Superior surface integrity by hybrid dry cutting-hydrostatic burnishing for controlled biofluid corrosion performance of novel biodegradable Magnesium-Calcium implants
Compared to Young's modulii (3~30 GPa) of bones, the higher modulii (100~200 GPa) of traditional permanent metallic implants cause stress shielding and result in artificial osteoporosis. To avoid the progress of this undesirable side effect, annually many second surgeries with all their social and economical consequences must be performed to remove the permanent implants after one or two years. Magnesium-Calcium (Mg-Ca) alloys are a promising alternative to tackle the aforementioned issues since they have modulus of elasticity (40 GPa) close to those of bones and they are biodegradable. However, these alloys corrode very fast and produce a large amount of dissolved Mg cations, a large volume of hydrogen, and a remarkable increase in local pH value which will cause significant imbalance in physiological reactions. To develop Mg-Ca alloys as a successful orthopedic material, corrosion rate of these alloys should be adjusted to match the healing rate of bone tissue and local absorption rate of corrosion by-products. In this context, hybrid cutting-burnishing process has been utilized to tailor surface integrity of the Mg-Ca implants in such a way which results in gaining control on the corrosion kinetics. A synergistic numerical-experimental investigation was conducted to study process mechanics in hybrid dry cutting-hydrostatic burnishing and to characterize the induced surface integrity on processed implants. A series of short-term and long-term in-vitro corrosion tests were performed to evaluate the effects of the hybrid technique on degradation kinetics and to find the likely correlations between process parameters, surface integrity characteristics, and bioperformance of the processed Mg-Ca0.8 implants. Hybrid dry cutting-hydrostatic burnishing technique was able to reduce the in-vitro degradation kinetics. The amount of this reduction was also adjustable depending on the selected process parameters and the subsequent induced surface integrity characteristics on the implants. These results suggest that it is feasible to tailor degradation kinetics of the Mg-Ca0.8 implants at the manufacturing stage so that the degradation rate matches healing rate of the bone trauma and absorption rate of the corrosion products under various physiological conditions. Magnitude of the compressive residual stresses and the depth of the compressed layer were key parameters affecting the bioperformance.