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Characteristics of surface modified Ti-6Al-4V alloy by a series of YAG laser irradiation
Optics and Laser Technology 98 (2018) 106–112
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Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Characteristics of surface modified Ti-6Al-4V alloy by a series of YAG laser irradiation Xian Zeng a,b, Wenqin Wang b,c,⇑, Tomiko Yamaguchi b, Kazumasa Nishio d a
School of Materials Science, Wuhan University of Technology, Wuhan 430070, China Kyushu Institute of Technology, Kitakyushu 804-8550, Japan c School of Mechanical and Electrical Engineering, Nanchang University, Nanchang 330031, China d Nishinippon Institute of Technology, Kitakyushu 800-0394, Japan b
a r t i c l e
i n f o
Article history: Received 27 February 2017 Received in revised form 31 May 2017 Accepted 28 July 2017
Keywords: Ti (C, N) Laser irradiation Hardness Wear Ion release
a b s t r a c t In this study, a double-layer Ti (C, N) film was successfully prepared on Ti-6Al-4V alloy by a series of YAG laser irradiation in nitrogen atmosphere, aiming at improving the wear resistance. The effects of laser irradiation pass upon surface chemical composition, microstructures and hardness were investigated. The results showed that the surface chemicals were independent from laser irradiation pass, which the up layer of film was a mixture of TiN and TiC0.3N0.7, and the down layer was nitrogen-rich a-Ti. Both the surface roughness and hardness increased as raising the irradiation passes. However, surface deformation and cracks happened in the case above 3 passes’ irradiation. The wear resistance of laser modified sample by 3 passes was improved approximately by 37 times compared to the as received substrate. Moreover, the cytotoxic V ion released from laser modified sample was less than that of as received Ti-6Al-4V alloy in SBF, suggesting the potentiality of a new try to modify the sliding part of Ti-based hard tissue implants in future biomedical application. Ó 2017 Published by Elsevier Ltd.
1. Introduction Commercially Ti-6Al-4V alloy is extensively explored in many fields, such as aeronautical, marine, automobile and chemical industry, due to its excellent properties of low density, high fatigue strength and good corrosion resistance. Moreover, its acceptable biocompatibility and higher fatigue strength than pure titanium make it remain the dominant titanium alloy used in medical materials [1,2]. However, its low hardness and poor wear resistance limit its use in many applications. For instance, being used as hard tissue implant materials of femoral head, the heavy abrasion of sliding part not only reduce the component’s service life, but also bring out the potential risk that the alloying element of V and Al might be released into the human bodies as metal ion from wear debris, resulting in the neurotoxicity and cytotoxic effect [3]. Thus, to effectively decrease the debris and avoid the elution of these toxic elements, surface modification is strongly suggested. Ti(C, N) containing the merits of both TiN and TiC, is a promising reinforcing material due to its superior properties such as high hardness, good corrosion resistance and biocompatibilities. Tradi-
tional surface modification approaches to obtain such film covered a wide range of techniques, such as physical vapour deposition (PVD) [4,5], chemical vapour deposition (CVD) [6,7], ion implantation [8] and so on. However, regarding to the manufacturing cost, efficiency, flexibility and accuracy, these methods have been still considered as unsatisfied. Owing to high energy density and directionality, laser surface modification having capability for rapid processing, net-shape manufacture, and low heat effect on substrate, aroused researchers’ extensive study [9,10–13]. Thus, the present study is trying to synthesize Ti(C, N) film by laser irradiation, aiming at improving the wear resistance of Ti-6Al-4V alloy. The effects of laser irradiation pass upon chemical composition, microstructures and hardness were discussed. What’s more, the release of Al and V ions from the laser modified sample (by 3 passes) in simulated body fluid (SBF) was also measured for the purpose of a new try to modify the surface of Ti-based hard tissue implants in future biomedical application. 2. Materials and methods 2.1. Materials and laser processing
⇑ Corresponding author at: School of Mechanical and Electrical Engineering, Nanchang University, Nanchang 330031, China. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.optlastec.2017.07.048 0030-3992/Ó 2017 Published by Elsevier Ltd.
Ti-6Al-4V alloy sheets with size of 80 40 2 mm were used. The sheet’s surface was grinded by emery paper (SiC, 1200#) to
X. Zeng et al. / Optics and Laser Technology 98 (2018) 106–112
remove the oxide layer, and then cleaned by acetone. Before the laser irradiation, a graphite layer was sprayed on the cleaned surface equably, using an anti-adhesive agent (Ishihara Chemicals Co., Ltd. Japan), which contains graphite (5–10 wt.%), ethanol (35–45 wt.%) and butane (20–30 wt.%). And then the surface was air dried at room temperature. A Nd: YAG laser processor (MW2000, Sumitomo Heavy Industry. Japan) with continuous wave mode was used for laser irradiation. The specimen was put in a gas tight chamber with a quartz glass window which the laser beam can penetrate through and the sheet was irradiated inside the chamber. The fixed laser processing parameters were: laser output power of 1.6 kW, defocus distance of 70 mm giving a spot diameter of about 6 mm, laser travelling speed of 360 mm/min, and a shielding gas of pure nitrogen with a flow rate of 20 L/min. the samples were irradiated by 1, 3, 5 and 7 passes within the same irradiation track, separately. After each pass of irradiation, the residual graphite layer was cleaned by acetone, and then a new graphite layer was sprayed on the surface before the next irradiation.
