Mg 합금소재에 이온플레이팅한 Zn 박막의 전기화학적 특성

Abstract

Recently, mechanical materials are requested to have the properties of higher hardness, little wear, strong corrosion resistance, excellent quality and decoration with gradual development of industries.

In order to possess these properties, many kinds of material surface modification methods have been introduced and applied to the surface of bulk materials.

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. Moreover, it is plentiful element, consisting 2.7% of earth crust and 0.13% of major source in ocean. However, magnesium has not been applied as much as aluminum because of its poor corrosion resistivity in most of all environments. Recent tendency in electronics and mechanical engineering fields, aiming for high performance and energy savings, requires the material to be light weight. Furthermore, new materials usually mean new bulk materials in the traditional materials development. If a new surface coating technique can be developed to improve its corrosion resistivity, magnesium will be a new candidate in various fields applications.

In general, light metals, including magnesium, are difficult to plate using conventional coating techniques such as chemical or electrochemical processes. This is due to the presence of a easily-formed 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 dry conditions. It is well known that coated films, particularly those deposited by plasma-assisted vacuum coating technique, are usually quite different from the bulk material in their structures and properties. For this reason, the plasma-assisted techinques, physical vapor deposition such as sputtering method, has spreaded in various industrial applications. However, few studies have been reported dealing with magnesuim with new techniques.

In this work, Zn thin films were prepared on the magnesium alloy(AZ91D) substrate by environmental friendly coating technique, i.e. 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 investigated by X-ray diffraction and field emission scanning electron microscopy(SEM). In addition the effect of crystal orientation and morphology of the Zn thin films on corrosion behavior was evaluated by electro-chemical anodic polarization tests in deaerated 3% NaCl solution.

From the experimental results, all the deposited Zn films showed obviously good corrosion resistance compare to the substrate of AZ91D. It was observed that the morphology of Zn film was changed from columnar to granular structure with gas pressure. The morphology of the films was influenced not only by gas pressure but also by bias voltage. 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. Finally, it was concluded that the properties of Zn films can be improved greatly by controlling the crystal orientation and morphology with effective use of the plasma ion plating technique.