18]. This phase is reported on the exhibits the cubic V8C7 crystalline phase structure2(De ofg therees NaCl) type [18]. This phase is reported on the PDF 00-025-1002 card, which assigned the highest intensities to planes (111) and (200). The coating PDF 00-025-1002 card, which assigned the highest intensities to planes (111) and (200). The coating grown from ferro-chrome (M3) has two crystalline phases: one for Cr23C6 belonging to the structure grown from ferro-chrome (M3) has two crystalline phases: one for Cr23C6 belonging to the structure of NaCl (PDF 00-035-0783) with planes (420), (422), and (511) of high intensity, and the other for of NaCl (PDF 00-035-0783) with planes (420), (422), and (511) of high intensity, and the other for chromium carbide (Cr7C3, PDF 00-036-1482), with an orthorhombic phase that highlights planes (150), chromium carbide (Cr7C3, PDF 00-036-1482), with an orthorhombic phase that highlights planes (150), (112), (151), and (060). Due the complex structure of the XRD patterns of the M1 and M2 samples, (112), (151), and (060). Due the complex structure of the XRD patterns of the M1 and M2 samples, in in Figure4, the deconvolution of these patterns is shown in detail, where we see the presence of Figure 4, the deconvolution of these patterns is shown in detail, where we see the presence of two for two for chromium carbide and one fors vanadium carbide. These results suggest that the samples chromium carbide and one fors vanadium carbide. These results suggest that the samples with the with the greatest content of Fe-Cr allow a chemical reaction between the carbon of the steel and the greatest content of Fe-Cr allow a chemical reaction between the carbon of the steel and the chromium chromium of the bath. The presence of vanadium carbide in the M1 and M2 samples in the coatings of the bath. The presence of vanadium carbide in the M1 and M2 samples in the coatings can be can be explained by the greater thermodynamic stability in forming vanadium carbide, because the explained by the greater thermodynamic stability in forming vanadium carbide, because the energy energy of formation of vanadium carbide (−24 Kcal / mol) [19] is less than that of chromium carbide of formation of vanadium carbide (−24 Kcal / mol) [19] is less than that of chromium carbide (−18 (−18 Kcal/mol) [20]. Kcal/mol) [20].

200 0 -200 -400 -600 max -800 x y -1000 z

-1200 stress(MPa) Contact -1400 0,000 0,001 0,002 0,003 0,004 Depth(mm)

Coatings 2019, 9, 215 5 of 13

Coatings 2019, 8, x FOR PEER REVIEW 5 of 12 Cr7C3(150) Cr7C3(201) Cr7C3(112) Cr7C3(060) Cr7C3(202) Cr7C3(331) Cr7C3(222) Cr7C3(161) Cr7C3(260) Cr23C6(420) Cr23C6(422) Cr23C6(511) Cr23C6(440) Cr23C6(531) V8C7(222) V8C7(400)

Fe(111) M3

M1 Intensity (a.u) M2

M4 0 35 40 45 50 55

2(Degrees) Figure 3. XRD patterns of different carbides. Figure 3. XRD patterns of different carbides.

Cr7C3 (112)

a) M1 V8C7 (400) Cr23C6 (511) Cr7C3 (151) superposition

42,0 42,5 43,0 43,5 44,0 44,5 45,0 45,5 46,0

b) M2

Intensity (a.u.)

42,0 42,5 43,0 43,5 1 44,0 44,5 45,0 45,5 46,0 2(Degrees)

