Effect of Ca/P ratio on the mechanochemical preparation of carbonate-substituted hydroxyapatite

Mechanochemical solid-state reactions to produce carbonate-subtituted hydroxyapatite powder (CHA_p) using CaHPO₄.2H₂O and CaCO₃ mixtures with different Ca/P ratios of 1.67, 1.85 and 2.00 were investigated. The resulting materials were tested by x-ray diffraction (XRD), fourier transformed infrared spectroscopy (FTIR), x-ray fluoroscence (XRF), CHN elemental analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively. Results showed that all the solid-state reactions for CHA_p were almost completed within 15 to 24 hours of mechanical alloying at 400rpm with a ball to powder weight ratio equal to 10:1. Increasing milling time improved the formation of apatite phase due to higher impact energy transference to the compounds accelerating the kinetic of the phase transformation. The synthesized powders were preferentially CHA _p of the B type in the form of agglomerates containing 6.45 - 13.35 wt% of carbonate ions in the lattice. Consequently, dry milling process proved to be advantageous since a liquid medium is not necessary in the preparation of pure CHA_p and the powder obtained is nanocrystalline with crystallite size less than 50nm.