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Wettability and corrosion tests of diamond films grown on Ti6Al4V alloy
Surface & Coatings Technology 194 (2005) 271 – 275 www.elsevier.com/locate/surfcoat
Wettability and corrosion tests of diamond films grown on Ti6Al4V alloy Adriana F. Azevedoa,b,*, Evaldo J. Corata, N.G. Ferreirac, Vladimir J. Trava-Airoldia a b
Instituto Nacional de Pesquisas Espaciais-INPE-Sa˜o Jose´ dos Campos, SP, Brazil Faculdade de Engenharia Quı´mica de Lorena-Faenquil/Demar-Lorena, SP, Brazil c Centro Te´cnico Aeroespacial (CTA)-Sa˜o Jose´ dos Campos, SP, Brazil Received 26 December 2003; accepted in revised form 6 October 2004 Available online 14 November 2004
Abstract Diamond thin films were successfully deposited on both sides of jetted substrates Ti6Al4V alloy, without intermediate layers deposition, by using an enhanced 2.45 GHz microwave-assisted technique. This system is based on discharge surface-wave guide, Surfatron system. It was used as a bias-enhanced nucleation (BEN) applied between the plasma shell and the substrate, reaching a nucleation density around 5109 parts cm2 in just 5 min. These films showed a good quality and a total residual stress around 2.4 GPa that was evaluated by Raman scattering spectroscopy. In spite of the high residual stress, the films adhesion on substrate was excellent, even when a load of 250 kgf was applied. Wettability of these films designed a small hydrophobicity that is very similar to those of other carbon structures used as biological implants. The surface energy value of 50.5 mJ m2 indicated that is possible to get a good tissue adhesion on diamond films. These films were also exposed to various biological fluids, including isotonic NaCl and Ringer’s solution for 1 month and acid–water solution for 2 months. Diamond surface chemical stability was analyzed from films micrographs by scanning electron microscopy (SEM). The results revealed that the diamond films surfaces were not degraded by environmental agents. D 2004 Elsevier B.V. All rights reserved. Keywords: Wettability; Diamond film; Nucleation; Titanium alloy; Adhesion
1. Introduction Titanium alloys have demonstrated physical, mechanical and chemical properties for many structural applications [1]. For these applications, CVD diamond coatings become a goal for solving their low wear resistance and high fatigue [2,3]. The main disadvantage of this system, diamond on titanium alloys, is its poor film adhesion that occurs partially due to the thermal expansion mismatch between diamond and substrate [2–4]. The adhesion study has already been explored in previous works [5,6] and specific results for such a films are shown elsewhere [7], where the indentation * Corresponding author. Mailing address: Instituto Nacional de Pesquisas Espaciais-INPE, Laborato´rio Associado de Sensores e Materiais-LAS, Av dos Astronautas, 1758, C. P. 515-12201-970, Sa˜o Jose´ dos Campos, SP, Brazil. Tel.: +55 12 3945 6905; fax: +55 12 3945 6717. E-mail address: [email protected] (A.F. Azevedo). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.007
tests showed that diamond films can support a load of 250 kgf without delamination. Diamond films on Ti6Al4V alloy have interesting properties for many applications, mainly the biological applications [8,9]. In agreement with the recent literature and other authors [10,11], the biomaterials for human body should satisfy the following requirements: wettability, biostability and chemical stability. Besides, implant system must present excellent adhesion and very good mechanical characteristics. Pinzari et al. [12] and Djemia et al. [13] investigated the wettability of diamond films grown on silicon and Ti6Al4V alloy in different growth conditions, respectively. The water contact angles (h) varied from 758 to 968 for films deposited on silicon substrate and from 598 to 648 for films deposited on Ti6Al4V alloy. Both of them observed that wettability change is due to the surface contaminant removal, graphitic sp2 carbon and hydrogen inclusions, which depends on the
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A.F. Azevedo et al. / Surface & Coatings Technology 194 (2005) 271–275
deposition conditions and surface topography (roughness, morphology, etc.). In a more detailed study, Ostrovskaya et al. [14], based on the wettability data of auto-sustained diamond films, studied the variation in diamond films surface tension induced by different methods of surface treatment (hydrogenation and oxidation). They concluded that hydrogenation of the diamond films surface increases the wettability angle to 938 as compared with diamond surface oxidized by air. However, it was, in a recent work, that Kaibara et al. [11] showed the importance of the wettability control for biological applications. They analyzed, in terms of wettability on the nanoscale, the hydrogen-terminated and oxidized diamond surfaces. They concluded that the difference in wettability between diamond surfaces could be applied in the fields of medicine and biotechnology, i.e. for the fabrication of DNA or protein tips. All these studies indicated the importance of wettability and its relationship with the studies that it comes being developed for biological application. The purpose of this study was to evaluate if CVD diamond thin films deposited on jetted Ti6Al4V substrates, by using BEN during 5 min with a deposition time of 5 h, could be used for biological application. There are no reports in the literature with similar results by using these experimental conditions, substrate pretreatments and film growth, for biological application. Besides, these films are very adherent to substrate which is one of the conditions demanded to be able use them as biomaterial.
