Densification of hydroxyapatite/zirconia nanocomposites fabricated via low-temperature mineralization sintering...

Hydroxyapatite/zirconia (HAP/ZrO₂) composites were fabricated via the low-temperature mineralization sintering process (LMSP) at an extremely low temperature of 130 °C to enhance the mechanical properties of HAP and broaden its practical applications. For this purpose, 5-20 vol% calcia-stabilized ZrO₂ were introduced into HAP, and HAP/ZrO₂ nanoparticles, mixed with simulated body fluid, were densified under a uniaxial pressure of 800 MPa at 130 °C. At 10 vol% ZrO₂, the relative density of the HAP/ZrO₂ composite was determined to be 88.3 ± 1.1%. Additionally, it exhibited the highest values of mechanical properties such as the Vickers hardness (3.68 ± 0.18 GPa), fracture toughness (1.11 ± 0.10 MPa·m^1/2), biaxial flexural strength (63.72 ± 2.35 MPa), and Young's modulus (83.91 ± 1.93 GPa) among the composite samples. These values were considerably higher than those of the pure HAP matrix due to the adequate reinforcement by ZrO₂ nanoparticles. Notably, owing to the low sintering temperature, phase decomposition of HAP, normally observed at high sintering temperatures above 1200 °C, was not observed. These results suggest that LMSP enables the incorporation of reinforcing ceramic materials with high sintering temperatures into bioactive materials at significantly lower temperatures, thereby improving their properties.