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Ion-beam assisted deposited C–N coating on magnesium alloys
Surface & Coatings Technology 202 (2008) 5737–5741
Contents lists available at ScienceDirect
Surface & Coatings Technology 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 / s u r f c o a t
Ion-beam assisted deposited C–N coating on magnesium alloys J.X. Yang a, F.Z. Cui a,⁎, In-Seop Lee b, Y.P. Jiao a, Q.S. Yin c, Y. Zhang c a b c
Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Institute of Physics and Applied Physics, and Atomic-Scale Surface Science Research Center, Yonsei University, Seoul 120-749, South Korea Department of Orthopedics, General Hospital of Guangzhou Military Command of PLA, Guangzhou 510010, China
A R T I C L E
I N F O
Available online 18 June 2008 Keywords: Medical magnesium alloy CN coating IBAD
A B S T R A C T In this study, thin carbon nitride (CN) coating was made on magnesium (Mg) alloy (AZ31) substrate by ion beam assisted deposition. The morphology, composition and phase structure of the coating were investigated by scanning electron microscopy with an energy dispersive spectroscopy, X-ray diffraction and Raman spectroscopy. Atomic force microscopy and contact angle test were used to analyze surface properties. In addition, mechanical properties were investigated by nanoindentation technique. The results of the properties of CN coating on the Mg alloy substrate provide useful information for the biological applications in the future. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, metallic implants and devices have become essential biomaterials to assist with the repair of hard tissue, apply in vascular tissue and other clinical applications. It is well known that permanent materials in human body would induce decease or metallic allergy or affect new tissue growth, and need to be removed by a second surgical procedure [1,2]. As a degradable metal biomaterial, magnesium (Mg) alloys have shown potential advantages like similar mechanical properties to natural bone and high stability. Moreover, degradation products of Mg alloy should be biocompatible because Mg ion is the fourth most abundant cation in the human body and it's essential to human metabolism [3]. However, Mg alloys are exposed to human environment completely and easy to degradation, which imitate their application. One of the most effective methods is to form a protective coating on the substrate surface. Carbon nitride coating (CN) is expected to use as biocompatible hard coating on biomedical implants, which are due to not only their excellent properties but also their chemical composition containing only carbon and nitrogen, which are biologically compatible [4–6]. Among various CN coating techniques [7–9], IBAD [10] is currently one of the most promising techniques due to a low temperature, a strong interfacial adhesion, and freedom in choice of substrates. Recently, there is a valuable investigation on CN coating which is formed on traditional metallic substrate and degradable polymer substrate to improve biocompatible and reduce the rate of degradation [11,12]. However, no report about CN coating on degradable Mg alloy substrate is available. In this work, AZ31 was selected as the Mg alloy substrate due to its suitable physical and mechanical properties for medical implants. To ⁎ Corresponding author. Fax: +86 10 62772850. E-mail address: [email protected] (F.Z. Cui). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.116
the best of our knowledge report on IBAD CN coating on Mg alloy substrate has not been available so far. In this paper, Low and high N concentration coatings were formed according to different processing parameters for IBAD. We studied the morphology, phase structure, and composition of the coatings. Moreover, comparison of surface properties and mechanical properties between different atomic composition ratios of nitride to carbon (N/C) were also investigated. As the system of CN coating on Mg alloy substrate has a number of potential values in clinical applications, such as hard carbon coatings for surgical instruments, implanted fittings and semi-permanent components, it has been a very attraction system for future biomedical applications. Accordingly, the analysis of the IBAD CN coating in this paper should be indispensable for the further research like biocompatibility. 2. Materials and methods 2.1. Sample preparation Metallic substrates were commercial AZ31 (Al 3%, Zn 1%, Mn 0.2%, Feb0.005%), which are machined into discs with a size of 1 mm thickness, 15 mm diameter. Subsequently, the discs were polished by Grit 1000 abrasive papers. Then all discs were successively cleaned sequentially in an ultrasonic bath of deionized water, aether, and acetone each for 8 min before introduced into the vacuum chamber. 2.2. Coating deposition A polyfunctional IBAD system was used to form CN coating, which had been described in ref. [10]. Briefly in the IBAD system a high pure graphite target was sputtered by 3 keV Ar+ beam to deposit the C coatings, and the resulting coatings were simultaneously bombarded
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J.X. Yang et al. / Surface & Coatings Technology 202 (2008) 5737–5741
Table 1 Coating conditions for the deposition of the groups Sample
CN 1#
CN 2#
Base pressure (Pa) Operating pressure (Pa) Sputtering ion energy (keV) Sputtering ion current (mA) Bombarding ion energy (eV) Bombarding ion current (mA) Deposition time (min) Deposition rate (nm/s)
5 × 10− 4 1 × 10− 2 3 100 600 20 60 0.06
5 × 10− 4 1 × 10− 2 3 100 600 50 60 0.06
by N ion beam from another ion source to produce an atomic mixed interface. Before deposition sample was cleaned with an Ar+ beam of 15 mA and 1 keV for 3 min. N ion bombard was employed at room temperature. Table 1 shows the used coating parameters for the deposition of the samples of two groups. The samples were labeled as naked substrate, CN 1# coated and CN 2# coated sample.
