Cr-based sputtered decorative coatings for automotive industry

materials Article Cr-Based Sputtered Decorative Coatings for Automotive Industry Edgar Carneiro 1,* , Nuno M. G. Parreira 1, Todor Vuchkov 2,3 , Albano Cavaleiro 2,3 , Jorge Ferreira 4, Martin Andritschky 1and Sandra Carvalho 2   Citation: Carneiro, E.; Parreira, N.M.G.; Vuchkov, T.; Cavaleiro, A.; Ferreira, J.; Andritschky, M.; Carvalho, S. Cr-Based Sputtered Decorative Coatings for Automotive Industry. Materials 2021,14, 5527. https:// doi.org/10.3390/ma14195527 Academic Editor: Annett Dorner-Reisel Received: 31 August 2021 Accepted: 21 September 2021 Published: 24 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1CFUM-UP, Physics Department, University of Minho, 4800-058 Guimarães, Portugal; [email protected] (N.M.G.P.); [email protected] (M.A.) 2CEMMPRE—Centre for Mechanical, Department of Mechanical Engineering, Engineering Materials and Processes, University of Coimbra, Rua Luís Reis Santos, 3030-788 Coimbra, Portugal; todor[email protected] (T.V.); albano.cavaleir[email protected] (A.C.); [email protected] (S.C.) 3LED & MAT-IPN—Laboratory for Wear, Testing and Materials, Instituto Pedro Nunes, Rua Pedro Nunes, 3030-199 Coimbra, Portugal 4 Engineering Department, KLC—Technical Plastics, 2430-021 Marinha Grande, Portugal; Jor[email protected] *Correspondence: [email protected] Abstract: The present work aims to study the impact of O and N addition on Cr-sputtered coatings on plastic (polycarbonate, PC) used in automobile parts, as a promisor alternative for auto part metallization, while eliminating the usage of toxic hexavalent chromium. The coatings were deposited using DC magnetron sputtering from a single pure Cr target in a reactive atmosphere (N 2 and/or O 2 ). The deposition of the coatings was performed maintaining the total pressure constant and close to 1 Pa by tuning Ar pressure while reactive gases were added. The target current density was kept at J W = 20 mA · cm −2 . Structural characterization revealed a mixture of α -Cr, δ -Cr, β -Cr 2 N, and CrN crystalline structures as well as amorphous oxides. The coating hardness ranged from 9 GPa for the CrON coating to 15 GPa for the CrN coating. All deposited coatings showed a particularly good interface adhesion; adjusting the amount of O and N made it possible to tune the optical properties of the Cr-based coatings as desired. The promising results open future industrialization of sputtered Cr-based coatings for automotive industries. Keywords: chromium coatings; reactive sputtering; decorative coatings; coatings on plastics 1. Introduction Many objects found daily in automobile or decorative industries are metal-coated plastic parts that replace traditional metallic materials. The benefits of these metallized plastics are the combination of low density, flexibility, design versatility, and low production cost of the plastics, while maintaining the shiny finish, high reflectivity, and conductivity of metals [ 1 , 2 ]. Most of these parts are manufactured using injection molding and are subsequently metallized using electroplating and electroless methods [ 3 ]. However, these practices of electroplating nonconductive parts include the use of highly toxic hexavalent chromium during surface activation [ 4 , 5 ] or in the chrome plating itself [ 6 ]. Hence, the European Union created a Legislation Directive to reduce the use of this human health and environmentally hazardous compound [ 7 ], that drives the needs in industry to develop other methods for deposition of coatings on polymers. Industry and researchers are now focused on finding new surface technologies based on more environmentally friendly processes to substitute chrome plating. Various processes offer themselves as alternatives to chromium plating. Chemical vapor deposition (CVD) and physical vapor deposition (PVD), which include sputtering techniques and thermal spraying (HVOF) that are used to alter the surface of different substrates with different coatings, depending on the application, for valves, decorative surfaces, or tools [ 8 – 11 ]. Magnetron sputtering is one such technique, which can be used on a wide range of available polymers, and has a reduced environmental impact and is becoming an increasingly Materials 2021,14, 5527. https://doi.org/10.3390/ma14195527 https://www.mdpi.com/journal/materials Materials 2021,14, 5527 2 of 18 attractive industrial process for polymer metallization, especially for the deposition of chromium nitride (CrN) films [1–3]. The most common polymers used in the automobile industry are acrylonitrile butadiene styrene (ABS), polycarbonate (PC), and PC/ABS blends, there is also an interest in fiber-glass reinforced polyamide (PA) [ 12 ]. Typically, PC can be used in a variety of optical and technical applications, and its demand is increasing year on year. It is widely used in optical data storage devices, bulletproof windows, and food packaging. Because of its good properties, such as optical transmittance, excellent thermal and flame resistance, high impact strength, and high stability to different environmental conditions, PC is used in a wide range of industrial applications, such as in the automotive industry [13]. The metallization of polymers with a thin metallic layer using magnetron sputtering started in 1994, by Grimberg et al. [ 14 ], who deposited TiN onto ABS and stated that, in order to coat the polymer with TiN, two conditions should be met: (i) adhesion must be assured, as Cu and Ni layers were added by electroplating and (ii) that the deposition must be performed at a low temperature to avoid polymer degradation. Respecting