Fabrication And Investigation Of Coupling Biomimetic Hydrophobic Coating On Magnesium Alloys Surface

Magnesium alloys have been employed as increasingly as the requests of functionalsurface on the hostile protective environment in recent. For this reason, it is beingmore and more discussed widely about the fabrication and investigation on themagnesium ally surface with unique functions. And the wettability is very importantproperty of solid surface, which has the significance for improving performance onmagnesium ally surface, such as anti-stietion, friction reduction, self-cleaning,corrosion resistance and wear resistance. In our study, the hydrophobic,stable andprotective coatings were prepared on magnesium ally surface by electrodeposition,electroless nickel plating and chemical etching method. Then, characteristics of thehydrophobic coatings were tested and investigated, such as morphology analysis,wettability and wear resistance.Direct electroless Ni plating of magnesium alloy was obtained from nickel sulfateplating for the first time. The magnesium alloy samples were etched first in a solutionof chromate and nitric acid and than soaked in HF solution to form a conversion filmbefore electroless nickel deposition. The n-ZrO₂/Ni composite coatings were preparedby electro-deposition on AZ91D magnesium alloy surfaces which was plated by Ni-P.The microhardnesses of n-ZrO₂/Ni composite electro-deposition coatings on AZ91DMg alloy surface were harder than AZ91D Mg alloy matrix and pure Ni coating. Themicrohardness of coatings increased and then decreased with contents of n-ZrO₂inthe electrodeposition electrolyte increasing. And the results showed that the n-ZrO₂/Nicomposite coatings were more compact and smooth and its grains were smaller thanpure Ni coatings. The n-ZrO₂/Ni composite coatings had an evident passivation regionand showed excellent corrosion resistance. The biomimetic hydrophobic coatings on AZ91D magnesium alloy surface werefabricated by wet-chemical combining electroless. The sample was immersed intochemical etching solution in wet-chemical method firstly, and then the biomimetichydrophobic coatings were fabricated by electroless. In our study, it was elaborated todescribe the causes and affecting factors for biomimetic hydrophobic coatings. Due tothe effect of modifying surface characteristics and Ag deposition on AZ91Dmagnesium alloy surface, the postion and distribution of Nickel ions adsorption waschanged during deposition course of biomimetic hydrophobic coatings. X-raydiffraction spectrums showed that the XRD peaks become broader for and theamorphous structure is improved gradually. The results indicated that the biomimetichydrophobic coatings with uniform crystalline, dense structure could be obtained onAZ91D magnesium alloy. The microstructure was observed by SEM. The contactangles were measured by JGW360a contact angle tester. The results of contact anglevalues indicated that the biomimetic nano-composite coating was hydrophobicaccording to the high contact angle values（109°）.To fabricate hydrophobic modified layer on AZ91D magnesium alloy surface, it waschoosen that the chemical etching solution of the single concentration and thestepping wet-chemical method. Scanning Electron Microscopy （SEM） was used toexamine the changes of the surface morphology of magnesium after the wet-chemicalmethods. As the SEM images shown, the hollow nature of porous and dendriticcrystal structure was found on AZ91D magnesium alloy surface. Energy dispersiveX-ray Spectroseopy（EDS） was used to examine the element component of differentregion, it was obviously found that the element distribution difference inside themicroporous on the magnesium surface. It was shown that the Ag elementconcentrations to distribute in modified layer superficially, while Ag element was notfound in the microporous structure. The influence of the variety of experimentconditions on the wettability of the substrates was investigated by a contact anglegoniometry with water as a probe liquid. Comparing to the data of the experiment ofwettability, our results indicated that the hydrophobic modified layer was obtained in high-surface-energy materials by the stepping wet-chemical routes （the maximumcontact angle obtained is130.8°）. From the view of energy function, it was elaboratedto describe the causes and affecting factors for hydrophobic modified layer.