Control of magnesium in vitro degradation based on ultrafine-grained surface gradient structure using ultrasonic nanocrystalline surface modification

Here, a gradient structure based on an ultrafine-grained Mg surface was prepared using ultrasonic nanocrystalline surface modification (UNSM). UNSM is expected to be commercially attractive because it enhances mechanical properties and does not lead to surface roughening, which promotes corrosion. The modified surface increases corrosion resistance by accelerating the development of compact corrosion products that provide protection. The ultrafine-grained (UFG) surface gradient structure improved corrosion resistance and maintaining strength, which increased as the grain size became smaller. In particular, the UFG structure generated by the UNSM process exhibits a dramatic decrease in corrosion rate compared to the fine-grained structure, resulting in the slope change in the plot of corrosion rate versus grain size in the UFG regime. The surface gradient structure is an optimized approach for early-stage fast bone healing and later-stage fast degradation considering the relationship between grain size, pH, and strength change.