Figure 4. Deconvolution of the XRD patterns for samples (a)) M1 and ((bb)) M2.M2. Figure5 shows K α and Kβ emission lines of the chromium (5.41 and 5.95 keV), vanadium (4.95 Figure 5 shows Kα and Kβ emission lines of the chromium (5.41 and 5.95 keV), vanadium (4.95 and 5.43 keV), aluminum (1.48 and 1.56 keV), and iron (6.40 and 7.06 keV) in the coatings. The Kα and and 5.43 keV), aluminum (1.48 and 1.56 keV), and iron (6.40 and 7.06 keV) in the coatings. The Kα Kβ lines are not detectable using the EDS technique because the energy of those lines is about 0.185 keV. and Kβ lines are not detectable using the EDS technique because the energy of those lines is about Table2 summarizes the content of each element in at%. These results allow us to establish that the M1 0.185 keV. Table 2 summarizes the content of each element in at%. These results allow us to establish sample, even though it was sinterized with a greater ferro-chromium content than ferro-vanadium, that the M1 sample, even though it was sinterized with a greater ferro-chromium content than ferro- contains more V than Cr in the coating. This is because the formation energy of vanadium carbide vanadium, contains more V than Cr in the coating. This is because the formation energy of vanadium is more negative than that of chromium carbide. In the M2 and M3 samples, the percentage of Cr carbide is more negative than that of chromium carbide. In the M2 and M3 samples, the percentage increases, because the ferro-chromium increases from 26% to 30% wt. Finally, the M4 sample has a of Cr increases, because the ferro-chromium increases from 26% to 30% wt. Finally, the M4 sample high content of vanadium because of the lack of ferro-chromium in the fabrication process. has a high content of vanadium because of the lack of ferro-chromium in the fabrication process.

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Figure 5. EDS spectra of the carbides grown onon D2 steel.steel. ((a)a) M2;M2; ((b)b) M1;M1; ((c)c) M3;M3; ((d)d) M4.M4. Table 2. Chemical composition of the carbides grown on D2 steel evaluated via EDS. Table 2. Chemical composition of the carbides grown on D2 steel evaluated via EDS. V (Wt %) Cr (Wt %) C (Wt %) Fe (Wt %) Al (Wt %) O (Wt%)

M1V 63.88(Wt %) Cr ( 4.14Wt %) C (W 4.78t %) Fe (W 21.26t %) Al (W 1.95t %) O (W 3.97t%) M2M1 63.88 9.46 54.044.14 4.78 3.09 21.26 29.73 1.9 1.885 3.97 1.90 M3 — 53.48 2.91 31.77 1.60 10.24 M4M2 66.049.46 54.04 – 3.09 10.08 29.73 6.84 1.88 — 1. 17.0490 M3 --- 53.48 2.91 31.77 1.60 10.24 Figure6M4 shows 66 XPS.04 spectra for-- the Cr carbide10.08 coatings6.84 of the M1-V--- sample17.04 deposited on tool steel. Figure6a shows the extended XPS spectrum. In this spectrum, binding energies can be identified Figure 6 shows XPS spectra for the Cr carbide coatings of the M1-V sample deposited on tool corresponding to vanadium, chromium, oxygen, carbon, and iron on the coating’s surface. Figure6b steel. Figure 6a shows the extended XPS spectrum. In this spectrum, binding energies can be shows the high-resolution XPS spectrum of C1s. Within the spectrum, four contributions can be identified corresponding to vanadium, chromium, oxygen, carbon, and iron on the coating’s surface. identified. The first is centered on 284.4 eV, which corresponds to the aliphatic carbon found on the Figure 6b shows the high-resolution XPS spectrum of C1s. Within the spectrum, four contributions coating’s surface; the second is centered on 285.6 eV, which corresponds to the binding energy of C in can be identified. The first is centered on 284.4 eV, which corresponds to the aliphatic carbon found the vanadium carbide; and the last two contributions are focused on 286.5 and 287.4 eV, corresponding on the coating’s surface; the second is centered on 285.6 eV, which corresponds to the binding energy to the binding energy of C in the chromium carbide [21]. of C in the vanadium carbide; and the last two contributions are focused on 286.5 and 287.4 eV, In Figure6c, the 2p peak of Cr is exhibited, formed by the contributions of two peaks centered at corresponding to the binding energy of C in the chromium carbide [21]. 576.12 and 574.4 eV. These energies correspond to chromium oxide (Cr O ) and chromium carbide In Figure 6c, the 2p peak of Cr is exhibited, formed by the contributions2 3 of two peaks centered (Cr3C2). Figure6d shows the V2p and O1s peaks, which are composed of two contributions: V 2p at 576.12 and 574.4 eV. These energies correspond to chromium oxide (Cr2O3) and chromium carbide exhibits binding energies centered at 512.6 and 514.2 eV. These energies correspond to the V–C (Cr3C2). Figure 6d shows the V2p and O1s peaks, which are composed of two contributions: V 2p bonding, and the other peak corresponds to vanadium oxide. The O1s peak has four contributions: i) exhibits binding energies centered at 512.6 and 514.2 eV. These energies correspond to the V–C one centered at 530.1 eV, corresponding to Cr O [22]; ii) one centered at 531.4 eV, which is the energy bonding, and the other peak corresponds to vanadium2 3 oxide. The O1s peak has four contributions: i) of an OH group; iii) one centered at 532 eV, which is the energy corresponding to the water absorbed one centered at 530.1 eV, corresponding to Cr2O3 [22]; ii) one centered at 531.4 eV, which is the energy by the coating (when it is exposed to the atmosphere); and iv) one centered at 533 eV, which is the of an OH group; iii) one centered at 532 eV, which is the energy corresponding to the water absorbed energy of the oxygen absorbed into the vanadium carbide [23]. by the coating (when it is exposed to the atmosphere); and iv) one centered at 533 eV, which is the The hardness values obtained for the carbide coatings are shown in Table3. The lowest hardness energy of the oxygen absorbed into the vanadium carbide [23]. value for M3 is obtained in the sample corresponding to chromium carbide (14.7 GPa), and for the The hardness values obtained for the carbide coatings are shown in Table 3. The lowest hardness M4 sample, vanadium carbide values above 20 GPa were recorded, which are consistent with other value for M3 is obtained in the sample corresponding to chromium carbide (14.7 GPa), and for the studies [24,25]. Overall, a significant increase in hardness for the chromium–vanadium systems (M1 M4 sample, vanadium carbide values above 20 GPa were recorded, which are consistent with other and M2) was not observed, possibly because there is no substantial atomic substitution between studies [24,25]. Overall, a significant increase in hardness for the chromium–vanadium systems (M1 the crystal lattices of the materials that form the coatings. Table3 also shows the wear rate of the and M2) was not observed, possibly because there is no substantial atomic substitution between the carbide coatings. These values show evidence that all the coatings have higher wear resistance than crystal lattices of the materials that form the coatings. Table 3 also shows the wear rate of the carbide the substrate. Furthermore, the M3 sample exhibits the lowest wear resistance, possibly due to the low coatings. These values show evidence that all the coatings have higher wear resistance than the degree of hardness. substrate. Furthermore, the M3 sample exhibits the lowest wear resistance, possibly due to the low degree of hardness.

