Grain structure dependent self-assembled bioactive coating on Mg-2Zn-2Gd alloy: Mechanism of degradation at biointerfaces

Metallic biomedical devices with ultrafine-sized grains (UFGs) provide surfaces that are different from their coarse-grained (CG) (tens of micrometer) counterpart in terms of increased fraction of grain boundaries (UFG > 50%; CG <2-3%). A novel concept of severe plastic deformation involving multiaxial forging and annealing was used to obtain a wide range of grain structures, starting from the UFG regime to the CG regime, to elucidate that the grain structure significantly impacts bioactivity and degradation behavior at biointerfaces. Experiments on the interplay between grain structure from the UFG regime to CG regime and bioactivity in a magnesium alloy indicated that the fundamental mechanism associated with the biodegradation process was strongly governed by the grain structure such that the ultrafine-grained alloy with dendritic growth of apatite exhibited slowest degradation in comparison to the coarse-grained counterpart. The differences observed in the biodegradation behavior with respect to grain structure are attributed to differences in surface energy that led to the formation of a stable surface oxide and apatite layer with dendritic morphology, which suppressed surface degradation at longer times in the UFG alloy. The degradation rate expressed in terms of corrosion rate was observed to decrease from 10.74 mm/year to 3.77 mm/year, with decrease in grain size from similar to 44 mu m to similar to 0.7 mu m. The study underscores and lays the foundation of a new branch of ultrafine grained magnesium alloy for biomedical devices. (C) 2017 Elsevier B.V. All rights reserved.