Influence of the carbon content on the corrosion and tribocorrosion performance of Ti-DLC coatings for biomedical alloys

Tribology International 88 (2015) 115–125

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Influence of the carbon content on the corrosion and tribocorrosion performance of Ti-DLC coatings for biomedical alloys R. Bayón a,n, A. Igartua a, J.J. González b, U. Ruiz de Gopegui a a b

Fundación IK4-Tekniker, Eibar, Spain Department of Metallurgical and Materials Engineering, University of Basque Country, Spain

art ic l e i nf o

a b s t r a c t

Article history: Received 3 October 2014 Received in revised form 27 February 2015 Accepted 3 March 2015 Available online 19 March 2015

Biomedical alloys are prone to suffer corrosion and wear phenomena coming from the hostile environment of the body and friction processes, respectively. Diamond-like carbon (DLC) coatings are known to be excellent candidates for using as protective coatings on biomedical alloys, not only due to their excellent tribological properties but also due to their chemical composition and stability. In this work, three Ti-DLC PVD coatings with different compositions were deposited on Ti6Al4V alloy and their corrosion and tribocorrosion responses were evaluated in simulated body fluid. Excellent tribocorrosion response has been found especially in case of coatings with high carbon content. Additionally significative reduction of friction and wear has been obtained in comparison to the substrate response. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Tribocorrosion PVD Ti6Al4V Corrosion

1. Introduction In the biomedical area, the implantation of artificial joints is almost a routine surgical procedure. The hip and the knee joints are the two most common replacements and have proven to be very effective in relieving pain. Moreover, the success rates are as high as 95% at least in the short time. Nowadays, the major problem of artificial joints concerns to the long-term durability: on the average, a total hip joint replacement lasts only 10–15 years [1]. The materials of the implants are exposed to the interaction with the body cells and fluids and to potential corrosive activity from the materials of the body. The body fluid contains 1% NaCl between other salts, and constitutes a corrosive environment for the metallic implants. Joint implants are also exposed to sliding wear, fatigue and other mechanical solicitations [2–4]. The interactions of the implants with the body cells, the products of the corrosion, and of the wear debris can have adverse effects on the body and on the implants. These effects can include cellular damage, infections, blood coagulation (potentially leading to thrombosis) and failure of the implants. In order to accomplish their function, the implants should not cause infections, prevent uncontrolled cell growth, maintain their integrity inside the body, and avoid formation of debris. In certain cases it is useful to have the implants interact in a controllable way with the biological environment, e.g., to promote growth of bone cells on implants [5,6].

n

Corresponding author. Tel.: þ 34 636991983; fax: þ34 943202757. E-mail address: [email protected] (R. Bayón).

http://dx.doi.org/10.1016/j.triboint.2015.03.007 0301-679X/& 2015 Elsevier Ltd. All rights reserved.

Metallic implants can release metal ions and wear debris into the surrounding tissue and these can lead to osteolysis (bone resorption, loss) and loosening and failure of the implant. Hip and knee implants are exposed to sliding movements, which can cause wear of the surfaces in contact. In hip prosthesis, the acetabular cup (socket) is usually made of ultra-high molecular weight polyethylene (UHMWPE) and the femoral head is made of metallic alloys (CoCrMo, stainless steel, or Ti6Al4V) or ceramics (Al2O3 or ZrO2). The tibiae component of the knee prosthesis is also made of polyethylene and the femoral component is usually made of CoCrMo alloy. Ti and its alloys are among the most biocompatible metals but their wear resistance is relatively low. CoCrMo alloys are wear resistant; however, the UHMWPE counterpart wears down over time due to movement [7,8]. The debris particles generated by the wear can cause inflammations of the tissue and can lead to osteolysis around the implant. The aseptic loosening of the fixation, which can be caused by the osteolysis, is one of the main causes of failures of the joint implants [9]. A solution for preventing the failure of metallic artificial joints is the use of hard, wearreducing and corrosion-reducing coating, which should be also biocompatible. Diamond-like carbon (DLC) coatings are excellent candidates for use as biocompatible coatings on biomedical implants due to their excellent properties. This type of protective coatings provide to the metallic substrates interesting properties such as high hardness, low coefficient of friction, chemical inertness, high electrical resistivity, etc. [10,11]. This work describes the corrosion, wear and tribocorrosion behavior of three PVD (Physical Vapor Deposition) coatings applied on biomedical grade Ti6Al4V substrates. Ti-DLC coatings were deposited

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by means of cathodic-arc evaporation system. Different deposition process parameters were changed in order to modify coatings carbon and nitrogen content for improving the mechanic-chemical properties of the system.

