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Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium
Optical Materials xxx (2017) 1e5
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium Xiaoyuan Chen a, Litong Guo a, b, *, Xuemei Liu a, Wei Feng a, Baoe Li c, Xueyu Tao a, Yinghuai Qiang a a b c
China University of Mining and Technology, Xuzhou 221116, PR China Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 21 January 2017 Accepted 25 January 2017 Available online xxx
Bonding porcelain layer was fused on Ti surface by laser cladding process using a 400 W pulse CO2 laser. The specimens were studied by field-emission scanning electron microscopy, X-ray diffraction and bonding tests. During the laser fusion process, the porcelain powders were heated by laser energy and melted on Ti to form a chemical bond with the substrate. When the laser scanning speed decreased, the sintering temperature and the extent of the oxidation of Ti surface increased accordingly. When the laser scanning speed is 12.5 mm/s, the bonding porcelain layers were still incomplete sintered and there were some micro-cracks in the porcelain. When the laser scanning speed decreased to 7.5 mm/s, vitrified bonding porcelain layers with few pores were synthesized on Ti. © 2017 Published by Elsevier B.V.
Keywords: Biomaterials Microstructure Titanium Laser processing Sintering
1. Introduction Titanium was an alternative metal substrate for porcelain fused to metal because of its excellent performance, such as good corrosion resistance, biocompatibility, mechanical strength and low cost [1e3]. However, compared to conventional NiCr alloyporcelain, the poor bonding strength restricted its application [4e6]. Laser cladding is the fusion of a powder on a substrate and the laser energy melts the cladding material forming a metallurgical bond with the substrate [7e10]. More recently, laser cladding technology is a promising way to prepare coatings on Ti due to its limited heat affected zone, low dilution ratio and metallurgical bond between the coating and substrate [11,12]. This process allows for a coating built on titanium substrates to be built layer by layer and may prevent unwanted phase changes in the titanium [13,14]. However, there were no available reports on using laser cladding process to synthesize porcelain layers on Ti. In addition, the coefficient of thermal expansion of the bonding
* Corresponding author. China University of Mining and Technology, Xuzhou 221116, PR China. E-mail address: [email protected] (L. Guo).
porcelain (a ¼ 9.4 106/ C) used in this research is slightly lower than that of titanium (9.5 106/ C) [15], which will minimize the thermal stress between Ti-porcelain and avoid the formation of crack during high temperature cladding and rapid solidification. During the laser cladding process, the porcelain powders, which are glassy solid, were heated by laser energy and melted on Ti to form a chemical bond with the substrate. Microstructure of the coating and the interface between the coating and the substrate is of greatest interest [16]. Therefore, laser parameters were optimized to synthesize a crack-free coating on titanium. In addition, acid etching and anodization processes were combined to improve the surface roughness of Ti and chemical bonding between porcelain-Ti. 2. Experimental procedures ASTM grade II CP titanium was cast, ground and polished to prepare plate-shaped specimens (F13 mm 0.5 mm). The specimens without any treatment were used as control. 2.1. Anodization of titanium In a typical anodization process, the electrolyte was prepared by
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Please cite this article in press as: X. Chen, et al., Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.046
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Fig. 1. F-SEM microphotographs of titanium surface (a), (b) the untreated Ti (c), (d) the Ti just treated in 40 wt% HF, (e) and (f) Ti surface anodized at 20 V for 15 min after pretreatment in 40 wt% HF.
adding 0.3 wt% of ammonium fluoride (NH4F, Sinopharm Chemical Reagent Co. Ltd., AR) and 1.25 vol% of distilled water into ethylene glycol (C2H6O2, Sinopharm Chemical Reagent Co. Ltd., AR). In a typical preparation procedure, the titanium specimens (>99% purity, thickness of 0.5 mm) were pre-treated in 40 wt% HF acid, and then anodized in the electrolyte solution using a graphite counter electrode at 20 V for 15 min at room temperature.
