High-pressure plasma spraying of hydroxyapatite powders

Abstract

Thermal spray processes with their wide array of operating parameters are flexible manufacturing tools in the production of bioactive hydroxyapatite (HA) coatings. In this study, two spherical HA powders, namely, spray-dried hydroxyapatite and spheroidized hydroxyapatite powders were sprayed with suitable parameters in a controlled atmosphere plasma spraying (CAPS) system. This unique system was operated in three distinct modes: High-pressure plasma spraying (HPPS), atmospheric plasma spraying (APS) and inert plasma spraying. The HPPS mode has three different pressure settings up to 250 kPa. The APS mode is operated at normal atmospheric pressure of 100 kPa (sea level), and, the other modes utilised argon gas to provide the inert atmosphere during plasma spray. These were applied in order to assess the influence of chamber pressure and chamber atmosphere on the deposition of HA coatings. The microstructures and phase compositions of the plasma sprayed HA coatings are evaluated using standard X-ray diffraction (XRD) and electron microscopy techniques. These established the influence of the plasma spray parameters in the CAPS chamber. HPPS led to enhanced heating of the powder and dense HA coatings with a high content of the amorphous calcium phosphate phase. Small amounts of other calcium phosphates, tetracalcium phosphate and tricalcium phosphate were detected. Calcium oxide was not detected. Such coatings are useful for subsequent investigation of biological and mechanical properties where phase composition and porosity are deciding factors. It is found that the degree of melting of the HA powder can be controlled with CAPS system. This has the advantage of tailoring the final coating microstructure.