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Silver implanted diamond-like carbon coatings
Vacuum 110 (2014) 78e86
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Silver implanted diamond-like carbon coatings D. Batory a, b, *, J. Gorzedowski a, B. Rajchel c, W. Szymanski a, L. Kolodziejczyk a a
Institute of Materials Science and Engineering, Lodz University of Technology, 1/15 Stefanowskiego St, 90-924 Lodz, Poland Institute for Nanomaterials, Advanced Technologies and Innovation, Technical University of Liberec, 2 Studentska St, 461 17 Liberec, Czech Republic c Institute of Nuclear Physics, Polish Academy of Sciences, 152 Radzikowskiego St., 31-342 Krakow, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 May 2014 Received in revised form 3 September 2014 Accepted 4 September 2014 Available online 16 September 2014
Modern technologies in surface engineering allow for the creation of new and modification of existing types of coatings, thereby improving their performance, and often adding extra features, such as antibacterial properties. In presented work a possibility of application of ion implantation method for the modification of the physicochemical properties of carbon coatings is presented. Gradient a-C:H/Ti coatings were subsequently implanted with two doses of silver ions: 7 and 10 1016 ion/cm2, respectively. Physicochemical characteristics like chemical structure and composition, morphology, surface topography, surface free energy, hardness and adhesion were determined. Silver depth concentration profiles measured using AES show correlation between applied dose and surface chemical composition. Optimal depth distribution of silver was observed for both values of Agþ doses. Implantation of silver in the DLC coating smoothens the surface, changes the ID/IG ratio and affects the surface free energy by changing both values of polar and dispersive component. Part of it may be also related to the possible hydrogen release occurring during the implantation. Mechanical properties of silver implanted carbon coatings were slightly lower, without noticeable differences between both Agþ doses. Ion implantation technique appears to be an effective way for surface modification and tailoring the properties of carbon coatings. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Diamond-like carbon Silver Implantation CVD PVD
1. Introduction Growing number of reports related to allergy issues of patients with medical implants creates the need of a development of new biocompatible biomaterials. New materials with controlled bioactivity and other useful properties that can improve the quality of life and the healing processes of patients suffering from various types of allergies are of special interest [1,2]. In this regard diamond-like carbon (DLC) coatings due to their very special physicochemical [3e6] and mechanical [6e11] properties combined with good biocompatibility [11,12] and bactericidal activity [13,14] have been considered as a good candidate for variety of biomedical applications [8,15,16]. Although the techniques of the deposition of DLC coatings are well known and described elsewhere the intensive studies on the synthesis and modification of
* Corresponding author. Institute of Materials Science and Engineering, Lodz University of Technology, 1/15 Stefanowskiego St, 90-924 Lodz, Poland. Tel.: þ48 42 631 30 69. E-mail addresses: [email protected] (D. Batory), jeremiasz.gorzedowski@ gmail.com (J. Gorzedowski), [email protected] (B. Rajchel), witold. [email protected] (W. Szymanski), [email protected] (L. Kolodziejczyk). http://dx.doi.org/10.1016/j.vacuum.2014.09.001 0042-207X/© 2014 Elsevier Ltd. All rights reserved.
carbon coatings which prove their high application potential are still conducted. One of these focuses on the enhanced bactericidal activity. For that purpose silver seems to be a perfect candidate. It is known to be an antibacterial agent with a very broad spectrum of activity and has a long history of application in medicine [17,18]. Therefore, over the past few years a growing interest in evaluation of modern and hybrid techniques for the synthesis of variety of silver incorporated DLC coatings is seen. Marciano et al. [19] produced silver incorporated diamond-like carbon coatings using Plasma Assisted Chemical Vapour Deposition (PACVD) method. Silver nanoparticles were introduced to the coating by spraying its water solution obtained by electrodeposition method. After silver deposition, another DLC thin interlayer (~25 nm) was synthesized to fix the silver nanoparticles. Raman spectra showed that the phase composition of obtained coatings was not affected significantly due to the incorporation of silver. It was also shown that silver nanoparticles cannot chemically bind with carbon, but must be captured by another top DLC layer. In work [20] RF reactive magnetron sputtering technique was applied to synthesize Ag doped DLC coatings. Obtained silver concentrations varied between 1 and 12 at.%. Authors reported significant changes in the chemical structure of obtained coatings, especially increasing intensity of the sp2 and decreasing intensity of the sp3 hybridized carbon, as well as
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Table 1 Chemical composition of AISI 316L (%wt.). Element
C
Si
Mn
P
S
Cr
Ni
Mo
Fe
AISI 316LVM
Max. 0.022
Max. 0.583
Max. 1.669
Max. 0.021
Max. 0.022
16.48
13.38
2.49
Rest
the overall increase in the sp2/sp3 ratio. Pulsed filtered cathodic vacuum arc deposition system was applied by Kwok et al. [21] for the synthesis of silver incorporated diamond-like carbon coatings using AgeC co-axial target. Authors obtained rather high concentrations of silver in carbon coatings. Changes in chemical structure of obtained samples, particularly sp3/sp2 ratio, were in authors' opinion rather attributed to negative sample bias than to the silver atomic concentration. The contact angle value for water varied between 71 for the highest silver concentration through 79 for the medium and finally 67 for the lowest Ag concentration, however also here the strong influence of negative bias on the amount of sp3 hybridized carbon should be considered. Choi and co-workers [22,23] proposed carbon coatings deposited with use of the end-Hall-type hydrocarbon ion gun (using benzene) simultaneously doped with silver by means of DC magnetron sputtering technique. The concentration of silver varied between 0.1 and 9.7 at.%. It was shown that the total surface free energy of Ag-doped DLC films is reduced with silver concentration increase. Authors also observed an increase of the graphite-like bonds in amorphous carbon matrix with increased concentration of silver. In our recent work [24] we elaborated similar hybrid deposition technique. It combines carbon coating, synthesized with use of RF PACVD method, at the same time doped with silver delivered via pulsed magnetron sputtering technique. The obtained results showed that doping of the DLC coatings with silver by magnetron sputtering process lowers the hardness and increases the surface development parameter RL0. In works [25,26] authors proposed plasma immersion ion implantation (PIII) as a deposition method of Agdoped DLC gradient nanocomposite coatings. The study showed that Ag-doping has a slight effect on carbon hybridization and disordering while having smoothing effect on the surface topography. What was interesting, authors also reported that silver doping increases the strength of hydrogen-free DLC films and simultaneously decreases the strength of hydrogenated DLC films. There were three common conclusions that were reported in majority of cited papers. Authors of works [21e25] unanimously reported that silver atoms incorporated into amorphous carbon matrix occur mostly in the metallic state, thus they do not form any chemical bonds with carbon. Usually, the authors have not found significant changes in mechanical performance of the coatings. At the same time a decrease of residual stress accompanied with slightly lower hardness of the coatings was noticed [19,23e25]. Despite wide range of technologies used for the synthesis of silver doped diamond-like carbon coatings there is lack of information on the use of ion implantation method to modify the preformed carbon coating with silver. Similar technique was
reported in Ref. [27], however in that work silver ions were implanted to substrates made of pyrolytic carbon, mostly used in cardiac surgery applications. Therefore authors of this work propose some new complex surface modification utilizing highly biocompatible diamond-like carbon coatings subsequently implanted with silver ions. Two doses of silver ions were implanted into hydrogenated DLC layers and their basic physicochemical characteristics was performed. The results show that proposed method of modification, even though complicated and expensive, provides ample possibilities of selective surface modification of materials for variety of applications with special regard to those requiring the antibacterial activity (foreseen in our further studies). 2. Materials and methods Studied samples were made of AISI316LVM austenitic steel (confirmed by the X-ray fluorescence spectroscopy - SRS 303 Siemens, see Table 1). Mirror polished flat discs of 16 mm in diameter and 6 mm thick were used. Before the modification samples were cleaned in methanol bath in ultrasonic cleaner for 10 min and then dried using compressed air. Carbon coatings for the ion implantation processes were synthesized with use of the Radio Frequency Plasma Assisted Chemical Vapour Deposition/Magnetron Sputtering (RF PCVD/MS) system described in Refs. [5,28]. A proper adhesion of DLC coatings was ensured by the application of gradient TieTixCyeDLC interlayer with a thickness of ca. 200 nm. Synthesized carbon coatings, with the thickness of 700 nm, were subsequently implanted with Agþ ions using a 70 kV ion implanter described in Ref. [29]. The implantation processes were carried out with the use of silver ions with energy of 15 keV. In order to determine the influence of silver ion implantation process onto overall physicochemical properties of carbon coatings two ion doses of 7 and 10 1016 Agþ/cm2 were applied (denoted from here as DLC/Ag1, DLC/Ag2 respectively). The individual doses of ions were selected in such a way as to obtain coatings with silver concentration equal to 2 and 4 at.% respectively, assuming a uniform distribution through the film. A beam of ions, prior to the implantation process was filtered using the magnetic separator (to avoid multiply charged Ag ions) and additional masks were used to give it the final dimensions of 10 70 mm. The Agþ flux density for each dose was adjusted separately, so as to avoid excessive heating of the sample during the process, and was less than few mA/cm2 (as measured with use of the Faraday cage mounted below the sample holder). In addition, to obtain the uniform distribution of silver, the sample holder was oscillating. The silver implantation profile was also simulated with use of SRIM
