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Electrochemical response of ZrCN-Ag-a(C,N) coatings in simulated body fluids
Electrochimica Acta 176 (2015) 898–906
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical response of ZrCN-Ag-a(C,N) coatings in simulated body fluids S. Calderon Va,b,* , A. Cavaleirob , S. Carvalhoa,b a b
Physics Department, University of Minho, 4800-058 Guimarães, Portugal SEG-CEMUC Mechanical Engineering Department, University of Coimbra, 3030-788 Coimbra, Portugal
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 January 2015 Received in revised form 22 June 2015 Accepted 14 July 2015 Available online 23 July 2015
In this study, Zr-C-N-Ag coatings were deposited by magnetron sputtering aiming to provide an enhanced corrosion resistance to stainless steel 316 L, while adding antibacterial capabilities using silver, a wellknown antibacterial agent. The films were analysed by X-ray diffraction, Raman spectroscopy, electron probe microanalysis and X-ray photoelectron spectroscopy (XPS) in relation to the chemical composition and structural characteristics. Additionally, electrochemical impedance spectroscopy and potentiodynamic tests were carried out as a function of the immersion time, in Hanks’ balanced salt solution with 10 g/L of Bovine serum albumin (BSA), to evaluate the electrochemical characteristics of the samples. The results revealed that the coatings are predominantly composed by ZrCxN1x, Ag and amorphous carbon phases. A large deterioration of the electrochemical stability of the films was evidenced as the silver content increased. The presence of amorphous carbon phases negatively influenced the polarization resistant, behavior ascribed to morphological changes. The immersion test showed a progressive increment of the polarization resistance with time, attributed to surface and pores passivation, due to the formation of both ZrO2 and albumin protective layers on the surface, as shown by XPS. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: ZrCN Silver Corrosion Albumin XPS
1. Introduction Thin protective layers have been used as an efficient process in surface engineering to provide additional properties to a base material or to improve the existing ones, such as wear and corrosion resistance, and biocompatibility. Nowadays, this methodology is suitable to solve problems identified in materials used for medical devices. Low wear resistance in titanium alloys [1], toxicity in CrCo alloys [2] and limited localized corrosion resistance in 316 L stainless steel [3], the most used materials for medical applications [4,5], showed the need of producing a multifunctional material that can enhance the mechanical, tribological, chemical and biological properties. These characteristics have been modified by applying chemical inert coatings with low-reactive ceramic material, such as diamond-like-carbon materials [6,7], nitrides [8–10], carbides [11], carbonitrides [12], or oxides [9,13]; carbonitrides present the best compromise between the mechanical, triboglogical and corrosion resistance performance. Moreover, in order to potentiate the materials biocompatibility, antimicrobial
* Corresponding author at: Universidade do Minho, Dept. Física, Campus de Azurém, 4800-058 Guimarães, Portugal. Tel: +351 253510175x517465; fax: +351 253510461. E-mail address: [email protected] (S. Calderon). http://dx.doi.org/10.1016/j.electacta.2015.07.083 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
agents, such as silver, can be included [14–19]. Therefore, our group has been exploring the possibility of combining transition metal carbonitrides with silver nanoparticles as promising materials to provide medical devices with multifunctional capabilities for an enhanced performance [20–22]. Transition metal carbonitrides have been extensively used as protective coatings, due to their great mechanical, tribological and chemical properties [12,21,23–30], making them potential candidates to exceed the properties of current bio-materials. ZrC1xNx, for instance, is biocompatible and its mechanical and tribological properties, as well as chemical stability, can be modified by tuning the chemical composition [12,28–34], which provides different mixtures of phases and nano-composite structures. Further changes can be achieved in respect to antimicrobial effect by introducing silver into the ceramic ZrC1xNx matrix. Although ZrCxN1x coatings have been recognized to improve the corrosion resistance of the base material in different fluids [35,36], little is known about the ability of the films for protection in simulated body fluids. This complex Ag-ZrCxN1x system allows the co-existence of a large variety of phases, such as ZrCxN1x, amorphous carbon, metallic silver and some residual oxides, which may easily alter the electrochemical behavior of the system. Previously we have reported that the increase of silver metallic phase in Ag-ZrCxN1x coatings, accompanied by a reduction of
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ZrCxN1x grain size and the increase of amorphous phases in the material, could explain the more electrochemical active behavior of the samples [20]. In the present report, the electrochemical behavior of Ag-ZrCxN1x coatings with and without amorphous phases, deposited on 316 L stainless steel, is reported; the evolution of the samples as a function of the immersion time in simulated body fluid, with composition close to the synovial fluid present in the body joints, is emphasized. 2. Material and methods 2.1. Coating production Ag-ZrCxN1x coatings were deposited by direct current (DC) unbalance reactive dual magnetron sputtering technique with two highly pure targets (Zr and Ag) of 100 200 6 mm3. The targets were located in opposite direction, pointing towards a rotational substrate holder placed in the center of the chamber at 70 mm of each target. The incorporation of carbon and nitrogen in the films was carried out through acetylene and nitrogen gases, by controlling the fluxes which ranged from 1.2–2.4 and 3–9 sccm, respectively, as shown in Table 1. The silver content, on the other hand, was varied by changing the Ag target current density between 0.25 and 0.75 mA cm2, maintaining the Zr current density at 10 mA cm2. The bias potential (50 V), substrate rotation speed (8 rpm) and chamber temperature (373 K) were monitored and kept constant for all the depositions. The 316 L stainless steel (Fe 68.40 wt%, Cr 16.40 wt%, Ni 11.1 wt%, Mo 2.28 wt%, Mn 1.3 wt%, Si 0.39 wt%, P = 0.06 wt%, and C < 0.03 wt %) was grinded using emery paper from 240 to 2400 grit, and then, mirror polished with a diamond solution, which allowed to reach a final surface roughness (Ra) around 8 nm. Thereafter, a cleaning procedure was carried out using ultrasonic baths in distilled water, ethanol and acetone during 10 min for each solvent, in order to remove dust and impurities. 2.2. Physical and chemical characterization The coatings structure was studied by X-ray diffraction (XRD) in a PANalytical XPert PRO micro-diffractometer with Cu Ka (1.540598 Å) radiation in grazing angle mode. Raman spectroscopy was carried out to investigate the amorphous carbon phases. Measurements were performed using a LabRAM Horiba Jobin Yvon spectrometer equipped a He–Ne laser (532 nm) at 5 mW. The coating bulk chemical composition was analyzed by a Cameca SX 50 electron probe micro-analyzer (EPMA), using statistical analysis of 5 randomly selected spots in each sample. Films thickness were evaluated by the Calotest method utilizing a rotation sphere 20 mm in diameter, at 1000 rpm for 90 s, in order to obtain the desired wear. The surface morphology was assessed using a NanoSEM – FEI Nova 200 scanning electron microscope. The superficial defects in the coatings were analyzed by optical
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microscopy. The defects are represented as the percentage of the area covered by surface defects. For each coating, 10 different areas were imaged at 50X magnification and analyzed with ImageJ v.1.48 software. The surface composition, before and after immersion tests, was assessed by X-ray photoelectron spectrometer Kratos AXIS Ultra HSA, with VISION software for data acquisition and CASAXPS software for data analysis. A monochromatic Al Ka X-ray source (1486.7 eV), operating at 15 kV (150 W), in FAT mode (Fixed Analyser Transmission), with a pass energy of 40 eV for regions ROI and 80 eV for survey. Data acquisition was performed with a pressure lower than 1 106 Pa, and a charge neutralisation system was used. The effect of the electric charge was corrected by reference to the carbon peak (285 eV). The deconvolution was carried out using CasaXPS program; peak fitting was performed using Gaussian-Lorentzian peak shape and Shirley type background subtraction. 2.3. Contact angle and hydrophobicity The surface free energy and the hydrophobicity parameters of the surfaces were determined by the sessile drop contact angle technique, by means of an OCA 15 Plus device. The measurements were performed at room temperature, using 2 mL of water, formamide and a-bromonaphtalene with known surface energy components, as reference liquids. The surface tension parameters were calculated using the van Oss approach [37]. 2.4. Electrochemical characterization The electrochemical behavior of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) at open circuit potential, for frequencies ranging from 100 kHz to 5 mHz, with a 10 mV (rms) AC perturbation, using EIS300 software in a Gamry potentiostat REF600. All electrochemical experiments were performed in a classic three electrodes corrosion cell with platinum and saturated calomel (SCE) as counter and reference electrodes, respectively, in Hank's balanced salt solution (0.137 M NaCl, 5.4 mM KCl, 0.25 mM Na2HPO4, 0.44 mM KH2PO4, 1.3 mM CaCl2, 1.0 mM MgSO4 and 4.2 NaHCO3) with 10 g L1 of bovine serum albumin (BSA) at 37 C with stabilized pH to 7.0 0.2 in equilibrium with air. All measurements were acquired after achieving a stable open circuit potential, at about 1 h. Simulation of the experimental data was also performed with Gamry software, using a Randles equivalent circuit, in which the capacitance element was replaced by a constant phase element in order to consider the surface roughness and heterogeneities [38– 40], with Rp representing the polarization resistance element, i.e. the charge transfer resistance of the material. The samples with best corrosion resistance properties were utilized for immersion tests to observe the evolution of the electrochemical parameters for 15 days.