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a) overview b) C 1s Fe 2p V 2s CrxCy(285.9) Cr 2p O 1s CrxCy V 2p Cf(284.5)

Intensity (a.u.) C 1s

Intensity (a.u.)

1200 1000 800 600 400 200 0 292 290 288 286 284 282 280 278 Binding Energy (eV) Binding Energy (eV)

c) Cr 2p CrxCy(575.1) d) O 1s y V2p O1s Cr2O3(577.05) H O(532.8) 2 V 2p OH(532.2) O (533.5) a Cr2O3(530.5) V2O5(517.1)

Intensity (a.u.) V C Intensity (a.u.) 8 7(513.6)

590 585 580 575 570 540 530 520 510 500 Binding Energy (eV) Binding Energy (eV) Figure 6. 6. XPSXPS spectra spectra of ofthe the coatings coatings deposited deposited on the on M2 the sample. M2 sample. (a) survey (a) spectra;survey (b spectra) high; resolution (b) high resolutionof C 1s; (c) of high C 1s resolution ; (c) high ofresolution Cr 2p; (d )of high Cr 2p resolution; (d) high ofresolution 0 1s. of 0 1s.

Table 3. Hardness of the coatings deposited on AISI D2. Table 3. Hardness of the coatings deposited on AISI D2.

