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Microstructure, wear resistance and cell proliferation ability of in situ synthesized Ti–B coating produced by laser alloying
Optics & Laser Technology 67 (2015) 176–182
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Microstructure, wear resistance and cell proliferation ability of in situ synthesized Ti–B coating produced by laser alloying Y. Wu a,b, A.H. Wang a,n, Z. Zhang a, H.B. Xia b, Y.N. Wang b a State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China b The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) & Key Laboratory of Oral Biomedicine Ministry of Education, Wuhan University, Wuhan 430079, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 July 2014 Received in revised form 9 October 2014 Accepted 12 October 2014 Available online 10 November 2014
Ti–B titanium compound coating with satisfactory wear resistance and good cell proliferation ability was in situ synthesized by a laser alloying technique on a Ti6Al4V substrate. Microstructural analysis indicated that defect-free coating and good interface between the coating and the substrate were obtained. Microstructure and phase structure analysis revealed that the produced coating mainly exhibited dot-like crystals and included α-Ti, TiB, AlTi, AlTi2 and AlTi3, respectively. Microhardness and wear resistance test indicated that the microhardness and the wear resistance were improved greatly by laser alloying treatment in comparison with the substrate. Evaluation of cell proliferation showed that the produced coating was suitable for cell proliferation. & 2014 Elsevier Ltd. All rights reserved.
Keywords: In situ synthesized Ti–B coating Tribology property Cell proliferation
1. Introduction Ti and titanium alloys present excellent mechanical properties, corrosion resistance and biocompatibility [1,2]. These features have made Ti and titanium alloys clinically accepted orthopedic implant material. However, poor wear resistance and high friction coefficients of Ti and titanium alloys limit their applications towards articulating surfaces [3–6]. Therefore, to improve the wear resistance of Ti and titanium alloys is of great importance. Surface modification is often adopted to improve wear resistance of Ti and titanium alloys. Laser treatment is believed to be a promising method, since this method has many advantages. For example, it exhibits capability to in situ synthesize reinforcement phase and excellent bonding strength between coating and substrate. Research works regarding improving properties of Ti and titanium alloys by laser treatment have been carried out by many researchers [4–16]. Titanium boride is considered as excellent reinforcement for Ti and titanium alloys due to its high hardness and excellent wear resistance [17]. Tian et al. [4] used a CO2 laser to fabricate boride layers on Ti6Al4V alloy. They found that the laser-alloyed titanium boride layers have high microhardness and excellent wear resistance. They also pointed out that laser scanning speed has a remarkable influence on the size and the morphology of borides. Kühnle and Partes [5] synthesized titanium boride and titanium carbide on a pure titanium substrate by
n
Corresponding author. Tel.: þ 86 27 87180507. E-mail address: [email protected] (A.H. Wang).
http://dx.doi.org/10.1016/j.optlastec.2014.10.009 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
Selective Laser Melting. Results from their study revealed that dilution of molten pool could influence the products of in-situ reaction significantly. Das et al. [6] fabricated TiB–TiN reinforced Ti6Al4V alloy composite coatings on Ti by using laser based additive manufacturing technology. They found that the composite coatings possessed excellent wear resistance, high stiffness and suitable biocompatibility. Tian et al. [17] fabricated composite coatings on Ti6Al4V which contained titanium carbides and borides compounds by laser surface alloying technique. They believed it was the in-situ-formed titanium borides and carbides that led to the excellent wear resistance of the coatings. Badini et al. [18] studied laser boronizing of titanium alloys by adopting a CO2 laser. Their results revealed that the coating fabricated by CO2 laser was rough and uneven. Boron is a trace element which plays an important role in many life processes [19,20]. Recent studies have indicated that boron containing materials can be used as potential biomaterials. Das et al. [6] found that TiB–TiN reinforced Ti6Al4V alloy composite coatings on Ti were non-toxic and present similar cell materials interactions to that of CP-Ti. Wu et al. [19] prepared porous mesopore bioactive glass scaffolds by using sol–gel method. They found that boron plays an important role in enhancing osteoblast proliferation in the scaffold. Gorustovich et al. [20] evaluated the neoformed bone tissue around boron-modified bioactive glass particles. Results of their study showed that boron-modified bioactive glass could enhance bone formation. Majumdar et al. [21] prepared Ti–35Nb–5.7Ta–7.2Zr and Ti–35Nb–5.7Ta–7.2Zr– 0.5B alloys by arc melting. They found that both of the alloys had better cell adhesion and spreading than polystyrene.
Y. Wu et al. / Optics & Laser Technology 67 (2015) 176–182
Based on the facts discussed above, it is clear that little research work has been conducted about cell proliferation ability of the laser-alloyed boride coating. Besides, the laser-alloyed boride coatings fabricated in the previous research works usually exhibited rough surface [4,18]. In this work, laser alloying of Ti6Al4V alloy was carried out to obtain in situ synthesized Ti–B coating with smooth surface. The wear resistance and cell proliferation ability of the coating were studied.