2.2. Preparation of titaniumeporcelain test specimens Then thin layer of bonding porcelain (Ti-bond porcelain, Dentsply, USA) was brushed on Ti surface and the thickness were controlled to about 0.5 mm. Laser cladding was carried out by using a LCY-400 (Wuhan Huagong Laser Technology Co., Ltd, China.) 400 W pulse CO2 laser. The laser was operated by smart MC software with Argon as the feeding gas. The laser cladding parameters were selected as 100e200 W laser power, 120 HZ frequency, 0.3e0.5 mm beam diameter and 7.5e15 mm/s traverse speed. The surface roughness (Ra) of titanium was measured using a JB4C surface roughness tester. The cross-section of the specimens were ground and polished successively. Microstructural characterization of laser-cladding ceramic layers was observed by using a Jeol JSM6400 scanning electron microscope. Universal testing machine (DSS-25T, Shimadzu, Japan) was used to evaluate the tensile bond strength between coating-Ti. The bond strength was
calculated by dividing the force (Newton) to the coating area (mm2).
3. Results and discussion Fig. 1 shows the F-SEM microphotographs of titanium surface (a), (b) the untreated Ti (c), (d) the Ti just treated in 40 wt% HF, (e) and (f) Ti surface anodized at 20 V for 15 min after pre-treatment in 40 wt% HF. Compared with the untreated Ti, a compact oxide layer with hybrid structures consisting of parallel micro-stripes and compact nano-protuberances was obtained after pretreatment in 40 wt% HF, as shown in Fig. 1(c) and (d). The size of the nanoprotuberance was about 10 nm, while the spaces between the stripes were about 1 mm. Hybrid structures with submicron rows of leaf-like embossments and nano-pores were synthesized after anodization of titanium at 20 V for 15 min with pre-treatment in 40 wt% HF, as shown in Fig. 1(e) and (f). The spaces between the rows of leaf-like embossments in Fig. 1(e) are the same order of magnitude as the spaces between the micro-stripes in Fig. 1(c). The formation of leaf-like embossment rows and nano-pores attributed to the hybrid effect of HF acid etching and anodization processes [3]. The selective corrosion of the titanium surface resulted in the formation of leaf-like embossment rows. The surface roughness of titanium was increased from 0.24 ± 0.02 mm to 0.82 ± 0.09 mm after anodization, which was
Please cite this article in press as: X. Chen, et al., Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.046
X. Chen et al. / Optical Materials xxx (2017) 1e5
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Fig. 2. SEM microphotographs of the bonding porcelain layers on Ti synthesized at different laser scanning speed (a) 15 mm/s, (b) 12.5 mm, (c) 10 mm/s and (d) 7.5 mm/s.
Fig. 3. XRD patterns of Ti surface after laser treatment with different scanning speed (a) 15 mm/s, (b) 12.5 mm/s, (c) 10 mm/s and (d) 7.5 mm/s.
mainly due to the existence of leaf-like embossments rows. In the previous research, the increased surface roughness of titanium increased the contact areas and mechanical bonding between titanium and porcelain [17]. In addition, anodization of titanium will help to increase the wettability between porcelain-Ti during laser fusion process. Therefore, the specimens were anodized with pretreatment in HF acid in this study. Fig. 2 shows the SEM microphotographs of the bonding
porcelain layers on Ti synthesized at different laser scanning speed (a) 15 mm/s, (b) 12.5 mm/s, (c) 10 mm/s and (d) 7.5 mm/s. During the laser fusion process, the porcelain powders absorbed the laser energy and began to melt when the temperature was higher than the softening point of the porcelain. When the laser scanning speed is 15 mm/s, as shown in Fig. 2(a), unmelted areas and half-melted areas coexisted in the bonding porcelain layer. There were also some micro-pores and micro-cracks in the bonding porcelain layer. When the laser scanning speed decreased, the sintering temperature increased accordingly and there were more time for the porcelain to complete the sintering densification process. At the