Effect of Heat treatment and Phase Change on BFS of Two Hydroxyapatite Ceramic Powders

Abstract

Thermal stability and sintering behaviour of two hydroxyapatite (HAP) powders
where designated. AHAP and BHAP have been investigated at fired sintering
temperature range from 1100 to 1450 oC for 3 hrs using XRD and IR spectroscopy. The
fired compacts have been characterised by measuring their bulk density, firing shrinkage
and biaxial flexural strength (BFS).
BHAP has been found to transform to oxyhydroxyapatite (OHAP) at T≥1200oC, and at
T≥1300oC started to decompose to β-tricalcium phosphate (β-TCP). AHAP has been
found to be marginally less stable than BHAP with OHAP forming at T≥1150oC and β-
TCP forming at T≥1200oC. In both materials, β-TCP transformed to α-TCP at T
≥1350oC. Maximum densification (∼95% of theoretical density) occurred at a lower
firing temperature for AHAP (1150oC) than for BHAP (1250oC), with AHAP also
showing a slightly higher firing shrinkage (23% compared with 20% for BHAP). The
BFS of both materials have been found to increase with increasing sintering temperature
up to 1300oC, reaching a maximum value of ∼90-100 MPa. At T≥1300oC, this is
followed by rapid reduction in BFS due to cracks forming as a result of a volume
increase associated with the transformation of β-TCP into α-TCP.
It has been concluded that BHAP is marginally more thermally stable than AHAP, while
the latter densifies at a lower firing temperature, this is not a disadvantage and
comparable properties can be achieved for both powders, although different firing
conditions need to be used.