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Ti composite on superplastic titanium alloy (Ti–6Al–4V)
Materials Science and Engineering A 527 (2010) 5831–5836
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Embedment of HA/Ti composite on superplastic titanium alloy (Ti–6Al–4V) Adibah Haneem Mohamad Dom ∗ , Iswadi Jauhari, Sanaz Yazdanparast, Hidayah Mohd Khalid Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history: Received 16 March 2010 Received in revised form 18 May 2010 Accepted 23 May 2010
Keywords: Bonding strength Elemental diffusion Embedment HA/Ti composite Superplastic Ti–6Al–4V
a b s t r a c t In this research, embedment of HA-2 wt.% Ti composite powder on superplastic Ti–6Al–4V was conducted through continuous pressing technique to improve the low strength of the conventional pure HA coatings. The embedment process was carried out at temperature below the allographic temperature. The bond strength evaluation of HA-2 wt.% Ti composite layer was performed using wear test method under different applied pressure. The experimental results indicated that the HA-2 wt.% Ti composite layer had uniform thickness and well bonded to the substrate. EDX and line scanning analysis revealed that HA elements were detected at the substrate indicating embedment process was successful. The wear test results proved that the strength of the embedded layer on the superplastic Ti–6Al–4V was much better than the as-received Ti–6Al–4V. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys have been extensively used in biomedical applications because of their favourable biocompatibility and mechanical properties. Nevertheless, like other metallic implants, titanium is bioinert. Hence its abilities to bond to bone and to guide bone growth are distinctly smaller as compared to the other biomaterials with bioactive properties such as hydroxyapatite (HA). This will lead to a slower healing process. In spite of this, HA is a low toughness brittle material which limits its use in load bearing applications. For this reason, HA is widely employed as coating on metal substrates such as Ti–6Al–4V. A review article by Albrektsson [1] stated the case against the use of HA coated implants. It has been recognized that the mechanical stability of the interface between the HA coating and Ti–6Al–4V substrate could be a problem, due to the poor mechanical properties of HA [2,3]. One way to improve the low strength of pure HA coatings is to form a composite coating by reinforcing HA coating with a tough secondary phase such as Ti [4]. It is expected that the incorporation of Ti particles as reinforcement within the coating will improve the bonding strength between the coating and the substrate. Thus, fabrication of biomedical composites coating composed of HA and Ti can offer combination of bioactive properties as well as superior mechanical properties.
∗ Corresponding author. Fax: +60 3 79675317. E-mail addresses: adibah [email protected] (A.H. Mohamad Dom), [email protected] (I. Jauhari). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.05.064
Several methods are available for the application of HA/Ti coating onto Ti–6Al–4V and plasma spraying is the most common method [5]. However, it is difficult to prepare a uniform coating on the Ti–6Al–4V implant with a complex structure by this technique. Moreover, the coating has a tendency to degrade from the titanium substrate, due to low bonding strength [6]. Ti–6Al–4V alloy with fine equiaxed microstructures shows superplastic properties that allow for large plastic deformation under certain conditions. Superplastic Ti–6Al–4V can be effectively utilized for forming extremely complex shapes which is useful in the applications for implant materials. The objective of this study is to produce HA/Ti composite layer on superplastic Ti–6Al–4V using continuous pressing technique. It is expected that through this method, deformation of the Ti–6Al–4V substrate surface asperities will strongly hold the embedded layer. Thus, an improved HA/Ti composite layer on Ti6Al4V is expected to be produced. The bonding strengths of HA/Ti layer on superplastic Ti–6Al–4V was measured by wear test method.
