Fabrication and compressive strength of macrochannelled tetragonal zirconia polycrystals with calcium phosphate coating
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Fabrication and compressive strength of macrochannelled tetragonal zirconia polycrystals with calcium phosphate coating layer Young-Hag Koh,a) Hae-Won Kim, and Hyoun-Ee Kim School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea
John W. Halloran Materials Science and Engineering Department, University of Michigan, Ann Arbor, Michigan, 48109-2136 (Received 10 January 2003; accepted 4 June 2003)
Macrochannelled tetragonal zirconia polycrystals (TZP) coated with a calcium phosphate layer were fabricated using a coextrusion process to produce strong and bioactive porous bioceramics. The initial feedrod, composed of three materials [TZP (shell), calcium phosphate (intermediate layer), and carbon black (core)], was coextruded through a 750-m orifice at 120 °C, producing a continuous and flexible filament. Each sheet, which was composed of a unidirectional array of filaments, was stacked and then warm-pressed at 140 °C with 10 MPa. After binder burnout, the green billet was sintered between 1350 and 1600 °C for 1 h in air, leaving uniform macrochannels clad on the inside with bioactive calcium phosphate on a strong TZP body. The compressive strength of the specimen was much higher than that of calcium phosphate with a similar structure. Porous calcium phosphates have been extensively studied as bone graft materials because those bioceramics show a crystallographic and chemical similarity with various calcified tissues of vertebrates.1–5 As a result, these bioceramics can form a direct bond with the neighboring bones, enhancing resorption and osteoconductive behavior.6–8 Many efforts have been made to fabricate and characterize these porous bioceramics using several manufacturing techniques, such as hydrothermal exchange9 and the pyrolysis of organic particles.10 However, these conventional methods have the limited control of the pore geometry (such as pore size and distribution) and a relatively low compressive strength.11,12 Recently, a new fabrication method for porous bioceramics was developed to improve the control of pore geometry using a coextrusion process.13 However, the compressive strength of the bioceramics produced by this method requires more improvement for actual applications. Among the bioceramics, tetragonal zirconia polycrystals (TZP-3 mol% yttria-zirconia) have excellent mechanical properties, such as high strength, toughness, and compressive strength.1 Despite these merits, its bioinert
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Present address: University of Michigan, Ann Arbor, MI. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 9, Sep 2003
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nature has limited its wider applications. Therefore, in this paper, the macrochannelled TZP coated with a calcium phosphate layer was fabricated to both enhance the compressive strength and ensure active osteoconductive behavior. For comparison, macrochannelled calcium phosphate was also fabricated. Sintered samples were analyzed by x-ray diffraction (XRD), optical microscopy, scanning elec
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