2. Experimental procedure Films were grown by using the Surfatron system that has been described elsewhere [6,15]. Basically, the surfaguide set is composed of the launcher and two coaxial dielectric tubes. The inner tube is made of quartz and used as a discharge tube. This tube has an abrupt transition that is essential to finish the surface-wave (SW) propagation and create expanded hemispherical plasma shell slightly bellow the transition. The plasma shell is uniform with a thickness of few millimeters, and has high energy density. The substrate is placed close to this plasma shell, 0.5 mm approximately. Depositions were carried out on both sides of Ti6Al4V 11 cm2 and 1.2 mm thick substrate. We have taken 10 samples that were mechanically pretreated by glass microspheres jet in order to improve the film adhesion. This treatment resulted in a substrate surface roughness lower than 0.3 Am, obtained by Surface Perfilometer, model Alpha-Step 500 of TERCOR Instruments. For all experiments, the samples were cleaned with ethanol and prepared by ultrasonic hexane bath with 0.25 Am diamond powder during 60 min. Substrate temperature, T dep, was kept at 700 8C with a gas flow rate of 100 sccm and a pressure inside the reactor
of 27 Torr by setting the microwave power at 2.5 kW for all experiments. Negative bias of 400 V was applied during 5 min (t b) by using a gas mixture of 8.0% methane in hydrogen. Then, the films were grown from a gas mixture of 0.7% methane in hydrogen for a deposition time, t dep, of 150 min, separately, in each side of the substrate. After the deposition on the first side of samples, the microwave power supply started to be decreased and the mass flow controllers were turned off. So, the substrate temperature was decreased around 40 min. After 1 h inside of the reactor turned off completely, the substrate reaches the environment temperature and the sample is placed in its backside for the second deposition. It was not necessary to do a new cleaning in the sample because the growth atmosphere is very clean. All diamond films showed the same characteristics, such as high density nucleation, good adhesion on Ti6Al4V alloy and growth rate around 0.7 Am h1. These films grow in the format of agglomerates grains due to substrates superficial roughness and growth temperature. They presented a roughness (R a) of 0.5 Am that it was measured by nanoscope atomic force microscopy (AFM) system. The nucleation density as a function of BEN parameters, deposition time and sample preparation are subjects of another submitted work [16].
3. Results and discussions 3.1. Raman spectrum The film quality was evaluated by Renishaw Raman spectrometer with a 514.5 nm Ar-ion laser. In order to improve the Raman data statistics, a large number of spectra were recorded from each sample. Five points in a sample area were chosen and the spectra were scanned five times on each point of such area. The Raman spectra of diamond coatings grown on Ti6Al4V alloy are very similar for both sample sides and one of them is depicted in Fig. 1. The diamond films peaks values were around 1339F2 cm1. By considering the natural diamond of 1332 cm1, this Raman shift corresponds to a total residual stress around 2.4F0.7 GPa [3,4,6]. Although this stress value seems to be significant, it is lower when compared with the thermal stress of 6.0 GPa for this growth temperature. Raman band centered at 1550 cm1 is attributed mainly to amorphous sp2-bond. As the film is very thin and a high methane concentration was used during BEN step, the very wide Raman band is due to incorporation of non-diamond carbon phase in the interface. 3.2. Contact angle and surface tension measurements The contact angle (h) measurements have been performed in order to evaluate films surface tension (c) in liquids whose surface tension components are known [12]. The films surface tension components were calculated by
A.F. Azevedo et al. / Surface & Coatings Technology 194 (2005) 271–275
273
Fig. 1. Raman spectrum of diamond film deposited on Ti6Al4V for 150 min.