SampleSample HardnessHardness (Gpa)(Gpa) RateRate Wear wear AISI D2 6.45 ± 0.28 324 × 10−6 AISI D2 6.45 ± 0.28 324 × 10−6 M1 23.3 ± 0.91 26 × 10−6 M1M2 23.3 25.1 ±± 0.911.1 23 ×2610 ×− 106 −6 ± × −6 M2M3 14,725.1 ± 0.651.1 47 2310 × 10−6 M4 24.82 ± 0.69 26 × 10−6 M3 14,7 ± 0.65 47 × 10−6 M4 24.82 ± 0.69 26 × 10−6 Figure7 shows the curves of the friction coefficient for each of the carbides deposited on AISI D2. FromFigure the 7 shows graph, the it can curves be established of the friction that thecoefficient coefficient for each of friction of the (COF) carbides ofthe deposited grown carbideson AISI D2.is between From the 0.3 graph and 0.4,, it can while be forestablished the substrate that the it is coefficient greater than of friction 0.6. These (COF) values of the agree grown with carbides results isobtained between in 0.3 other and studies 0.4, while [26]. for The the decrease substrate in the it is coefficient greater than of friction 0.6. These of the values coatings agree compared with results with obtainedthe substrate in other could studies be explained [26]. The by d consideringecrease in the that coefficient there is free of f carbonriction of on the the coatings surface, whichcompared probably with theacts substrate as a lubricant could when be explain the ball-on-disked by considering test is performed. that there is free carbon on the surface, which probablyFigure acts8 shows as a lubricant the scar when wear the for ball the-on M1-disk and test M2 is samples. performed. The figure shows parallel lines or channels,Figure characteristic 8 shows the of scar adhesive wear wear.for the According M1 and toM2 Adachi sampl andes. The Hutchings, figure shows this situation parallel is lines known or channels,as grooving characteristic wear, or the of abrasion adhesive of wear. two bodies According [27]. Theseto Adachislots and arise Hutchings, from the cutting this situation action of is knownabrasive as microparticles grooving wear, that or arethe fundamentally abrasion of two embedded bodies [27]. in theThese ball slots with arise which from the the test cutting is conducted. action ofThe abrasive microparticles micropa wererticles possibly that are produced fundamentally by the mechanisms embedded of inthe the contact ball with fatigue which and the the plastic test is conducted.deformation The that microparticles are generated were during possibly the contact produced of the by tribological the mechanisms pair. These of the types contact of wearfatigue can and be thecombined plastic withdeformation an oxidation that wearare generated mechanism during promoted the contact by the of formation the tribological of oxides pair. due These to the types increase of wearin temperature can be combined and the with reaction an oxidation of elements wear such mechanism as chromium promoted and vanadium by the formation with the of oxygen oxides in due the toenvironment, the increase possibly in temperature forming and a film the that reaction prevents of elements the continuous such as removal chromium of material. and vanadium with the oxygen in the environment, possibly forming a film that prevents the continuous removal of material.

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D2 0,7 D2 0,7 M4 M4M3 0,6 M3 0,6 M2 M2 0,5 M1 0,5 M1 0,4 0,4 0,3 COF 0,3

COF 0,2 0,2 0,1 0,1 0,0 0,0 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 Time (s) Time (s) Figure 7. Coefficient of friction versus time for the carbides deposited on AISI D2. FigureFigure 7. 7.CoefficientCoefﬁcient of of friction friction versus versus time time for the carbidescarbides depositeddeposited on on AISI AISI D2. D2.

