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Laser alloying of Ti–Si compound coating on Ti–6Al–4V alloy for the improvement of bioactivity
Applied Surface Science 305 (2014) 16–23
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Laser alloying of Ti–Si compound coating on Ti–6Al–4V alloy for the improvement of bioactivity Y. Wu a,b , A.H. Wang a,∗ , Z. Zhang a , R.R. Zheng a , H.B. Xia b , Y.N. Wang b a State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) and Key Laboratory of Oral Biomedicine Ministry of Education, Wuhan University, Wuhan 430079, PR China
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Article history: Received 13 September 2013 Received in revised form 24 February 2014 Accepted 24 February 2014 Available online 7 March 2014 Keywords: Laser alloying Ti–Si compound coating Microstructure Microhardness Corrosion properties Bioactivity
a b s t r a c t Laser alloying of Ti–Si compound coating on Ti–6Al–4V alloy is carried out by a pulsed Nd:YAG laser. The corresponding microstructure, phase structure, microhardness profiles, corrosion properties and bioactivity of the laser-alloyed coatings are investigated to optimize the atomic ratio of Ti–Si. The laser alloyed Ti–Si compound coatings are free of cracks, and primarily present block-like crystals, lath-like crystals and dendrite crystals. The phase structures of both laser-alloyed Ti + Si and 5Ti + 3Si coatings are mainly consisted of ␣-Ti and Ti5 Si3 , while the laser-alloyed Si coating is mainly consisted of TiSi2 and Ti5 Si3 . Microhardness test indicates that the laser-alloyed Si coating has the highest microhardness. Also, corrosion resistance measurement reveals that the corrosion resistance of the laser-alloyed Si coating is much better than that of the Ti–6Al–4V alloy. Evaluation of bioactivity shows that cell growth on the laser-alloyed Si coating with high volume fraction of Ti–Si compounds is faster than that of the Ti–6Al–4V alloy. © 2014 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing need of biomaterials with the growth of the world population, especially those used for replacement surgeries and revision surgeries [1]. Ti and the titanium alloys are usually chosen for replacement surgeries and revision surgeries due to their corrosion resistance and bioactivity [2,3]. Up to now, many research works have been carried out to improve the properties of Ti and titanium alloys. Wear resistance and bioactivity are of great value to Ti and titanium alloys when they are used as biomaterials [1]. However, Ti and titanium alloys have poor wear resistance and high friction coefficients [4–6]. On the other hand, the previous studies revealed that the surface chemical composition of biomaterials can be manipulated to improve the bioactivity, for instance by using laser surface alloying [7–13]. Compared with the conventional methods, laser surface alloying is capable of improving the wear resistance and modifying the chemical composition. This method has high efficiency, excellent bonding strength between coating and substrate, as well as the ability to precisely control the
∗ Corresponding author. Tel.: +86 27 87180507. E-mail address: [email protected] (A.H. Wang). http://dx.doi.org/10.1016/j.apsusc.2014.02.140 0169-4332/© 2014 Elsevier B.V. All rights reserved.
thickness of coating (0.01–1 mm). Many research works on improving the properties of Ti and titanium alloys by laser alloying technique have been reported [14–21]. Ti–Si compounds have numerous attractive properties such as excellent oxidation and wear resistance, high hardness, relative low density, and high melting point. Several research works have been conducted to synthesize Ti–Si compounds on Ti or titanium alloys. Alhammad et al. [20] used laser alloying technique to produce a coating which contains Ti5 Si3 and ␣-Ti mixtures on Ti–6Al–V alloy. They also found that laser scanning speeds had an obvious influence on microstructure and microhardness. Liu et al. [21] produced coatings on ␥-TiAl alloy with NiCr–Si mixed powders. They thought that the formation of the reinforced primary Ti5 Si3 , ␥/TiSi eutectics and the refinement of the microstructure could modify the wearresisting property. Majumdar et al. [22] fabricated coatings on pure Ti with Si, Al or Si + Al by laser surface alloying method. They pointed out that the formation of Ti5 Si3 could effectively improve the wear resistance of Ti. Tian et al. [23] investigated laser surface alloying of Ti–6Al–4V with C + Si mixed powders. The results indicated that the hardness and the wear resistance increased remarkably after adopting laser surface alloying process. Although these researchers reported that laser alloying with Si on the surface of Ti or titanium alloys was beneficial to the modification of substrate, little effort was made on the bioactivity of the alloyed coating.
Y. Wu et al. / Applied Surface Science 305 (2014) 16–23
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Fig. 1. SEM images with lower magnification of the cross-section of the laser-alloyed coatings. (a) Si coating. (b) Ti + Si coating. (c) 5Ti + 3Si coating.
Table 1 Chemical composition of the substrate (wt.%).
Table 2 Composition proportion of prepared powder specimens. Mixture No.
Al
V
Fe
C
N
H
O
Ti
6.100
4.000
0.100
0.010
0.010
0.002
0.090
Bal.