2. Materials and methods A Ti6Al4V alloy, of size 20 mm 20 mm 3 mm, was employed as the substrate for laser alloying treatment. It was polished by SiC grit paper to reduce surface roughness prior to coating operation. 99.9% purity boron powders, with an average particle size of about 46 μm, blended with diluted cellulose acetate solution, were preplaced on the substrate. Concentration of the powders in the solution was about 210 g/L. The solution was sprayed on the substrate by using an airbrush. Average thickness of the preplaced coating was approximately 0.2 mm. Laser alloying experiments were carried out by a HG Laser LCY-400 pulsed Nd:YAG laser. Large area coating is achieved by overlapping treatment and the specification of the laser is listed in Table 1. Argon was utilized as shield gas during laser alloying process, and a special shielding gas apparatus was adopted as shown in Ref. [22]. Surface roughness (Ra) was measured from center line of the sample, by using a Mahr MarSurf PS1 surface roughometer. The laser-alloyed coating was machined by means of wire-cut to reveal the microstructure of the cross-section. The sample was slightly grinded on SiC grit paper, and then polished by a polishing machine. Subsequently, it was etched in a solution of HF, HNO3 and H2O in volume ratio of 1:1:1 to reveal the microstructure. A JEOL JSM-5610LV scanning electron microscope (SEM) incorporating energy dispersive X-ray analysis (EDX) was utilized to characterize cross-section of the laser-alloyed coating, with an electron beam diameter of 8 nm. Phase structure of the laser-alloyed coating was identified using a Bruker D8 ADVANCE X-ray diffractometer (XRD) with Cu Kα radiation (voltage was 2.2 kV, scanning speed was 4 1/min, and current was 20 mA). Microhardness of the laser-alloyed coating was evaluated using a Buehler Micromet II Microhardness Tester, with a load of 100 gf for 15 s. For the microhardness measurement, on an average of six different readings were used. Wear test was carried out using a Henxu MMS-1G pin-on-disc wear tester, with a load of 300 N for sliding distance of 3000 m. Sliding velocity was 10 m/s and a 52100 steel honing wheel was used as wear couple. For wear loss measurement, reduction in weight was measured using an electronic balance after the wear test. Samples were cleaned ultrasonically in ethyl alcohol before the wear loss measurement. Friction coefficient was recorded by the tester system continuously. In the wear test, sandblast acid-etched (SA) Ti6Al4V was adopted for comparison. To get SA Ti6Al4V, Ti6Al4V was blasted with 0.25–0.50 mm corundum grit at 5 bar for 1 min, and then etched in a solution of HCl and H2SO4 in volume ratio of 1:1 at 60 1C for 30 min. The Ra values of the laser-alloyed coating and the
177
SA Ti6Al4V were 2.75–2.85 mm and 2.37–2.51 mm, respectively. Each three samples of the laser-alloyed samples and the SA Ti6Al4V samples were used in the wear test. Cell proliferation ability of the laser-alloyed coating was evaluated by using MC3T3-E1 subclone 4 (ATCC CRL-2593, Lot no. 3225550), derived from newborn mouse calvaria. They were cultured in alphaMinimum Essential Medium (α-MEM) at 37 1C in humidified air with 5% CO2. The α-MEM was supplemented with 10% fetal bovine serum (FBS), 100 unit/ml penicillin and 100 mg/ml streptomycin. After 1 and 3 days, the samples were washed thrice with phosphate buffer saline (PBS), and fixed with 95% alcohol for 10 min at 4 1C. Before visualization on a Leica DM4000B microscope, the cultured cells were stained for 5 min with 0.025% acridine orange (AO) and washed with 1% calcium chloride solution for 1 min. Each three samples of the laseralloyed samples and the SA Ti6Al4V samples were adopted to evaluate cell proliferation. Cell counting was performed manually. The percentage of viable cells was compared, normalized by setting the number of cells on the SA Ti6Al4V surface as 100%. Student's t-test was used to perform statistical analysis, with po0.05 considered statistically significant.
3. Results 3.1. Investigation of laser alloying parameters Fig. 1 shows the influence of average power on the surface morphology of the laser-alloyed coatings while other laser parameters are fixed. Uneven surface was observed at an average laser power less than 210 W, which indicated that the pre-placed powder was not melted completely because of the insufficient laser energy, as shown in Fig. 1(a). Even surface was produced at an average laser power of 210 W, but average laser power higher than 210 W resulted in rough surface again since much higher laser power caused evaporation and mass loss of the pre-placed powder, as shown in Fig. 1(b)–(d). Fig. 2 displays the effect of scanning rate on the surface morphology of the laser-alloyed coatings while other laser parameters are fixed. As shown in Fig. 2(a) and (b), rough surface is obtained at the scanning rates of 150 mm/min and 200 mm/min, respectively, which indicated that high laser energy caused evaporation and mass loss of the pre-placed powder. Even surface was produced at the scanning rate of 250 mm/min, but scanning rate up to 300 mm/min resulted to uneven surface again because insufficient laser energy could not melt the pre-placed powder completely, as respectively shown in Fig. 2(c) and (d). Fig. 3 shows the surface morphology of the laser-alloyed coatings at different pulse durations while other laser parameters are fixed. It can be seen from Fig. 3(a) that pulse duration lower than 1.5 ms created rough surface since high laser energy caused evaporation and mass loss of the pre-placed powder. Even surface was produced at the pulse duration of 1.5 ms, but pulse duration higher than 1.5 ms could induce rough surface again since insufficient laser energy could not melt the pre-placed powder completely, as shown in Fig. 3(b)–(d), respectively. Therefore, the suitable parameters adopted to perform laser alloying process are obtained and listed in Table 2, and the surface morphology produced by the optimal parameters is smooth and uniform, as shown in Fig. 4. 3.2. Microstructure and phase structure
Table 1 Specification of laser used in the experiments. Wavelength Maximum (nm) average power (W)
Focal length (mm)
Plus duration (ms)
Frequency Maximum pulse energy (Hz) (J)
1064
100
0.2–20
80
400
1–200
Fig. 5 shows the microstructure of the cross-section of the laseralloyed coating at different magnifications. It is clear that the coating is smoother than the coatings in Refs. [4] and [18]. No defects were observed from the cross-sectional view of the laseralloyed coating, and the corresponding microstructure was mainly composed of dot-like crystal. EDX analysis was carried out in an
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Fig. 1. Surface morphology of the laser-alloyed coatings at different average laser powers (scanning velocity is 250 mm/min, pulse duration is 1.5 ms): (a) 180 W, (b) 210 W, (c) 240 W, and (d) 270 W.
