In the preparation of bioactive composites containing hydroxyapatite, Ca5(PO4)3(OH), and an oxide it has been a problem to prevent the hydroxyapatite from decomposing in the sintering process. This is because H2O is evolved when hydroxyapatite is heated, implying that the occupied OH- positions in hydroxyapatite structure are partly replaced by vacancies and O2- ions. The thermal stability of hydroxyapatite was found to depend on the fraction of vacancies and O2- ions present. The decomposition of the hydroxyapatite is initiated when a critical fraction of the OH- ions has been lost, and it is not specifically related to the temperature applied or atmosphere used. The decomposition temperature of hydroxyapatite and fluoride-containing apatite, Ca5(PO4)3(OH)1-xFx, in the presence of alumina has been studied and found to increase with increasing x value in Ca5(PO4)3(OH)1-xFx. By combining this observation with thermogravimetric studies of hydroxyapatite and Ca5(PO4)3OH1-xFxsamples, it was concluded that the decomposition of hydroxyapatite in the presence of alumina can be described by the following reactions:
Ca5(PO4)3(OH) --> Ca5(PO4)3(OH)1-xOx/2 + x/2 H20
2 Ca5(PO4)3(OH)1-xOx/2+ Al2O3 --> 3 Ca3(PO4)2 + CaAl2O4 + (1-x) H2O
With the use of a closed system for sintering the aluminañapatite composites, the loss of water can be reduced. The equilibrium in the first reaction will then be shifted to the left, and the second reaction will not occur. This implies that a higher sintering temperature can be used to densify an aluminañhydroxyapatite composite. Accordingly, composites of alumina and zirconia, respectively, with hydroxyapatite could be hot isostatically pressed (HIPed) in a closed system at 1200oC and at a pressure of 160 MPa without any detectable decomposition of the hydroxyapatite. Another way to avoid excess formation of vacancies is to replace some of the OH-ions with F-. This implies that the equilibrium in the first reaction given above is shifted to the left, thus improving the thermal stability of the apatite.
The main result of this thesis work is a more detailed understanding of the reaction between the oxide and hydroxyapatite, which has made it possible to prepare densified oxide-hydroxyapatite composites without decomposition of the hydroxyapatite phase.
Stockholm: Stockholm University , 1999. , 67 p.