In this work, laser alloying of different atomic ratios of Ti–Si coatings on Ti–6Al–4V plate was carried out by a pulsed Nd:YAG laser. The microstructure characteristics and phase structure of different laser-alloyed coatings were analyzed to optimize the ratio of Ti–Si. In this way, laser-alloyed coating with the highest volume content of Ti–Si compounds can be produced. The microhardness of different laser-alloyed coatings was tested to evaluate the impact of the ratio of Ti–Si on hardness. Finally, the corrosion resistance in simulated body fluid and the bioactivity of the coating which has the highest volume content of Ti–Si compounds was tested to confirm whether it could be used as biomaterial. 2. Experimental procedure A cast Ti–6Al–4V alloy was employed as the substrate for laser alloying treatment and the composition was given in Table 1. The samples were machined into sheets with a dimension of 20 mm × 20 mm × 3 mm by means of wire-cut. Then they were
Components (at. %)
1 2 3
Si
Ti
1 1 3
– 1 5
polished to reduce their surface roughness. And they were also cleaned with alcohol before pre-placing the powder mixtures. The 99.9% purity Ti powder with a mean particle size of 46 m and the 99.9% purity Si powder with a size range of 46–75 m were used as raw materials. The components of different powder mixtures were listed in Table 2. The mixed powder coatings were pre-placed on the substrate by diluted cellulose acetate solution and then oven-dried before laser alloying treatment. The thickness of the pre-placed powder was set to be 0.15 mm. Laser alloying experiments were carried out by a HG Laser LCY400 pulsed Nd:YAG laser. The specifications of the laser were given in Table 3. The parameters of laser alloying process were listed in Table 4. The cross-sections of the laser-alloyed samples were characterized by a JEOL JSM-5610LV scanning electron microscopy (SEM). The phase structures of the laser-alloyed coatings were identified
Table 3 Specification of laser used in the experiments. Wavelength (nm)
Maximum power (W)
Focal length (mm)
Plus duration (ms)
Frequency (Hz)
1064
400
100
0.2–20
1–200
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Y. Wu et al. / Applied Surface Science 305 (2014) 16–23
Table 4 Parameters of laser alloying process. Power (W)
Scanning rate (mm/min)
Plus duration (ms)
Frequency (Hz)
Beam diameter (mm)
Overlapping ratio (%)
210
300
1.5
45
2
35
Table 5 Chemical composition of Hank’s solution. Chemical composition
NaCl
CaCl2
KCl
NaHCO3
Glucose
MgCl2 ·6H2 O
Na2 HPO4 ·12H2 O
KH2 PO4
MgSO4 ·7H2 O
Concentration (g/L)
8.0
0.14
0.4
0.35
1.0
0.1
0.06
0.06
0.06
using a Bruker D8 ADVANCE X-ray diffraction (XRD) with Cu K␣ radiation. The microhardness of the laser-alloyed samples was determined using a Buehler Micromet II Microhardness Tester with 15 s load application time and under loads of 100 g. The corrosion resistance in a simulated body fluid (see Table 5) was tested in Hank’s solution at pH = 7.4. Saturated calomel electrode was used as reference electrode and platinum was used as counter electrode. Polarization was conducted at a scan rate of 1 mV/s. The corroded samples were analyzed by SEM incorporating energy dispersive X-ray analysis (EDX). Cell proliferation was conducted to evaluate the bioactivity of the laser-alloyed coatings. MC3T3-E1 subclone 4 (ATCC CRL-2593, Lot No. 3225550), derived from newborn mouse calvaria, were used in this study. They were grown in alpha-Minimum Essential Medium (␣-MEM) containing 10% fetal bovine serum (FBS), 100 unit/ml penicillin, 100 g/ml streptomycin, at 37 ◦ C in humidified air with 5% CO2 . Acridine orange (AO) was used to stain the MC3T3-E1 cells on the laser-alloyed coatings after 1, 3, and 5 days of culture. The samples were washed with phosphate buffer saline
(PBS) for three times, and then fixed with 95% alcohol for 10 min at 4 ◦ C. The samples were stained with 0.025% AO for 5 min in dark, and then washed with 1% calcium chloride solution for 1 min. A Leica DM4000B microscope was used to obtain fluorescence images.
3. Results 3.1. Microstructural analysis of the laser-alloyed coatings Fig. 1 shows the entire cross-sections of the laser-alloyed Si, Ti + Si and 5Ti + 3Si coatings. Evidently, all three kinds of coatings are free of pores or cracks. The interface between the alloyed coating and the substrate is smooth, continuous and well-bonded. Figs. 2–4 show the microstructure of three kinds of laser-alloyed coatings changing from interface to surface, respectively. All of the laser-alloyed coatings present block-like crystals and lath-like crystals in the top zones. Besides, it can be noticed that all of the
Fig. 2. Microstructures of the bottom zones of the laser-alloyed coatings. (a) Si coating. (b) Ti + Si coating. (c) 5Ti + 3Si coating.
Y. Wu et al. / Applied Surface Science 305 (2014) 16–23
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Fig. 3. Microstructures of the medium zones of the laser-alloyed coatings. (a) Si coating. (b) Ti + Si coating. (c) 5Ti + 3Si coating.
laser-alloyed coatings display block-like crystals, lath-like crystals and dendrite crystals in the medium zones. The interfacial zones exhibit a metallurgical bond and are characterized by epitaxially grown planar crystals from non-molten substrate. The crystals in this zone change from lath-like crystals + block-like crystals to block-like crystals with a decrease in the atomic ratio of Si–Ti. Fig. 5 shows XRD patterns of the laser-alloyed coatings. The laser-alloyed 5Ti + 3Si and Ti + Si coatings (Fig. 5(a) and (b)) both consist of ␣-Ti and Ti5 Si3 , and the laser-alloyed Si coating mainly consists of TiSi2 , Ti5 Si3 and some residual ␣-Ti and Si. The height of main peaks of Ti5 Si3 decreases as the content of Si increases in the powder mixtures. When the content of Si increases to a certain level, the characteristic peak of TiSi2 appears. The results reveal that the laser-alloyed Si coating has the highest volume content of Ti–Si compounds. 3.2. Microhardness profiles of the laser-alloyed coatings Fig. 6 presents the microhardness profile in the transverse crosssection of the laser-alloyed coatings. The microhardness of the laser-alloyed Si coating is in a range of 1500–1000 HV0.1 , which is approximately 4–3 times higher compared with the Ti–6Al–4V alloy substrate (about 400 HV0.1 ). Besides, the microhardness of the laser-alloyed Ti + Si coating is lower than that of the laser-alloyed Si coating, which is in a range of 900–700 HV0.1 . Moreover, the microhardness of the laser-alloyed 5Ti + 3Si coating ranges from 800–650 HV0.1 at the region near the surface and then decreases sharply to 400 HV0.1 . It can also be seen from Fig. 6 that the microhardness of the laser-alloyed coatings increases with an increase in Si content. The results confirm that the laser-alloyed Si coating exhibits the highest microhardness.