2. Experimental setup 2.1. Deposition process The Ti-DLC PVD coatings were deposited by a cathodic arc evaporation method in the laboratory equipment MIDAS 775 developed by IK4-Tekniker. This system has 12 circular evaporators of 100 mm and a working intensity range of 60–140 A. In the cleanliness step, the samples were sprayed with a solvent product and cleaned in an alkaline detergent by means of an ultrasound method. When samples were loaded into the PVD chamber, they were heated up to reach a temperature in the substrate of 400–500 1C. Then a cleanliness named Glow Discharge was applied to them. This process consists of an electrical discharge to create plasma around the samples by using a gas mixture of argon and hydrogen. For improving the adhesion of the Ti-DLC coating to the substrate a previous thin titanium layer (0–100 nm thickness) was applied directly on the substrate surface. This layer promotes the adhesion in three ways: the high energy Ti-ions bombardment cleans the surface of the substrate, an ion implantation of titanium takes place in the substrate, and finally it is the beginning of the functional DLC coating. The three Ti-DLC coatings were made by adding the necessary reactive gases, nitrogen and acetylene, with a gas flow of between 50 and 200 sccm. A titanium target for the arc evaporator was used in order to fulfill the biocompatibility of the coating. The chamber pressure before starting to coat was 10  6 mbar and the pressure supported during the deposition process was kept between 10  2– 10  3 mbar. The arc intensity of the titanium target was set between 70 and 140 A and the bias voltage between 400 and 30 V. The different deposition parameters applied to develop three types of Ti-DLC coatings are summarized in Table 1. 2.2. Coatings characterization In this study, titanium alloy Ti6Al4V ELI was selected as reference substrate. The mechanical properties and composition meet the ASTM F136-02 standard specification for wrought Ti6Al4V-ELI (extra low interstitial) alloy for surgical implant applications (UNS R56401) [12]. Surfaces were characterized by different techniques: roughness was measured with a Mahr Perthometer equipment. Hardness was obtained by nanoindentation measurements with a Fischer nanoindenter. The adhesion of coatings was tested using procedure VDI 3198 Rockwell test and the thickness of each coating was determined by Calotest method. Composition in depth was obtained by using Glow Discharge Optical Emission Spectroscopy, GD-OES (Horiba Jobin Yvon) equipment and morphology of the coatings was observed by Field Emission Scanning Electron Microscope FE-SEM microscope.

Table 1 Process parameters for Ti-DLC cathodic arc coatings. Coatings

Ar (sccm)a

N2 (sccm)a

C2H2 (sccm)a

Iarc, (A)

DLC 1 DLC2 DLC 3

200 200 200

50 – 50

200 200 200

100 100 75

a

sccm: standard cm3/min at a pressure of 1 bar.

2.3. Corrosion tests Corrosion behavior of uncoated and Ti-DLC coated Ti6Al4V alloy was studied by an Electrochemical Impedance Spectroscopy (EIS) technique. The measurements were performed in a threeelectrode electrochemical cell using a Ag/AgCl (KCl 3 M) reference electrode (SSC) and a platinum wire as counter electrode. Tests were done under room temperature and aerated conditions on an exposed area of 1.8 cm2. For simulating body fluids a Phosphate Buffered Solution (PBS), pH 7.4 was used. The composition of the PBS electrolyte was the following: 0.14 M NaCl, 1 mM KH2PO4, 3 mM KCl, 10 mM Na2HPO4. For EIS measurements, a sinusoidal AC perturbation of 710 mV amplitude was applied to the electrode at a frequency range from 100 kHz to 1 mHz. The impedance spectra measured at open circuit potential was registered at different exposure times. All potentials in this work were referred to Ag/AgCl electrode (0.207 V vs. SHE). The tests were repeated at least three times for each surface. 2.4. Tribocorrosion tests Tribocorrosion behavior of Ti6Al4V and Ti-DLC coatings was studied by using uni-directional ball on disc tribometer. Sliding wear conditions against alumina balls (10 mm diameter) were established in PBS solution. Experiments were performed at open circuit potential and under room temperature and aerated conditions in a special electrochemical cell placed on the rotatory tribometer. The exposed surface was in this case 2 cm2. The applied normal load was 5 N and the sliding speed was 0.063 m/s. The track radius was 6 mm and the total number of cycles or contact events were 3000. Surfaces potentials as well as friction coefficients were registered before, during and after the wear process. Two electrochemical impedance spectroscopies were performed during tribocorrosion tests. The first one was measured before the sliding process, after 20 min of stabilization in the electrolyte. The second one at the end of the test, some minutes after the rubbing ends. Impedance measurements were done at a frequency range between 10 kHz and 10 mHz with a signal amplitude of 10 mV. Additionally, techniques as optical microscopy and confocal profilometry were used to estimate the surfaces wear morphologies after tests and the total material loss due to tribocorrosion effects.