this last condition, the author kept the deposition temperature around 100–110 ◦ C. Later, Sukwisute et al. [ 15 ] deposited CrN coatings on ABS substrates using a reactive DC magnetron to achieve a higher wear resistance for the ABS surface. In that study, a hardness of up to 9.6 GPa was reached, despite the occurrence of voids and cracks. Recently, Pedrosa et al. [ 16 ] studied the impact of reactive magnetron sputtering conditions on the metallization of Cr-N coating onto ABC substrates to reach desirable protective and decorative properties, comparable to those obtained traditionally by electroplating. In the present study, the authors aim to develop a hexavalent chromium free coating for polymeric substrates to be used in the automobile industry as an alternative to electroplated chromium coatings. The coatings are deposited by reactive magnetron sputtering from a chromium target using N 2 and O 2 as reactive gases onto polycarbonate (PC) substrates. In addition to depositing a pure Cr layer, CrN and chromium oxynitrides were produced to improve the hardness and allow a variation of the decorative appearance [ 17 ]. Adding oxygen to CrN coatings provides a wide range of shades of grey without sacrificing wear and corrosion resistance [ 18 – 20 ]. Among the typical difficulties of the deposition of metallic coatings on polymers is adhesion; with that in mind, an interlayer was used and the impact of multilayers was also studied. 2. Materials and Methods The coatings were deposited using DC reactive magnetron sputtering, using a homemade vacuum chamber, onto monocrystalline silicon wafers (100 P-type) supplied by SIEGERT WAFER GmbH (Aachen Germany), and polycarbonate (PC) (LEXAN TM ) samples were supplied by SABIC (Al-Jubail, Saudi Arabia). The PC substrates were coated with a UV-cured basecoat prior to metallization to improve both the adhesion and decorative appearance of the coating. The samples were cleaned with isopropanol and then loaded into the deposition chamber. During the sputtering process, a single Cr target (99.5% purity) was used, Ar gas was used as the sputtering agent and reactive gases N 2 , O 2 or a gas mix N 2 + O 2 (85% N 2 and 15% O 2 ) were added at different flow rates, as indicated in Table 1. During the depositions, no external heating was used to keep the deposition temperature as low as possible and a 3.5 rpm rotation was applied to the substrates. The base pressure in the chamber was approximately 1 × 10 −3 Pa and the working pressure was kept at approximately 1 Pa. This is a relatively high pressure for reactive magnetron sputtering and was selected to reduce the energy of the species that bombard the substrates, therefore reducing the temperature increase during deposition. Before every deposition, an etching process was performed using a pulsed power source at 200 kHz, with a pulse width of 1536 ns and 400 mA current for 15 min. A chromium interlayer was deposited prior to coating for 50 s to enhance adhesion. The deposition time for each sample is also presented in Table 1. In essence, total deposition time was 360 s, except for the multilayer coatings Materials 2021,14, 5527 3 of 18 which was 480 s. The multilayer coatings were deposited using similar conditions as indicated, with each layer being deposited for 60 s. The last layer of CrN/CrO was Cr x O y and for CrN/CrON it was Cr x O y N z . No bias voltage was applied during deposition to protect the substrates. Table 1. Deposition conditions of the coatings (gas flow and deposition times), deposition rate and chemical composition characterization of the deposited coatings. Coating Gas Flow (sccm) Deposition Time (s) Thickness Deposition Rate Chemical Composition (at.%) Ar N2O2N2+ O2Interlayer Coating (nm) (nm/s) Cr O N Cr 100 360 603 1.7 92 8 - gCrN 70 5→25 360 629 1.8 66 6 28 CrN 70 25 50 310 638 1.8 60 8 32 CrO 100 15 50 310 862 2.4 69 31 - CrON 70 40 50 310 497 1.4 46 21 33 CrN/CrO 70 25 15 50 480 1020 1.9 63 25 12 CrN/CrON 70 25 40 50 480 858 1.6 49 18 33 The thickness and morphology of the coatings was analyzed using a NanoSEM FEI Nova 200, equipped with a Pegasus X4M for EDS chemical composition analysis. The structure of the coatings was analyzed by X-ray diffraction in a X 0 Pert Pro MPD diffractometer operating with Cu K α radiation ( λ = 1.5406 Å at a grazing incidence angle of α = 3 ◦ and in a 2 θ interval of 30–80 ◦ ). Peak deconvolution was performed in Origin(Pro)9 (OriginLab Corporation, Northampton, MA, USA) using a Pseudo-Voigt function. The hardness (H) and reduced Young’s modulus (Er) were measured by nanoindentation (Micro Materials Nano Test platform, Wrexham, UK)), with a Berkovich diamond pyramid indenter, applying a load of 3 mN to ensure that the indentation depth was less than 10% of the coating thickness. A total of 16 indentations were performed and the average was calculated. The adhesion of the coatings on the PC substrates was tested using a cross-cut test following the ISO 2409 standard. Two sets of 11 cuts with a spacing of 1 mm were made perpendicular to each other, thus making a grid of 100 small blocks. Then, a standardized tape (Tesa ® 4657) was applied on the crosscut and pulled off with a constant force. The number of blocks removed was an indication of the adhesion, following the standard classification between 0 to 5, 0 being a perfect adhesion coating where the edges of the cuts are completely smooth and none of the squares of the lattice detached, and a classification of 5 corresponding to a coating that flaked along the edges of the cuts in large ribbons and/or squares that detached partly or wholly in a proportion