Fowkes’ equation [9,10] using measured values of a wetting angle: qffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffi ð1Þ cdl cdf þ cpl cpf Wa ¼ cl ð1 þ coshÞ ¼ 2 where Wa is the adhesion work and cpl , cdl , cpf and cdf are the polar and dispersed components of the liquid and solid phases, respectively. Thus, Fowkes assumed that the surface tension (c i) could be determinate by superposition of polar and dispersed components: ci ¼ cpi þ cdi
ð2Þ
The contact angle measurements were carried out using a Rame´–Hart contact angle goniometer (Model 100-00) by the sessile drop method at 25 8C in an environmental chamber. The liquids used were deionised water, as polar liquid, and di-iodomethane, as apolar liquid, those were laid onto the solid surfaces by a Gilmont micro-syringe. This procedure consisted to deposit five droplets at different regions for two samples of diamond film and two samples of Ti6Al4V substrate, to get reliable contact angle data. This water contact angle is correlated with the hydrophobicity that should be higher than 908 for keeping this property. The contact angles measured on diamond film were h=938 for water and h=68 for di-iodomethane. The water contact angle indicates a small hydrophobicity that is very similar to those of other carbon structures as graphite, carbon fibers or DLC [12,14]. This occurs because the polycrystalline CVD diamond may include on the surface sp2-bonded carbon and hydrogen inclusions that decrease surface energy, giving more hydrophobic films. So, its
wettability behavior is sensitively different of value found for natural diamond [14]. For Ti6Al4V substrate, the contact angles were h=748 for water and h=438 for di-iodomethane. This result shows that Ti6Al4V surface has hydrophilic characteristic as was shown by Ponsonnet et al. [17]. They have found water contact angle of 508 for titanium-polished surface with a roughness lower than 0.015 Am. The difference between these values of contact angle may be attributed to titanium surface roughness. The polar and dispersion components and surface tension have been calculated to be c p=2.1 mJ m2, c d=48.4 mJ m2 and c=50.5 mJ m2 for diamond film and c p=11.6 mJ m2, c d=36.6 mJ m2 and c=48.2 mJ m2 for Ti6Al4V alloy, respectively. As it can be seen, the surface tension values of diamond film and substrate are very close and indicate that is possible to get a good tissue adhesion on diamond film because the c values were higher than 45 mJ m2 [10]. 3.3. Preclinical investigations The recommended experimental methods for qualifying a material biocompatibility are the analyses of its tendency to degradation, i.e. its interactions with the surrounding media under the real conditions of exposure as well as the resulting deterioration of both components. The interactions of the implants with the body cells, the products of the corrosion, and of the wear debris can have adverse effects on the body and on the implants. These effects can include cellular damage, infections and blood coagulation. The assessment of the degradation tendency of biomaterials should consist of the some examination stages, which are characterized by a decreasing amount of the materials
274
A.F. Azevedo et al. / Surface & Coatings Technology 194 (2005) 271–275
involved and some chemical reaction. These stages are: (1) in vitro corrosion and wear examination in an inorganic media and soon after in a mixed inorganic–organic media; (2) examinations using cell and organ cultures; and (3) in vivo examinations based on implantation in animal organisms [10]. Investigations using purely inorganic testing media represent the simplest tests and at least provide a meaningful degree of testing the corrosion or degradation behavior of materials that will be used as implants. In general, corrosion process on these materials can be described as destruction of
the material based on electrochemical process and starting on the surface. On closer examination, one must distinguish between uniform corrosion, uniformly attacking the whole surface and localized corrosion. Previous works [8,10] have also used the solutions described below in these tests because they simulate fluids present in the human body. The tests used were: One month immersed in a isotonic NaCl solution (9 g l1 NaCl); ! One month immersed in a Ringer’s physiological solution. It was used in an acidic version (pH=2.5) to simulate decreases in pH caused by inflammation process (8 g l1 NaCl, 0.2 g l1 CaCl2, 0.2 g l1 KCl, 1.0 g l1 NaHCO3 and HCl to decrease the pH); ! Two months in the weak acid–water solution (2% HCl).