3. Results and discussion 3.1. Coatings characterization Ti6Al4V substrates were mechanized in discs shape with a diameter of 24 mm and 7.9 mm thickness and then, they were carefully polished till reaching on their surfaces a final roughness of 0.05 μm. Three different deposition processes were accomplished in order to develop three different DLC coatings by varying some process parameters such as gas flows (nitrogen and acetylene) and arc intensity. DLC 1 and DLC 3 had same argon and nitrogen content but arc intensity of the process varied from 100 A in case of DLC 1 to 75 A in case of DLC 3. DLC 2 has no nitrogen content on its structure. Fig. 1 shows detail of one of the coatings cross section structure. In the three cases, final roughness was increased by the presence of Ti drops on the external part of the films. All coatings showed a roughness higher than 0.5 μm. The roughness is almost independent to the acetylene quantity during the deposition, just a slightly increase of the roughness when increasing the acetylene could be observed. However, when the nitrogen is not used (DLC 2), an increase of the roughness is observed. This is

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Fig. 1. SEM cross section images of DLC 3 coating.

3.2. Corrosion tests

Table 2 Surfaces characterization results. Surfaces

Thickness (μm)

Roughness (μm)

Hardness (GPa)

Adhesion (HF)

Ti6Al4V DLC 1 DLC 2 DLC 3

– 3.28 3.05 3.86

0.05 0.40 0.60 0.42

3 22 8

– HF 1 HF 1 HF 1

because a more carbon poisoned target leading to an increase of higher droplets emission. Trying to reduce the final coatings roughness for a better comparison of their wear, corrosion and tribocorrosion response, the coated surfaces were polished with a 1 μm polishing cloth. After polishing, the coating thickness was checked again by a Calotest method and it was found that the thicknesses of the coatings were reduced in 0.2 μm. Thickness, final roughness, hardness and adhesion tests results are summarized in Table 2. The thicknesses of the three coatings varied from 3.05 μm to 3.86 μm. Differences in terms of hardness were detected as consequence of the parameters imposed in the deposition process. Absence of nitrogen in the film structure notably enhances the hardness. All coatings showed excellent adhesion properties. Fig. 2 shows some microscopic images of the DLC surfaces. Observing the pictures, the presence of high quantity of droplets distributed on the whole surfaces is noticeable. The SEM-EDS analysis of DLC coatings revealed that the composition of these droplets was mainly carbon and titanium (Fig. 3). Oxygen was also detected on the three surfaces. The image of Fig. 4 shows a detail of DLC 3 cross section. In the middle of the layer, the EDS analysis revealed the presence of Ti and a small peak of carbon. No oxygen was detected inside the coating structure. By means of GD-OES, it was possible to analyze the composition in depth for the three coatings. Fig. 5 shows the GD-OES profiles of the three DLC coatings. The topmost part of three DLC is mainly composed by carbon (almost the 60%). Carbon content is quite constant in the external part of around 2 μm of thickness, then the carbon content decreases progressively along the film. Titanium is also present in the most external part of the coatings in minor quantity and when carbon starts to decrease, a pick of titanium is observed in all cases. DLC 2 does not contain nitrogen and in this case, the coating is a TiC compound. DLC 1 and DLC 3 show the structure of a TiCN. The presence of carbon in the external part of TiCN coatings (DLC 1 and DLC 3) reduces the hardness of the coatings but in case of CTi coating (DLC 2), the combination of C and Ti increases hardness and probably, the tribological properties of the film.