Table 2 Parameters of the synthesis process. Phase
Plasma etching
Pulsed magnetron sputtering
DLC deposition
Ion implantation
Details
Pressure: 2 Pa Time: 10 min RF bias: 800 V Ar flow: 15 sccm Temp.: <200 C
Pressure: 0.5 Pa Time: 12 min RF bias: 300 V Power: 2.2e2.7 kW Ar flow: 9 sccm CH4 flow: 1e7 sccm Temp.: <200 C
Pressure: 20 Pa Time: 50 min RF bias: 300 V Precursor: CH4 Temp.: <200 C
Pressure: 1 105 Pa Energy: 15 keV Dose: 7 1016 Agþ/cm2 10 1016 Agþ/cm2 Temp.: 25 C
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software. The details of the synthesis of carbon coatings and implantation procedure are gathered in Table 2. Structure and qualitative chemical composition of manufactured coatings were examined using ultra high-resolution scanning electron microscope (Carl Zeiss ULTRA Plus) equipped with Aztec Energy EDS analyser. Quantitative chemical composition analysis was made with use of the Microlab 350 (Thermo VG Scientific, UK) high-resolution Auger system. Analysis of the chemical structure of implanted coatings was made by means of Raman spectroscopy. A confocal micro-Raman spectrometer (Nicolet Almega XR) working with 532 nm wavelength and power of 2.5 mW was applied. Surface morphology and topography were measured using atomic force microscope (Bruker Corporation, USA). The data acquisition and image processing were done using Nanoscope 7.3 and MountainsMap 5 software, respectively. Areas of sizes 2 2 mm and 10 10 mm were scanned for all samples, wherein the latter were used to define their roughness parameters. Surface free energy (SFE) was determined according to the OwenseWendt method [30] that uses a contact angle measurement. Contact angles were measured with KRUSS Contact Angle Measuring Instrument, using sessile drop method. The investigation was made for deionized water (specific conductivity: 0.1e1 mS/cm) and diiodomethane (Sigma Aldrich, USA). Each time a standard volume of 0.8 ml was applied on the sample surface. The measurement was repeated three times and the results were averaged. Hardness, elasticity modulus and adhesion of Ag implanted carbon coatings were measured using nanoindentation technique on Nano Indenter G200 system (Agilent Technologies, USA). Hardness and Young's modulus tests were performed using a diamond Berkovich tip and the continuous stiffness measurement (CSM) mode, described in details in work [31]. For each sample nine experiments were conducted and the data were analysed using the Oliver and Pharr [32] approach. A diamond cone (apex angle equal to 87.7 and radius equal to 0.91 mm) was used for the adhesion evaluation. A step change in the friction coefficient during the measuring process was used to determine the critical delamination force. More detailed description of the nanoindentation and AFM methodology can be also found in our previous work [24]. Additionally in all tests DLC coated and pure AISI 316LVM substrates were examined for comparison. 3. Results and discussion 3.1. Auger electron spectroscopy Obtained silver surface concentrations increased with increasing ion dose and were equal to 1.5 and 3.8 at.% respectively. Silver concentration depth profiles obtained for both doses are presented in Fig. 1a, whereas in Fig. 1b is presented simulated silver
implantation depth distribution profile. The inset graph of Fig. 1a presents the raw spectra in derivative mode plotted as a function of energy. According to authors of works [33,34], when the exposure is increased, i.e. the dose increases, both target and implanted ions are removed during the sputtering process. Eventually, an equilibrium may be reached, where as many implanted atoms are removed by sputtering as are replenished by the implantation. This is particularly important when heavy ions are implanted into matrices made of lighter atoms, which is exactly the case of presented research. In our case, for both doses we obtained the most expected profiles also reported in Refs. [33,34]. Both profiles are slightly shifted that the maximum concentration of silver is just below the surface. An increase of the dose resulted in changed mutual share of the etching and implantation, which led to more uniform distribution of silver with the profile slightly shifted into shallower regions of the sample. Increased ion dose resulted in higher concentration of silver as expected. 3.2. Scanning electron microscopy In Fig. 2 are presented the results of SEM analysis of samples implanted with the doses of 7 1016 and 10 1016 Agþ/cm2, respectively. Surface of implanted samples appears to be uniform and free of any defects (e.g. local delamination areas or cracks originated from residual stress). The results of the chemical composition showed increased silver concentration with increasing ion dose, which is in agreement with the results of AES. Surface of both samples is rather uniform with small (less than 1 mm) randomly distributed silver conglomerates. However, detailed EDS analysis showed that silver is also dissolved (implanted) in amorphous carbon matrix. Note that the size of registered Ag conglomerates was significantly smaller compared to those, produced by hybrid RF PACVD/MS method, described in our work [24]. The tendency of silver to form aggregates was also reported in works [19,35]. In authors opinion, the reason for this phenomenon was the high mobility of silver atoms when absorbed on the surface, which could lead to island-shape structure. As in the case of this work, authors of both papers also synthesized silver incorporated carbon coatings without additional substrate heating. A cross-section view of silver implanted carbon layer is presented in Fig. 3. Concentration gradients of Ti (the adhesion promoting gradient interlayer) and Ag are schematically marked. Since there were no differences in the structure and morphology for both doses and changes in the thickness were in the range of measurement error, presented cross-section view is for sample implanted with the higher dose (10 1016 Agþ/cm2). It seems that diamond-like carbon coatings appear to be insensitive to the implantation procedure, showing no cracks or
Fig. 1. Silver concentration depth profiles for silver doped samples obtained from Auger spectrograms analysis and raw spectra in derivative mode plotted as a function of energy (inset) (a) and simulated silver implantation depth distribution profile (b).
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Fig. 2. BSE mode scanning electron microscopy of samples implanted with the doses of 7 1016 Agþ/cm2 (a,b) and 10 1016 Agþ/cm2 (c,d), surface EDS qualitative spectra of sample implanted with the dose of 7 1016 Agþ/cm2 (e).
voids in the structure. Bright artefact visible in the middle of the cross section originates from the conductive resin used for the sample preparation. 3.3. Surface free energy In Table 3 are presented the contact angles of polar and nonpolar liquid drops for the analysed samples. These values show the increase in hydrophobicity. The contact angle between the Table 3 Contact angles of polar and non-polar liquid drops for the analysed samples. Sample
Fig. 3. Cross-section view of the sample implanted with the dose of 10 1016 Agþ/cm2.
AISI316LVM DLC DLC/Ag1 DLC/Ag2
Water
Diiodomethane
Avg. contact angle
SD
Avg. contact angle
SD
49.0 81.5 91.2 88.0
0.3 0.1 0.9 1.5
42.8 35.0 44.8 45.4
0.5 0.4 0.8 1.1
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hemocompatibility [21]. This and the above indirectly indicates that, besides silver concentration, the applied Ag ions dose affects some other properties that may influence the overall physicochemical parameters of examined coatings. Furthermore, a termination of dangling bonds by hydrogen atoms is the major reason of high hydrophobicity of a-C:H coatings. It is possible that incident silver ions through the etching/implantation phenomenon enable the hydrogen release from the top surface atomic layers of silver implanted DLC, which may influence the contact angle and surface free energy. On the other hand, as reported in work [37], the contact angle and surface free energy of a-C:H layers change in time and observed values of those parameters may undergo further changes, since this phenomenon is dominated by the hydrogen originating from the hydrogenated carbon. Fig. 4. Polar and dispersive component and total surface free energy vs. silver surface concentration.