2.2. Characterization The samples were cut transversely to the laser traverse direction and then polished by usual metallographic procedure, etched by a corrosion solution (HF: HNO3: H2O equal to 2:5:100 in volume). The microstructure was observed by optical microscope (OM, Nikon-L150) and field emission scanning electron microscope (FE-SEM, JSM-6701 JEO). The chemical composition was examined by X-ray diffraction (XRD, JDX-3500K), field emission electron probe micro analyzer (FE-EPMA, JXA-8530F) and electron backscattered diffraction (EBSD, JSM-6701 JEO). The hardness distribution crossing the depth of prepared samples was measured by Nano-indentation hardness testing device (ENT-110a) using a berkovich indenter with a load of 0.49 N. The first point was measured at a depth of 2 lm. Within 0–18 lm depth, the measurement interval was set as 4 lm. Within 18–198 lm depth, the interval was set as 10 lm. The wear behavior was tested by a ball-on-plate dry sliding friction player (FPR-2000) with a reciprocating sliding mode against ZrO2 ball with diameter of 5 mm at room temperature. The applied load was 5 N, the reciprocating angle was 30°, and the reciprocating radius was 30 mm with a line speed of 2 cm s1, sliding time of 1, 2, 4 and 8 h.
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3. Results and discussions 3.1. Surface morphology and chemical composition The surface irradiated zone was in a dark purple colour after laser movement. Fig. 1 shows the X-ray diffraction results of samples irradiated by 0, 1, 3, 5 and 7 passes respectively, which demonstrates that the as received Ti-6Al-4V (0 pass) was composed of a-Ti and b-Ti, and the surface chemicals of laser treated samples were independent from laser irradiation passes. All the laser treated samples exhibit nearly the same diffraction patterns of TiN, TiC0.3N0.7 and a-Ti. However, a slightly right shift occurred on the peaks marked as TiN and TiC0.3N0.7, when increasing the irradiation passes. Since there is a subtle difference (less than 1°) between the standard diffraction angles of TiN (06-0642) and TiC0.3N0.7 (42-1488), which TiN is slightly bigger than TiC0.3N0.7, it might suggest the amount of TiN increased as raising the irradiation passes. The optical cross-section microstructure of samples treated by different laser irradiation pass was shown in Fig. 2. it was seen that the as received substrate (0 pass) had a typical microstructure of fine equiaxed grains, and the cross-sectional microstructure of all the laser treated samples could be divided into three parts, which from top to bottom were film, acicular area and coarsening area, separately. Furthermore, the surface roughness increased as raising the irradiation pass. Some cracks and surface deformation were observed in the case above 3 passes. Fig. 3 shows the near surface microstructures observed by scanning electron microscope. According to Fig.3(a), the microstructure of as received substrate (0 pass), more precisely, was composed of equiaxed a-Ti and intergranular b-Ti. And from Fig.3(b)–(e), it was seen that the synthesized films under all cases were in a doublelayer structure, which the up layer was porous, and conversely the down layer was compact. Beneath the double-layer film, there was so-called acicular area, which more accurately was formed by the growth of down layer towards inner substrate. The thickness of both the up layer and the down layer increased as raising the irradiation pass. To further determine the chemical composition of these modified surfaces, EPMA analysis was carried out on both the modified samples and as-received Ti-6Al-4V alloy. For instance, Fig. 4 shows the element distribution of as-received substrate. The backscattering electron (BSE) image of Fig. 4 shows that the un-treated substrate composed of grey equiaxed a-Ti phase and white
2.3. Ionic release test The laser irradiated Ti-6Al-4V alloy by 3 passes and as-received Ti-6Al-4V alloy were cut into square pieces of 10 10 2 mm in size and polished slightly using emery paper (SiC, 1200#). After cleaning by ethanol and drying in air, the square pieces were separately soaked in a 30 mL SBF solution at 36.5 °C for 28 days. The SBF with inorganic ion concentrations (Na+ 142.0, K+ 5.0, Mg2+ 2 1.5, Ca2+ 2.5, Cl 147.8, HCO 1.0, SO2 0.5 mol/m3, 3 4.2, HPO4 4 pH 7.40) was prepared by dissolving NaCl, NaHCO3, KCl, K2HPO4 3H2O, MgCl2 6H2O, CaCl2, and Na2SO4 (Nacalai Tesque, Inc., Kyoto, Japan) in ultrapure water in order, and then buffering at pH 7.40 with tris (hydroxymethyl) amino methane ((CH2OH)3CNH2, Nacalai Tesque, Inc.) and an appropriate amount of a 1 kmol/m3 HCl aqueous solution. The concentration of Ti, Al and V ions released from the sample into the SBF solution were determined by inductively coupled plasma atomic emission spectrometers (ICP-AES, ICPF-9820). The surface morphology and chemical composition of samples before and after soaking were separately analysed by scanning electron microscope (SEM, JSM-7000F) and X-ray diffraction (XRD, JDX-3500K).
Fig. 1. Surface XRD patterns of Ti-6Al-4V alloys modified by different laser irradiation passes.
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Fig. 2. Cross-section OM images of samples modified by different laser irradiation pass.
Fig. 3. Near-surface SEM images of samples modified by different laser irradiation pass: (a) 0 pass, (b) 1 passes, (c) 3 passes, (d) 5 passes and (e) 7 passes.
intergranular b-Ti phase. The line analysis result of dotted line A shows that the element of V was enriched in b-Ti, and Al was enriched in a-Ti. In addition, it was unexpected that the detected
nitrogen concentration of non-treated substrate ranged about 6.1–8.4% along the dotted line A, giving an average concentration of approximately 6.9% rather than zero. This can be explained that
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Fig. 4. EPMA line analysis result of as received Ti-6Al-4V alloy.
