Slow growth of microcracks in hydroxyapatite during aging

Hydroxyapatite (Ca-10(PO4)(6)(OH)(2) or HA) is a brittle material that is subject to environmentally assisted slow crack growth. While most slow crack growth studies are carried out after aging, this study examines the slow growth of radial cracks induced by Vickers indentation in dense HA (94 % of theoretical density) during aging in ambient air, where the observed crack growth is consistent with a process in which residual stress drives crack growth. For indentation loads of 0.98, 1.96, 2.94, and 4.91 N, the average radial crack length increased exponentially with time for indentation loads of 0.98, 1.96, 2.94, and 4.91 N, with crack lengths saturating within 1 h following indentation. However, no radial crack growth was observed for 9.81 N loads. The load dependence of radial crack growth is proposed to be linked to the partitioning of residual strain energy by the lateral crack growth, which has not been reported in the literature.