Monotonic and cyclic Hertzian fracture of a glass coating on titanium-based implants

Monotonic and cyclic loading fracture by Hertzian (spherical) indentation of a glass-based coating on a Ti6Al4V alloy has been investigated. This coating is designed to be used as the inner layer of a bioactive bilayer coating in order to improve the fixation to the hosting bone. The glass used is of the SiO2–CaO–MgO–Na2O–K2O–P2O5 system and it was obtained by an enamelling technique. In particular, this work focuses on the damage induced under Hertzian monotonic and cyclic contact loading in the coating as well as in sintered and fused monolithic glasses with the same chemical composition as the coating. Hertzian contact tests under cyclic loading were performed using as a damage criterion the first appearance of brittle damage (ring crack). A comparison between the damage in the coating under cyclic contact loading and under static loading in air clearly shows a larger degradation under cyclic loading conditions. This is discussed in terms of the existence of a damaged zone ahead of the crack tip due to microcracking between the sintered glass particles which is degraded under cyclic loading. It is also shown that under cyclic loading the time to complete a ring crack is strongly dependent on the peak load per cycle, but it has a very weak dependence on the load ratio. The results show for the first time the occurrence of cyclic fatigue in sintered glass, in contrast to fused glass where cyclic fatigue does not usually occur.