Effect of MgF₂ on hot pressed hydroxylapatite and monoclinic zirconia composites

Hydroxylapatite (HA) has been widely used in biomedical applications because of its excellent biocompatibility in the human body. A total of 25 wt% monoclinic (m) zirconia–HA composites (with and without 5 wt% MgF₂) were synthesized to investigate their mechanical properties and phase stability. In HA–m-ZrO₂ composites, HA and m-ZrO₂ reacted to form CaZrO₃ when there was no F⁻ present in the composite and m-ZrO₂ partially transformed to tetragonal ZrO₂. When MgF₂ was added into the system, it improved the thermal stability of the phases, densification, hardness, and fracture toughness of the composites and it caused the m-ZrO₂ to transform completely to t-ZrO₂ by incorporating the Mg²⁺ ions present in MgF₂ in the ZrO₂. Moreover, the stability of HA was improved by incorporating the F⁻ ions from MgF₂ in place of OH⁻ ions in HA. Substitution of OH⁻ by F⁻ ions was verified by the change in HA’s hexagonal lattice parameters. A fracture toughness of 2.0 MPa√m was calculated for the composite containing MgF₂.