initial stage of sintering densification of the porcelain, the bubbles in the porcelain layer began to remove and the volume of the porcelain gradually decreased. When the laser scanning speed decreased to 12.5 mm/s, the bonding porcelain layers were still incomplete sintered and some microcracks began to form due to the volume shrinkage and thermal stress arising from the laser heating and cooling process, as shown in Fig. 2(b). With the sintering densification of the porcelain progressed, the micro-cracks were gradually sealed by the molten porcelain, which could be seen from Fig. 2(c) and (d). The filling of the apparent pores resulted in the densification of the porcelain. When the laser scanning speed is 7.5 mm/s, vitrified bonding porcelain layers without any obvious microcracks were synthesized as shown in Fig. 2(d). Because the temperature of the porcelain decreased soon after the laser scanning, the pores which had not been completely removed still remained in the porcelain layer. Fig. 3 shows the XRD patterns of Ti surface after laser treatment with different scanning speed (a) 15 mm/s, (b) 12.5 mm/s, (c) 10 mm/s and (d) 7.5 mm/s. The XRD results revealed that the major
Please cite this article in press as: X. Chen, et al., Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.046
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X. Chen et al. / Optical Materials xxx (2017) 1e5
porcelain layer near the interface. Then the tensile bond strength between porcelain-Ti was evaluated for the specimens after laser treatment at 7.5 mm/s. After surface modification of Ti, the bonding strength of Ti-porcelain was significantly improved from 9.68 ± 1.21 MPa to 35.21 ± 2.42 MPa. The improvement of the bonding strength is mainly due to the improvement of the interface quality. The parallel cracks will led to spallation and substantial damage to the overall porcelain layers during bonding strength tests. In the further work, pre-heating and other laser parameters optimizing will be employed to reduce the excessive thermal stress, porosity and microcracks at the interface of Ti-porcelain, so that to improve the bonding between Ti-porcelain. 4. Conclusions Hybrid structures with submicron rows of leaf-like embossments and nano-pores were synthesized after anodization of titanium at 20 V for 15 min with pre-treatment in 40 wt% HF. By optimizing laser scanning speed, vitrified bonding porcelain layers with few pores and microcracks were synthesized on Ti surface. The surface modification of Ti significantly improved the surface roughness and bonding between Ti-porcelain. Acknowledgements This research was supported by the Natural Science Foundation of Jiangsu Province (No. BK20161182), QingLan Project of Jiangsu Province and Fundamental Research Funds for the Central Universities (No. 2015XKMS064). References
Fig. 4. SEM micrograph of the cross sections of the Ti-porcelain specimens synthesized by laser cladding process, (a) the untreated specimen and (b) specimen after surface modification.
phase of all the 4 specimens was a-Ti [JCPDS 11e0218], whilst the rutile [JCPDS 21e1276] and an amorphous TiO phase as secondary phases. The sintering temperature increased as the laser scanning speed decreased. When the laser scanning speed decreased to 12.5 mm/s and below, the rutile phase began to form and the intensities of the peaks of rutile phase gradually increased as the laser scanning speed decreased, while on the contrary the intensities of the peaks of Ti phase decreased accordingly. It revealed that a rutile layer was formed on the titanium surface after laser scanning and the oxidation of Ti surface increased with the decreased scanning speed. Fig. 4 shows the SEM micrograph of the cross sections of the Tiporcelain specimens synthesized by laser cladding process, (a) the untreated specimen and (b) specimen after surface modification. For the untreated specimen, as shown in Fig. 4(a), a microcrack about 2 mm was observed along the Ti-porcelain interface and some pores in the bonding porcelain layer near the interface. While for the specimen pretreated in 40 wt% HF, as shown in Fig. 4(b), the bonding porcelain layer adhered tightly to the Ti substrate and there were only few micro-cracks along the Ti-porcelain interface. At the same time, there were also some pores in the bonding
