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Mechanical properties and platelet adhesion behavior of diamond-like carbon films synthesized by pulsed vacuum arc plasma deposition
Surface Science 531 (2003) 177–184 www.elsevier.com/locate/susc
Mechanical properties and platelet adhesion behavior of diamond-like carbon films synthesized by pulsed vacuum arc plasma deposition Y.X. Leng *, J.Y. Chen, P. Yang, H. Sun, G.J. Wan, N. Huang
*
School of Materials Science & Engineering, Southwest Jiaotong University, Sichuan, Chengdu 610031, China Received 25 November 2002; accepted for publication 17 March 2003
Abstract Diamond-like carbon (DLC) is an attractive biomedical material due to its high inertness and excellent mechanical properties. In this study, DLC films were fabricated on Ti6Al4V and Si(1 0 0) substrates at room temperature by pulsed vacuum arc plasma deposition. By changing the argon flow from 0 to 13 sccm during deposition, the effects of argon flow on the characteristics of the DLC films were systematically examined to correlate to the blood compatibility. The microstructure and mechanical properties of the films were investigated using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) surface analysis, a nano-indenter and pin-on-disk tribometer. The blood compatibility of the films was evaluated using in vitro platelet adhesion investigation, and the quantity and morphology of the adherent platelets was investigated employing optical microscopy and scanning electron microscopy. The Raman spectroscopy results showed a decreasing sp3 fraction (an increasing trend in ID =IG ratio) with increasing argon flow from 0 to 13 sccm. The sp3 :sp2 ratio of the films was evaluated from the deconvoluted XPS spectra. We found that the sp3 fraction decreased as the argon flow was increased from 0 to 13 sccm, which is consistent with the results of the Raman spectra. The mechanical properties results confirmed the decreasing sp3 content with increasing argon flow. The Raman D-band to G-band intensity ratio increased and the platelet adhesion behavior became better with higher flow. This implies that the blood compatibility of the DLC films is influenced by the sp3 :sp2 ratio. DLC films deposited on titanium alloys have high wear resistance, low friction and good adhesion. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond; Carbon; X-ray photoelectron spectroscopy; Noble gases; Raman scattering spectroscopy
1. Introduction
* Corresponding authors. Tel.: +86-28-87600728; fax: +8628-87600625. E-mail addresses: [email protected] (Y.X. Leng), [email protected] (N. Huang).
Diamond-like carbon (DLC) coatings possess high hardness, wear resistance and corrosion resistance properties similar to those of diamond [1– 3]. The low friction coefficient and good wear resistance of this film material makes it suitable in many tribological and wear-resistant applications such as cutting tools and magnetic storage systems
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00487-4
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[4]. For improving the properties of medical implants, for instance, in artificial heart valves, stents, hip and rotary blood pumps, DLC is a very promising coating material because it is chemically inert, extremely hard, wear resistant and biocompatible [5–12]. In spite of the potential benefits offered by DLC as a blood contacting material, however, the relationship between the sp3 content of DLC and its blood compatibility is not well understood. The mechanical properties of DLC films such as hardness, YoungÕs modulus and density depend on the concentration ratio of sp3 - to sp2 -bonded carbon atoms. This ratio can be varied over a wide range, enabling films with customized properties to be deposited for special application. It is now recognized that a wide variety of methods can be used for their preparation, such as pulsed laser deposition [13–15], ion beam deposition [14], chemical vapor deposition [16], and cathodic vacuum arc deposition [17–19]. In the work described here, hydrogen-free DLC films were deposited onto (1 0 0) silicon and Ti6Al4V substrates using pulsed vacuum arc plasma deposition and the effects of the argon flow on the film characteristics was evaluated. The effect of argon flow on the blood compatibility characteristics of the DLC films was systematically examined also. The relationship between the sp3 and sp2 content ratio, the mechanical properties, and the adherent blood platelet quantity was specifically investigated.
2. Experiment Hydrogen-free DLC films were deposited onto Si(1 0 0) wafers and Ti6Al4V substrates by pulsed vacuum arc plasma deposition. By changing the argon flow during deposition, films with different ratios of sp3 to sp2 bonded carbon atoms were formed. Fig. 1 shows a schematic of the experimental set-up used in this study. The base pressure of the system was 6 104 Pa. The deposition conditions were as follows: arc pulse frequency 40 Hz, arc pulse width 1.5 ms, arc current 170 A, substrate bias voltage )50 V, substrate temperature <100 °C, and deposition time 20 min. The
Fig. 1. Schematic diagram of the pulse vacuum arc plasma deposition equipment.
