Controlled microstructural evolution of Hydroxyapatite–Bioglass® nanocomposites via two-step sintering

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Hydroxyapatite (HA)-Bioglass® (45S) nanocomposites were fabricated via two-step sintering (TSS) to optimize densification, phase stability, and mechanical performance for potential bone regeneration applications. Composite nanopowders were prepared by a sol-gel route, compacted into pellets, and subjected to a TSS protocol with systematically varied temperatures and holding times. The selected two-step sintering (TSS2) parameters were identified as an initial temperature of 1150 °C with a 15 min hold, followed by a seconday treatment at 1050 °C for 25 h, which yielded the best balance of densification and phase stability. X-ray diffraction and scanning electron microscopy revealed that HA remained the primary phase, while β-tricalcium phosphate (β-TCP) formation increased with Bioglass® content, enhancing fracture toughness via crack-bridging and transformation-induced local compressive stresses. Bulk density and nanoindentation measurements showed that Bioglass® acted as an effective sintering aid, promoting densification and improving hardness, elastic modulus, and toughness. Among the studied compositions, the 10 wt% Bioglass® composite processed under selected TSS2 conditions exhibited the highest density and superior mechanical properties, while maintaining nanoscale grains (<100 nm). These results demonstrate that controlled TSS can effectively tailor the microstructure and performance of HA- Bioglass® composites, offering a promising strategy for advanced bioceramic implants.