3.3. Corrosion properties Since the laser-alloyed Si coating mainly consists of Ti–Si compounds, the laser-alloyed Si coating and the Ti–6Al–4V alloy were selected to evaluate the corrosion resistance in simulated body fluid. The potentio-dynamic polarization curves are displayed in Fig. 7. As seen, the laser-alloyed Si coating significantly improves the corrosion resistance of Ti–6Al–4V alloy. The corrosion potential (Ecorr ) increases from −398 mV to −290 mV, and the corrosion current density (Icorr ) decreases from 11 A/cm2 to 0.903 A/cm2 . This result confirms that the laser-alloyed Si coating exhibits good corrosion resistance in simulated body fluid. Fig. 8 shows the surface morphologies and the EDX patterns of the potentio-dynamic polarization test samples. Both of the corroded samples present coarse, cracked and discontinuous surfaces. Besides, the surface of the corroded substrate is much rougher than that of the laser-alloyed sample. The result of EDX reveals the presence of Ti, Si, Al and O elements on the surface after the potentio-dynamic polarization test. 3.4. Bioactivity Based on the fact that the laser-alloyed Si coating is mainly composed of Ti–Si compounds, the laser-alloyed Si coating and the Ti–6Al–4V alloy were selected to evaluate the bioactivity of the laser-alloyed coating. The results of the AO fluorescence staining of MC3T3-E1 cells cultured on different samples are given in Fig. 9. On the first day, cell growth seemed to be the same for both of the samples. Cell growth was much denser on the laser-alloyed Si coating than on the substrate when the cells were cultured for 3 days and 5 days. This indicates that the laser-alloyed Si coating exhibits much
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Fig. 4. Microstructures of the top zones of the laser-alloyed coatings. (a) Si coating. (b) Ti + Si coating. (c) 5Ti + 3Si coating.
better bioactivity. The results also verify that the laser-alloyed Si coating exhibits good bioactivity. 4. Discussion The microstructural morphology and phase structure in those laser-alloyed coatings are mainly determined by atomic ratio of Si–Ti and solidification cooling condition. The Si contents
Fig. 5. XRD patterns of the laser-alloyed 5Ti + 3Si coating, the laser-alloyed Ti + Si coating, the laser-alloyed Si coating and the Ti–6Al–4V substrate.
in three kinds of laser-alloyed coatings are in a sequence of Si > Ti + Si > 5Ti + 3Si. It can be seen from Figs. 2–4 that the interfacial regions of all laser-alloyed coatings have epitaxially-grown planar crystals. This can be attributed to the large temperature gradient and high nucleation rate in the interfacial zone [24]. Besides, all of the laser-alloyed coatings are characterized by block-like crystals, lathlike crystals and dendrite crystals. During solidification of molten pool, the laser-alloyed coatings firstly form block-like crystals.
Fig. 6. Microhardness profiles of the laser-alloyed coatings from surface to substrate.
Y. Wu et al. / Applied Surface Science 305 (2014) 16–23
Fig. 7. Potentio-dynamic polarization curves of different samples in Hank’s solution.
And then the block-like crystals will grow up as lath shape. With the progress of solidification, dendrite crystals will form eventually. According to the Ti–Si phase diagram [20], Ti5 Si3 solidifies as primary phase during solidification of the laser-alloyed 5Ti + 3Si and Ti + Si coatings. And then eutectic reaction (i.e., L → Ti5 Si3 + ␣Ti) happened in the coatings. As for the laser-alloyed Si coating, Ti5 Si3 + ␣-Ti eutectic structure and TiSi2 + Si eutectic structure will form after the solidification of primary phase. From Ti–Si phase diagram, the equilibrium phases in the laseralloyed Ti + Si and 5Ti + 3Si coatings are in accordance with the phase diagram and the equilibrium phases in the laser-alloyed
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Si coating are incompatible with the phase diagram. Logically, the phase structure in the laser-alloyed Si coating should be Ti5 Si3 + Ti5 Si4 , or Ti5 Si4 + TiSi, or TiSi + TiS2 . However, only TiSi2 and Ti5 Si3 are detected by XRD in the laser-alloyed Si coating, which is inconsistent with the Ti–Si phase diagram. This may be attributed to inhomogeneous concentration of Si in the molten pool. During laser alloying process, the solidification rate is so rapid that the mixed powders do not have enough time to uniformly dissolve into the molten pool, which results in Ti-rich and Si-rich melt. In the Si-rich region, the following reaction should take place according to the phase diagram: L → TiSi2 + Si, while the following reaction should take place according to the phase diagram in the Ti-rich region: L → ␣-Ti + Ti5 Si3 . As a result, there is no TiSi or Ti5 Si4 in the laser-alloyed Si coating. The laser-alloyed coatings can improve the hardness of Ti–6Al–4V alloy remarkably. It is believed that the formation of Ti5 Si3 is beneficial to the increase of hardness [20–22]. The enhancement in the microhardness can also be ascribed to refinement during non-equilibrium solidification [25]. Besides, the microhardness of the laser-alloyed coatings shows an upward trend with an increase in Si content. This can be attributed to the fact that more titanium silicides will form when precursor mixed powders contain more Si. Moreover, the microhardness in the top zone of the laser-alloyed Si coating shows a fluctuation. The variation of the microhardness reveals that there is a change in the microstructure. The formation and inhomogeneous distribution of soft TiSi2 (microhardness 360 HV0.1 ) in the alloyed coating is thought to be responsible for hardness variation. Potentio-dynamic polarization curves show that the laseralloyed Si coating can improve the corrosion resistance of Ti–6Al–4V alloy. XRD analysis indicates that the laser-alloyed Si
Fig. 8. Morphologies and EDX result of corroded surfaces of different samples. (a) Si coating. (b) Ti–6Al–4V substrate. (c) EDX result of Si coating.
