| Due to many excellent properties,such as low density and high strength,making magnesium（Mg）alloys widely to use in aviation,automotive,medical,electronics,interior design and decoration,sports equipment and other fields.However,the high electronegativity and low surface hardness of Mg alloys make it prone to severe galvanic corrosion and friction wear.As we all know,brightly colored products can meet peoples visual needs for color.Therefore,Mg alloys products with colored surface effects will have more core competitiveness.Therefore,the devices manufactured by Mg alloys materials should not only enhance corrosion resistance and wear resistance,but also have the function of color adjustment.Surface coating technology has been widely used to enhance the corrosion resistance and surface hardness of Mg alloys.However,the current surface coatings used to enhance the corrosion resistance and surface hardness of Mg alloys cannot give it color.Therefore,there is an urgent need to develop and research the coatings with triple functions of anti-corrosion,hardening and color rendering on Mg alloys surface.Hard coating can effectively isolate the substrate from the corrosive medium,enhance the surface hardness of the substrate and improve its corrosion resistance.Structural colors can be produced by visible light and micro-nano materials,such as nano-coatings,through visible light interference.Therefore,in this thesis,combining the strategy of film interference structure color and hard coating hardening and anticorrosion,an Al/Sibased multilayer coatings with AlN as color top layer,Al/Sibased hard middle layer and Alas metal bottom layer was designed on the surface of Mg alloy.Therefore,this thesis firstly uses the magnetron sputtering technology to prepare the full-range color AlN film,studies the full-color mechanism of the AlN film.Then,taking the AZ31B magnesium alloy as the research object,the color-tunable AlN/Si/Alcoatings was prepared through the interlayer design of the coating,the mechanism of color rendering and control was explored.Next,the SiN/AlSiN/AlN intermediate layer was embedded at the Si/Alinterface,the mechanical properties of the coating were improved through the internal optimization of the coating,and the mechanism of hardening was investigated.Finally,on the basis of the AlN/Si/SiN/AlSiN/AlN/Alcoating,and the anti-corrosion performance of the coating is enhanced through the internal structure optimization and interface strengthening of coating,the mechanism of anti-corrosion of coatings was investigated.The details and results are as follows:1.First,the magnetron sputtering was used to deposit AlN coating on silicon wafer by sputtering an Altarget in Ar/N2 atmosphere.The surface of AlN coating is smooth and grows stably.By controlling the deposition time to adjust the coating thickness,AlN coatings with full-color range were obtained.By investigating the relationship between coating thickness and color,it is found that when the coating thickness gradually increases to 165nm,the coating will show changes in violet,indigo,blue,green,yellow,orange and red.When the thickness continues to increase to 281nm,these colors will repeat in the same order due to the integral change of the optical path difference.When the thickness of the coating exceeds 467 nm,the colors of the coating overlap,resulting in a color mixing effect.2.Then,the AlN/Alcoating was deposited on the Mg alloy according to the full color parameters of the AlN coating,and it was found that the AlN/Alcoating could not exhibit structural color.Therefore,the AlN/Si/Almultilayer coating was prepared by embedding the Silayer at the AlN/Alinterface,and found that the AlN/Si/Almultilayer coating could exhibit structural color.When the thickness of the AlN top layer was continuously increased from 165 to 290 nm,the AlN/Si/Alcoatings exhibited violet,indigo,blue,green,yellow,orange and red,respectively.However,the embedding of the Silayer makes a huge difference in hardness between the Si/Allayers,and the hardening of the coating fails.3.Next,a SiN/SiAlN/AlN intermediate layers was embedded at the Si/Alinterface of the AlN/Si/Alcoating to synthesis the AlN/Si/SiN/AlSiN/AlN/Alnanocomposite gradient multilayer coating.It is found that the embedding of the SiN/SiAlN/AlN layers can