Nanoindentation and nano-scratching of hydroxyapatite coatings for resorbable magnesium alloy bone implant applications

The corrosion rate of Mg alloys is currently too high for viable resorbable implant applications. One possible solution is to coat the alloy with a hydroxyapatite (HA) layer to slow the corrosion and promote bone growth. As such coatings can be under severe stresses during implant insertion, we present a nano-mechanical and nano-tribological investigation of RF-sputtered HA films on AZ31 Mg alloy substrates. EDX and XRD analysis indicate that as-deposited coatings are amorphous and Ca-deficient whereas rapid thermal annealing results in c-axis orientation and near-stoichiometric composition. Analysis of the nanoindentation data using a thin film model shows that annealing increases the coating's intrinsic hardness (H) and strain at break (H/E) values, from 2.7 GPa to 9.4 GPa and from 0.043 to 0.079, respectively. In addition, despite being rougher, the annealed samples display better wear resistance; a sign that the rapid thermal annealing does not compromise their interfacial strength and that these systems have potential for resorbable bone implant applications.