!
After this period, the samples were washed in water by ultrasonic bath. The material exposed in these types of liquids used to present pitting corrosion. This corrosion is a strongly localized type of attack, which leads to the formation of deep and narrow cavities. So, a detailed study by SEM was done in different magnifications and points of each sample with the objective of visualizing some pitting corrosion. The images revealed that the films are unchanged, without pitting corrosion and they maintained the same characteristics of as-deposited films, independent of the fluids used in tests. Fig. 2a–c shows the images of diamond films surface for samples immersed in isotonic NaCl, Ringer’s physiological and acid–water solutions, respectively. These tests confirmed that the diamond films deposited on jetted Ti6Al4V substrates have a very good chemical inertia to aggressive liquid and they can be used in applications that the materials need to have a good resistance to corrosion and chemical attack.
4. Conclusion
Fig. 2. SEM images of diamond films deposited on Ti6Al4V alloy after (a) 1 month immersed in an isotonic NaCl solution, (b) 1 month immersed in a Ringer’s physiological solution, and (c) 2 months in the 2% HCl.
Diamond films with high-density nucleation were successfully deposited on both sides of Ti6Al4V alloy and in spite of the high total residual stress, 2.4 GPa, they presented a good adherence to the substrate. It has been demonstrated that our films have a small hydrophobicity around h=938 for water contact angle. This value is very similar to the values of auto-sustained films obtained by Ostrovskaya et al. and to other carbon structures that are used in biological applications. Also, the results obtained from chemical resistance tests showed that the diamond films were not degraded by aggressive environmental and, thus, they are a promising biomaterial. In order to conclude this study, more investigation will be done, including examinations using cell, organ cultures and, in the future, in vivo tests based on implantation in animal organisms.
A.F. Azevedo et al. / Surface & Coatings Technology 194 (2005) 271–275
Acknowledgements The authors are very grateful to FAPESP and CNPq for financial support, to Mr. Jognes Panasiewicz Junior (INPE) for SEM analyses and to Mrs. Elidiane C. Rangel (Faculdade de Engenharia de Guaratingueta´) for wettability tests.
References [1] M. Rahman, Y.S. Wong, A.R. Zareena, JSME International Journal. Series C, Mechanical Systems, Machine Elements and Manufacturing 46 (1) (2003) 107. [2] M.I. de Barros, V. Serin, L. Vandenbulcke, G. Botton, P. Andreazza, M.W. Phaneuf, Diamond and Related Materials 11 (2002) 1544. [3] Y. Fu, H. Du, C.Q. Sun, Thin Solid Films 424 (2003) 107. [4] M.I. de Barros, L. Vandenbulcke, J.J. Blechet, Wear 249 (2001) 68. [5] A.F. Azevedo, V.J. Trava-Airoldi, E.J. Corat, N.F. Leite, Diamond and Related Materials 11 (2002) 550. [6] A.F. Azevedo, E.J. Corat, N.F. Leite, N.G. Ferreira, V.J. TravaAiroldi, Materials Research 6 (1) (2003) 51.
