# Bio-Corrosion Behavior of Ceramic Coatings Containing Hydroxyapatite on Mg-Zn-Ca Magnesium Alloy
## 3 Results and Discussion  
### Microstructure, Thickness, and Micro-Hardness of the Coatings  
Figure 1 shows the surface morphologies of the coatings formed at various concentrations of HA. One can see that the coatings are porous with a large number of lamellar, spherical pieces, presenting a  
typical feature of MAO coating. It should be noted that HA additive may considerably influence the surface morphologies. Figure 1b,c display the morphology of the coating added with 0.2 and 0.4 g/L HA in the electrolyte, respectively. Interestingly, samples prepared in the electrolyte with HA exhibited fewer cracks, smoother surface, and smaller pore size compared with samples without additional HA, as shown in Figure 1a.  
The electrical conductivity of the HA-containing complex electrolyte slightly decreased after the addition of a small amount of insulating HA particles. The voltage distribution on the specimen became somewhat low. Thus, instantaneous breakdown energy was small, and the discharge channel was narrow. Furthermore, the HA particles may enter into the discharge channel with the HA-suspended complex electrolyte, which raises the dielectric breakdown potential of the growing oxide film, resulting in the formation of smooth surface morphology.  
As HA concentrations were increased to 0.6 g/L, on the one hand, the original uniform pore-structure gradually disappeared and was accompanied by the appearance of few large holes; on the other hand, some deep penetrating cracks were generated, and a small amount of white, sintered granules adhered to the surface of the coatings, as evidenced in Figure 1d. Thus, the surface quality of the coating decreased as HA concentrations were increased. This observation may be due to the large number of insulating HA particles that may hinder the movement of conductive ions in the electrolyte, resulting in the decrease in both the electrochemical reaction rate and migration of charged particles in the solution. Moreover, in the HA-suspended complex electrolyte, extensive heat that was generated during the electrical breakdown process cannot be easily spread, decreasing the cooling capacity of molten material.  
The XRD pattern of the HA-containing coatings is shown in Figure 2. The coating consists mainly of MgO, MgSiO\({}_{3}\), and a small amount of Mg\({}_{3}\)(PO\({}_{4}\))\({}_{2}\) and HA. HA cannot be easily identified from the full spectrum because of its small amount. In the inset in Figure 2, some characteristic diffraction peaks of HA are presented, indicating that HA was incorporated into the coatings.  
Figure 1: Surface morphologies of coatings formed at hydroxyapatite (HA) concentrations of (**a**) 0; (**b**) 0.2; (**c**) 0.4; and (**d**) 0.6 g/L.  
The average micro-hardness of the HA-containing coatings (0.4 g/L, hereinafter) was approximately 215.8 \(\pm\) 5.3 HV, slightly higher than that of the coatings without HA additive (hereinafter referred as MAO coating) which was about 194.7 \(\pm\) 4.8 HV. The hardness of the HA-containing coatings might be enhanced by some dispersed HA particles as the second phase within the coatings. The thickness of the HA-containing coatings was 13.5 \(\pm\) 0.2 \(\upmu\)m; as for the MAO coatings, the thickness was about 12.9 \(\pm\) 0.2 \(\upmu\)m. There was no obvious variation in the thickness which might be interpreted as the same electric parameters during the MAO process.