[1] L.T. Guo, J.L. Tian, J. Wu, B. Li, Y.B. Zhu, C. Xu, Y.H. Qiang, Effect of nano-porous film on the bonding strength of titanium-porcelain, Mater. Lett. 109 (2013) 140e142. [2] K.D. Yun, Y.Z. Yang, H.P. Lim, G.J. Oh, J.T. Koh, I.H. Bae, J.H. Kim, K.M. Lee, S.W. Park, Effect of nanotubular-micro-roughened titanium surface on cell response in vitro and osseointegration in vivo, Mater. Sci. Eng. C 30 (2010) 27e33. [3] L.T. Guo, X.Y. Chen, X.M. Liu, W. Feng, C. Lin, B. Li, Y.H. Qiang, Surface modifications and Nano-composite coatings to improve the bonding strength of titanium-porcelain, Mater. Sci. Eng. C 61 (2016) 143e148. [4] M.G. Troia Jr., G.E. Henriques, M.F. Mesquita, W. Fragoso, The effect of surface modifications on titanium to enable titanium-porcelain bonding, Dent. Mater 24 (2008) 28e33. [5] L.T. Guo, W. Feng, X.M. Liu, C. Lin, B. Li, Y.H. Qiang, Solegel synthesis of antibacterial hybrid coatings on titanium, Mater. Lett. 160 (2015) 448e451. [6] L.T. Guo, J.L. Tian, J. Wu, B. Li, Y.B. Zhu, Y.H. Qiang, Sol-gel synthesis of hybrid coating to improve titanium-porcelain bonding, Mater. Manuf. Process. 29 (2014) 1025e1029. [7] I.V. Shishkovsky, I. Smurov, Titanium base functional graded coating via 3D laser cladding, Mater. Lett. 73 (2012) 32e35. [8] H.C. Li, D.G. Wang, C.Z. Chen, F. Weng, Effect of CeO2 and Y2O3 on microstructure, bioactivity anddegradability of laser cladding CaO-SiO2 coating on titanium alloy, Coll. Surf. B Biointerfaces 127 (2015) 15e21. [9] Y.H. Diao, K.M. Zhang, Microstructure and corrosion resistance of TC2 Ti alloy by laser cladding with Ti/TiC/TiB2 powders, Appl. Surf. Sci. 352 (2015) 163e168. [10] M. Li, J. Huang, Y.Y. Zhu, Z.G. Li, Effect of heat input on the microstructure of in-situ synthesized TiNeTiB/Ti based composite coating by laser cladding, Surf. Coat. Tech. 206 (2012) 4021e4026. [11] J.M. Wilson, N. Jones, L. Jin, Y.C. Shin, Laser deposited coatings of Co-Cr-Mo onto Ti-6Al-4V and SS316L substrates for biomedical applications, J. Biomed. Mater. Res. B 101B (2013) 1124e1132. [12] H. Paydas, A. Mertens, R. Carrus, J. Lecomte-Beckers, J. Tchoufang Tchuindjang, Laser cladding as repair technology for Tie6Ale4V alloy: influence of building strategy on microstructure and hardness, Mater. Des. 85 (2015) 497e510. [13] X.L. Lu, X.B. Liu, P.C. Yu, S.J. Qiao, Y.J. Zhai, M.D. Wang, Y. Chen, D. Xu, Synthesis and characterization of Ni60-hBN high temperature self-lubricating anti-wear composite coatings on Ti6Al4V alloy by laser cladding, Opt. Laser Techol. 78 (2016) 87e94. [14] K.P. O'Flynn, K.T. Stanton, Laser sintering and crystallization of a bioactive
Please cite this article in press as: X. Chen, et al., Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.046
X. Chen et al. / Optical Materials xxx (2017) 1e5 glass-ceramic, J. Non-Cryst. Solid 360 (2013) 49e56. [15] L.T. Guo, Y. Shi, L.Z. Guo, Q. Zhang, C. Xu, Y.B. Zhu, T.W. Guo, Preparation and characterization of a titanium bonding porcelain, Mater. Sci. Eng. C 32 (2012) 1531e1535. [16] Y.L. Yang, N. Guo, J.F. Li, Synthesizing, microstructure and microhardness
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distribution of TieSieCeN/TiCN composite coating on Tie6Ale4V by laser cladding, Surf. Coat. Techol. 219 (2013) 1e7. [17] L.T. Guo, J.L. Tian, J. Wu, B. Li, Y.B. Zhu, C. Xu, Y.H. Qiang, Effect of surface texturing on the bonding strength of titanium-porcelain, Mater. Lett. 131 (2014) 321e323.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium Xiaoyuan Chen a, Litong Guo a, b, *, Xuemei Liu a, Wei Feng a, Baoe Li c, Xueyu Tao a, Yinghuai Qiang a a b c
China University of Mining and Technology, Xuzhou 221116, PR China Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 21 January 2017 Accepted 25 January 2017 Available online xxx
