Thermal spraying of Ti6Al4V/hydroxyapatite composites coatings: powder processing and post-spray treatment

Abstract

Hydroxyapatite (HA) is known for its attractive bioactive properties. Thermal spray techniques (plasma spray and high velocity oxy-fuel) are employed to deposit HA on titanium implants because of their high thermal efficiency and relative economy. However, some of the bioactive properties of HA are lost during thermal spraying. Generally, HA has poor mechanical properties. Titanium is a light metal that has been applied to biomedical engineering because of its non-toxicity and low density. A composite that can elicit the combined bioactive property of HA and the mechanical properties of Ti6Al4V to provide an implant that is both biocompatible and mechanically strong would certainly be desirable. Thermal spray techniques are employed in the present study to process Ti6Al4V/HA composite coatings. The Ti6Al4V/HA coatings can be sprayed onto existing implants to improve post-operation healing. This paper reports the thermal spraying of Ti6Al4V/HA composite coatings using powder feedstock prepared by two powder processing techniques: (i) mechanical alloying and (ii) the ceramic slurry mixing method. The effects of post-spray treatment by hot isostatic pressing (HIP) on the microstructure and other physical properties are investigated also. The surface morphology and microstructure of the as-sprayed coatings and HIPped coatings are examined by scanning electron microscopy. The investigation shows that the as-sprayed coating microstructure is comprised, predominantly, of HA lamella sandwiched between the Ti6Al4V lamellae. The coatings, in particular the HA-rich regions, suffer from high porosity levels. A mercury intrusion porosimeter is used to study the pore-size distribution of the as-sprayed and HIPped samples, the results indicating that the majority of the micropores are drastically reduced. The improvement in the physical properties of the composite was attributed to this reduction. Overall, the results showed that HIP can effectively enhance the mechanical properties of the as-sprayed coatings and improve the porosity levels.