Fabrication of Hydroxyapatite–Zirconia Composites for Orthopedic Applications

Dense hydroxyapatite–zirconia (HAp–ZrO2) composites are expected to have desired mechanical and biological properties for orthopedic applications. However, due to some processing problems, to date, this material can only be prepared by special techniques. In this paper, we report for the first time a facile route to prepare HAp–ZrO2 composites. Initially, HAp and ZrO2 powders were dispersed in aqueous media with polyacrylic acid and glutamic acid as the dispersants. The slurries exhibited a well-stabilized state at a high solid content (>50 vol%) and therefore green samples with high density (>60%) can be obtained after slip casting. These HAp–ZrO2 green samples can be easily densified by pressureless sintering at 1450°C with 2 h holding. After sintering, only hydroxyoxyapatite Ca10(PO4)6Ox (OH)2(1−x) (HOA), ZrO2, and trace amounts of α-tricalcium phosphate phases were detected. No obvious reactions between HAp and ZrO2 phase were observed. The HAp–ZrO2 samples showed excellent mechanical and biological properties. For 40 vol% HAp–60 vol% ZrO2 samples sintered at 1450°C, the flexural strength and toughness were 220 MPa and 4.37 MPa·m1/2, respectively. In addition, we observed the attachment, spreading, and proliferation of mesenchymal stem cells on the HAp–ZrO2 samples' surface. The results showed that the proposed colloidal processing and pressureless sintering process is feasible for preparing HAp–ZrO2 composites with high mechanical properties and promising bioactivity for orthopedic applications.