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Fig. 9. AO fluorescence staining of MC3T3-E1 cells cultured on the laser-alloyed Si coating and Ti–6Al–4V substrate. (a) Si coating for 1day. (b) Ti–6Al–4V substrate for 1day. (c) Si coating for 3 days. (d) Ti–6Al–4V substrate for 3 days. (e) Si coating for 5 days. (f) Ti–6Al–4V substrate for 5 days.
coating is mainly covered by Ti5 Si3 and TiSi2 . The laser-alloyed Si coating exhibits good corrosion resistance. Thus, the formation of the Ti–Si compound contributes to the increase of Ecorr , and the decrease of Icorr . Furthermore, microstructural refinement and well-bonded interfaces stimulated by rapid solidification during the laser alloying process can lead to the improvement in corrosion resistance as well [26–28]. As a result, the corrosion resistance of the laser-alloyed Si coating is better than that of the Ti–6Al–4V substrate. The results of the AO fluorescence staining of MC3T3-E1 cells cultured on different samples reveal that the laser-alloyed Si coating exhibits a satisfying bioactivity. According to the previous research works, Si has a remarkable impact on stimulating attachment and proliferation of cells [7,9–13]. It is supposed that the Si on the surface of the coatings provides favorable sites for apatite nucleation and the early formation of extracellular matrix. Since Si could hold the proteins which are capable of promoting cell attachment through interaction with the integrins on the cells. It
is also worth mentioning that cell growth is denser on the laseralloyed Si coating when the cells are cultured for 3 days and 5 days. This may be caused by the formation of Si acid in the culture medium. The Si acid can lead to alkalinization of the medium through ion exchanging. And the alkalinization of medium has an influence on the sensitivity of voltage-activated calcium channels and cellular calcium entry, which will finally result in the increased proliferation of cells [13]. To sum up, it is possible that the chemical composition (Si) contributes to the improvement of bioactivity. 5. Conclusions (1) The laser-alloyed Ti–Si compound coatings with metallurgical bond to the substrate are free of pores or cracks. (2) The laser-alloyed coatings mainly present block-like crystals, lath-like crystals and dendrite crystals. Phase structure analysis confirms that the laser-alloyed Ti + Si and 5Ti + 3Si coatings primarily consist of ␣-Ti and Ti5 Si3 , and the laser-alloyed Si
Y. Wu et al. / Applied Surface Science 305 (2014) 16–23
coating chiefly consists of TiSi2 and Ti5 Si3 . This means that the laser-alloyed Si coating has the highest volume content of Ti–Si compounds. (3) The microhardness increases significantly after laser alloying process. The microhardness is in a range of 1500–1000 HV0.1 , 900–700 HV0.1 and 800–650 HV0.1 achieved by the laseralloyed coatings of Si, Ti + Si and 5Ti + 3Si, respectively. (4) The corrosion resistance of the laser-alloyed Si coating is better than that of the Ti–6Al–4V alloy. The Ecorr of the laser-alloyed Si coating increases from −398 mV to −290 mV (which means 27% improvement). Besides, the Icorr of the laser-alloyed Si coating is about 1/10 times in comparison with the substrate. (5) In the evaluation of bioactivity, cell growth seems to be the same for both of the samples on the first day. With the progress of the test, cell growth on the laser-alloyed Si coating is faster than that of the Ti–6Al–4V alloy. Acknowledgements The author would like to thank the financial support provided by the key project of Wenzhou City science and technology plan on laser and optoelectronic industry cluster, and Open Research Fund Program of Hubei-MOST KLOS & KLOBME. References [1] M.A. Gepreel, M. Niinomi, Biocompatibility of Ti-alloys for long-term implantation, J. Mech. Behav. Biomed. Mater. 20 (2013) 407–415. [2] M. Calin, A. Gebert, A.C. Ghinea, P.F. Gostin, S. Abdi, C. Mickel, J. Eckert, Designing biocompatible Ti-based metallic glasses for implant applications, Mater. Sci. Eng. C Mater. Biol. Appl. 33 (2013) 875–883. [3] K. Venkateswarlu, N. Rameshbabu, D. Sreekanth, A.C. Bose, V. Muthupandi, S. Subramanian, Fabrication and characterization of micro-arc oxidized fluoride containing titania films on Cp Ti, Ceram. Int. 39 (2013) 801–812. [4] C. Huang, Y. Zhang, R. Vila, J. Shen, Dry sliding wear behavior of laser clad TiVCrAlSi high entropy alloy coatings on Ti–6Al–4V substrate, Mater. Des. 41 (2012) 338–343. [5] P. Jiang, X.L. He, X.X. Li, L.G. Yu, H.M. Wang, Wear resistance of a laser surface alloyed Ti–6Al–4V alloy, Surf. Coat. Technol. 130 (2000) 24–28. [6] C. Guo, J. Zhou, J. Zhao, L. Wang, Y. Yu, J. Chen, H. Zhou, Improvement of the oxidation and wear resistance of pure Ti by laser-cladding Ti3 Al coating at elevated temperature, Tribol. Lett. 42 (2011) 151–159. [7] Y. Ramaswamy, C. Wu, H. Zhou, H. Zreiqat, Biological response of human bone cells to zinc-modified Ca-Si-based ceramics, Acta. Biomater. 4 (2008) 1487–1497. [8] H. Zreiqat, P. Evans, C.R. Howlett, Effect of surface chemical modification of bioceramic on phenotype of human bone-derived cells, J. Biomed. Mater. Res. 44 (1999) 389–396.