Fig. 2. Surface morphology of the laser-alloyed coatings produced at different scanning rates (average power is 210 W, pulse duration is 1.5 ms): (a) 150 mm/min, (b) 200 mm/min, (c) 250 mm/min, and (d) 300 mm/min.
Fig. 3. Surface morphology of the laser-alloyed coatings produced at different pulse durations (scanning rate is 250 mm/min, average power is 210 W): (a) 1.2 ms, (b) 1.5 ms, (c) 2.0 ms, and (d) 2.3 ms.
Table 2 Parameters of laser alloying process. Average power (W)
Scanning rate (mm/min)
Pulse duration (ms)
Frequency (Hz)
Beam diameter (mm)
Track-to-track overlap ratio (%)
Pulse-to-pulse overlap ratio (%)
210
250
1.5
45
2
35
95
3.3. Microhardness test and tribological property
Fig. 4. Surface morphology of the laser-alloyed coating produced at suitable processing parameters.
area of 10 20 μm2 to measure the mean content of elements in the laser-alloyed coating, as shown in Fig. 5(c). The mean content of main elements in the laser-alloyed coating was 5.58Al–25.39B– 69.03Ti (at%). XRD analysis indicated the laser-alloyed coating was composed of α-Ti, TiB, AlTi, AlTi2 and AlTi3, as shown in Fig. 6. A combined analysis of EDX and XRD showed that the dot-like crystal contained TiB, which means that the TiB phase could be in-situ synthesized by laser alloying of pre-placing B powder onto the Ti6Al4V substrate.
Fig. 7 illustrates microhardness profile of the laser-alloyed coating. Obviously, the laser-alloyed zone displayed much higher microhardness (1000–1500 HV0.1), and the microhardness decreased gradually from the surface to the substrate (about 400 HV0.1). The decrease in microhardness along depth direction of the laser-alloyed coating is caused by uneven distribution of TiB. The top region of the molten pool should have a higher B concentration, since the density of B (2.3 g/cm3) is lower than that of liquid Ti (4.13 g/cm3). As a result, more TiB was synthesized in the top region, leading to higher microhardness. The friction coefficient of the laser-alloyed coating (about 0.22–0.26) was much lower than that of the Ti6Al4V substrate (about 0.35–0.5), as shown in Fig. 8. The worn surface of the laser-alloyed coating was characterized by the presence of wear debris while the worn surface of the Ti6Al4V substrate presented rough surface and intense plastic deformation. The measurement of wear loss revealed that the average weight loss of the Ti6Al4V substrate (about 180 mg) is approximately 11 times higher than that of the coating (about 16.2 mg). Consequently, the in situ synthesized Ti–B coating increased the wear resistance of the Ti6Al4V substrate significantly.
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Fig. 5. Overview and microstructure of the cross-section of the laser-alloyed coating. (a) Overview of the laser-alloyed coating. (b) Mediate zone of the laser-alloyed coating (content of main elements in the marked point is 71.72B–27.07Ti–1.21Al (at%)). (c) Top zone of the laser-alloyed coating.
Fig. 7. Microhardness profile of the laser-alloyed coating from surface to substrate. Fig. 6. XRD pattern of the laser-alloyed coating.
3.4. Cell proliferation ability of the laser-alloyed coating Images of the AO fluorescence staining of MC3T3-E1 cells cultured on the laser-alloyed coating and the Ti6Al4V substrate are shown in Fig. 10. Viability of the cells on the laser-alloyed coating and the Ti6Al4V substrate is shown in Fig. 11. On the first day, cell growth was denser on the substrate than on the laseralloyed coating. There was no significant difference (p40.05) in cell growth between the Ti6Al4V substrate and the laser-alloyed coating after 3 days of culture, which suggests that the cells on the surface of the Ti6Al4V substrate and the surface of the laser-alloyed coating
have similar viability. This observation indicates that the surface of the laser-alloyed coating is suitable for cell proliferation.
