Surface biodegradation behavior of rare earth- containing magnesium alloys with different microstructure: the impact on apatite coating formation on the surface

Abstract

Magnesium alloys have significant potential as load-bearing biodegradable implants. However, controlling the biodegradation rate in the physiological environment is a challenge. In this study, we explore three magnesium alloys containing rare earth elements in terms of biodegradable behavior in simulated body fluid. The chemistry and processing of these alloys (Mg-2Zn-2Gd, Mg-3Sn-1Mn-0·6La, Mg-3Gd-1·7Y-0·5Zr) was designed to exhibit different strength based on grain size-strength relationship in addition to precipitation strengthening. Surface studies indicated the formation of rapid and stable growth of apatite layer on ultrafine grained Mg-2Zn-2Gd alloy as compared to relatively coarse-grained Mg-3Sn-1Mn-0·6La and Mg-3Gd-1·7Y-0·5Zr alloys. While, the pH change was initially higher for all the three alloys, but changed with time. Mg-2Zn-2Gd exhibited least degradation rate as compared to other two alloys. The differences in the biodegradation behavior of the alloys is attributed to the degree of stability of apatite layer on the surface, which was governed by grain size, rare-earth containing precipitates along the grain boundaries (including matrix) and surface energy.