| Magnesium alloy is the lightest structural material with many superior performances and has wide application prospect.However,the poor corrosion resistance of magnesium alloy is a key factor that restricting its application.Improving the corrosion resistance of magnesium alloy is an important topic in the research and development of magnesium alloy and the coating is an important way to improve the corrosion resistance of magnesium alloy.Magnetron sputtering has been considered as one of the best method for the preparation of surface coatings due to its high deposition rate,uniform film thickness,high density and purity and strong adhesion,in various of coating preparation technology.In this work,high purity aluminum film was prepared on the surface of AZ91 magnesium alloy by magnetron sputtering technique for the good corrosion resistance of aluminum and the preparation process,microstructure and corrosion resistance of the film were studied.Firstly,using orthogonal test method,the corrosion resistance of the film and the interfacial adhesion force,film thickness and roughness were taken as the index,sputtering power,substrate temperature,negative bias voltage and other parameters were optimized.The order of parameters of affecting the experimental properties and the parameters combination of good comprehensive performance were obtained.Then single factor experiments were carried out on the main process parameters that affecting the performance.The effects of various parameters on the microstructure and properties of the coating were studied and the influence of the microstructure on the corrosion resistance was analyzed based on the corrosion mechanism of the coating.The results of orthogonal test show that the order of magnetron sputtering process parameters that affect coating performance is: sputtering power,substrate temperature,substrate negative bias voltage,Ar gas pressure and sputtering time.The optimum process parameters of magnetron sputtering are sputtering power 60 W,sputtering time 120 min,substrate temperature 200 ℃,Ar gas pressure 2.1Pa,substrate negative bias voltage-150 V.The observation of the microstructure of the aluminum film shows that the complete film can be roughly divided into the equiaxed crystal region on the bottom side,the transition zone of equiaxed crystal to columnar crystal,and the outer columnar region.Changing the process parameters overall thickness of the film and the size of each region will be changed.With the increase of sputtering power,the thickness of the film can be increased,the columnar crystal region will be enlarged,and the transition zone of equiaxed crystal to columnar crystal will be narrowed.When the substrate heating temperature is increased and the sputtering time is prolonged,the film structure will also be changed similarly.The process parameters also have a significant effect on the interfacial adhesion force.With the increase of the sputtering power and the heating temperature of the substrate and sputtering time,the interfacial adhesion force shows the trend of increases and then decreases.When the sputtering power is 60 W,the substrate heating temperature is 200 ℃,the sputtering time is 120 min,interfacial adhesion force is the strongest.The analysis of the corrosion process shows that the magnetron sputtering aluminum film can play a protective role on the magnesium alloy matrix.For the simulated sweat corrosion of aluminum film,corrosion occurs preferentially at the grain boundary of film surface and develops along the grain boundary to the inside of the film.Once the film local corrosion penetration,the corrosive medium will flow into the interface between substrate and aluminum film,then forming primary battery accelerated corrosion and leading to the peeling of the film from the substrate.Therefore,the film more thick,mainly equiaxed crystal region and transitional region and interfacial adhesion force stronger are the important conditions for the film with excellent corrosion performance. |
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