Table 1 Deposition parameters and coatings properties.
Ag0 Ag6 Ag7 Ag9 Ag13 Ag20
Ag current density [mA cm2]
C2H2 [sccm]
N2 [sccm]
Thickness [mm]
Deposition Rate [mm h1]
ZrCN Grain Size [nm]
Zr [%]
N [%]
C [%]
Ag [%]
O [%]
0 0.25 0.25 0.5 0.5 0.75
1.2 1.2 1.2 2.4 2.4 1.2
3 3 9 3 9 3
2.3 1.8 1.1 1.6 1.5 2.0
1.9 1.7 1.0 1.5 1.4 2.1
7.2 6.9 3.8 11.6 NA 5.4
49 46 32 35 21 38
33 24 39 18 30 20
15 18 18 29 27 16
0 6 7 9 13 20
3 6 4 9 9 6
Standard deviation of the composition are between 0.5 and 1.2 at.%.
CþN Zr
1.0 0.9 1.8 1.4 2.8 1.0
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3. Results and discussion 3.1. Chemical and physical characterization
Fig. 1. a) Coatings X-ray diffraction patterns; ZrC0.5N0.5 (ICDD-03-065-8779) and Ag (ICDD- 00-087-0719) standard patterns are shown for comparison and b) coatings Raman spectroscopy spectra.
Additionally, potentiodynamic tests were performed in two stages (after 1 h and 336 h after immersion in HBSS + A), using a scanning rate of 60 mV/min from 300 mV vs. OCP to +1000 mV vs. SCE. The results of the electrochemical test were calculated as the average and standard deviations of at least three separated samples. All the potentials are expressed with respect to SCE electrode.
Table 1 shows the acetylene and nitrogen fluxes, thickness, deposition rate, ZrCxN1x grain size and the chemical composition of the films sorted from the lowest to the largest amount of silver content in the films, as measured by EPMA. The silver content is affected not only by the increase of the current density applied to the silver target, but also by the variation of acetylene and nitrogen fluxes, which give rise to a variety of film compositions and phases arrangements. More detailed information about the effect of the deposition conditions on the chemical and phase composition can be found elsewhere [41]. Fig. 1 displays the X-ray diffraction (XRD) and Raman spectroscopy results for all the samples. The results were sorted as a function of the silver content in the films, which evidenced that there is not a simple correlation between the silver content and the structural characteristics of the coatings. In fact, the silver variation was achieved not only by increasing the silver target power, but also by changing the gas fluxes in the chamber. The increase of reactive gas fluxes promotes the total consumption of zirconium and the formation of C-rich phases. A face-centered cubic phase of ZrC1xNx is identified, showing different degrees of crystallinity. The metallic silver phase is not clearly evidenced for low silver content coatings, but the broadening of the (2 0 0) peak of the ZrC1xNx phase, at the lower angles side, allows to conclude that the FCC silver phase is present in the films. The samples with the highest silver content clearly evidenced the (111) diffraction plane of silver. Raman spectroscopy, on the other hand, revealed the existence of both an amorphous carbon (a-(C,N)) and the ZrC1xNx phase. Samples Ag0 and Ag6 did not show evidence of the amorphous carbon phase whereas Ag13 did not show the characteristics peaks of the ZrCxN1x phase, revealing its amorphous state. The amorphous carbon phases present two
Fig. 2. Surface and cross-section SEM images for Ag-ZrCxN1x.
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Fig. 3. Open circuit potential evolution of Ag-ZrCxN1x coatings as a function of immersion time.
typical peaks attributed to disordered (D) and graphitic carbon (G) [5], while the ZrCxN1x phase showed the peaks corresponding to the acoustic phonons at low frequency (230 cm1) and optical phonons at higher frequencies (490 cm1), as well as small peak around 750 cm1, associated to the residual oxygen that enters the ZrCxN1x lattice [42]. A more detailed structural characterization can be found in previous publications [43]. These results are in agreement with the composition of the samples achieved by EPMA, where the samples Ag0, Ag6 and Ag20 showed a ratio between Zr and (C + N) close to the unity. The architecture (substrate/Zr interlayer/ZrCN-Ag layer) of the films can be observed at the cross-sectional SEM images, displayed in Fig. 2. Pure ZrCxN1x crystalline phase is characterized by a columnar growth, as previously reported by Silva et al. [12], and observed also in sample Ag6; however, as the silver content and/or the amorphous carbon phases increase the columns are interrupted being evident a more granular-like growth. In addition, small bright spots are evidenced in the images that can be attributed to silver nanoparticles. This granular growth is promoted by the mixture of immiscible phases, such as ZrCxN1x, Ag and amorphous carbon. This arrangement is expected to play an important role on the electrochemical behavior of the coatings and will be further discussed in the electrochemical characterization section.
