RF plasma processing of ultra-fine hydroxyaptite powders

The study of biocompatible materials has intensified over the last decade driven by the growing awareness of an aging population. The need for alternative medical solutions has not only extended the requirement for biomedical devices to offer more than just functionality but also bioactivity. A new generation of bioactive materials has emerged promising better properties over existing biomaterials because of their ability to promote intimate bone growth and rapid fixation. In particular, hydroxyapatite (HA) has been recognised as one bioactive material having the potential and opportunity for development as bone substitutes. Although HA has a similar chemical composition to that of natural bone, it lacks sufficient strength and toughness for use in load bearing applications. Very often it requires blending with a low modulus polymer to achieve adequate toughness. However, the properties of HA composites is highly dependent on the particle size and morphology of the HA filler. Much has been suggested on the benefit of nano particulate materials in achieving higher mechanical properties. The need now arises in developing and processing HA of sufficient fineness for this purpose. As such the synthesis of ultra-fine HA was initiated using RF induction suspension plasma spraying with a wet suspension of HA as feedstock. This was axially injected into the RF plasma at various plate powers (plasma energies), chamber pressures, probe distances and plasma gas flow rates. The processed powders varied in size according to the cyclones designed to collect the powders from medium to ultra-fine. The chamber collecting ultra-fine powder contained particles ranging from 10 nm to 4 μm. The particle size, morphology and phase concentration of the powders were characterised using SEM, TEM, XRD and FTIR. In general, the particle size decreased with increasing plate power, while the reverse was observed for the volume of nano-sized particles produced. Decomposition into other phases such as tricalcium phosphate (TCP), tetracalcium phosphate (TTCP) and calcium oxide (CaO) increased with increasing plate power. This study suggests that the processing parameters associated with the production of the ultra-fine powders interact in a complex manner but can be rationalised by considering the overall thermal treatment experienced by the particulates during plasma treatment.