Table 1 Instrumental parameters Sample number
#1
#2
#3
#4
Argon flow rate (sccm)
0
3
6
13
argon flow was varied from 0 to 13 sccm (see Table 1). The thickness of the films was about 200 nm. Raman spectroscopy were employed to characterize the chemical bonding and the microstructure of the DLC films, and graphite was used as a control in the analyses. Sometime the Raman results are not definitive enough to discern the DLC films [20]. In order to obtain more structural information, X-ray photoelectron spectroscopy (XPS) was used [21,22]. Ball-on-disc rotation wear test was performed to evaluate the tribological properties of the coatings, and scanning electron microscopy (SEM) was used to observe the ball-on-disc wear tracks. A continuous indentation technique was utilized to investigate the nanohardness properties of the DLC films. The adhesion of the films was examined using the indentor test [23,24]. The area of the indentation was studied using an optical microscope to determine film failure. Platelet adhesion tests were performed on the DLC films. The quantity and morphology of adhered platelet were examined to study the surface thrombogenicity. Blood was obtained from a healthy adult volunteer. Whole blood was collected in an acid citrate dextrose medium. After centrifugation, red cells and platelets were sepa-
Y.X. Leng et al. / Surface Science 531 (2003) 177–184 5000 4500 4000
Intensity (arb.)
rated and platelet-rich plasma was obtained. The samples were immersed in the platelet-rich plasma and incubated at 37 °C for 30 or 120 min. After rinsing, fixing and critical point drying, the specimens with platelets on the surface were coated with a gold layer 10–20 nm thick and examined by optical microscopy and SEM. The quantity, morphology, aggregation, and pseudopodium of the adherent platelets were examined to investigate the surface thrombogenicity. Twenty fields of view were chosen at random to obtain good statistics.
Fig. 3 shows the XPS C1s peaks of DLC samples deposited at different argon flow rates. It can be seen that the peaks move to lower energy as the argon flow increases, implying that the sp2 bonded carbon atom fraction increases also, because the bonding energy of sp2 carbon atoms is of lower energy. Fitting of the DLC C1s peaks was per-
3000 2500 2000
#4 #3 #2
1500
500
#1 graphic
0 800
1000
(a)
Intensity (arb.)
1200
1400
1600
1800
2000
Raman shift (cm -1)
Data: LYX1 Model: Gauss Chi^2 = 1888.58414 y0 0 ±0 xc1 1529.52183 3000 w1 209.23482 A1 567155.57412 2500 xc2 1350.8766 w2 341.3242 A2 446905.751 2000
3.1. Raman spectroscopy
3.2. XPS C1s core level spectra analysis
3500
1000
3. Results and discussion
Fig. 2a shows Raman spectra of the DLC films deposited on Si(1 0 0). It is clear from the figure that there are no significant differences between the spectra. Compared with graphite, all the spectra show a broad peak at around 1530 cm1 and a lower frequency shoulder at approximately 1350 cm1 , commonly referred to as the G-band and Dband, respectively [25]. Fig. 2b shows a typical deconvoluted Raman spectrum fitted by Gaussians after subtracting the background. Based on the fitting parameters, the peak positions and the ratio of the integrated areas under the D and G peaks (ID =IG ) were obtained, and are summarized in Table 2. Our results show that a higher argon flow leads to a shift of the two peaks toward higher wave numbers and a slight increase of ID =IG ratios. Since the ID =IG ratio is related to the sp3 :sp2 ratio [26], it can be concluded that the sp3 content decreases with increasing argon flow––the film structure becomes more graphite-like with increasing argon flow.
179
#1 ±0.52623 ±1.94929 ±19554.20318 ±7.65821 ±5.88087 ±20075.82023
G
1500
D 1000 500 0 -500 800
1000
1200
1400
1600
1800
2000
-1
(b)
Raman shift(cm )
Fig. 2. (a) Raman spectra of DLC films fabricated at different argon flow and (b) typical Raman spectrum fitted by Gaussians.
Table 2 Experimental results from Raman spectroscopy Sample no.
Peak position (cm1 ) D-band
G-band
1 2 3 4
1350.9 1357.7 1360.8 1363.5
1529.5 1531.0 1530.8 1532.0
a
ID =IG a 0.79 0.87 0.86 0.9
Intensity ratio of D-band to G-band.
formed by using three components (each being a mixture of 80% Gaussian and 20% Lorentzian) and by approximating the contribution of background by the Shirley method [21,22]. The fittings for samples #1 and #4, deposited at the lowest and highest (respectively) argon partial pressures, are presented in Fig. 4. The first component at 285.3 eV
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Y.X. Leng et al. / Surface Science 531 (2003) 177–184 50000 45000
0.9
40000
30000 25000 20000
#1 (0sccm)
15000
#2(3sccm)
10000
#4(13sccm)
5000
atom content(%)
Counts
2
sp
3
0.8
35000
0 275
sp
0.7 0.6 0.5 0.4 0.3 0.2
280
285
290
295
B.E. (eV)
0
2
4
6
8
10
12
14
argon inlet(sccm)
Fig. 3. XPS C1s peaks of DLC thin films obtained at different argon flow rates.
