Influence of sodium phosphate additives on densification, phase evolution and mechanical properties of hydroxyapatite

Hydroxyapatite (HA) is widely used as a biomaterial; however, its limited densification and mechanical strength restrict its application in load-bearing structures. This study investigates the effects of sodium phosphate-based sintering additives on the densification, phase evolution, microstructure, and mechanical properties of HA ceramics. Na₄P₂O₇, (NaPO₃)n, and Na₅P₃O₁₀ were separately added to HA at 5 wt% and sintered at 1000°C, 1100°C, and 1200°C for 2 and 4 h. The results showed that sintering at 1000°C and 1100°C resulted in insufficient densification, whereas improved densification was achieved at 1200°C. The highest relative density (88%) and lowest porosity (11.44%) were obtained for Na₄P₂O₇-containing HA sintered at 1200°C for 4 h. The same sample exhibited the highest hardness (2.98 GPa) and flexural strength (54.49 MPa). XRD analysis revealed temperature-dependent phase evolution, while SEM observations confirmed improved particle bonding and reduced pore content at higher sintering temperatures. Among the investigated additives, Na₄P₂O₇ showed the most effective enhancement of HA densification and mechanical performance under the studied conditions.