significantly improve the hardness of the coating,The hardness of the coating is increased from 9.5 to 20.6 GPa,the hardness is increased by17 times compared to the AZ31B.This is due to the hardness of the SiN/AlSiN/AlN layers is between that of the Siand Allayer,alleviating effectively the hardness difference between the Si/Allayers,leading to the formation of a hardness gradient structure inside the AlN/Si/SiN/AlSiN/AlN/Almultilayer coating.The improved mechanical properties can be attributed to the coordinated mechanism of the internal hardness gradient effect and multi-interface effect of the coating.Meanwhile,the AlN/Si/SiN/AlSiN/AlN/Almultilayer coating can still present the full range of structural colors via controlling the thickness of the AlN top layer.In addition,the anti-corrosion performance of the AlN/Si/SiN/AlSiN/AlN/Almultilayer coating has also been improved,but the coarse and continuous columnar defects between the SiAlN/AlN layers lead to the anti-corrosion failure of the coating.4.So,an AlSialloy target（AT）was used instead Aland Sielemental target（ST）to deposit AlSiN layer,it was found that the continuous columnar defects between SiAlN（ST）/AlN layers were effectively suppressed by changing the growth mode of the AlSiN layer.As the continuous growth between layers is suppressed,the continuous columnar structure inside the coating disappears,the AlSiN（AT）layer is more dense,the AlN grains are effectively refined,and the large particles on the coating surface are reduced.The improvement of surface and continuous columnar defects increases the penetration resistance of corrosive medium,the corrosion resistance of the coating is improved,and the corrosion current density(Icorr)of the coating/substrate decreases from 3.03×10-8 to 6.53×10-9A/cm2.Although the AlSiN（AT）layer inhibits the continuous growth with the AlN bottom layer,the coarse columnar crystal defects of the AlN bottom layer itself still exist,and the coating eventually fails.5.Then,the AlN bottom layer was deposited by using the RF power supply instead of the DC power supply,and found that the coarse columnar crystal defects of the AlN bottom layer could be effectively suppressed by changing the deposition method of the AlN（RF）layer.The AlN（RF）layer is a composite growth structure with an average grain size of 6 nm,so the growth of the AlN（RF）bottom layer is not affected by the Allayer.Due to the continuous growth inhibition between the AlN（RF）/Alinterlayer and the ultrafine nanocrystalline refinement effect of the AlN（RF）layer,the columnar structure of the AlN bottom layer is changed to a dense non-columnar structure,and the surface roughness of the coating decreases.The corrosion resistance of the coating was improved,and the Icorr of the coating/substrate decreased from 6.53×10-9 to 2.23×10-9 A/cm2.6.Finally,we propose an efficient and simple interface treatment method for multi-layer coatings,which can completely suppress the continuous growth defects between AlSiN/AlN/Allayers,that is,after the Allayer is deposited,the surface pre-oxidation treatment is performed,and then the AlN bottom layer is deposited,and so on,the method is named as interlayer interface pre-oxidation,and the/symbol indicates the interface pre-oxidation.Found that after the interlayer interface pre-oxidation treatment,the anti-corrosion performance of the AlN/Si/SiN/AlSiN/AlN/Alcoating is greatly improved.And the continuous columnar structure inside the coating disappears,the grains are continuously refined,the vertical growth disappears,the large particles on the surface are reduced,and the interface is more obvious,that is,the surface,internal and interface defects of the coating are improved at the same time,so the anti-corrosion performance of coating significantly improved.The Icorr is reduced from 3.03×10-8 to 4.75×10-10 A/cm2,which is nearly 6 orders of magnitude lower than that of the AZ31B.At the same time,the internal grain refinement of AlN increases the hardness of the coating from 20.6 to 23.0 GPa.In addition,the interfacial pre-oxidation did not affect the structural coloring function of the AlN/Sisurface/interface system,and the AlN/Si/SiN/AlSiN/AlN/Almultilayer coating could still exhibit a full range of structural colors. |
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