Fig. 2. XRD patterns of naked Mg alloy, CN 1# and CN 2# coatings on Mg alloy substrate.
2.3. Characterization
2.4. Mechanical characterization
The Morphology of CN coatings were characterized by scanning electron microscopy (SEM; LEO-1530, Germany) at 20 kV with an energy dispersive spectroscopy (EDS; Germany) at 133 eV, which was used to investigate the N/C ratio of the coatings. The phase structure and molecular composition of the coatings were performed by X-ray diffraction (XRD; Rigaku Corporation, Japan) using a diffractometer with a CuKα target. Chemical structure was carried out by Raman spectroscopy (RS; RM2000, Renishaw, England) with an argon–ion laser operating at 514.5 nm. Atomic force microscopy (AFM; MAC. Picoscan, Arizona) was used to investigate the surface roughness. Contact angle test (OCA; OCA20, Germany) was also performed to evaluate the surface property.
Micromechanical characterizations of the coatings were measured by a nanoindentation technique (XP, MTS Corporation, USA). The average values were calculated from ten readings on different points of the coatings. 3. Result and discussion 3.1. Morphology and composition of CN coating Fig. 1 shows that a number of grains, with varying size, cover the whole substrate surface after deposition by IBAD. The particle-like
Fig. 1. SEM photographs and EDS spectrum for CN 1# (a) and CN 2# (b) coatings. Scale bar is 1 μm.
J.X. Yang et al. / Surface & Coatings Technology 202 (2008) 5737–5741
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Table 2 Contact angle and N/C ratio of the samples Sample
AZ31
CN 1#
CN 2#
N/C ratio (atm) Contact angle (°)
/ 67.0
0.24 107.8
0.38 112.1
The EDS analyses indicate that the elements of the coatings are the same, mainly compound of Mg, N, and C. But the N/C ratio are different, 0.24 and 0.38, respectively, corresponding to bombarding beam current density of 20, 50 mA. It reveals that higher bombarding beam current density could increase the nitrogen concentration in the coatings. 3.2. Structure and composition of the coatings
Fig. 3. Raman spectrum of the CN 1# and CN 2 # coatings.
grains of CN 1# coating are estimated to be 600–800 nm (Fig. 1(a)), while for CN 2# the grains are larger. Furthermore, CN 2# coating presents denser in appearance than that of CN 1#. It is worthwhile to point out that the coating changed from small to large grain with increasing of bombarding beam current density. The thickness of coating was calculated approximately 240 nm according to the parameters.
We have compared the XRD patterns of the naked and the coated samples as shown in Fig. 2. No new diffraction peaks were clearly observed in the patterns of the coated samples. These results reveal that the CN films are amorphous, which is similar to our previous study on CN coatings on Ti alloy and other substrates [11,14]. Typical RS of the CN coatings are shown in Fig. 3. In the Raman shift range of 500 to 4000 cm− 1, two peaks of CN 1# at 1374.2 and 1558.4 cm− 1 and the other two peaks of CN 2# at 1320.9 and 1550.7 cm− 1 were observed. They are corresponding to higher binding energies D (1300–1450 cm− 1; disorder) and G (1550–1580 cm− 1; graphite) peaks typical for amorphous CN coatings [13,14]. A slight shift in Raman peaks to a lower frequency side is due to a higher concentration of nitrogen in the C–N coating [15]. Furthermore, in the
Fig. 4. AFM image and three-dimensional topography of the surfaces of (a) naked substrate (b) CN 1# coating (c) CN 2# coating. Scale bar is 1 μm.
