Introduction
Magnesium alloys are under intensive investigation because of their possible use in the biomedical field thanks to their good biocompatibility and mechanical properties which are similar to human bones and biodegradation. For instance, they have been used to produce a biodegradable bone fixator that does not require a second surgery. The main issue is the high corrosion rate in respect to tissue remodelling. The principal strategies to overcome this problem are: tailoring the alloy composition, inducing microstructural changes, employing surface treatments and coatings.
Several authors observed that the Layered Double Hydroxides (LDH) coatings improve the biocorrosion behaviour of the Mg-alloys and also permit drug delivery.
The aim of present work is to investigate how the microstructural changes induced by Equal Channel Angular Pressing (ECAP) affect the LDH films growth on the AZ31 surface.
Methods
The commercial AZ31 alloy was processed by 0, 1, 2 and 4 ECAP passes.
LDH structures were grown on AZ31 samples using the co-precipitation technique. The nutrient solution was: Zn(NO3)2*6H2O (5 mM) and urea (15 mM) in 150 mL of distilled water. After preliminary cleaning the samples were immersed in the nutrient solutions, and kept there for 12 h at 90 C. Finally, they were cooled in the nutrient solution, extracted from the reactor, rinsed in distilled water and ethanol and air-dried.
The microstructural characterisation of the samples was performed by LM, XRD, SEM, EBSD technique.
Results
The LDH film was successfully grown on the surface of all the samples. The LDH crystals mostly present a dendritical shape. It was observed that the LDH film was more uniform after 1 ECAP pass with an average crystal size of 200 nm.
Conclusions
The microstructural evolution induced by ECAP favours a more homogeneous LDH growth due to the best trade-off between texture and presence of nucleation sites (twins, dislocations, GB etc.).