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Laser alloying of Ti–Si compound coating on Ti–6Al–4V alloy for the improvement of bioactivity Y. Wu a,b , A.H. Wang a,∗ , Z. Zhang a , R.R. Zheng a , H.B. Xia b , Y.N. Wang b a State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) and Key Laboratory of Oral Biomedicine Ministry of Education, Wuhan University, Wuhan 430079, PR China
a r t i c l e
i n f o
Article history: Received 13 September 2013 Received in revised form 24 February 2014 Accepted 24 February 2014 Available online 7 March 2014 Keywords: Laser alloying Ti–Si compound coating Microstructure Microhardness Corrosion properties Bioactivity
a b s t r a c t Laser alloying of Ti–Si compound coating on Ti–6Al–4V alloy is carried out by a pulsed Nd:YAG laser. The corresponding microstructure, phase structure, microhardness profiles, corrosion properties and bioactivity of the laser-alloyed coatings are investigated to optimize the atomic ratio of Ti–Si. The laser alloyed Ti–Si compound coatings are free of cracks, and primarily present block-like crystals, lath-like crystals and dendrite crystals. The phase structures of both laser-alloyed Ti + Si and 5Ti + 3Si coatings are mainly consisted of ␣-Ti and Ti5 Si3 , while the laser-alloyed Si coating is mainly consisted of TiSi2 and Ti5 Si3 . Microhardness test indicates that the laser-alloyed Si coating has the highest microhardness. Also, corrosion resistance measurement reveals that the corrosion resistance of the laser-alloyed Si coating is much better than that of the Ti–6Al–4V alloy. Evaluation of bioactivity shows that cell growth on the laser-alloyed Si coating with high volume fraction of Ti–Si compounds is faster than that of the Ti–6Al–4V alloy. © 2014 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing need of biomaterials with the growth of the world population, especially those used for replacement surgeries and revision surgeries [1]. Ti and the titanium alloys are usually chosen for replacement surgeries and revision surgeries due to their corrosion resistance and bioactivity [2,3]. Up to now, many research works have been carried out to improve the properties of Ti and titanium alloys. Wear resistance and bioactivity are of great value to Ti and titanium alloys when they are used as biomaterials [1]. However, Ti and titanium alloys have poor wear resistance and high friction coefficients [4–6]. On the other hand, the previous studies revealed that the surface chemical composition of biomaterials can be manipulated to improve the bioactivity, for instance by using laser surface alloying [7–13]. Compared with the conventional methods, laser surface alloying is capable of improving the wear resistance and modifying the chemical composition. This method has high efficiency, excellent bonding strength between coating and substrate, as well as the ability to precisely control the
∗ Corresponding author. Tel.: +86 27 87180507. E-mail address: [email protected] (A.H. Wang). http://dx.doi.org/10.1016/j.apsusc.2014.02.140 0169-4332/© 2014 Elsevier B.V. All rights reserved.
thickness of coating (0.01–1 mm). Many research works on improving the properties of Ti and titanium alloys by laser alloying technique have been reported [14–21]. Ti–Si compounds have numerous attractive properties such as excellent oxidation and wear resistance, high hardness, relative low density, and high melting point. Several research works have been conducted to synthesize Ti–Si compounds on Ti or titanium alloys. Alhammad et al. [20] used laser alloying technique to produce a coating which contains Ti5 Si3 and ␣-Ti mixtures on Ti–6Al–V alloy. They also found that laser scanning speeds had an obvious influence on microstructure and microhardness. Liu et al. [21] produced coatings on ␥-TiAl alloy with NiCr–Si mixed powders. They thought that the formation of the reinforced primary Ti5 Si3 , ␥/TiSi eutectics and the refinement of the microstructure could modify the wearresisting property. Majumdar et al. [22] fabricated coatings on pure Ti with Si, Al or Si + Al by laser surface alloying method. They pointed out that the formation of Ti5 Si3 could effectively improve the wear resistance of Ti. Tian et al. [23] investigated laser surface alloying of Ti–6Al–4V with C + Si mixed powders. The results indicated that the hardness and the wear resistance increased remarkably after adopting laser surface alloying process. Although these researchers reported that laser alloying with Si on the surface of Ti or titanium alloys was beneficial to the modification of substrate, little effort was made on the bioactivity of the alloyed coating.
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Fig. 1. SEM images with lower magnification of the cross-section of the laser-alloyed coatings. (a) Si coating. (b) Ti + Si coating. (c) 5Ti + 3Si coating.
Table 1 Chemical composition of the substrate (wt.%).
Table 2 Composition proportion of prepared powder specimens. Mixture No.
Al
V
Fe
C
N
H
O
Ti
6.100
4.000
0.100
0.010
0.010
0.002
0.090
Bal.