Impedance measurements of the three DLC coatings and the substrate were registered at different immersion times (2 h, 24 h, 168 h and 192 h). Evolution of surfaces open circuit potentials (Fig. 6) and corrosion resistances with time were analyzed for each surface. The potential in case of titanium increases with immersion time from  0.122 V at 2 h of immersion to 0.102 V after 192 h. This evolution indicates a surface passivation process where a protective oxide film is forming on its surface. The potentials of three Ti-DLC coatings are similar for the three deposition processes and remains more or less constant for the total immersion time in PBS media. Electrochemical impedance data at 2 h of immersion are plotted in Fig. 7. Titanium substrate shows a capacitive behavior and elevated corrosion resistance. When increasing the exposure time, the corrosion resistance of bare titanium increases due to the formation of a dense and thick layer of titanium oxide. In case of DLC coatings, impedance diagrams show a very similar shape. At high frequencies, Bode diagrams reveal a capacitive behavior and at low frequencies additional effects should be taken into account. After several analyses with all the experimental data registered for each coating at every immersion time, the best fit was obtained by using a diffusion element in the equivalent circuit instead of a second pair (QR). The reason of this solution is related to the high corrosion resistance of the substrate which makes difficult to distinguish its contribution through coatings pores. Assuming that, in case the electrolyte was reaching the Ti6Al4V substrate, the proportional area of the alloy exposed to the PBS media would be passivated and localized corrosion would not happen. Thus, the equivalent circuit B of Fig. 7 was used for a better analysis of the experimental data according with [13]. In case of Ti6Al4V substrates the best fitting at all immersion times was reached by using the equivalent circuit A of Fig. 7 named Randles circuit. Randles circuit is composed by three elements: Rs that represents the electrolyte resistance between working (Ti6Al4V) and reference electrode. CPE 1 is the constant phase element that represents the passive surface capacitance. This element is used for replacing the double layer capacitance in fitting process of experimental data because takes into account the deviation from the true capacitance due to the surface inhomogeneities. R1 is the polarization resistance of the alloy. Some authors reported experimental impedance measurements on Ti family alloys in biological media and results were fitted with two time constants. This interpretation describes a bilayer structure of oxide film composed by a dense inner layer of TiO2 and a porous outer layer [14,15]. In this case, a better fit was reached by using only one time constant which corresponds to a dense oxide film formed on titanium alloy immersed in PBS electrolyte [16].

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Fig. 2. Coated surfaces appearance and droplets density of DLC 1 (left), DLC 2 (center) and DLC 3 (right).

Fig. 3. SEM image of DLC 3 cross section and surface droplets on silicon wafer. EDS analysis inside the droplet.

Fig. 4. Detail of DLC 3 cross section on silicon wafer. EDS analysis in the middle of the film.

Fig. 5. GD-OES analysis in depth of three DLC coatings.

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The bode of coatings exhibit similar tendency in the three surfaces shape and this behavior does not change with immersion time. Thus difussional processes govern the electrochemical response of DLC coatings at low frequencies for all immersion times. Experimental parameters obtained after a fitting process are displayed in Table 3. Corrosion resistance (R1) of Ti6Al4V increases when increasing the immersion time in PBS. According to the positive evolution of the open circuit potential with time, it is expected that a dense passive oxide layer is continuously being formed on the surface promoting in this way the excellent corrosion resistance of the alloy. The low values of Yo-CPE1 element as well as n values close to 1 obtained are related to a capacitive behavior of the surface. When immersion time increases, CPE 1 and n parameters tend to increase softly. This effect can be attributed to an increment

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in the surface roughness because of the formation of oxide film and also because the inhomogeneities of this film. Diffusion elements present in three DLC coatings are mainly related to the coatings microstructure that allows oxygen diffusion from the electrolyte to the substrate. The impedance element for representing semi-infinite length Warburg diffusion is given by: Zω ¼ σω1/2 (1-i), where σ is the Warburg coefficient and ω the frequency. The Warburg impedance is an example of a constant phase element for which the phase angle is a constant 451 and

Table 3 Parameters obtained from fitting Impedance data at different exposure times in PBS.

