PVD법에 의한 Zn 및 Mg계 금속박막의 제작과 그 특성 규명

Abstract

Magnesium is the lightest metal among the practical metals(2/3 of Al, 1/4 of Fe) and it has outstanding merits such as in EMR shield effect, vibration resistance and a good strength-to-weight ratio, etc.. Moreover, it is plentiful element consisting about 1.93% mass of the earth crust and 2,300mg/l of major source in ocean. Accordingly, it forms the basis for commercial Mg alloys that have found to be used in wide variety of application such as aviation-space, nuclear power generation, automobile, note book computer and cellular phone case etc..

On the other hand, magnesium requires surface plating because of weak corrosion resistance when exists as the type of chlorides and in acid environment. Therefore, in general, magnesium is used after coating by wet process such as chromate and anodizing surface treatment etc., in order to cope with its problem inevitably. But it is known that these coating techniques entails the durable and environmental problems.

Meanwhile, PVD(Physical Vapour Deposition) method, i.e ion plating and sputtering, etc., is able to produce the superior quality films compared to other wet process, due to its ability to make a good adhesive and homogeneous film. Recently, it is drawing more noticeable attention than before with the requirement of environmental friendly coating technique.

In PVD method, the properties of the deposited film depend on the deposition parameters such as substrate temperature, gas pressure, bias voltage, etc., and these, in turn depend critically on the morphology and crystal orientation of the films.

The influence of the substrate temperature on the micro structure of evaporated thick coatings has been examined by B. A. Movchan and A. V. Demchishin who reported the structure zone model. J. A. Thornton has upgraded this model to include the effect of the gas pressure and found the fourth zone structures. R. Messier also found that the fourth zone boundary of J. A. Thornton varies in a fashion similar to the film bias voltage as a function of gas pressure. And some current computer simulation studies have shown the mechanism on the formation and growth of micro structure of the PVD film. However, there have not still enough investigation model for explaining the change in morphology with crystal orientation of the films. Therefore, it is important to clarify that the nucleation occurrence and growth stage for the crystal orientation and morphology of the film are affected by deposition conditions.

In this work, Mg, Zn and Zn-Mg alloy thin films were prepared onto the Mg alloy substrate(AZ91D) by PVD ion plating technique. And the gas pressure and substrate bias voltage on their morphology, crystal orientation and composition of the deposited films was investigated by SEM, XRD and EDX, respectively. In addition, the effect of composition, crystal orientation and morphology of these films on corrosion resistance was evaluated by potentiostat. The main results obtained are as follows

where x ranged from 0 to about 38 atomic%. The alloy films show microcrystalline and grain structures respectively in accordance with preparation conditions such as composition ratio of zinc and magnesium or gas pressures etc.. And X-ray diffraction analysis indicates not only the presence of the Zn-Mg thin films with forced solid solution but also the one of MgZn2 alloy films partly.

In addition the influence of magnesium in the alloy film, morphology and crystal orientation of the Zn-Mg alloy films on corrosion behavior is evaluated by electro-chemical anodic polarization tests in deaerated 3% NaCl solution. From the measured results, all the prepared Zn-Mg alloy films obviously showed good corrosion resistance compared with 99.99% Zn and Mg ingots for evaporation metal. It is, therefore, thought that Mg element in the Zn-Mg films, produces smaller and denser grain structure so that they may improve the formation of homogeneous passive layer in corrosion environment.

In conclusion, the characteristics of all the films can be improved greatly by controlling the composition, morphology and crystal orientation with effective use of the PVD ion plating technique.

In case of Mg and Zn thin film prepared by PVD method

The morphology of Mg and Zn films changed from columnar to granular structure with an increase in gas pressure, and surface crystal grain size decreased. In this case, the preferred orientation of the films tend to be exhibited (002) and the diffraction peaks of the films became less sharp and broadened. From the results it is obvious that the gas pressure play a large part in affecting the formation of the crystal orientation and morphology of deposited films. That is, it is considered that remained gas(Ar, O2, H2O, etc.) and the evaporated metal gas in vacuum chamber attach on the substrate in the PVD method. Therefore, it is thought that they affect on the crystal growth and morphology of the film acting as adsorption inhibitor.

The effect of crystal orientation and morphology of the deposited Mg and Zn films on corrosion behaviors was estimated by measuring anodic polarization curves in deaerated 3% NaCl solution. Among all the deposited Mg and Zn thin films, the films of granular structure which prepared in condition of high gas pressure had the highest corrosion resistance.

In case of Zn-Mg thin film prepared by PVD method

(100-x)ZnㆍxMg alloy films are prepared onto magnesium alloy substrates(AZ91D)