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

Biomedical implants using metals, polymers, ceramics and composites are inc reasingly being designed to restore body function and to facilitate healing . It is often the material design aspects of such implants that will determ ine the stability of the device. A hydroxyapatite (HA) coating on Ti-6Al-4V alloy is a common implant used in orthopaedics applications. IIA being a b io-active material combined with non-toxicity and bio-inert Ti-6Al-4V provi des an implant that is bio-compatible, light and stable. The plasma spray t echnique is a popular method of depositing HA onto Ti-6Al-4V. However, ther e are processing problems associated with this technique and as a result th ere are inconsistencies in the subsequent in vitro and in vivo investigatio ns. Addressing these inconsistencies from a materials processing standpoint would help to alleviate the problems faced. This paper investigates the in fluence of plasma spraying parameters on the characteristics of the composi te coatings. The plasma spraying parameters under study are the spraying cu rrent and the gun transverse speed. Under these spraying parameters, a comp arison is made between two different kinds of composite feedstock, namely c alcined HA and Ti-6Al-3V in the ratio of 80:20 wt% and 50:50 wt%, respectiv ely. Composite powders in the size range of 75-106 mu m were prepared by th e ceramic slurry mixing method. The powder was deposited onto Ti-6Al-4V sub strate by means of a robot-controlled 100 kW plasma gun (Praxair Surface Te chnologies, 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 m icrostructure of the composite layers were examined by a back-scattered sca nning electron microscope. The phase composition and crystallinity were det ermined by an X-ray diffractometer. The investigation shows that the as-spr ayed coating of the individual layers possesses a higher porosity level in the HA-rich regions. In other words, a porous coating is achieved using com posite powders in the ratio of 80:20 wt%. The tensile adhesion test reveale d a higher bond strength for the composite coatings compared to that for pu re HA coatings. A lower spraying current and a faster gun transverse speed are also considered accountable for the porous microstructure and low bondi ng strength, (C) 1999 Elsevier Science S.A. All rights reserved.