higher than 65% of the tested area. Dry sliding reciprocating tests were performed using the RTEC MFT-5000 platform (RTEC INSTRUMENTS, San Jose, CA, USA). Al 2 O 3 balls with a diameter of 10 mm were used as counterparts. The normal load was set to 1 N, resulting in an initial contact stress of 37 MPa. The stroke was set to 2.2 mm. The reciprocating frequency was 5 Hz. The test duration was limited to 1 min, resulting in a total sliding distance of 1.21 m. For assessing the wear rate, an Alicona 3D optical profilometer was used was used. The specific wear rates (K i ) were obtained using Equation (1), where ∆ V i is wear volume, F N is the normal load, and Lthe sliding distance [21,22]: Ki= ∆Vi FN×L(1) The color and reflectance of the coatings were measured using a Minolta CM-2600d portable spectrophotometer equipped with a 52-mm diameter integrating sphere, 3 pulsed xenon lamps, and a wavelength range of 400–700 nm. The color coordinates were measured Materials 2021,14, 5527 4 of 18 in the CIELab-1976 color space at a viewing angle of 10 ◦ and using the primary illuminant D65 (specular component included—SCI). 3. Results and Discussion 3.1. Functional Characterization: Colour and Adhesion As previously defined, these coatings were to be used decoratively in the automobile industry, in which color and adhesion are important and critical factors. The color coordinates and reflectance of the films were measured to check if, and which of, the deposited coatings complied with end-user specifications. It is important to mention that color is not only dependent on the material composition, but is also related to surface roughness. Once this analysis was done for similar substrates, roughness impact could be excluded for analysis. The color coordinates of the deposited coatings in the CIE L*a*b* system are shown in Figure 1. Note that L* is the brightness, where 0 = black and 100 = white; a* represents red and green on the positive and negative axes, respectively and b* represents yellow and blue on positive and negative axes, respectively [23]. Materials 2021, 14, x FOR PEER REVIEW 5 of 19 Figure 1. CIELAB color coordinates of the sputtered coatings onto PC substrates. Figure 2. Reflectance evolution of the sputtered coatings deposited onto PC substrates. Other important end-user specifications in the development of coatings onto polymers is to have good adhesion to the substrate. Therefore, the adhesion of the coatings was tested using the cross-cut tape test and the results are presented in Figure 3. According to the scale defined by the standard, the adhesion of the deposited coatings was rated between 0 (perfect adhesion, where the edges of the cut were completely smooth and none of the squares of the grid were detached after removal of the tape) and 1. A special mention should be made regarding the gradient coating, gCrN, which showed a detachment of small flakes of the coating at the intersection of the cuts in an area smaller than 5%. From these results, the use of gradient CrN coatings seems to be uninteresting for further research. The CrO coating also presented a poor adhesion when compared to the other coatings. Color also played a part and in the case of end-user preference, as this -1 0 1 2 3 4 5 6 7 8 9 0 10 20 30 40 50 60 70 80 90 100 Cr gCrN CrN CrO CrON CrN/CrO CrN/CrON a* and b* L* L* a* b* Figure 1. CIELAB color coordinates of the sputtered coatings onto PC substrates. Typically, with the increase of non-metallic elements, N or O, the brightness (L* coordinate) of the samples suffers a decrease [ 24 ]. With our samples, having up to 40 at.% non-metallic elements, coatings Cr, gCrN, CrN, and CrO, (see Table 1for composition reference) showed similar brightness: close to 70. The color appearance differs, driven by the a* and b* coordinates, in particular the b* coordinate that increases from 0.5 for Cr to 8 for CrO coating gives it a yellow tonality. The CrON coating is the one that showed a darker appearance, displaying a tonality of dark grey. Note that this coating was the only one with a higher non-metallic element (>50 at.%). The multilayer coatings, CrN/CrO and CrN/CrON, did not exhibit major differences in color in comparison with the monolithic coatings. These coatings presented a tonality and color coordinates, in particular L*, ruled by the coating’s top layer, which was CrO for the CrN/CrO coating and CrON for the CrN/CrON coating. The reflectance behavior, Figure 2, was consistent with the results presented by the brightness; in fact, Cr, gCrN, CrN, CrO, and CrN/CrO showed similar reflectance with a value between 35 and 50% in the range of 400 to 700 nm, and CrON and CrN/CrON presented a similar reflectance which was lower than the previous group with a reflectance value between 20 to 30% in the defined range. Materials 2021,14, 5527 5 of 18 Materials 2021, 14, x FOR PEER REVIEW 5 of 19 Figure 1. CIELAB color coordinates of the sputtered coatings onto PC substrates. Figure 2. Reflectance evolution of the sputtered coatings deposited onto PC substrates. Other important end-user specifications in the development of coatings onto polymers is to have good adhesion to the substrate. Therefore, the adhesion of the coatings was tested using the cross-cut tape test and the results are presented in Figure 3. According to the scale defined by the standard, the adhesion of the deposited coatings was rated between 0 (perfect adhesion, where the edges of the cut were completely smooth and none of the squares of the grid were detached after removal of the tape) and 1. A special mention should be made regarding the gradient coating, gCrN, which showed