Bonding porcelain layer was fused on Ti surface by laser cladding process using a 400 W pulse CO2 laser. The specimens were studied by field-emission scanning electron microscopy, X-ray diffraction and bonding tests. During the laser fusion process, the porcelain powders were heated by laser energy and melted on Ti to form a chemical bond with the substrate. When the laser scanning speed decreased, the sintering temperature and the extent of the oxidation of Ti surface increased accordingly. When the laser scanning speed is 12.5 mm/s, the bonding porcelain layers were still incomplete sintered and there were some micro-cracks in the porcelain. When the laser scanning speed decreased to 7.5 mm/s, vitrified bonding porcelain layers with few pores were synthesized on Ti. © 2017 Published by Elsevier B.V.
Keywords: Biomaterials Microstructure Titanium Laser processing Sintering
1. Introduction Titanium was an alternative metal substrate for porcelain fused to metal because of its excellent performance, such as good corrosion resistance, biocompatibility, mechanical strength and low cost [1e3]. However, compared to conventional NiCr alloyporcelain, the poor bonding strength restricted its application [4e6]. Laser cladding is the fusion of a powder on a substrate and the laser energy melts the cladding material forming a metallurgical bond with the substrate [7e10]. More recently, laser cladding technology is a promising way to prepare coatings on Ti due to its limited heat affected zone, low dilution ratio and metallurgical bond between the coating and substrate [11,12]. This process allows for a coating built on titanium substrates to be built layer by layer and may prevent unwanted phase changes in the titanium [13,14]. However, there were no available reports on using laser cladding process to synthesize porcelain layers on Ti. In addition, the coefficient of thermal expansion of the bonding
* Corresponding author. China University of Mining and Technology, Xuzhou 221116, PR China. E-mail address: [email protected] (L. Guo).
porcelain (a ¼ 9.4 106/ C) used in this research is slightly lower than that of titanium (9.5 106/ C) [15], which will minimize the thermal stress between Ti-porcelain and avoid the formation of crack during high temperature cladding and rapid solidification. During the laser cladding process, the porcelain powders, which are glassy solid, were heated by laser energy and melted on Ti to form a chemical bond with the substrate. Microstructure of the coating and the interface between the coating and the substrate is of greatest interest [16]. Therefore, laser parameters were optimized to synthesize a crack-free coating on titanium. In addition, acid etching and anodization processes were combined to improve the surface roughness of Ti and chemical bonding between porcelain-Ti. 2. Experimental procedures ASTM grade II CP titanium was cast, ground and polished to prepare plate-shaped specimens (F13 mm 0.5 mm). The specimens without any treatment were used as control. 2.1. Anodization of titanium In a typical anodization process, the electrolyte was prepared by
http://dx.doi.org/10.1016/j.optmat.2017.01.046 0925-3467/© 2017 Published by Elsevier B.V.
Please cite this article in press as: X. Chen, et al., Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.046
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Fig. 1. F-SEM microphotographs of titanium surface (a), (b) the untreated Ti (c), (d) the Ti just treated in 40 wt% HF, (e) and (f) Ti surface anodized at 20 V for 15 min after pretreatment in 40 wt% HF.
adding 0.3 wt% of ammonium fluoride (NH4F, Sinopharm Chemical Reagent Co. Ltd., AR) and 1.25 vol% of distilled water into ethylene glycol (C2H6O2, Sinopharm Chemical Reagent Co. Ltd., AR). In a typical preparation procedure, the titanium specimens (>99% purity, thickness of 0.5 mm) were pre-treated in 40 wt% HF acid, and then anodized in the electrolyte solution using a graphite counter electrode at 20 V for 15 min at room temperature.
