Sintering behaviour and mechanical properties of hydroxyapatite and dicalcium phosphate

Abstract

The sintering behaviour of powders of two calcium phosphates, namely hydroxyapatite (HA) and dicalcium phosphate (DCP), were studied at various temperatures and in various environments. The density, flexural strength and Knoop hardness of HA sintered in air for 4 h initially increased with the sintering temperature, reaching maxima at around 1150°C, and then decreased due to decomposition of HA into tri- (TCP) and tetracalcium phosphates. Sintering in vacuum caused decomposition of HA at lower temperatures, and consequently the mechanical properties were poorer than those of HA sintered in air. The densification and mechanical properties of DCP sintered in air and vacuum showed similar behaviour to those of HA. In air DCP underwent phase transformation from γ- to β- and to α-phases. In vacuum DCP started to decompose into tricalcium phosphate at 1000°C. To reduce dehydroxylation, HA powder was sintered in moisture at various temperatures up to 1350°C and X-ray diffraction study did not indicate any decomposition at the highest sintering temperature. The density, flexural strength and hardness of HA sintered in moisture increased with the sintering temperature and eventually reached plateaux at about 1300°C, but below 1200°C they were lower than those of HA sintered in air at corresponding temperatures. Thus, it is seen that dehydroxylation did not hinder sintering of HA. On the other hand, decomposition obstructed sintering of both HA and DCP.