Phase transformation, microstructural and mechanical properties of hydroxyapatite/alumina nanocomposite scaffolds produced by freeze casting

Freeze casting is an effective fabrication technique that allows producing scaffolds with variable porosity, pore size, pore orientation and compressive strength. To our knowledge, the present study is the first investigation on the replacement effects of microparticles with nanoparticles in bone scaffolds prepared through the freeze casting method. In this study, the effect of nano-alumina content on phase transformation, microstructural and mechanical properties of hydroxyapatite/nano-alumina (HA/nAl2O3) nanocomposite scaffolds, fabricated through the freeze casting method at different cooling rates, has been investigated. In the first stage, slurries with 15 vol% solid loading and different nano-alumina content were prepared. In the next stage, cooling rates of 1 and 4 K/min were applied to synthesize the porous scaffolds, followed by sintering at 1250 and 1350 °C. The characteristics of the initial powders, and phase composition, microstructure, pore size, pore distribution and mechanical strength of the scaffolds were assessed. The porosity of the synthesized scaffolds was in a range of 78–85%, and the compressive strength varied from 0.2 to 4 MPa as a function of nAl2O3 concentration, cooling rate, and sintering temperature. Surprisingly, further addition of nAl2O3 not only affected the microstructural features, but also provided mechanisms to improve the mechanical strength of the scaffolds.