In this work, laser alloying of different atomic ratios of Ti–Si coatings on Ti–6Al–4V plate was carried out by a pulsed Nd:YAG laser. The microstructure characteristics and phase structure of different laser-alloyed coatings were analyzed to optimize the ratio of Ti–Si. In this way, laser-alloyed coating with the highest volume content of Ti–Si compounds can be produced. The microhardness of different laser-alloyed coatings was tested to evaluate the impact of the ratio of Ti–Si on hardness. Finally, the corrosion resistance in simulated body fluid and the bioactivity of the coating which has the highest volume content of Ti–Si compounds was tested to confirm whether it could be used as biomaterial. 2. Experimental procedure A cast Ti–6Al–4V alloy was employed as the substrate for laser alloying treatment and the composition was given in Table 1. The samples were machined into sheets with a dimension of 20 mm × 20 mm × 3 mm by means of wire-cut. Then they were
Components (at. %)
1 2 3
Si
Ti
1 1 3
– 1 5
polished to reduce their surface roughness. And they were also cleaned with alcohol before pre-placing the powder mixtures. The 99.9% purity Ti powder with a mean particle size of 46 m and the 99.9% purity Si powder with a size range of 46–75 m were used as raw materials. The components of different powder mixtures were listed in Table 2. The mixed powder coatings were pre-placed on the substrate by diluted cellulose acetate solution and then oven-dried before laser alloying treatment. The thickness of the pre-placed powder was set to be 0.15 mm. Laser alloying experiments were carried out by a HG Laser LCY400 pulsed Nd:YAG laser. The specifications of the laser were given in Table 3. The parameters of laser alloying process were listed in Table 4. The cross-sections of the laser-alloyed samples were characterized by a JEOL JSM-5610LV scanning electron microscopy (SEM). The phase structures of the laser-alloyed coatings were identified
Table 3 Specification of laser used in the experiments. Wavelength (nm)
Maximum power (W)
Focal length (mm)
Plus duration (ms)
Frequency (Hz)
1064
400
100
0.2–20
1–200
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Y. Wu et al. / Applied Surface Science 305 (2014) 16–23
Table 4 Parameters of laser alloying process. Power (W)
Scanning rate (mm/min)
Plus duration (ms)
Frequency (Hz)
Beam diameter (mm)
Overlapping ratio (%)
210
300
1.5
45
2
35
Table 5 Chemical composition of Hank’s solution. Chemical composition
NaCl
CaCl2
KCl
NaHCO3
Glucose
MgCl2 ·6H2 O
Na2 HPO4 ·12H2 O
KH2 PO4
MgSO4 ·7H2 O
Concentration (g/L)
8.0
0.14
0.4
0.35
1.0
0.1
0.06
0.06
0.06
using a Bruker D8 ADVANCE X-ray diffraction (XRD) with Cu K␣ radiation. The microhardness of the laser-alloyed samples was determined using a Buehler Micromet II Microhardness Tester with 15 s load application time and under loads of 100 g. The corrosion resistance in a simulated body fluid (see Table 5) was tested in Hank’s solution at pH = 7.4. Saturated calomel electrode was used as reference electrode and platinum was used as counter electrode. Polarization was conducted at a scan rate of 1 mV/s. The corroded samples were analyzed by SEM incorporating energy dispersive X-ray analysis (EDX). Cell proliferation was conducted to evaluate the bioactivity of the laser-alloyed coatings. MC3T3-E1 subclone 4 (ATCC CRL-2593, Lot No. 3225550), derived from newborn mouse calvaria, were used in this study. They were grown in alpha-Minimum Essential Medium (␣-MEM) containing 10% fetal bovine serum (FBS), 100 unit/ml penicillin, 100 g/ml streptomycin, at 37 ◦ C in humidified air with 5% CO2 . Acridine orange (AO) was used to stain the MC3T3-E1 cells on the laser-alloyed coatings after 1, 3, and 5 days of culture. The samples were washed with phosphate buffer saline
(PBS) for three times, and then fixed with 95% alcohol for 10 min at 4 ◦ C. The samples were stained with 0.025% AO for 5 min in dark, and then washed with 1% calcium chloride solution for 1 min. A Leica DM4000B microscope was used to obtain fluorescence images.
3. Results 3.1. Microstructural analysis of the laser-alloyed coatings Fig. 1 shows the entire cross-sections of the laser-alloyed Si, Ti + Si and 5Ti + 3Si coatings. Evidently, all three kinds of coatings are free of pores or cracks. The interface between the alloyed coating and the substrate is smooth, continuous and well-bonded. Figs. 2–4 show the microstructure of three kinds of laser-alloyed coatings changing from interface to surface, respectively. All of the laser-alloyed coatings present block-like crystals and lath-like crystals in the top zones. Besides, it can be noticed that all of the
Fig. 2. Microstructures of the bottom zones of the laser-alloyed coatings. (a) Si coating. (b) Ti + Si coating. (c) 5Ti + 3Si coating.
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Fig. 3. Microstructures of the medium zones of the laser-alloyed coatings. (a) Si coating. (b) Ti + Si coating. (c) 5Ti + 3Si coating.
laser-alloyed coatings display block-like crystals, lath-like crystals and dendrite crystals in the medium zones. The interfacial zones exhibit a metallurgical bond and are characterized by epitaxially grown planar crystals from non-molten substrate. The crystals in this zone change from lath-like crystals + block-like crystals to block-like crystals with a decrease in the atomic ratio of Si–Ti. Fig. 5 shows XRD patterns of the laser-alloyed coatings. The laser-alloyed 5Ti + 3Si and Ti + Si coatings (Fig. 5(a) and (b)) both consist of ␣-Ti and Ti5 Si3 , and the laser-alloyed Si coating mainly consists of TiSi2 , Ti5 Si3 and some residual ␣-Ti and Si. The height of main peaks of Ti5 Si3 decreases as the content of Si increases in the powder mixtures. When the content of Si increases to a certain level, the characteristic peak of TiSi2 appears. The results reveal that the laser-alloyed Si coating has the highest volume content of Ti–Si compounds. 3.2. Microhardness profiles of the laser-alloyed coatings Fig. 6 presents the microhardness profile in the transverse crosssection of the laser-alloyed coatings. The microhardness of the laser-alloyed Si coating is in a range of 1500–1000 HV0.1 , which is approximately 4–3 times higher compared with the Ti–6Al–4V alloy substrate (about 400 HV0.1 ). Besides, the microhardness of the laser-alloyed Ti + Si coating is lower than that of the laser-alloyed Si coating, which is in a range of 900–700 HV0.1 . Moreover, the microhardness of the laser-alloyed 5Ti + 3Si coating ranges from 800–650 HV0.1 at the region near the surface and then decreases sharply to 400 HV0.1 . It can also be seen from Fig. 6 that the microhardness of the laser-alloyed coatings increases with an increase in Si content. The results confirm that the laser-alloyed Si coating exhibits the highest microhardness.