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J.X. Yang et al. / Surface & Coatings Technology 202 (2008) 5737–5741
wettability and estimate the surface energy. Each sample was measured three times and each data point was the average of three measurements. It can be determined that the contact angle values from big down to small are as follows: AZ31 b CN 1# b CN 2#. It shows that the CN 1# coating is more hydrophilic than CN 2# coating. Hydrophobic surface is beneficial for the cells' proliferation [12]. From the results of the contact angle, the surface energy of AZ31 is greater than that of CN coatings, the main effect factor may be the surface structure. For CN coatings only, the contact angle increases as the N content increases. That is to say, the wettability and surface energy decreases with the increase of nitrogen content. In general, the main factors of surface affecting biocompatibility of materials are surface roughness, surface energy, and surface corrosion resistance. For blood compatibility, the rougher a surface of an implant, the greater the area exposed to blood. This is easy to result in blood coagulation. However, it is a very different case for a hard tissue implant. Rougher surface is beneficial to the adhesion between osteoblast and the implants. For a blood implanted material, low surface energy is beneficial to its hemocompatibility [19]. CN coatings possess lower surface energy than AZ31 do. Concerning only CN coatings, high nitrogen content is corresponding to a low surface energy. Therefore, high nitrogen content is beneficial to blood compatibility of the coating. Surface corrosion resistance is another key factor to affect biocompatibility. In comparison, inert bioceramic coatings such as carbon nitride exhibit good surface corrosion resistance and mechanical property for hard tissue implants. 3.4. Mechanical characterization
Fig. 5. Microhardness–displacement curves and modulus–displacement curves of CN coating: (a) hardness profile for CN 1# and CN 2# coatings; (b) elastic modulus profile for CN 1# and CN 2# coatings.
case of the nitrogen-containing coatings, the width of the D band is wider than that of the G band. From the process parameters, XRD and Raman spectra and the literature data [16–18], it follows that the amorphous CN coating were deposited. 3.3. Surface properties Typical AFM surface morphologies of naked Mg alloy, CN 1# and CN 2# coating are shown in Fig. 4(a) to (c). The naked surface is relatively rough, about 32.9 nm, probably due to the discs were polished by SiC sand papers (Fig. 4(a)). Both CN 1# and CN 2# coatings exhibit smooth surfaces composed of fine grains, and the average roughness are 29.5 nm and 28.9 nm, respectively. On the basis of the AFM observations, we revealed that the deposition of CN coatings could improve the rough nature of Mg alloy surface. In addition, CN 1# coating (Fig. 4(b)) is smoother than CN 2# (Fig. 4(c)), which possibly because higher N ion bombarding beam current density can help to improve surface roughness. Table 2 lists the contact angles of CN coatings together with AZ31 naked sample. The contact angle is commonly used to describe the
Fig. 5 shows the hardness and elastic modulus of AZ31 substrate are about 1.24 GPa and 33.42 GPa. The abrupt increase of the values at the initial stage may be due to water adsorption, coating surface roughness, and oxide of Mg produced in the air [20]. For CN 1# and CN 2# coating, hardness rise to a maximum in the initial stage, which are approximately 13.9 GPa and 15.3 GPa (Fig. 5(a)), respectively. The hardness keep invariable with increasing load applied to the indenter. Fig. 5(b) shows the elastic modulus of the coatings, the maximum are 196.0 GPa and 202.1 GPa, respectively. The distribution of elastic modulus is similar to that of hardness. This relatively high hardness value is indicative of the formation of the C3N4. Unfortunately, in this way, it was thought that the effect of the substrate hardness and elastic modulus on the measured value of the coating hardness could not be completely removed. However, the same substrate was used for the coating formation, thus we obtained considerable difference of mechanical properties that CN 2# are superior to that of CN 1# coating. Accordingly, CN 2# coating is more beneficial to be the coating of hard tissue implants. 4. Conclusions CN coatings were successfully deposited by IBAD on the Mg alloy substrate. Through adjusting parameter, coating with high N/C ratio of 0.38 was produced. The CN coating is a composition of amorphous C3N4. The coating improved the roughness of naked substrate from 32.9 nm to 28.9 nm. Moreover, the percentage increases of hardness and elastic modulus induced by coating are more than 90.6% and 82.8%, respectively. As a hard IBAD CN coating was formed on the Mg alloy substrate, further study is worthwhile to perform in order to check biocompatibility of these coatings. Acknowledgments This work was supported by the R&D project of Guangdong Province of China Grant No. 2007A090302043, Analysis Fund of Tsinghua University of China and by the Brain Korea 21 Project of the Ministry of Education of Korea.