FigureFigure 8. 8.SEMSEM micrographs micrographs of of the the wear wear tracks tracks for the ((aa)) M1;M1; ( b(b)) M2; M2; ( c()c) M3; M3 and; and (d ()d M4) M4 samples. samples. Figure 8. SEM micrographs of the wear tracks for the (a) M1; (b) M2; (c) M3; and (d) M4 samples. To determine the corrosion rate of the coating–substrate system, the potential vs. current curves, To determine the corrosion rate of the coating–substrate system, the potential vs. current curves, calledTo determine potentiodynamic the corrosion polarization rate of and the Tafelcoating polarization,–substrate cansystem, be used. the potential Figure9 shows vs. current the curves curves, called potentiodynamic polarization and Tafel polarization, can be used. Figure 9 shows the curves calledobtained potentiodynamic for uncoated AISIpolarization D2 steel andand for Tafel the coatings.polarization Table, can4 summarizes be used. Figure the parameters 9 shows obtainedthe curves obtained for uncoated AISI D2 steel and for the coatings. Table 4 summarizes the parameters obtained obtainedthrough for this uncoated measurement. AISI D2 From steel theand values for the in coatings. the table, Tab it canle 4 besummarizes established the that parameters the coated obtained steels through this measurement. From the values in the table, it can be established that the coated steels throughexhibit this increased measurement. corrosion From resistance, the values since in the the corrosion table, it can current be establish Icorr decreasesed that bythe ancoated order steels of exhibit increased corrosion resistance, since the corrosion current Icorr decreases by an order of exhibitmagnitude increased with corrosion respect to resistance,the Icorr values since for the the corrosion AISI D2. current Additionally, Icorr the decreases results showby an that order the of magnitude with respect to the Icorr values for the AISI D2. Additionally, the results show that the magnitudecoatings thatwith grow respect in a to salt the bath Icorr with values the highest for the concentration AISI D2. Additionally, of vanadium the exhibit results lower show corrosion that the coatings that grow in a salt bath with the highest concentration of vanadium exhibit lower corrosion coatingsresistance, that whilegrow thein a coatingssalt bath with with chromium the highest contents concentration exhibit theof vanadium best protection exhibit against lower corrosion corrosion resistance, while the coatings with chromium contents exhibit the best protection against corrosion resistance,These results while can the be coatings explained with by consideringchromium contents the XPSanalysis, exhibit the which best showed protection the formation against corrosion of two These results can be explained by considering the XPS analysis, which showed the formation of two Thesetypes results of oxide can inbe theexplained coating: by chromium considering oxide the (Cr XPS2O analysis,3) and vanadium which showed oxide (Vthe2O formation5). Chromium of two typesoxide of hasoxide a high in the degree coating: of chemical chromium stability oxide and (Cr is2O used3) and in vanadium industry as oxide a protective (V2O5).material Chromium against oxide types of oxide in the coating: chromium oxide (Cr2O3) and vanadium oxide (V2O5). Chromium oxide has a high degree of chemical stability and is used in industry as a protective material against hasdegradation a high degree in layers of chemical or thin ﬁlms stability [28,29 ], and and is vanadium used in oxideindustry is chemically as a protective unstable material and therefore against degradation in layers or thin films [28,29], and vanadium oxide is chemically unstable and therefore degradationdoes not have in layers the ability or thin to servefilms as[28,29], a protective and vanadium layer [30, 31oxide]. is chemically unstable and therefore does not have the ability to serve as a protective layer [30,31]. does not have the ability to serve as a protective layer [30,31].

Coatings 2019, 9, 215 9 of 13 Coatings 2019, 8, x FOR PEER REVIEW 9 of 12

0,2 AISI D2 M2 0,0 M1 M3 -0,2 M4

-0,4

-0,6

Vf(V vs Ref) -0,8

-1,0

-1,2

100p 1n 10n 100n 1µ 10µ 100µ 1m 10m Current (A) Figure 9. Linear pol polarizationarization curves for carbide coatings.

Table 4. Parameters obtained from the linear polarization curves for each of the coatings: current Table 4. Parameters obtained from the linear polarization curves for each of the coatings: current corrosion (Icor), corrosion potential (Ecor), and the anodic (βa) and cathodic (βc) Tafel slopes. corrosion (Icor), corrosion potential (Ecor), and the anodic (βa) and cathodic (βc) Tafel slopes.

SampleSample IcorrIcorr (A) (A) Error Error Ecorr Ecorr(mV) (mV) Error Error Βa (V) Ba (V)Βc (V) Bc (V) M1 1.28 × 10−7 2.68 × 10−8 −489.3 2.1 0.18 0.49 M1 1.28 × 10−7 2.68 × 10−8 −489.3 2.1 0.18 0.49 M2 1.63 × 10−7 1.81 × 10−8 −544.7 1.5 0.19 0.36 M3 M2 1.221.63× 10× 10−7−7 1.812.98 × ×1010−8 −8 −544.7458.6 1.5 3.50.19 0.170.36 0.5297 M4 6.66 × 10−7 9.50 × 10−8 −692.7 5.2 0.29 0.438 M3 1.22 × 10−7 2.98 × 10−8 458.6 3.5 0.17 0.5297 AISI D2 1.19 × 10−6 1.07 × 10−7 −807 1.2 0.362 0.251 M4 6.66 × 10−7 9.50 × 10−8 −692.7 5.2 0.29 0.438