polar liquid drop and the surface noticeably increases with DLC coating and slightly increases when compared to silver doped DLC. Here a direct relation can be observed between the contact angle and applied dose of Agþ. The higher dose was applied the lower contact angle was registered. Nevertheless, even for the highest dose the water contact angle was still higher than for unmodified DLC. In Fig. 4 is presented a chart showing the changes of polar and dispersive component as well as the total SFE of samples with different surface concentration of silver. The total value of surface free energy for stainless steel is equal to 57.5 mJ/m2 (not presented in the chart), for DLC coating is equal to 42 mJ/m2, and for Ag-doped DLC coatings decreases to 38.2 and 38.6 mJ/m2 with increasing silver concentration. The variation in both values of SFE is in the rage of standard deviation thus it can be stated that there is no difference between both applied doses. Hence, the total surface free energy is decreased by about 27% after DLC deposition and then by another 12% after Ag implantation. At the same time for both, DLC deposited and silver doped samples, the contribution of polar component of the total surface free energy was decreased. Interesting change of the polar component was observed for both applied silver ions doses. For the dose of 7 1016 Agþ/cm2 polar component of SFE decreases (compared to DLC) and then increases for the dose of 10 1016 Agþ/cm2, but still it is lower than one for DLC. Opposite results were reported in works [21] and [22]. Authors observed the highest water contact angle for the sample with the highest surface concentration of silver. Moreover, in both cited works, authors observed changes in the chemical structure (particularly sp2/sp3 ratio) for different concentrations of silver that might also affect results of the contact angle measurement. Note that the change of the polar component is in agreement with ID/IG trend registered using the Raman spectroscopy (Table 4 discussed in 3.4 Section). The same correlation between the polar component of the SFE and ID/IG ratio was reported in work [36]. Nevertheless, values of the polar component of the SFE reported in works [21,22] varied between 8 and 23 mJ/m2. That is still much higher than those registered in presented work, which is expected to enhance the
Table 4 Experimental results from Raman spectroscopy of the deposited diamond-like carbon and silver implanted coatings. Sample
DLC DLC/Ag7 DLC/Ag10
G band
ID/IG
Peak (cm1)
FWHM (cm1)
1569 1542 1542
140 ± 1.01 170 ± 1.01 170 ± 1.03
0.72 0.49 0.51
3.4. Raman spectroscopy Fig. 5a presents a comparison of the Raman spectra of all examined carbon films. At first sight spectra do not differ significantly from each other and closely resemble those of amorphous diamond-like carbon with broad asymmetric peak in the range 1400e1700 cm1 [3]. The spectra were deconvoluted using twopeak fitting method and two Gaussians, commonly attributed to D (disorder) and G (graphite) bands, were obtained. In Fig. 5b is presented the example of deconvolution showing the positions and intensities of these two components. Generally, Raman spectra depend on clustering of the sp2 phase, that may occur as rings or chains, bond length and bond angle disorder as well as sp2/sp3 ratio [38,39]. In case of diamond-like carbon films, the number and size of sp2 clusters can be determined basing on the ID/IG intensity ratio, whereas the information about the sp3 content is only a qualitative indication [26]. Following the authors of work [40], the another useful parameter to monitor the carbon bonding is the full-width at half maximum of the G peak (FWHM), which is related to the measure of disorder in the film. The increase in disorder, mainly manifested by the broadening of G peak, is assigned to higher sp3 content. The fitting parameters including ID/IG ratio, position and FWHM of G peak for examined coatings are summarized in Table 4. The initial intensity ratio for base carbon coating is equal to 0.72, which subsequently decreases for silver implanted samples, however there is almost no difference in ID/IG ratio between both applied ion doses. At the same time a significant broadening of G peak, accompanied with a shift toward lower wavenumbers, is observed. This indicates a slight disordering of the amorphous carbon matrix with implantation of silver ions. It appears that the change of the ion dose (to higher one), that results in increased concentration of silver, does not affect the chemical structure of the coating. However, authors keep in mind, that for implantation energy of 15 keV the overall penetration depth is rather low and is equal to ca. 14 nm, and are aware of the fact that it may not result in severe changes in the Raman spectra. Opposite trend was observed by Cui et al. in their work related to Ti implanted DLC. Authors reported induced graphitization processes, what we believe to be expected, confirmed by the increased ID/IG ratio, shift of the G peak toward higher wavenumbers and lower mechanical properties of the coating. Yet this could be also related to higher ions energy (30 keV) and high chemical affinity of titanium and carbon [41]. In our case this disordering process should result in higher mechanical properties, which certainly did not happen (as discussed later), but again one has to keep in mind very shallow penetration depth of silver ions. Following Ferrari, the possible explanation may be the second stage of a hysteresis phenomenon, where the quality of sp2 bonded carbon can be changed independently from the sp2/sp3 ratio. The accelerated Agþ beam might induce the topological
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Fig. 5. a) Raman spectra of examined samples, b) deconvolution of a Raman spectra for DLC sample.
disorder of the surface. The bonding was still mainly sp2, but the weaker bonds soften the vibrational modes, so the shift of G peak was toward lower wavenumbers. The ID/IG ratio was also decreased, since it is proportional to the number of aromatic rings [38,42]. As for the chemical composition point of view, Endrino et al. reported a slight disordering of the a-C coatings with silver addition. In their opinion, a possible cause could be either the hybridization between carbon and silver or the energetic condensation of silver ions originating from the filtered cathodic arc [26]. In turn, Wang et al. claim that change of nanograins size in an Ag-DLC film might induce the intensity change of both D and G bands [43]. The shift in the G peak into lower wavenumbers of functionally gradient DLCAg nanocomposite coatings was reported by Narayan [44]. However in this work there is no information about the shape (broadening or narrowing) of the former. In his opinion one of the reasons for this change in G-band position may be the reduced compressive stress, being an effect of significantly smaller elastic modulus of silver nanoparticles that may absorb compressive stress from the diamond-like carbon matrix. Nevertheless, all three papers are related to the simultaneous work of carbon and silver sources unlike the one analysed. Thus authors consider another phenomenon that can be partially explained on the basis of results presented in works [25,38,42] together. Considering that the influence of silver onto chemical structure and properties of diamond-like carbon coatings depends on their hydrogenation (discussed later in 3.6 section), as stated in Section 3.3, it is possible that during the implantation process hydrogen is released from the top surface atomic layers of silver implanted DLC. This in turn may change the quality of sp2 organized carbon and indirectly affect the residual stress of the coating. This hypothesis, however, needs further investigations. Summarizing, because of the possible twofold influence of implantation process onto overall parameters of implanted layers, an increase in disorder of diamond-like carbon coatings should be considered in terms of changes in chemical structure and chemical composition as well. 3.5. Atomic force microscopy AFM images (Fig. 6) as well as examples of surface profiles variations (Fig. 7) clearly show changes in morphologies of investigated surfaces. To describe surface geometric structure commonly used roughness parameters Ra and RMS were selected [45]. Fig. 6a presents polished medical steel substrate for the synthesis of DLC coating. Single scratches, originated from polishing process, can be noted on this surface. Despite this, Ra and RMS parameters are rather low - 1.46 and 1.90 nm, respectively. Surface of DLC coating synthesized on medical steel is shown in Fig. 6b. Its morphology is typical for such coating type with distinctive features similar to globular conglomerates. RMS parameter of investigated DLC coating is equal to 1 nm (Fig. 8) which is also in the range of values
reported in the literature (0.1e1.13 nm) [24,46,47]. Further implantation of silver in the DLC coating smoothens the surface, which can be seen on AFM scans (Fig. 6c and d). Smoothing of the DLC surface after ion implantation by titanium was observed by Cui, where RMS value decreased from 1.589 to 0.595 nm for implanted coatings [41]. The study of Lipinski et al. presents the changes in morphology of the polymer doped with silver. The authors describe the smoothing of the polymer with respect to the starting substrate. This was due to coating the polymer with a very thin layer of metal as a result of aggregation of Ag atoms on the surface [48]. According to graph in Fig. 8, a decrease of roughness parameters of ion implanted coatings compared to steel and DLC substrates can be found. Average value of Ra for the DLC and Ag implanted samples decreased by 68% and 38%, respectively. Similar trend can be observed for RMS parameter: 68% and 39% decrease, respectively. 3.6. Hardness, modulus and adhesion Generally, the influence of silver incorporation onto mechanical properties of amorphous carbon matrix should be regarded in terms of the hydrogen content in the latter (a-C and a-C:H), as suggested in works [23] and [26]. In non-hydrogenated carbon films heterogeneous structures, formed by silver nanoparticles embedded in the amorphous carbon matrix, may possibly decrease the plastic flow and in consequence, under particular conditions, increase hardness. Whereas in hydrogenated carbon films dangling bonds are occupied by hydrogen thus the formation of bonds between C and Ag (increased hybridization) is strongly limited, therefore an increase in hardness of hydrogenated carbon coatings is not observed. Fig. 9 presents a comparison of hardness and modulus for all examined samples. Hardness of DLC coating is almost four times higher than that of steel substrate, as expected. Furthermore, an evident decrease in hardness can be observed between DLC and DLC/Ag samples. The decrease in hardness for this case when compared to DLC sample was ~14%. Barely noticeable is a lowering trend of modulus with increasing dose of silver ions, however in this case there is a lack of statistical significance between particular samples. Changes in hardness and modulus are not consistent with Raman spectroscopy, which suggest slight disordering with the implantation of Agþ ions, regardless the applied dose. Neither they correlate with applied dose of Agþ ions and it appears that increased ion dose does not affect this parameter. This, in turn, is contrary to the fact of decreased mechanical parameters of hydrogenated carbon coatings with increasing concentration of silver, which was reported by other authors [19,23], and is in agreement with our experience [24]. Thus, in authors opinion, the interaction of Agþ with sample surface is a dominant phenomenon affecting the mechanical properties of silver implanted DLC, which is in agreement with results presented in work [41]. At the same time hardness and modulus are rather
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Fig. 6. AFM images of a) substrate AISI 316L stainless steel, b) substrate coated with DLC coating, c - d) silver ions doped DLC coatings DLC/Ag1 e 1.5 at.% and DLC/Ag2 e 3.8 at.% Ag, respectively. Scan size of 2 2 mm.
related to the bulk properties of the coating, which during the measurement can be affected by the influence of thin top layer modified by the implantation procedure. The common feature for both silver incorporated a-C and a-C:H films is decreased level of internal stress, that can be explained by the formation of silver nanoclusters, which are characterized by the low value of elastic modulus and the ability of stress accumulation [25,44]. The reduced bonding ability between the nanocrystalline and amorphous phase enables their mutual sliding and multiple-
shifts of grain-matrix interface under the influence of applied external stress that in consequence may lead to increased ductility of the film [43]. Fig. 10 presents the results of scratch tested samples. For undoped DLC the critical load reaches 48.2 ± 6.1 mN while for samples implanted with doses of 7 and 10 1016 Agþ/cm2 the critical load is noticeably decreased to 41.3 ± 0.6 and 44.4 ± 1.9 mN, respectively. Here the dose of 7 1016 Agþ/cm2 resulted in the lowest overall concentration of silver and the lowest value of
Fig. 7. Variations of selected surface profiles for investigated samples.