2. Experimental procedure In this study, titanium alloy, Ti–6Al–4V with chemical composition shown in Table 1 was used as the embedment substrate. Electrical discharge machine (EDM) was used to cut the as-received material to a dimension of 8 mm × 8 mm × 15 mm for the length, width and thickness, respectively. Two types of microstructures were prepared; superplastic Ti–6Al–4V with fine grain microstructure and as-received Ti–6Al–4V with coarse grain microstructure. The superplastic Ti–6Al–4V was obtained by thermo-mechanical treatment pro-
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Fig. 1. Schematic diagram of thermo-mechanical treatment process for superplastic Ti–6Al–4V. Table 1 Chemical composition of Ti–6Al–4V. Element wt.%
Titanium, Ti 88.05
Aluminum, Al 6.73
Vanadium, V 3.90
Carbon, C 2.12
cess where the as-received Ti–6Al–4V was heated at 1373 K for 30 min and ice quenched to room temperature. Then the substrate was reheated at 1198 K, held for 5 min for homogenization and compressed at a constant strain rate of 2 × 10−4 s−1 until a height reduction of 50%. The compression process was conducted using a compression test machine (Instron) equipped with a hightemperature furnace in Argon gas atmosphere. Fig. 1 schematically shows the thermo-mechanical treatment process. Surface to be embedded was then ground by emery paper until grade 1200 grit. The specimen surface will be cleaned with alcohol to remove any impurities. In this research, HA (Merck, US) and Ti (Aldrich Chemical Company, UK) powder were used as the embedment agent. The mixed powders of HA and Ti with 2 wt.% Ti used in this study were blended by ball milling for 3 h with rotational speed of 300 rpm.
Fig. 2. Schematic illustration of the experimental apparatus.
The purposes of the milling were to mix the two different powders homogeneously as much as possible and to reduce the initial particle size of Ti. The size of the HA-2 wt.% Ti composite powder used in the experiment was less than 20 m. Embedment process was conducted at 1123 K for 4 h under compression strain of 3.64 × 10−4 in a controlled atmospheric condition. Argon was used as the control gas throughout the process to prevent oxidation. Fig. 2 shows the schematic diagram of the embedment process’ set-up. The movable crosshead was kept at the same position throughout the process so that the compression
Fig. 3. Microstructure of the Ti6Al4V substrate (a) as-received and (b) superplastic.
Fig. 4. SEM surface morphologies of HA-2 wt.% Ti layer on superplastic Ti–6Al–4V (a) low magnification and (b) high magnification.
A.H. Mohamad Dom et al. / Materials Science and Engineering A 527 (2010) 5831–5836
Fig. 5. Cross-sectional observations of HA-2 wt.% Ti composite embedded layer.
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strain could be maintained. In this way, the plastic deformation of the substrate could be restricted to the asperities area which is in contact with the HA-2 wt.% Ti composite powder. Thus, the bulk deformation of the substrate could be avoided. The furnace was then air cooled to room temperature. For comparison, embedment of HA-2 wt.% Ti composite on as-received was also produced. A Philips XL 30 scanning electron microscopy (SEM) operating with an accelerating voltage of 25 kV was used to examine the morphology of the embedded surface and to evaluate the embedded layer thickness. EDX line analysis was conducted to observe the distribution of HA and Ti–6Al–4V elements (OXFORD Instrument, INCA Energy 400). The phase composition of the layer was analyzed by X-ray diffraction (XRD) using a Rigaku D-MAX diffractometer (CuK␣). The XRD data were collected at a room temperature over the 2Â range of 10–80◦ at a step size of 0.02◦ .
Fig. 6. EDX spectra on HA-2 wt.% Ti layer on superplastic Ti6Al4V (a) layer and (b) substrate.
Fig. 7. The line scans analysis of the cross- section HA-2 wt.% Ti layer.
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Fig. 8. XRD patterns for HA-2 wt.% Ti composite layer.
Wear test was performed using a modified polishing machine with the speed of rotation at 165 rpm. This test was conducted against polishing cloth under water spraying condition. Different pressures of 2, 4, 8 and 16 kPa were applied on the specimens for 30 min. The wear distance is approximately 2805 m. Layer thickness was measured after wear tests for each load condition.
