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°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.