J.X. Yang et al. / Surface & Coatings Technology 202 (2008) 5737–5741
References [1] T.N. Kim, Q.L. Feng, Z.S. Luo, F.Z. Cui, J.O. Kim, Surf. Coat. Technol. 9 (1998) 20. [2] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (2006) 1728. [3] Water, electrolyte mineral and acid/base metabolism. Section 2. Endocrine & Metabolic Disorders. Merk Manual of Diagnosis and Therapy [Chapter 12]. [4] C.L. Zheng, F.Z. Cui, B. Meng, J. Ge, D.P. Liu, I.-S. Lee, Surf. Coat. Technol. 193 (2005) 361. [5] M. Allen, F. Law, N. Rushton, Clin. Mater. 17 (1994) 1. [6] C. Du, X.W. Su, F.Z. Cui, X.D. Zhu, Biomaterials 19 (1998) 651. [7] M.A. Baker, P. Hammer, Surf. Interface Anal. 25 (1997) 301. [8] A. Bousetta, M. Lu, A. Bansaoula, J. Vac. Sci. Technol., A, Vac. Surf. Films 13 (1995) 1639. [9] W. Dawei, F. Dejun, G. Huaixi, Z. Zhilong, M. Xianquan, F. Xiangjun, Phys. Rev., B 56 (1997) 4949.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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F.Z. Cui, Z.S. Luo, Surf. Coat. Technol. 112 (1999) 278. F.Z. Cui, X.L. Qing, D.J. Li, J. Zhao, Surf. Coat. Technol. 200 (2005) 1009. Z.R. Wu, M. Zhang, F.Z. Cui, Surf. Coat. Technol. 201 (2007) 5710. M. Zlatanovic, N. Popovic, S. Zlatanovic, Surf. Coat. Technol. 200 (2005) 1445. W. Yang, F.Z. Cui, X.L. Qing, Curr. Appl. Phys. 6 (2006) 827. C.T. Kuo, J.Y. Wu, T.R. Lu, Mater. Chem. Phys. 72 (2001) 251. J.G. LEE, S.P. LEE, J. Electroceram. 13 (2004) 321. K. Kutasi, Z. Donko, M. Mohai, L. Nemes, G. Marosi, Vacuum 68 (2003) 311. J. Pereira, I.G. Grenier, V.M. Guilbaud, A. Plain, Surf. Coat. Technol. 482 (2005) 226. C.M.J.M. Pypen, H. Plenk Jr, M.F. Ebel, R. Svagera, J. Wernisch, J. Mater. Sci., Mater. Med. 8 (1997) 781. [20] E. Liu, X. Shi, H.S. Tan, L.K. Cheah, Z. Sun, B.K. Tay, J.R. Shi, Surf. Coat. Technol. 120 (1999) 601.
Contents lists available at ScienceDirect
Surface & Coatings Technology 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 / s u r f c o a t
Ion-beam assisted deposited C–N coating on magnesium alloys J.X. Yang a, F.Z. Cui a,⁎, In-Seop Lee b, Y.P. Jiao a, Q.S. Yin c, Y. Zhang c a b c
Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Institute of Physics and Applied Physics, and Atomic-Scale Surface Science Research Center, Yonsei University, Seoul 120-749, South Korea Department of Orthopedics, General Hospital of Guangzhou Military Command of PLA, Guangzhou 510010, China
A R T I C L E
I N F O
Available online 18 June 2008 Keywords: Medical magnesium alloy CN coating IBAD
A B S T R A C T In this study, thin carbon nitride (CN) coating was made on magnesium (Mg) alloy (AZ31) substrate by ion beam assisted deposition. The morphology, composition and phase structure of the coating were investigated by scanning electron microscopy with an energy dispersive spectroscopy, X-ray diffraction and Raman spectroscopy. Atomic force microscopy and contact angle test were used to analyze surface properties. In addition, mechanical properties were investigated by nanoindentation technique. The results of the properties of CN coating on the Mg alloy substrate provide useful information for the biological applications in the future. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, metallic implants and devices have become essential biomaterials to assist with the repair of hard tissue, apply in vascular tissue and other clinical applications. It is well known that permanent materials in human body would induce decease or metallic allergy or affect new tissue growth, and need to be removed by a second surgical procedure [1,2]. As a degradable metal biomaterial, magnesium (Mg) alloys have shown potential advantages like similar mechanical properties to natural bone and high stability. Moreover, degradation products of Mg alloy should be biocompatible because Mg ion is the fourth most abundant cation in the human body and it's essential to human metabolism [3]. However, Mg alloys are exposed to human environment completely and easy to degradation, which imitate their application. One of the most effective methods is to form a protective coating on the substrate surface. Carbon nitride coating (CN) is expected to use as biocompatible hard coating on biomedical implants, which are due to not only