2.2. Preparation of titaniumeporcelain test specimens Then thin layer of bonding porcelain (Ti-bond porcelain, Dentsply, USA) was brushed on Ti surface and the thickness were controlled to about 0.5 mm. Laser cladding was carried out by using a LCY-400 (Wuhan Huagong Laser Technology Co., Ltd, China.) 400 W pulse CO2 laser. The laser was operated by smart MC software with Argon as the feeding gas. The laser cladding parameters were selected as 100e200 W laser power, 120 HZ frequency, 0.3e0.5 mm beam diameter and 7.5e15 mm/s traverse speed. The surface roughness (Ra) of titanium was measured using a JB4C surface roughness tester. The cross-section of the specimens were ground and polished successively. Microstructural characterization of laser-cladding ceramic layers was observed by using a Jeol JSM6400 scanning electron microscope. Universal testing machine (DSS-25T, Shimadzu, Japan) was used to evaluate the tensile bond strength between coating-Ti. The bond strength was
calculated by dividing the force (Newton) to the coating area (mm2).
3. Results and discussion Fig. 1 shows the F-SEM microphotographs of titanium surface (a), (b) the untreated Ti (c), (d) the Ti just treated in 40 wt% HF, (e) and (f) Ti surface anodized at 20 V for 15 min after pre-treatment in 40 wt% HF. Compared with the untreated Ti, a compact oxide layer with hybrid structures consisting of parallel micro-stripes and compact nano-protuberances was obtained after pretreatment in 40 wt% HF, as shown in Fig. 1(c) and (d). The size of the nanoprotuberance was about 10 nm, while the spaces between the stripes were about 1 mm. Hybrid structures with submicron rows of leaf-like embossments and nano-pores were synthesized after anodization of titanium at 20 V for 15 min with pre-treatment in 40 wt% HF, as shown in Fig. 1(e) and (f). The spaces between the rows of leaf-like embossments in Fig. 1(e) are the same order of magnitude as the spaces between the micro-stripes in Fig. 1(c). The formation of leaf-like embossment rows and nano-pores attributed to the hybrid effect of HF acid etching and anodization processes [3]. The selective corrosion of the titanium surface resulted in the formation of leaf-like embossment rows. The surface roughness of titanium was increased from 0.24 ± 0.02 mm to 0.82 ± 0.09 mm after anodization, which was
Please cite this article in press as: X. Chen, et al., Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.046
X. Chen et al. / Optical Materials xxx (2017) 1e5
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Fig. 2. SEM microphotographs of the bonding porcelain layers on Ti synthesized at different laser scanning speed (a) 15 mm/s, (b) 12.5 mm, (c) 10 mm/s and (d) 7.5 mm/s.
Fig. 3. XRD patterns of Ti surface after laser treatment with different scanning speed (a) 15 mm/s, (b) 12.5 mm/s, (c) 10 mm/s and (d) 7.5 mm/s.