Fig. 8. Value of selected roughness parameters (Ra and RMS) and Ag surface concentration.
D. Batory et al. / Vacuum 110 (2014) 78e86
Fig. 9. Hardness and modulus of investigated samples.
critical load. Moreover, registered critical loads for both doses are lower than that of DLC which confirms our earlier hypothesis that both the implantation process and the chemical composition change affect the overall parameters of silver doped carbon coatings. Opposite trend of positive correlation between silver concentration and adhesion (up to ~10% at.) was reported by Wang et al. [43] for samples synthesized by dual magnetron sputtering. On the other hand, a noticeable decrease in adhesion was observed for silver incorporated DLC coatings deposited by the hybrid RF PACVD/MS technique. However, lower critical delamination force was accompanied by different delamination character (layer crumbling was not observed). 4. Conclusions Ion implantation technique was successfully applied for the modification of diamond-like carbon coatings. Implanted ions with energy of 15 keV and two doses resulted in similar implantation range and increasing trend in silver concentration. Both surface and bulk properties of obtained coatings are affected by two independent phenomena (change in chemical composition and overall interaction between silver beam and modified surface), however at this stage of investigation it is impossible to determine which one is dominant and what exactly happens upon it. In authors opinion hydrogen may also play an important role in this regard. Implantation of silver ions decreases the roughness parameters and surface free energy of DLC layers, thereby making them more hydrophobic in comparison to pure diamond-like carbon coatings and less attractive for the possible bacterial colonization. Raman spectroscopy revealed slight disorder of implanted films. On the other hand, implantation of silver ions slightly decreased the
Fig. 10. Critical load to full delamination for DLC and Ag-doped DLC coatings.
85
mechanical parameters of obtained layer (note that for modulus and adhesion the changes were not statistically significant). Further increase of the dose of Ag ions (form 7 to 10 1016 ion/cm2) results in increased Ag surface concentration but does not result in further changes in surface roughness, ID/IG intensity ratio, surface free energy and mechanical properties. Summarising, presented technology allows obtaining the assumed concentration and depth distribution of silver in the previously prepared carbon coatings, allowing at the same time the modification of their mechanical and physicochemical properties. Our results also demonstrated that the physicochemical properties of DLC/Ag layers appear to be insensitive to different doses of silver ions, and at the same time surface concentrations of silver (at least to 4 at.% of Ag). Silver implanted carbon coatings with sufficient mechanical properties, very smooth surface, increased hydrophobicity and decreased surface free energy suggest a possibility of their implementation as biomaterial. Acknowledgement This work has been supported by the National Centre for Research and Development under grant no. ERA-NET/MNT/CARSILA/1/2010 and in part by the Project OP VaVpI Centre for Nanomaterials, Advanced Technologies and Innovation CZ.1.05/2.1.00/ 01.0005 and by the Project Development of Research Teams of R&D Projects at the Technical University of Liberec CZ.1.07/2.3.00/ 30.0024. References [1] Christel P. Biological and biomechanical performance of biomaterials. In: Proceedings of the 5. Europ. Conference on biomaterials, Paris, France, Sept. 4e6. Elsevier; 1985. Amsterdam u.a., 1986. [2] Mitura S, Niedzielski P, Walkowiak B, editors. NANODIAM, new technologies for medical applications: studying and production of carbon surfaces allowing for controllable bioactivity. Warsaw: PWN; 2006. [3] Robertson J. Diamond-like amorphous carbon. Mat Sci Eng R 2002;37: 129e281. [4] Dorner-Reisel A, Schürer C, Irmer G, Müller E. Electrochemical corrosion behavior of uncoated and DLC coated medical grade Co28Cr6Mo. Surf Coat Tech 2004;2002(177e178):830e7. [5] Batory D, Blaszczyk T, Clapa M, Mitura S. Investigation of anti-corrosion properties of TieC gradient layers manufactured in hybrid deposition system. J Mater Sci 2008;10:3385e91. [6] Dorner A, Schurer C, Reisel G, Irmer G, Seidel O, Muller E. Diamond-like carbon-coated Ti6Al4V: influence of the coating thickness on the structure and the abrasive wear resistance. Wear 2001;249:489e97. [7] Kaczorowski W, Szymanski W, Batory D, Niedzielski P. Tribological properties and characterization of diamond like carbon coatings deposited by MW/RF and RF plasma-enhanced CVD method on poly(ether-ether-ketone). Plasma Processes Polym 2014;11:878e87. [8] Hauert R. An overview on the tribological behavior of diamond-like carbon in technical and medical applications. Tribol Int 2004;37:991e1003. [9] Yamauchi N, Okamoto A, Tukahara H, Demizu K, Ueda N, Sone T, et al. Friction and wear of DLC films on 304 austenitic stainless steel in corrosive solutions. Surf Coat Tech 2003;174-175:465e9. [10] Huang L, Xu K, Lu J, Guelorget B, Chen H. Nano-scratch and fretting wear study of DLC coatings for biomedical application. Diam Relat Mater 2001;10: 1448e56. [11] Hauert R, Müller U. An overview on tailored tribological and biological behavior of diamond-like carbon. Diam Relat Mater 2003;12:171e7. [12] Cui F, Li D. A review of investigations on biocompatibility of diamond-like carbon and carbon nitride films. Surf Coat Tech 2000;131:481e7. [13] Liu C, Zhao Q, Liu Y, Wang S, Abel EW. Reduction of bacterial adhesion on modified DLC coatings. Colloid Surf B 2008;61:182e7. [14] Jakubowski W, Bartosz G, Niedzielski P, Szymanski W, Walkowiak B. Nanocrystalline diamond surface is resistant to bacterial colonization. Diam Relat Mater 2004;13:1761e3. [15] Shirakura A, Nakaya M, Koga Y, Kodama H, Hasebe T, Suzuki T. Diamond-like carbon films for PET bottles and medical applications. Thin Solid Films 2006;494:84e91. [16] Dorner-Reisel A, Schürer C, Müller E. The wear resistance of diamond-like carbon coated and uncoated Co28Cr6Mo knee prostheses. Diam Relat Mater 2004;13:823e7. [17] Simchi A, Tamjid E, Pishbin F, Boccaccini AR. Recent progress in inorganic and composite coatings with bactericidal capability for orthopaedic applications. Nanomedicine 2011;7:22e39.
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[18] Bosetti M, Masse A, Tobin E, Cannas M. Silver coated materials for external fixation devices: in vitro biocompatibility and genotoxicity. Biomaterials 2002;23:887e92. [19] Marciano FR, Bonetti LF, Santos LV, Da-Silva NS, Corat EJ, Trava-Airoldi VJ. Antibacterial activity of DLC and Ag-DLC films produces by PECVD technique. Diam Relat Mater 2009;18:1010e4. [20] Ahmed SF, Moon MW, Lee KR. Effect of silver doping on optical property of diamond like carbon films. Thin Solid Films 2009;517:4035e8. [21] Kwok SCH, Zhang W, Wan GJ, McKenzie DR, Bilek MMM, Chu PK. Hemocompatibility and anti-bacterial properties of silver doped diamond-like carbon prepared by pulsed filtered cathodic vacuum arc deposition. Diam Relat Mater 2007;16:1353e60. [22] Choi HW, Dauskardt RH, Lee SC, Lee KR, Oh KH. Characteristic of silver doped DLC films on surface properties and protein adsorption. Diam Relat Mater 2008;17:252e7. [23] Choi HW, Choi JH, Lee KR, Ahn JP, Oh KH. Structure and mechanical properties of Ag-incorporated DLC films prepared by a hybrid ion beam deposition system. Thin Solid Films 2007;516:248e51. [24] Batory D, Czerniak-Reczulska M, Kolodziejczyk L, Szymanski W. Gradient titanium and silver based carbon coatings deposited on AISI316L. Appl Surf Sci 2013;275:303e10. [25] Zhang HS, Endrino JL, Anders A. Comparative surface and nano-tribological characteristics of nanocomposite diamond-like carbon thin films doped by silver. Appl Surf Sci 2008;255:2551e6. [26] Endrino JL, Escobar Galindo R, Zhang HS, Allen M, Gago R, Espinosa A, et al. Structure and properties of silver-containing a-C(H) films deposited by plasma immersion ion implantation. Surf Coat Tech 2008;202:3675e82. [27] Zhao J, Feng HJ, Tang HQ, Zheng JH. Bactericidal properties of silver implanted pyrolytic carbon. Nucl Instrum Methods B 2006;243:299e303. [28] Batory D, Stanishevsky A, Kaczorowski W. The effect of deposition parameters on the properties of gradient a-C: H/Ti layers. J Achiev Mater Manuf Eng 2009;37:381e6. ska E, Wierba M. Adaptation of the 70 kV ion [29] Rajchel B, Drwiega M, Lipin implanter to the IBAD technique. Nucl Instrum Methods B 1994;89:342e5. [30] Owens DK, Wendt RC. Estimation of the surface free energy of polymers. J Appl Polym Sci 1969;13:1741e7. [31] Li X, Bhushan B. A review of nanoindentation continuous stiffness measurement technique and its applications. Mater Charact 2002;48:11e36. [32] Pharr GM, Oliver WC, Brotzen FR. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7:613e7.