since the Ka line of N is somewhat overlapped with Li, Lg line of Ti in EPMA analysis, the detected N concentration would not be zero due to the effect of substrate. A typical element distribution of laser modified sample by 5 passes is shown in Fig. 5. According to the mapping and line analysis results in Fig. 5(a) and (b), it was observed that the concentration of C and N exhibited a declining trend from surface to inner substrate, which the up layer (L1) possessed notably high C and N concentration, and the down layer (L2) possessed barely any C and a little higher N concentration than the inner substrate. As for the distribution of Al and V, being in contrast with C and N, the concentrations exhibited an increasing trend, which the up layer possessed the lowest V and Al concentration, and the down layer possessed less V and Al concentration. Concerning the effect of substrate mentioned above, the detected N concentration was modified directly minus 6.9%. Based on the EPMA analysis results, it was obtained that the maximum N concentration of the up layer increased as raising the laser irradiation passes, with modified values of 15.7% for 1 pass, 19.1% for 3 passes, 19.8% for 5 passes, and
20.3% for 7 passes. However, the change of the maximum N concentration in down layer and inner substrate were not so obvious, with modified values ranging about 1.1–1.3% in the down layer and 0.2–0.5% in the substrate. Moreover, attributing to the surface impurities and porous structure which showed even high C concentration, the change of maximum C concentration in up layer against laser irradiation passes was irregular. Since the coexistence of C and N in up layer was confirmed, moreover, the maximum N concentrations in up layer (above 3 passes) was larger than the theoretical N concentration of 15.9% in pure TiC0.3N0.7, the chemical composition of the up layer was considered as a mixture of TiN and TiC0.3N0.7, which was in accordance with the XRD results. Because the N concentration in down layer was ranging 1.1–1.3% approximately, it was considered that the down layer was a nitrogen-rich a-Ti layer according to the Ti-N binary phase diagram [14]. And these assumptions were further proved by the EBSD analysis. Fig. 6(a) and (b) shows the phase images and inverse pole figure (IPF) images of samples treated by different laser irradiation pass. According to it, the up layer of film
Fig. 5. EPMA analysis result of laser modified sample by 5 passes: (a) mapping analysis, (b) line analysis of dotted line B marked in (a).
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to Eq. (1-5). Since the nucleation and growth of a0 phase was strongly affected by the temperature and nitrogen concentration gradients, the growth direction of acicular phase was random but showing a general trend towards inner substrate. After laser movement, the acicular area next to the down-layer film would be generated by the rapid cooling from a0 + b binary phase area into a phase area according to Eq. (1-6). And the coarsening area would be generated by the b phase with poor nitrogen concentration directly transferred into a-Ti according to Eq. (1-7) after laser movement.
Fig. 6. EBSD analysis result of samples modified by different laser irradiation pass: (a) phase image, (b) IPF image.
a-TiðsÞ ! b-TiðsÞ
ð1-4Þ
b-TiðsÞ þ ½N ! a0 -TiðsÞ
ð1-5Þ
ða0 þ bÞ-TiðsÞ ! a-TiðsÞ
ð1-6Þ
b-TiðsÞ ! a-TiðsÞ
ð1-7Þ
The reason for cracks and surface deformation happening above 3 passes was thought to be related to the shrinking of the irradiation zone due to the chilling effect after laser movement. It was considered that the shrinking of the Ti (C, N) film was bigger than the substrate, resulting in a residual tensile stress acted on the surface. When increasing the laser irradiation passes, a thicker Ti (C, N) film was formed, and the shrinking of the Ti (C, N) film would be further intensified. The residual tensile stress increased till exceeding the breaking strength, leading the cracks to produce above 3 passes. 3.2. Hardness and wear
was a fine crystallized layer composed of blue TiN and purple TiC0.3N0.7, and the down layer was an equiaxed layer sharing the same green colour with the inner substrate, which referred to aTi. In addition, none special crystal orientation was observed on neither the up layer nor the down layer based on the IPF. Thus, the formation mechanism of the up-layer film in this study can be described as follows:
Laser þ TiðsÞ ! TiðlÞ
ð1-1Þ
TiðlÞ þ ½N ! TiN
ð1-2Þ
TiðlÞ þ 0:3½C þ 0:7½N ! TiC0:3 N0:7
ð1-3Þ
Own to the high-energy density of laser beams, the outmost few microns of substrate reacted with the laser beams forming liquid titanium which immersed up and encircled the pre-placed graphite particles, according to Eq. (1-1). The up-layer film composed of TiN and TiC0.3N0.7 was synthesized by the liquid titanium reacting with diffused N and C, according to Eqs. (1-2) and (1-3). The porous structure of the up-layer film might be generated by the incomplete immersion and reaction of liquid titanium to graphite particles, where the residual graphite was remained and exhibited higher C concentration. The formation of the down-layer film and the change of microstructure in substrate can be described based on Eqs. (1-4) to (1-7). Attributing to the rapid heating during laser irradiation, the substrate would transit from a-Ti to b-Ti according to Eq. (14), within the area where the temperature was above the b transit point and below the solidus temperature. Meanwhile, accompanied with the diffusion of nitrogen atoms into substrate, there was a nitrogen concentration gradient formed in the heat transport direction. Thus, based on the Ti-N binary phase diagram [14], the high-temperature a’ phase with higher nitrogen concentration would nucleate near the up-layer film, and grow up towards inner b phase forming the down-layer film and acicular phase according
The nano-indentation hardness distribution crosssing the depth of laser modified samples is shown in Fig. 7. For all cases, the crossing hardness distribution exhibited an abrupt declining trend from surface to inner substrate, due to a Ti(C, N) ceremic film with extremly high hardness was formed. What’s more, it was noticed that the hardness of the accicular area (depth < 100 lm) was slightly higher than the coarsening area. This may be related to the solution strengthening of dissolved nitrogen in accicular area. The outmost hardness increased as increasing the laser irradiation passes, which was 27 GPa for 1 pass, 42.6 GPa for 3 passes, 43.7 GPa for 5 passes and 44.3 GPa for 7 passes. In view of the cracks and surface deformation happened above 3 passes, the wear
Fig. 7. Cross sectional nano-indentation hardness distribution of laser modified samples with different irradiation pass.