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resulting OCP is located between the OCP of both phases. Thus, the increase of the OCP for the coatings containing Ag and amorphous carbon phases is a consequence of the larger potential exhibited by these phases when compared to the pure ZrCxN1x phase. When those phases are incorporated in the coatings, a nanocomposite material is created, where the formation of couples such as, ZrCxN1x /Ag, ZrCxN1x /a-(C,N), Ag/a-(C,N), promotes different electrochemical processes, which contribute to the open circuit potential values in the cell. In Fig. 3, the OCP for a pure Ag films and a-C films were added for comparison. The potentiodynamic curves for all the samples and the bared 316 L stainless steel are shown in Fig. 4. Almost one order of magnitude reduction in density currents for the film with the lowest silver content (6 at%) was observed, compared to SS316L, evidencing an improved corrosion resistance. For higher silver content (7 and 9 at%) the reduction is also noticed, but only around the corrosion potential (Ecorr 100 mV); after this value, the density current is comparable to the stainless steel or even larger. Films with high silver content exhibit a more electrochemical active behavior as it is shown by the shift of the curves to higher density. Furthermore, the lower slope of the anodic branch of the curves makes that, for very small potential alteration, large variations in the current are achieved. The breakdown potential, usually associated to pitting for the stainless steel, is not clear in any of the coated samples for the reported range of potential, revealing a larger resistance to pitting formation. Fig. 5 depicts the results of the EIS experiments, using both Nyquist plot and the phase vs frequency in Fig. 5a and b, respectively. Fig. 5a shows a reduction of the impedance as the silver content is increased, with the exception of sample Ag0 that exhibits lower impedance values than Ag6 film. Fig. 5b, on the other hand, shows that the coatings, with the exception of Ag13, evidenced one time constant, suggesting a dense structure of the coating. The behavior of Ag13 is typical of an electrochemical porous film, which permits the penetration of the electrolyte creating a second time constant attributed to the pores. In consequence, the data were fitted using two equivalent circuits, shown in Fig. 5b inset. The equivalent circuit located in the top part of the inset was used for all the samples, including the uncoated stainless steel, whereas sample Ag13 results were fitted using the second equivalent circuit. The capacitance elements were replaced
3.2. Electrochemical characterization The electrochemical characterization is divided into two sections. The first section is focused on the effect of phase composition on the electrochemical behavior, whereas, the second section discusses the electrochemical behavior of a selected set of samples as a function of immersion time in Hank's balanced salt solution with 10 g L1 of albumin from bovine serum. 3.2.1. Electrochemical characterization vs. phase composition In all samples, the open circuit potential shows a very stable behavior, varying between 70 to 150 mV for the coatings with silver, while the film without silver clearly exhibited lower open circuit potential, around 220 mV (Fig. 3). This behavior can be understood on the basis of a mixture of electrical conductive phases and can be explained by the mixed potential theory which states that, for two conductive phases in electrical contact, the
Fig. 4. Potentiodynamic curves of Ag-ZrCxN1x coatings in Hank's balanced salt solution with 10 g of albumin per liter.
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Fig. 5. Nyquist plots of Ag-ZrCxN1x coated SS316L steel, using Hank's balanced salt solution with 10 g L1 of albumin at 37 C. Inset: Equivalent circuit used to fit the EIS data, where the RE, Rsol, Rp, CPE and WE represents the reference electrode (SCE), solution resistance, polarization resistance, a constant phase element and the working electrode (sample), respectively. The lines represent the fitting results and the symbols correspond to the experimental values in a and b.
by a constant phase element (CPE) in order to consider the surface roughness and heterogeneities [38–40]. Rp represents the polarization resistance element, which characterizes the charge transfer resistance of the material in both circuits. Moreover, two elements are added to the second circuit to take into consideration the high porosity in the Ag13 film. The polarization resistance, displayed in Fig. 6, is reduced as the silver content increases, behavior associated to the more electrochemical active silver phase compared to ZrCxN1x and amorphous carbon phases [20]. Although the silver content of the films is a decisive parameter affecting their electrochemical activity, the morphology and the phase structure promoted by compositional variations should not be disregarded. The carbon phases, for example, are also detrimental, as shown by the fact that, for similar silver content (Ag6 with 6 at% and Ag7 with 7 at%), the polarization resistance and the potentiodynamic curves revealed a worse corrosion resistance in Ag7 sample, the one with higher amount of amorphous carbon phase. Moreover, this trend may also be explained by either the reduction of the thickness (i.e. Ag7) or the increment of the diffusion path for the electrolyte to contact a larger area in the films. As the coatings become less columnar-like, the number of boundaries between the columns or grains increases, developing a granular morphology, as schematically
Fig. 6. Bar plot of the polarization resistance of uncoated and Ag-ZrCxN1x coated SS316L steel, in Hank's balanced salt solution with 10 g L1 of albumin at 37 C. Scatter plot of the area of defects on the material surface observed by optical microscopy.
shown in Fig. 7. These boundaries can create zones with nanoporosity that permit the flux of ions, which alters the electrochemical behavior of the coatings. The boundaries regions can act as passivation zones, which improve the corrosion behavior of the material or behave as diffusion paths when the material is chemically inert, depending on the electrochemical activity of the phases. Both scenarios can be foreseen in the present materials due to the different phase compositions. Firstly, ZrCxN1x phase can passivate at the boundaries regions, while in regions with pure carbon phases, the high inertness of the carbon may work purely as diffusion paths, explaining the additional deterioration of the films with a granular growth, which was promoted by the amorphous carbon phases and the silver nanoparticles. Finally, the addition of silver also promotes the intensification of macro-defects (Fig. 6) such as pin-holes and droplets on the surface of the films, which are known to produce larger contact areas between film and the electrolyte, producing higher electrochemical activity [38,44–46]. The enhanced corrosion behavior of Ag6 sample when compared to Ag0 suggests that a material with a pure ZrC0.5N0.5
Fig. 7. Scheme of the evolution of the film growth from columnar to granular and the impact on the boundary regions.
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Table 2 EIS fitting parameters of Ag-ZrCxN1x coatings in Hank’s balanced salt solution with 10 g L1 of albumin.
SS316L Ag0 Ag6 Ag7 Ag9 Ag13 Ag20
Rsol (V cm2)
Rp (MV cm2)
44.3 0.2 42.5 2.2 64.1 2.1 35.8 4.0 38.9 2.6 33.1 0.1 41.9 1.3
8.3 1.1 11.3 4.1 18.6 2.1 7.0 1.2 4.5 1.6 3.5 1.9 1.0 0.1
Rpor (kV cm2)
5.2 0.3
CPE Yo (mS sa cm2)
a-CPE
25.9 0.1 20.8 1.6 7.7 0.2 10.9 1.8 12.9 0.2 17.0 0.7 15.5 1.8
0.92 0.1 0.91 0.0 0.94 0.0 0.88 0.0 0.91 0.0 0.76 0.0 0.87 0.0
(Ag6) phase, with no amorphous phases, presents higher corrosion resistance than ZrCxN1x phases (Ag0) with other stoichiometries (x value). In fact, despite the ratio C+N/Zr is similar in both cases, C/ N for Ag0 is almost half of the Ag6; additional research must be carried out to confirm this trend. The CPE parameter (Table 2), on the other hand, reflects the capacitive behavior of the interface between the fluid and the films, but both CPE and a parameters must be taken into consideration. a values are close to 1 for low content silver films, revealing a more capacitive behavior. However, the admittance increases, as silver is introduced, probably due to the higher contact areas between the films and the electrolyte induced by the more granular behavior, which also explains the higher CPE admittance in sample Ag13.
CPE2 Yo (mS sa cm2)
17.0 0.7
a-CPE2
0.69 0.0
Goodness of fit 2.0 104 4.6 105 4.2 105 5.7 104 1.6 104 7.2 104 3.0 103
since lower albumin adsorption and none silver ion release are expected from Ag0 due to its hydrophilic character and 0 at.% of silver, respectively, as later explained. An overlap of the anodic polarization curves for the sample after 0 h and 336 h of immersion is presented in Fig. 9. After the immersion tests, and in agreement with the OCP results, the coatings tested showed a decrease in the corrosion potential when compared to the sample with 0 h immersion time, as well as a more
3.2.2. Electrochemical Characterization vs immersion time in HBSS. Three samples were selected to evaluate the effect of the electrolyte as a function of the immersion time. The samples were selected taking into consideration, by the one hand, the analysis made in the previous section, and on the other, the low content of silver and amorphous phase in the coatings. Fig. 8 shows the open circuit potential (OCP) as a function of the immersion time, evidencing similar tendencies for all the studied samples. A decrease of the OCP with increasing immersion time is observed, with a steeper trend for samples with silver. Similar behavior has been reported for albumin-containing electrolytes and metallic samples [47]. Thus, the decrease of the OCP may be explained as a combination of three factors; the formation of a thicker zirconium oxide layer, the adsorption of albumin on the surface and the reduction of silver, due to its release from the material surface, which will be later corroborated by XPS. Additionally, the larger reduction on the films containing silver support the hypotheses,
Fig. 9. Potentiodynamic curves of Ag0, Ag6 and Ag9 in Hank's balanced salt solution with 10 g of albumin per liter after 1 h and 336 h of immersion.
