Implantation of HA into Superplastic Ti-6Al-4V: Kinetics and Mechanical Behaviors of Implanted Layer
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NTRODUCTION
IN recent years, the use of ceramic coatings on metallic substrates has been receiving more attention in the medical implant industry because of the combination of bioactive properties of ceramics and beneficial mechanical properties of metallic substrates.[1,2] Among ceramics, HA is widely used as the coating material because its chemical composition is very similar to the mineral part of the bone.[3] Titanium alloys have been used in implant materials as the substrate because of their excellent mechanical properties, chemical stability, and acceptable biocompatibility.[4] Furthermore, titanium alloys have the ability to be deformed in superplastic condition. In the medical field, the low flow stress of superplastic titanium is beneficial in forming of implants.[5–7] Several methods exist for coating HA on a titanium alloy substrate.[8–11] The significant problem of coating is associated with the lack of adhesion strength between the coated layer and the titanium substrate, which is a critical factor in long-term stability of the implant material.[12] One way to solve this problem is through implantation of hydroxyapatite into titanium alloy. This will result in mutual diffusion of HA and titanium alloy elements. Nonami et al.[6] reported a method for implantation of HA granules into Ti-4.5Al-3V-2Fe-2Mo, in which the implantation process was performed in the superplastic temperature. In their work, the granules SANAZ YAZDAN PARAST, MSc Student, and ISWADI JAUHARI, Senior Lecturer, are with the Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, 50603, Malaysia. Contact e-mail: [email protected] MOHSEN ASLE ZAEEM, Postdoctoral Research Associate, is with the Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759. Manuscript submitted May 21, 2010. Article published online October 19, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
(32–50 lm in diameter) were individually implanted into the substrate and they did not produce a uniform implanted layer on the substrate that is favorable as a biomaterial implant in medical applications. In this article, we present a method for implantation of HA into a titanium alloy substrate to produce an implanted layer. The effects of different implantation temperatures on the thickness of the implanted layer are investigated. The bonding strength between the implanted layer and the substrate is examined by using a friction wear test, and the effects of superplasticity and implanted layer thickness on the wear resistance are studied. Details of the experimental procedure are presented in the next section.
II.
EXPERIMENTS
A. Implantation Process Ti-6Al-4V specimens with chemical composition shown in Table I and size of 10 9 10 9 8 mm3 are used as the substrate. The as-received Ti-6Al-4V specimens initially have coarse grains with an average grain size approximately 13 lm. By applying a superplastic condition to an as-received Ti-6Al-4V, we can obtain a fine grain size (approximately 3 lm) that results in more d
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