Formation mechanism of new corrosion resistance magnesium thin films by PVD method

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1 Surface and Coatings Technology (2003) Formation mechanism of new corrosion resistance magnesium thin films by PVD method a, a a a b M.H. Lee *, I.Y. Bae, K.J. Kim, K.M. Moon, T. Oki a Korea Maritime University, 1. Dongsam-Dong, Youngdo-gu, Busan , South Korea b Nagoya University, Chikusa Nagoya, Japan Abstract Magnesium thin films were deposited on cold-rolled steel by physical vapor deposition sputtering technique. The crystal orientation and morphology of the deposited films were investigated by using XRD and scanning electron microscopy, respectively. The effect of crystal orientation and morphology of magnesium films on corrosion behaviors was estimated by measuring anodic polarization curves in deaerated 3% NaCl solution. With the increase of argon gas pressure, the morphology of the deposited magnesium films changed from columnar to granular structure and the diffraction peaks of the film became little sharp and broad. And all the sputtered magnesium films obviously showed good corrosion resistance compared with 99.99% magnesium target metal. Formation mechanism on the crystal orientation and morphology of magnesium films can be explained by applying the effects of adsorption and occlusion of argon gas Elsevier Science B.V. All rights reserved. Keywords: PVD; Magnesium film; Corrosion resistance; Morphology; Crystal orientation; Adsorption 1. Introduction Recent tendency in electronics and mechanical engineering fields aiming for high performance and energy savings requires the material to be light weight w1x. Magnesium is the sixth most abundant element among the natural resources in the earth. This metal is 35% lighter than aluminum and has a good vibration resistivity. However, magnesium has not been applied as much as aluminum because of its poor corrosion resistivity in all environments. If a new surface coating technique can be developed to improve its corrosion resistivity, magnesium metal will find new applications in various fields. In general, light metal, particularly magnesium, is difficult to plate using conventional coating techniques such as chemical or electrochemical process. This is due to the presence of the 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 conditions or in air. It is well known that thin films, particularly those *Corresponding author. Tel.: q ; fax: q address: leemh@hanara.kmaritime.ac.kr (M.H. Lee). deposited from plasma-assisted vacuum coating techniques, are usually quite different from the respective bulk material as to their structure and properties w2x. For this reason, the use of plasma-assisted vacuum process, e.g. physical vapor deposition method such as sputtering technique, has spread into various types of industrial applications w3x. However, few studies have been reported dealing with magnesium metal and using the new techniques w4x. In this work, magnesium films were prepared onto the cold-rolled steel substrates by RF magnetron sputtering technique. Finally, it was shown that the corrosion resistance of magnesium films can be improved by controlling crystal orientation and morphology. 2. Experimental procedures Deposition was made in a RF magnetron sputtering system equipped with a 4 inch diameter magnesium target. The magnesium target used in this experiment was 99.99% purity. The cold-rolled steel plates used as substrates were progressively polished up to final abrasive size of 0.05 mm Al2O 3, thereafter, ultrasonically cleaned in a bath of acetone for 30 min, before mounting in the vacuum chamber. Prior to sputtering process, the /03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. PII: S Ž

2 M.H. Lee et al. / Surface and Coatings Technology (2003) substrate was ion cleaned in an argon glow discharge at a pressure of 1.0=10 Torr with a bias voltage of y700 V for approximately 20 min. Also the magnesium target was presputter-cleaned in argon gas pressure of 1.0=10 Torr at closed shutter. After the argon ion sputter cleaning, the system was pumped down to a y6 base pressure of 5.0=10 Torr again. Once the system reached stable condition, the sputter-evaporation typically lasted 30 min. Argon gas pressures of 1.0=10, 5.0=10 and 1.0=10 Torr and bias voltages of 0, y150 and 00 V were used in RF magnetron sputtering arrangement. The top surface and cross-section of the obtained films were examined by scanning electron microscopy (SEM). X-ray diffraction meter with Cu Ka radiation was used to study the crystal structure and the preferred orientation of the films. And, anodic polarization measurements of each specimen by masking all 1.0 cm 2 2 (1.0=1.0 cm ) area without polishing were carried out at 2 mvys in deaerated 3% NaCl solutions at room temperature. Potentials were measured vs. silver chloride electrode. 3. Results and discussion 3.1. Magnesium films deposited at various argon gas pressures and bias voltages In order to investigate the influence of argon gas pressure and bias voltage on the crystal orientation and morphology of magnesium films, the specimens were prepared by changing argon gas pressure from 1.0=10 to 1.0=10 Torr at bias voltage from 0 to 00 V, respectively. The SEM photographs of top surface and crosssection for deposited magnesium films as a function of argon gas pressure at the different bias voltages are shown in Fig. 1. The magnesium films which were deposited at no bias voltage exhibited the change from columnar to a granular structure with porosity and defects as the increase of argon gas pressure. However, the morphology of the sputtered films deposited at the bias voltage of y0.2 kv showed the change into a granular structure without defects or pinholes with the increase of argon gas pressure as shown in Fig. 1. In the case of the bias voltages of 00 V, column width of the film sputtered at low argon gas pressure became larger compared with other bias voltage conditions. From the results of Fig. 1, it can be clearly seen that increasing bias voltage has the similar effect as decreasing argon gas pressure. The X-ray diffraction patterns as a function of argon gas pressure at the different bias voltage for the deposited magnesium films are shown in Fig. 2. The X-ray diffraction patterns of the magnesium films regardless of the negative bias voltage exhibited (002) preferred orientation as the argon gas pressure increased to 1.0=10 from 1.0=10 Torr. Here, the diffraction peaks of the film deposited at high argon gas pressure of 1.0=10 Torr became little sharp and broad. Also, it can be seen that the crystal orientation of the films was influenced not only by argon gas pressure but also substrate bias voltage Corrosion behavior of the deposited magnesium films The result from corrosion test of the deposited magnesium films as a function of argon gas pressure is shown in Fig. 3. It shows the anodic potentio-dynamic polarization curves for the films measured in deaerated 3% NaCl solution. As shown in Fig. 3, the deposited magnesium films nearly tend to show low corrosion current (passive current density) compared with 99.99% magnesium target of the sputter-evaporation metal. And, the passive current density of all the deposited magnesium films tend to decrease with the increase of argon gas pressure. That is, the magnesium films which had fine granular structure with (0 0 2) preferred orientation showed good corrosion resistance Formation mechanism on the crystal orientation and morphology of the deposited magnesium films In general, the properties of the deposited films depend on the deposition condition and these, in turn, depend critically on the crystal orientation and morphology of the films w5 8x. Therefore, it is important to clarify that the nucleation occurrence and growth stage for the crystal orientation and morphology of the film affected by deposition conditions. In these experimental results, the morphology of magnesium films tend to change from columnar structure to a granular structure with the increase of argon gas pressure, even if the substrate temperature was constant. In this case, the preferred orientation of the films exhibited (0 0 2) and the diffraction peaks of the films became little sharp and broad. From these results, it is obvious that the adsorption or occlusion of argon gas on the substrate surface during the deposition plays a large part in affecting the formation change of the crystal orientation and morphology of the growing films. Since the cold-rolled steel plate of substrate which was used in this work is polycrystal, the orientation of the nuclei would have been random at the early growth stages. Consequently, the film orientation depends on the growth of the nuclei. Generally, the differences of growth rate of different crystal planes are expected. In this experiment, it is possible that the growth rate of different crystal planes becomes different. When the magnesium film was deposited by sputtering at low argon gas pressure compared with relatively high argon