3.3. Corrosion properties Since the laser-alloyed Si coating mainly consists of Ti–Si compounds, the laser-alloyed Si coating and the Ti–6Al–4V alloy were selected to evaluate the corrosion resistance in simulated body fluid. The potentio-dynamic polarization curves are displayed in Fig. 7. As seen, the laser-alloyed Si coating significantly improves the corrosion resistance of Ti–6Al–4V alloy. The corrosion potential (Ecorr ) increases from −398 mV to −290 mV, and the corrosion current density (Icorr ) decreases from 11 A/cm2 to 0.903 A/cm2 . This result confirms that the laser-alloyed Si coating exhibits good corrosion resistance in simulated body fluid. Fig. 8 shows the surface morphologies and the EDX patterns of the potentio-dynamic polarization test samples. Both of the corroded samples present coarse, cracked and discontinuous surfaces. Besides, the surface of the corroded substrate is much rougher than that of the laser-alloyed sample. The result of EDX reveals the presence of Ti, Si, Al and O elements on the surface after the potentio-dynamic polarization test. 3.4. Bioactivity Based on the fact that the laser-alloyed Si coating is mainly composed of Ti–Si compounds, the laser-alloyed Si coating and the Ti–6Al–4V alloy were selected to evaluate the bioactivity of the laser-alloyed coating. The results of the AO fluorescence staining of MC3T3-E1 cells cultured on different samples are given in Fig. 9. On the first day, cell growth seemed to be the same for both of the samples. Cell growth was much denser on the laser-alloyed Si coating than on the substrate when the cells were cultured for 3 days and 5 days. This indicates that the laser-alloyed Si coating exhibits much
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Fig. 4. Microstructures of the top zones of the laser-alloyed coatings. (a) Si coating. (b) Ti + Si coating. (c) 5Ti + 3Si coating.
better bioactivity. The results also verify that the laser-alloyed Si coating exhibits good bioactivity. 4. Discussion The microstructural morphology and phase structure in those laser-alloyed coatings are mainly determined by atomic ratio of Si–Ti and solidification cooling condition. The Si contents
Fig. 5. XRD patterns of the laser-alloyed 5Ti + 3Si coating, the laser-alloyed Ti + Si coating, the laser-alloyed Si coating and the Ti–6Al–4V substrate.
in three kinds of laser-alloyed coatings are in a sequence of Si > Ti + Si > 5Ti + 3Si. It can be seen from Figs. 2–4 that the interfacial regions of all laser-alloyed coatings have epitaxially-grown planar crystals. This can be attributed to the large temperature gradient and high nucleation rate in the interfacial zone [24]. Besides, all of the laser-alloyed coatings are characterized by block-like crystals, lathlike crystals and dendrite crystals. During solidification of molten pool, the laser-alloyed coatings firstly form block-like crystals.
Fig. 6. Microhardness profiles of the laser-alloyed coatings from surface to substrate.
Y. Wu et al. / Applied Surface Science 305 (2014) 16–23
Fig. 7. Potentio-dynamic polarization curves of different samples in Hank’s solution.
And then the block-like crystals will grow up as lath shape. With the progress of solidification, dendrite crystals will form eventually. According to the Ti–Si phase diagram [20], Ti5 Si3 solidifies as primary phase during solidification of the laser-alloyed 5Ti + 3Si and Ti + Si coatings. And then eutectic reaction (i.e., L → Ti5 Si3 + ␣Ti) happened in the coatings. As for the laser-alloyed Si coating, Ti5 Si3 + ␣-Ti eutectic structure and TiSi2 + Si eutectic structure will form after the solidification of primary phase. From Ti–Si phase diagram, the equilibrium phases in the laseralloyed Ti + Si and 5Ti + 3Si coatings are in accordance with the phase diagram and the equilibrium phases in the laser-alloyed
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Si coating are incompatible with the phase diagram. Logically, the phase structure in the laser-alloyed Si coating should be Ti5 Si3 + Ti5 Si4 , or Ti5 Si4 + TiSi, or TiSi + TiS2 . However, only TiSi2 and Ti5 Si3 are detected by XRD in the laser-alloyed Si coating, which is inconsistent with the Ti–Si phase diagram. This may be attributed to inhomogeneous concentration of Si in the molten pool. During laser alloying process, the solidification rate is so rapid that the mixed powders do not have enough time to uniformly dissolve into the molten pool, which results in Ti-rich and Si-rich melt. In the Si-rich region, the following reaction should take place according to the phase diagram: L → TiSi2 + Si, while the following reaction should take place according to the phase diagram in the Ti-rich region: L → ␣-Ti + Ti5 Si3 . As a result, there is no TiSi or Ti5 Si4 in the laser-alloyed Si coating. The laser-alloyed coatings can improve the hardness of Ti–6Al–4V alloy remarkably. It is believed that the formation of Ti5 Si3 is beneficial to the increase of hardness [20–22]. The enhancement in the microhardness can also be ascribed to refinement during non-equilibrium solidification [25]. Besides, the microhardness of the laser-alloyed coatings shows an upward trend with an increase in Si content. This can be attributed to the fact that more titanium silicides will form when precursor mixed powders contain more Si. Moreover, the microhardness in the top zone of the laser-alloyed Si coating shows a fluctuation. The variation of the microhardness reveals that there is a change in the microstructure. The formation and inhomogeneous distribution of soft TiSi2 (microhardness 360 HV0.1 ) in the alloyed coating is thought to be responsible for hardness variation. Potentio-dynamic polarization curves show that the laseralloyed Si coating can improve the corrosion resistance of Ti–6Al–4V alloy. XRD analysis indicates that the laser-alloyed Si
Fig. 8. Morphologies and EDX result of corroded surfaces of different samples. (a) Si coating. (b) Ti–6Al–4V substrate. (c) EDX result of Si coating.