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[7] A.F. Azevedo, E.J. Corat, N.G. Ferreira, V.J. Trava-Airoldi, Journal of Metastable and Nanocrystalline Materials 20–21 (2004) 753. [8] S. Mitura, A. Mitura, P. Niedzielki, P. Couvrat, Chaos, Solitons and Fractals 10 (12) (1999) 2165. [9] S. Rupprecht, A. Bloch, S. Rosiwal, F.W. Neukam, J. Wiltfang, International Journal of Oral and Maxillofacial Implants 17 (6) (2002) 778. [10] J.A. Helsen, H.J. Breme, Metals as Biomaterials, John Wiley & Sons, New York, 1998. [11] Y. Kaibara, K. Sugata, M. Tachiki, H. Umezawa, H. Kawarada, Diamond and Related Materials 12 (2003) 560. [12] F. Pinzari, P. Ascarelli, E. Cappelli, G. Mattei, R. Giorgi, Diamond and Related Materials 10 (2001) 781. [13] P. Djemia, F. Te´tard, F. Ganot, C. Met, M.I. de Barros, L. Vandenbulcke, Surface and Coatings Technology 151–152 (2002) 170. [14] L. Ostrovskaya, V. Perevertailo, V. Ralchenko, A. Dementjev, O. Loginova, Diamond and Related Materials 11 (2002) 845. [15] C.F.M. Borges, L. St-Onge, M. Moisan, A. Gicquel, Thin Solid Films 274 (1996) 3. [16] A.F. Azevedo, E.J. Corat, N.G. Ferreira, V.J. Trava-Airoldi, Materials Letters, submitted for publication (2004). [17] L. Ponsonnet, K. Reybier, N. Jaffrezic, V. Comte, C. Lagneau, M. Lissac, C. Martelet, Materials Science and Engineering C 23 (2003) 551.
Wettability and corrosion tests of diamond films grown on Ti6Al4V alloy Adriana F. Azevedoa,b,*, Evaldo J. Corata, N.G. Ferreirac, Vladimir J. Trava-Airoldia a b
Instituto Nacional de Pesquisas Espaciais-INPE-Sa˜o Jose´ dos Campos, SP, Brazil Faculdade de Engenharia Quı´mica de Lorena-Faenquil/Demar-Lorena, SP, Brazil c Centro Te´cnico Aeroespacial (CTA)-Sa˜o Jose´ dos Campos, SP, Brazil Received 26 December 2003; accepted in revised form 6 October 2004 Available online 14 November 2004
Abstract Diamond thin films were successfully deposited on both sides of jetted substrates Ti6Al4V alloy, without intermediate layers deposition, by using an enhanced 2.45 GHz microwave-assisted technique. This system is based on discharge surface-wave guide, Surfatron system. It was used as a bias-enhanced nucleation (BEN) applied between the plasma shell and the substrate, reaching a nucleation density around 5109 parts cm2 in just 5 min. These films showed a good quality and a total residual stress around 2.4 GPa that was evaluated by Raman scattering spectroscopy. In spite of the high residual stress, the films adhesion on substrate was excellent, even when a load of 250 kgf was applied. Wettability of these films designed a small hydrophobicity that is very similar to those of other carbon structures used as biological implants. The surface energy value of 50.5 mJ m2 indicated that is possible to get a good tissue adhesion on diamond films. These films were also exposed to various biological fluids, including isotonic NaCl and Ringer’s solution for 1 month and acid–water solution for 2 months. Diamond surface chemical stability was analyzed from films micrographs by scanning electron microscopy (SEM). The results revealed that the diamond films surfaces were not degraded by environmental agents. D 2004 Elsevier B.V. All rights reserved. Keywords: Wettability; Diamond film; Nucleation; Titanium alloy; Adhesion
1. Introduction Titanium alloys have demonstrated physical, mechanical and chemical properties for many structural applications [1]. For these applications, CVD diamond coatings become a goal for solving their low wear resistance and high fatigue [2,3]. The main disadvantage of this system, diamond on titanium alloys, is its poor film adhesion that occurs partially due to the thermal expansion mismatch between diamond and substrate [2–4]. The adhesion study has already been explored in previous works [5,6] and specific results for such a films are shown elsewhere [7], where the indentation * Corresponding author. Mailing address: Instituto Nacional de Pesquisas Espaciais-INPE, Laborato´rio Associado de Sensores e Materiais-LAS, Av dos Astronautas, 1758, C. P. 515-12201-970, Sa˜o Jose´ dos Campos, SP, Brazil. Tel.: +55 12 3945 6905; fax: +55 12 3945 6717. E-mail address: [email protected] (A.F. Azevedo). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.007