4. Discussion TiB possesses the orthorhombic structure (B27), characterized by zigzagging chains of boron along the [010] direction [23–25]. TiB usually has a needle-shaped morphology with a large aspect ratio, since TiB has much faster growth rate along the [010] direction [23,24]. However, the result in this study shows that
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TiB crystal exhibits a nearly equiaxed morphology. This is attributed to the influence of B diffusion on the formation and growth of TiB. Fan et al. [23] found that B atoms are responsible for the growth of TiB. Therefore, the supply of B atoms will affect the growth rates along different metallographic directions significantly [24]. According to the XRD pattern, the content of Ti in the laser-alloyed coating is much higher than that of B. The formation of titanium boride nuclei consumes a large number of B atoms. Therefore, the lack of B atoms makes the growth of TiB difficult. As a result, the difference between growth rates along different metallographic directions is not that significant, and the TiB crystal will form a fairly equiaxed morphology [24]. It is known that crystal morphology has a remarkable effect on alloy properties. Many research works pointed out that dendritic compounds can increase stress in the coating and are apt to promote cracking [17,23]. Thus, the dot-like crystals formed during laser alloying process are beneficial for mechanical properties of the laseralloyed coating. Further, the XRD pattern reveals that the main titanium boride formed in the laser-alloyed coating is TiB. This result indicates the pre-placed B powders can mix well with Ti in the molten pool. Besides, according to Ti–B binary diagram, TiB-β(Ti) precipitates from the molten pool after solidification of primary phase during solidification of the laser-alloyed coating. As shown in Fig. 7, microhardness of the Ti6Al4V substrate is much lower than that of the laser-alloyed coating. Combining the XRD result, it is believed that the phase constituent of the laser-
alloyed coating contributes to the increase in microhardness. It means that formation of TiB hard phase and titanium aluminides intermetallic compounds leads to the enhancement of the microhardness [17,26,27]. The results of the wear test reveal that the laser-alloyed coating exhibits excellent wear resistance. The high hardness of the laser-alloyed coating may contribute to its low wear loss, since it is believed that sliding wear resistance is proportional to the hardness of the wearing area [7]. In this study, microhardness of the laser-alloyed coating is much higher than that of the substrate. Thus, the laser-alloyed coating is able to withstand the external load. Meanwhile, low friction coefficient of the laser-alloyed coating can reduce friction, leading to low weight loss. Besides, TiB in the laser-alloyed coating is in situ synthesized, which improves the bonding strength between TiB and the matrix. This is also beneficial for the improvement of wear resistance [26]. In addition, it can be seen from Fig. 9(b) that some areas of the laser-alloyed coating are covered with debris layer, which indicates that three-body abrasive wear occurs. That is, wear debris produced during the wear test can occupy the space between the laser-alloyed coating and the counterpart. They can improve the wear resistance greatly by acting as lubricants [28]. Thus, the presence of the third body also contributes to the excellent wear resistance of the laser-alloyed coating. The surface topography and chemical composition of biomaterials have a remarkable influence on biological property [29]. In this study, both samples have similar surface roughness. Previous research works reveal that boron can influence bone-related gene expression and the formation of mineralized nodules [19]. Besides, it is found that boron can affect cell proliferation and growth in form of H3BO3 [20]. Moreover, Majumdara et al. [21] reported that boron in the Ti–35Nb–5.7Ta–7.2Zr–0.5B alloy may result in change in the surface characteristics of the alloy. Further, Okazaki et al. [30] found that ions of Al and V could jeopardize the viabilities of cells. Thus, it is possible that the chemical composition of the coating leads to the variation in cell proliferation ability. In our previous study, Ti–Si coating was in situ synthesized on the Ti6Al4V by laser alloying [31]. The results reveal that although the two different coatings exhibit similar microhardness, the laseralloyed Ti–Si coating has better cell proliferation ability in comparison with the laser-alloyed Ti–B coating.
5. Conclusions
Fig. 8. Friction coefficients of the Ti6Al4V substrate and the laser-alloyed coating.
Titanium compound coating reinforced with in situ synthesized titanium boride was fabricated on Ti6Al4V substrate by using a laser alloying technique. The fabricated coating has no pores or cracks,
Fig. 9. Morphologies of the worn surfaces: (a) Ti6Al4V substrate, and (b) laser-alloyed coating.
Y. Wu et al. / Optics & Laser Technology 67 (2015) 176–182
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Fig. 10. AO fluorescence staining of MC3T3-E1 cells cultured on the Ti6Al4V substrate and the laser-alloyed coating. (a) Ti6Al4V substrate for 1day. (b) Laser-alloyed coating for 1day. (c) Ti6Al4V substrate for 3 days. (d) Laser-alloyed coating for 3 days.
Acknowledgments The author would like to thank the financial support provided by the key project of Wenzhou City Science (Grant no. J20120020) and Technology Plan on laser and optoelectronic industry cluster, and Open Research Fund Program of Hubei-MOST KLOS & KLOBME (Grant no. 201203). References
Fig. 11. Viability of cells on the Ti6Al4V substrate and the laser-alloyed coating for 1 and 3 days of culture.
and exhibits good interface between the coating and the substrate. The laser-alloyed coating mainly presents dot-like crystals and contains α-Ti and TiB, AlTi, AlTi2 and AlTi3. The microhardness of the laser-alloyed coating is in the range of 1000–1500 HV0.1, which is much higher than that of the substrate. Compared with the substrate, the laser-alloyed coating possesses better wear resistance. In the evaluation of cell proliferation ability, cell growth on the substrate is faster than that of the laser-alloyed coating on the first day. With the progress of the test, cell growth seems to be the same for both of the samples.
