Processing and characterization of hydroxyapatite coatings on titanium produced by magnetron sputtering
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J. Koike Department of Materials Science, Tohoku University, 02 Aoba-yama, Aoba-ku, Sendai 980-8579, Japan (Received 30 April 2001; accepted 31 August 2001)
Hydroxyapatite (HA) coatings with different thicknesses were produced on Ti and Si substrates using the radio frequency magnetron sputtering method. The mechanical properties, for example, modulus and hardness, of the coatings were measured using nanoindentation. The measured values of modulus and hardness were close to the upper limit of that reported for bulk HA, indicating a fully dense structure. Interfacial strengths between the HA coatings and substrates were also evaluated using a nanoscratch technique. The HA–Ti interface appeared to be stronger than the HA–Si interface. The microstructures of the HA coating and the HA–Ti interface were examined using high-resolution electron microscopy. Chemical compositions of the HA coating and the HA–Ti interface were also analyzed using x-ray energy dispersive spectrometer and electron energy loss spectroscopy. The results indicated that the strong HA–Ti bonding is associated with an outward diffusion of Ti into HA layer and concomitant formation of TiO2 at or near the interface.
I. INTRODUCTION
Bone and teeth are mineralized tissues whose primary function is “load bearing.” Wet cortical bone is composed of 22 wt% organic matrix, 69 wt% mineral, and 9 wt% water.1 The major subphase of the mineral consists of submicroscopic crystals of an apatite of calcium and phosphate, resembling hydroxyapatite (HA) crystal structure. The apatite crystals are usually formed as slender needles, 20– 40 nm in length by 1.5–7 nm in thickness.2 Hydroxyapatite [Ca10(PO4)6(OH)2] has the unit cell dimensions of a ⳱ 0.9432 to 0.9418 nm and c ⳱ 0.6881 to 0.6884 nm, and the maximum x-ray diffraction plan is (211).3 The ideal Ca/P ratio of HA is 1.67 and the calculated density is 3.219 g/cm3.4 Calcium phosphatebased bioceramics have been in use in medicine and dentistry for over 20 years, because of its excellent biocompatibility with human tissues. Applications of fully dense bioceramics include dental implants, percutaneous devices, and use in periodontal treatment, alveolar ridge augmentation, orthopedics, maxillofacial surgery, otolaryngology, and spinal surgery.5 Whereas porous hydroxyapatite has been successfully synthesized for non-load-bearing bioimplants,6 it is still considered not suitable for load-bearing application because of its low mechanical strength. In contrast, hydroxyapatite coating on metallic implants, e.g., Ti and Co 3238
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J. Mater. Res., Vol. 16, No. 11, Nov 2001 Downloaded: 24 Apr 2015
alloys, to improve surface biocompatibility is considered to be an alternative solution. The coating approach is more practical than designing a new monolithic hydroxyapatite structure, since metallic implants have been used for many years and their mechanical responses are well understood. A hydroxyapatite-coated metallic implant is expected to extend its useful life. Various coating methods, includ
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