Hydroxyapatite-zirconia composites have received much attention during the last decade
due to their combination of the desirable mechanical properties of zirconia and the excellent
bioactivity of hydroxyapatite (HA). However, thermal decomposition of the hydroxyapatite phase and
reaction between the zirconia phase and the hydroxyapatite phase remain a major problem in the
hydroxyapatite-zirconia composites. In this study, thermally stable and fluorine-substituted
hydroxyapatite (Ca10(PO4)6(OH)0.8F1.2; coded as HA06F) was prepared by a sol-gel method to replace
the hydroxyapatite. Yttria-stabilized zirconia (YTZP) was also prepared by a sol-gel method in order
to produce HA06F-YTZP composites with 5, 10, 15, 20, 40, and 60 wt% YTZP by simple and
cost-effective pressureless sintering. Thermogravimetric analysis (TGA) and x-ray diffraction (XRD)
of the HA06F-YTZP composites showed that the thermal stability of the HA06F matrices could be
maintained when the YTZP content did not exceed 20 wt% and for sintering temperatures less than
1400 oC. Dilatometric analysis and microstructural observation revealed that the YTZP phase in the
HA06F-YTZP composites retarded the densification of the composites if the zirconia content was
over 20 wt%. Electron scanning microscopy (SEM) and high resolution transmission electron
microscopy (HR-TEM) of the HA06F-YTZP composites showed that the YTZP second phase had a
size in the nanometer scale and the reaction between the HA06F phase and the zirconia phase was
suppressed. Mechanical properties including the Knoop hardness, the Young’s modulus, and the
fracture toughness of the HA06F-YTZP composites increased with the YTZP content until the
optimal content of 20 wt%; higher YTZP contents led to low mechanical properties due to poor
densification of the composites and the severe thermal decomposition of the HA06F phase. The
optimal HA06F-20YTZP composite also showed desirable attachment and proliferation of osteoblast
cells. Nevertheless, the study of the composite system indicated the limitations of the pressureless
sintering technique. To achieve the full potential of the composites for medium or low load bearing
applications, a pressure-assisted sintering technique would still be necessary.
The Effect of Yttria-Stabilized Zirconia on the Properties of the Fluorine-Substituted Hydroxyapatite Ceramics Prepared by Pressureless Sintering