The poor mechanical properties of hydroxyapatite (HA) can be enhanced by forming a composite with a bioinert and mechanically strong metal alloy such as Ti-6A1-4V. Biomedical composites composed of titanium alloys and HA can offer concomitant bioactive properties as well as good mechanical strength and toughness. This paper describes an attempt to improve coating mechanical properties by forming a composite composed of HA and Ti-6A1-4V. Several compositions (20, 33, and 80 wt % HA) were prepared. Subsequent examination of the plasma-sprayed coatings revealed alternating HA-rich and titanium-rich lamella microstructures. The HA-rich regions appeared porous as a result of poor interparticle adhesion, with the 80 wt% HA coatings having the highest porosity. Mechanical property analysis showed the 20 wt% HA coating to have the highest storage modulus (∼60 GPa). This coating also had the highest bond strength (≥20 MPa max). The coatings tended to exhibit increased bond strength at thicknesses less than or equal to 60 μm. The excellent bond strength of the Ti-6A1-4V/HA composite is caused by the superior interfacial bond between the Ti-6Al-4V-rich splats and the substrate. The encouraging development of this composite raises the possibility of its use as a bond coat for plasma-sprayed HA on titanium-alloy implants.

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.

A series of Ti6Al4V/hydroxyapatite (HA) composite coating was produced by plasma spraying. Several compositions (20, 33 and 80 wt.% HA) were prepared. Subsequent examination of the coatings showed them to have a high level of porosity. However, some amount of porosity, within a specified size range, may be desirable in biomedical applications to enhance bony tissue ingrowth, although inter-lamella pores in the size range 10–300 nm (100–3000 Å) in the plasma sprayed coatings are detrimental to their mechanical properties, and these small pores should be reduced drastically in order for the coatings to have sound mechanical strength. Hot isostatic pressing (HIP) is applied in this study to reduce the amount of micropores in the plasma sprayed coatings. The influence of HIP temperature on the pore size distribution, microstructure and other physical properties of the composite coatings is investigated. Scanning electron microscopy revealed that the lamellae in the HIPped samples appeared ‘compressed’ because of the plastic deformation of the Ti6Al4V phase. A mercury intrusion porosimeter measured the pore size distribution of the HIPped samples, the results indicating that the majority of the micro-pores, most likely inter-lamella pores, are reduced drastically after HIP. A dynamic mechanical analyser is employed to measure the storage modulus of the composites by a 3-point bend fixture, the results showing that the storage modulus of the 20 and 33 wt.% HA coatings improved with HIP and that there is a corresponding increase with the HIP temperature employed. Other physical properties such as density and microhardness also improved with HIP. Overall, the results demonstrate that HIP can effectively enhance the mechanical properties of the Ti6Al4V/HA composite coatings. Tensile adhesive bond tests show the interface between the coating and the substrate to be improved. The mode of failure apparently transferred from adhesive failure in the as-sprayed coatings to a predominantly cohesive mode of failure in the HIPed samples and suggests that the influence of HIP is greater in the enhancement of the coating/substrate interface than in inter-lamellae strengthening.