Fig. 5. sp2 and sp3 carbon atom content as determined from fitting of the XPS C1s peaks, as a function of argon flow rate.
corresponds to sp2 carbon atoms, while the second component at 286.1 eV corresponds to sp3 carbon atoms. A third peak of much smaller intensity at 287.4 eV has is also seen and is attributed to some C–O contamination formed at the surface of the samples due to air exposure. The fractional sp3 and sp2 carbon atom content was then determined as the ratio of the corresponding peak area to the total C1s peak area. Fig. 5 shows the sp3 and sp2 content as a function of argon flow. At 0 sccm, the fractional sp3 content is about 78%, and decreases to about 59% with increasing argon flow to 13 sccm. The XPS analysis for sp3 content is simply a semi-quantity method. We only use these data to compare relatively content of all the DLC films. Though we get the data of sp3 content, we are not assured the actual sp3 content in the DLC films. We only use these data for comparing the sp3 content between all the DLC films. So these results show that the sp3 fraction decreases as the argon flow increases. This is consistent with the results of the Raman spectra. 3.3. Nano-hardness
Fig. 4. Deconvolution of the XPS 1s peaks of DLC films deposited at (a) 0 and (b) 13 sccm.
The mechanical properties of DLC films are of great interest for its use as a coating material. However, it is hard to obtain reliable values of mechanical properties using conventional mechanical testers. For example, the conventional microhardness tester requires direct imaging of the
Y.X. Leng et al. / Surface Science 531 (2003) 177–184
indentations to obtain hardness, and large errors can be introduced in measurement of the diagonal lengths, especially when the indentations are small. Recently, a nano-hardness technique has been used to investigate mechanical properties of thin films. The nano-hardness tester uses an indenter tip driven into a specific site on the material by applying an increasing normal load. When a pre-set maximum value is reached, the normal load is reduced until partial or complete relaxation occurs. For each loading/unloading cycle, the applied load value is plotted with respect to the corresponding position of the indenter. The resulting load/displacement curves provide data specific to the mechanical nature of the material. In the work described here the maximum load was 0.5 mN and the loading/unloading rate was 1 mN/ min. In this way the hardness of the DLC films can be measured without ÔinterferenceÕ from the substrate. Fig. 6 shows the load/displacement curves for samples #1 and #4. It can be seen that the slope is approximately constant for the two samples from 0 to 0.5 mN load. This means that the sample is in the elastic deformation regime. The slope for sample #1 is greater than that for sample #4; this means that sample #1 has a higher YoungÕs modulus and hardness. The nano-hardness of samples #1 and #4 were 72.1 and 68 GPa, respectively. The results indicate that the nanohardness of the films decrease with increasing argon flow. This corresponds to the observed decrease of sp3 bonded carbon atoms with increasing argon flow.
#1
0.5
181
3.4. Adhesion tests The adhesion of the films was examined using the ‘‘Knoop’’ indentor test on Ti6Al4V substrates. Around the Knoop indentation crater, no peeling or flaking of the coating material was observed for 100, 200, 300, 500–1000 g loads. This means that the DLC films have good adhesion to the Ti6Al4V substrate. 3.5. Ball-on-disc tests Dry ball-on-disc tribological tests of DLC films coated on Ti6Al4V substrates were performed with a 0.98 N normal load, 10 cm/s rotation speed and 6 mm rotation radius, with 6-mm-diameter SiC balls as the counterpart material. The results showed that the wear resistance decreased when the argon flow increased. With increasing argon flow, the hardness of the films decreased and the films can more easily delaminate at 0.98 N normal load. Fig. 7 shows the friction coefficients for DLC coatings on Ti6Al4V substrates. It can be seen that the minimum coefficient is obtained for sample #1, where the initial coefficient of friction is 0.17 and the steady state coefficient of friction is 0.07. After 600,000 cycles, the friction coefficient for sample #1 was very stable and in the low range of 0.06– 0.09. SEM was used to observe the ball-ondisc wear track, shown in Fig. 8(a). After 600,000 cycles, the sample #1 film was not penetrated and the film had maintained its integrity. But for
#4
0.4
mN
0.3 0.2 0.1 0.0 0
2
4
6
8
10 12 14 16 18 20 22 24 nm
Fig. 6. Loading/unloading cycle of the DLC films.
Fig. 7. Friction coefficient of DLC films on Ti6Al4V substrate.
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3.6. Blood compatibility Adhered platelets are usually measured to assess hemocompatibility, but activation of platelets is a more important parameter indicating the interaction of blood with materials than the adhesion behavior. In our experiments, the hemocompatibility of the DLC films was studied. Deformation of the adhered platelets on the DLC surface was studied semi-quantitatively to evaluate the hemocompatibility [27], and the results are shown in Table 3. The number of adherent platelets, the degree of deformation (pseudopodium) and aggregation of the adhered platelets on the DLC film surface decreases with increasing argon flow. Fig. 9 displays the morphology of the adhered platelets on the DLC films. It can be observed that the number of platelets adhered on the #4 DLC film (Figs. 9e,f) is slightly less than for the #3 DLC film (Fig. 9c,d) and markedly less than that on #1 DLC film (Fig. 9a,b). The degree of deformation (pseudopodium) and aggregation of the adhered platelets are also less on #3 and #1 DLC films. The result show that the platelet adhesion behavior of the DLC films become better with increasing argon flow. The Raman and XPS results showed that sp3 :sp2 decreased with increasing argon flow. It
Fig. 8. SEM morphology of the wear tracks: (a) sample #1 after 600,000 rotation cycles and (b) sample #4 after 160,000 rotation cycles.