3. Results and discussions 3.1. Microstructure of the Ti–6Al–4V substrate Fig. 3 shows SEM images of the as-received and superplastic Ti–6Al–4V substrates before the embedment process. In the as-
Fig. 9. Cross section images of HA-2 wt.% Ti layer on superplastic Ti–6Al–4V (a) before wear test (b) 2 kPa (c) 4 kPa (d) 8 kPa and (e) 16 kPa.
A.H. Mohamad Dom et al. / Materials Science and Engineering A 527 (2010) 5831–5836
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Fig. 10. Cross section images of HA-2 wt.% Ti layer as-received Ti–6Al–4V (a) before wear test (b) 2 kPa and (c) 4 kPa.
received conditions, the microstructure (Fig. 3(a)) was composed of primary and secondary ␣-phase with very little -phase contained between the secondary ␣ plates. The average grain size of the as-received Ti–6Al–4V was estimated to be about 14.0 ± 0.5 m. Fig. 3(b) shows fine equiaxed grain microstructure of (␣ + )-phase with average grain size of about 3.0 ± 0.5 m after superplastic deformation. Fig. 4 shows surface morphology of the embedded layer obtained after the embedment process. The embedded surface exhibits fine and well-melted splats with small agglomerates of HA and Ti particles. Under higher magnification in Fig. 4(b), a porous surface can be observed. Porous surface is necessary to promote new bone tissue growth and to optimize the integration into surrounding tissue. 3.2. Embedded layer characterizations Fig. 5 shows cross-sectional SEM image of the HA-2 wt.% Ti composite layer formed on superplastic Ti–6Al–4V. The average layer thickness is approximately 8.0 ± 0.5 m. It can be seen that homogenous thin layer was formed on the substrate without the formation of cracks or delaminations. Delamination of the coating layer is usually observed in the plasma spraying process [7]. The occurrence of delamination of the coating will cause failure to the specimen. EDX analysis was performed to visualize the appearance of HA and Ti–6Al–4V elements in the layer and the substrate. Fig. 6 presents EDX spectra of HA-2 wt.% Ti layer on superplastic Ti–6Al–4V. It could be seen that the presence of compounds made of elements coming from HA (Ca and P) and Ti–6Al–4V substrate existed in the layer and the substrate. This showed that elemental diffusion occurred at the layer/substrate interface. Fig. 7 shows line scanning profiles of the various elements across the embedded layer/superplastic Ti–6Al–4V substrate interface after embedment process. The element line scanning results
revealed that Ca and P content in the layer gradually decreased from the layer to the substrate. The width of the Ca and P penetration into the substrate was about 4.0 ± 0.5 m. Diffusion in metals was generally accelerated by factors such as vacancies, dislocations and grain boundaries. The application of continuous pressing onto the fine grain microstructure of superplastic Ti–6Al–4V substrate used in this experiment is expected to provide those factors. This is because the continuous compression condition throughout the process would plastically deform the surface asperities and create higher densities of vacancies and dislocations. On the other hand, diffusion of Ti element from the substrate to the layer is easier to occur due to the porous HA-2 wt.% Ti layer. The mutual diffusion of the three main elements (Ca, P and Ti) could further enhance the bonding of the embedded layer to the substrate. The X-ray diffraction pattern showed the presence of TiO2 and Ti phase on the embedded surface (Fig. 8). Despite existence of Ca and P by analysis of EDX (Fig. 6), calcium phosphate phase has not been detected in the HA-2 wt.% Ti layer by XRD. It is reasonable to assume that Ca and P exist in TiO2 matrix in an amorphous form [8]. Previous studies by Ning and Zhou stated that TiO2 phase has the ability to induce apatite formation [9].
3.3. Bonding strength evaluation The HA/Ti layer will provide a bioactive surface on a metal implant for bone ongrowth. Therefore, it is important that the bonding strength between the layer and the substrate should be sufficiently high to withstand the interfacial stresses encountered in the in vivo environment. Fig. 9 presents SEM images of the embedded layer after wear test at different pressures. It could be observed that with increasing applied pressure, the thickness of the embedded layer decreased. Nevertheless, a thin HA-2 wt.% Ti composite layer of 2.0 ± 0.5 m still existed after applied pressure of 16 kPa (Fig. 9(e)).