[33] Stepanov AL, Zhikharev VA, Khaibullin IB. Depth profiles of metal ions implanted in dielectrics at low energies. Phys Solid State 2001;4:766e71. [34] Stepanov AL, Zhikharev VA, Hole DE, Townsend PD, Khaibullin IB. Depth distribution of Cu, Ag and Au ions implanted at low energy into insulators. Nucl Instrum Methods 2000;166-167:26e30. [35] Chekan NM, Beliauski NM, Akulich VV, Pozdniak LV, Sergeeva EK, Chernov AN,Kazbanov VV, et al. Biological activity of silver-doped DLC films. Diam Relat Mater 2009;18:1006e9. [36] Ostrovskaya L, Perevertailo V, Ralchenko V, Dementjev A, Loginova O. Wettability and surface energy of oxidized and hydrogen plasma-treated diamond films. Diam Relat Mater 2002;11:845e50. [37] Grabarczyk J. Long-term termination of carbon layers synthesized using RF PACVD method onto metal substrates. Diam Relat Mater 2011;20:1133e6. [38] Ferrari AC. Determination of bonding in diamond-like carbon by Raman spectroscopy. Diam Relat Mater 2002;11:1053e61. [39] Ferrari AC, Robertson J. Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond. Phil Trans R Soc Lond A 2004;362: 2477e512. [40] Casiraghi C, Piazza F, Ferrari AC, Grambole D, Robertson J. Bonding in hydrogenated diamond-like carbon by Raman spectroscopy. Diam Relat Mater 2005;14:1098e102. [41] Cui L, Guoqing L, Wenwu C, Zongxin M, Chengwu Z, Liang W. The study of doped DLC films by Ti ion implantation. Thin Solid Films 2005;475:279e82. [42] Ferrari A, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000;61:14095e107. [43] Wang C, Yu X, Hua M. Microstructure and mechanical properties of Agcontaining diamond-like carbon films in mid-frequency dual-magnetron sputtering. Appl Surf Sci 2009;256:1431e5. [44] Narayan RJ. Pulsed laser deposition of functionally gradient diamondlike carbon-metal nanocomposites. Diam Relat Mater 2005;14:1319e30. [45] Webb HK, Truong VK, Hasan J, Fluke C, Crawford RJ, Ivanova EP. Roughness parameters for standard description of surface nanoarchitecture. Scanning 2012;34:257e63. [46] Sun Z. Morphological features of diamond-like carbon films deposited by plasma-enhanced CVD. J Non Cryst Solids 2000;261:211e7. [47] Sheeja D, Tay B, Krishnan S, Nung L. Tribological characterization of diamondlike carbon (DLC) coatings sliding against DLC coatings. Diam Relat Mater 2003;12:1389e95. [48] Lipinski P, Bielinski D, Okroj W, Jakubowski W, Klimek L, Jagielski J. Biomedical aspects of ion bombardment of polyethylene. Vacuum 2009;83: 200e3.
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Silver implanted diamond-like carbon coatings D. Batory a, b, *, J. Gorzedowski a, B. Rajchel c, W. Szymanski a, L. Kolodziejczyk a a
Institute of Materials Science and Engineering, Lodz University of Technology, 1/15 Stefanowskiego St, 90-924 Lodz, Poland Institute for Nanomaterials, Advanced Technologies and Innovation, Technical University of Liberec, 2 Studentska St, 461 17 Liberec, Czech Republic c Institute of Nuclear Physics, Polish Academy of Sciences, 152 Radzikowskiego St., 31-342 Krakow, Poland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 May 2014 Received in revised form 3 September 2014 Accepted 4 September 2014 Available online 16 September 2014
Modern technologies in surface engineering allow for the creation of new and modification of existing types of coatings, thereby improving their performance, and often adding extra features, such as antibacterial properties. In presented work a possibility of application of ion implantation method for the modification of the physicochemical properties of carbon coatings is presented. Gradient a-C:H/Ti coatings were subsequently implanted with two doses of silver ions: 7 and 10 1016 ion/cm2, respectively. Physicochemical characteristics like chemical structure and composition, morphology, surface topography, surface free energy, hardness and adhesion were determined. Silver depth concentration profiles measured using AES show correlation between applied dose and surface chemical composition. Optimal depth distribution of silver was observed for both values of Agþ doses. Implantation of silver in the DLC coating smoothens the surface, changes the ID/IG ratio and affects the surface free energy by changing both values of polar and dispersive component. Part of it may be also related to the possible hydrogen release occurring during the implantation. Mechanical properties of silver implanted carbon coatings were slightly lower, without noticeable differences between both Agþ doses. Ion implantation technique appears to be an effective way for surface modification and tailoring the properties of carbon coatings. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Diamond-like carbon Silver Implantation CVD PVD
1. Introduction Growing number of reports related to allergy issues of patients with medical implants creates the need of a development of new biocompatible biomaterials. New materials with controlled bioactivity and other useful properties that can improve the quality of life and the healing processes of patients suffering from various types of allergies are of special interest [1,2]. In this regard diamond-like carbon (DLC) coatings due to their very special physicochemical [3e6] and mechanical [6e11] properties combined with good biocompatibility [11,12] and bactericidal activity [13,14] have been considered as a good candidate for variety of biomedical applications [8,15,16]. Although the techniques of the deposition of DLC coatings are well known and described elsewhere the intensive studies on the synthesis and modification of
* Corresponding author. Institute of Materials Science and Engineering, Lodz University of Technology, 1/15 Stefanowskiego St, 90-924 Lodz, Poland. Tel.: þ48 42 631 30 69. E-mail addresses: [email protected] (D. Batory), jeremiasz.gorzedowski@ gmail.com (J. Gorzedowski), [email protected] (B. Rajchel), witold. [email protected] (W. Szymanski), [email protected] (L. Kolodziejczyk). http://dx.doi.org/10.1016/j.vacuum.2014.09.001 0042-207X/© 2014 Elsevier Ltd. All rights reserved.
carbon coatings which prove their high application potential are still conducted. One of these focuses on the enhanced bactericidal activity. For that purpose silver seems to be a perfect candidate. It is known to be an antibacterial agent with a very broad spectrum of activity and has a long history of application in medicine [17,18]. Therefore, over the past few years a growing interest in evaluation of modern and hybrid techniques for the synthesis of variety of silver incorporated DLC coatings is seen. Marciano et al. [19] produced silver incorporated diamond-like carbon coatings using Plasma Assisted Chemical Vapour Deposition (PACVD) method. Silver nanoparticles were introduced to the coating by spraying its water solution obtained by electrodeposition method. After silver deposition, another DLC thin interlayer (~25 nm) was synthesized to fix the silver nanoparticles. Raman spectra showed that the phase composition of obtained coatings was not affected significantly due to the incorporation of silver. It was also shown that silver nanoparticles cannot chemically bind with carbon, but must be captured by another top DLC layer. In work [20] RF reactive magnetron sputtering technique was applied to synthesize Ag doped DLC coatings. Obtained silver concentrations varied between 1 and 12 at.%. Authors reported significant changes in the chemical structure of obtained coatings, especially increasing intensity of the sp2 and decreasing intensity of the sp3 hybridized carbon, as well as
D. Batory et al. / Vacuum 110 (2014) 78e86
79
Table 1 Chemical composition of AISI 316L (%wt.). Element
C
Si
Mn
P
S
Cr
Ni
Mo
Fe
AISI 316LVM
Max. 0.022
Max. 0.583
Max. 1.669
Max. 0.021
Max. 0.022
16.48
13.38
2.49
Rest
the overall increase in the sp2/sp3 ratio. Pulsed filtered cathodic vacuum arc deposition system was applied by Kwok et al. [21] for the synthesis of silver incorporated diamond-like carbon coatings using AgeC co-axial target. Authors obtained rather high concentrations of silver in carbon coatings. Changes in chemical structure of obtained samples, particularly sp3/sp2 ratio, were in authors' opinion rather attributed to negative sample bias than to the silver atomic concentration. The contact angle value for water varied between 71 for the highest silver concentration through 79 for the medium and finally 67 for the lowest Ag concentration, however also here the strong influence of negative bias on the amount of sp3 hybridized carbon should be considered. Choi and co-workers [22,23] proposed carbon coatings deposited with use of the end-Hall-type hydrocarbon ion gun (using benzene) simultaneously doped with silver by means of DC magnetron sputtering technique. The concentration of silver varied between 0.1 and 9.7 at.%. It was shown that the total surface free energy of Ag-doped DLC films is reduced with silver concentration increase. Authors also observed an increase of the graphite-like bonds in amorphous carbon matrix with increased concentration of silver. In our recent work [24] we elaborated similar hybrid deposition technique. It combines carbon coating, synthesized with use of RF PACVD method, at the same time doped with silver delivered via pulsed magnetron sputtering technique. The obtained results showed that doping of the DLC coatings with silver by magnetron sputtering process lowers the hardness and increases the surface development parameter RL0. In works [25,26] authors proposed plasma immersion ion implantation (PIII) as a deposition method of Agdoped DLC gradient nanocomposite coatings. The study showed that Ag-doping has a slight effect on carbon hybridization and disordering while having smoothing effect on the surface topography. What was interesting, authors also reported that silver doping increases the strength of hydrogen-free DLC films and simultaneously decreases the strength of hydrogenated DLC films. There were three common conclusions that were reported in majority of cited papers. Authors of works [21e25] unanimously reported that silver atoms incorporated into amorphous carbon matrix occur mostly in the metallic state, thus they do not form any chemical bonds with carbon. Usually, the authors have not found significant changes in mechanical performance of the coatings. At the same time a decrease of residual stress accompanied with slightly lower hardness of the coatings was noticed [19,23e25]. Despite wide range of technologies used for the synthesis of silver doped diamond-like carbon coatings there is lack of information on the use of ion implantation method to modify the preformed carbon coating with silver. Similar technique was
