Plasma sprayed hydroxyapatite(HA) coatings produced with flame spheroidised powders

Hydroxyapatite (HA) is known for its ability to bond chemically with bone. It is therefore termed as a bone-bonding or bioactive material. The plasma spray process is widely used to coat metallic implants with HA. However, processing this material remains an arduous task because of its tendency to breakdown during plasma spray into other calcium phosphate compounds that do not have the bioactive properties of HA. This setback can be addressed by tailoring powder characteristics to suit the plasma spray process. The raw HA powder feedstock normally has an angular morphology and a wide particle size range. As a result, there are difficulties in powder transport from the hopper to the plasma spray gun and, in nonuniform melting of the powders in the plasma flame. The very small particles will be overheated or vapourised while the large particles will not melt completely. Hence, spherical particles within a narrow particle size range are better suited for the plasma spray processes. The present study used flame spheroidised HA as the feedstock for plasma spraying. Results show that coatings generated from spheroidised HA (SHA) powders have unique surface and microstructure characteristics. SEM micrograph of the coating surface reveals well-formed splats that spread and flatten to a disc configuration without disintegration. This reflects adequate melting of the HA in the plasma and subsequent deposition consistency. The surface topography is generally flat with good overlapping of subsequent spreading droplets. Porosity in the form of macropores is substantially reduced and can be further improved using a more accurate control of the plasma energy input during the spraying process. The cross-section microstructure reveals a highly dense coating comprising of randomly stacked lamellae. The paper also presents the tensile bond strength of the SHA coatings and, the phase composition and characteristics of the coatings generated with different particle size ranges (125-75 μm, 45-75 μm, 20-45 μm and 5-20 μm).