X. Zeng et al. / Optics and Laser Technology 98 (2018) 106–112
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3.3. Ionic release
Fig. 8. Volume loss against time in wear test of as received Ti-6Al-4V alloy and laser treated sample with 3 irradiation passes.
Table 1 Ionic concentration of Ti, Al and V determined by ICP-AES (lg/L). Sample
Ti
Al
V
As received Ti-6Al-4V Laser modified Ti-6Al-4V
285 290
0 0
30 25
behavior test was only performed on the sample treated by 3 passes and as received substrate. Fig. 8 shows the volume loss against wear time. It was observed that the volume loss of both samples increased linearly with the wear time. However, the laser treated sample exhibited a much smaller growth rate, which suggested a better wear resistance as compared to the as received substrate. After 8 h test, the volume loss of laser treated sample by 3 passes was 0.08 mm3, which was nearly 37 times lower than that of as received substrate.
The ionic release test was performed on the sample with 3 irradiation passes and as received substrate. The determined concentration of Ti, Al and V ions in SBF after 28-days soaking is shown in Table 1, which illustrated that the ionic release from laser modified Ti-6Al-4V and as received Ti-6Al-4V were very similar. The V ionic concentration in SBF of laser modified sample was slightly smaller than that of as received Ti-6Al-4V. In addition, the determined Al ionic concentration in SBF of both samples was zero. Fig. 9 shows the surface morphology of as received Ti-6Al-4V and laser modified sample before and after soaking. The surface morphologies of as received Ti-6Al-4V and laser modified sample were barely changed after soaking. The surfaces were as clean as the ones before test. In order to evaluate the surface chemical change, the XRD analysis was performed and the corresponding results are shown in Fig. 10. Comparing the diffraction patterns to each other, it can be concluded that there were no abnormal chemicals detected on both samples’ surface, excepting for a small amount of AlV2O4. Though the ionic concentration of Al in SBF was zero, the existence of AlV2O4 made it clear that the elements of Al and V did release from both samples during the soaking. According to report by Takao Hanawa [15], the behaviour of ion release can be divided into two types. One type is the released metal ions react with the hydroxyl or anion in solution forming oxides or salts, which can reduce the toxicity. And another type is the released metal ions remain the form of free ions, which might cause toxic effect. In this study, it was considered that the released Al and one part of released V were coexisted in the type of AlV2O4 on samples’ surface, and the residual part of released V was existed in the type of free V ions in SBF. In addition, it was confirmed that no apatite (Ca10(PO4)6(OH)2) was formed on surfaces of both samples, which means no ontogenesis ability. This illustrates the possibility of keeping the appearance shape unchanged when been used as sliding parts of hard tissue implant in vivo environment. Moreover, regarding to the improvement on surface hardness and wear resis-
Fig. 9. Surface morphology of (a) as received Ti-6Al-4V before soaking, (b) as received Ti-6Al-4V after soaking, (c) laser modified Ti-6Al-4V before soaking, and (d) laser modified Ti-6Al-4V after soaking.
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characteristics was investigated. The results showed that the chemicals of film were independent from laser irradiation pass, which the up layer of film consisted of TiN and TiC0.3N0.7, and the down layer was nitrogen-rich a-Ti layer. Both the surface roughness and hardness increased as raising the laser irradiation pass. Surface deformation and some cracks happened in the case above 3 pass, due to the thermal mismatch between the Ti (C, N) film and the substrate. The wear resistance of laser treated sample (3 passes) was improved approximately by 37 times as compared to as received substrate. In addition, the ion release test showed the potentiality of a new try to modify the surface of Ti-based hard tissue implant in future biomedical application. References
Fig. 10. Surface XRD patterns of samples before and after soaking: (a) as received Ti-6Al-4V, (b) laser modified Ti-6Al-4V.
tance, it was considered that the cytotoxic ion release from laser modified sample could be reduced effectively by decreasing the wear debris. 4. Conclusions A double-layer Ti (C, N) film was successfully synthesized on the surface of Ti-6Al-4V alloy by YAG laser irradiation in nitrogen atmosphere. The effect of laser irradiation pass upon the surface
[1] T. Akahori, M. Niinomi, Fracture characteristics of fatigued Ti–6Al–4V ELI as an implant material, Mater. Sci. Eng. A 243 (1–2) (1998) 237–243. [2] Xuanyong Liu, Paul K. Chub, Chuanxian Ding, Surface modification of titanium, titanium alloys, and related materials for biomedical applications, Mater. Sci. Eng. R 47 (2004) 49–121. [3] B.S. Ng, I. Annergren, A.M. Soutar, K.A. Khor, A.E.W. Jarfors, Characterization of a duplex TiO2/CaP coatings on Ti6Al4V for hard tissue replacement, Biomaterials 26 (2005) 1087–1095. [4] V. Sáenz de Viteri, M.G. Barandika, U. Ruiz de Gopegui, R. Bayón, C. Zubizarreta, X. Fernández, A. Igartua, F. Agullo-Rueda, Characterization of Ti-C-N coatings deposited on Ti6Al4V for biomedical applications, J. Inorg. Biochem. 117 (2012) 359–366. [5] K. Balázsi, I.E. Lukács, S. Gurbán, M. Menyhárd, L. Bacáková, M. Vandrovcová, C. Balázsi, Structural, mechanical and biological comparison of TiC and TiCN nanocomposites films, J. Eur. Ceram. Soc. 33 (2013) 2217–2221. [6] S.J. Bulla, D.G. Bhat, M.H. Staia, Properties and performance of commercial TiCN coatings. Part 1: coating architecture and hardness modelling, Surf. Coat. Technol. 