The mechanical properties of hydroxyapatite (HAp) bioceramic material were improved significantly by adding tetragonal-zirconia-polycrystalline (TZP) powder coated with Al 2 O 3 . The coating was done by dispersing the TZP powder in a boehmite suspension and adjusting the pH to cause heterocoagulation. The coating effectively reduced the deleterious reaction between the HAp and the TZP that tended to form tricalcium phosphate (TCP) and fully stabilized cubic zirconia. When 15 vol% zirconia and 30 vol% alumina were added to the HAp by the coating method, the strength and the fracture toughness of the composite were 300 MPa and 3 MPa⅐m 1/2 , respectively, which are about 3 times higher than those of pure HAp.
A thin hydroxyapatite (HA) layer was coated on a microarc oxidized titanium (MAO-Ti) substrate by means of the sol-gel method. The microarc oxidation (anodizing) enhanced the biocompatibility of the Ti, and the bioactivity was improved further by the sol-gel HA coating on the anodized Ti. The HA sol was aged fully to obtain a stable and phase-pure HA, and the sol concentration was varied to alter the coating thickness. Through the sol-gel HA coating, the Ca and P concentrations in the coating layer increased significantly. However, the porous morphology and roughness of the MAO-Ti was altered very little by the sol-gel treatment. The proliferation and alkaline phosphatase (ALP) activity of the osteoblast-like cells on the MAO/HA sol-gel-treated Ti were significantly higher than those on the MAO-Ti without the HA sol-gel treatment.
additions on the mechanical properties of hydroxyapatite (HAp) were investigated. The addition of Ni 3 Al particles increased the strength as well as the fracture toughness of HAp. However, the improvements in the properties were limited because of the formation of microcracks around the metal particles. The microcracks were formed because of the large difference in the coefficients of thermal expansion between HAp and Ni 3 Al, and because of the relatively large size of Ni 3 Al particles (∼20 µm). The addition of submicrometer Al 2 O 3 powder was also effective in increasing the mechanical properties. The flexural strength and the fracture toughness were increased from about 100 MPa and 0.7 MPaؒm 1/2 , respectively, to 200 MPa and 1.5 MPaؒm 1/2 by the addition of 20 vol% Al 2 O 3 . When Ni 3 Al and Al 2 O 3 were added together, the fracture toughness was further increased to 2.3 MPaؒm 1/2 . This increase in the fracture toughness was attributed to the synergistic effect of matrix strengthening and crack interactions with the metal particles.
A hydroxyapatite [HAp; Ca10(PO4)6(OH)2] coating layer was formed on a Ti-based alloy by the electron-beam deposition method. When pure HAp was used as a target for the deposition, an amorphous layer was formed on the metal substrate. By heat treatment in a vacuum at 630 °C, the layer was crystallized into tricalcium phosphate [Ca3(PO4)2]. The crystallization improved the dissolution rate of the layer remarkably; however, at the same time, it deteriorated the bond strength with the substrate. When extra CaO (up to 25 wt%) was added to the target and processed under the same conditions, a layer compositionally close to crystalline HAp was deposited. Before the heat treatment, even though the layer was in amorphous state, the dissolution rate in the physiological solution was extremely low. Furthermore, the bond strength increased remarkably compared to the layer formed by the pure HAp target. Compositional and structural resemblance of the layer with the crystalline HAp was attributed to these improvements in properties.
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