their excellent properties but also their chemical composition containing only carbon and nitrogen, which are biologically compatible [4–6]. Among various CN coating techniques [7–9], IBAD [10] is currently one of the most promising techniques due to a low temperature, a strong interfacial adhesion, and freedom in choice of substrates. Recently, there is a valuable investigation on CN coating which is formed on traditional metallic substrate and degradable polymer substrate to improve biocompatible and reduce the rate of degradation [11,12]. However, no report about CN coating on degradable Mg alloy substrate is available. In this work, AZ31 was selected as the Mg alloy substrate due to its suitable physical and mechanical properties for medical implants. To ⁎ Corresponding author. Fax: +86 10 62772850. E-mail address: [email protected] (F.Z. Cui). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.116
the best of our knowledge report on IBAD CN coating on Mg alloy substrate has not been available so far. In this paper, Low and high N concentration coatings were formed according to different processing parameters for IBAD. We studied the morphology, phase structure, and composition of the coatings. Moreover, comparison of surface properties and mechanical properties between different atomic composition ratios of nitride to carbon (N/C) were also investigated. As the system of CN coating on Mg alloy substrate has a number of potential values in clinical applications, such as hard carbon coatings for surgical instruments, implanted fittings and semi-permanent components, it has been a very attraction system for future biomedical applications. Accordingly, the analysis of the IBAD CN coating in this paper should be indispensable for the further research like biocompatibility. 2. Materials and methods 2.1. Sample preparation Metallic substrates were commercial AZ31 (Al 3%, Zn 1%, Mn 0.2%, Feb0.005%), which are machined into discs with a size of 1 mm thickness, 15 mm diameter. Subsequently, the discs were polished by Grit 1000 abrasive papers. Then all discs were successively cleaned sequentially in an ultrasonic bath of deionized water, aether, and acetone each for 8 min before introduced into the vacuum chamber. 2.2. Coating deposition A polyfunctional IBAD system was used to form CN coating, which had been described in ref. [10]. Briefly in the IBAD system a high pure graphite target was sputtered by 3 keV Ar+ beam to deposit the C coatings, and the resulting coatings were simultaneously bombarded
5738
J.X. Yang et al. / Surface & Coatings Technology 202 (2008) 5737–5741
Table 1 Coating conditions for the deposition of the groups Sample
CN 1#
CN 2#
Base pressure (Pa) Operating pressure (Pa) Sputtering ion energy (keV) Sputtering ion current (mA) Bombarding ion energy (eV) Bombarding ion current (mA) Deposition time (min) Deposition rate (nm/s)
5 × 10− 4 1 × 10− 2 3 100 600 20 60 0.06
5 × 10− 4 1 × 10− 2 3 100 600 50 60 0.06
by N ion beam from another ion source to produce an atomic mixed interface. Before deposition sample was cleaned with an Ar+ beam of 15 mA and 1 keV for 3 min. N ion bombard was employed at room temperature. Table 1 shows the used coating parameters for the deposition of the samples of two groups. The samples were labeled as naked substrate, CN 1# coated and CN 2# coated sample.
Fig. 2. XRD patterns of naked Mg alloy, CN 1# and CN 2# coatings on Mg alloy substrate.
2.3. Characterization
2.4. Mechanical characterization
The Morphology of CN coatings were characterized by scanning electron microscopy (SEM; LEO-1530, Germany) at 20 kV with an energy dispersive spectroscopy (EDS; Germany) at 133 eV, which was used to investigate the N/C ratio of the coatings. The phase structure and molecular composition of the coatings were performed by X-ray diffraction (XRD; Rigaku Corporation, Japan) using a diffractometer with a CuKα target. Chemical structure was carried out by Raman spectroscopy (RS; RM2000, Renishaw, England) with an argon–ion laser operating at 514.5 nm. Atomic force microscopy (AFM; MAC. Picoscan, Arizona) was used to investigate the surface roughness. Contact angle test (OCA; OCA20, Germany) was also performed to evaluate the surface property.
