Sintering effects on the strength of hydroxyapatite

Mechanisms underlying temperature-strength interrelations for dense (> 95% dense, pores closed) hydroxyapatite (HAp) were investigated by comparative assessment of temperature effects on tensile strength, Weibull modulus, apparent density, decomposition (HAp:tricalcium phosphate ratio), dehydroxylation and microstructure. Significant dehydroxylation occurred above ~800 °C. Strength peaked at ~80 MPa just before the attainment of closed porosity (~95% dense). For higher temperatures (closed porosity), the strength dropped sharply to ~60 MPa due to the closure of dehydroxylation pathways, and then stabilized at ~60 MPa. At very high temperatures (> 1350 °C), the strength dropped catastrophically to ~10 MPa corresponding to the decomposition of HAp to tricalcium phosphate and the associated sudden release of the remaining bonded water.