Crystal defects induced grain refinement and enhanced mechanical properties of hydroxyapatite ceramics

Hydroxyapatite (HA) ceramics are one of the most widely used biomaterials due to their high biocompatibility and bioactivity. However, the inherent brittleness limits their biomedical applications as load‐bearing components for hard tissue repair. Herein, we reported a 127% enhancement in fracture toughness ( K IC increased from 0.62 to 1.41 MPa·m 1/2 ) of defective Al‐doped HA (D‐Al‐HA) ceramics through defect‐engineering mechanism. The theoretical and experimental studies indicated that Al substitution induced lattice distortion and defects such as generation of Ca vacancy, rotation of PO 4 3− group, dislocation of Ca ions and disorder of OH − chains. These defects acted as potent Zener pinning sites, suppressing grain boundary mobility during sintering and yielding a refined microstructure. The average grain size decreased from 2.2 µm for HA to 1.3 µm for D‐Al‐HA ceramics. This grain refinement caused a remarkable increase in fracture toughness of D‐Al‐HA ceramics through crack deflection, branching, and bridging mechanism. Concurrently, the compressive strength and flexural strength increased by 43% and 21%, achieving 363 ± 86 MPa and 77.4 ± 14.0 MPa, respectively, through Hall‒Petch mechanism. This study not only provided the first insights into the effects of Al‐induced defects on the enhanced mechanical performances of HA and the positive role of Al element for HA materials, but also offered a promising pathway for developing stronger and tougher bioceramics for demanding hard tissue implants.