Table 3 Morphology of the platelets on the DLC films after 30 and 120 min incubation time Sample no.
sample #4, the friction coefficient was about 0.2 during the initial Ôrun-inÕ stage, then rapidly increased, correlating to a sudden and premature coating failure. Fig 8(b) shows the ball-on-disc wear tracks for sample #4 after 160,000 cycles. After about 75,000 cycles with a 0.98 N normal load, the friction coefficient of sample #4 sharply increased to 0.7 (showed as Fig. 7), with considerable localized spallation resulted from fatigue, and failed catastrophically almost immediately. All these results indicate that the wear resistance of the DLC films decreases with increasing argon flow, though still much better than for Ti alloy substrates.
1 2 3 4
30 min
III II II II
120 min IV III III III
Note: The degree of deformation of the adhesive platelets on the surface of the films can be categorized into four types [24]. I:II There are adhered platelets on the surface of the sample, but the platelets are not activated. I*: the number of adhered platelets is less; I**, more; I***, much more; II:I The adhered platelets are activated and begin to exhibit pseudopodium. II*: a portion of platelets exhibit pseudopodium; II**: many platelets show pseudopodium; III: Adhered platelets activated further and aggregated. III*: a portion of platelets aggregated; III**: many platelets aggregated. IV: Aggregated platelets forming net structures with fibrin. IV*: aggregated platelets net structure; IV**: erythrocytes adhered onto the net structure.
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Fig. 9. Morphology of adherent platelets (incubation time ¼ 30 min) on DLC films: (a) sample #1, (b) sample #1, (c) sample #3, (d) sample #3, (e) sample #4 and (f) sample #4.
can be inferred that the platelet adhesion behavior of our DLC films is influenced by the sp3 :sp2 ratio, and the hemocompatibility may improves with decreasing sp3 :sp2 . This result is basically consistent with the results by Chen, who synthesized DLC films with hydrogen by plasma immersion ion implantation-deposition [12]. The blood compatibility of the DLC films is influenced by the ratio of sp3 to sp2 , not by the absolute sp3 or sp2 content because the materials blood compatibility is affected by many factors such as surface energy, electrical properties, composition. The platelet adhesion behavior better is only one indication of
anti-thrombogenicity, the other aspects of antithrombogenicity, such as coagulation factors, should be done later on.
4. Conclusion Diamond-like carbon thin films were synthesized by pulsed vacuum arc plasma deposition. The Raman and XPS results showed that the sp3 content changed as a function of argon flow, with the sp3 carbon atom content decreasing as the argon flow increases. DLC films have good adhesion to
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Ti6Al4V substrates. Our results showed that the blood adhesion behavior of the DLC films is influenced by the sp3 :sp2 ratio. The platelet adhesion behavior of the DLC films increased with increasing argon flow. Unfortunately, the hardness and wear resistance decreased with increasing argon flow, though still much better than for Ti alloy substrates, which result from decreasing sp3 :sp2 . In order to fully exploit the full potential of DLC films as blood contacting biomaterials, the films must have proper sp3 :sp2 ratio, so as to achieve good blood compatibility and mechanical properties.
Acknowledgements The authors would like to thank Dr. Ian G. Brown for reviewing this paper. The work described in this paper was jointly supported by Chinese NSFC 39870199, National Research Fund for fundamental key Projects#G1999064706, High Tech Research and Development (863) Program 102-12-09-1.