Ti6Al4V

DLC 1

DLC 2

DLC 3

Time (h)

OCP (V vs. Ag/AgCl)

Yo-CPE1 n1 Yo-W R1 (kΩ cm2) (μmF/cm2) (CPE1) (Ω  1 cm-2 s1/2)

2 24 168 192 2 24 168 192 2 24 168 192 2 24 168 192

 0.122  0.020 0.021 0.102 0.019 0.046 0.038 0.036 0.025 0.023 0.031 0.033 0.030 0.015 0.034 0.037

31,250 29,040 46,000 63,000 95 76 58 40 412 1604 9540 11,140 76 270 312 328

18.96 19.34 30.12 32.35 92.47 108.40 123.40 121.60 28.85 35.11 36.26 36.64 54.33 68.67 64.68 68.41

0.901 0.905 0.907 0.896 0.854 0.850 0.838 0.841 0.901 0.890 0.877 0.875 0.871 0.859 0.861 0.855

1.41E-05 1.39E-05 1.81E-05 1.75E-05 2.13E-06 1.81E-06 1.83E-06 2.18E-06 6.50E-06 6.22E-06 4.68E-06 5.28E-06

Fig. 6. Open circuit potential evolution with immersion time in PBS.

Fig. 7. Impedance data of Ti6Al4V and DLC coatings after 2 h of immersion in PBS and equivalent circuits used for simulation: (A) bare substrate, (B) DLC coatings.

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magnitude of the impedance (|Zω| ¼ σ21/2/ω1/2) is inversely proportional to the square root of the frequency (1/ω1/2) as a CPE element with an n-value of 0.5. The Warburg coefficient can be expressed in terms of admittance as follows:   σ ¼ 1= Y o  21=2 Then, the magnitude of the impedance |Zω| can be rewritten in terms of Yo being j Z ω j ¼ 1=Y o ω1=2 ; ½Y o  ¼ Ω

 1 1=2

s

Low Yo values imply high diffusion impedance (Zω) and high Warburg coefficient (σ). As σ coefficient is inversely proportional to the diffusion coefficient, D [17], then low Yo-W values imply low diffusion coefficient which is desirable to enhance the corrosion resistance of the coatings. Attending diffusion processes, it can be observed in Fig. 8 (right) that DLC 1 presents the highest values of Yo-W which tend to increase with immersion time. That implies a higher diffusion activity through the film microstructure. In case of DLC 2 and DLC 3, Yo-W values are much lower, especially in case of DLC 2 and these values decrease with time in both coatings. Diffusion impedance values of DLC coatings agree with corrosion resistance (R1) values measured. DLC 1 shows the lowest corrosion resistance that tends to decreases when immersion time increases. For DLC 2 and DLC 3 the values of R1 increase when immersion time increases. Especially high values of corrosion resistance were achieved with DLC 2 that also is the coating with lowest Yo-W values. DLC 2 and DLC 3 registered more or less constant CPE1 values along the total immersion time (Fig. 8 (left)). In case of DLC 1, these values are one order of magnitude higher than those measured in the other two coatings. Lowest values of Yo-CPE1 were found for DLC 2 which means a better capacitive response. The n1 values related to CPE1 element are close to the unit and no special variation with time was observed in any case. For a better comparison of the corrosion tendency in Ti6Al4V and the three DLC coatings, Fig. 9 represents R1 values evolution with immersion time. It can be clearly observed an increase of R1 values with time in the substrate, DLC 2 and DLC 3. 3.3. Tribocorrosion tests Combined corrosion and wear tests were performed on uncoated Ti6Al4V substrates and the three Ti-DLC coatings according with the experimental procedure described in Section 2.4. After 20 min of immersion in PBS and before the tribological process, all the surfaces showed a stable potential and a passivated surface. The evolution of