Micromechanical characterizations of the coatings were measured by a nanoindentation technique (XP, MTS Corporation, USA). The average values were calculated from ten readings on different points of the coatings. 3. Result and discussion 3.1. Morphology and composition of CN coating Fig. 1 shows that a number of grains, with varying size, cover the whole substrate surface after deposition by IBAD. The particle-like
Fig. 1. SEM photographs and EDS spectrum for CN 1# (a) and CN 2# (b) coatings. Scale bar is 1 μm.
J.X. Yang et al. / Surface & Coatings Technology 202 (2008) 5737–5741
5739
Table 2 Contact angle and N/C ratio of the samples Sample
AZ31
CN 1#
CN 2#
N/C ratio (atm) Contact angle (°)
/ 67.0
0.24 107.8
0.38 112.1
The EDS analyses indicate that the elements of the coatings are the same, mainly compound of Mg, N, and C. But the N/C ratio are different, 0.24 and 0.38, respectively, corresponding to bombarding beam current density of 20, 50 mA. It reveals that higher bombarding beam current density could increase the nitrogen concentration in the coatings. 3.2. Structure and composition of the coatings
Fig. 3. Raman spectrum of the CN 1# and CN 2 # coatings.
grains of CN 1# coating are estimated to be 600–800 nm (Fig. 1(a)), while for CN 2# the grains are larger. Furthermore, CN 2# coating presents denser in appearance than that of CN 1#. It is worthwhile to point out that the coating changed from small to large grain with increasing of bombarding beam current density. The thickness of coating was calculated approximately 240 nm according to the parameters.
We have compared the XRD patterns of the naked and the coated samples as shown in Fig. 2. No new diffraction peaks were clearly observed in the patterns of the coated samples. These results reveal that the CN films are amorphous, which is similar to our previous study on CN coatings on Ti alloy and other substrates [11,14]. Typical RS of the CN coatings are shown in Fig. 3. In the Raman shift range of 500 to 4000 cm− 1, two peaks of CN 1# at 1374.2 and 1558.4 cm− 1 and the other two peaks of CN 2# at 1320.9 and 1550.7 cm− 1 were observed. They are corresponding to higher binding energies D (1300–1450 cm− 1; disorder) and G (1550–1580 cm− 1; graphite) peaks typical for amorphous CN coatings [13,14]. A slight shift in Raman peaks to a lower frequency side is due to a higher concentration of nitrogen in the C–N coating [15]. Furthermore, in the
Fig. 4. AFM image and three-dimensional topography of the surfaces of (a) naked substrate (b) CN 1# coating (c) CN 2# coating. Scale bar is 1 μm.
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J.X. Yang et al. / Surface & Coatings Technology 202 (2008) 5737–5741
wettability and estimate the surface energy. Each sample was measured three times and each data point was the average of three measurements. It can be determined that the contact angle values from big down to small are as follows: AZ31 b CN 1# b CN 2#. It shows that the CN 1# coating is more hydrophilic than CN 2# coating. Hydrophobic surface is beneficial for the cells' proliferation [12]. From the results of the contact angle, the surface energy of AZ31 is greater than that of CN coatings, the main effect factor may be the surface structure. For CN coatings only, the contact angle increases as the N content increases. That is to say, the wettability and surface energy decreases with the increase of nitrogen content. In general, the main factors of surface affecting biocompatibility of materials are surface roughness, surface energy, and surface corrosion resistance. For blood compatibility, the rougher a surface of an implant, the greater the area exposed to blood. This is easy to result in blood coagulation. However, it is a very different case for a hard tissue implant. Rougher surface is beneficial to the adhesion between osteoblast and the implants. For a blood implanted material, low surface energy is beneficial to its hemocompatibility [19]. CN coatings possess lower surface energy than AZ31 do. Concerning only CN coatings, high nitrogen content is corresponding to a low surface energy. Therefore, high nitrogen content is beneficial to blood compatibility of the coating. Surface corrosion resistance is another key factor to affect biocompatibility. In comparison, inert bioceramic coatings such as carbon nitride exhibit good surface corrosion resistance and mechanical property for hard tissue implants. 3.4. Mechanical characterization
Fig. 5. Microhardness–displacement curves and modulus–displacement curves of CN coating: (a) hardness profile for CN 1# and CN 2# coatings; (b) elastic modulus profile for CN 1# and CN 2# coatings.