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[8] K. Gutensohn, C. Baeythein, J. Bau, et al., Thromb. Res. 99 (6) (2000) 577. [9] M.I. Jones, I.R. McColl, D.M. Grant, K.G. Parker, T.L. Parker, Diamond Relat. Mater. 8 (1999) 457. [10] I. Dion, X. Roques, C. Baquey, E. Baudet, B. Cathalinat, N. More, Biomed. Mat. Eng. 3 (1) (1993) 51. [11] L.A. Thomson, F.G. Law, N. Rushton, J. Franks, Biomaterials 2 (1) (1991) 37. [12] J.Y. Chen, L.P. Wang, K.Y. Fu, N. Huang, Y. Leng, Y.X. Leng, P. Yang, A.J. Wang, G.J. Wan, H. Sun, X.B. Tian, P.K. Chu, Surf. Coat. Technol. 156 (2002) 289. [13] P. Merel, M. Tabbal, M. Chaker, S. Moisa, J. Margot, Appl. Surf. Sci. 136 (1998) 105. [14] D. Schneider, C.F. Meyer, H. Mai, B. Schoneich, H. Ziegele, H.J. Scheibe, Y. Lifshitz, Diamond Relat. Mater. 7 (1998) 973. [15] M. Tabbal, P. Merel, M. Chaker, M.A. Elkhahani, E.G. Herbert, B.N. Lucas, M.E. OÕhern, Surf. Coat. Technol. 116–119 (1999) 452. [16] S. Kumar, C.M.S. Rauthan, K.M.K. Srivatsa, P.N. Dixit, R. Bhattacharyya, Appl. Surf. Sci. 182 (2001) 326. [17] H. Ronkainen, S. Varjus, J. Koskinen, K. Holmberg, Wear 249 (2001) 260. [18] M. Chhowalla, Diamond Relat. Mater. 10 (2001) 1011. [19] P.X. Yan, X.C. Wu, Mater. Chem. Phys. 60 (1999) 44. [20] L. Li, H. Zhang, Y. Zhang, P.K. Chu, X. Tian, L. Xia, X. Ma, Mater. Sci. Eng. B 94 (2002) 95–101. [21] P. Merel, M. Tabbal, M. Chaker, S. Moisa, J. Margot, Appl. Surf. Sci. 136 (1998) 105. [22] A.K. Sharma, R.J. Narayan, J. Narayan, K. Jagannadham, Mat. Sci. Eng. B 77 (2000) 139. [23] P.C. Jindal, D.T. Quinto, G.J. Wolfe, Thin Solid Films 154 (1987) 361. [24] M. Nesladek, K. Vandierendonck, C. Quaeyhaegens, M. Kerhofs, L.M. Stals, Thin Solid Films 270 (1995) 184. [25] J. Robertson, Prog. Solid State Chem. 21 (1991) 199. [26] J. Robertson, Mat. Sci. Eng. Reports 37 (2002) 129. [27] Y.X. Leng, N. Huang, P. Yang, J.Y. Chen, H. Sun, J. Wang, G.J. Wan, Y. Leng, P.K. Chu, Thin Solid Films 420–421 (2002) 408–413.
Mechanical properties and platelet adhesion behavior of diamond-like carbon films synthesized by pulsed vacuum arc plasma deposition Y.X. Leng *, J.Y. Chen, P. Yang, H. Sun, G.J. Wan, N. Huang
*
School of Materials Science & Engineering, Southwest Jiaotong University, Sichuan, Chengdu 610031, China Received 25 November 2002; accepted for publication 17 March 2003
Abstract Diamond-like carbon (DLC) is an attractive biomedical material due to its high inertness and excellent mechanical properties. In this study, DLC films were fabricated on Ti6Al4V and Si(1 0 0) substrates at room temperature by pulsed vacuum arc plasma deposition. By changing the argon flow from 0 to 13 sccm during deposition, the effects of argon flow on the characteristics of the DLC films were systematically examined to correlate to the blood compatibility. The microstructure and mechanical properties of the films were investigated using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) surface analysis, a nano-indenter and pin-on-disk tribometer. The blood compatibility of the films was evaluated using in vitro platelet adhesion investigation, and the quantity and morphology of the adherent platelets was investigated employing optical microscopy and scanning electron microscopy. The Raman spectroscopy results showed a decreasing sp3 fraction (an increasing trend in ID =IG ratio) with increasing argon flow from 0 to 13 sccm. The sp3 :sp2 ratio of the films was evaluated from the deconvoluted XPS spectra. We found that the sp3 fraction decreased as the argon flow was increased from 0 to 13 sccm, which is consistent with the results of the Raman spectra. The mechanical properties results confirmed the decreasing sp3 content with increasing argon flow. The Raman D-band to G-band intensity ratio increased and the platelet adhesion behavior became better with higher flow. This implies that the blood compatibility of the DLC films is influenced by the sp3 :sp2 ratio. DLC films deposited on titanium alloys have high wear resistance, low friction and good adhesion. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond; Carbon; X-ray photoelectron spectroscopy; Noble gases; Raman scattering spectroscopy
1. Introduction
* Corresponding authors. Tel.: +86-28-87600728; fax: +8628-87600625. E-mail addresses: [email protected] (Y.X. Leng), [email protected] (N. Huang).
Diamond-like carbon (DLC) coatings possess high hardness, wear resistance and corrosion resistance properties similar to those of diamond [1– 3]. The low friction coefficient and good wear resistance of this film material makes it suitable in many tribological and wear-resistant applications such as cutting tools and magnetic storage systems
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00487-4
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[4]. For improving the properties of medical implants, for instance, in artificial heart valves, stents, hip and rotary blood pumps, DLC is a very promising coating material because it is chemically inert, extremely hard, wear resistant and biocompatible [5–12]. In spite of the potential benefits offered by DLC as a blood contacting material, however, the relationship between the sp3 content of DLC and its blood compatibility is not well understood. The mechanical properties of DLC films such as hardness, YoungÕs modulus and density depend on the concentration ratio of sp3 - to sp2 -bonded carbon atoms. This ratio can be varied over a wide range, enabling films with customized properties to be deposited for special application. It is now recognized that a wide variety of methods can be used for their preparation, such as pulsed laser deposition [13–15], ion beam deposition [14], chemical vapor deposition [16], and cathodic vacuum arc deposition [17–19]. In the work described here, hydrogen-free DLC films were deposited onto (1 0 0) silicon and Ti6Al4V substrates using pulsed vacuum arc plasma deposition and the effects of the argon flow on the film characteristics was evaluated. The effect of argon flow on the blood compatibility characteristics of the DLC films was systematically examined also. The relationship between the sp3 and sp2 content ratio, the mechanical properties, and the adherent blood platelet quantity was specifically investigated.
