The influence of the microstructure of sintered hydroxyapatite on the properties of hardness, fracture toughness, thermal expansion and the dielectric permittivity

Sintered hydroxyapatite specimens with a 58–98% relative density range and average grain sizes between 1.64 μm and 7.44 μm were made with a commercial hydroxyapatite powder. No decomposition of the hydroxyapatite was detected, by x-ray diffraction, for any specimen. Vickers hardness (Hv) of the sintered hydroxyapatite specimens was successfully fit, as a function of the volume fraction porosity (P), to the minimum solid area model (MSA): H_V = H_V0 e−bP. The zero porosity hardness value H_V0 = 6.00 ± 0.79 GPa and the material dependent constant b = 6.03. Fracture toughness (K_Ic0) of the sintered hydroxyapatite specimens, measured by indentation methods, diverged from the MSA model. Fracture toughness increased as the porosity increased. K_Ic0 ≅ 0.5 MPa m1/2 for sintered hydroxyapatite specimens of 97% relative density increasing to K_Ic0 ≅ 2.6 MPa m1/2 for specimens of 72% relative density. Thermal expansion coefficients for sintered hydroxyapatite, measured by dilatometry = 12.9 × 10−6/°C at 100°C, 13.8 × 10−6/°C at 200°C, 16.3 at 400°C and 19.8 × 10−6/°C at 600°C. The relative dielectric permittivity was measured for several sintered hydroxyapatite specimens of varying porosity and the zero porosity relative dielectric permittivity was calculated using a dielectric mixing law to k₀ = 17.63 ± 0.70.