case of the nitrogen-containing coatings, the width of the D band is wider than that of the G band. From the process parameters, XRD and Raman spectra and the literature data [16–18], it follows that the amorphous CN coating were deposited. 3.3. Surface properties Typical AFM surface morphologies of naked Mg alloy, CN 1# and CN 2# coating are shown in Fig. 4(a) to (c). The naked surface is relatively rough, about 32.9 nm, probably due to the discs were polished by SiC sand papers (Fig. 4(a)). Both CN 1# and CN 2# coatings exhibit smooth surfaces composed of fine grains, and the average roughness are 29.5 nm and 28.9 nm, respectively. On the basis of the AFM observations, we revealed that the deposition of CN coatings could improve the rough nature of Mg alloy surface. In addition, CN 1# coating (Fig. 4(b)) is smoother than CN 2# (Fig. 4(c)), which possibly because higher N ion bombarding beam current density can help to improve surface roughness. Table 2 lists the contact angles of CN coatings together with AZ31 naked sample. The contact angle is commonly used to describe the
Fig. 5 shows the hardness and elastic modulus of AZ31 substrate are about 1.24 GPa and 33.42 GPa. The abrupt increase of the values at the initial stage may be due to water adsorption, coating surface roughness, and oxide of Mg produced in the air [20]. For CN 1# and CN 2# coating, hardness rise to a maximum in the initial stage, which are approximately 13.9 GPa and 15.3 GPa (Fig. 5(a)), respectively. The hardness keep invariable with increasing load applied to the indenter. Fig. 5(b) shows the elastic modulus of the coatings, the maximum are 196.0 GPa and 202.1 GPa, respectively. The distribution of elastic modulus is similar to that of hardness. This relatively high hardness value is indicative of the formation of the C3N4. Unfortunately, in this way, it was thought that the effect of the substrate hardness and elastic modulus on the measured value of the coating hardness could not be completely removed. However, the same substrate was used for the coating formation, thus we obtained considerable difference of mechanical properties that CN 2# are superior to that of CN 1# coating. Accordingly, CN 2# coating is more beneficial to be the coating of hard tissue implants. 4. Conclusions CN coatings were successfully deposited by IBAD on the Mg alloy substrate. Through adjusting parameter, coating with high N/C ratio of 0.38 was produced. The CN coating is a composition of amorphous C3N4. The coating improved the roughness of naked substrate from 32.9 nm to 28.9 nm. Moreover, the percentage increases of hardness and elastic modulus induced by coating are more than 90.6% and 82.8%, respectively. As a hard IBAD CN coating was formed on the Mg alloy substrate, further study is worthwhile to perform in order to check biocompatibility of these coatings. Acknowledgments This work was supported by the R&D project of Guangdong Province of China Grant No. 2007A090302043, Analysis Fund of Tsinghua University of China and by the Brain Korea 21 Project of the Ministry of Education of Korea.
J.X. Yang et al. / Surface & Coatings Technology 202 (2008) 5737–5741
References [1] T.N. Kim, Q.L. Feng, Z.S. Luo, F.Z. Cui, J.O. Kim, Surf. Coat. Technol. 9 (1998) 20. [2] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (2006) 1728. [3] Water, electrolyte mineral and acid/base metabolism. Section 2. Endocrine & Metabolic Disorders. Merk Manual of Diagnosis and Therapy [Chapter 12]. [4] C.L. Zheng, F.Z. Cui, B. Meng, J. Ge, D.P. Liu, I.-S. Lee, Surf. Coat. Technol. 193 (2005) 361. [5] M. Allen, F. Law, N. Rushton, Clin. Mater. 17 (1994) 1. [6] C. Du, X.W. Su, F.Z. Cui, X.D. Zhu, Biomaterials 19 (1998) 651. [7] M.A. Baker, P. Hammer, Surf. Interface Anal. 25 (1997) 301. [8] A. Bousetta, M. Lu, A. Bansaoula, J. Vac. Sci. Technol., A, Vac. Surf. Films 13 (1995) 1639. [9] W. Dawei, F. Dejun, G. Huaixi, Z. Zhilong, M. Xianquan, F. Xiangjun, Phys. Rev., B 56 (1997) 4949.
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