tests showed that diamond films can support a load of 250 kgf without delamination. Diamond films on Ti6Al4V alloy have interesting properties for many applications, mainly the biological applications [8,9]. In agreement with the recent literature and other authors [10,11], the biomaterials for human body should satisfy the following requirements: wettability, biostability and chemical stability. Besides, implant system must present excellent adhesion and very good mechanical characteristics. Pinzari et al. [12] and Djemia et al. [13] investigated the wettability of diamond films grown on silicon and Ti6Al4V alloy in different growth conditions, respectively. The water contact angles (h) varied from 758 to 968 for films deposited on silicon substrate and from 598 to 648 for films deposited on Ti6Al4V alloy. Both of them observed that wettability change is due to the surface contaminant removal, graphitic sp2 carbon and hydrogen inclusions, which depends on the
272
A.F. Azevedo et al. / Surface & Coatings Technology 194 (2005) 271–275
deposition conditions and surface topography (roughness, morphology, etc.). In a more detailed study, Ostrovskaya et al. [14], based on the wettability data of auto-sustained diamond films, studied the variation in diamond films surface tension induced by different methods of surface treatment (hydrogenation and oxidation). They concluded that hydrogenation of the diamond films surface increases the wettability angle to 938 as compared with diamond surface oxidized by air. However, it was, in a recent work, that Kaibara et al. [11] showed the importance of the wettability control for biological applications. They analyzed, in terms of wettability on the nanoscale, the hydrogen-terminated and oxidized diamond surfaces. They concluded that the difference in wettability between diamond surfaces could be applied in the fields of medicine and biotechnology, i.e. for the fabrication of DNA or protein tips. All these studies indicated the importance of wettability and its relationship with the studies that it comes being developed for biological application. The purpose of this study was to evaluate if CVD diamond thin films deposited on jetted Ti6Al4V substrates, by using BEN during 5 min with a deposition time of 5 h, could be used for biological application. There are no reports in the literature with similar results by using these experimental conditions, substrate pretreatments and film growth, for biological application. Besides, these films are very adherent to substrate which is one of the conditions demanded to be able use them as biomaterial.
2. Experimental procedure Films were grown by using the Surfatron system that has been described elsewhere [6,15]. Basically, the surfaguide set is composed of the launcher and two coaxial dielectric tubes. The inner tube is made of quartz and used as a discharge tube. This tube has an abrupt transition that is essential to finish the surface-wave (SW) propagation and create expanded hemispherical plasma shell slightly bellow the transition. The plasma shell is uniform with a thickness of few millimeters, and has high energy density. The substrate is placed close to this plasma shell, 0.5 mm approximately. Depositions were carried out on both sides of Ti6Al4V 11 cm2 and 1.2 mm thick substrate. We have taken 10 samples that were mechanically pretreated by glass microspheres jet in order to improve the film adhesion. This treatment resulted in a substrate surface roughness lower than 0.3 Am, obtained by Surface Perfilometer, model Alpha-Step 500 of TERCOR Instruments. For all experiments, the samples were cleaned with ethanol and prepared by ultrasonic hexane bath with 0.25 Am diamond powder during 60 min. Substrate temperature, T dep, was kept at 700 8C with a gas flow rate of 100 sccm and a pressure inside the reactor
of 27 Torr by setting the microwave power at 2.5 kW for all experiments. Negative bias of 400 V was applied during 5 min (t b) by using a gas mixture of 8.0% methane in hydrogen. Then, the films were grown from a gas mixture of 0.7% methane in hydrogen for a deposition time, t dep, of 150 min, separately, in each side of the substrate. After the deposition on the first side of samples, the microwave power supply started to be decreased and the mass flow controllers were turned off. So, the substrate temperature was decreased around 40 min. After 1 h inside of the reactor turned off completely, the substrate reaches the environment temperature and the sample is placed in its backside for the second deposition. It was not necessary to do a new cleaning in the sample because the growth atmosphere is very clean. All diamond films showed the same characteristics, such as high density nucleation, good adhesion on Ti6Al4V alloy and growth rate around 0.7 Am h1. These films grow in the format of agglomerates grains due to substrates superficial roughness and growth temperature. They presented a roughness (R a) of 0.5 Am that it was measured by nanoscope atomic force microscopy (AFM) system. The nucleation density as a function of BEN parameters, deposition time and sample preparation are subjects of another submitted work [16].