−6 −7 Figure 10AISI shows D2 the1 electrochemical.19 × 10 1.07 impedance× 10 − spectroscopy807 1.2 (EIS) spectra0.362 obtained0.251 for the steel and theFigure carbides 10 shows after the 7 days electrochemical of corrosion impedance treatment in spectroscopy a solution of (EIS) 3% NaCl.spectra The obtained equivalent for the circuits steel (EC)and the proposed carbides for after the 7 fitdays of theof corrosion experimental treatment spectra in a are solution shown of in 3% Figure NaCl. 11 The. The equivalent fit between circuits the theoretical(EC) proposed and experimentalfor the fit of spectrathe experimental was performed spectra with are Gamry shown Echem in Figure Analyist 11. The software, fit between version the 7. Thetheoretical EC corresponding and experimental to AISI spectra D2 (Figure was 11performa) hased a resistance, with Gamry Rsol, Echem which Analyist is the solution software resistance,, version connected7. The EC corresponding in series with to two AISI parallel D2 (Fig elements:ure 11a) has a constant a resistance phase, R elementsol, which (CPE) is the with solution an exponent resistance, n, inconnected which a in value series of 1with indicates two parallel that that elements: element ofa constant the proposed phase circuit element is completely (CPE) with capacitive, an exponent and n a, valuein which of0 a indicatesvalue of 1 that indicates that circuit that that element element is resistive. of the proposed Values between circuit is 0 completely and 1 indicate capacitive, a capacitive and anda value resistive of 0 indicates behavior. that Rpol that iscircuit the polarization element is resistive. resistance, Values which between is the resistance0 and 1 indicate to the a passagecapacitive of chargeand resistive through behavior. the substrate-electrolyte Rpol is the polarization interface. resistance, For the which coated is steel the resistance (Figure 11 tob), the the passage proposed of circuitcharge representsthrough the a coatingsubstrate material-electrolyte with interface. pores. The For EC the has coated an Rsol steel that (Fig representsure 11b), thethe resistanceproposed ofcircuit the electrolyticrepresents a solution, coating amaterial constant with phase pores. element The EC CPE-c has a withn Rsol its that exponent represents m that the represents resistance the of coating-solutionthe electrolytic solution, interface, a andconstant Rpor, phase which element represents CPE the-c resistancewith its exponent to charge m transfer that represents through the pores.coating Additionally,-solution interface, a second and constant Rpor, which phase represents CPE-s is included,the resistance with to its charge exponent transfer n along through with the corrosionpores. Additionally, resistance Rcora second describing constant the phase electrochemical CPE-s is included, behavior andwith the its resistanceexponent ton along charge with transfer the atcorrosion the coating–substrate resistance Rcor interface. describing the electrochemical behavior and the resistance to charge transferBode at plotsthe coating show a–substrate polarization interface. resistance at 0.01 Hz in the range of 5 kΩ for the AISI D2 substrate (FigureBode 10 plots), while show for a thepolarization coatings, resistance the impedance at 0.01 valuesHz in the are range about of 105 kΩ kΩ forat the the AISI same D2 frequency. substrate In(Fig general,ure 10), the while impedance for the coatings, decreases the with impedance the exposure values time are forabout all cases,10 kΩ whichat the couldsame frequency. be due to the In increasegeneral, inthe porosity impedance over time,decreases which with allows the the exposure electrolyte time to for penetrate all cases, and which reach thecould coating–substrate be due to the interface.increase in Additionally, porosity over the time, figure which that shows allows the the behavior electrolyte of the to phase penetr asate a function and reach of the the frequency coating– showssubstrate two interface relaxation. Additionally, times for the the coatings. figure that This shows is evident the behavior in the coating of the submerged phase as afor function 7 days of (blue the curvefrequency triangles). shows Thesetwo relaxation relaxation times times for are the given coatings. by [32 ]This is evident in the coating submerged for 7 days (blue curve triangles). These relaxation times are given by [32]