reported in Ref. [27], however in that work silver ions were implanted to substrates made of pyrolytic carbon, mostly used in cardiac surgery applications. Therefore authors of this work propose some new complex surface modification utilizing highly biocompatible diamond-like carbon coatings subsequently implanted with silver ions. Two doses of silver ions were implanted into hydrogenated DLC layers and their basic physicochemical characteristics was performed. The results show that proposed method of modification, even though complicated and expensive, provides ample possibilities of selective surface modification of materials for variety of applications with special regard to those requiring the antibacterial activity (foreseen in our further studies). 2. Materials and methods Studied samples were made of AISI316LVM austenitic steel (confirmed by the X-ray fluorescence spectroscopy - SRS 303 Siemens, see Table 1). Mirror polished flat discs of 16 mm in diameter and 6 mm thick were used. Before the modification samples were cleaned in methanol bath in ultrasonic cleaner for 10 min and then dried using compressed air. Carbon coatings for the ion implantation processes were synthesized with use of the Radio Frequency Plasma Assisted Chemical Vapour Deposition/Magnetron Sputtering (RF PCVD/MS) system described in Refs. [5,28]. A proper adhesion of DLC coatings was ensured by the application of gradient TieTixCyeDLC interlayer with a thickness of ca. 200 nm. Synthesized carbon coatings, with the thickness of 700 nm, were subsequently implanted with Agþ ions using a 70 kV ion implanter described in Ref. [29]. The implantation processes were carried out with the use of silver ions with energy of 15 keV. In order to determine the influence of silver ion implantation process onto overall physicochemical properties of carbon coatings two ion doses of 7 and 10 1016 Agþ/cm2 were applied (denoted from here as DLC/Ag1, DLC/Ag2 respectively). The individual doses of ions were selected in such a way as to obtain coatings with silver concentration equal to 2 and 4 at.% respectively, assuming a uniform distribution through the film. A beam of ions, prior to the implantation process was filtered using the magnetic separator (to avoid multiply charged Ag ions) and additional masks were used to give it the final dimensions of 10 70 mm. The Agþ flux density for each dose was adjusted separately, so as to avoid excessive heating of the sample during the process, and was less than few mA/cm2 (as measured with use of the Faraday cage mounted below the sample holder). In addition, to obtain the uniform distribution of silver, the sample holder was oscillating. The silver implantation profile was also simulated with use of SRIM
Table 2 Parameters of the synthesis process. Phase
Plasma etching
Pulsed magnetron sputtering
DLC deposition
Ion implantation
Details
Pressure: 2 Pa Time: 10 min RF bias: 800 V Ar flow: 15 sccm Temp.: <200 C
Pressure: 0.5 Pa Time: 12 min RF bias: 300 V Power: 2.2e2.7 kW Ar flow: 9 sccm CH4 flow: 1e7 sccm Temp.: <200 C
Pressure: 20 Pa Time: 50 min RF bias: 300 V Precursor: CH4 Temp.: <200 C
Pressure: 1 105 Pa Energy: 15 keV Dose: 7 1016 Agþ/cm2 10 1016 Agþ/cm2 Temp.: 25 C
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D. Batory et al. / Vacuum 110 (2014) 78e86
software. The details of the synthesis of carbon coatings and implantation procedure are gathered in Table 2. Structure and qualitative chemical composition of manufactured coatings were examined using ultra high-resolution scanning electron microscope (Carl Zeiss ULTRA Plus) equipped with Aztec Energy EDS analyser. Quantitative chemical composition analysis was made with use of the Microlab 350 (Thermo VG Scientific, UK) high-resolution Auger system. Analysis of the chemical structure of implanted coatings was made by means of Raman spectroscopy. A confocal micro-Raman spectrometer (Nicolet Almega XR) working with 532 nm wavelength and power of 2.5 mW was applied. Surface morphology and topography were measured using atomic force microscope (Bruker Corporation, USA). The data acquisition and image processing were done using Nanoscope 7.3 and MountainsMap 5 software, respectively. Areas of sizes 2 2 mm and 10 10 mm were scanned for all samples, wherein the latter were used to define their roughness parameters. Surface free energy (SFE) was determined according to the OwenseWendt method [30] that uses a contact angle measurement. Contact angles were measured with KRUSS Contact Angle Measuring Instrument, using sessile drop method. The investigation was made for deionized water (specific conductivity: 0.1e1 mS/cm) and diiodomethane (Sigma Aldrich, USA). Each time a standard volume of 0.8 ml was applied on the sample surface. The measurement was repeated three times and the results were averaged. Hardness, elasticity modulus and adhesion of Ag implanted carbon coatings were measured using nanoindentation technique on Nano Indenter G200 system (Agilent Technologies, USA). Hardness and Young's modulus tests were performed using a diamond Berkovich tip and the continuous stiffness measurement (CSM) mode, described in details in work [31]. For each sample nine experiments were conducted and the data were analysed using the Oliver and Pharr [32] approach. A diamond cone (apex angle equal to 87.7 and radius equal to 0.91 mm) was used for the adhesion evaluation. A step change in the friction coefficient during the measuring process was used to determine the critical delamination force. More detailed description of the nanoindentation and AFM methodology can be also found in our previous work [24]. Additionally in all tests DLC coated and pure AISI 316LVM substrates were examined for comparison. 3. Results and discussion 3.1. Auger electron spectroscopy Obtained silver surface concentrations increased with increasing ion dose and were equal to 1.5 and 3.8 at.% respectively. Silver concentration depth profiles obtained for both doses are presented in Fig. 1a, whereas in Fig. 1b is presented simulated silver
implantation depth distribution profile. The inset graph of Fig. 1a presents the raw spectra in derivative mode plotted as a function of energy. According to authors of works [33,34], when the exposure is increased, i.e. the dose increases, both target and implanted ions are removed during the sputtering process. Eventually, an equilibrium may be reached, where as many implanted atoms are removed by sputtering as are replenished by the implantation. This is particularly important when heavy ions are implanted into matrices made of lighter atoms, which is exactly the case of presented research. In our case, for both doses we obtained the most expected profiles also reported in Refs. [33,34]. Both profiles are slightly shifted that the maximum concentration of silver is just below the surface. An increase of the dose resulted in changed mutual share of the etching and implantation, which led to more uniform distribution of silver with the profile slightly shifted into shallower regions of the sample. Increased ion dose resulted in higher concentration of silver as expected. 3.2. Scanning electron microscopy In Fig. 2 are presented the results of SEM analysis of samples implanted with the doses of 7 1016 and 10 1016 Agþ/cm2, respectively. Surface of implanted samples appears to be uniform and free of any defects (e.g. local delamination areas or cracks originated from residual stress). The results of the chemical composition showed increased silver concentration with increasing ion dose, which is in agreement with the results of AES. Surface of both samples is rather uniform with small (less than 1 mm) randomly distributed silver conglomerates. However, detailed EDS analysis showed that silver is also dissolved (implanted) in amorphous carbon matrix. Note that the size of registered Ag conglomerates was significantly smaller compared to those, produced by hybrid RF PACVD/MS method, described in our work [24]. The tendency of silver to form aggregates was also reported in works [19,35]. In authors opinion, the reason for this phenomenon was the high mobility of silver atoms when absorbed on the surface, which could lead to island-shape structure. As in the case of this work, authors of both papers also synthesized silver incorporated carbon coatings without additional substrate heating. A cross-section view of silver implanted carbon layer is presented in Fig. 3. Concentration gradients of Ti (the adhesion promoting gradient interlayer) and Ag are schematically marked. Since there were no differences in the structure and morphology for both doses and changes in the thickness were in the range of measurement error, presented cross-section view is for sample implanted with the higher dose (10 1016 Agþ/cm2). It seems that diamond-like carbon coatings appear to be insensitive to the implantation procedure, showing no cracks or
Fig. 1. Silver concentration depth profiles for silver doped samples obtained from Auger spectrograms analysis and raw spectra in derivative mode plotted as a function of energy (inset) (a) and simulated silver implantation depth distribution profile (b).
D. Batory et al. / Vacuum 110 (2014) 78e86
81
Fig. 2. BSE mode scanning electron microscopy of samples implanted with the doses of 7 1016 Agþ/cm2 (a,b) and 10 1016 Agþ/cm2 (c,d), surface EDS qualitative spectra of sample implanted with the dose of 7 1016 Agþ/cm2 (e).
voids in the structure. Bright artefact visible in the middle of the cross section originates from the conductive resin used for the sample preparation. 3.3. Surface free energy In Table 3 are presented the contact angles of polar and nonpolar liquid drops for the analysed samples. These values show the increase in hydrophobicity. The contact angle between the Table 3 Contact angles of polar and non-polar liquid drops for the analysed samples. Sample
Fig. 3. Cross-section view of the sample implanted with the dose of 10 1016 Agþ/cm2.