163–164 (2003) 499–506. [7] Mingdong Bao, Xu. Xuebo, Haijun Zhang, Xiaoping Liu, Linhai Tian, Zhaoxin Zeng, Yubin Song, Tribological behavior at elevated temperature of multilayer TiCN/TiC/TiN hard coatings produced by chemical vapor deposition, Thin Solid Films 520 (2011) 833–836. [8] P. Huber, D. Manova, S. Mändl, B. Rauschenbach, Formation of TiN, TiC and TiCN by metal plasma immersion ion implantation and deposition, Surf. Coat. Technol. 174–175 (2003) 1243–1247. [9] R. Filip, J. Sieniawski, E. Pleszakov, Formation of surface layer on Ti-6Al-4V titanium alloy by laser alloy, Surf. Eng. 22 (2006) 53–57. [10] Yu-Chi Lin, Yuan-Ching Lin, Yong-Chwang Chen, Evolution of the microstructure and tribological performance of Ti-6Al-4V cladding with TiN powder, Mater. Des. 36 (2012) 584–589. [11] R.M. Mahamood, E.T. Akinlabi, Laser metal deposition of functionally graded Ti6Al4V/TiC, Mater. Des. 84 (2015) 402–410. [12] Z.D. Liu, X.C. Zhang, F.Z. Xuan, Z.D. Wang, S.T. Tu, In situ synthesis of TiN/Ti3Al intermetallic matrix composite coatings on Ti6Al4V alloy, Mater. Des. 37 (2012) 268–273. [13] Xian Zeng, Tomiko Yamaguchi, Kazumasa Nishio, Characteristics of Ti(C, N)/TiB composite layer on Ti–6Al–4V alloy produced by laser surface melting, Opt. Laser. Technol. 80 (2016) 84–91. [14] Titanium and nitrogen phase diagram. Computational Thermodynamics inc., USA, Available at http://www.calphad.com/pdf/Ti_N_Phase_Diagram.pdf (assessed 16 June, 2016). [15] Takao Hanawa, Surface phenomena and surface modification of metallic materials in vivo, Surf. Sci. 20 (1999) 607–612 (Japanese).
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Characteristics of surface modified Ti-6Al-4V alloy by a series of YAG laser irradiation Xian Zeng a,b, Wenqin Wang b,c,⇑, Tomiko Yamaguchi b, Kazumasa Nishio d a
School of Materials Science, Wuhan University of Technology, Wuhan 430070, China Kyushu Institute of Technology, Kitakyushu 804-8550, Japan c School of Mechanical and Electrical Engineering, Nanchang University, Nanchang 330031, China d Nishinippon Institute of Technology, Kitakyushu 800-0394, Japan b
a r t i c l e
i n f o
Article history: Received 27 February 2017 Received in revised form 31 May 2017 Accepted 28 July 2017
Keywords: Ti (C, N) Laser irradiation Hardness Wear Ion release
a b s t r a c t In this study, a double-layer Ti (C, N) film was successfully prepared on Ti-6Al-4V alloy by a series of YAG laser irradiation in nitrogen atmosphere, aiming at improving the wear resistance. The effects of laser irradiation pass upon surface chemical composition, microstructures and hardness were investigated. The results showed that the surface chemicals were independent from laser irradiation pass, which the up layer of film was a mixture of TiN and TiC0.3N0.7, and the down layer was nitrogen-rich a-Ti. Both the surface roughness and hardness increased as raising the irradiation passes. However, surface deformation and cracks happened in the case above 3 passes’ irradiation. The wear resistance of laser modified sample by 3 passes was improved approximately by 37 times compared to the as received substrate. Moreover, the cytotoxic V ion released from laser modified sample was less than that of as received Ti-6Al-4V alloy in SBF, suggesting the potentiality of a new try to modify the sliding part of Ti-based hard tissue implants in future biomedical application. Ó 2017 Published by Elsevier Ltd.
1. Introduction Commercially Ti-6Al-4V alloy is extensively explored in many fields, such as aeronautical, marine, automobile and chemical industry, due to its excellent properties of low density, high fatigue strength and good corrosion resistance. Moreover, its acceptable biocompatibility and higher fatigue strength than pure titanium make it remain the dominant titanium alloy used in medical materials [1,2]. However, its low hardness and poor wear resistance limit its use in many applications. For instance, being used as hard tissue implant materials of femoral head, the heavy abrasion of sliding part not only reduce the component’s service life, but also bring out the potential risk that the alloying element of V and Al might be released into the human bodies as metal ion from wear debris, resulting in the neurotoxicity and cytotoxic effect [3]. Thus, to effectively decrease the debris and avoid the elution of these toxic elements, surface modification is strongly suggested. Ti(C, N) containing the merits of both TiN and TiC, is a promising reinforcing material due to its superior properties such as high hardness, good corrosion resistance and biocompatibilities. Tradi-
tional surface modification approaches to obtain such film covered a wide range of techniques, such as physical vapour deposition (PVD) [4,5], chemical vapour deposition (CVD) [6,7], ion implantation [8] and so on. However, regarding to the manufacturing cost, efficiency, flexibility and accuracy, these methods have been still considered as unsatisfied. Owing to high energy density and directionality, laser surface modification having capability for rapid processing, net-shape manufacture, and low heat effect on substrate, aroused researchers’ extensive study [9,10–13]. Thus, the present study is trying to synthesize Ti(C, N) film by laser irradiation, aiming at improving the wear resistance of Ti-6Al-4V alloy. The effects of laser irradiation pass upon chemical composition, microstructures and hardness were discussed. What’s more, the release of Al and V ions from the laser modified sample (by 3 passes) in simulated body fluid (SBF) was also measured for the purpose of a new try to modify the surface of Ti-based hard tissue implants in future biomedical application. 2. Materials and methods 2.1. Materials and laser processing
⇑ Corresponding author at: School of Mechanical and Electrical Engineering, Nanchang University, Nanchang 330031, China. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.optlastec.2017.07.048 0030-3992/Ó 2017 Published by Elsevier Ltd.