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Microstructure, wear resistance and cell proliferation ability of in situ synthesized Ti–B coating produced by laser alloying Y. Wu a,b, A.H. Wang a,n, Z. Zhang a, H.B. Xia b, Y.N. Wang b a State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China b The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) & Key Laboratory of Oral Biomedicine Ministry of Education, Wuhan University, Wuhan 430079, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 July 2014 Received in revised form 9 October 2014 Accepted 12 October 2014 Available online 10 November 2014
Ti–B titanium compound coating with satisfactory wear resistance and good cell proliferation ability was in situ synthesized by a laser alloying technique on a Ti6Al4V substrate. Microstructural analysis indicated that defect-free coating and good interface between the coating and the substrate were obtained. Microstructure and phase structure analysis revealed that the produced coating mainly exhibited dot-like crystals and included α-Ti, TiB, AlTi, AlTi2 and AlTi3, respectively. Microhardness and wear resistance test indicated that the microhardness and the wear resistance were improved greatly by laser alloying treatment in comparison with the substrate. Evaluation of cell proliferation showed that the produced coating was suitable for cell proliferation. & 2014 Elsevier Ltd. All rights reserved.
Keywords: In situ synthesized Ti–B coating Tribology property Cell proliferation
1. Introduction Ti and titanium alloys present excellent mechanical properties, corrosion resistance and biocompatibility [1,2]. These features have made Ti and titanium alloys clinically accepted orthopedic implant material. However, poor wear resistance and high friction coefficients of Ti and titanium alloys limit their applications towards articulating surfaces [3–6]. Therefore, to improve the wear resistance of Ti and titanium alloys is of great importance. Surface modification is often adopted to improve wear resistance of Ti and titanium alloys. Laser treatment is believed to be a promising method, since this method has many advantages. For example, it exhibits capability to in situ synthesize reinforcement phase and excellent bonding strength between coating and substrate. Research works regarding improving properties of Ti and titanium alloys by laser treatment have been carried out by many researchers [4–16]. Titanium boride is considered as excellent reinforcement for Ti and titanium alloys due to its high hardness and excellent wear resistance [17]. Tian et al. [4] used a CO2 laser to fabricate boride layers on Ti6Al4V alloy. They found that the laser-alloyed titanium boride layers have high microhardness and excellent wear resistance. They also pointed out that laser scanning speed has a remarkable influence on the size and the morphology of borides. Kühnle and Partes [5] synthesized titanium boride and titanium carbide on a pure titanium substrate by
n
Corresponding author. Tel.: þ 86 27 87180507. E-mail address: [email protected] (A.H. Wang).
http://dx.doi.org/10.1016/j.optlastec.2014.10.009 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
Selective Laser Melting. Results from their study revealed that dilution of molten pool could influence the products of in-situ reaction significantly. Das et al. [6] fabricated TiB–TiN reinforced Ti6Al4V alloy composite coatings on Ti by using laser based additive manufacturing technology. They found that the composite coatings possessed excellent wear resistance, high stiffness and suitable biocompatibility. Tian et al. [17] fabricated composite coatings on Ti6Al4V which contained titanium carbides and borides compounds by laser surface alloying technique. They believed it was the in-situ-formed titanium borides and carbides that led to the excellent wear resistance of the coatings. Badini et al. [18] studied laser boronizing of titanium alloys by adopting a CO2 laser. Their results revealed that the coating fabricated by CO2 laser was rough and uneven. Boron is a trace element which plays an important role in many life processes [19,20]. Recent studies have indicated that boron containing materials can be used as potential biomaterials. Das et al. [6] found that TiB–TiN reinforced Ti6Al4V alloy composite coatings on Ti were non-toxic and present similar cell materials interactions to that of CP-Ti. Wu et al. [19] prepared porous mesopore bioactive glass scaffolds by using sol–gel method. They found that boron plays an important role in enhancing osteoblast proliferation in the scaffold. Gorustovich et al. [20] evaluated the neoformed bone tissue around boron-modified bioactive glass particles. Results of their study showed that boron-modified bioactive glass could enhance bone formation. Majumdar et al. [21] prepared Ti–35Nb–5.7Ta–7.2Zr and Ti–35Nb–5.7Ta–7.2Zr– 0.5B alloys by arc melting. They found that both of the alloys had better cell adhesion and spreading than polystyrene.
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Based on the facts discussed above, it is clear that little research work has been conducted about cell proliferation ability of the laser-alloyed boride coating. Besides, the laser-alloyed boride coatings fabricated in the previous research works usually exhibited rough surface [4,18]. In this work, laser alloying of Ti6Al4V alloy was carried out to obtain in situ synthesized Ti–B coating with smooth surface. The wear resistance and cell proliferation ability of the coating were studied.
