Influence of processing parameters in the plasma spraying of hydroxyapatite/Ti–6Al–4V composite coatings

Abstract

Biomedical implants using metals, polymers, ceramics and composites are increasingly being designed to restore body function and to facilitate healing. It is often the material design aspects of such implants that will determine the stability of the device. A hydroxyapatite (HA) coating on Ti–6Al–4V alloy is a common implant used in orthopaedics applications. HA being a bio-active material combined with non-toxicity and bio-inert Ti–6Al–4V provides an implant that is bio-compatible, light and stable. The plasma spray technique is a popular method of depositing HA onto Ti–6Al–4V. However, there are processing problems associated with this technique and as a result there are inconsistencies in the subsequent in vitro and in vivo investigations. Addressing these inconsistencies from a materials processing standpoint would help to alleviate the problems faced. This paper investigates the influence of plasma spraying parameters on the characteristics of the composite coatings. The plasma spraying parameters under study are the spraying current and the gun transverse speed. Under these spraying parameters, a comparison is made between two different kinds of composite feedstock, namely calcined HA and Ti–6Al–4V in the ratio of 80:20 wt% and 50:50 wt%, respectively. Composite powders in the size range of 75–106 μm were prepared by the ceramic slurry mixing method. The powder was deposited onto Ti–6Al–4V substrate by means of a robot-controlled 100 kW plasma gun (Praxair Surface Technologies, USA) equipped with an advanced computerized closed-loop powder feed system. The mechanical properties of the individual layers were tested using the tensile adhesion test (ASTM C-633). The surface morphology and microstructure of the composite layers were examined by a back-scattered scanning electron microscope. The phase composition and crystallinity were determined by an X-ray diffractometer. The investigation shows that the as-sprayed coating of the individual layers possesses a higher porosity level in the HA-rich regions. In other words, a porous coating is achieved using composite powders in the ratio of 80:20 wt%. The tensile adhesion test revealed a higher bond strength for the composite coatings compared to that for pure HA coatings. A lower spraying current and a faster gun transverse speed are also considered accountable for the porous microstructure and low bonding strength.