3 672 M.H. Lee et al. / Surface and Coatings Technology (2003) Fig. 1. SEM photographs of Mg films prepared by RF magnetron sputtering method. gas pressure, the growth rate of (101) face which has the highest surface energy would be larger than that of (002) face. As a result, the area possession of (002) on film surface will increase w6x. Therefore, it is considered that the film shows (002) orientation. It can be seen that the films exhibiting granular and fine structure with (0 0 2) preferred orientation can be obtained by sputtering method at relatively higher argon gas pressure.

4 M.H. Lee et al. / Surface and Coatings Technology (2003) the substrate temperature, the kinetic mechanism between metal atoms and argon gas, and even the presence of impurities. We recognize, by this empirical measurements, that the system is not fully characterized in film formation mechanism but offers an understanding of film growth for reciprocal relation between crystal orientation and morphology of films with added explanation related to the effects of adsorption and occlusion of argon gas. 4. Conclusion (1) All the deposited magnesium films obviously showed good corrosion resistance compared with Fig. 2. X-ray diffraction patterns of Mg films deposited at various Ar gas pressures of (a) 1.0=10 Torr; (b) 5.0=10 Torr and (c) 1.0=10 Torr. Increasing bias voltage will enhance the mobilities of adatom diffusivities. Consequently, the effect of increasing bias voltage will be similar to that of reducing argon gas pressure as shown in Figs. 1 and 2. The formation mechanism on nucleation growth for morphology and crystal structure of the films is very complex in a sputtering technique because it depend on Fig. 3. Anodic polarization curves of magnesium films deposited at various bias voltages (0, y150, 00 V) measured in deaerated 3% NaCl solution.

5 674 M.H. Lee et al. / Surface and Coatings Technology (2003) % magnesium target of the sputter-evaporation metal. And, the magnesium film of fine granular structure which was obtained at high argon gas pressure of 1.0=10 Torr had the highest corrosion resistance. (2) The crystal orientation and 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 formation mechanism of crystal orientation and morphology can be explained by applying the effects of adsorption, occlusion of argon gas. (3) The properties of all the films can be improved greatly by controlling the crystal orientation and morphology with effective use of the plasma sputtering technique. References w1x J. Feuling, Magnesium 12 (1989) 1. w2x R. Glang, The nature of thin films, in: L.I. Maissel, R. Glang (Eds.), Handbook of Thin Films Tech, Mc Graw-Hill, New York, 1970, chapter 8 (1994). w3x C.M. Egert, D.G. Scott, J. Vac. Sci. Technol. A5 (4) (1987) w4x S. Akavipat, E.B. Hale, C.E. Habermann, P.L. Hagans, Mat. Sci. Eng. 69 (1985) 311. w5x B.A. Movchan, A.V. Demchishin, Phys. Met. Metallogr. 28 (1969) 83. w6x J.A. Thornton, J. Vac. Sci. Technol. 11 (1974) 666. w7x M.H. Lee, Y. Hasegawa, T. Oki, J. Jpn. Inst. Met. 57 (1993) 686. w8x R. Messier, A.P. Giliand, R.A. Roy, J. Vac. Sci. Technol. A2 (1984) 500.