Thermal decomposition of synthesised carbonate hydroxyapatite

Heat treatments are used when sintering hydroxyapatite to make porous blocks and granules and during plasma spraying of coatings. Calcium : phosphorus ratio is known to affect the thermal decomposition behavior of hydroxyapatite. Hydroxyapatite with carbonate ions substituted for phosphate ions is more similar in composition to bone mineral. While it has been shown that carbonate apatite may be sintered, relatively little is known about its high temperature stability. Various atmospheres have been used in investigations into the thermal stability of hydroxyapatites and carbonate hydroxyapatites, including nitrogen, wet carbon dioxide air, water vapor and wet oxygen, but few of these studies were directly comparable. Previous work has shown that loss of carbonate from CHA at high temperature is time dependent, which suggests that rapid high temperature treatment may prevent carbonate loss during processing. This study investigated the effect of dry carbon dioxide, carbon dioxide containing 3% water, nitrogen and nitrogen containing 3% water on the phase composition of hydroxyapatite containing between 1.0 and 11.5 wt % carbonate rapidly heated to temperatures of between 700 and 1400 degreesC. Carbonate ion substitution was observed to decrease the temperature at which crystallisation occurred to a minimum of 700 degreesC for 11.8 wt % carbonate apatite heated in wet atmospheres. Atmosphere was found to appreciably affect the crystallization temperature and phase transformations of carbonate apatite containing 7.8 wt % carbonate. In wet and dry carbon dioxide atmospheres, crystallisation began in this material at 1100 and 900 degreesC, betaTCP was formed at 1500 and 1300 degreesC respectively. The high temperature decomposition of carbonate hydroxyapatite would appear to depend on the composition of the apatite and the atmosphere in which it is heated. (C) 2002 Kluwer Academic Publishers.