Structural and microstructural characterizations of nanocrystalline hydroxyapatite synthesized by mechanical alloying

Single phase nanocrystalline hydroxyapatite (HAp) powder has been synthesized by mechanical alloying the stoichiometric mixture of CaCO3 and CaHPO4 powders in open air at room temperature, for the first time, within 2h of milling. Nanocrystalline hexagonal single crystals are obtained by sintering of 2h milled sample at 500°C. Structural and microstructural properties of as-milled and sintered powders are revealed from both the X-ray line profile analysis and transmission electron microscopy. Shape and lattice strain of nanocrystalline HAp particles are found to be anisotropic in nature. Particle size of HAp powder remains almost invariant up to 10h of milling and there is no significant growth of nanocrystalline HAp particles after sintering at 500°C for 3h. Changes in lattice volume and some primary bond lengths of as-milled and sintered are critically measured, which indicate that lattice imperfections introduced into the HAp lattice during ball milling have been reduced partially after sintering the powder at elevated temperatures. We could achieve ~96.7% of theoretical density of HAp within 3h by sintering the pellet of nanocrystalline powder at a lower temperature of 1000°C. Vickers microhardness (VHN) of the uni-axially pressed (6.86MPa) pellet of nanocrystalline HAp is 4.5GPa at 100gm load which is close to the VHN of bulk HAp sintered at higher temperature. The strain-hardening index (n) of the sintered pellet is found to be >2, indicating a further increase in microhardness value at higher load. Hydroxyapatite (HAp) has been synthesized by mechanical alloying in polycrystalline form within 2h of milling in which some important bond lengths are measured. After annealing some single crystals are grown up and changes in bond lengths are observed. [Display omitted] •HAp phase is completed after 2h of milling.•HAp nanoparticles are anisotropic in nature.•Both the lattice parameters a and c of HAp lattice increases with milling time.•HAp single crystals found at 500°C.•Vickers microhardness of HAp pellet sintered at 700°C is 4.5GPa.