High velocity suspension flame sprayed (HVSFS) potassium-based bioactive glass coatings with and without TiO2 bond coat

Titanium plates were coated by high-velocity suspension flame spraying (HVSFS) technique using a novel bioactive glass composition based on the K2O―CaO―P2O5―SiO2 composition (Bio-K). On half of the samples, an atmospheric plasma sprayed (APS) TiO2 bond coat was preliminarily deposited; suspensions of attrition-milled micron-sized glass powders, dispersed in a water + isopropanol mixture, were then sprayed onto both bare and bond-coated plates using five different process parameter sets. The microstructure of the coatings is independent of the presence of the bond coat but is strongly influenced by the deposition parameters. If the latter result in surface temperatures larger than the glass transition temperature of the Bio-K composition, large-scale viscous flow allows the expansion of the air entrained in the porosities, developing large rounded pores. When this phenomenon is avoided, denser layers are obtained. In tensile adhesion tests, porous layers fail cohesively at low loads, whereas adhesive/cohesive failure occurs in denser layers. In this latter case, the adhesion strength is significantly improved by the bond coat, reaching maximum values of 17 MPa. When immersed in simulated body fluid (SBF), the coating surface is rapidly converted into a silica gel because of ion leaching. A hydroxyapatite layer starts precipitating on top of it after 3 days and grows into a uniform film (of ≈ 10 μm thickness) after 2 weeks.