Effect of ball milling on the processing of bone substitutes with calcium phosphate powders

Decreasing the microscale morphology of synthetic bone substitutes is of primary importance in order to enhance the morphology of the surface of the material, which is directly in contact with osteoconductive cells when it is implanted in bone. The aim of this study was to investigate the influence of ball milling of slurries on the microscale morphology of hydroxyapatite and tricalcium phosphate bone substitutes and the influence on their processing. Ball milling appeared to be a successful method in order to raise the sintering reactivity of the powders, that is, to decrease the sintering temperature and microstructural morphology of the material. However, it was demonstrated that ball milling had a great influence on dispersion, which became very difficult under long milling times because of dissolution of the calcium phosphate powders. Due to dissolution, ionic species were generated in the slurry and interfered with the dispersing agent. Moreover a reprecipitation process occurred simultaneously, and large particles of the most stable phase (HAP) formed. The presence of such large particles generated stress gradients and cracks in the material during the sintering stage.