2. Experiment Hydrogen-free DLC films were deposited onto Si(1 0 0) wafers and Ti6Al4V substrates by pulsed vacuum arc plasma deposition. By changing the argon flow during deposition, films with different ratios of sp3 to sp2 bonded carbon atoms were formed. Fig. 1 shows a schematic of the experimental set-up used in this study. The base pressure of the system was 6 104 Pa. The deposition conditions were as follows: arc pulse frequency 40 Hz, arc pulse width 1.5 ms, arc current 170 A, substrate bias voltage )50 V, substrate temperature <100 °C, and deposition time 20 min. The
Fig. 1. Schematic diagram of the pulse vacuum arc plasma deposition equipment.
Table 1 Instrumental parameters Sample number
#1
#2
#3
#4
Argon flow rate (sccm)
0
3
6
13
argon flow was varied from 0 to 13 sccm (see Table 1). The thickness of the films was about 200 nm. Raman spectroscopy were employed to characterize the chemical bonding and the microstructure of the DLC films, and graphite was used as a control in the analyses. Sometime the Raman results are not definitive enough to discern the DLC films [20]. In order to obtain more structural information, X-ray photoelectron spectroscopy (XPS) was used [21,22]. Ball-on-disc rotation wear test was performed to evaluate the tribological properties of the coatings, and scanning electron microscopy (SEM) was used to observe the ball-on-disc wear tracks. A continuous indentation technique was utilized to investigate the nanohardness properties of the DLC films. The adhesion of the films was examined using the indentor test [23,24]. The area of the indentation was studied using an optical microscope to determine film failure. Platelet adhesion tests were performed on the DLC films. The quantity and morphology of adhered platelet were examined to study the surface thrombogenicity. Blood was obtained from a healthy adult volunteer. Whole blood was collected in an acid citrate dextrose medium. After centrifugation, red cells and platelets were sepa-
Y.X. Leng et al. / Surface Science 531 (2003) 177–184 5000 4500 4000
Intensity (arb.)
rated and platelet-rich plasma was obtained. The samples were immersed in the platelet-rich plasma and incubated at 37 °C for 30 or 120 min. After rinsing, fixing and critical point drying, the specimens with platelets on the surface were coated with a gold layer 10–20 nm thick and examined by optical microscopy and SEM. The quantity, morphology, aggregation, and pseudopodium of the adherent platelets were examined to investigate the surface thrombogenicity. Twenty fields of view were chosen at random to obtain good statistics.
Fig. 3 shows the XPS C1s peaks of DLC samples deposited at different argon flow rates. It can be seen that the peaks move to lower energy as the argon flow increases, implying that the sp2 bonded carbon atom fraction increases also, because the bonding energy of sp2 carbon atoms is of lower energy. Fitting of the DLC C1s peaks was per-
3000 2500 2000
#4 #3 #2
1500
500
#1 graphic
0 800
1000
(a)
Intensity (arb.)
1200
1400
1600
1800
2000
Raman shift (cm -1)
Data: LYX1 Model: Gauss Chi^2 = 1888.58414 y0 0 ±0 xc1 1529.52183 3000 w1 209.23482 A1 567155.57412 2500 xc2 1350.8766 w2 341.3242 A2 446905.751 2000
3.1. Raman spectroscopy
3.2. XPS C1s core level spectra analysis
3500
1000
3. Results and discussion
Fig. 2a shows Raman spectra of the DLC films deposited on Si(1 0 0). It is clear from the figure that there are no significant differences between the spectra. Compared with graphite, all the spectra show a broad peak at around 1530 cm1 and a lower frequency shoulder at approximately 1350 cm1 , commonly referred to as the G-band and Dband, respectively [25]. Fig. 2b shows a typical deconvoluted Raman spectrum fitted by Gaussians after subtracting the background. Based on the fitting parameters, the peak positions and the ratio of the integrated areas under the D and G peaks (ID =IG ) were obtained, and are summarized in Table 2. Our results show that a higher argon flow leads to a shift of the two peaks toward higher wave numbers and a slight increase of ID =IG ratios. Since the ID =IG ratio is related to the sp3 :sp2 ratio [26], it can be concluded that the sp3 content decreases with increasing argon flow––the film structure becomes more graphite-like with increasing argon flow.
179
#1 ±0.52623 ±1.94929 ±19554.20318 ±7.65821 ±5.88087 ±20075.82023
G
1500
D 1000 500 0 -500 800
1000
1200
1400
1600
1800
2000
-1
(b)
Raman shift(cm )
Fig. 2. (a) Raman spectra of DLC films fabricated at different argon flow and (b) typical Raman spectrum fitted by Gaussians.
Table 2 Experimental results from Raman spectroscopy Sample no.