the open circuit potential before, during and after the wear process is displayed in Fig. 10. When the load is applied, potentials decrease in all cases to more negative values indicating an activation of the track surface due to the sliding process. This behavior is related to the total o partial removal of the passive layer from the surfaces as a consequence of the mechanical interaction. Potential in case of Ti6Al4V decreases to the lowest potential values in relation with DLC coated surfaces. During the sliding process, it fluctuates and when the loading stops, potential increases quickly to a more noble value ( 0.281 V vs. Ag/ AgCl) but then remains constant till the end of the test, and does not reach its initial value ( 0.123 V vs. Ag/AgCl). Fluctuations of the open circuit potential of titanium substrate during sliding are due to the constant increase of the active area as consequence of the mechanical damage provoked progressively by the counter-material. DLC coatings show very similar potentials behavior under sliding conditions. Their potentials slightly decrease when the load is applied. In case of DLC 2 and DLC 3, potentials increases again (repasivation of surfaces at the beginning of the test) during the first cycles of sliding and then remains stable without fluctuations during the whole wear process. When unloading, DLCs potentials increase quickly to values close to the initials ones and then remain constant till the end of the test. Final potential values for DLC coatings where:  0.060 V (DLC 1),  0.001 V (DLC 2), and  0.005 V (DLC 3). The initial values registered before the sliding were: 0.047 V (DLC 1), 0.107 V (DLC 2) and 0.090 V (DLC 3). Open circuit potential variations are closely related to electrochemical phenomena that take place on the surfaces exposed to combined corrosion and rubbing effects. In this way, titanium substrate seems to be more affected by the mechanical interaction. Friction coefficients registered simultaneously during sliding, show very low values in case of DLC coatings as it was expected from the well known friction properties of this kind of PVD films (Fig. 11). The carbon content of the topmost section of the coatings seems to have a lubricant effect avoiding the formation of third particles in the contact area and reducing the friction. The mean values of friction coefficients obtained were: 0.473 for Ti6Al4V, 0.180 for DLC 1, 0.202 for DLC 2 and 0.172 for DLC 3. Impedance measurements before and after the sliding are plotted in Fig. 12 for all surfaces. In this case, Nyquist diagrams were selected to show how the electrochemical state of the surface changed as consequence of the induced wear. In order to compare the surfaces' electrochemical state before and after sliding, the impedance data were not normalized to the surfaces area because after sliding it is supposed to have two different area situations: passive zone outside the track and active-repassivated zone inside the friction zone. Ti6Al4V substrate data were simulated by using the equivalent circuit A from Fig. 13. As in case of corrosion tests performed

Fig. 8. Evolution with time of surfaces capacitance (left) and evolution of coatings Warburg impedance (right).

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Fig. 9. Evolution with time of surfaces capacitance (left) and evolution of coatings Warburg impedance (right).

Fig. 10. Open circuit potential during sliding and friction coefficients.

Fig. 11. Open circuit potential during sliding and friction coefficients.

previously, the impedance response of this alloy in PBS can be represented by a Randles circuit both before and after sliding. Reduction on corrosion resistance of the alloy (R1) is noticeable. Wear process strongly influenced on the electrochemical response

of this alloy as it was expected. The reduction of the corrosion resistance of Titanium substrate is related with the size of the active area which contributes to the total electrochemical response of the exposed surface composed by a passive area outside the track and the active area inside the wear track. Thus, as bigger the wornactive area is, lower is the total corrosion resistance of the total exposed surface. Noticeable is that, in this case the increase of YoCPE value which is related to a decrease of the passive area. The experimental data registered in case of the three DLC coatings were fitted by using the circuit B from Fig. 13. In this case, no diffusion processes were observed at low frequencies. This was due to the frequency range used when performing the impedance measurements before and after tribocorrosion tests. Tests here were performed till 10 mHz of frequency in order to reduce the total test duration. In case of corrosion tests performed at different exposure times reported in previous section, the frequency range used for impedance measurements was 100 kHz to 1 mHz. Diffusion processes can be better monitored at low frequencies between 10–1 mHz, thus in these tests it was not possible to register low frequency processes and only data from high-medium frequencies could be analyzed. Other noticeable difference between impedance data in corrosion and tribocorrosion tests of DLC coatings is the presence of the small and similar values R1 as pore resistance in all cases (Table 4). These R1 values (less than 100 Ω) could be related to the formation of an external oxide film on the top of DLC coatings. Due to the high titanium content of coatings it is expected the formation of an oxide film on their surfaces when exposed to PBS solution. Then the R1 parameter could be related to the pore resistance through this oxide film formed on the top of the coatings. The no homogeneity of the coating structure and the presence of different size drops on their surfaces would be the causes of the non dense and porous film formed on during the first minutes of immersion in the PBS solution. R2 parameter is associated in this case with the corrosion resistance of DLC coatings. Attending the values obtained for this parameter, DLC 3 seems to have higher corrosion resistance before sliding and also after sliding. The corrosion resistance in coatings seems to be a little

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Fig. 12. Impedance diagrams before and after the wear process of Ti6Al4V and DLC coatings.