Ti-6Al-4V alloy sheets with size of 80 40 2 mm were used. The sheet’s surface was grinded by emery paper (SiC, 1200#) to
X. Zeng et al. / Optics and Laser Technology 98 (2018) 106–112
remove the oxide layer, and then cleaned by acetone. Before the laser irradiation, a graphite layer was sprayed on the cleaned surface equably, using an anti-adhesive agent (Ishihara Chemicals Co., Ltd. Japan), which contains graphite (5–10 wt.%), ethanol (35–45 wt.%) and butane (20–30 wt.%). And then the surface was air dried at room temperature. A Nd: YAG laser processor (MW2000, Sumitomo Heavy Industry. Japan) with continuous wave mode was used for laser irradiation. The specimen was put in a gas tight chamber with a quartz glass window which the laser beam can penetrate through and the sheet was irradiated inside the chamber. The fixed laser processing parameters were: laser output power of 1.6 kW, defocus distance of 70 mm giving a spot diameter of about 6 mm, laser travelling speed of 360 mm/min, and a shielding gas of pure nitrogen with a flow rate of 20 L/min. the samples were irradiated by 1, 3, 5 and 7 passes within the same irradiation track, separately. After each pass of irradiation, the residual graphite layer was cleaned by acetone, and then a new graphite layer was sprayed on the surface before the next irradiation.
2.2. Characterization The samples were cut transversely to the laser traverse direction and then polished by usual metallographic procedure, etched by a corrosion solution (HF: HNO3: H2O equal to 2:5:100 in volume). The microstructure was observed by optical microscope (OM, Nikon-L150) and field emission scanning electron microscope (FE-SEM, JSM-6701 JEO). The chemical composition was examined by X-ray diffraction (XRD, JDX-3500K), field emission electron probe micro analyzer (FE-EPMA, JXA-8530F) and electron backscattered diffraction (EBSD, JSM-6701 JEO). The hardness distribution crossing the depth of prepared samples was measured by Nano-indentation hardness testing device (ENT-110a) using a berkovich indenter with a load of 0.49 N. The first point was measured at a depth of 2 lm. Within 0–18 lm depth, the measurement interval was set as 4 lm. Within 18–198 lm depth, the interval was set as 10 lm. The wear behavior was tested by a ball-on-plate dry sliding friction player (FPR-2000) with a reciprocating sliding mode against ZrO2 ball with diameter of 5 mm at room temperature. The applied load was 5 N, the reciprocating angle was 30°, and the reciprocating radius was 30 mm with a line speed of 2 cm s1, sliding time of 1, 2, 4 and 8 h.
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3. Results and discussions 3.1. Surface morphology and chemical composition The surface irradiated zone was in a dark purple colour after laser movement. Fig. 1 shows the X-ray diffraction results of samples irradiated by 0, 1, 3, 5 and 7 passes respectively, which demonstrates that the as received Ti-6Al-4V (0 pass) was composed of a-Ti and b-Ti, and the surface chemicals of laser treated samples were independent from laser irradiation passes. All the laser treated samples exhibit nearly the same diffraction patterns of TiN, TiC0.3N0.7 and a-Ti. However, a slightly right shift occurred on the peaks marked as TiN and TiC0.3N0.7, when increasing the irradiation passes. Since there is a subtle difference (less than 1°) between the standard diffraction angles of TiN (06-0642) and TiC0.3N0.7 (42-1488), which TiN is slightly bigger than TiC0.3N0.7, it might suggest the amount of TiN increased as raising the irradiation passes. The optical cross-section microstructure of samples treated by different laser irradiation pass was shown in Fig. 2. it was seen that the as received substrate (0 pass) had a typical microstructure of fine equiaxed grains, and the cross-sectional microstructure of all the laser treated samples could be divided into three parts, which from top to bottom were film, acicular area and coarsening area, separately. Furthermore, the surface roughness increased as raising the irradiation pass. Some cracks and surface deformation were observed in the case above 3 passes. Fig. 3 shows the near surface microstructures observed by scanning electron microscope. According to Fig.3(a), the microstructure of as received substrate (0 pass), more precisely, was composed of equiaxed a-Ti and intergranular b-Ti. And from Fig.3(b)–(e), it was seen that the synthesized films under all cases were in a doublelayer structure, which the up layer was porous, and conversely the down layer was compact. Beneath the double-layer film, there was so-called acicular area, which more accurately was formed by the growth of down layer towards inner substrate. The thickness of both the up layer and the down layer increased as raising the irradiation pass. To further determine the chemical composition of these modified surfaces, EPMA analysis was carried out on both the modified samples and as-received Ti-6Al-4V alloy. For instance, Fig. 4 shows the element distribution of as-received substrate. The backscattering electron (BSE) image of Fig. 4 shows that the un-treated substrate composed of grey equiaxed a-Ti phase and white
2.3. Ionic release test The laser irradiated Ti-6Al-4V alloy by 3 passes and as-received Ti-6Al-4V alloy were cut into square pieces of 10 10 2 mm in size and polished slightly using emery paper (SiC, 1200#). After cleaning by ethanol and drying in air, the square pieces were separately soaked in a 30 mL SBF solution at 36.5 °C for 28 days. The SBF with inorganic ion concentrations (Na+ 142.0, K+ 5.0, Mg2+ 2 1.5, Ca2+ 2.5, Cl 147.8, HCO 1.0, SO2 0.5 mol/m3, 3 4.2, HPO4 4 pH 7.40) was prepared by dissolving NaCl, NaHCO3, KCl, K2HPO4 3H2O, MgCl2 6H2O, CaCl2, and Na2SO4 (Nacalai Tesque, Inc., Kyoto, Japan) in ultrapure water in order, and then buffering at pH 7.40 with tris (hydroxymethyl) amino methane ((CH2OH)3CNH2, Nacalai Tesque, Inc.) and an appropriate amount of a 1 kmol/m3 HCl aqueous solution. The concentration of Ti, Al and V ions released from the sample into the SBF solution were determined by inductively coupled plasma atomic emission spectrometers (ICP-AES, ICPF-9820). The surface morphology and chemical composition of samples before and after soaking were separately analysed by scanning electron microscope (SEM, JSM-7000F) and X-ray diffraction (XRD, JDX-3500K).
