Investigation into microstructural properties of fluorapatite Nd-YAG laser clad coatings with PVA and WG binders

Biomedical implants are generally coated with a thin layer of hydroxyapatite (HA) using a plasma spraying or pulsed laser deposition method. However, the bonding strength between the coating layer and the substrate is relatively low. Moreover, the high temperature of the deposition process causes a structural instability of the HA, and therefore degrades its bioactivity and reduces the service life of the coating layer following implantation. Accordingly, the present study replaces HA with fluorapatite (FA) as the coating material, and examines the effects of two different binders, namely polyvinyl alcohol (PVA) and water glass (WG), on the properties of FA coatings deposited on Ti–6Al–4V substrates using an Nd-YAG laser cladding process. Scanning electron microscopy (SEM) observations reveal that the FA coating has a coral- and dendrite-like structure when mixed with the PVA binder, but a pure dendrite-like structure when mixed with the WG binder. In addition, the Ca/P value of the WG-based FA coating is significantly higher than that of the PVA-based coating. Fluorapatite, Al2O3, CaTiO3 and Ca3(PO4)2 phases are observed in both coating layers. However, the XRD analysis results indicate that the microstructure of the WG sample is dominated by Ca3(PO4)2 phase while that of the PVA sample is dominated by FA. After soaking in SBF, it was observed that the specimen produced with PVA binder or less laser output power possessed a better apatite induction ability.

Research Highlights
► Coating fluorapatite (FA) on Ti–6Al–4V by Nd-YAG laser cladding technology.
► Polyvinyl alcohol (PVA) and water glass (WG) as coating binders.
► WG sample has a relatively higher Ca/P ratio than PVA sample.
► Coating products contain residual FA, CaTiO3, Al2O3 and Ca3(PO4)2 phases.
► PVA sample possessed a better apatite induction ability than WG sample in SBF.