Post-spray hot isostatic pressing of plasma sprayed Ti-6A1-4V/hydroxyapatite composite coatings

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.