Structural, mechanical and biocompatibility studies of hydroxyapatite-derived composites toughened by zirconia addition

Hydroxyapatite(OHAp)-based ceramic composites with added ZrO sub 2 have been prepared both by sintering at 1400 deg C and by hot isostatic pressing (HIP) at 1450 deg C and 140 MPa pressure (Ar atmosphere). The development of the crystalline phases and the microstructure of the composites have been examined using X-ray diffraction, electron microscopy, infrared and magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopic techniques. The fracture toughness and biocompatibility of the composites have also been studied. The effect of the addition of CeO sub 2 - and Y sub 2 O sub 3 -stabilized ZrO sub 2 and of simple monoclinic ZrO sub 2 to the initial physical mixture, on the structure and properties of the resulting composites has been investigated. In most of the sintered or HIP samples, the OHAp decomposes into tricalcium phosphate ( beta -TCP). CaO, which forms as a product of decomposition, dissolves completely in ZrO sub 2 and stabilizes the latter in its cubic/tetragonal phase. Presence of the beta -TCP phase in the product seems to be the result of a structural synergistic effect of hexagonal OHAp. Two structurally distinct orthophosphate groups have been identified in the composites by MASNMR of exp 31 P and attributed to decomposition products of OHAp at higher temperatures. The composites possess high K sub Ic values (two to three times higher than that of pure OHAp). Decomposition of hydroxyapatite gives rise to differences in microstructure between HIP and simply sintered composites although fracture toughness values are similar in magnitude indicating the presence of several toughening mechanisms. The in vitro SP2-O cell test suggests that these composites possess good biocompatibility. The combination of good biocompatibility, desirable microstructure and easy availability of initial reactants indicates that the simply sintered composite of OHAp and monoclinic ZrO sub 2 (ZAP-30) appears to be the most suitable for prosthetic applications.