Fig. 8. Open circuit potential as a function of the immersion time of Ag-ZrCxN1x coatings in Hank's balanced salt solution with 10 g/L of albumin at 37 C
Fig. 10. Nyquist plot for a representative coating (Ag6) as a function of the immersion time, using Hank's balanced salt solution with 10 g/L of albumin at 37 C.
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Fig. 11. Polarization resistance (Rp) of Ag-ZrCxN1x samples as a function of immersion time in Hank's balanced salt solution with 10 g/L of albumin at 37 C.
passive behavior, also explained by the formation of the zirconium oxide, the silver release and the protective albumin layer. Electrochemical impedance spectroscopy experiments carried out with increasing immersion time, revealed an increment of the impedance as observed in the Nyquist plot in Fig. 10. These results were also fitted using the equivalent circuit in the inset of Fig. 5b, as previously described. The polarization resistance increased as a function of time for all the samples (cf. Fig. 11), during the first 72 h of immersion, reaching then a plateau-like zone. This behavior is commonly associated to a passivation film that may be formed on the material surface, inducing the blocking of the charge transfer
by impeding the ionic exchange between the electrolyte and the material. The surface modified zone was analyzed by X-ray photoelectron spectroscopy (XPS). Surface oxidation is expected due to the high affinity of Zr to form oxides, particularly ZrO2 with an enthalpy of formation around 1097.46 kJ/mol compared to ZrN (365.26 kJ/ mol) or ZrC (196.65 kJ/mol) [48]. Zr–O bonds were detected by XPS around 182.3 eV for Zr and 530.2 eV for O (Fig. 12a and e). In addition, the formation of a layer rich in C–N and C–O and C–N–O, related to the adsorption of the albumin on the surface of the material is observed. This albumin layer can be confirmed by the attenuation of the Zr–O, N–Zr, Zr–N and Ag–Ag bonds, shown in Fig. 12, before and after immersion and can be roughly qualitatively characterized by the N–C (399.8 0.2 eV – associated to the amine or amide groups of the protein) peak intensity changes before and after immersion [49]. This peak showed that the films with silver (Ag6 and Ag9) have 1.4 times more albumin adsorption than the sample without silver (Ag0). These differences of albumin adsorption are justified by the difference in the surface energy of the samples with and without silver (Table 3). The sample without Ag is the only hydrophilic, and hence, less albumin adsorption is predictable, since the albumin adsorbs more easily onto hydrophobic surfaces [50]. Moreover, the relative intensity of silver is more largely reduced after immersion, compared to the attenuation of Zr–O, N–Zr and Zr–N, as shown in Fig. 12, attributed to a silver ion release to the electrolyte. The silver release has been previously demonstrated for similar materials, in very low quantities, in the first 7 to 15 nm of the material [21]. As a result, silver not only shows the characteristic binding energy of pure metallic state (Ag–Ag 368.4 eV) after immersion, but also an oxidized state at 367.7 eV. The increase of the charge transfer resistance of the materials is a consequence of the three factors described above, providing protection to the coatings and enhancing the corrosion resistance
Fig. 12. XPS spectra of Zr 3d, Ag 3d, O 1s, N 1s and C 1s core levels of Ag-ZrCxN1x samples before and after 168 h of immersion in Hank's balanced salt solution with 10 g L1 of albumin at 37 C. Every element is displayed at different scales to clearly identify the peaks. Each scale is shown below the element name.
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Table 3 Water, formamide and bromonaphtalene contact angles, and surface interation (free Energy DG) between the water and the surface (DGLW = apolar Lifshitz–van der Waals component; DGAB = electron acceptor component). Interaction solid liquid (mJ m2)
SS316L Ag0 Ag6 Ag7 Ag9 Ag13 Ag20
Contact angle (degrees)
DG
DG LW
DG AB
Water
Formamide
Bromonaphtalene
85.1 105.1 85.1 74.3 77.5 86.6 83.2
75.4 80.0 76.9 72.9 72.1 74.1 76.8
9.6 25.1 8.2 1.3 5.5 12.5 6.4
86 5 64 9 98 2 101 3 104 1 96 1 95 3
68 3 36 5 62 3 74 6 70 1 60 2 64 3
36 3 23 3 32 6 41 2 43 1 39 2 32 5
of the material, which, in the case of Ag6, attains values almost one order of magnitude larger after 336 h of immersion. 4. Conclusion ZrCN-Ag-a(C,N) coatings were successfully deposited on 316 L stainless steel by magnetron sputtering technique with varied amounts of silver and amorphous carbon phases. The electrochemical characterization revealed an improvement of the corrosion resistance, in Hank's balanced salt solution with 10 g L1 of BSA, of stainless steel when coated with ZrCN-Ag-a(C,N) coatings with low contents of silver and amorphous carbon. When increasing the silver and amorphous carbon contents, electrochemical porous material is produced, inducing a reduction of the charge transfer resistance. Immersion test, on the other hand, showed a constant increase of the polarization resistance of the material for the first 72 h of immersion, attaining a constant value thereafter. This behavior was attributed to a passivation layer created by the surface oxidation of both Zr and Ag, as well as the adsorption of albumin onto the surface. In addition, a low silver release was detected, which also decreased the electrochemical activity of the surfaces. Acknowledgements This research is partially sponsored by FEDER funds through the program COMPETE- Programa Operacional Factores de Competitividade and by Portuguese national funds through FCT-Fundação para a Ciência e a Tecnologia, under the projects ANTIMICROBCOAT - PTDC/CTM/102853/2008 and in the framework of the Strategic Projects PEST-C/FIS/UI607/2013”, PEST-C/EME/UI0285/2013 and SFRH/BD/80947/2011. References [1] F. Borgioli, E. Galvanetto, F. Iozzelli, G. Pradelli, Improvement of wear resistance of Ti–6Al–4V alloy by means of thermal oxidation, Materials Letters 59 (2005) 2159–2162. [2] Y.S. Hedberg, B. Qian, Z. Shen, S. Virtanen, I. Odnevall Wallinder, In vitro biocompatibility of CoCrMo dental alloys fabricated by selective laser melting, Dental Materials 30 (2014) 525–534. [3] K. Yaya, Y. Khelfaoui, B. Malki, M. Kerkar, Numerical simulations study of the localized corrosion resistance of AISI 316L stainless steel and pure titanium in a simulated body fluid environment, Corrosion Science 53 (2011) 3309–3314. [4] J.R. Davis, Handbook of Materials for Medical Devices, ASM International, 2003341. [5] L. Mattei, F. Di Puccio, B. Piccigallo, E. Ciulli, Lubrication and wear modelling of artificial hip joints: A review, Tribology International 44 (2011) 532–549. [6] R. Hauert, K. Thorwarth, G. Thorwarth, An overview on diamond-like carbon coatings in medical applications, Surface and Coatings Technology 233 (2013) 119–130. [7] N.K. Manninen, F. Ribeiro, A. Escudeiro, T. Polcar, S. Carvalho, A. Cavaleiro, Influence of Ag content on mechanical and tribological behavior of DLC coatings, Surface and Coatings Technology 232 (2013) 440–446. [8] B. Subramanian, C.V. Muraleedharan, R. Ananthakumar, M. Jayachandran, A comparative study of titanium nitride (TiN), titanium oxy nitride (TiON) and titanium aluminum nitride (TiAlN), as surface coatings for bio implants, Surface and Coatings Technology 205 (2011) 5014–5020.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical response of ZrCN-Ag-a(C,N) coatings in simulated body fluids S. Calderon Va,b,* , A. Cavaleirob , S. Carvalhoa,b a b
Physics Department, University of Minho, 4800-058 Guimarães, Portugal SEG-CEMUC Mechanical Engineering Department, University of Coimbra, 3030-788 Coimbra, Portugal
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 January 2015 Received in revised form 22 June 2015 Accepted 14 July 2015 Available online 23 July 2015
In this study, Zr-C-N-Ag coatings were deposited by magnetron sputtering aiming to provide an enhanced corrosion resistance to stainless steel 316 L, while adding antibacterial capabilities using silver, a wellknown antibacterial agent. The films were analysed by X-ray diffraction, Raman spectroscopy, electron probe microanalysis and X-ray photoelectron spectroscopy (XPS) in relation to the chemical composition and structural characteristics. Additionally, electrochemical impedance spectroscopy and potentiodynamic tests were carried out as a function of the immersion time, in Hanks’ balanced salt solution with 10 g/L of Bovine serum albumin (BSA), to evaluate the electrochemical characteristics of the samples. The results revealed that the coatings are predominantly composed by ZrCxN1x, Ag and amorphous carbon phases. A large deterioration of the electrochemical stability of the films was evidenced as the silver content increased. The presence of amorphous carbon phases negatively influenced the polarization resistant, behavior ascribed to morphological changes. The immersion test showed a progressive increment of the polarization resistance with time, attributed to surface and pores passivation, due to the formation of both ZrO2 and albumin protective layers on the surface, as shown by XPS. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: ZrCN Silver Corrosion Albumin XPS
