Synthesis of n‐HAP using different precursors and dependence of Vickers hardness on the structure by tuning sintering temperature

Abstract Three sets of starting materials Ca(NO ₃ ) ₂ ·4H ₂ O and (NH ₄ ) ₂ HPO ₄ ; Ca(OH) ₂ and H ₃ PO ₄ ; Ca(OH) ₂ and (NH ₄ ) ₂ HPO ₄ were chosen for the preparation of different precursors of hydroxyapatite nanoparticles (n‐HAP) by wet chemical precipitation technique denoted by HAP‐1, HAP‐2, HAP‐3. X‐ray diffraction (XRD) studies show that n‐HAP of all three types exhibited single‐phase in the as‐prepared condition. Transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) studies of as‐prepared samples showed characteristic needle‐like particles with a width of 5 nm, which were fully crystalline exhibited by the lattice fringe in HRTEM, and single‐phase demonstrated by SAED patterns. The samples were pelletized and annealed at varying temperatures in the range of 600°C–1400°C. The results indicated that HAP‐1 and HAP‐3 samples showed phase decomposition at 1400°C to form β‐tricalcium phosphate (β‐TCP) while for HAP‐2 no such decomposition occurred up to 1400°C. Data from the Vickers hardness test revealed that the hardness value up to 2.5 GPa was attained at the sintering temperature of 1200°C for HAP‐1 and HAP‐3. The hardness values are consistently higher for HAP‐2 than HAP‐1 and HAP‐3 at all sintering temperatures and attained a value of 3 GPa at 1200°C. Thus, synthesizing n‐HAP by using H ₃ PO ₄ ; Ca(OH) ₂ bestows highest Vickers hardness.