a detachment of small flakes of the coating at the intersection of the cuts in an area smaller than 5%. From these results, the use of gradient CrN coatings seems to be uninteresting for further research. The CrO coating also presented a poor adhesion when compared to the other coatings. Color also played a part and in the case of end-user preference, as this -1 0 1 2 3 4 5 6 7 8 9 0 10 20 30 40 50 60 70 80 90 100 Cr gCrN CrN CrO CrON CrN/CrO CrN/CrON a* and b* L* L* a* b* Figure 2. Reflectance evolution of the sputtered coatings deposited onto PC substrates. Other important end-user specifications in the development of coatings onto polymers is to have good adhesion to the substrate. Therefore, the adhesion of the coatings was tested using the cross-cut tape test and the results are presented in Figure 3. Materials 2021, 14, x FOR PEER REVIEW 6 of 19 composition/color is necessary to optimize adhesion by using a multilayer configuration that, as our results in Figure 3 point out, is promising In general, the adhesion of these coatings is very good for industrial applications and it is important to mention that the presence of the acrylate layer between the coating and the polymer substrate enhanced the adhesion. Figure 3. Adhesion behavior assessment of coatings deposited onto PC substrates. 3.2. Deposition Conditions and Basic Characterization In the previous sections, we showed the possibility of tuning the color of Cr-based coatings deposited via sputtering onto polycarbonate substrates, and the good adhesion that we were able to achieve for most of these coatings. In this section, some of the fundamental characterizations (chemical composition, morphology, and structure) will be presented and discussed. 3.2.1. Chemical Composition The chemical composition for coatings deposited on Si-wafer substrates was measured using WDS, as shown in Table 1. We observed a high amount of oxygen in of our all samples, even in the pure Cr coating. This can be explained by the fact that the deposition of the coatings occurred at a relatively high pressure, around 1 Pa, and that at these working pressures residual oxygen from the chamber can be incorporated into the films. It is also likely that the oxygen presence came from the degasification of the plastic substrates or from the epoxy layer that acted as a pre-coating. It is worth mentioning that, in the chemical composition results presented for graded and multilayered growth, the stated composition is the combination of several different layers of composition, and not individual layers. The individual results revealed that: (1) gCrN and CrN coatings: For graded CrN coatings, gCrN, deposited with increasing nitrogen flux presented lower N content in comparison with the CrN coating. The 33 Figure 3. Adhesion behavior assessment of coatings deposited onto PC substrates. Materials 2021,14, 5527 6 of 18 According to the scale defined by the standard, the adhesion of the deposited coatings was rated between 0 (perfect adhesion, where the edges of the cut were completely smooth and none of the squares of the grid were detached after removal of the tape) and 1. A special mention should be made regarding the gradient coating, gCrN, which showed a detachment of small flakes of the coating at the intersection of the cuts in an area smaller than 5%. From these results, the use of gradient CrN coatings seems to be uninteresting for further research. The CrO coating also presented a poor adhesion when compared to the other coatings. Color also played a part and in the case of end-user preference, as this composition/color is necessary to optimize adhesion by using a multilayer configuration that, as our results in Figure 3point out, is promising. In general, the adhesion of these coatings is very good for industrial applications and it is important to mention that the presence of the acrylate layer between the coating and the polymer substrate enhanced the adhesion. 3.2. Deposition Conditions and Basic Characterization In the previous sections, we showed the possibility of tuning the color of Cr-based coatings deposited via sputtering onto polycarbonate substrates, and the good adhesion that we were able to achieve for most of these coatings. In this section, some of the fundamental characterizations (chemical composition, morphology, and structure) will be presented and discussed. 3.2.1. Chemical Composition The chemical composition for coatings deposited on Si-wafer substrates was measured using WDS, as shown in Table 1. We observed a high amount of oxygen in of our all samples, even in the pure Cr coating. This can be explained by the fact that the deposition of the coatings occurred at a relatively high pressure, around 1 Pa, and that at these working pressures residual oxygen from the chamber can be incorporated into the films. It is also likely that the oxygen presence came from the degasification of the plastic substrates or from the epoxy layer that acted as a pre-coating. It is worth mentioning that, in the chemical composition results presented for graded and multilayered growth, the stated composition is the combination of several different layers of composition, and not individual layers. The individual results revealed that: (1) gCrN and CrN coatings: For graded CrN coatings, gCrN, deposited with increasing nitrogen flux presented lower N content in comparison with the CrN coating. The 33 at.% N introduced in the CrN coating on a N 2 /Ar flow-rate ratio of 0.35 was in line with previous work by Mayhofer et al. [25]; (2) CrO coating: By adding O 2 as the reactive gas inside the deposition chamber, it is possible to incorporate a significate amount of oxygen (30 at.