Graded coatings on ceramic substrates for biomedical applications

Bioactive glasses and particles reinforced composites were used to coat alumina substrates, in order to combine the mechanical properties of the high-strength alumina with the bioactivity of the coatings. The coatings were either monolithic glass or glass-matrix/zirconia particle composite and were prepared by a low-cost firing method. A multilayer approach was applied to minimize crack propagation at the interface between the coating and the substrate. Functionally graded structures were developed to achieve a compliant material to withstand the stresses due to the expansion coefficient mismatch between the substrate and the coatings. The sequential coating of the alumina with glass-matrix/zirconia particle composite layers produced a structurally stable composite structure. A systematic study revealed that multiple layers were necessary to provide a gradual compliance of the thermal expansion coefficient. The glass-matrix/zirconia particles composites layers were also essential for the control of the Al3+ diffusion from the substrate through the glass. This is in accordance with the experimental results of previous works. Thus, the alumina content in the coating should be maintained as low as possible in order to preserve its bioactivity. The composite layers were further coated by a glass belonging to the system SiO2–CaO–P2O5–Na2O–MgO–F−, known for its bioactivity. The experimental results were substantiated by optical and scanning electron microscopy (SEM) with compositional analysis (EDS) and by a mechanical characterization. The in vitro behavior of the coated samples was investigated by means of soaking in simulated body fluid (SBF) followed by SEM observation and XRD analysis.