Delamination under Hertzian cyclic loading of a glass coating on Ti6al4v for implants

In this work, the interfacial response of a glass-based coating on Ti6Al4V to monotonic and cyclic Hertzian (spherical) indentation is investigated. This coating belongs to the SiO2-CaO-MgO-Na2O-K2O-P2O5 system and it is specifically designed to be used as the inner layer of a bioactive bilayer coating with an outer layer of lower SiO2 content to ensure bioactivity. During Hertzian monotonic loading, delamination of the coating occurs, which is revealed in the microscope by the presence of a colour pattern of interference fringes at the interface. Hertzian cyclic loading at maximum loads, Pmax, lower than the monotonic delamination load, PDel, also generates delamination damage. A plot of Pmax versus the number of cycles for delamination shows a two-slope curve with a “knee” for Pmax close to the critical load to induce a radial crack from the interface, PRc. The analysis of the delamination morphology and the results for different load ratios, R = Pmin/Pmax, confirmed the existence of two different delamination mechanisms with a common feature of plastic deformation of the substrate but with a different dependence with the maximum applied load: for Pmax > PRc the process is mostly controlled by the presence of the radial cracks (Pmax dependent), meanwhile, for Pmax < PRc radial cracks are not observed and delamination is attributed to the residual stress at the interface induced by the cyclic plastic deformation of the substrate and the elastic recovery of the coating during unloading part.