## 5 Surface Modification Used for Mg Alloys
Zinc phosphate conversion into Zn(OH)  
Figure 5: Biomimetic deposition of CaP on the surface of a poly(dopamine) (PDA) film in addition to subsequent graphene oxide coating [91]. Figure 6: (**A**) MAO device, (**B**) growth model of MAO coating, (**C**) thin coating with through pores, (**D**) thicker coating with complex pores. restrained infiltration corrosion, fostering a favorable microenvironment for cell activities. The zinc phosphate-doped MAO coating also enhanced wear resistance, making it a promising choice for Mg-based bone fixation implants. * Sol-gel coating  
The sol-gel process, also known as chemical solution deposition, has been used widely in material science and ceramic engineering. This technique initially uses a chemical solution as a precursor to produce an integrated network of discrete particles or network polymers [100; 101]. Generally, the sol-gel process involves four steps: (1) hydrolysis, (2) condensation and polymerization of monomers for the formation of chains and particles, (3) growth of particles, and (4) accumulation of polymer structures followed by a continuous network formation in a liquid medium which increases viscosity for gel formation [102; 103]. The schematics of preparation for sol-gel coatings are shown in Figure 7. The technique has the benefits of low cost, low processing temperature, and the ability to coat different materials with complex shapes [99; 104]. While adding bioactive silica-glasses (58S and 68S) on AZ91D alloy through the sol-gel technique, [PERSON] et al. [105] discovered that, after 17 days of immersion, 58S provided superior corrosion protection. Based on cell attachment and proliferation tests, AZ91D alloy with either 58S or 68S coatings is deemed suitable for use as temporary implant material. * Ion implantation: Ion implantation is a surface modification technique in which target materials are converted to an ion beam in a vacuum, which is later sputtered onto a modified material. Finally, a layer with a specific composition and structure is formed on the surface of the substrate [99; 106]. Implanting specific ions onto the Mg substrate can increase the corrosion rate and improve mechanical performance and biocompatibility. For the biodegradable material (Mg alloy), metal ions from iron, cerium, zinc, zirconium, and strontium, and non-metallic ions, for instance, carbon, oxygen, and nitrogen are currently used for ion implantation [86; 89; 107; 108; 109; 110; 111; 102]. The advantages of this technique include  
Figure 7: Sol-gel coating process. For Mg alloy implants, the object would be subjected to dip coating or electro-deposition. convenience, controllability, and flexibility. In addition to improving physical and chemical properties, ion implantation improves alloys' biological and antibacterial abilities [24]. [PERSON] et al. [111] used titanium ion implantation to change the surface of pure magnesium and create a self-layered hydrothermal film. Results showed that the inner layer of the film provided good corrosion protection, while the outer layer improved the cell adhesion and cytocompatibility properties. Surface microstructural modification:  
This technique involves the deformation of the metal surface through mechanical processing so that the surface acquires a microstructure and mechanical performance different from the bulk matrix material. Mechanical processing increases the mechanical characteristics, surface hardness, and corrosion resistance of Mg alloys by processes that lead to grain refinement and changes in the distribution of second phases or intermetallic compounds [88; 94]. The process usually does not involve chemical reactions. Below are the details of microstructural modification techniques used for Mg alloys. Surface mechanical grinding treatment:  
Surface mechanical grinding treatment, also known as surface mechanical attrition treatment (SMAT), is a surface nano-crystallization technique that refines grains in the nanoscale and forms gradient nanostructures without changing material composition.