(5) c  RporCc cor  RcorCs

Coatings 2019, 9, 215 10 of 13 Coatings 2019, 8, x FOR PEER REVIEW 10 of 12

c s whereCoatings C is2019 the, 8, capacitancex FOR PEER REVIEW of coatings and C is the capacitance of the substrate–coating interface10 of 12. The relaxation time at high frequenciesτc = RporC representsc τthecor dielectric= RcorCs coating performance (Cc, Rpor),(5) andw thehere relaxation Cc is the capacitance time at low of frequencies coatings and represents Cs is the capacitance the properties of the of substratethe substrate/coating–coating interface interface. where Cc is the capacitance of coatings and Cs is the capacitance of the substrate–coating interface. (Cs, RcorThe) [33]. relaxation Bode diagramstime at high of frequenciesthe M4 sample represents do not the show dielectric good coating corrosion performance resistance, (C c,since Rpor), the The relaxation time at high frequencies represents the dielectric coating performance (Cc, Rpor), sampleand the has relaxation a layer of time vanadium at low frequencies oxide on the represents surface ,the which properties has little of the chemical substrate/coating stability (Figure interface 9b), and the relaxation time at low frequencies represents the properties of the substrate/coating interface while(Cs, theRcor M2) [33]. sample Bode exhibitsdiagrams high of the corrosion M4 sample resistance do not show due togood the corrosion presence resistance, of a surface since layer the of (Cs, Rcor) [33]. Bode diagrams of the M4 sample do not show good corrosion resistance, since the chromiumsample has oxide, a layer which of vanadium has excellent oxide chemical on the surface stability, which and actshas littleas a chemicalprotective stability coating. (Figure The values 9b), sample has a layer of vanadium oxide on the surface, which has little chemical stability (Figure9b), of thewhile parameters the M2 sample obtained exhibits after highthe fit corrosion of the proposed resistance equivalent due to the circuits presence are ofsummarized a surface layer in T able of while the M2 sample exhibits high corrosion resistance due to the presence of a surface layer of 5 forchromium the coatings oxide, produced, which has after excellent exposure chemical times stability of 168 andh. The acts result as a protective shows that coating. Rpor andThe Rcorvalues are chromium oxide, which has excellent chemical stability and acts as a protective coating. The values of lowerof the for parameters the M4 sample obtained, indicating after the low fit corrosionof the proposed resistance equivalent for this circuits coating. are This summarized could be inexplained Table the parameters obtained after the ﬁt of the proposed equivalent circuits are summarized in Table5 for by 5the for formation the coatings of produced,only vanadium after exposure oxide in times the coatingof 168 h during. The result its production shows that ,Rpor as seen and in Rcor the are XPS the coatings produced, after exposure times of 168 h. The result shows that Rpor and Rcor are lower spectralower, wforhereas the M4 the sample coatings, indicating with high low corrosionchromium resistance content for have this higher coating. values This could of Rcor be explained and Rpor , forby the the M4 formation sample, of indicating only vanadium low corrosion oxide in resistance the coating for during this coating. its production This could, as be seen explained in the XPS by the which could be explained by the presence of Cr23C6 and Cr7C3 along with the formation of the stable formationspectra, w ofhereas only vanadiumthe coatings oxide with inhigh the chromium coating during content its have production, higher values as seen of inRcor the and XPS Rpor spectra,, oxide Cr2O3, creating a passive layer on the coating that protects the substrate from electrolyte whereaswhich could the coatings be explained with highby the chromium presence of content Cr23C6 have and Cr higher7C3 along values with of the Rcor formation and Rpor, of whichthe stable could penetration. beoxide explained Cr2O3 by, creat theing presence a passive of Cr layer23C6 and on the Cr7 coatingC3 along that with protects the formation the substrate of the stablefrom electrolyte oxide Cr2 O3, creatingpenetration. a passive layer on the coating that protects the substrate from electrolyte penetration. 80 (a) AISI D2(1h) M4(1h) (c) M3(1h) 10k 60 AISI D2(1d) 60 (b) M3(1d) ) M4(1d) 100k AISI D2(2d) ) M3(2d)80 60 (a) AISI D2(1h) M4(1h)M4(2d) ) (c) M3(1h) AISI D2 (7d) 10k 60 M3(7d) AISI D2(1d) 60 (b) M3(1d) ) 1k 40 M4(1d)M4(7d) 40 100k AISI D2(2d) ) 10k M3(2d) 60 M4(2d) ) 40 AISI D2 (7d) 1k M3(7d) 1k 40 M4(7d) 40

Degrees

( 10k 20 20 40 ( Degrees 1k  20

( 1k 



Degrees

( 100 20 20 (

Z mod(ohms)

Degrees

Z mod(ohms) 1k  20

( 0 0 0  Z mod(ohms) 100 100  100

Z mod(ohms) Z mod(ohms) 0 0 0 10m 100m 1 10 100 1k 10k 100k 10m 100m 1 10 100 1k 10k 100k Z mod(ohms) 10010m 100m 1 10 100 1k 10k 100k 100 (Hz) (Hz) 10m 100m 1 (Hz)10 100 1k 10k 100k 10m 100m 1 10 100 1k 10k 100k 10m 100m 1 10 100 1k 10k 100k (Hz) (Hz) (Hz) 80 100k 80 (d) M1(1h) (e) M2(1h)