% of O), even with a much lower gas flow than for N 2 -presence coatings, which is explained by the higher reactivity of oxygen to Cr [26]; (3) CrON coating: When a gas mixture of 85% N 2 + 15% O 2 was added to the atmosphere for the CrON coating, the number of reactive species inside the deposition chamber became higher and this led to the highest non-metallic elements amount being added to the coatings in the present study (~21 at.% of O and ~33 at.% of N). Note that, proportionally, the amount of O in the coating was higher than the amount of O 2 in the gas mixture (85% N 2 and 15% O 2 ) which can be explained by the higher affinity of O to Cr than N to Cr, as indicated by the standard molar enthalpy of formation of Cr 2 O 3 and CrN: ∆f H ◦ (Cr 2 O 3 ) = − 1139.7 kJ/mol and ∆f H ◦ (CrN) = − 117.2 kJ/mol [ 26 ], also observed in previous studies from our research team on oxynitrides coatings [27]; (4) CrN/CrO and CrN/CrON coatings: In the case of multilayered coatings, CrN/CrO and CrN/CrON, the chemical composition measured using this method showed a value similar to the average of the chemical composition of individual layers (CrN Materials 2021,14, 5527 7 of 18 and CrO or CrON), which indicated that the analytical method had been influenced by the several layers of depth. The thickness of the coatings was measured by cross-section analysis of SEM micrographs on silicon and after the deposition rate was calculated. These results are shown in Table 1and Figure 4as a function of the deposition conditions. Materials 2021, 14, x FOR PEER REVIEW 8 of 19 Figure 4. Scanning electron cross-section micrographs of the coatings deposited on silicon. 3.2.2. Morphology The scanning electron cross-section morphology of the deposited coatings is shown in Figure 4. Cr coatings showed a well-defined columnar structure, and the same was observed for the gCrN and CrN coatings. In fact, this is typical behavior for coatings deposited up to 1 Pa (high pressure) without external heating (low deposition temperature) [32,33]. For the CrON coating, a columnar structure was still observed, but was less pronounced. The obtained CrO coating observed in the SEM micrographs showed an interesting phenomenon, which was a multilayer growth (Figure 5). This multilayer morphology can be explained by the deposition conditions, namely the depositions were carried out using only one Cr target in a non-poisoned mode (transition zone) and the substrate holder rotated at 3.5 rpm. Therefore, the formation of a differentiated plasma surrounding the substrate holder could be expected. When the samples were facing the Cr target, a fresh metallic Cr layer was deposited, and the amount of oxygen was reduced due to the high flux of Cr atoms coming from the target. In the remaining time, when the samples were turned around, the oxidative plasma environment (Ar + O2) promoted the oxidation and adsorption of oxygen by the films, which resulted in an increase in oxygen on the surface of the Cr layer. This phenomenon is clear in Figure 5, where charged (brighter) and uncharged (darker) layers are visible, which correspond to O-rich and O-deficient layers, respectively. This behavior was also found in other similar systems in previous studies from our research group [34]. Figure 4. Scanning electron cross-section micrographs of the coatings deposited on silicon. The coating thickness ranged between 500 nm and 1020 nm and the deposition rate varied between 1.4 and 2.4 nm/s depending on the deposition conditions. Under the defined conditions, the deposition rate of pure Cr coatings was close to 1.7 nm/s and for gCrN and CrN coatings it showed a value of 1.8 nm/s. This clearly showed that the deposition was done in a metallic mode, even up to 33 at. % addition of N. Combining the deposition rate value and the chemical composition, this clearly shows that the deposition is still far from the full poisoning of the target and so the stoichiometric composition of CrN was not reached [ 28 ]. For samples deposited with oxygen as the reactive gas (CrO coating), the thickness increased and the deposition ratio reached 2.4 nm/s. This is typical behavior of Cr-O coatings when deposition conditions remain in the metallic mode and it is a consequence of the incorporation of O atoms in the Cr lattice, which leads to a structural change with the formation of new phases having different atomic arrangements, which give rise to a lower specific molecular weight and consequently to an increase in the coating volume and final coating thickness. This behavior was confirmed by Rothhaar et al. [ 29 ] and also observed in Mo-O [30] and W-O systems [31]. Multilayer coating of CrN/CrO and CrN/CrON showed a deposition ratio that was very close to the average deposition ratio of CrN and CrO or CrON, respectively. In fact, as will be presented in the discussion on morphology, these multilayer coatings showed two clear visible layers (see Figure 4) for CrN and CrO or CrON, respectively. 