Fig. 1. Surface XRD patterns of Ti-6Al-4V alloys modified by different laser irradiation passes.
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Fig. 2. Cross-section OM images of samples modified by different laser irradiation pass.
Fig. 3. Near-surface SEM images of samples modified by different laser irradiation pass: (a) 0 pass, (b) 1 passes, (c) 3 passes, (d) 5 passes and (e) 7 passes.
intergranular b-Ti phase. The line analysis result of dotted line A shows that the element of V was enriched in b-Ti, and Al was enriched in a-Ti. In addition, it was unexpected that the detected
nitrogen concentration of non-treated substrate ranged about 6.1–8.4% along the dotted line A, giving an average concentration of approximately 6.9% rather than zero. This can be explained that
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Fig. 4. EPMA line analysis result of as received Ti-6Al-4V alloy.
since the Ka line of N is somewhat overlapped with Li, Lg line of Ti in EPMA analysis, the detected N concentration would not be zero due to the effect of substrate. A typical element distribution of laser modified sample by 5 passes is shown in Fig. 5. According to the mapping and line analysis results in Fig. 5(a) and (b), it was observed that the concentration of C and N exhibited a declining trend from surface to inner substrate, which the up layer (L1) possessed notably high C and N concentration, and the down layer (L2) possessed barely any C and a little higher N concentration than the inner substrate. As for the distribution of Al and V, being in contrast with C and N, the concentrations exhibited an increasing trend, which the up layer possessed the lowest V and Al concentration, and the down layer possessed less V and Al concentration. Concerning the effect of substrate mentioned above, the detected N concentration was modified directly minus 6.9%. Based on the EPMA analysis results, it was obtained that the maximum N concentration of the up layer increased as raising the laser irradiation passes, with modified values of 15.7% for 1 pass, 19.1% for 3 passes, 19.8% for 5 passes, and
20.3% for 7 passes. However, the change of the maximum N concentration in down layer and inner substrate were not so obvious, with modified values ranging about 1.1–1.3% in the down layer and 0.2–0.5% in the substrate. Moreover, attributing to the surface impurities and porous structure which showed even high C concentration, the change of maximum C concentration in up layer against laser irradiation passes was irregular. Since the coexistence of C and N in up layer was confirmed, moreover, the maximum N concentrations in up layer (above 3 passes) was larger than the theoretical N concentration of 15.9% in pure TiC0.3N0.7, the chemical composition of the up layer was considered as a mixture of TiN and TiC0.3N0.7, which was in accordance with the XRD results. Because the N concentration in down layer was ranging 1.1–1.3% approximately, it was considered that the down layer was a nitrogen-rich a-Ti layer according to the Ti-N binary phase diagram [14]. And these assumptions were further proved by the EBSD analysis. Fig. 6(a) and (b) shows the phase images and inverse pole figure (IPF) images of samples treated by different laser irradiation pass. According to it, the up layer of film
Fig. 5. EPMA analysis result of laser modified sample by 5 passes: (a) mapping analysis, (b) line analysis of dotted line B marked in (a).
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to Eq. (1-5). Since the nucleation and growth of a0 phase was strongly affected by the temperature and nitrogen concentration gradients, the growth direction of acicular phase was random but showing a general trend towards inner substrate. After laser movement, the acicular area next to the down-layer film would be generated by the rapid cooling from a0 + b binary phase area into a phase area according to Eq. (1-6). And the coarsening area would be generated by the b phase with poor nitrogen concentration directly transferred into a-Ti according to Eq. (1-7) after laser movement.
Fig. 6. EBSD analysis result of samples modified by different laser irradiation pass: (a) phase image, (b) IPF image.
