Bioactivity and mechanical properties of bioactive glass coatings fabricated by flame spraying

In this study, bioactive glass powders were synthesized from four different types of oxides (SiO2, P2O5, CaO and MgO). These oxides were mixed, melted and milled to produce two materials of 31SiO2-11P2O5-(58-x)-CaO-xMgO, where the values x = 0 and x = 2 describe powders P0 and P2, respectively. The powders were sieved to obtain a particle size distribution of 8.8-66.2 μm for P0 and 5.1-61.6 μm for P2. The powders were flame sprayed onto AISI 316L stainless steel and Ti6Al4V alloy substrates using a modified Terodyn 2000 (Eutetic-Castolín) gun. The flame-forming gas was a mixture of O2-C2H2 (38/22 L/min). The microstructure of the powders and the obtained coatings was examined by SEM. X-ray diffraction (XRD) was used to identify the crystalline phases in the powders and coatings. The hardness and fracture toughness of the coatings were determined by Vickers micro-indentation tests. According to the feedstock powder, the substrate material and thermal spraying parameters, the microhardness of the coatings was between 4.1 ± 0.5 and 5.2 ± 0.6 GPa, and their fracture toughness was between 4.6 ± 0.9 and 6.6 ± 1.1 MPa·m1/2. The bioactivity of coatings was evaluated from hydroxyapatite layer produced in their surface by exposure with simulated body fluid (SBF) during 1, 9 and 15 days. The hydroxyapatite layer was characterized by X-ray diffraction and SEM-EDS.