Optimization of milling parameters for the mechanosynthesis of nanocrystalline hydroxyapatite

Here we report synthesis of nanocrystalline hydroxyapatite (HAP), a bone substitute, through dry mechano-chemical method using phosphorous pentoxide and calcium hydroxide powders as starting materials, in a specially designed high-energy dual-drive planetary mill which is able to generate a strong gravitational (around 100 g) field inside the planetary mill thereby reducing the synthesis time significantly. Effects of various milling parameters such as milling time, ball to powder weight ratio, ball size and critical speed of the mill were investigated to understand their effect on the milling process and formation of nanocrystalline HAP. The study shows that the optimum conditions for the synthesis of nanocrystalline HAP is achieved when grinding is performed at a critical speed of 60% using a 4 mm diameter ball at a charge ratio of 20:1 for 15 h. X-ray diffraction and Fourier transform infra red (FT-IR) analysis confirmed phase purity of the nanocrystalline HAP powders. Electron microscopy of the milled HAP powder shows that powder particles are spherical and around 100 nm in size.