Hydrogenated amorphous carbon films deposited on 316L stainless steel

Diamond & Related Materials 19 (2010) 533–536

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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d

Hydrogenated amorphous carbon films deposited on 316L stainless steel A. Kluba, D. Bociaga, M. Dudek ⁎ Institute of Materials Science and Engineering, Technical University of Lodz, Stefanowskiego 1/15, 90-924 Lodz, Poland

a r t i c l e

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Available online 11 January 2010 Keywords: RF PECVD Methane Hydrogenated amorphous carbon film 316L stainless steel

a b s t r a c t Research on hydrogen amorphous carbon films (a-C:H), which possess the diamond-like characteristic, has been stimulated for many years by need to simultaneously optimizing the mechanical, optical and biological properties, and by challenges related to the deposition of a-C:H films on medical implants. In the present work, we investigate the structure, optical and mechanical properties (hardness, elastic modulus and stress) of a-C:H films deposited on 316L stainless steel substrate by the radio frequency plasma enhanced chemical vapor deposition (RF PECVD). The negative self-bias voltages significantly influence on temperature of steel substrates during the deposition process and films properties. Specifically, the high energetic deposition leads also to stabilization of the sp2 content and thermally-activated relaxation in the stress of a-C:H films. Presented correlation between the obtained results and literature analysis let deem the Raman spectra as a good tool to control the properties of implants made of 316L stainless steel with a-C:H film for general use. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Among the metallic materials used for orthopedic devices, 316L stainless steel is one of the most commonly used. It is frequently used for temporary devices in orthopedic surgery because of its relative low cost and acceptable biocompatibility [1,2]. However, this alloy may corrode inside the body under certain conditions; for example in a highly stressed and oxygen-depleted region. Then releases iron in quantity to the neighboring tissues, inducing fibrosis around the implant [3,4]. Protective and bioactive coatings can inhibit ion diffusion in the stainless steel and induce a bioactive surface on the metallic prostheses. It is well documented that well adhering thin diamondlike carbon (DLC) films could significantly improve the 316L stainless steel performance and lifetime as a biomaterial [5,6]. DLC films have been defined as amorphous carbon with significant fraction of sp3 bonds and, in most cases, included hydrogen. The hydrogenated amorphous carbons (a-C:H) with the same sp3 and H content show different optical, electronic and mechanical properties according to the clustering of the sp2 phase [7–9]. The a-C:H films have been generally deposited by the radio frequency plasma enhanced chemical vapor deposition (RF PECVD) method, in which self-bias voltage of the substrate constitutes a dominant factor determining the structure and properties of fabricated films. The value of that voltage in a specific reactor is directly proportional to the rf power and inversely proportional to the pressure in the system [10,11]. Varying bias of the substrate does not

⁎ Corresponding author. Tel.: +48 42 631 32 63x33; fax: +48 42 636 67 90. E-mail address: [email protected] (M. Dudek).

only affect the phenomena taking place on the surface of the growing film but it also influences the fragmentation of the precursor molecules in the plasma [12]. In this work, a-C:H films have been deposited on the 316L stainless steel substrates by the RF PECVD system using pure methane. The effect of substrate biasing on the structural arrangement, optical and mechanical properties was investigated. 2. Experimental The a-C:H films were prepared on 316L stainless steel substrate with the help of radio frequency (rf, 13.56 MHz) plasma discharge in the vacuum system described in detail elsewhere [12–14]. The substrates were in the form of rectangle cut from thin stainless steel sheets (35 × 25 × 0.50 mm) and cylindrical shape (ϕ15 × 3.0 mm). Prior to the deposition process substrates were ultrasonically cleaned and washed in acetone. Further cleaning was obtained by ion sputtering in a high intensity rf plasma discharge for 160 s at − 700 V and 5 Pa of residual pressure. The deposition processes were performed at 40 and 50 sccm methane flow and various values of the negative self-bias voltage (from 300 to 600 V). After the deposition the film was smooth; the roughness was lower than 3 nm. Typically, the thickness of the deposited layers, as measured by surface profilometry, varied between 60 and 190 nm. The microstructures of the films were investigated using the JobinYvon T64000 Raman spectrometer equipped with an Ar+ ion laser (λ = 514 nm) arranged in a backscattering geometry. The power of the laser was up to 200 mW with a 1 μm spot diameter. The investigated wavenumber ranged from 700 cm− 1 to 1800 cm− 1. All measurements were carried out at room temperature and in air atmosphere.

