Fluoride substitution within
hydroxyapatite is an important occurrence for
biological apatites and is a promising approach for the chemical modification of synthetic
hydroxyapatite. Limited information on the influence of
fluoride substitution for
hydroxyl groups on the mechanical properties has provided the rationale for this study. Hydroxyfluorapatites with 0%, 20%, 40%, 60%, 80% and 100% replacement of
hydroxyl groups with
fluoride ions were assessed for hardness, elastic modulus, fracture toughness and brittleness using microindentation of sintered pellets. The production of samples with a similar grain size and density allowed the influence of
fluoride on mechanical properties to be determined. It was found that the hardness remains unaffected until 80% replacement of
hydroxyl groups with
fluoride, after which the hardness rapidly increases. The elastic modulus increases linearly with
fluoride content. Fracture toughness is improved with
fluoride incorporation into the lattice and reaches a peak of 1.8 for a 95% dense sintered pellet with a 60%
fluoride replacement, followed by a rapid decrease at higher
fluoride concentrations. The brittleness index is lowered to a minimum at 60%, after which a rapid increase occurs. High
fluoride levels are unfavourable from a mechanical perspective, are not recommended for
biomaterials, and can lead to a higher incidence of fracture where
sodium fluoride, for treatment of
osteoporosis, may produce a highly
fluoridated hydroxyapatite.