3.2.2. Morphology The scanning electron cross-section morphology of the deposited coatings is shown in Figure 4. Cr coatings showed a well-defined columnar structure, and the same was observed for the gCrN and CrN coatings. In fact, this is typical behavior for coatings deposited up to 1 Pa (high pressure) without external heating (low deposition temperature) [ 32 , 33 ]. For the CrON coating, a columnar structure was still observed, but was less pronounced. The obtained CrO coating observed in the SEM micrographs showed an interesting phenomenon, which was a multilayer growth (Figure 5). This multilayer morphology can be explained by the deposition conditions, namely the depositions were carried out using only one Cr target in a non-poisoned mode (transition zone) and the substrate holder rotated at 3.5 rpm. Therefore, the formation of a differentiated plasma surrounding the Materials 2021,14, 5527 8 of 18 substrate holder could be expected. When the samples were facing the Cr target, a fresh metallic Cr layer was deposited, and the amount of oxygen was reduced due to the high flux of Cr atoms coming from the target. In the remaining time, when the samples were turned around, the oxidative plasma environment (Ar + O 2 ) promoted the oxidation and adsorption of oxygen by the films, which resulted in an increase in oxygen on the surface of the Cr layer. This phenomenon is clear in Figure 5, where charged (brighter) and uncharged (darker) layers are visible, which correspond to O-rich and O-deficient layers, respectively. This behavior was also found in other similar systems in previous studies from our research group [34]. Materials 2021, 14, x FOR PEER REVIEW 9 of 19 Figure 5. Cross-section SEM micrograph of the CrO coatings. We observed a multilayer-like structure with charged and uncharged layers, corresponding to layers deficient and rich in O, respectively. The cross-section SEM images of the multilayer coatings in secondary electron (SE) and backscattered electron (BSED) modes are shown in Figure 6. The CrN/CrO coating, as shown in Figure 6a, shows, bands similar to CrO interrupted by a darker “solid” band in the SE mode. This is even more evident in the BSE mode, where brighter and darker areas are more contrasting. In the BSED mode, lighter areas correspond to heavier elements or metallic ones [35], meaning that from the bottom-up, one can see a Cr interlayer, CrN layer, and a layer of Cr-O that, in fact, are four bilayers of Cr/CrO due to the phenomenon described earlier for CrO coatings. This is repeated four times, which corresponds to the deposition of 60 s of each layer for a total time of 480 s, as per Table 1. Regarding CrN/CrON coatings, a similar but not so evident behavior was observed. On the left of Figure 6b, using the BSED mode (right), a Cr interlayer was present at the bottom and, after, the 4 bilayers of CrN/CrON were observed, as expected. Figure 5. Cross-section SEM micrograph of the CrO coatings. We observed a multilayer-like structure with charged and uncharged layers, corresponding to layers deficient and rich in O, respectively. The cross-section SEM images of the multilayer coatings in secondary electron (SE) and backscattered electron (BSED) modes are shown in Figure 6. The CrN/CrO coating, as shown in Figure 6a, shows, bands similar to CrO interrupted by a darker “solid” band in the SE mode. This is even more evident in the BSE mode, where brighter and darker areas are more contrasting. In the BSED mode, lighter areas correspond to heavier elements or metallic ones [ 35 ], meaning that from the bottom-up, one can see a Cr interlayer, CrN layer, and a layer of Cr-O that, in fact, are four bilayers of Cr/CrO due to the phenomenon described earlier for CrO coatings. This is repeated four times, which corresponds to the deposition of 60 s of each layer for a total time of 480 s, as per Table 1. Materials 2021,14, 5527 9 of 18 Materials 2021, 14, x FOR PEER REVIEW 10 of 19 Figure 6. Cross-section SEM micrography in secondary electron (left) and backscattered electron modes (right) for (a) CrN/CrO and (b) CrN/CrON multilayer coatings. 3.2.3. Structural Characterization The crystallographic structure was evaluated using X-ray diffraction (XRD) and the results are shown in Figure 7a for all deposited coatings. Figure 6. Cross-section SEM micrography in secondary electron ( left ) and backscattered electron modes ( right ) for (a) CrN/CrO and (b) CrN/CrON multilayer coatings. Regarding CrN/CrON coatings, a similar but not so evident behavior was observed. On the left of Figure 6b, using the BSED mode (right), a Cr interlayer was present at the bottom and, after, the 4 bilayers of CrN/CrON were observed, as expected. 3.2.3. Structural Characterization The crystallographic structure was evaluated using X-ray diffraction (XRD) and the results are shown in Figure 7a for all deposited coatings. Materials 2021,14, 5527 16 of 18 Author Contributions: E.C., investigation, writing—original draft, visualization; N.M.G.P., formal analysis, writing—original draft, visualization; T.V., investigation, writing—original draft; J.F.: funding, validation; A.C., supervision, writing—review and editing; M.A., supervision, writing—review and editing, project administration; S.C., supervision, writing—review and editing, project administration. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by COMPETE 2020 a Portuguese and European Union initiative through the Project POCI-01-0247-FEDER-042785, acronym “GREENCoat”. This research was sponsored by Norte2020, through European Social Fund (FSE), under the National Doctoral Program in “Surfaces Engineering and Protection”, NORTE-08-5369-FSE-000047. This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of Strategic Funding (co-financed via UIDB/00285/2020 and UIDB/04650/2020). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. References 1. Reheem, A.A.; Maksoud, M.I.A.A.; Ashour, A. Surface modification and metallization of polycarbonate using low energy ion beam. Radiat. Phys. Chem. 2016,125, 171–175. [CrossRef] 2. Olivera, S.; Muralidhara, H.B.; Venkatesh, K.; Gopalakrishna, K.; Vivek, C.S. Plating on acrylonitrile–butadiene–styrene (ABS) plastic: A review. J. Mater. Sci. 2016,51, 3657–3674. [CrossRef] 3. Charbonnier, M.; Romand, M.; Harry, E.; Alami, M. Surface plasma functionalization of polycarbonate: Application to electroless nickel and copper plating. J. Appl. Electrochem. 