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Fig. 9. AO fluorescence staining of MC3T3-E1 cells cultured on the laser-alloyed Si coating and Ti–6Al–4V substrate. (a) Si coating for 1day. (b) Ti–6Al–4V substrate for 1day. (c) Si coating for 3 days. (d) Ti–6Al–4V substrate for 3 days. (e) Si coating for 5 days. (f) Ti–6Al–4V substrate for 5 days.
coating is mainly covered by Ti5 Si3 and TiSi2 . The laser-alloyed Si coating exhibits good corrosion resistance. Thus, the formation of the Ti–Si compound contributes to the increase of Ecorr , and the decrease of Icorr . Furthermore, microstructural refinement and well-bonded interfaces stimulated by rapid solidification during the laser alloying process can lead to the improvement in corrosion resistance as well [26–28]. As a result, the corrosion resistance of the laser-alloyed Si coating is better than that of the Ti–6Al–4V substrate. The results of the AO fluorescence staining of MC3T3-E1 cells cultured on different samples reveal that the laser-alloyed Si coating exhibits a satisfying bioactivity. According to the previous research works, Si has a remarkable impact on stimulating attachment and proliferation of cells [7,9–13]. It is supposed that the Si on the surface of the coatings provides favorable sites for apatite nucleation and the early formation of extracellular matrix. Since Si could hold the proteins which are capable of promoting cell attachment through interaction with the integrins on the cells. It
is also worth mentioning that cell growth is denser on the laseralloyed Si coating when the cells are cultured for 3 days and 5 days. This may be caused by the formation of Si acid in the culture medium. The Si acid can lead to alkalinization of the medium through ion exchanging. And the alkalinization of medium has an influence on the sensitivity of voltage-activated calcium channels and cellular calcium entry, which will finally result in the increased proliferation of cells [13]. To sum up, it is possible that the chemical composition (Si) contributes to the improvement of bioactivity. 5. Conclusions (1) The laser-alloyed Ti–Si compound coatings with metallurgical bond to the substrate are free of pores or cracks. (2) The laser-alloyed coatings mainly present block-like crystals, lath-like crystals and dendrite crystals. Phase structure analysis confirms that the laser-alloyed Ti + Si and 5Ti + 3Si coatings primarily consist of ␣-Ti and Ti5 Si3 , and the laser-alloyed Si
Y. Wu et al. / Applied Surface Science 305 (2014) 16–23
coating chiefly consists of TiSi2 and Ti5 Si3 . This means that the laser-alloyed Si coating has the highest volume content of Ti–Si compounds. (3) The microhardness increases significantly after laser alloying process. The microhardness is in a range of 1500–1000 HV0.1 , 900–700 HV0.1 and 800–650 HV0.1 achieved by the laseralloyed coatings of Si, Ti + Si and 5Ti + 3Si, respectively. (4) The corrosion resistance of the laser-alloyed Si coating is better than that of the Ti–6Al–4V alloy. The Ecorr of the laser-alloyed Si coating increases from −398 mV to −290 mV (which means 27% improvement). Besides, the Icorr of the laser-alloyed Si coating is about 1/10 times in comparison with the substrate. (5) In the evaluation of bioactivity, cell growth seems to be the same for both of the samples on the first day. With the progress of the test, cell growth on the laser-alloyed Si coating is faster than that of the Ti–6Al–4V alloy. Acknowledgements The author would like to thank the financial support provided by the key project of Wenzhou City science and technology plan on laser and optoelectronic industry cluster, and Open Research Fund Program of Hubei-MOST KLOS & KLOBME. References [1] M.A. Gepreel, M. Niinomi, Biocompatibility of Ti-alloys for long-term implantation, J. Mech. Behav. Biomed. Mater. 20 (2013) 407–415. [2] M. Calin, A. Gebert, A.C. Ghinea, P.F. Gostin, S. Abdi, C. Mickel, J. Eckert, Designing biocompatible Ti-based metallic glasses for implant applications, Mater. Sci. Eng. C Mater. Biol. Appl. 33 (2013) 875–883. [3] K. Venkateswarlu, N. Rameshbabu, D. Sreekanth, A.C. Bose, V. Muthupandi, S. Subramanian, Fabrication and characterization of micro-arc oxidized fluoride containing titania films on Cp Ti, Ceram. Int. 39 (2013) 801–812. [4] C. Huang, Y. Zhang, R. Vila, J. Shen, Dry sliding wear behavior of laser clad TiVCrAlSi high entropy alloy coatings on Ti–6Al–4V substrate, Mater. Des. 41 (2012) 338–343. [5] P. Jiang, X.L. He, X.X. Li, L.G. Yu, H.M. Wang, Wear resistance of a laser surface alloyed Ti–6Al–4V alloy, Surf. Coat. Technol. 130 (2000) 24–28. [6] C. Guo, J. Zhou, J. Zhao, L. Wang, Y. Yu, J. Chen, H. Zhou, Improvement of the oxidation and wear resistance of pure Ti by laser-cladding Ti3 Al coating at elevated temperature, Tribol. Lett. 42 (2011) 151–159. [7] Y. Ramaswamy, C. Wu, H. Zhou, H. Zreiqat, Biological response of human bone cells to zinc-modified Ca-Si-based ceramics, Acta. Biomater. 4 (2008) 1487–1497. [8] H. Zreiqat, P. Evans, C.R. Howlett, Effect of surface chemical modification of bioceramic on phenotype of human bone-derived cells, J. Biomed. Mater. Res. 44 (1999) 389–396.
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