AISI316LVM DLC DLC/Ag1 DLC/Ag2
Water
Diiodomethane
Avg. contact angle
SD
Avg. contact angle
SD
49.0 81.5 91.2 88.0
0.3 0.1 0.9 1.5
42.8 35.0 44.8 45.4
0.5 0.4 0.8 1.1
82
D. Batory et al. / Vacuum 110 (2014) 78e86
hemocompatibility [21]. This and the above indirectly indicates that, besides silver concentration, the applied Ag ions dose affects some other properties that may influence the overall physicochemical parameters of examined coatings. Furthermore, a termination of dangling bonds by hydrogen atoms is the major reason of high hydrophobicity of a-C:H coatings. It is possible that incident silver ions through the etching/implantation phenomenon enable the hydrogen release from the top surface atomic layers of silver implanted DLC, which may influence the contact angle and surface free energy. On the other hand, as reported in work [37], the contact angle and surface free energy of a-C:H layers change in time and observed values of those parameters may undergo further changes, since this phenomenon is dominated by the hydrogen originating from the hydrogenated carbon. Fig. 4. Polar and dispersive component and total surface free energy vs. silver surface concentration.
polar liquid drop and the surface noticeably increases with DLC coating and slightly increases when compared to silver doped DLC. Here a direct relation can be observed between the contact angle and applied dose of Agþ. The higher dose was applied the lower contact angle was registered. Nevertheless, even for the highest dose the water contact angle was still higher than for unmodified DLC. In Fig. 4 is presented a chart showing the changes of polar and dispersive component as well as the total SFE of samples with different surface concentration of silver. The total value of surface free energy for stainless steel is equal to 57.5 mJ/m2 (not presented in the chart), for DLC coating is equal to 42 mJ/m2, and for Ag-doped DLC coatings decreases to 38.2 and 38.6 mJ/m2 with increasing silver concentration. The variation in both values of SFE is in the rage of standard deviation thus it can be stated that there is no difference between both applied doses. Hence, the total surface free energy is decreased by about 27% after DLC deposition and then by another 12% after Ag implantation. At the same time for both, DLC deposited and silver doped samples, the contribution of polar component of the total surface free energy was decreased. Interesting change of the polar component was observed for both applied silver ions doses. For the dose of 7 1016 Agþ/cm2 polar component of SFE decreases (compared to DLC) and then increases for the dose of 10 1016 Agþ/cm2, but still it is lower than one for DLC. Opposite results were reported in works [21] and [22]. Authors observed the highest water contact angle for the sample with the highest surface concentration of silver. Moreover, in both cited works, authors observed changes in the chemical structure (particularly sp2/sp3 ratio) for different concentrations of silver that might also affect results of the contact angle measurement. Note that the change of the polar component is in agreement with ID/IG trend registered using the Raman spectroscopy (Table 4 discussed in 3.4 Section). The same correlation between the polar component of the SFE and ID/IG ratio was reported in work [36]. Nevertheless, values of the polar component of the SFE reported in works [21,22] varied between 8 and 23 mJ/m2. That is still much higher than those registered in presented work, which is expected to enhance the
Table 4 Experimental results from Raman spectroscopy of the deposited diamond-like carbon and silver implanted coatings. Sample
DLC DLC/Ag7 DLC/Ag10
G band
ID/IG
Peak (cm1)
FWHM (cm1)
1569 1542 1542
140 ± 1.01 170 ± 1.01 170 ± 1.03
0.72 0.49 0.51
3.4. Raman spectroscopy Fig. 5a presents a comparison of the Raman spectra of all examined carbon films. At first sight spectra do not differ significantly from each other and closely resemble those of amorphous diamond-like carbon with broad asymmetric peak in the range 1400e1700 cm1 [3]. The spectra were deconvoluted using twopeak fitting method and two Gaussians, commonly attributed to D (disorder) and G (graphite) bands, were obtained. In Fig. 5b is presented the example of deconvolution showing the positions and intensities of these two components. Generally, Raman spectra depend on clustering of the sp2 phase, that may occur as rings or chains, bond length and bond angle disorder as well as sp2/sp3 ratio [38,39]. In case of diamond-like carbon films, the number and size of sp2 clusters can be determined basing on the ID/IG intensity ratio, whereas the information about the sp3 content is only a qualitative indication [26]. Following the authors of work [40], the another useful parameter to monitor the carbon bonding is the full-width at half maximum of the G peak (FWHM), which is related to the measure of disorder in the film. The increase in disorder, mainly manifested by the broadening of G peak, is assigned to higher sp3 content. The fitting parameters including ID/IG ratio, position and FWHM of G peak for examined coatings are summarized in Table 4. The initial intensity ratio for base carbon coating is equal to 0.72, which subsequently decreases for silver implanted samples, however there is almost no difference in ID/IG ratio between both applied ion doses. At the same time a significant broadening of G peak, accompanied with a shift toward lower wavenumbers, is observed. This indicates a slight disordering of the amorphous carbon matrix with implantation of silver ions. It appears that the change of the ion dose (to higher one), that results in increased concentration of silver, does not affect the chemical structure of the coating. However, authors keep in mind, that for implantation energy of 15 keV the overall penetration depth is rather low and is equal to ca. 14 nm, and are aware of the fact that it may not result in severe changes in the Raman spectra. Opposite trend was observed by Cui et al. in their work related to Ti implanted DLC. Authors reported induced graphitization processes, what we believe to be expected, confirmed by the increased ID/IG ratio, shift of the G peak toward higher wavenumbers and lower mechanical properties of the coating. Yet this could be also related to higher ions energy (30 keV) and high chemical affinity of titanium and carbon [41]. In our case this disordering process should result in higher mechanical properties, which certainly did not happen (as discussed later), but again one has to keep in mind very shallow penetration depth of silver ions. Following Ferrari, the possible explanation may be the second stage of a hysteresis phenomenon, where the quality of sp2 bonded carbon can be changed independently from the sp2/sp3 ratio. The accelerated Agþ beam might induce the topological
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Fig. 5. a) Raman spectra of examined samples, b) deconvolution of a Raman spectra for DLC sample.
disorder of the surface. The bonding was still mainly sp2, but the weaker bonds soften the vibrational modes, so the shift of G peak was toward lower wavenumbers. The ID/IG ratio was also decreased, since it is proportional to the number of aromatic rings [38,42]. As for the chemical composition point of view, Endrino et al. reported a slight disordering of the a-C coatings with silver addition. In their opinion, a possible cause could be either the hybridization between carbon and silver or the energetic condensation of silver ions originating from the filtered cathodic arc [26]. In turn, Wang et al. claim that change of nanograins size in an Ag-DLC film might induce the intensity change of both D and G bands [43]. The shift in the G peak into lower wavenumbers of functionally gradient DLCAg nanocomposite coatings was reported by Narayan [44]. However in this work there is no information about the shape (broadening or narrowing) of the former. In his opinion one of the reasons for this change in G-band position may be the reduced compressive stress, being an effect of significantly smaller elastic modulus of silver nanoparticles that may absorb compressive stress from the diamond-like carbon matrix. Nevertheless, all three papers are related to the simultaneous work of carbon and silver sources unlike the one analysed. Thus authors consider another phenomenon that can be partially explained on the basis of results presented in works [25,38,42] together. Considering that the influence of silver onto chemical structure and properties of diamond-like carbon coatings depends on their hydrogenation (discussed later in 3.6 section), as stated in Section 3.3, it is possible that during the implantation process hydrogen is released from the top surface atomic layers of silver implanted DLC. This in turn may change the quality of sp2 organized carbon and indirectly affect the residual stress of the coating. This hypothesis, however, needs further investigations. Summarizing, because of the possible twofold influence of implantation process onto overall parameters of implanted layers, an increase in disorder of diamond-like carbon coatings should be considered in terms of changes in chemical structure and chemical composition as well. 3.5. Atomic force microscopy AFM images (Fig. 6) as well as examples of surface profiles variations (Fig. 7) clearly show changes in morphologies of investigated surfaces. To describe surface geometric structure commonly used roughness parameters Ra and RMS were selected [45]. Fig. 6a presents polished medical steel substrate for the synthesis of DLC coating. Single scratches, originated from polishing process, can be noted on this surface. Despite this, Ra and RMS parameters are rather low - 1.46 and 1.90 nm, respectively. Surface of DLC coating synthesized on medical steel is shown in Fig. 6b. Its morphology is typical for such coating type with distinctive features similar to globular conglomerates. RMS parameter of investigated DLC coating is equal to 1 nm (Fig. 8) which is also in the range of values
reported in the literature (0.1e1.13 nm) [24,46,47]. Further implantation of silver in the DLC coating smoothens the surface, which can be seen on AFM scans (Fig. 6c and d). Smoothing of the DLC surface after ion implantation by titanium was observed by Cui, where RMS value decreased from 1.589 to 0.595 nm for implanted coatings [41]. The study of Lipinski et al. presents the changes in morphology of the polymer doped with silver. The authors describe the smoothing of the polymer with respect to the starting substrate. This was due to coating the polymer with a very thin layer of metal as a result of aggregation of Ag atoms on the surface [48]. According to graph in Fig. 8, a decrease of roughness parameters of ion implanted coatings compared to steel and DLC substrates can be found. Average value of Ra for the DLC and Ag implanted samples decreased by 68% and 38%, respectively. Similar trend can be observed for RMS parameter: 68% and 39% decrease, respectively. 3.6. Hardness, modulus and adhesion Generally, the influence of silver incorporation onto mechanical properties of amorphous carbon matrix should be regarded in terms of the hydrogen content in the latter (a-C and a-C:H), as suggested in works [23] and [26]. In non-hydrogenated carbon films heterogeneous structures, formed by silver nanoparticles embedded in the amorphous carbon matrix, may possibly decrease the plastic flow and in consequence, under particular conditions, increase hardness. Whereas in hydrogenated carbon films dangling bonds are occupied by hydrogen thus the formation of bonds between C and Ag (increased hybridization) is strongly limited, therefore an increase in hardness of hydrogenated carbon coatings is not observed. Fig. 9 presents a comparison of hardness and modulus for all examined samples. Hardness of DLC coating is almost four times higher than that of steel substrate, as expected. Furthermore, an evident decrease in hardness can be observed between DLC and DLC/Ag samples. The decrease in hardness for this case when compared to DLC sample was ~14%. Barely noticeable is a lowering trend of modulus with increasing dose of silver ions, however in this case there is a lack of statistical significance between particular samples. Changes in hardness and modulus are not consistent with Raman spectroscopy, which suggest slight disordering with the implantation of Agþ ions, regardless the applied dose. Neither they correlate with applied dose of Agþ ions and it appears that increased ion dose does not affect this parameter. This, in turn, is contrary to the fact of decreased mechanical parameters of hydrogenated carbon coatings with increasing concentration of silver, which was reported by other authors [19,23], and is in agreement with our experience [24]. Thus, in authors opinion, the interaction of Agþ with sample surface is a dominant phenomenon affecting the mechanical properties of silver implanted DLC, which is in agreement with results presented in work [41]. At the same time hardness and modulus are rather
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Fig. 6. AFM images of a) substrate AISI 316L stainless steel, b) substrate coated with DLC coating, c - d) silver ions doped DLC coatings DLC/Ag1 e 1.5 at.% and DLC/Ag2 e 3.8 at.% Ag, respectively. Scan size of 2 2 mm.