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grain – is considered as the main reason for the strong layer produced. 4. Conclusions The results obtained from this experiment showed that:
Fig. 11. Thickness variation of the embedded layer on as-received and superplastic Ti–6Al–4V before and after wear test under different pressures.
Wear test was also conducted for HA-2 wt.% Ti composite layer on the as-received Ti–6Al–4V. The same trend as in the superplastic Ti–6Al–4V (Fig. 9) was also observed here (Fig. 10). However, total removal of embedded layer occurred at a much lower pressure of 4 kPa (Fig. 10(c)). Fig. 11 shows the thickness variations of as-received and superplastic Ti–6Al–4V after wear test at applied pressures of 2, 4, 8 and 16 kPa. From the results, it was expected that through continuous pressing of HA-2 wt.% Ti composite, the superplastic deformation of the Ti–6Al–4V surface asperities will strongly hold the embedded composite layer which finally lead to the improvement of the bonding strength between embedded layer and substrate. The present process has similarities with the solid-state diffusion bonding concept. It is understood that during the early stage of solid-state diffusion bonding process the asperities on each of the faying surfaces deform plastically as the pressure is applied. At the following stage, mainly creep and diffusion of atoms are taken place. In this study, the surface asperities of the superplastic Ti–6Al–4V substrate can be easily plastically deformed through the continuous pressing and therefore expedite the embedment between HA-2 wt.% Ti composite powder and the substrate. The fine grain microstructure of the alloy would then accelerate the following diffusion stage. The combination of these two elements – superplastic deformation of substrate surface asperities and fine
(1) Smooth and homogenous thin layer of HA-2 wt.% Ti composite has been successfully embedded on superplastic Ti–6Al–4V substrate through continuous pressing technique. (2) According to the EDX and line scanning results, there was a mutual diffusion between HA and Ti–6Al–4V elements across the interface. (3) Embedded layer on superplastic Ti–6Al–4V could withstand higher pressure than as-received Ti–6Al–4V substrate where a thin layer of HA-2 wt.% Ti (2.0 ± 0.5 m) still existed after applied pressure of 16 kPa. On the other hand, total removal of embedded layer on as-received occurs at a much lower pressure of 4 kPa. (4) Improvement of the layer was due to the superplastic deformation of substrate surface asperities and fine grain of Ti–6Al–4V. Acknowledgements This work is part of research programs financed by the Ministry of Science, Technology and Innovation, Malaysia (Project No. 1202-03-2052) and Postgraduate Research Fund University Malaya (Project No. PS027-2009A). The characterization and mechanical tests were conducted in the Department of Mechanical Engineering Facility, University of Malaya, Malaysia. References [1] T. Albrektsson, J. Oral Maxillofac. Surg. 56 (11) (1998) 1312–1326. [2] X.B. Zheng, M.H. Huang, C.X. Ding, Biomaterials 21 (8) (2000) 841–849. [3] C.A. Simmons, N. Valiquette, R.M. Pilliar, J. Biomed. Mater. Res. 47 (2) (1999) 127–138. [4] Xiu Feng Xiao, Rong Fang Liu, Yang Zeng Zheng, Mater. Lett. 59 (2005) 1660–1664. [5] W.R. Lacefield, in: L.L. Hench, J. Wilson (Eds.), An Introduction to Bioceramics, World Scientific Publishing Co., 1993, pp. 223–237. [6] J. Jansen, J. Van Waerden, J. Wolke, K. de Groot, J. Biomed. Mater. Res. 25 (1991) 973–989. [7] Fu Liu, Fuping Wang, Tadao Shimizu, Kaoru Igarashi, Liancheng Zhao, Surf. Coat. Technol. 199 (2005) 220–224. [8] Fu Liu, Ying Song, Fuping Wang, Tadao Shimizu, Kaoru Igarashi, Liancheng Zhao, J. Biosci. Bioeng. 100 (2005) 100–104. [9] C.Q. Ning, Y. Zhou, Biomaterials 23 (2002) 2909–2915.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Embedment of HA/Ti composite on superplastic titanium alloy (Ti–6Al–4V) Adibah Haneem Mohamad Dom ∗ , Iswadi Jauhari, Sanaz Yazdanparast, Hidayah Mohd Khalid Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history: Received 16 March 2010 Received in revised form 18 May 2010 Accepted 23 May 2010