Fig. 13. Equivalent circuit used for simulating experimental impedance data before and after the sliding. Circuit A used in case of Ti6Al4V substrate and circuit B used for DLC coatings.

Table 4 Parameters obtained from fitting Impedance data at before and after sliding in PBS. Time

OCP

R1

Yo CPE1

(h)

(Vn)

(kΩ)

(μF)

 0.123  0.281 0.047  0.060 0.107  0.001 0.090  0.005

4560 299 0.023 0.037 0.051 0.024 0.026 0.037

39.34 56.61 136.1 140.8 57.83 92.61 58.63 86.72

Ti6Al4V Before After DLC 1 Before After DLC 2 Before After DLC 3 Before After

n1 (CPE1)

R2

Yo CPE2

n2 (CPE2)

(KΩ) (μF) 0.923 0.893 0.746 0.723 0.834 0.851 0.798 0.865

344 322 987 627 1202 834

17.54 51.84 15.73 50.18 59.61 80.97

Fig. 14. Corrosion resistance evolution due to tribological effect for bare Ti6Al4V and DLC coatings.

0.981 0.875 0.987 0.919 0.805 0.790

Vn ¼ volts referred to Ag/AgCl reference electrode.

affected by the wear but much lower than in case of bare substrate. The CPE1 and CPE 2 elements in case of coatings related to their capacitive response increase after wear in all cases. The increase in CPE 1 element was expected since this parameter depends on the passive and capacitive area. Thus a reduction of a total passive area

due to the presence of the worn area in the total surface exposed explains the increase of CPE1 values observed after sliding. Fig. 14 shows the variation of the surfaces polarization resistance after the sliding. For coated samples, polarization resistance was considered as the sum of R1 þR2 although R1 value is negligible in comparison with R2. The corrosion resistance of the titanium substrate has been reduced around 93% due to the mechanical degradation at the wear track. DLC 1 shows the lower corrosion resistance reduction as consequence of wear. Fig. 15 shows the optical microscopy pictures of the wear tracks generated on titanium and DLC coated surfaces. Tracks pictures revealed differences in the wear behavior of coated and uncoated surfaces. Titanium substrate shows a big damage and the thickest worn area. The main wear mechanism was in this case abrasion generated by hard ceramic counterbody which promotes the formation of debris particles in the frictional area. Wear particles

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Fig. 15. Optical microscope pictures of the wear tracks on titanium substrate and DLC 1, 2 and 3 coatings.

trapped in the contact zone induce the third body effect increasing the total wear of the surface [18]. Also corrosion products were found inside the track making the corrosion-abrasion combination as the principal degradation mechanism observed in case of the titanium substrate. DLC 1 showed a small track almost inappreciable and very much minor that the substrate one. Any corrosion signs or abrasion lines were found inside the coating track area. DLC 2 surface does no present any damage. Any track evidence was found after microscopic inspection neither confocal analysis. DLC 3 surface exhibits a very small, no homogeneous and narrow track, hardly distinguishable from the rest of unworn surface. The wear volumes on the samples were determined by measuring the track cross-section area using confocal microscopy, and multiplying it by the length of the wear track (2πR). At least four positions along the length of the wear track were measured to obtain the average area. In case of DLC 2 no wear track was detected and then no wear volume could be calculated. Fig. 16 shows the track profiles of titanium substrate and DLC coatings. Titanium surface is the most degraded one due to the synergistic effect of wear and corrosion suffered whereas coatings show negligible wear under the same conditions being their scar depth smaller than their total thicknesses. The wear coefficient (K) was also calculated by dividing the wear volume between the applied normal force and the sliding distance [19,20]. The total wear due to tribocorrosion effect as well as the wear coefficients obtained for Ti6Al4V and DLC coatings are summarized in Table 5. DLC 2 did not suffer tribocorrosion wear. At least the mechanical contribution to the total tribocorrosion damage was null. DLC 1 reduces titanium wear two orders of magnitude whereas DLC 3 reduces this value in three orders. From the results obtained in tribocorrosion tests, it is noticeable the improvement of tribocorrosion performance of Ti6Al4V biomedical alloy when Ti-DLC based coatings are applied on it (Fig. 17). Electrochemical response of three coatings in PBS media is almost