3. Results and discussions 3.1. Raman spectrum The film quality was evaluated by Renishaw Raman spectrometer with a 514.5 nm Ar-ion laser. In order to improve the Raman data statistics, a large number of spectra were recorded from each sample. Five points in a sample area were chosen and the spectra were scanned five times on each point of such area. The Raman spectra of diamond coatings grown on Ti6Al4V alloy are very similar for both sample sides and one of them is depicted in Fig. 1. The diamond films peaks values were around 1339F2 cm1. By considering the natural diamond of 1332 cm1, this Raman shift corresponds to a total residual stress around 2.4F0.7 GPa [3,4,6]. Although this stress value seems to be significant, it is lower when compared with the thermal stress of 6.0 GPa for this growth temperature. Raman band centered at 1550 cm1 is attributed mainly to amorphous sp2-bond. As the film is very thin and a high methane concentration was used during BEN step, the very wide Raman band is due to incorporation of non-diamond carbon phase in the interface. 3.2. Contact angle and surface tension measurements The contact angle (h) measurements have been performed in order to evaluate films surface tension (c) in liquids whose surface tension components are known [12]. The films surface tension components were calculated by
A.F. Azevedo et al. / Surface & Coatings Technology 194 (2005) 271–275
273
Fig. 1. Raman spectrum of diamond film deposited on Ti6Al4V for 150 min.
Fowkes’ equation [9,10] using measured values of a wetting angle: qffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffi ð1Þ cdl cdf þ cpl cpf Wa ¼ cl ð1 þ coshÞ ¼ 2 where Wa is the adhesion work and cpl , cdl , cpf and cdf are the polar and dispersed components of the liquid and solid phases, respectively. Thus, Fowkes assumed that the surface tension (c i) could be determinate by superposition of polar and dispersed components: ci ¼ cpi þ cdi
ð2Þ
The contact angle measurements were carried out using a Rame´–Hart contact angle goniometer (Model 100-00) by the sessile drop method at 25 8C in an environmental chamber. The liquids used were deionised water, as polar liquid, and di-iodomethane, as apolar liquid, those were laid onto the solid surfaces by a Gilmont micro-syringe. This procedure consisted to deposit five droplets at different regions for two samples of diamond film and two samples of Ti6Al4V substrate, to get reliable contact angle data. This water contact angle is correlated with the hydrophobicity that should be higher than 908 for keeping this property. The contact angles measured on diamond film were h=938 for water and h=68 for di-iodomethane. The water contact angle indicates a small hydrophobicity that is very similar to those of other carbon structures as graphite, carbon fibers or DLC [12,14]. This occurs because the polycrystalline CVD diamond may include on the surface sp2-bonded carbon and hydrogen inclusions that decrease surface energy, giving more hydrophobic films. So, its
wettability behavior is sensitively different of value found for natural diamond [14]. For Ti6Al4V substrate, the contact angles were h=748 for water and h=438 for di-iodomethane. This result shows that Ti6Al4V surface has hydrophilic characteristic as was shown by Ponsonnet et al. [17]. They have found water contact angle of 508 for titanium-polished surface with a roughness lower than 0.015 Am. The difference between these values of contact angle may be attributed to titanium surface roughness. The polar and dispersion components and surface tension have been calculated to be c p=2.1 mJ m2, c d=48.4 mJ m2 and c=50.5 mJ m2 for diamond film and c p=11.6 mJ m2, c d=36.6 mJ m2 and c=48.2 mJ m2 for Ti6Al4V alloy, respectively. As it can be seen, the surface tension values of diamond film and substrate are very close and indicate that is possible to get a good tissue adhesion on diamond film because the c values were higher than 45 mJ m2 [10]. 3.3. Preclinical investigations The recommended experimental methods for qualifying a material biocompatibility are the analyses of its tendency to degradation, i.e. its interactions with the surrounding media under the real conditions of exposure as well as the resulting deterioration of both components. The interactions of the implants with the body cells, the products of the corrosion, and of the wear debris can have adverse effects on the body and on the implants. These effects can include cellular damage, infections and blood coagulation. The assessment of the degradation tendency of biomaterials should consist of the some examination stages, which are characterized by a decreasing amount of the materials