0925-9635/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.12.020

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A. Kluba et al. / Diamond & Related Materials 19 (2010) 533–536

The spectroscopic ellipsometry (SE) measurements were made at 75° incidence (from substrate normal) using the M-2000 (J.A. Woollam Co.) rotating compensator ellipsometer on an ex-situ fixed angle base. The spectra extended from 245 to 1650 nm (5− 0.75 eV) and consisted of 610 wavelengths. The optical properties of a-C:H films were parametrized using two Tauc–Lorentz oscillators [15]. Once a sufficiently good fit was obtained, film thickness was fixed and reflective index (n) and extinction coefficient (k) were fitted point by point (at each wavelength individually) to the raw SE data. From these fitted values, the absorption coefficient (α) was calculated as a function of photon energy and the optical gap of each film was determined. The hardness value of films was determined by nanoindentation exploiting the continuous stiffness measurement (CSM) technique, using the Nano Indenter G200 (MTS Systems Corp.) equipped with the Berkovich pyramid, which is particularly suitable for the determination of the mechanical properties of thin films. The internal residual stress was obtained quantitatively by the bending beam method from well-known Stoney equation [16]: 2

σf =

1 Es hs 6 1−νs hf



1 1 − Rc Rs

 ð1Þ

where σf is the internal stress, Es is the Young's modulus of the substrate, νs is the Poisson's ratio of the substrate, hs and hf are the thickness of the substrate and film, respectively, Rs and Rc are the initial central deflection radius of the substrate and after the film deposition, respectively, measured by surface profilometry (Hammel Tester T1000 profilometer). For 316L stainless steel Es = 193 GPa, νs = 0.3 [17,18]. 3. Results and discussion The Raman spectra of films prepared at various self-bias voltages display two main features — the broad G peak as well as D peak. The G peak at around 1580–1600 cm− 1 has been assigned to the scattering by sp2 bonded carbon associated with graphite. The D peak at around 1350 cm− 1 is associated with the disorder due to the formation of sp3 bonded between graphite adjacent planes [19,20]. In order to obtain more quantitative information, the D and G peaks of films spectra were fitted by two Gaussian lines. Fig. 1 is the evolution, as a function of substrate bias, of the ratio of the D to G peak areas (ID/IG) calculated from the fitting procedure. In both cases deposition pressure of methane (correspond with 40 and 50 sccm methane flow), the ID/IG ratio increases from 0.67 ± 0.01 to 1.95 ± 0.02 for films deposited at

Fig. 1. Variation of the ID/IG factor determined by the fitting of the Raman spectra as function of self-bias voltage.

Fig. 2. Evolution of the G position as a function of self-bias voltage.

300 and 600 V, respectively. Fig. 2 shows that the G peak position (ωG) reveals a behavior similar to that ID/IG ratio; it grows up from 1537 to 1569 cm− 1. It is well know that the ID/IG ratio and ωG are a measure of the sp3 content for deposited a-C:H films. Thereby, to be based on reference where relationship between the Raman parameters and sp3 content directly measured by electron-energy-loss spectroscopy [7] and nuclear magnetic resonance [9] has been showed, we obtained that the content of sp3 decreases from 52 to 25% for films deposited at 300 and 600 V, respectively. For as-deposited a-C:H films there is a general relationship between optical gap and sp2 content (indirectly on the sp3 content) [21,22]. Fig. 3 shows ωG as function of the optical gap obtained from SE data. Similar correlation between the same parameters has been reported in reference [7,9]. Thus, the measurements of optical gap in a-C:H films confirm analysis of the sp3 content from the visible Raman spectra showed above. Smaller optical gaps (Fig. 3) corresponded to larger sp2 clusters, which in turn have more graphite-like Raman spectra. The main factor reducing the sp2 clusters in a-C:H films is content of H which decreases with bias voltage increase [9,23]. Thus, higher sp3 content is achieved at low bias (−300 V) considered in the present work. The total hydrogen content critically determines a-C:H film structure (most sp3 sites are bonded to hydrogen), and therefore