Peak position (cm1 ) D-band
G-band
1 2 3 4
1350.9 1357.7 1360.8 1363.5
1529.5 1531.0 1530.8 1532.0
a
ID =IG a 0.79 0.87 0.86 0.9
Intensity ratio of D-band to G-band.
formed by using three components (each being a mixture of 80% Gaussian and 20% Lorentzian) and by approximating the contribution of background by the Shirley method [21,22]. The fittings for samples #1 and #4, deposited at the lowest and highest (respectively) argon partial pressures, are presented in Fig. 4. The first component at 285.3 eV
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0.9
40000
30000 25000 20000
#1 (0sccm)
15000
#2(3sccm)
10000
#4(13sccm)
5000
atom content(%)
Counts
2
sp
3
0.8
35000
0 275
sp
0.7 0.6 0.5 0.4 0.3 0.2
280
285
290
295
B.E. (eV)
0
2
4
6
8
10
12
14
argon inlet(sccm)
Fig. 3. XPS C1s peaks of DLC thin films obtained at different argon flow rates.
Fig. 5. sp2 and sp3 carbon atom content as determined from fitting of the XPS C1s peaks, as a function of argon flow rate.
corresponds to sp2 carbon atoms, while the second component at 286.1 eV corresponds to sp3 carbon atoms. A third peak of much smaller intensity at 287.4 eV has is also seen and is attributed to some C–O contamination formed at the surface of the samples due to air exposure. The fractional sp3 and sp2 carbon atom content was then determined as the ratio of the corresponding peak area to the total C1s peak area. Fig. 5 shows the sp3 and sp2 content as a function of argon flow. At 0 sccm, the fractional sp3 content is about 78%, and decreases to about 59% with increasing argon flow to 13 sccm. The XPS analysis for sp3 content is simply a semi-quantity method. We only use these data to compare relatively content of all the DLC films. Though we get the data of sp3 content, we are not assured the actual sp3 content in the DLC films. We only use these data for comparing the sp3 content between all the DLC films. So these results show that the sp3 fraction decreases as the argon flow increases. This is consistent with the results of the Raman spectra. 3.3. Nano-hardness
Fig. 4. Deconvolution of the XPS 1s peaks of DLC films deposited at (a) 0 and (b) 13 sccm.
The mechanical properties of DLC films are of great interest for its use as a coating material. However, it is hard to obtain reliable values of mechanical properties using conventional mechanical testers. For example, the conventional microhardness tester requires direct imaging of the
Y.X. Leng et al. / Surface Science 531 (2003) 177–184
indentations to obtain hardness, and large errors can be introduced in measurement of the diagonal lengths, especially when the indentations are small. Recently, a nano-hardness technique has been used to investigate mechanical properties of thin films. The nano-hardness tester uses an indenter tip driven into a specific site on the material by applying an increasing normal load. When a pre-set maximum value is reached, the normal load is reduced until partial or complete relaxation occurs. For each loading/unloading cycle, the applied load value is plotted with respect to the corresponding position of the indenter. The resulting load/displacement curves provide data specific to the mechanical nature of the material. In the work described here the maximum load was 0.5 mN and the loading/unloading rate was 1 mN/ min. In this way the hardness of the DLC films can be measured without ÔinterferenceÕ from the substrate. Fig. 6 shows the load/displacement curves for samples #1 and #4. It can be seen that the slope is approximately constant for the two samples from 0 to 0.5 mN load. This means that the sample is in the elastic deformation regime. The slope for sample #1 is greater than that for sample #4; this means that sample #1 has a higher YoungÕs modulus and hardness. The nano-hardness of samples #1 and #4 were 72.1 and 68 GPa, respectively. The results indicate that the nanohardness of the films decrease with increasing argon flow. This corresponds to the observed decrease of sp3 bonded carbon atoms with increasing argon flow.
#1
0.5
181
3.4. Adhesion tests The adhesion of the films was examined using the ‘‘Knoop’’ indentor test on Ti6Al4V substrates. Around the Knoop indentation crater, no peeling or flaking of the coating material was observed for 100, 200, 300, 500–1000 g loads. This means that the DLC films have good adhesion to the Ti6Al4V substrate. 3.5. Ball-on-disc tests Dry ball-on-disc tribological tests of DLC films coated on Ti6Al4V substrates were performed with a 0.98 N normal load, 10 cm/s rotation speed and 6 mm rotation radius, with 6-mm-diameter SiC balls as the counterpart material. The results showed that the wear resistance decreased when the argon flow increased. With increasing argon flow, the hardness of the films decreased and the films can more easily delaminate at 0.98 N normal load. Fig. 7 shows the friction coefficients for DLC coatings on Ti6Al4V substrates. It can be seen that the minimum coefficient is obtained for sample #1, where the initial coefficient of friction is 0.17 and the steady state coefficient of friction is 0.07. After 600,000 cycles, the friction coefficient for sample #1 was very stable and in the low range of 0.06– 0.09. SEM was used to observe the ball-ondisc wear track, shown in Fig. 8(a). After 600,000 cycles, the sample #1 film was not penetrated and the film had maintained its integrity. But for
#4
0.4
mN
0.3 0.2 0.1 0.0 0
2
4
6
8
10 12 14 16 18 20 22 24 nm
Fig. 6. Loading/unloading cycle of the DLC films.