Keywords: Bonding strength Elemental diffusion Embedment HA/Ti composite Superplastic Ti–6Al–4V
a b s t r a c t In this research, embedment of HA-2 wt.% Ti composite powder on superplastic Ti–6Al–4V was conducted through continuous pressing technique to improve the low strength of the conventional pure HA coatings. The embedment process was carried out at temperature below the allographic temperature. The bond strength evaluation of HA-2 wt.% Ti composite layer was performed using wear test method under different applied pressure. The experimental results indicated that the HA-2 wt.% Ti composite layer had uniform thickness and well bonded to the substrate. EDX and line scanning analysis revealed that HA elements were detected at the substrate indicating embedment process was successful. The wear test results proved that the strength of the embedded layer on the superplastic Ti–6Al–4V was much better than the as-received Ti–6Al–4V. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys have been extensively used in biomedical applications because of their favourable biocompatibility and mechanical properties. Nevertheless, like other metallic implants, titanium is bioinert. Hence its abilities to bond to bone and to guide bone growth are distinctly smaller as compared to the other biomaterials with bioactive properties such as hydroxyapatite (HA). This will lead to a slower healing process. In spite of this, HA is a low toughness brittle material which limits its use in load bearing applications. For this reason, HA is widely employed as coating on metal substrates such as Ti–6Al–4V. A review article by Albrektsson [1] stated the case against the use of HA coated implants. It has been recognized that the mechanical stability of the interface between the HA coating and Ti–6Al–4V substrate could be a problem, due to the poor mechanical properties of HA [2,3]. One way to improve the low strength of pure HA coatings is to form a composite coating by reinforcing HA coating with a tough secondary phase such as Ti [4]. It is expected that the incorporation of Ti particles as reinforcement within the coating will improve the bonding strength between the coating and the substrate. Thus, fabrication of biomedical composites coating composed of HA and Ti can offer combination of bioactive properties as well as superior mechanical properties.
∗ Corresponding author. Fax: +60 3 79675317. E-mail addresses: adibah [email protected] (A.H. Mohamad Dom), [email protected] (I. Jauhari). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.05.064
Several methods are available for the application of HA/Ti coating onto Ti–6Al–4V and plasma spraying is the most common method [5]. However, it is difficult to prepare a uniform coating on the Ti–6Al–4V implant with a complex structure by this technique. Moreover, the coating has a tendency to degrade from the titanium substrate, due to low bonding strength [6]. Ti–6Al–4V alloy with fine equiaxed microstructures shows superplastic properties that allow for large plastic deformation under certain conditions. Superplastic Ti–6Al–4V can be effectively utilized for forming extremely complex shapes which is useful in the applications for implant materials. The objective of this study is to produce HA/Ti composite layer on superplastic Ti–6Al–4V using continuous pressing technique. It is expected that through this method, deformation of the Ti–6Al–4V substrate surface asperities will strongly hold the embedded layer. Thus, an improved HA/Ti composite layer on Ti6Al4V is expected to be produced. The bonding strengths of HA/Ti layer on superplastic Ti–6Al–4V was measured by wear test method.
