Titanio

Abstract: Titanium (Ti) implants coated with hydroxyapatite (HA), combining the ability of HA to bond with natural bone and excellent mechanical properties of titanium, have been successfully used in clinics. To improve the crystallinity of the coatings, post-heattreatment was applied on the HA coating-titanium system. Several groups [1-4] have post-heat-treated plasma-sprayed and sputtered HA coatings, to increase their crystallinity and lower their solubility compared with the as-received coatings. Ducheyne et al. [5] employed a vacuum heat treatment on electrophoretically deposited HA coatings to investigate the induced changes on their structures. Besides the benefits produced by heat treatment, there also exist some negative side-effects. Ducheyne et al. [5] found that the heat treated layers contain distinctively different compositions from the as-received coatings. The present authors [3] also reported that the plasma-sprayed HA coatings began to decompose at 800 °C during vacuum heat treatment. It is well known that the HA structure is thermally stable up to 1250 °C in air [6] and to 1050 °C in vacuum [7]. So it is hypothesized that the destruction of HA structure in H A T i or HA-TiO: systems at lower temperatures than 1000°C is completely related to the titanium and its dioxide. In the present study, HA powder was pre-heat-treated at 1200 °C for 120 min (the XRD pattern showing no decomposition) and then mechanically mixed with titanium and titanium dioxide powders. The mixtures were pressed into circular plates and sintered for 30 rain in vacuum and for 60 min in air, separately, at different temperatures. The stability of the starting H A powder was also proved by heating it in vacuum (<1.333 x 10 -3 Pa) at 1000 °C for 30 min, showing the same reflection patterns as those of the standard HA structure. The XRD patterns were determined on a RIGAKU diffractometer (D/max-y A) with Cu-Ko~ radiation at 45-50 kV and 140160 mA. The XRD patterns in Fig. 1 exhibit phase compositions of the mixture of HA and titanium sintered at different temperatures in vacuum. The decomposition of HA induced by titanium begins at 800 °C with the appearance of traces of o: tricalcium phosphate (oL-TCP) and tetracalcium phosphate (TCPM). With the increase of temperature, this process becomes more extensive and no other phase is produced besides cr-TCP and TCPM. The results are identical with our previous reports [3] concerning the thermal