2001,31, 57–63. [CrossRef] 4. White, J.; Tenore, C.; Pavich, A.; Scherzer, R.; Stagon, S. Environmentally benign metallization of material extrusion technology 3D printed acrylonitrile butadiene styrene parts using physical vapor deposition. Addit. Manuf. 2018,22, 279–285. [CrossRef] 5. Tang, X.; Wang, J.; Wang, C.; Shen, B. A novel surface activation method for Ni/Au electroless plating of acrylonitrile–butadiene– styrene. Surf. Coat. Technol. 2011,206, 1382–1388. [CrossRef] 6. Garcia, A.; Berthelot, T.; Viel, P.; Mesnage, A.; Jégou, P.; Nekelson, F.; Roussel, S.; Palacin, S. ABS Polymer Electroless Plating through a One-Step Poly(acrylic acid) Covalent Grafting. ACS Appl. Mater. Interfaces 2010,2, 1177–1183. [CrossRef] [PubMed] 7. European Parliament. Directive 2005/90/EC of the European Parliament and of the Council; European Parliament: Brussels, Belgium, 2006. 8. Vernhes, L.; Azzi, M.; Klemberg-Sapieha, J. Alternatives for hard chromium plating: Nanostructured coatings for severe-service valves. Mater. Chem. Phys. 2013,140, 522–528. [CrossRef] 9. Sahraoui, T.; Fenineche, N.-E.; Montavon, G.; Coddet, C. Alternative to chromium: Characteristics and wear behavior of HVOF coatings for gas turbine shafts repair (heavy-duty). J. Mater. Process. Technol. 2004,152, 43–55. [CrossRef] 10. Forsich, C.; Dipolt, C.; Heim, D.; Mueller, T.; Gebeshuber, A.; Holecek, R.; Lugmair, C. Potential of thick a-C:H:Si films as substitute for chromium plating. Surf. Coat. Technol. 2014,241, 86–92. [CrossRef] 11. Yasbandha, H. Surface Engineering of Coinage Dies. Ph.D. Thesis, Faculty of Engineering, University of Wollongong, Wollongong, Australia, 2001. 12. Bewilogua, K.; Bräuer, G.; Dietz, A.; Gäbler, J.; Goch, G.; Karpuschewski, B.; Szyszka, B. Surface technology for automotive engineering. CIRP Ann. Manuf. Technol. 2009,58, 608–627. [CrossRef] 13. Antonakou, E.V.; Achilias, D.S. Recent Advances in Polycarbonate Recycling: A Review of Degradation Methods and Their Mechanisms. Waste Biomass Valoriz. 2013,4, 9–21. [CrossRef] 14. Grimberg, I.; Bouaifi, B.; Draugelates, U.; Soifer, K.; Weiss, B. Microstructure and adhesion mechanisms of TiN coatings on metallized acrylonitrile-butadiene-styrene. Surf. Coat. Technol. 1994,68–69, 166–175. [CrossRef] 15. Sukwisute, P.; Sakdanuphab, R.; Sakulkalavek, A. Hardness and wear resistance improvement of ABS surface by CrN thin film. Mater. Today Proc. 2017,4, 6553–6561. [CrossRef] 16. Pedrosa, P.; Rodrigues, M.S.; Neto, M.A.; Oliveira, F.J.; Silva, R.; Borges, J.; Amaral, M.; Ferreira, A.; Godinho, L.H.; Carvalho, S.; et al. Properties of CrN thin films deposited in plasma-activated ABS by reactive magnetron sputtering. Surf. Coat. Technol. 2018 , 349, 858–866. [CrossRef] 17. Carneiro, E.; Castro, J.D.; Marques, S.; Cavaleiro, A.; Carvalho, S. REACH regulation challenge: Development of alternative coatings to hexavalent chromium for minting applications. Surf. Coat. Technol. 2021,418, 127271. [CrossRef] 18. Subramanian, B.; Jayachandran, M. Preparation of chromium oxynitride and chromium nitride films by DC reactive magnetron sputtering and their material properties. Corros. Eng. Sci. Technol. 2011,46, 554–561. [CrossRef] Materials 2021,14, 5527 17 of 18 19. Yuan, Y.; Zhang, B.; Sun, J.; Jonnard, P.; Le Guen, K.; Tu, Y.; Yan, C.; Lan, R. Structure and optical properties of CrOxNy films with composition modulation. Surf. Eng. 2020,36, 411–417. [CrossRef] 20. Mientus, R.; Grötschel, R.; Ellmer, K. Optical and electronic properties of CrO x N y films, deposited by reactive DC magnetron sputtering in Ar/N2/O2(N2O) atmospheres. Surf. Coat. Technol. 2005,200, 341–345. [CrossRef] 21. Kato, K. Friction and wear of passive metals and coatings. In Tribocorrosion of Passive Metals and Coatings; Woodhead Publishing: Sawston, UK, 2011; pp. 65–99. [CrossRef] 22. Colombo, D.A.; Echeverría, M.D.; Laino, S.; Dommarco, R.C.; Massone, J.M. Sliding Wear Behavior of PVD CrN and TiN Coated Austempered Ductile Iron. ISIJ Int. 2014,54, 2860–2867. [CrossRef] 23. Beck, U.; Reiners, G.; Kopacz, U.; Jehn, H. Decorative hard coatings: Interdependence of optical, stoichiometric and structural properties. Surf. Coat. Technol. 1993,60, 389–395. [CrossRef] 24. Budke, E.; Krempel-Hesse, J.; Maidhof, H.; Schüssler, H. Decorative hard coatings with improved corrosion resistance. Surf. Coat. Technol. 1999,112, 108–113. [CrossRef] 25. Mayrhofer, P.; Tischler, G.; Mitterer, C. Microstructure and mechanical/thermal properties of Cr–N coatings deposited by reactive unbalanced magnetron sputtering. Surf. Coat. Technol. 2001,142–144, 78–84. [CrossRef] 26. Handbook of chemistry and physics. J. Frankl. Inst. 1930,209, 847. [CrossRef] 27. Parreira, N.M.G.; Polcar, T.; Martin, N.; Banakh, O.; Cavaleiro, A. Optical and Electrical Properties of W-O-N Coatings Deposited by DC Reactive Sputtering. Plasma Process. Polym. 2007,4(Suppl. 1), S69–S75. [CrossRef] 28. Xu, J.; Umehara, H.; Kojima, I. Effect of deposition parameters on composition, structures, density and topography of CrN films deposited by r.f. magnetron sputtering. Appl. Surf. Sci. 2002,201, 208–218. [CrossRef] 29. Rothhaar, U.; Oechsner, H. R.f. magnetron sputter deposition of Cr 2 O 3 layers on ceramic Al 2 O 3 substrates. Surf. Coat. Technol. 