Slow crack growth in bioactive ceramics

Due to their biocompatibility and bioactivity, hydroxyapatite (Ca-10(PO4)(6)(OH)(2), HAP) and P-Calcium phosphate (Ca-3(PO4)(2), beta-TCP) ceramics are attractive materials for biological implants. However, although their biological properties are very important, their mechanical behavior has also to be characterized. In this work, double torsion tests were performed to investigate slow crack growth behavior of dense HAP and beta-TCP materials. A crack rate, V, versus stress intensity factor, K-I, law was obtained in air for each ceramics. The presence of a threshold defining a safety range of use was observed. Stress assisted corrosion by water molecules in oxide ceramics is accounted for slow crack growth and delayed failure. The different propagation stages could be analyzed here in relation to this process. Ageing of the hydroxyapatite and the beta-calcium phosphate for six weeks in water at 37degreesC is associated with fall of both densities and mechanical properties. This kind of bio-ceramics should be considered with care as structural materials since they exhibit poor crack resistance.