Preparation of Fe–W Amorphous Films by an Electroplating Method

JOURNALOFMATERIALSSCIENCELETTERS 1 4 (1 9 9 5 ) 1 – 3

Preparation of Fe–W amorphous ﬁlms by an electroplating method

Y.NISHI,Y.MOGI,K.OGURI Department of Materials Science, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa, 259-12, Japan

T.WATANABE Department of Industrial Chemistry, Tokyo Metropolitan University, Minamiohsawa, Hachiohji-shi, Tokyo, 192-03, Japan

Iron and tungsten alloy films increase the life of dies ray diffraction. Elemental analysis was carried out by and tools because of their high resistance to wear. use of an X-ray fluorescent method. Crystalline Fe–W alloy films usually show the Fig. 2 shows the X-ray diffraction patterns for the highest resistance to wear: the resistance of crystal- Fe–W alloys. The crystalline peaks are {1 1 0} ␣-Fe lized films is about two times higher than that of Fe– and {1 1 1} Cu, the latter arising from the electrode W amorphous films and five times higher than that of substrate. Thus, the crystalline phase in the film is ␣- commercial chromium films [1]. It is difficult to Fe. The ␣-Fe peak cannot be found in the amorphous make homogenous as-deposited crystalline Fe–W single phase. films. However, if the electroplated film is deposited Fig. 3 shows the change in the full width at half in the amorphous state and then crystallized, the maximum (FWHM) against the tungsten concentra- crystalline film is homogenous. Since electroplating tion of the Fe–W alloys. The high FWHM value is one of the most convenient processes for preparing increases very rapidly for tungsten concentrations homogenous amorphous films [2, 3], the process above 11.5 at %. We define the amorphous phase as conditions for preparing Fe–W amorphous films by structures for which the FWHM is over 5. the electroplating method have been studied and are Fig. 4 shows the glass formation map of Fe–W reported here. alloys. The vertical axis indicates the plating current Fig. 1 shows a schematic drawing of the electro- density (I) and the horizontal axis indicates the plating bath. The electroplating conditions for Fe–W potential of hydrogen (pH). The lower the pH value, alloys are shown in Table I. The total ion concentra- the stronger the acid becomes. If the pH value is tion of Fe and W in the bath was fixed at 0.26 mol/l. below 3, amorphous films can be prepared over a A copper plate was used as substrate and the plating range of current densities (I) in the strong acid bath. temperature was 308 K. The structure of the

deposited alloy ﬁlm was analysed by means of X- )

Co -- K 111

Pt electrode Cu (

Sensor 110) Fe ( Cu electrode

Heater Crystal

N2 gas

Mixture Water

Figure 1 Schematic drawing of the electroplating equipment.

TABLE I Electroplating conditions for Fe–W alloy Amorphous FeSO4 0.052 mol/l Na2WO4 0.208 mol/l C3H4(OH)(COONa)3 0.260 mol/l pH 3–6 Current density 20–80 mA/cm2 Bath temperature 308 K 45 50 55 60 Anode Pt plate 2θ (degrees) Cathode Cu plate

Figure 2 X-ray diffraction patterns for Fe–W alloys. 0261-8028 5 1995 Chapman & Hall 1 7 1.0

6 Amorphous

4 0.5 3 FWHM (degrees) 2 Deposition rate (mg/min) 1 Crystal 0 15 20 0 5 10 2 3 4 5 6 7 W constant (at%) pH Figure 3 Change in full width at half maximum (FWHM) with tungsten concentration for Fe–W alloys. Crystal—ﬁlled symbols; Figure 5 Change in deposition rate with potential of hydrogen (pH) for mixture—half-ﬁlled symbols; amorphous—open symbols. pH value: Fe–W alloys electroplated at 40 mA/cm2 current density. ᮀ 6; ᭺ 5; ᭝ 4.5; ᭛ 4; ᭞ 3.

7 On the other hand, the frequency is high at low hydrogen ion concentrations, thus it is easy to form the crystal. This explains why most samples had a 6 crystalline structure at low hydrogen concentrations Crystal Mixture (pH Ͼ 5) (see Fig. 4). At high current density, the deposition rate is high. 5 Since there is insufﬁcient time for the metal ions to

pH enter a crystal lattice site, it is easy to form the 4 Mix. amorphous phase. This explains why the glass phase was always formed when the current density was Amorphous high (see Fig. 4) and why a single-phase crystalline 3 structure was formed at low current density (I Ͻ 30 mA/cm2). Since the tungsten addition probably decreases the 2 atom diffusibility, it is difficult for atoms to move in 0 20 40 60 80 100 the bath and to arrange on the surface for crystal- 2 / (mA/cm ) lization. In addition, the liquidus temperature is Figure 4 Relationship between current density and potential of below 1600 ЊC up to about 16 at % W [4], indicating hydrogen (pH) for Fe–W alloys. that the liquid is more stable in this composition range. The low liquidus temperature and low atom diffusibility favour glass formation, therefore amor- A high current density (I) increases the deposition phous films are formed at I = 40 mA/cm2 in the rate of plating and the surface roughness of the plate. weakly acidic bath near pH 7 (see Fig. 4). From an From an engineering point of view, this is useful. engineering point of view, it is useful to electroplate When the pH value is close to 7, an amorphous in a weakly acidic bath, because plated layers having film can be formed at I = 40 mA/cm2. Crystalline low resistance to corrosion can be produced. films are obtained for I below 30 mA/cm2 and above In conclusion, we have studied the plating 60 mA/cm2 in a weakly acidic bath near pH 7, as conditions for preparing Fe–W amorphous alloys by shown in Fig. 4. an electroplating method. Amorphous films can be Fig. 5 shows the change in the deposition rate with prepared in a strongly acidic bath over a range of pH value at I = 40 mA/cm2 for the Fe–W amorphous current densities. In addition, amorphous films can be alloys. A low pH value decreases the deposition rate. formed at I = 40 mA/cm2 in a weakly acidic bath In a strongly acidic bath, the hydrogen ion near pH 7. Thus, the conditions have been estab- concentration is high near the surface of electrode, lished for producing an amorphous plated layer therefore it is difficult for the metal ion to move to having a low resistance to corrosion. the electrode surface. This corresponds to the deposition rate at a low pH value. The high concentration (low pH) hydrogen ions prevent metal Acknowledgement ions from entering the crystal site. Thus, it is easy to This work is supported by a Grant in Aid for form the amorphous phase at low pH (pH Ͻ 3) as Scientific Research of the Ministry of Education, shown in Fig. 5. Science and Culture (05640660). 2 References 3. A.NARITA,T.WATANABE and Y.TANABE, ibid. p. 133. 1. T.WATANABE and M.WATANABE, private communica- 4. M.HANSEN , ‘‘Constitution of binary alloys’’, 2nd Edn tion. (McGraw-Hill, New York, 1958) pp. 732–737. 2. T.WATANABE and Y.TANABE, in Proceedings of the 5th International Conference on Rapidly Quenched Metals, edited by S. Steeb and H. Warlimont (North-Holland, Amsterdam, Received 16 January 1985) p. 127. and accepted 17 August 1994