1. Introduction Thin protective layers have been used as an efficient process in surface engineering to provide additional properties to a base material or to improve the existing ones, such as wear and corrosion resistance, and biocompatibility. Nowadays, this methodology is suitable to solve problems identified in materials used for medical devices. Low wear resistance in titanium alloys [1], toxicity in CrCo alloys [2] and limited localized corrosion resistance in 316 L stainless steel [3], the most used materials for medical applications [4,5], showed the need of producing a multifunctional material that can enhance the mechanical, tribological, chemical and biological properties. These characteristics have been modified by applying chemical inert coatings with low-reactive ceramic material, such as diamond-like-carbon materials [6,7], nitrides [8–10], carbides [11], carbonitrides [12], or oxides [9,13]; carbonitrides present the best compromise between the mechanical, triboglogical and corrosion resistance performance. Moreover, in order to potentiate the materials biocompatibility, antimicrobial
* Corresponding author at: Universidade do Minho, Dept. Física, Campus de Azurém, 4800-058 Guimarães, Portugal. Tel: +351 253510175x517465; fax: +351 253510461. E-mail address: [email protected] (S. Calderon). http://dx.doi.org/10.1016/j.electacta.2015.07.083 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
agents, such as silver, can be included [14–19]. Therefore, our group has been exploring the possibility of combining transition metal carbonitrides with silver nanoparticles as promising materials to provide medical devices with multifunctional capabilities for an enhanced performance [20–22]. Transition metal carbonitrides have been extensively used as protective coatings, due to their great mechanical, tribological and chemical properties [12,21,23–30], making them potential candidates to exceed the properties of current bio-materials. ZrC1xNx, for instance, is biocompatible and its mechanical and tribological properties, as well as chemical stability, can be modified by tuning the chemical composition [12,28–34], which provides different mixtures of phases and nano-composite structures. Further changes can be achieved in respect to antimicrobial effect by introducing silver into the ceramic ZrC1xNx matrix. Although ZrCxN1x coatings have been recognized to improve the corrosion resistance of the base material in different fluids [35,36], little is known about the ability of the films for protection in simulated body fluids. This complex Ag-ZrCxN1x system allows the co-existence of a large variety of phases, such as ZrCxN1x, amorphous carbon, metallic silver and some residual oxides, which may easily alter the electrochemical behavior of the system. Previously we have reported that the increase of silver metallic phase in Ag-ZrCxN1x coatings, accompanied by a reduction of
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ZrCxN1x grain size and the increase of amorphous phases in the material, could explain the more electrochemical active behavior of the samples [20]. In the present report, the electrochemical behavior of Ag-ZrCxN1x coatings with and without amorphous phases, deposited on 316 L stainless steel, is reported; the evolution of the samples as a function of the immersion time in simulated body fluid, with composition close to the synovial fluid present in the body joints, is emphasized. 2. Material and methods 2.1. Coating production Ag-ZrCxN1x coatings were deposited by direct current (DC) unbalance reactive dual magnetron sputtering technique with two highly pure targets (Zr and Ag) of 100 200 6 mm3. The targets were located in opposite direction, pointing towards a rotational substrate holder placed in the center of the chamber at 70 mm of each target. The incorporation of carbon and nitrogen in the films was carried out through acetylene and nitrogen gases, by controlling the fluxes which ranged from 1.2–2.4 and 3–9 sccm, respectively, as shown in Table 1. The silver content, on the other hand, was varied by changing the Ag target current density between 0.25 and 0.75 mA cm2, maintaining the Zr current density at 10 mA cm2. The bias potential (50 V), substrate rotation speed (8 rpm) and chamber temperature (373 K) were monitored and kept constant for all the depositions. The 316 L stainless steel (Fe 68.40 wt%, Cr 16.40 wt%, Ni 11.1 wt%, Mo 2.28 wt%, Mn 1.3 wt%, Si 0.39 wt%, P = 0.06 wt%, and C < 0.03 wt %) was grinded using emery paper from 240 to 2400 grit, and then, mirror polished with a diamond solution, which allowed to reach a final surface roughness (Ra) around 8 nm. Thereafter, a cleaning procedure was carried out using ultrasonic baths in distilled water, ethanol and acetone during 10 min for each solvent, in order to remove dust and impurities. 2.2. Physical and chemical characterization The coatings structure was studied by X-ray diffraction (XRD) in a PANalytical XPert PRO micro-diffractometer with Cu Ka (1.540598 Å) radiation in grazing angle mode. Raman spectroscopy was carried out to investigate the amorphous carbon phases. Measurements were performed using a LabRAM Horiba Jobin Yvon spectrometer equipped a He–Ne laser (532 nm) at 5 mW. The coating bulk chemical composition was analyzed by a Cameca SX 50 electron probe micro-analyzer (EPMA), using statistical analysis of 5 randomly selected spots in each sample. Films thickness were evaluated by the Calotest method utilizing a rotation sphere 20 mm in diameter, at 1000 rpm for 90 s, in order to obtain the desired wear. The surface morphology was assessed using a NanoSEM – FEI Nova 200 scanning electron microscope. The superficial defects in the coatings were analyzed by optical
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microscopy. The defects are represented as the percentage of the area covered by surface defects. For each coating, 10 different areas were imaged at 50X magnification and analyzed with ImageJ v.1.48 software. The surface composition, before and after immersion tests, was assessed by X-ray photoelectron spectrometer Kratos AXIS Ultra HSA, with VISION software for data acquisition and CASAXPS software for data analysis. A monochromatic Al Ka X-ray source (1486.7 eV), operating at 15 kV (150 W), in FAT mode (Fixed Analyser Transmission), with a pass energy of 40 eV for regions ROI and 80 eV for survey. Data acquisition was performed with a pressure lower than 1 106 Pa, and a charge neutralisation system was used. The effect of the electric charge was corrected by reference to the carbon peak (285 eV). The deconvolution was carried out using CasaXPS program; peak fitting was performed using Gaussian-Lorentzian peak shape and Shirley type background subtraction. 2.3. Contact angle and hydrophobicity The surface free energy and the hydrophobicity parameters of the surfaces were determined by the sessile drop contact angle technique, by means of an OCA 15 Plus device. The measurements were performed at room temperature, using 2 mL of water, formamide and a-bromonaphtalene with known surface energy components, as reference liquids. The surface tension parameters were calculated using the van Oss approach [37]. 2.4. Electrochemical characterization The electrochemical behavior of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) at open circuit potential, for frequencies ranging from 100 kHz to 5 mHz, with a 10 mV (rms) AC perturbation, using EIS300 software in a Gamry potentiostat REF600. All electrochemical experiments were performed in a classic three electrodes corrosion cell with platinum and saturated calomel (SCE) as counter and reference electrodes, respectively, in Hank's balanced salt solution (0.137 M NaCl, 5.4 mM KCl, 0.25 mM Na2HPO4, 0.44 mM KH2PO4, 1.3 mM CaCl2, 1.0 mM MgSO4 and 4.2 NaHCO3) with 10 g L1 of bovine serum albumin (BSA) at 37 C with stabilized pH to 7.0 0.2 in equilibrium with air. All measurements were acquired after achieving a stable open circuit potential, at about 1 h. Simulation of the experimental data was also performed with Gamry software, using a Randles equivalent circuit, in which the capacitance element was replaced by a constant phase element in order to consider the surface roughness and heterogeneities [38– 40], with Rp representing the polarization resistance element, i.e. the charge transfer resistance of the material. The samples with best corrosion resistance properties were utilized for immersion tests to observe the evolution of the electrochemical parameters for 15 days.
