Microstructural evolution, strengthening mechanisms, and tribological behavior in Ti–ZrO2–nHA biocomposites consolidated by SPS

Titanium–ceramic composites offer a promising pathway for improving the mechanical reliability and biological performance of implants. In this work, Ti, Ti–4 vol% ZrO2, and Ti–4 vol% ZrO2–6 vol% nano-hydroxyapatite (nHA) composites were fabricated via spark plasma sintering (1200 °C, vacuum) to elucidate the coupled effects of ZrO2 and nHA on densification, microstructure, mechanical properties, tribological behavior, and in vitro bioactivity. Sol-gel-derived nHA exhibited a nanometric particle size (24.5 ± 6.5 nm), while the incorporation of ZrO2 and nHA effectively suppressed grain growth in the Ti matrix. X-ray diffraction and Rietveld refinement confirmed that lattice expansion is governed primarily by oxygen interstitials, while ZrO2 remains a stable dispersed phase without dissolution. Hardness increased from 238.4 ± 4 VHN in pure Ti to 538.6 ± 7 VHN in T-Z-H due to combined grain-refinement strengthening, oxygen-driven solid-solution strengthening, and the load-bearing role of ZrO2 particles. Tribological testing revealed a progressive transition from severe adhesive wear in Ti to mixed adhesive–abrasive wear in Ti–ZrO2, and ultimately to tribochemical wear in T-Z-H, accompanied by the lowest wear rate (3.89 × 10−4 mm3/Nm) and friction coefficient (0.42 ± 0.04). The superior wear resistance of the T-Z-H nanocomposite is attributed to the formation of a compact tribochemical layer enriched with calcium-phosphate species. In vitro SBF immersion confirmed the bioactivity of the T-Z-H nanocomposite through dense apatite deposition after 14 days. Overall, the synergistic incorporation of ZrO2 and nHA provides a balanced enhancement of mechanical, tribological, and biological performance, while further evaluations are required for load-bearing implant applications.