microstructural and in‑vitro characteristics of functional calcium silicate topcoat on hydroxyapatite coating for bio‑implant applications

The delayed tissue–implant interactions in metallic implants coated with hydroxyapatite (ha) paved the way for the develop- ment of alternative bioactive coatings. in this study, bi-layered functional gradient (ha-cs) coating was formulated by the atmospheric plasma spray (aps) process on ti6al4v alloy. the ha layer was applied at the metal interface to ensure long- term stability, while the calcium silicate (cs) outer layer was applied to achieve fast tissue–implant interactions. moreover, single-layered ha and cs coating were also formulated for comparative analysis. the phase compositions, coating micro- structure, chemical properties, microhardness, porosity, surface roughness, and in-vitro bioactivity were investigated. the cs top layer showed high porosity and surface roughness with respect to the inner ha layer, which constitutes an optimum microstructure to promote bioactivity. the microhardness of the outer cs layer of ha-cs was 520.3 ± 80.8 hv, while the corresponding value for the inner ha layer was 291.7 ± 45.7 hv. ha-cs and cs coatings demonstrated higher in-vitro bioactivity compared to ha coating. on the contrary, ha coating (3.76 mpy) displayed better corrosion resistance than the ha-cs (4.17 mpy) and cs coatings (4.34 mpy). the in-vitro results indicated that the ha-cs coating could promote the healthy development of osteoblast-like mg-63.