constant with immersion time. Their excellent tribological response makes them practically inalterable when a mechanical load is applied on their surface. The wear did not affect their electrochemical response. It is known that the total tribocorrosion degradation of a material depends on its inherent chemical and mechanical properties. The chemical properties are related to the material ability for generating a spontaneous protective oxide film on its surface when is exposed to the aggressive media. Mechanical properties are related mainly with its microstructure and composition. Ti6Al4V alloy has excellent electrochemical response in several aqueous media especially in biological fluids, but its poor mechanical properties make it susceptible to failure when it works under tribocorrosion conditions. By the contrary, Ti-DLC coatings showed, during corrosion and tribocorrosion tests, a bit lower corrosion resistance values than bare titanium, although they also presented high corrosion resistance. The improvement of their corrosion behavior with time suggests they have a very good electrochemical stability and this stability is not affected when they are submitted simultaneously to tribological events. Additionally to their electrochemical stability, they have an excellent tribological behavior reflected in negligible wear rates and low friction coefficients under the testing conditions applied in this work. For all this, Ti based DLC coatings could be a good alternative for protecting biomedical alloys under synergistic corrosion-wear effects.

4. Conclusions New Ti-DLC based PVD coatings were developed and deposited on a common biomedical alloy named Ti6Al4V. In this case, modifications in the deposition process where assed in order to check behavior differences on coatings with different carbon and nitrogen content and also different arc intensities. According to

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Fig. 16. Profiles of the wear tracks generated during sliding against alumina balls for titanium and DLC coatings.

Table 5 Wear track analysis after tribocorrosion tests. Samples Depth (mm) Width (mm)

Ti6Al4V DLC 1 DLC 2 DLC 3

13.69 7 1.04 1.43 7 0.07 – 0.497 0.02

1036.2 7 36.7 212.2 7 8.4 – 17.5 7 1.8

Wtr (  103 mm3)

K (  1015 m3 N  1 m  1)

420.3375.36 9.03 7 0.58 – 0.25 7 0.04

743.0 7 7.0 16.0 7 1.3 – 0.5 7 0.1

Fig. 17. Reduction of total surface damage due to tribocorrosion effect.

these objectives, three different DLC coatings were evaluated under corrosion and tribocorrosion conditions. Corrosion behavior was studied in function of the immersion time in PBS solution by using electrochemical impedance spectroscopy technique for a total immersion time of 192 h. Results obtained for the three Ti-DLC coatings suggested a very good corrosion response and high electrochemical stability with time. DLC coating formed in

absence of nitrogen and DLC coating created with low arc current (DLC 2 and DLC 3, respectively), behaved especially good and an increase of corrosion resistance was observed when increasing the immersion time for both coatings. Ti6Al4V was also tested as reference substrate reporting elevated corrosion resistance and passive behavior in the same electrolytic media. Despite of this, its corrosion resistance strongly decreased when corrosion and wear processes were imposed simultaneously. The breakdown of the titanium passive film on the wear track leaded to the alloy dissolution during sliding. As consequence high wear rates were measured after tribocorrosion tests. Under the same testing conditions, DLCs behavior was completely different from the substrate one. Three DLC coatings reduced notably friction and wear during tribocorrosion tests. The coatings had excellent tribological properties and because of that, they did not suffer mechanical damage during sliding test in PBS. The absence of mechanical contribution to the total material loss and their electrochemical resistance were the causes of their excellent response under tribocorrosion conditions. Special good results were reported in case of DLC 2 and DLC 3. Elevated wear resistance was expected in case of DLC 2 since this coating was the harder one due to its differentiated TiC microstructure. The differences between DLC 1 and 3 were the arc intensity in the deposition process, the hardness and the final thickness although differences in thickness and hardness were no significant. Nevertheless, the difference of hardness between DLC 2 and DLC 3 (22 GPa and 8 GPa respectively) was big and no noticeable tribocorrosion response was detected between both coatings.

Acknowledgments The author would like to acknowledge to the Spanish Government for the financial received across the project CSD2008-00323 FUNCOAT, which belongs to the CONSOLIDER INGENIO-2010 program.

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