Thermal behaviour of hydroxyapatite intended for medical applications

Abstract

Four commercial hydroxyapatites (both natural and synthetic) were tested to assess transformations of the chemical and crystalline structure following variation of temperature from 20 to 1600°C. The thermal behaviour of hydroxyapatite is relevant for biomedical applications such as plasma spraying of metallic implants. Thermogravimetric analysis showed a weight loss from each hydroxyapatite specimen, due to a release of structural H 20 molecules; all the specimens up to 1300°C were made of crystalline hydroxyapatite, determined by X-ray diffraction; at 1470°C they were made of both hydroxyapatite and calcium phosphate, but at 1570°C of calcium phosphate exclusively. The diffractograms of the hydroxyapatite coatings showed the same peaks as the original powders, so at the chosen plasma-spray procedure level no new phases were formed. The peak height was nevertheless lower in the plasma-sprayed hydroxyapatites for all interplanar spacing values, which indicated a lower degree of crystallinity, associated with a random structure derived from an alteration to the original crystalline network.