related to the bulk properties of the coating, which during the measurement can be affected by the influence of thin top layer modified by the implantation procedure. The common feature for both silver incorporated a-C and a-C:H films is decreased level of internal stress, that can be explained by the formation of silver nanoclusters, which are characterized by the low value of elastic modulus and the ability of stress accumulation [25,44]. The reduced bonding ability between the nanocrystalline and amorphous phase enables their mutual sliding and multiple-
shifts of grain-matrix interface under the influence of applied external stress that in consequence may lead to increased ductility of the film [43]. Fig. 10 presents the results of scratch tested samples. For undoped DLC the critical load reaches 48.2 ± 6.1 mN while for samples implanted with doses of 7 and 10 1016 Agþ/cm2 the critical load is noticeably decreased to 41.3 ± 0.6 and 44.4 ± 1.9 mN, respectively. Here the dose of 7 1016 Agþ/cm2 resulted in the lowest overall concentration of silver and the lowest value of
Fig. 7. Variations of selected surface profiles for investigated samples.
Fig. 8. Value of selected roughness parameters (Ra and RMS) and Ag surface concentration.
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Fig. 9. Hardness and modulus of investigated samples.
critical load. Moreover, registered critical loads for both doses are lower than that of DLC which confirms our earlier hypothesis that both the implantation process and the chemical composition change affect the overall parameters of silver doped carbon coatings. Opposite trend of positive correlation between silver concentration and adhesion (up to ~10% at.) was reported by Wang et al. [43] for samples synthesized by dual magnetron sputtering. On the other hand, a noticeable decrease in adhesion was observed for silver incorporated DLC coatings deposited by the hybrid RF PACVD/MS technique. However, lower critical delamination force was accompanied by different delamination character (layer crumbling was not observed). 4. Conclusions Ion implantation technique was successfully applied for the modification of diamond-like carbon coatings. Implanted ions with energy of 15 keV and two doses resulted in similar implantation range and increasing trend in silver concentration. Both surface and bulk properties of obtained coatings are affected by two independent phenomena (change in chemical composition and overall interaction between silver beam and modified surface), however at this stage of investigation it is impossible to determine which one is dominant and what exactly happens upon it. In authors opinion hydrogen may also play an important role in this regard. Implantation of silver ions decreases the roughness parameters and surface free energy of DLC layers, thereby making them more hydrophobic in comparison to pure diamond-like carbon coatings and less attractive for the possible bacterial colonization. Raman spectroscopy revealed slight disorder of implanted films. On the other hand, implantation of silver ions slightly decreased the
Fig. 10. Critical load to full delamination for DLC and Ag-doped DLC coatings.
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mechanical parameters of obtained layer (note that for modulus and adhesion the changes were not statistically significant). Further increase of the dose of Ag ions (form 7 to 10 1016 ion/cm2) results in increased Ag surface concentration but does not result in further changes in surface roughness, ID/IG intensity ratio, surface free energy and mechanical properties. Summarising, presented technology allows obtaining the assumed concentration and depth distribution of silver in the previously prepared carbon coatings, allowing at the same time the modification of their mechanical and physicochemical properties. Our results also demonstrated that the physicochemical properties of DLC/Ag layers appear to be insensitive to different doses of silver ions, and at the same time surface concentrations of silver (at least to 4 at.% of Ag). Silver implanted carbon coatings with sufficient mechanical properties, very smooth surface, increased hydrophobicity and decreased surface free energy suggest a possibility of their implementation as biomaterial. Acknowledgement This work has been supported by the National Centre for Research and Development under grant no. ERA-NET/MNT/CARSILA/1/2010 and in part by the Project OP VaVpI Centre for Nanomaterials, Advanced Technologies and Innovation CZ.1.05/2.1.00/ 01.0005 and by the Project Development of Research Teams of R&D Projects at the Technical University of Liberec CZ.1.07/2.3.00/ 30.0024. References [1] Christel P. Biological and biomechanical performance of biomaterials. In: Proceedings of the 5. Europ. Conference on biomaterials, Paris, France, Sept. 4e6. Elsevier; 1985. Amsterdam u.a., 1986. [2] Mitura S, Niedzielski P, Walkowiak B, editors. NANODIAM, new technologies for medical applications: studying and production of carbon surfaces allowing for controllable bioactivity. Warsaw: PWN; 2006. [3] Robertson J. Diamond-like amorphous carbon. Mat Sci Eng R 2002;37: 129e281. [4] Dorner-Reisel A, Schürer C, Irmer G, Müller E. Electrochemical corrosion behavior of uncoated and DLC coated medical grade Co28Cr6Mo. Surf Coat Tech 2004;2002(177e178):830e7. [5] Batory D, Blaszczyk T, Clapa M, Mitura S. Investigation of anti-corrosion properties of TieC gradient layers manufactured in hybrid deposition system. J Mater Sci 2008;10:3385e91. [6] Dorner A, Schurer C, Reisel G, Irmer G, Seidel O, Muller E. Diamond-like carbon-coated Ti6Al4V: influence of the coating thickness on the structure and the abrasive wear resistance. Wear 2001;249:489e97. [7] Kaczorowski W, Szymanski W, Batory D, Niedzielski P. Tribological properties and characterization of diamond like carbon coatings deposited by MW/RF and RF plasma-enhanced CVD method on poly(ether-ether-ketone). Plasma Processes Polym 2014;11:878e87. [8] Hauert R. An overview on the tribological behavior of diamond-like carbon in technical and medical applications. Tribol Int 2004;37:991e1003. [9] Yamauchi N, Okamoto A, Tukahara H, Demizu K, Ueda N, Sone T, et al. Friction and wear of DLC films on 304 austenitic stainless steel in corrosive solutions. Surf Coat Tech 2003;174-175:465e9. [10] Huang L, Xu K, Lu J, Guelorget B, Chen H. Nano-scratch and fretting wear study of DLC coatings for biomedical application. Diam Relat Mater 2001;10: 1448e56. [11] Hauert R, Müller U. An overview on tailored tribological and biological behavior of diamond-like carbon. Diam Relat Mater 2003;12:171e7. [12] Cui F, Li D. A review of investigations on biocompatibility of diamond-like carbon and carbon nitride films. Surf Coat Tech 2000;131:481e7. [13] Liu C, Zhao Q, Liu Y, Wang S, Abel EW. Reduction of bacterial adhesion on modified DLC coatings. Colloid Surf B 2008;61:182e7. [14] Jakubowski W, Bartosz G, Niedzielski P, Szymanski W, Walkowiak B. Nanocrystalline diamond surface is resistant to bacterial colonization. Diam Relat Mater 2004;13:1761e3. [15] Shirakura A, Nakaya M, Koga Y, Kodama H, Hasebe T, Suzuki T. Diamond-like carbon films for PET bottles and medical applications. Thin Solid Films 2006;494:84e91. [16] Dorner-Reisel A, Schürer C, Müller E. The wear resistance of diamond-like carbon coated and uncoated Co28Cr6Mo knee prostheses. Diam Relat Mater 2004;13:823e7. [17] Simchi A, Tamjid E, Pishbin F, Boccaccini AR. Recent progress in inorganic and composite coatings with bactericidal capability for orthopaedic applications. Nanomedicine 2011;7:22e39.
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