Fig. 3. G position versus optical gap for a-C:H film.

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bias). It is interesting to note that the stress values obtained with the lower substrate bias (b450 V) are closely zero. The lower stress values in the region between 300 and 400 V of substrate bias are achieved when the G peak position ωG remain below 1550 cm− 1. This indicated that the high frequency of the G peak is due to its high compressive macroscopic stress. On the other hand, the higher value ωG suggests stabilization of sp2 carbon phase. It means that a high energetic deposition leads to increase of the sp2 content and thus reduces the compressive stress (Fig. 5), what was reported for a-C:H films deposited by PE CVD method [23,29]. 4. Conclusions

Fig. 4. Variation of the refractive index (at 500 nm) as a function of self-bias voltage.

the physical properties of the films. Fig. 4 shows value of reflective index, measured at 550 nm, as a function of substrate bias, as deduced from analysis of the spectroscopic ellipsometry results. These values are fluctuated between 1.73 and 1.93, thus indicate that our a-C:H films have a density typically reported [23–25] what is confirmed by hardness measurement (12.3–16.1 GPa). These results indicate that the microstructure of carbon film is more porous when film is deposited at higher substrate bias. This behavior can be explained as a result of the competition between deposition and etching processes; the higher substrate bias promote to the etching process activated among other things by increase of substrate temperature [26,27]. In the last part of the work we investigated the stress of the a-C:H films deposited on 316L stainless steel substrate. Fig. 5 shows value of stress as a function of substrate bias, as deduced from Stoney Eq. (1). The behavior of the internal stress exhibits a relatively sharp peak in the region between 450 and 550 V with maximum compressive stress values by the 6.0 to 6.4 GPa range. These values are comparable to those found in a-C:H films deposited by PE CVD methods [23,28,29], although are differences in the position of the maximum of compressive stress (maximum was observed at lower substrate

This work has demonstrated that increased (absolute) value of self-bias voltage of rf powered electrode leads to stabilization of the sp2 content and thermally-activated relaxation in the stress of a-C:H films deposited on 316L stainless steel substrate. The behavior of the internal stress exhibits a relatively sharp peak in the region between 450 and 550 V with maximum compressive stress values by the 6.0 to 6.4 GPa range. These values are comparable to those found in a-C:H films commonly deposited on silicon substrate by PE CVD methods, although the position of the maximum of compressive stress was observed at higher substrate bias. The analysis presented in this paper has demonstrated that exists the correlation between structure and optical properties of the a-C:H films deposited on 316L stainless steel substrate observed prior for silicon substrate. We would like to point out that the visible Raman spectroscopy (which was presented) is reliably tools to predict the properties of these films without any additional (in particular expensive) measurements. Moreover, most of these tests are feasible in the case of films deposited on flat silicon wafer but unfeasible for films deposited on implant made of 316L stainless steel with a complicated shape. This is the most important that distribution of substrate temperature during the RF PECVD process is not homogeneous and affects on the structure and films properties. Acknowledgments We thank Prof. S. Mitura, Dr. J. Klemberg-Sapieha and Dr. O. Zabeida for stimulating discussions, Dr. M. Kozanecki for Raman measurements and Dr. W. Szymański for nanoindenter measurements. These works were financed by Polish Ministry of Science and Higher Education (Grant no. 3T08C06129). References

Fig. 5. Residual stress as a function of self-bias voltage for films deposited with 40 sccm methane flow.

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