Densification and mechanical response of cold-sintered hydroxyapatite with phosphoric-acid as a transient liquid phase

Hydroxyapatite (HAp) was consolidated by the cold sintering process at 200 °C under 600 MPa for 30 min using phosphoric-acid transient liquid phases containing either 5 or 10 wt% solution at concentrations of 1 or 2 mol L −1 . The influence of liquid content and molarity on phase stability, densification, mechanical behavior, and fracture morphology was investigated. X-ray diffraction confirmed preservation of the HAp phase for all conditions, with only minor variations in lattice parameters and apparent crystallite size. The relative density ranged from 72.28 to 78.55%, and no statistically significant difference was observed among the four processing conditions. In contrast, the mechanical response was strongly affected by the transient-liquid formulation. The condition containing 10 wt% phosphoric-acid solution at 1 mol L −1 exhibited the highest biaxial flexural strength, reaching 36.08 ± 8.88 MPa, and was significantly superior to the other groups. Vickers microhardness was higher for the 2 mol L −1 conditions, with values of 2.133 ± 0.241 GPa for 5 wt% solution and 2.036 ± 0.502 GPa for 10 wt% solution. Weibull analysis of the hardness distribution showed the highest modulus for the condition containing 5 wt% solution at 2 mol L −1 ( m = 9.68 ), whereas the condition containing 10 wt% solution at 2 mol L −1 exhibited the lowest uniformity ( m = 4.13 ). Scanning electron microscopy (SEM) revealed a delaminated fracture morphology in all specimens, limited grain growth, and clear differences in particle connectivity and cavity distribution. The results demonstrate that phosphoric-acid content and molarity primarily controlled interparticle bonding and local mechanical response rather than average densification. The interpretation is limited by the unavailability of specimen-level flexural-strength data and by the use of one representative SEM field per condition. Among the investigated formulations, the condition containing 10 wt% solution at 1 mol L −1 provided the most favorable balance between flexural strength, microstructural continuity, and low-temperature phase preservation.