100k 80 ) 100k M2(1d)80

M1(1d) M2(1h) ) (d) M1(1h) 60 (e) M2(2d) 60 100k M1(2d) ) M2(1d)

M1(1d) 10k M2(7d) ) 60 M2(2d) 60 10k M1(7d) M1(2d) 40 10k M2(7d) 40 10k M1(7d) 40 40

Degrees

1k ( 1k

Degrees

20 20 (



Degrees

1k ( 1k



Degrees

20 20 (

Z mod(ohms)



Z mod(ohms) 100  0 Z mod(ohms) 100 0 Z mod(ohms) 100 0 100 0 10m 100m 1 10 100 1k 10k 100k 10m 100m 1 10 100 1k 10k 100k 10m 100m 1(Hz)10 100 1k 10k 100k 10m 100m 1 10(Hz)100 1k 10k 100k (Hz) (Hz) Figure 10. Bode plots for carbide coatings. (a)substrate; (b) M4: (c)M3: (d) M1: (e) M2 FigureFigure 10. 10.Bode Bode plots plots for for carbide carbide coatings.coatings. ((a)a)substrate substrate;; (b) (b )M4 M4;: (c ()cM3) M3;: (d) ( dM1:) M1; (e) (M2e) M2.

Figure 11. Equivalent circuits for electrochemical impedance spectroscopy (EIS) tests. (a) substrate Figure(bFigure) coating. 11. 11. Equivalent Equivalent circuits circuits for for electrochemical electrochemical impedanceimpedance spectroscopy ( EIS(EIS) )tests. tests. (a) (a) substrate substrate (b) (b) coatingcoating. .

TableTable 5. V5.alues Values of ofthe the parameters parameters obtained obtained afterafter thethe fitfit of the proposedproposed equivalentequivalent circuits circuits to to the the coatings produced, after exposure times of 168 h. CPE: constant phase element. coatings produced, after exposure times of 168 h. CPE: constant phase element. Sample Rsol (ohm) Rcor (ohm) Rpor (ohm) CPE-c m CPE-s n Sample Rsol (ohm) Rcor (ohm) Rpor (ohm) CPE-c m CPE-s n

Coatings 2019, 9, 215 11 of 13

Table 5. Values of the parameters obtained after the ﬁt of the proposed equivalent circuits to the coatings produced, after exposure times of 168 h. CPE: constant phase element.

Sample Rsol (ohm) Rcor (ohm) Rpor (ohm) CPE-c m CPE-s n M1 74.4 2176 3087 7.1 × 10−10 0.5 1.1 × 10−3 0.80 M2 68.2 11,160 14,010 1.8 × 10−10 0.76 1.95 × 10−5 0.83 M3 68.4 21,710 12,730 4.02 × 10−10 0.76 4.27 × 10−5 0.82 M4 103 11,000 90 2.13 × 10−6 0.77 4.17 × 10−5 0.63

4. Conclusions Carbides of vanadium, chromium, and chromium–vanadium were grown using the TRD technique on AISI D2 tool steels. The coatings exhibit a good uniformity of thickness values, between 12 and 15 microns, and a value of hardness greater than that of the substrate. Moreover, potentiodynamic polarization curves show better performance as a protective coating than those grown with greater chromium content in the salt bath treatment. The EIS spectra conﬁrm the nature of the ceramic carbide produced, which improves the electrochemical performance of the coatings compared with uncoated steel. The wear tests for samples M1 and M2 show the presence of parallel channels, characteristic of adhesive wear. This morphology is the result of wear or abrasion between two bodies. The friction coefﬁcients are lower for the carbides than for uncoated steel, mainly due to the presence of a hard coating on the surface.

Author Contributions: F.E.C., J.E.A. and J.J.O. conceived and designed the experiments; F.E.C. performed the experiments; F.E.C., J.E.A. and J.J.O. wrote the paper. Funding: The authors are grateful for the financial support of Colciencias through the project with code 1101-521-28337 and contract 338-2011. Conflicts of Interest: The authors declare no conflict of interest.

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