Polycrystalline Hydroxyapatite Materials Reinforced With Zirconia Inclusions
Waldemar Pyda, Anna Ślósarczyk, Zofia Paszkiewicz, Alicja Rapacz-Kmita, Maria Haberko, Anna Pyda Akademia Górniczo-Hutnicza, Wydział Inżynierii Materiałowej i Ceramiki, al. Mickiewicza 30, 30-059 Kraków
Annals 1 No. 2, 2001 pages 133-136
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abstract Hydroxyapatite ceramics containing dispersed particles of CaO-ZrO2 solid solution of tetragonal symmetry were prepared by hot-pressing a mixture of nanometric in size powders of hydroxyapatite (HAp) and pure zirconia. The zirconia and HAp powders were composed of isometric, agglomerated crystallites of ~25 nm and ~80 nm in size, respectively (Fig. 1). The former showed the specific surface area of 42.2±0.2 m2/g and the latter of 22.9±0.1 m2/g. The zirconia powder was composed of 68.2% of crystallites of monoclinic symmetry and the rest of monoclinic one. The composite powder had the specific surface area of 27.3±0.2 m2/g and the morphology imposed by the component powders. The incorporation of zirconia particles into the HAp powder decreased sinterability of the resultant composite powder when compared to the one of pure matrix as is shown in Fig. 2. The pure HAp sintered at temperatures higher than 1150ºC showed decreased densities, probably as a consequence of the initiation of HAp decomposition. The increased porosity was responsible for decreasing the modulus of elasticity and bending strength of the HAp materials with the sintering temperature (Fig. 3 and 5). The best densification (>99% theor. dens.) of the composite powder was achieved at 1250ºC. X-ray diffraction analysis revealed increased amounts of tetragonal and decreased amounts of monoclinic zirconia with the sintering temperature (Fig. 4). This was a result of the solid solution formation of CaO in ZrO2. Calcia could originate from decomposed HAp. Since HAp can maintain an appatite structure even if the Ca content is well below its stoichiometric value, the decomposition product TCP will only be formed when a critical amount of Ca is being transferred to the ZrO2 particles. The stabilization of tetragonal zirconia at 1250ºC requires 5.0 mole % CaO. In this conditions, the reduction of molar ratio Ca/P from 1.667 (stoichiometric HAp) to 1.649 is expected for the stabilization of 11.9 vol % ZrO2 particles introduced to the system. That is why evidences of HAp decomposition in the X-ray diffraction patterns are not observed (Fig. 4). Some excess of Ca introduced to the starting HAp powder was also conductive to this. The incorporation of zirconia particles into the HAp matrix inhibited greatly grain growth when compared to the pure HAp (Fig. 7). The zirconia inclusions received form of polycrystalline aggregates of crystallites of ~0.06 μm in size. Both transformation toughening and particle dispersion effects contributed to strengthening of the HAp+ZrO2 composites (Fig. 5). The former mechanism was especially effective in the composites with large amounts of the abstract tetragonal phase (Fig. 6; σ = 180±17 MPa).