2. Experimental procedure In this study, titanium alloy, Ti–6Al–4V with chemical composition shown in Table 1 was used as the embedment substrate. Electrical discharge machine (EDM) was used to cut the as-received material to a dimension of 8 mm × 8 mm × 15 mm for the length, width and thickness, respectively. Two types of microstructures were prepared; superplastic Ti–6Al–4V with fine grain microstructure and as-received Ti–6Al–4V with coarse grain microstructure. The superplastic Ti–6Al–4V was obtained by thermo-mechanical treatment pro-
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Fig. 1. Schematic diagram of thermo-mechanical treatment process for superplastic Ti–6Al–4V. Table 1 Chemical composition of Ti–6Al–4V. Element wt.%
Titanium, Ti 88.05
Aluminum, Al 6.73
Vanadium, V 3.90
Carbon, C 2.12
cess where the as-received Ti–6Al–4V was heated at 1373 K for 30 min and ice quenched to room temperature. Then the substrate was reheated at 1198 K, held for 5 min for homogenization and compressed at a constant strain rate of 2 × 10−4 s−1 until a height reduction of 50%. The compression process was conducted using a compression test machine (Instron) equipped with a hightemperature furnace in Argon gas atmosphere. Fig. 1 schematically shows the thermo-mechanical treatment process. Surface to be embedded was then ground by emery paper until grade 1200 grit. The specimen surface will be cleaned with alcohol to remove any impurities. In this research, HA (Merck, US) and Ti (Aldrich Chemical Company, UK) powder were used as the embedment agent. The mixed powders of HA and Ti with 2 wt.% Ti used in this study were blended by ball milling for 3 h with rotational speed of 300 rpm.
Fig. 2. Schematic illustration of the experimental apparatus.
The purposes of the milling were to mix the two different powders homogeneously as much as possible and to reduce the initial particle size of Ti. The size of the HA-2 wt.% Ti composite powder used in the experiment was less than 20 m. Embedment process was conducted at 1123 K for 4 h under compression strain of 3.64 × 10−4 in a controlled atmospheric condition. Argon was used as the control gas throughout the process to prevent oxidation. Fig. 2 shows the schematic diagram of the embedment process’ set-up. The movable crosshead was kept at the same position throughout the process so that the compression
Fig. 3. Microstructure of the Ti6Al4V substrate (a) as-received and (b) superplastic.
Fig. 4. SEM surface morphologies of HA-2 wt.% Ti layer on superplastic Ti–6Al–4V (a) low magnification and (b) high magnification.
A.H. Mohamad Dom et al. / Materials Science and Engineering A 527 (2010) 5831–5836
Fig. 5. Cross-sectional observations of HA-2 wt.% Ti composite embedded layer.
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strain could be maintained. In this way, the plastic deformation of the substrate could be restricted to the asperities area which is in contact with the HA-2 wt.% Ti composite powder. Thus, the bulk deformation of the substrate could be avoided. The furnace was then air cooled to room temperature. For comparison, embedment of HA-2 wt.% Ti composite on as-received was also produced. A Philips XL 30 scanning electron microscopy (SEM) operating with an accelerating voltage of 25 kV was used to examine the morphology of the embedded surface and to evaluate the embedded layer thickness. EDX line analysis was conducted to observe the distribution of HA and Ti–6Al–4V elements (OXFORD Instrument, INCA Energy 400). The phase composition of the layer was analyzed by X-ray diffraction (XRD) using a Rigaku D-MAX diffractometer (CuK␣). The XRD data were collected at a room temperature over the 2Â range of 10–80◦ at a step size of 0.02◦ .
Fig. 6. EDX spectra on HA-2 wt.% Ti layer on superplastic Ti6Al4V (a) layer and (b) substrate.
Fig. 7. The line scans analysis of the cross- section HA-2 wt.% Ti layer.
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Fig. 8. XRD patterns for HA-2 wt.% Ti composite layer.
Wear test was performed using a modified polishing machine with the speed of rotation at 165 rpm. This test was conducted against polishing cloth under water spraying condition. Different pressures of 2, 4, 8 and 16 kPa were applied on the specimens for 30 min. The wear distance is approximately 2805 m. Layer thickness was measured after wear tests for each load condition.
3. Results and discussions 3.1. Microstructure of the Ti–6Al–4V substrate Fig. 3 shows SEM images of the as-received and superplastic Ti–6Al–4V substrates before the embedment process. In the as-
Fig. 9. Cross section images of HA-2 wt.% Ti layer on superplastic Ti–6Al–4V (a) before wear test (b) 2 kPa (c) 4 kPa (d) 8 kPa and (e) 16 kPa.