a-TiðsÞ ! b-TiðsÞ
ð1-4Þ
b-TiðsÞ þ ½N ! a0 -TiðsÞ
ð1-5Þ
ða0 þ bÞ-TiðsÞ ! a-TiðsÞ
ð1-6Þ
b-TiðsÞ ! a-TiðsÞ
ð1-7Þ
The reason for cracks and surface deformation happening above 3 passes was thought to be related to the shrinking of the irradiation zone due to the chilling effect after laser movement. It was considered that the shrinking of the Ti (C, N) film was bigger than the substrate, resulting in a residual tensile stress acted on the surface. When increasing the laser irradiation passes, a thicker Ti (C, N) film was formed, and the shrinking of the Ti (C, N) film would be further intensified. The residual tensile stress increased till exceeding the breaking strength, leading the cracks to produce above 3 passes. 3.2. Hardness and wear
was a fine crystallized layer composed of blue TiN and purple TiC0.3N0.7, and the down layer was an equiaxed layer sharing the same green colour with the inner substrate, which referred to aTi. In addition, none special crystal orientation was observed on neither the up layer nor the down layer based on the IPF. Thus, the formation mechanism of the up-layer film in this study can be described as follows:
Laser þ TiðsÞ ! TiðlÞ
ð1-1Þ
TiðlÞ þ ½N ! TiN
ð1-2Þ
TiðlÞ þ 0:3½C þ 0:7½N ! TiC0:3 N0:7
ð1-3Þ
Own to the high-energy density of laser beams, the outmost few microns of substrate reacted with the laser beams forming liquid titanium which immersed up and encircled the pre-placed graphite particles, according to Eq. (1-1). The up-layer film composed of TiN and TiC0.3N0.7 was synthesized by the liquid titanium reacting with diffused N and C, according to Eqs. (1-2) and (1-3). The porous structure of the up-layer film might be generated by the incomplete immersion and reaction of liquid titanium to graphite particles, where the residual graphite was remained and exhibited higher C concentration. The formation of the down-layer film and the change of microstructure in substrate can be described based on Eqs. (1-4) to (1-7). Attributing to the rapid heating during laser irradiation, the substrate would transit from a-Ti to b-Ti according to Eq. (14), within the area where the temperature was above the b transit point and below the solidus temperature. Meanwhile, accompanied with the diffusion of nitrogen atoms into substrate, there was a nitrogen concentration gradient formed in the heat transport direction. Thus, based on the Ti-N binary phase diagram [14], the high-temperature a’ phase with higher nitrogen concentration would nucleate near the up-layer film, and grow up towards inner b phase forming the down-layer film and acicular phase according
The nano-indentation hardness distribution crosssing the depth of laser modified samples is shown in Fig. 7. For all cases, the crossing hardness distribution exhibited an abrupt declining trend from surface to inner substrate, due to a Ti(C, N) ceremic film with extremly high hardness was formed. What’s more, it was noticed that the hardness of the accicular area (depth < 100 lm) was slightly higher than the coarsening area. This may be related to the solution strengthening of dissolved nitrogen in accicular area. The outmost hardness increased as increasing the laser irradiation passes, which was 27 GPa for 1 pass, 42.6 GPa for 3 passes, 43.7 GPa for 5 passes and 44.3 GPa for 7 passes. In view of the cracks and surface deformation happened above 3 passes, the wear
Fig. 7. Cross sectional nano-indentation hardness distribution of laser modified samples with different irradiation pass.
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3.3. Ionic release
Fig. 8. Volume loss against time in wear test of as received Ti-6Al-4V alloy and laser treated sample with 3 irradiation passes.
Table 1 Ionic concentration of Ti, Al and V determined by ICP-AES (lg/L). Sample
Ti
Al
V
As received Ti-6Al-4V Laser modified Ti-6Al-4V
285 290
0 0
30 25
behavior test was only performed on the sample treated by 3 passes and as received substrate. Fig. 8 shows the volume loss against wear time. It was observed that the volume loss of both samples increased linearly with the wear time. However, the laser treated sample exhibited a much smaller growth rate, which suggested a better wear resistance as compared to the as received substrate. After 8 h test, the volume loss of laser treated sample by 3 passes was 0.08 mm3, which was nearly 37 times lower than that of as received substrate.
The ionic release test was performed on the sample with 3 irradiation passes and as received substrate. The determined concentration of Ti, Al and V ions in SBF after 28-days soaking is shown in Table 1, which illustrated that the ionic release from laser modified Ti-6Al-4V and as received Ti-6Al-4V were very similar. The V ionic concentration in SBF of laser modified sample was slightly smaller than that of as received Ti-6Al-4V. In addition, the determined Al ionic concentration in SBF of both samples was zero. Fig. 9 shows the surface morphology of as received Ti-6Al-4V and laser modified sample before and after soaking. The surface morphologies of as received Ti-6Al-4V and laser modified sample were barely changed after soaking. The surfaces were as clean as the ones before test. In order to evaluate the surface chemical change, the XRD analysis was performed and the corresponding results are shown in Fig. 10. Comparing the diffraction patterns to each other, it can be concluded that there were no abnormal chemicals detected on both samples’ surface, excepting for a small amount of AlV2O4. Though the ionic concentration of Al in SBF was zero, the existence of AlV2O4 made it clear that the elements of Al and V did release from both samples during the soaking. According to report by Takao Hanawa [15], the behaviour of ion release can be divided into two types. One type is the released metal ions react with the hydroxyl or anion in solution forming oxides or salts, which can reduce the toxicity. And another type is the released metal ions remain the form of free ions, which might cause toxic effect. In this study, it was considered that the released Al and one part of released V were coexisted in the type of AlV2O4 on samples’ surface, and the residual part of released V was existed in the type of free V ions in SBF. In addition, it was confirmed that no apatite (Ca10(PO4)6(OH)2) was formed on surfaces of both samples, which means no ontogenesis ability. This illustrates the possibility of keeping the appearance shape unchanged when been used as sliding parts of hard tissue implant in vivo environment. Moreover, regarding to the improvement on surface hardness and wear resis-
Fig. 9. Surface morphology of (a) as received Ti-6Al-4V before soaking, (b) as received Ti-6Al-4V after soaking, (c) laser modified Ti-6Al-4V before soaking, and (d) laser modified Ti-6Al-4V after soaking.
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characteristics was investigated. The results showed that the chemicals of film were independent from laser irradiation pass, which the up layer of film consisted of TiN and TiC0.3N0.7, and the down layer was nitrogen-rich a-Ti layer. Both the surface roughness and hardness increased as raising the laser irradiation pass. Surface deformation and some cracks happened in the case above 3 pass, due to the thermal mismatch between the Ti (C, N) film and the substrate. The wear resistance of laser treated sample (3 passes) was improved approximately by 37 times as compared to as received substrate. In addition, the ion release test showed the potentiality of a new try to modify the surface of Ti-based hard tissue implant in future biomedical application. References
Fig. 10. Surface XRD patterns of samples before and after soaking: (a) as received Ti-6Al-4V, (b) laser modified Ti-6Al-4V.
tance, it was considered that the cytotoxic ion release from laser modified sample could be reduced effectively by decreasing the wear debris. 4. Conclusions A double-layer Ti (C, N) film was successfully synthesized on the surface of Ti-6Al-4V alloy by YAG laser irradiation in nitrogen atmosphere. The effect of laser irradiation pass upon the surface
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