Nanocrystalline hydroxyapatite with simultaneous enhancements in hardness and toughness

Using a series of dense hydroxyapatite (HA) bodies with well controlled grain sizes ranging from sub-micrometers to nanometers, we show that simultaneous improvements in hardness and toughness can be attained for nanocrystalline (nc) HA. It is demonstrated that the hardness of HA follows the Hall–Petch relationship as the grain size decreases from sub-micrometers to nanometers. In the same grain size range, the toughness of HA increases by as much as 74% because of the enhanced crack deflection associated with a transition from transgranular to intergranular cracking, promoted by the reduced grain size in the nanoscale. The mechanisms for simultaneous enhancements in the hardness and toughness of nc HA are discussed. It is anticipated that the principle of simultaneous improvements in hardness and toughness discovered in this study is also applicable to other nc ceramics, particularly non-cubic ceramics, with anisotropic elastic and thermal expansion properties.