Mechanical integrity of hydroxyapatite coatings on titanium implants

Hydroxyapatite (I-IA) coated Ti alloy is a promising material for biomedical implants. The successful applications of such implants rely on fundamental understandings of mechanical behavior of the HA coatings, particularly, the fatigue resistance. In this work the mechanical stability of IFA coatings under cyclic loading was systematically investigated using four-point bending tests and the indentation tests in both air and simulated body fluid (SBF). Also, a shear cyclic test was developed for the coatings, considering the coatings of implants often failure under shearing loading in human body. Experiments revealed the conventional cyclic bending tests could not effectively evaluate the fatigue resistance of the HA coatings on Ti substrates. The contact fatigue under indentation might represent the worst service conditions of coatings. The cyclic indentation revealed that the fatigue damages in the HA coatings were severer in SBF than in air, and thin HA coatings exhibited less fatigue damages than the thick ones. The newly developed shear test for the coatings successfully generated similar shear loading as the conventional ones, however it is easier for alignment and for adapting coating thickness variation. Using the new method, the coating resistance to cyclic shear loading was evaluated with S-N curves. Also the tests revealed the existence of threshold stress which was required to generate fatigue damages in the coating.