Grain-Size Dependence of the Hardness of Submicrometer and Nanometer Hydroxyapatite

Dense hydroxyapatite (HA) bodies (at 99% of the theoretical density or higher) with grain sizes ranging from 67 to 732 nm have been obtained via a process of morphology-enhanced low-temperature sintering at temperatures as low as 850 degrees C. The Knoop indentation study reveals that the hardness of HA exhibits a linear dependence on the inverse square root of the grain diameter, consistent with the Hall-Petch relationship. This is the first report of the grain-size dependence of the HA hardness in the nanometer range, and the phenomena observed suggest that the deformation of HA under indentation is dislocation mediated.