HVOF spraying of nanostructured hydroxyapatite for biomedical applications

Abstract

A nanostructured hydroxyapatite (HA) feedstock was thermally sprayed on Ti–6Al–4V substrates via high velocity oxy-fuel (HVOF process). The particle temperature and velocity for the HA particles in the HVOF jet were monitored during spraying and found to be 1826 ± 346 °C and 638 ± 82 m/s, respectively. The bioactivity of the coating was investigated in a simulated physiological environment, as an attempt to simulate the real incubation condition of an implant in the human body, via immersing the coating in a simulated body fluid (SBF) for 7 days. The phase content and crystallinity of the coating was evaluated using X-ray diffraction (XRD). Field emission scanning electron microscopy indicated the presence of three types of nanostructured zones in the HA coating. The results show that the coating is highly crystalline and exhibits no secondary phases. After 7 days of incubation, a uniform layer of apatite (∼35 μm) was formed over the HA coating surface, which was thicker than that found in early work on conventional air plasma sprayed HA coatings [R.S. Lima, B.R. Marple, K.A. Khor, H. Li, P. Cheang, Mechanical properties, microstructural characteristics and in vitro behavior of APS-sprayed nanostructured and conventional hydroxyapatite coatings, PDF file in the CD Proceedings of the International Thermal Spray Conference 2004, DVS-Verlag GmbH, Dusseldorf, Germany]. The HVOF-sprayed HA coating was dense and uniform, even surpassing the crystallinity levels and bond strength requirements of the standard ISO 13779-2 for HA coatings.