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

Abstract

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, P _max, lower than the monotonic delamination load, P _Del, also generates delamination damage. A plot of P _max versus the number of cycles for delamination shows a two-slope curve with a “knee” for P _max close to the critical load to induce a radial crack from the interface, P _Rc. The analysis of the delamination morphology and the results for different load ratios, R = P _min/P _max, 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 P _max > P _Rc the process is mostly controlled by the presence of the radial cracks (P _max dependent), meanwhile, for P _max < P _Rc 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.