Table 1 Deposition parameters and coatings properties.
Ag0 Ag6 Ag7 Ag9 Ag13 Ag20
Ag current density [mA cm2]
C2H2 [sccm]
N2 [sccm]
Thickness [mm]
Deposition Rate [mm h1]
ZrCN Grain Size [nm]
Zr [%]
N [%]
C [%]
Ag [%]
O [%]
0 0.25 0.25 0.5 0.5 0.75
1.2 1.2 1.2 2.4 2.4 1.2
3 3 9 3 9 3
2.3 1.8 1.1 1.6 1.5 2.0
1.9 1.7 1.0 1.5 1.4 2.1
7.2 6.9 3.8 11.6 NA 5.4
49 46 32 35 21 38
33 24 39 18 30 20
15 18 18 29 27 16
0 6 7 9 13 20
3 6 4 9 9 6
Standard deviation of the composition are between 0.5 and 1.2 at.%.
CþN Zr
1.0 0.9 1.8 1.4 2.8 1.0
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3. Results and discussion 3.1. Chemical and physical characterization
Fig. 1. a) Coatings X-ray diffraction patterns; ZrC0.5N0.5 (ICDD-03-065-8779) and Ag (ICDD- 00-087-0719) standard patterns are shown for comparison and b) coatings Raman spectroscopy spectra.
Additionally, potentiodynamic tests were performed in two stages (after 1 h and 336 h after immersion in HBSS + A), using a scanning rate of 60 mV/min from 300 mV vs. OCP to +1000 mV vs. SCE. The results of the electrochemical test were calculated as the average and standard deviations of at least three separated samples. All the potentials are expressed with respect to SCE electrode.
Table 1 shows the acetylene and nitrogen fluxes, thickness, deposition rate, ZrCxN1x grain size and the chemical composition of the films sorted from the lowest to the largest amount of silver content in the films, as measured by EPMA. The silver content is affected not only by the increase of the current density applied to the silver target, but also by the variation of acetylene and nitrogen fluxes, which give rise to a variety of film compositions and phases arrangements. More detailed information about the effect of the deposition conditions on the chemical and phase composition can be found elsewhere [41]. Fig. 1 displays the X-ray diffraction (XRD) and Raman spectroscopy results for all the samples. The results were sorted as a function of the silver content in the films, which evidenced that there is not a simple correlation between the silver content and the structural characteristics of the coatings. In fact, the silver variation was achieved not only by increasing the silver target power, but also by changing the gas fluxes in the chamber. The increase of reactive gas fluxes promotes the total consumption of zirconium and the formation of C-rich phases. A face-centered cubic phase of ZrC1xNx is identified, showing different degrees of crystallinity. The metallic silver phase is not clearly evidenced for low silver content coatings, but the broadening of the (2 0 0) peak of the ZrC1xNx phase, at the lower angles side, allows to conclude that the FCC silver phase is present in the films. The samples with the highest silver content clearly evidenced the (111) diffraction plane of silver. Raman spectroscopy, on the other hand, revealed the existence of both an amorphous carbon (a-(C,N)) and the ZrC1xNx phase. Samples Ag0 and Ag6 did not show evidence of the amorphous carbon phase whereas Ag13 did not show the characteristics peaks of the ZrCxN1x phase, revealing its amorphous state. The amorphous carbon phases present two
Fig. 2. Surface and cross-section SEM images for Ag-ZrCxN1x.
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Fig. 3. Open circuit potential evolution of Ag-ZrCxN1x coatings as a function of immersion time.
typical peaks attributed to disordered (D) and graphitic carbon (G) [5], while the ZrCxN1x phase showed the peaks corresponding to the acoustic phonons at low frequency (230 cm1) and optical phonons at higher frequencies (490 cm1), as well as small peak around 750 cm1, associated to the residual oxygen that enters the ZrCxN1x lattice [42]. A more detailed structural characterization can be found in previous publications [43]. These results are in agreement with the composition of the samples achieved by EPMA, where the samples Ag0, Ag6 and Ag20 showed a ratio between Zr and (C + N) close to the unity. The architecture (substrate/Zr interlayer/ZrCN-Ag layer) of the films can be observed at the cross-sectional SEM images, displayed in Fig. 2. Pure ZrCxN1x crystalline phase is characterized by a columnar growth, as previously reported by Silva et al. [12], and observed also in sample Ag6; however, as the silver content and/or the amorphous carbon phases increase the columns are interrupted being evident a more granular-like growth. In addition, small bright spots are evidenced in the images that can be attributed to silver nanoparticles. This granular growth is promoted by the mixture of immiscible phases, such as ZrCxN1x, Ag and amorphous carbon. This arrangement is expected to play an important role on the electrochemical behavior of the coatings and will be further discussed in the electrochemical characterization section.