274
A.F. Azevedo et al. / Surface & Coatings Technology 194 (2005) 271–275
involved and some chemical reaction. These stages are: (1) in vitro corrosion and wear examination in an inorganic media and soon after in a mixed inorganic–organic media; (2) examinations using cell and organ cultures; and (3) in vivo examinations based on implantation in animal organisms [10]. Investigations using purely inorganic testing media represent the simplest tests and at least provide a meaningful degree of testing the corrosion or degradation behavior of materials that will be used as implants. In general, corrosion process on these materials can be described as destruction of
the material based on electrochemical process and starting on the surface. On closer examination, one must distinguish between uniform corrosion, uniformly attacking the whole surface and localized corrosion. Previous works [8,10] have also used the solutions described below in these tests because they simulate fluids present in the human body. The tests used were: One month immersed in a isotonic NaCl solution (9 g l1 NaCl); ! One month immersed in a Ringer’s physiological solution. It was used in an acidic version (pH=2.5) to simulate decreases in pH caused by inflammation process (8 g l1 NaCl, 0.2 g l1 CaCl2, 0.2 g l1 KCl, 1.0 g l1 NaHCO3 and HCl to decrease the pH); ! Two months in the weak acid–water solution (2% HCl).
!
After this period, the samples were washed in water by ultrasonic bath. The material exposed in these types of liquids used to present pitting corrosion. This corrosion is a strongly localized type of attack, which leads to the formation of deep and narrow cavities. So, a detailed study by SEM was done in different magnifications and points of each sample with the objective of visualizing some pitting corrosion. The images revealed that the films are unchanged, without pitting corrosion and they maintained the same characteristics of as-deposited films, independent of the fluids used in tests. Fig. 2a–c shows the images of diamond films surface for samples immersed in isotonic NaCl, Ringer’s physiological and acid–water solutions, respectively. These tests confirmed that the diamond films deposited on jetted Ti6Al4V substrates have a very good chemical inertia to aggressive liquid and they can be used in applications that the materials need to have a good resistance to corrosion and chemical attack.
4. Conclusion
Fig. 2. SEM images of diamond films deposited on Ti6Al4V alloy after (a) 1 month immersed in an isotonic NaCl solution, (b) 1 month immersed in a Ringer’s physiological solution, and (c) 2 months in the 2% HCl.
Diamond films with high-density nucleation were successfully deposited on both sides of Ti6Al4V alloy and in spite of the high total residual stress, 2.4 GPa, they presented a good adherence to the substrate. It has been demonstrated that our films have a small hydrophobicity around h=938 for water contact angle. This value is very similar to the values of auto-sustained films obtained by Ostrovskaya et al. and to other carbon structures that are used in biological applications. Also, the results obtained from chemical resistance tests showed that the diamond films were not degraded by aggressive environmental and, thus, they are a promising biomaterial. In order to conclude this study, more investigation will be done, including examinations using cell, organ cultures and, in the future, in vivo tests based on implantation in animal organisms.
A.F. Azevedo et al. / Surface & Coatings Technology 194 (2005) 271–275
Acknowledgements The authors are very grateful to FAPESP and CNPq for financial support, to Mr. Jognes Panasiewicz Junior (INPE) for SEM analyses and to Mrs. Elidiane C. Rangel (Faculdade de Engenharia de Guaratingueta´) for wettability tests.
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