Fig. 7. Friction coefficient of DLC films on Ti6Al4V substrate.
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3.6. Blood compatibility Adhered platelets are usually measured to assess hemocompatibility, but activation of platelets is a more important parameter indicating the interaction of blood with materials than the adhesion behavior. In our experiments, the hemocompatibility of the DLC films was studied. Deformation of the adhered platelets on the DLC surface was studied semi-quantitatively to evaluate the hemocompatibility [27], and the results are shown in Table 3. The number of adherent platelets, the degree of deformation (pseudopodium) and aggregation of the adhered platelets on the DLC film surface decreases with increasing argon flow. Fig. 9 displays the morphology of the adhered platelets on the DLC films. It can be observed that the number of platelets adhered on the #4 DLC film (Figs. 9e,f) is slightly less than for the #3 DLC film (Fig. 9c,d) and markedly less than that on #1 DLC film (Fig. 9a,b). The degree of deformation (pseudopodium) and aggregation of the adhered platelets are also less on #3 and #1 DLC films. The result show that the platelet adhesion behavior of the DLC films become better with increasing argon flow. The Raman and XPS results showed that sp3 :sp2 decreased with increasing argon flow. It
Fig. 8. SEM morphology of the wear tracks: (a) sample #1 after 600,000 rotation cycles and (b) sample #4 after 160,000 rotation cycles.
Table 3 Morphology of the platelets on the DLC films after 30 and 120 min incubation time Sample no.
sample #4, the friction coefficient was about 0.2 during the initial Ôrun-inÕ stage, then rapidly increased, correlating to a sudden and premature coating failure. Fig 8(b) shows the ball-on-disc wear tracks for sample #4 after 160,000 cycles. After about 75,000 cycles with a 0.98 N normal load, the friction coefficient of sample #4 sharply increased to 0.7 (showed as Fig. 7), with considerable localized spallation resulted from fatigue, and failed catastrophically almost immediately. All these results indicate that the wear resistance of the DLC films decreases with increasing argon flow, though still much better than for Ti alloy substrates.
1 2 3 4
30 min
III II II II
120 min IV III III III
Note: The degree of deformation of the adhesive platelets on the surface of the films can be categorized into four types [24]. I:II There are adhered platelets on the surface of the sample, but the platelets are not activated. I*: the number of adhered platelets is less; I**, more; I***, much more; II:I The adhered platelets are activated and begin to exhibit pseudopodium. II*: a portion of platelets exhibit pseudopodium; II**: many platelets show pseudopodium; III: Adhered platelets activated further and aggregated. III*: a portion of platelets aggregated; III**: many platelets aggregated. IV: Aggregated platelets forming net structures with fibrin. IV*: aggregated platelets net structure; IV**: erythrocytes adhered onto the net structure.
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Fig. 9. Morphology of adherent platelets (incubation time ¼ 30 min) on DLC films: (a) sample #1, (b) sample #1, (c) sample #3, (d) sample #3, (e) sample #4 and (f) sample #4.
can be inferred that the platelet adhesion behavior of our DLC films is influenced by the sp3 :sp2 ratio, and the hemocompatibility may improves with decreasing sp3 :sp2 . This result is basically consistent with the results by Chen, who synthesized DLC films with hydrogen by plasma immersion ion implantation-deposition [12]. The blood compatibility of the DLC films is influenced by the ratio of sp3 to sp2 , not by the absolute sp3 or sp2 content because the materials blood compatibility is affected by many factors such as surface energy, electrical properties, composition. The platelet adhesion behavior better is only one indication of
anti-thrombogenicity, the other aspects of antithrombogenicity, such as coagulation factors, should be done later on.
4. Conclusion Diamond-like carbon thin films were synthesized by pulsed vacuum arc plasma deposition. The Raman and XPS results showed that the sp3 content changed as a function of argon flow, with the sp3 carbon atom content decreasing as the argon flow increases. DLC films have good adhesion to
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Ti6Al4V substrates. Our results showed that the blood adhesion behavior of the DLC films is influenced by the sp3 :sp2 ratio. The platelet adhesion behavior of the DLC films increased with increasing argon flow. Unfortunately, the hardness and wear resistance decreased with increasing argon flow, though still much better than for Ti alloy substrates, which result from decreasing sp3 :sp2 . In order to fully exploit the full potential of DLC films as blood contacting biomaterials, the films must have proper sp3 :sp2 ratio, so as to achieve good blood compatibility and mechanical properties.
Acknowledgements The authors would like to thank Dr. Ian G. Brown for reviewing this paper. The work described in this paper was jointly supported by Chinese NSFC 39870199, National Research Fund for fundamental key Projects#G1999064706, High Tech Research and Development (863) Program 102-12-09-1.
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