Effect of Al2O3 reinforcement on microstructure and mechanical properties of Ti–HA nanocomposites: a strengthening–porosity trade-off

This study investigates the influence of aluminum oxide (Al 2 O 3 ) reinforcement on the microstructure, densification, and mechanical properties of titanium–hydroxyapatite (Ti–HA) nanocomposites intended for load-bearing biomedical implant applications. Titanium composites containing 15 wt.% hydroxyapatite and varying Al 2 O 3 contents (0–7.5 wt.%) were fabricated using powder metallurgy, which included ball milling, cold compaction, and vacuum sintering at 1300 °C. The synthesized materials were characterized by X-ray diffraction, field-emission scanning electron microscopy with energy-dispersive spectroscopy, density measurements, Vickers microhardness testing, and uniaxial compression testing. X-ray diffraction analysis revealed that α-titanium remained the dominant metallic phase after sintering, while hydroxyapatite decomposed to form β-tricalcium phosphate. Increased Al 2 O 3 content promoted the formation of titanium dioxide and calcium aluminate phases through solid-state reactions during sintering. As Al 2 O 3 content increased from 0 to 7.5 wt.%, relative density decreased from 95.6% to 90.87%, and porosity increased from 4.4% to 9.13%. In contrast, microhardness increased significantly from 365 to 564.3 HV, due to dispersion strengthening and the formation of hard ceramic phases. The compressive strength reached a maximum of 234.35 MPa at 2.5 wt.% Al 2 O 3 , but declined to 139.48 MPa at higher reinforcement levels as a result of increased porosity, particle agglomeration, and the presence of brittle phases. The results indicate that a reinforcement level of 2.5 wt.% Al 2 O 3 achieves the optimal balance among hardness, compressive strength, and structural integrity. This study advances the development of titanium-based bioactive composites with improved mechanical properties and underscores their potential for future load-bearing orthopedic implant applications.