Magnesium is the lightest metal among all structural metals(35% lighter than aluminium) and has a good strength-to-weight ratio, vibration resistance and EMR shield, etc.. Moreover, it is plentiful element, consisting 2.7% of earths crust and 0.13% of major source in ocean. Accordingly, it forms the basis for commercial alloys that have found to be used in a wide variety of application such as automobile, aircraft components, note book computer case, cellular phone case etc..
However, magnesium has not been applied as much as aluminum because of its insufficient corrosion resistance in neutral and acid environment. Therefore, in general, magnesium is used for coating techniques by wet process such as chromate surface treatment, and anodizing etc.. But these coating techniques entail durable and environmental problems. Moreover, it raises recycling problems which are related to the mixing of impurities.
In general, light metals, particularly magnesium, are difficult to plate using conventional coating techniques such as chemical or electro-chemical processes. This is due to the presence of the easy formation of oxide layer. In order to limit oxidization during coating, vacuum deposition techniques can be used as an alternative to conventional techniques operating in wet or in air conditions. It is well known that coated films, particularly those deposited from plasma-assisted vacuum coating technique, are usually quite different from the respective bulk material as to their structures and properties. For this reason, the use of plasma-assisted techniques, e.g., physical vapor deposition such as ion-plating method, has spreaded into various types of industrial applications. However, few studies have been reported dealing with magnesium metal and using the new techniques.
In this work, magnesium thin films were prepared on magnesium alloy(AZ91D and AZ31) substrate by environmental friendly coating technique of thermo-electron activated ion-plating method. The influence of gas pressure and substrate bias voltages on the crystal orientation and morphology of the films was determined by using X-ray diffraction and field emission scanning electron microscopy(FE-SEM), respectively. And the effect of crystal orientation and morphology of the magnesium thin films on corrosion behavior was estimated by measuring electro-chemical anodic polarization curves in deaerated 3% NaCl solution. Besides, hardness of these films were measured by knoop micro-hardness tester.
From the experimental results, all the deposited magnesium films showed obviously good corrosion resistance to compare with 99.99% Mg-ingot for evaporation metal and AZ91D for substrate. And magnesium film of morphology changed from columnar to granular structure with an increase of gas pressure. And the diffraction peaks of magnesium film became less sharp and broadened with the increase of gas pressure. The morphology of the films depended not only on gas pressure but also on bias voltage, i.e., the effect of increasing bias voltage was similar to that of decreasing gas pressure. The influences of gas pressures and bias voltages can be explained by applying the effects of adsorption and occlusion and argon gas. Finally, it was shown that the properties of magnesium films can be improved greatly by controlling the crystal orientation and morphology with effective use of the plasma ion plating technique.