2. Materials and methods A Ti6Al4V alloy, of size 20 mm 20 mm 3 mm, was employed as the substrate for laser alloying treatment. It was polished by SiC grit paper to reduce surface roughness prior to coating operation. 99.9% purity boron powders, with an average particle size of about 46 μm, blended with diluted cellulose acetate solution, were preplaced on the substrate. Concentration of the powders in the solution was about 210 g/L. The solution was sprayed on the substrate by using an airbrush. Average thickness of the preplaced coating was approximately 0.2 mm. Laser alloying experiments were carried out by a HG Laser LCY-400 pulsed Nd:YAG laser. Large area coating is achieved by overlapping treatment and the specification of the laser is listed in Table 1. Argon was utilized as shield gas during laser alloying process, and a special shielding gas apparatus was adopted as shown in Ref. [22]. Surface roughness (Ra) was measured from center line of the sample, by using a Mahr MarSurf PS1 surface roughometer. The laser-alloyed coating was machined by means of wire-cut to reveal the microstructure of the cross-section. The sample was slightly grinded on SiC grit paper, and then polished by a polishing machine. Subsequently, it was etched in a solution of HF, HNO3 and H2O in volume ratio of 1:1:1 to reveal the microstructure. A JEOL JSM-5610LV scanning electron microscope (SEM) incorporating energy dispersive X-ray analysis (EDX) was utilized to characterize cross-section of the laser-alloyed coating, with an electron beam diameter of 8 nm. Phase structure of the laser-alloyed coating was identified using a Bruker D8 ADVANCE X-ray diffractometer (XRD) with Cu Kα radiation (voltage was 2.2 kV, scanning speed was 4 1/min, and current was 20 mA). Microhardness of the laser-alloyed coating was evaluated using a Buehler Micromet II Microhardness Tester, with a load of 100 gf for 15 s. For the microhardness measurement, on an average of six different readings were used. Wear test was carried out using a Henxu MMS-1G pin-on-disc wear tester, with a load of 300 N for sliding distance of 3000 m. Sliding velocity was 10 m/s and a 52100 steel honing wheel was used as wear couple. For wear loss measurement, reduction in weight was measured using an electronic balance after the wear test. Samples were cleaned ultrasonically in ethyl alcohol before the wear loss measurement. Friction coefficient was recorded by the tester system continuously. In the wear test, sandblast acid-etched (SA) Ti6Al4V was adopted for comparison. To get SA Ti6Al4V, Ti6Al4V was blasted with 0.25–0.50 mm corundum grit at 5 bar for 1 min, and then etched in a solution of HCl and H2SO4 in volume ratio of 1:1 at 60 1C for 30 min. The Ra values of the laser-alloyed coating and the
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SA Ti6Al4V were 2.75–2.85 mm and 2.37–2.51 mm, respectively. Each three samples of the laser-alloyed samples and the SA Ti6Al4V samples were used in the wear test. Cell proliferation ability of the laser-alloyed coating was evaluated by using MC3T3-E1 subclone 4 (ATCC CRL-2593, Lot no. 3225550), derived from newborn mouse calvaria. They were cultured in alphaMinimum Essential Medium (α-MEM) at 37 1C in humidified air with 5% CO2. The α-MEM was supplemented with 10% fetal bovine serum (FBS), 100 unit/ml penicillin and 100 mg/ml streptomycin. After 1 and 3 days, the samples were washed thrice with phosphate buffer saline (PBS), and fixed with 95% alcohol for 10 min at 4 1C. Before visualization on a Leica DM4000B microscope, the cultured cells were stained for 5 min with 0.025% acridine orange (AO) and washed with 1% calcium chloride solution for 1 min. Each three samples of the laseralloyed samples and the SA Ti6Al4V samples were adopted to evaluate cell proliferation. Cell counting was performed manually. The percentage of viable cells was compared, normalized by setting the number of cells on the SA Ti6Al4V surface as 100%. Student's t-test was used to perform statistical analysis, with po0.05 considered statistically significant.
3. Results 3.1. Investigation of laser alloying parameters Fig. 1 shows the influence of average power on the surface morphology of the laser-alloyed coatings while other laser parameters are fixed. Uneven surface was observed at an average laser power less than 210 W, which indicated that the pre-placed powder was not melted completely because of the insufficient laser energy, as shown in Fig. 1(a). Even surface was produced at an average laser power of 210 W, but average laser power higher than 210 W resulted in rough surface again since much higher laser power caused evaporation and mass loss of the pre-placed powder, as shown in Fig. 1(b)–(d). Fig. 2 displays the effect of scanning rate on the surface morphology of the laser-alloyed coatings while other laser parameters are fixed. As shown in Fig. 2(a) and (b), rough surface is obtained at the scanning rates of 150 mm/min and 200 mm/min, respectively, which indicated that high laser energy caused evaporation and mass loss of the pre-placed powder. Even surface was produced at the scanning rate of 250 mm/min, but scanning rate up to 300 mm/min resulted to uneven surface again because insufficient laser energy could not melt the pre-placed powder completely, as respectively shown in Fig. 2(c) and (d). Fig. 3 shows the surface morphology of the laser-alloyed coatings at different pulse durations while other laser parameters are fixed. It can be seen from Fig. 3(a) that pulse duration lower than 1.5 ms created rough surface since high laser energy caused evaporation and mass loss of the pre-placed powder. Even surface was produced at the pulse duration of 1.5 ms, but pulse duration higher than 1.5 ms could induce rough surface again since insufficient laser energy could not melt the pre-placed powder completely, as shown in Fig. 3(b)–(d), respectively. Therefore, the suitable parameters adopted to perform laser alloying process are obtained and listed in Table 2, and the surface morphology produced by the optimal parameters is smooth and uniform, as shown in Fig. 4. 3.2. Microstructure and phase structure
Table 1 Specification of laser used in the experiments. Wavelength Maximum (nm) average power (W)
Focal length (mm)
Plus duration (ms)
Frequency Maximum pulse energy (Hz) (J)
1064
100
0.2–20
80
400
1–200
Fig. 5 shows the microstructure of the cross-section of the laseralloyed coating at different magnifications. It is clear that the coating is smoother than the coatings in Refs. [4] and [18]. No defects were observed from the cross-sectional view of the laseralloyed coating, and the corresponding microstructure was mainly composed of dot-like crystal. EDX analysis was carried out in an
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Fig. 1. Surface morphology of the laser-alloyed coatings at different average laser powers (scanning velocity is 250 mm/min, pulse duration is 1.5 ms): (a) 180 W, (b) 210 W, (c) 240 W, and (d) 270 W.