1993,59, 183–186. [CrossRef] 30. Kharrazi, M.; Azens, A.; Kullman, L.; Granqvist, C. High-rate dual-target d.c. magnetron sputter deposition of electrochromic MoO3films. Thin Solid Film. 1997,295, 117–121. [CrossRef] 31. Parreira, N.; Carvalho, N.; Cavaleiro, A. Synthesis, structural and mechanical characterization of sputtered tungsten oxide coatings. Thin Solid Film. 2006,510, 191–196. [CrossRef] 32. Daniel, R.; Martinschitz, K.; Keckes, J.; Mitterer, C. The origin of stresses in magnetron-sputtered thin films with zone T structures. Acta Mater. 2010,58, 2621–2633. [CrossRef] 33. Arias, D.; Gómez, A.G.; Souza, R.; Velez, J.M. Residual stress gradient of Cr and CrN thin films. Mater. Chem. Phys. 2018 ,204, 269–276. [CrossRef] 34. Alves, C.A.; Marques, L.; Calderon V., S.; Ferreira, P.; Schneider, D.; Cavaleiro, A.; Carvalho, S. An experimental and theoretical study on the crystal structure and elastic properties of Ta1−xOxcoatings. Surf. Coat. Technol. 2019,364, 289–297. [CrossRef] 35. Kowoll, T.; Müller, E.; Fritsch-Decker, S.; Hettler, S.; Störmer, H.; Weiss, C.; Gerthsen, D. Contrast of Backscattered Electron SEM Images of Nanoparticles on Substrates with Complex Structure. Scanning 2017,2017, 4907457. [CrossRef] 36. Kimoto, K.; Nishida, I. An Electron Diffraction Study on the Crystal Structure of a New Modification of Chromium. J. Phys. Soc. Jpn. 1967,22, 744–756. [CrossRef] 37. Peralta, J.; Esteve, J.; Lousa, A. δ -A15 and bcc phases coexist in sputtered chromium coatings with moderate oxygen contents. Thin Solid Films 2020,693, 137676. [CrossRef] 38. Parreira, N.M.; Carvalho, N.J.; Cavaleiro, A. On the Structural Evaluation of Unbiased W-O-N Sputtered Coatings. Mater. Sci. Forum 2006,514–516, 825–832. [CrossRef] 39. Parreira, N.; Carvalho, N.; Vaz, F.; Cavaleiro, A. Mechanical evaluation of unbiased W–O–N coatings deposited by d.c. reactive magnetron sputtering. Surf. Coat. Technol. 2006,200, 6511–6516. [CrossRef] 40. Song, H.J.; Yan, Q. The Characteristics of CrN X Coatings with Different Interlayer. Adv. Mater. Res. 2011 ,204–210, 938–941. [CrossRef] 41. Marulanda, D.; Lousa, A.; Martinez-De-Olcoz, L.; Olaya, J. Microstructure characterization of nano-structured Cr/Cr 2 N multilayer films produced through radio frequency magnetron sputtering. Thin Solid Films 2014,550, 272–277. [CrossRef] 42. Suryanarayana, C.; Norton, M.G. X-ray Diffraction A Practical Approach; Plenum Press: New York, NY, USA, 1998. 43. Collard, S.; Küpfer, H.; Hecht, G.; Hoyer, W.; Moussaoui, H. The reactive magnetron deposition of CrN x O y films: First results of property investigations. Surf. Coat. Technol. 1999,112, 181–184. [CrossRef] 44. Hones, P.; Diserens, M.; Lévy, F. Characterization of sputter-deposited chromium oxide thin films. Surf. Coat. Technol. 1999 , 120–121, 277–283. [CrossRef] 45. Forniés, E.; Galindo, R.E.; Sánchez, O.; Albella, J. Growth of CrNx films by DC reactive magnetron sputtering at constant N 2 /Ar gas flow. Surf. Coat. Technol. 2006,200, 6047–6053. [CrossRef] 46. Hones, P.; Sanjines, R.; Levy, F. Characterization of sputter-deposited chromium nitride thin films for hard coatings. Surf. Coat. Technol. 1997,94–95, 398–402. [CrossRef] 47. Elangovan, T.; Kuppusami, P.; Thirumurugesan, R.; Ganesan, V.; Mohandas, E.; Mangalaraj, D. Nanostructured CrN thin films prepared by reactive pulsed DC magnetron sputtering. Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. 2010 ,167, 17–25. [CrossRef] 48. Pang, X.; Gao, K.; Volinsky, A. Microstructure and mechanical properties of chromium oxide coatings. J. Mater. Res. 2007 ,22, 3531–3537. [CrossRef] Materials 2021,14, 5527 18 of 18 49. Fernandes, F.; Yaqub, T.; Cavaleiro, A. Influence of Ag additions on the structure, mechanical properties and oxidation behaviour of Cr-O coatings deposited by HiPIMS. Surf. Coat. Technol. 2018,339, 167–180. [CrossRef] 50. Barshilia, H.C.; Rajam, K. Growth and characterization of chromium oxide coatings prepared by pulsed-direct current reactive unbalanced magnetron sputtering. Appl. Surf. Sci. 2008,255, 2925–2931. [CrossRef] 51. Castaldi, L.; Kurapov, D.; Reiter, A.; Shklover, V.; Schwaller, P.; Patscheider, J. Effect of the oxygen content on the structure, morphology and oxidation resistance of Cr–O–N coatings. Surf. Coat. Technol. 2008,203, 545–549. [CrossRef] 52. Hall, E.O. The Deformation and Ageing of Mild Steel: III Discussion of Results. Proc. Phys. Soc. Sect. B 1951 ,64, 747–753. [CrossRef] 53. Lehoczky, S.L. Retardation of Dislocation Generation and Motion in Thin-Layered Metal Laminates. Phys. Rev. Lett. 1978 ,41, 1814–1818. [CrossRef] 54. Marulanda, D.; Olaya, J.; Piratoba, U.; Mariño, A.; Camps, E. The effect of bilayer period and degree of unbalancing on magnetron sputtered Cr/CrN nano-multilayer wear and corrosion. Thin Solid Film. 2011,519, 1886–1893. [CrossRef] 55. Kot, M.; Rakowski, W.; Major, Ł.; Major, R.; Morgiel, J. Effect of bilayer period on properties of Cr/CrN multilayer coatings produced by laser ablation. Surf. Coat. Technol. 2008,202, 3501–3506. [CrossRef] 56. Arias, D.; Gómez, A.; Velez, J.M.; Souza, R.; Olaya, J. A mechanical and tribological study of Cr/CrN multilayer coatings. Mater. Chem. Phys. 2015,160, 131–140. [CrossRef] 57. Parreira, N.; Polcar, T.; Cavaleiro, A. Characterization of W–O coatings deposited by magnetron sputtering with reactive gas pulsing. Surf. Coat. Technol. 2007,201, 5481–5486. [CrossRef] 58. He, X.-M.; Baker, N.; Kehler, B.A.; Walter, K.C.; Nastasi, M.; Nakamura, Y. Structure, hardness, and tribological properties of reactive magnetron sputtered chromium nitride films. J. Vac. Sci. Technol. A 2000,18, 30–36. [CrossRef] 59. Urgen, M.; Ezirmik, V.; Senel, E.; Kahraman, Z.; Kazmanli, K. The effect of oxygen content on the temperature dependent tribological behavior of Cr–O–N coatings. Surf. Coat. Technol. 2009,203, 2272–2277. [CrossRef]