A.H. Mohamad Dom et al. / Materials Science and Engineering A 527 (2010) 5831–5836
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Fig. 10. Cross section images of HA-2 wt.% Ti layer as-received Ti–6Al–4V (a) before wear test (b) 2 kPa and (c) 4 kPa.
received conditions, the microstructure (Fig. 3(a)) was composed of primary and secondary ␣-phase with very little -phase contained between the secondary ␣ plates. The average grain size of the as-received Ti–6Al–4V was estimated to be about 14.0 ± 0.5 m. Fig. 3(b) shows fine equiaxed grain microstructure of (␣ + )-phase with average grain size of about 3.0 ± 0.5 m after superplastic deformation. Fig. 4 shows surface morphology of the embedded layer obtained after the embedment process. The embedded surface exhibits fine and well-melted splats with small agglomerates of HA and Ti particles. Under higher magnification in Fig. 4(b), a porous surface can be observed. Porous surface is necessary to promote new bone tissue growth and to optimize the integration into surrounding tissue. 3.2. Embedded layer characterizations Fig. 5 shows cross-sectional SEM image of the HA-2 wt.% Ti composite layer formed on superplastic Ti–6Al–4V. The average layer thickness is approximately 8.0 ± 0.5 m. It can be seen that homogenous thin layer was formed on the substrate without the formation of cracks or delaminations. Delamination of the coating layer is usually observed in the plasma spraying process [7]. The occurrence of delamination of the coating will cause failure to the specimen. EDX analysis was performed to visualize the appearance of HA and Ti–6Al–4V elements in the layer and the substrate. Fig. 6 presents EDX spectra of HA-2 wt.% Ti layer on superplastic Ti–6Al–4V. It could be seen that the presence of compounds made of elements coming from HA (Ca and P) and Ti–6Al–4V substrate existed in the layer and the substrate. This showed that elemental diffusion occurred at the layer/substrate interface. Fig. 7 shows line scanning profiles of the various elements across the embedded layer/superplastic Ti–6Al–4V substrate interface after embedment process. The element line scanning results
revealed that Ca and P content in the layer gradually decreased from the layer to the substrate. The width of the Ca and P penetration into the substrate was about 4.0 ± 0.5 m. Diffusion in metals was generally accelerated by factors such as vacancies, dislocations and grain boundaries. The application of continuous pressing onto the fine grain microstructure of superplastic Ti–6Al–4V substrate used in this experiment is expected to provide those factors. This is because the continuous compression condition throughout the process would plastically deform the surface asperities and create higher densities of vacancies and dislocations. On the other hand, diffusion of Ti element from the substrate to the layer is easier to occur due to the porous HA-2 wt.% Ti layer. The mutual diffusion of the three main elements (Ca, P and Ti) could further enhance the bonding of the embedded layer to the substrate. The X-ray diffraction pattern showed the presence of TiO2 and Ti phase on the embedded surface (Fig. 8). Despite existence of Ca and P by analysis of EDX (Fig. 6), calcium phosphate phase has not been detected in the HA-2 wt.% Ti layer by XRD. It is reasonable to assume that Ca and P exist in TiO2 matrix in an amorphous form [8]. Previous studies by Ning and Zhou stated that TiO2 phase has the ability to induce apatite formation [9].
3.3. Bonding strength evaluation The HA/Ti layer will provide a bioactive surface on a metal implant for bone ongrowth. Therefore, it is important that the bonding strength between the layer and the substrate should be sufficiently high to withstand the interfacial stresses encountered in the in vivo environment. Fig. 9 presents SEM images of the embedded layer after wear test at different pressures. It could be observed that with increasing applied pressure, the thickness of the embedded layer decreased. Nevertheless, a thin HA-2 wt.% Ti composite layer of 2.0 ± 0.5 m still existed after applied pressure of 16 kPa (Fig. 9(e)).