Fig. 2. Surface morphology of the laser-alloyed coatings produced at different scanning rates (average power is 210 W, pulse duration is 1.5 ms): (a) 150 mm/min, (b) 200 mm/min, (c) 250 mm/min, and (d) 300 mm/min.
Fig. 3. Surface morphology of the laser-alloyed coatings produced at different pulse durations (scanning rate is 250 mm/min, average power is 210 W): (a) 1.2 ms, (b) 1.5 ms, (c) 2.0 ms, and (d) 2.3 ms.
Table 2 Parameters of laser alloying process. Average power (W)
Scanning rate (mm/min)
Pulse duration (ms)
Frequency (Hz)
Beam diameter (mm)
Track-to-track overlap ratio (%)
Pulse-to-pulse overlap ratio (%)
210
250
1.5
45
2
35
95
3.3. Microhardness test and tribological property
Fig. 4. Surface morphology of the laser-alloyed coating produced at suitable processing parameters.
area of 10 20 μm2 to measure the mean content of elements in the laser-alloyed coating, as shown in Fig. 5(c). The mean content of main elements in the laser-alloyed coating was 5.58Al–25.39B– 69.03Ti (at%). XRD analysis indicated the laser-alloyed coating was composed of α-Ti, TiB, AlTi, AlTi2 and AlTi3, as shown in Fig. 6. A combined analysis of EDX and XRD showed that the dot-like crystal contained TiB, which means that the TiB phase could be in-situ synthesized by laser alloying of pre-placing B powder onto the Ti6Al4V substrate.
Fig. 7 illustrates microhardness profile of the laser-alloyed coating. Obviously, the laser-alloyed zone displayed much higher microhardness (1000–1500 HV0.1), and the microhardness decreased gradually from the surface to the substrate (about 400 HV0.1). The decrease in microhardness along depth direction of the laser-alloyed coating is caused by uneven distribution of TiB. The top region of the molten pool should have a higher B concentration, since the density of B (2.3 g/cm3) is lower than that of liquid Ti (4.13 g/cm3). As a result, more TiB was synthesized in the top region, leading to higher microhardness. The friction coefficient of the laser-alloyed coating (about 0.22–0.26) was much lower than that of the Ti6Al4V substrate (about 0.35–0.5), as shown in Fig. 8. The worn surface of the laser-alloyed coating was characterized by the presence of wear debris while the worn surface of the Ti6Al4V substrate presented rough surface and intense plastic deformation. The measurement of wear loss revealed that the average weight loss of the Ti6Al4V substrate (about 180 mg) is approximately 11 times higher than that of the coating (about 16.2 mg). Consequently, the in situ synthesized Ti–B coating increased the wear resistance of the Ti6Al4V substrate significantly.
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Fig. 5. Overview and microstructure of the cross-section of the laser-alloyed coating. (a) Overview of the laser-alloyed coating. (b) Mediate zone of the laser-alloyed coating (content of main elements in the marked point is 71.72B–27.07Ti–1.21Al (at%)). (c) Top zone of the laser-alloyed coating.
Fig. 7. Microhardness profile of the laser-alloyed coating from surface to substrate. Fig. 6. XRD pattern of the laser-alloyed coating.
3.4. Cell proliferation ability of the laser-alloyed coating Images of the AO fluorescence staining of MC3T3-E1 cells cultured on the laser-alloyed coating and the Ti6Al4V substrate are shown in Fig. 10. Viability of the cells on the laser-alloyed coating and the Ti6Al4V substrate is shown in Fig. 11. On the first day, cell growth was denser on the substrate than on the laseralloyed coating. There was no significant difference (p40.05) in cell growth between the Ti6Al4V substrate and the laser-alloyed coating after 3 days of culture, which suggests that the cells on the surface of the Ti6Al4V substrate and the surface of the laser-alloyed coating
have similar viability. This observation indicates that the surface of the laser-alloyed coating is suitable for cell proliferation.