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resulting OCP is located between the OCP of both phases. Thus, the increase of the OCP for the coatings containing Ag and amorphous carbon phases is a consequence of the larger potential exhibited by these phases when compared to the pure ZrCxN1x phase. When those phases are incorporated in the coatings, a nanocomposite material is created, where the formation of couples such as, ZrCxN1x /Ag, ZrCxN1x /a-(C,N), Ag/a-(C,N), promotes different electrochemical processes, which contribute to the open circuit potential values in the cell. In Fig. 3, the OCP for a pure Ag films and a-C films were added for comparison. The potentiodynamic curves for all the samples and the bared 316 L stainless steel are shown in Fig. 4. Almost one order of magnitude reduction in density currents for the film with the lowest silver content (6 at%) was observed, compared to SS316L, evidencing an improved corrosion resistance. For higher silver content (7 and 9 at%) the reduction is also noticed, but only around the corrosion potential (Ecorr 100 mV); after this value, the density current is comparable to the stainless steel or even larger. Films with high silver content exhibit a more electrochemical active behavior as it is shown by the shift of the curves to higher density. Furthermore, the lower slope of the anodic branch of the curves makes that, for very small potential alteration, large variations in the current are achieved. The breakdown potential, usually associated to pitting for the stainless steel, is not clear in any of the coated samples for the reported range of potential, revealing a larger resistance to pitting formation. Fig. 5 depicts the results of the EIS experiments, using both Nyquist plot and the phase vs frequency in Fig. 5a and b, respectively. Fig. 5a shows a reduction of the impedance as the silver content is increased, with the exception of sample Ag0 that exhibits lower impedance values than Ag6 film. Fig. 5b, on the other hand, shows that the coatings, with the exception of Ag13, evidenced one time constant, suggesting a dense structure of the coating. The behavior of Ag13 is typical of an electrochemical porous film, which permits the penetration of the electrolyte creating a second time constant attributed to the pores. In consequence, the data were fitted using two equivalent circuits, shown in Fig. 5b inset. The equivalent circuit located in the top part of the inset was used for all the samples, including the uncoated stainless steel, whereas sample Ag13 results were fitted using the second equivalent circuit. The capacitance elements were replaced
3.2. Electrochemical characterization The electrochemical characterization is divided into two sections. The first section is focused on the effect of phase composition on the electrochemical behavior, whereas, the second section discusses the electrochemical behavior of a selected set of samples as a function of immersion time in Hank's balanced salt solution with 10 g L1 of albumin from bovine serum. 3.2.1. Electrochemical characterization vs. phase composition In all samples, the open circuit potential shows a very stable behavior, varying between 70 to 150 mV for the coatings with silver, while the film without silver clearly exhibited lower open circuit potential, around 220 mV (Fig. 3). This behavior can be understood on the basis of a mixture of electrical conductive phases and can be explained by the mixed potential theory which states that, for two conductive phases in electrical contact, the
Fig. 4. Potentiodynamic curves of Ag-ZrCxN1x coatings in Hank's balanced salt solution with 10 g of albumin per liter.
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Fig. 5. Nyquist plots of Ag-ZrCxN1x coated SS316L steel, using Hank's balanced salt solution with 10 g L1 of albumin at 37 C. Inset: Equivalent circuit used to fit the EIS data, where the RE, Rsol, Rp, CPE and WE represents the reference electrode (SCE), solution resistance, polarization resistance, a constant phase element and the working electrode (sample), respectively. The lines represent the fitting results and the symbols correspond to the experimental values in a and b.
by a constant phase element (CPE) in order to consider the surface roughness and heterogeneities [38–40]. Rp represents the polarization resistance element, which characterizes the charge transfer resistance of the material in both circuits. Moreover, two elements are added to the second circuit to take into consideration the high porosity in the Ag13 film. The polarization resistance, displayed in Fig. 6, is reduced as the silver content increases, behavior associated to the more electrochemical active silver phase compared to ZrCxN1x and amorphous carbon phases [20]. Although the silver content of the films is a decisive parameter affecting their electrochemical activity, the morphology and the phase structure promoted by compositional variations should not be disregarded. The carbon phases, for example, are also detrimental, as shown by the fact that, for similar silver content (Ag6 with 6 at% and Ag7 with 7 at%), the polarization resistance and the potentiodynamic curves revealed a worse corrosion resistance in Ag7 sample, the one with higher amount of amorphous carbon phase. Moreover, this trend may also be explained by either the reduction of the thickness (i.e. Ag7) or the increment of the diffusion path for the electrolyte to contact a larger area in the films. As the coatings become less columnar-like, the number of boundaries between the columns or grains increases, developing a granular morphology, as schematically
Fig. 6. Bar plot of the polarization resistance of uncoated and Ag-ZrCxN1x coated SS316L steel, in Hank's balanced salt solution with 10 g L1 of albumin at 37 C. Scatter plot of the area of defects on the material surface observed by optical microscopy.
shown in Fig. 7. These boundaries can create zones with nanoporosity that permit the flux of ions, which alters the electrochemical behavior of the coatings. The boundaries regions can act as passivation zones, which improve the corrosion behavior of the material or behave as diffusion paths when the material is chemically inert, depending on the electrochemical activity of the phases. Both scenarios can be foreseen in the present materials due to the different phase compositions. Firstly, ZrCxN1x phase can passivate at the boundaries regions, while in regions with pure carbon phases, the high inertness of the carbon may work purely as diffusion paths, explaining the additional deterioration of the films with a granular growth, which was promoted by the amorphous carbon phases and the silver nanoparticles. Finally, the addition of silver also promotes the intensification of macro-defects (Fig. 6) such as pin-holes and droplets on the surface of the films, which are known to produce larger contact areas between film and the electrolyte, producing higher electrochemical activity [38,44–46]. The enhanced corrosion behavior of Ag6 sample when compared to Ag0 suggests that a material with a pure ZrC0.5N0.5
Fig. 7. Scheme of the evolution of the film growth from columnar to granular and the impact on the boundary regions.
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Table 2 EIS fitting parameters of Ag-ZrCxN1x coatings in Hank’s balanced salt solution with 10 g L1 of albumin.
SS316L Ag0 Ag6 Ag7 Ag9 Ag13 Ag20
Rsol (V cm2)
Rp (MV cm2)
44.3 0.2 42.5 2.2 64.1 2.1 35.8 4.0 38.9 2.6 33.1 0.1 41.9 1.3
8.3 1.1 11.3 4.1 18.6 2.1 7.0 1.2 4.5 1.6 3.5 1.9 1.0 0.1
Rpor (kV cm2)
5.2 0.3
CPE Yo (mS sa cm2)
a-CPE
25.9 0.1 20.8 1.6 7.7 0.2 10.9 1.8 12.9 0.2 17.0 0.7 15.5 1.8
0.92 0.1 0.91 0.0 0.94 0.0 0.88 0.0 0.91 0.0 0.76 0.0 0.87 0.0
(Ag6) phase, with no amorphous phases, presents higher corrosion resistance than ZrCxN1x phases (Ag0) with other stoichiometries (x value). In fact, despite the ratio C+N/Zr is similar in both cases, C/ N for Ag0 is almost half of the Ag6; additional research must be carried out to confirm this trend. The CPE parameter (Table 2), on the other hand, reflects the capacitive behavior of the interface between the fluid and the films, but both CPE and a parameters must be taken into consideration. a values are close to 1 for low content silver films, revealing a more capacitive behavior. However, the admittance increases, as silver is introduced, probably due to the higher contact areas between the films and the electrolyte induced by the more granular behavior, which also explains the higher CPE admittance in sample Ag13.
CPE2 Yo (mS sa cm2)
17.0 0.7
a-CPE2
0.69 0.0
Goodness of fit 2.0 104 4.6 105 4.2 105 5.7 104 1.6 104 7.2 104 3.0 103
since lower albumin adsorption and none silver ion release are expected from Ag0 due to its hydrophilic character and 0 at.% of silver, respectively, as later explained. An overlap of the anodic polarization curves for the sample after 0 h and 336 h of immersion is presented in Fig. 9. After the immersion tests, and in agreement with the OCP results, the coatings tested showed a decrease in the corrosion potential when compared to the sample with 0 h immersion time, as well as a more
3.2.2. Electrochemical Characterization vs immersion time in HBSS. Three samples were selected to evaluate the effect of the electrolyte as a function of the immersion time. The samples were selected taking into consideration, by the one hand, the analysis made in the previous section, and on the other, the low content of silver and amorphous phase in the coatings. Fig. 8 shows the open circuit potential (OCP) as a function of the immersion time, evidencing similar tendencies for all the studied samples. A decrease of the OCP with increasing immersion time is observed, with a steeper trend for samples with silver. Similar behavior has been reported for albumin-containing electrolytes and metallic samples [47]. Thus, the decrease of the OCP may be explained as a combination of three factors; the formation of a thicker zirconium oxide layer, the adsorption of albumin on the surface and the reduction of silver, due to its release from the material surface, which will be later corroborated by XPS. Additionally, the larger reduction on the films containing silver support the hypotheses,
Fig. 9. Potentiodynamic curves of Ag0, Ag6 and Ag9 in Hank's balanced salt solution with 10 g of albumin per liter after 1 h and 336 h of immersion.