mainly due to the existence of leaf-like embossments rows. In the previous research, the increased surface roughness of titanium increased the contact areas and mechanical bonding between titanium and porcelain [17]. In addition, anodization of titanium will help to increase the wettability between porcelain-Ti during laser fusion process. Therefore, the specimens were anodized with pretreatment in HF acid in this study. Fig. 2 shows the SEM microphotographs of the bonding
porcelain layers on Ti synthesized at different laser scanning speed (a) 15 mm/s, (b) 12.5 mm/s, (c) 10 mm/s and (d) 7.5 mm/s. During the laser fusion process, the porcelain powders absorbed the laser energy and began to melt when the temperature was higher than the softening point of the porcelain. When the laser scanning speed is 15 mm/s, as shown in Fig. 2(a), unmelted areas and half-melted areas coexisted in the bonding porcelain layer. There were also some micro-pores and micro-cracks in the bonding porcelain layer. When the laser scanning speed decreased, the sintering temperature increased accordingly and there were more time for the porcelain to complete the sintering densification process. At the initial stage of sintering densification of the porcelain, the bubbles in the porcelain layer began to remove and the volume of the porcelain gradually decreased. When the laser scanning speed decreased to 12.5 mm/s, the bonding porcelain layers were still incomplete sintered and some microcracks began to form due to the volume shrinkage and thermal stress arising from the laser heating and cooling process, as shown in Fig. 2(b). With the sintering densification of the porcelain progressed, the micro-cracks were gradually sealed by the molten porcelain, which could be seen from Fig. 2(c) and (d). The filling of the apparent pores resulted in the densification of the porcelain. When the laser scanning speed is 7.5 mm/s, vitrified bonding porcelain layers without any obvious microcracks were synthesized as shown in Fig. 2(d). Because the temperature of the porcelain decreased soon after the laser scanning, the pores which had not been completely removed still remained in the porcelain layer. Fig. 3 shows the XRD patterns of Ti surface after laser treatment with different scanning speed (a) 15 mm/s, (b) 12.5 mm/s, (c) 10 mm/s and (d) 7.5 mm/s. The XRD results revealed that the major
Please cite this article in press as: X. Chen, et al., Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.046
4
X. Chen et al. / Optical Materials xxx (2017) 1e5
porcelain layer near the interface. Then the tensile bond strength between porcelain-Ti was evaluated for the specimens after laser treatment at 7.5 mm/s. After surface modification of Ti, the bonding strength of Ti-porcelain was significantly improved from 9.68 ± 1.21 MPa to 35.21 ± 2.42 MPa. The improvement of the bonding strength is mainly due to the improvement of the interface quality. The parallel cracks will led to spallation and substantial damage to the overall porcelain layers during bonding strength tests. In the further work, pre-heating and other laser parameters optimizing will be employed to reduce the excessive thermal stress, porosity and microcracks at the interface of Ti-porcelain, so that to improve the bonding between Ti-porcelain. 4. Conclusions Hybrid structures with submicron rows of leaf-like embossments and nano-pores were synthesized after anodization of titanium at 20 V for 15 min with pre-treatment in 40 wt% HF. By optimizing laser scanning speed, vitrified bonding porcelain layers with few pores and microcracks were synthesized on Ti surface. The surface modification of Ti significantly improved the surface roughness and bonding between Ti-porcelain. Acknowledgements This research was supported by the Natural Science Foundation of Jiangsu Province (No. BK20161182), QingLan Project of Jiangsu Province and Fundamental Research Funds for the Central Universities (No. 2015XKMS064). References
Fig. 4. SEM micrograph of the cross sections of the Ti-porcelain specimens synthesized by laser cladding process, (a) the untreated specimen and (b) specimen after surface modification.
phase of all the 4 specimens was a-Ti [JCPDS 11e0218], whilst the rutile [JCPDS 21e1276] and an amorphous TiO phase as secondary phases. The sintering temperature increased as the laser scanning speed decreased. When the laser scanning speed decreased to 12.5 mm/s and below, the rutile phase began to form and the intensities of the peaks of rutile phase gradually increased as the laser scanning speed decreased, while on the contrary the intensities of the peaks of Ti phase decreased accordingly. It revealed that a rutile layer was formed on the titanium surface after laser scanning and the oxidation of Ti surface increased with the decreased scanning speed. Fig. 4 shows the SEM micrograph of the cross sections of the Tiporcelain specimens synthesized by laser cladding process, (a) the untreated specimen and (b) specimen after surface modification. For the untreated specimen, as shown in Fig. 4(a), a microcrack about 2 mm was observed along the Ti-porcelain interface and some pores in the bonding porcelain layer near the interface. While for the specimen pretreated in 40 wt% HF, as shown in Fig. 4(b), the bonding porcelain layer adhered tightly to the Ti substrate and there were only few micro-cracks along the Ti-porcelain interface. At the same time, there were also some pores in the bonding
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Please cite this article in press as: X. Chen, et al., Effect of laser parameters on the microstructure of bonding porcelain layer fused on titanium, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.046