4. Discussion TiB possesses the orthorhombic structure (B27), characterized by zigzagging chains of boron along the [010] direction [23–25]. TiB usually has a needle-shaped morphology with a large aspect ratio, since TiB has much faster growth rate along the [010] direction [23,24]. However, the result in this study shows that
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TiB crystal exhibits a nearly equiaxed morphology. This is attributed to the influence of B diffusion on the formation and growth of TiB. Fan et al. [23] found that B atoms are responsible for the growth of TiB. Therefore, the supply of B atoms will affect the growth rates along different metallographic directions significantly [24]. According to the XRD pattern, the content of Ti in the laser-alloyed coating is much higher than that of B. The formation of titanium boride nuclei consumes a large number of B atoms. Therefore, the lack of B atoms makes the growth of TiB difficult. As a result, the difference between growth rates along different metallographic directions is not that significant, and the TiB crystal will form a fairly equiaxed morphology [24]. It is known that crystal morphology has a remarkable effect on alloy properties. Many research works pointed out that dendritic compounds can increase stress in the coating and are apt to promote cracking [17,23]. Thus, the dot-like crystals formed during laser alloying process are beneficial for mechanical properties of the laseralloyed coating. Further, the XRD pattern reveals that the main titanium boride formed in the laser-alloyed coating is TiB. This result indicates the pre-placed B powders can mix well with Ti in the molten pool. Besides, according to Ti–B binary diagram, TiB-β(Ti) precipitates from the molten pool after solidification of primary phase during solidification of the laser-alloyed coating. As shown in Fig. 7, microhardness of the Ti6Al4V substrate is much lower than that of the laser-alloyed coating. Combining the XRD result, it is believed that the phase constituent of the laser-
alloyed coating contributes to the increase in microhardness. It means that formation of TiB hard phase and titanium aluminides intermetallic compounds leads to the enhancement of the microhardness [17,26,27]. The results of the wear test reveal that the laser-alloyed coating exhibits excellent wear resistance. The high hardness of the laser-alloyed coating may contribute to its low wear loss, since it is believed that sliding wear resistance is proportional to the hardness of the wearing area [7]. In this study, microhardness of the laser-alloyed coating is much higher than that of the substrate. Thus, the laser-alloyed coating is able to withstand the external load. Meanwhile, low friction coefficient of the laser-alloyed coating can reduce friction, leading to low weight loss. Besides, TiB in the laser-alloyed coating is in situ synthesized, which improves the bonding strength between TiB and the matrix. This is also beneficial for the improvement of wear resistance [26]. In addition, it can be seen from Fig. 9(b) that some areas of the laser-alloyed coating are covered with debris layer, which indicates that three-body abrasive wear occurs. That is, wear debris produced during the wear test can occupy the space between the laser-alloyed coating and the counterpart. They can improve the wear resistance greatly by acting as lubricants [28]. Thus, the presence of the third body also contributes to the excellent wear resistance of the laser-alloyed coating. The surface topography and chemical composition of biomaterials have a remarkable influence on biological property [29]. In this study, both samples have similar surface roughness. Previous research works reveal that boron can influence bone-related gene expression and the formation of mineralized nodules [19]. Besides, it is found that boron can affect cell proliferation and growth in form of H3BO3 [20]. Moreover, Majumdara et al. [21] reported that boron in the Ti–35Nb–5.7Ta–7.2Zr–0.5B alloy may result in change in the surface characteristics of the alloy. Further, Okazaki et al. [30] found that ions of Al and V could jeopardize the viabilities of cells. Thus, it is possible that the chemical composition of the coating leads to the variation in cell proliferation ability. In our previous study, Ti–Si coating was in situ synthesized on the Ti6Al4V by laser alloying [31]. The results reveal that although the two different coatings exhibit similar microhardness, the laseralloyed Ti–Si coating has better cell proliferation ability in comparison with the laser-alloyed Ti–B coating.
5. Conclusions
Fig. 8. Friction coefficients of the Ti6Al4V substrate and the laser-alloyed coating.
Titanium compound coating reinforced with in situ synthesized titanium boride was fabricated on Ti6Al4V substrate by using a laser alloying technique. The fabricated coating has no pores or cracks,
Fig. 9. Morphologies of the worn surfaces: (a) Ti6Al4V substrate, and (b) laser-alloyed coating.
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Fig. 10. AO fluorescence staining of MC3T3-E1 cells cultured on the Ti6Al4V substrate and the laser-alloyed coating. (a) Ti6Al4V substrate for 1day. (b) Laser-alloyed coating for 1day. (c) Ti6Al4V substrate for 3 days. (d) Laser-alloyed coating for 3 days.
Acknowledgments The author would like to thank the financial support provided by the key project of Wenzhou City Science (Grant no. J20120020) and Technology Plan on laser and optoelectronic industry cluster, and Open Research Fund Program of Hubei-MOST KLOS & KLOBME (Grant no. 201203). References
Fig. 11. Viability of cells on the Ti6Al4V substrate and the laser-alloyed coating for 1 and 3 days of culture.
and exhibits good interface between the coating and the substrate. The laser-alloyed coating mainly presents dot-like crystals and contains α-Ti and TiB, AlTi, AlTi2 and AlTi3. The microhardness of the laser-alloyed coating is in the range of 1000–1500 HV0.1, which is much higher than that of the substrate. Compared with the substrate, the laser-alloyed coating possesses better wear resistance. In the evaluation of cell proliferation ability, cell growth on the substrate is faster than that of the laser-alloyed coating on the first day. With the progress of the test, cell growth seems to be the same for both of the samples.
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