Fig. 8. Open circuit potential as a function of the immersion time of Ag-ZrCxN1x coatings in Hank's balanced salt solution with 10 g/L of albumin at 37 C
Fig. 10. Nyquist plot for a representative coating (Ag6) as a function of the immersion time, using Hank's balanced salt solution with 10 g/L of albumin at 37 C.
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Fig. 11. Polarization resistance (Rp) of Ag-ZrCxN1x samples as a function of immersion time in Hank's balanced salt solution with 10 g/L of albumin at 37 C.
passive behavior, also explained by the formation of the zirconium oxide, the silver release and the protective albumin layer. Electrochemical impedance spectroscopy experiments carried out with increasing immersion time, revealed an increment of the impedance as observed in the Nyquist plot in Fig. 10. These results were also fitted using the equivalent circuit in the inset of Fig. 5b, as previously described. The polarization resistance increased as a function of time for all the samples (cf. Fig. 11), during the first 72 h of immersion, reaching then a plateau-like zone. This behavior is commonly associated to a passivation film that may be formed on the material surface, inducing the blocking of the charge transfer
by impeding the ionic exchange between the electrolyte and the material. The surface modified zone was analyzed by X-ray photoelectron spectroscopy (XPS). Surface oxidation is expected due to the high affinity of Zr to form oxides, particularly ZrO2 with an enthalpy of formation around 1097.46 kJ/mol compared to ZrN (365.26 kJ/ mol) or ZrC (196.65 kJ/mol) [48]. Zr–O bonds were detected by XPS around 182.3 eV for Zr and 530.2 eV for O (Fig. 12a and e). In addition, the formation of a layer rich in C–N and C–O and C–N–O, related to the adsorption of the albumin on the surface of the material is observed. This albumin layer can be confirmed by the attenuation of the Zr–O, N–Zr, Zr–N and Ag–Ag bonds, shown in Fig. 12, before and after immersion and can be roughly qualitatively characterized by the N–C (399.8 0.2 eV – associated to the amine or amide groups of the protein) peak intensity changes before and after immersion [49]. This peak showed that the films with silver (Ag6 and Ag9) have 1.4 times more albumin adsorption than the sample without silver (Ag0). These differences of albumin adsorption are justified by the difference in the surface energy of the samples with and without silver (Table 3). The sample without Ag is the only hydrophilic, and hence, less albumin adsorption is predictable, since the albumin adsorbs more easily onto hydrophobic surfaces [50]. Moreover, the relative intensity of silver is more largely reduced after immersion, compared to the attenuation of Zr–O, N–Zr and Zr–N, as shown in Fig. 12, attributed to a silver ion release to the electrolyte. The silver release has been previously demonstrated for similar materials, in very low quantities, in the first 7 to 15 nm of the material [21]. As a result, silver not only shows the characteristic binding energy of pure metallic state (Ag–Ag 368.4 eV) after immersion, but also an oxidized state at 367.7 eV. The increase of the charge transfer resistance of the materials is a consequence of the three factors described above, providing protection to the coatings and enhancing the corrosion resistance
Fig. 12. XPS spectra of Zr 3d, Ag 3d, O 1s, N 1s and C 1s core levels of Ag-ZrCxN1x samples before and after 168 h of immersion in Hank's balanced salt solution with 10 g L1 of albumin at 37 C. Every element is displayed at different scales to clearly identify the peaks. Each scale is shown below the element name.
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Table 3 Water, formamide and bromonaphtalene contact angles, and surface interation (free Energy DG) between the water and the surface (DGLW = apolar Lifshitz–van der Waals component; DGAB = electron acceptor component). Interaction solid liquid (mJ m2)
SS316L Ag0 Ag6 Ag7 Ag9 Ag13 Ag20
Contact angle (degrees)
DG
DG LW
DG AB
Water
Formamide
Bromonaphtalene
85.1 105.1 85.1 74.3 77.5 86.6 83.2
75.4 80.0 76.9 72.9 72.1 74.1 76.8
9.6 25.1 8.2 1.3 5.5 12.5 6.4
86 5 64 9 98 2 101 3 104 1 96 1 95 3
68 3 36 5 62 3 74 6 70 1 60 2 64 3
36 3 23 3 32 6 41 2 43 1 39 2 32 5
of the material, which, in the case of Ag6, attains values almost one order of magnitude larger after 336 h of immersion. 4. Conclusion ZrCN-Ag-a(C,N) coatings were successfully deposited on 316 L stainless steel by magnetron sputtering technique with varied amounts of silver and amorphous carbon phases. The electrochemical characterization revealed an improvement of the corrosion resistance, in Hank's balanced salt solution with 10 g L1 of BSA, of stainless steel when coated with ZrCN-Ag-a(C,N) coatings with low contents of silver and amorphous carbon. When increasing the silver and amorphous carbon contents, electrochemical porous material is produced, inducing a reduction of the charge transfer resistance. Immersion test, on the other hand, showed a constant increase of the polarization resistance of the material for the first 72 h of immersion, attaining a constant value thereafter. This behavior was attributed to a passivation layer created by the surface oxidation of both Zr and Ag, as well as the adsorption of albumin onto the surface. In addition, a low silver release was detected, which also decreased the electrochemical activity of the surfaces. Acknowledgements This research is partially sponsored by FEDER funds through the program COMPETE- Programa Operacional Factores de Competitividade and by Portuguese national funds through FCT-Fundação para a Ciência e a Tecnologia, under the projects ANTIMICROBCOAT - PTDC/CTM/102853/2008 and in the framework of the Strategic Projects PEST-C/FIS/UI607/2013”, PEST-C/EME/UI0285/2013 and SFRH/BD/80947/2011. References [1] F. Borgioli, E. Galvanetto, F. Iozzelli, G. Pradelli, Improvement of wear resistance of Ti–6Al–4V alloy by means of thermal oxidation, Materials Letters 59 (2005) 2159–2162. [2] Y.S. Hedberg, B. Qian, Z. Shen, S. Virtanen, I. Odnevall Wallinder, In vitro biocompatibility of CoCrMo dental alloys fabricated by selective laser melting, Dental Materials 30 (2014) 525–534. [3] K. Yaya, Y. Khelfaoui, B. Malki, M. Kerkar, Numerical simulations study of the localized corrosion resistance of AISI 316L stainless steel and pure titanium in a simulated body fluid environment, Corrosion Science 53 (2011) 3309–3314. [4] J.R. Davis, Handbook of Materials for Medical Devices, ASM International, 2003341. [5] L. Mattei, F. Di Puccio, B. Piccigallo, E. Ciulli, Lubrication and wear modelling of artificial hip joints: A review, Tribology International 44 (2011) 532–549. [6] R. Hauert, K. Thorwarth, G. Thorwarth, An overview on diamond-like carbon coatings in medical applications, Surface and Coatings Technology 233 (2013) 119–130. [7] N.K. Manninen, F. Ribeiro, A. Escudeiro, T. Polcar, S. Carvalho, A. Cavaleiro, Influence of Ag content on mechanical and tribological behavior of DLC coatings, Surface and Coatings Technology 232 (2013) 440–446. [8] B. Subramanian, C.V. Muraleedharan, R. Ananthakumar, M. Jayachandran, A comparative study of titanium nitride (TiN), titanium oxy nitride (TiON) and titanium aluminum nitride (TiAlN), as surface coatings for bio implants, Surface and Coatings Technology 205 (2011) 5014–5020.
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