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grain – is considered as the main reason for the strong layer produced. 4. Conclusions The results obtained from this experiment showed that:
Fig. 11. Thickness variation of the embedded layer on as-received and superplastic Ti–6Al–4V before and after wear test under different pressures.
Wear test was also conducted for HA-2 wt.% Ti composite layer on the as-received Ti–6Al–4V. The same trend as in the superplastic Ti–6Al–4V (Fig. 9) was also observed here (Fig. 10). However, total removal of embedded layer occurred at a much lower pressure of 4 kPa (Fig. 10(c)). Fig. 11 shows the thickness variations of as-received and superplastic Ti–6Al–4V after wear test at applied pressures of 2, 4, 8 and 16 kPa. From the results, it was expected that through continuous pressing of HA-2 wt.% Ti composite, the superplastic deformation of the Ti–6Al–4V surface asperities will strongly hold the embedded composite layer which finally lead to the improvement of the bonding strength between embedded layer and substrate. The present process has similarities with the solid-state diffusion bonding concept. It is understood that during the early stage of solid-state diffusion bonding process the asperities on each of the faying surfaces deform plastically as the pressure is applied. At the following stage, mainly creep and diffusion of atoms are taken place. In this study, the surface asperities of the superplastic Ti–6Al–4V substrate can be easily plastically deformed through the continuous pressing and therefore expedite the embedment between HA-2 wt.% Ti composite powder and the substrate. The fine grain microstructure of the alloy would then accelerate the following diffusion stage. The combination of these two elements – superplastic deformation of substrate surface asperities and fine
(1) Smooth and homogenous thin layer of HA-2 wt.% Ti composite has been successfully embedded on superplastic Ti–6Al–4V substrate through continuous pressing technique. (2) According to the EDX and line scanning results, there was a mutual diffusion between HA and Ti–6Al–4V elements across the interface. (3) Embedded layer on superplastic Ti–6Al–4V could withstand higher pressure than as-received Ti–6Al–4V substrate where a thin layer of HA-2 wt.% Ti (2.0 ± 0.5 m) still existed after applied pressure of 16 kPa. On the other hand, total removal of embedded layer on as-received occurs at a much lower pressure of 4 kPa. (4) Improvement of the layer was due to the superplastic deformation of substrate surface asperities and fine grain of Ti–6Al–4V. Acknowledgements This work is part of research programs financed by the Ministry of Science, Technology and Innovation, Malaysia (Project No. 1202-03-2052) and Postgraduate Research Fund University Malaya (Project No. PS027-2009A). The characterization and mechanical tests were conducted in the Department of Mechanical Engineering Facility, University of Malaya, Malaysia. References [1] T. Albrektsson, J. Oral Maxillofac. Surg. 56 (11) (1998) 1312–1326. [2] X.B. Zheng, M.H. Huang, C.X. Ding, Biomaterials 21 (8) (2000) 841–849. [3] C.A. Simmons, N. Valiquette, R.M. Pilliar, J. Biomed. Mater. Res. 47 (2) (1999) 127–138. [4] Xiu Feng Xiao, Rong Fang Liu, Yang Zeng Zheng, Mater. Lett. 59 (2005) 1660–1664. [5] W.R. Lacefield, in: L.L. Hench, J. Wilson (Eds.), An Introduction to Bioceramics, World Scientific Publishing Co., 1993, pp. 223–237. [6] J. Jansen, J. Van Waerden, J. Wolke, K. de Groot, J. Biomed. Mater. Res. 25 (1991) 973–989. [7] Fu Liu, Fuping Wang, Tadao Shimizu, Kaoru Igarashi, Liancheng Zhao, Surf. Coat. Technol. 199 (2005) 220–224. [8] Fu Liu, Ying Song, Fuping Wang, Tadao Shimizu, Kaoru Igarashi, Liancheng Zhao, J. Biosci. Bioeng. 100 (2005) 100–104. [9] C.Q. Ning, Y. Zhou, Biomaterials 23 (2002) 2909–2915.