1068-1302/02/0506-0296$27.00 2002 Plenum Publishing Corporation 296

Powder Metallurgy and Metal Ceramics, Vol. 41, Nos. 5-6, 2002

ADVANCING AND RECEDING CONTACT ANGLES OF TIN AND ITS ALLOYS ON THE SURFACE OF A SOLIDIFIED GLASS MASS

V. I. Nizhenko, Yu. I. Smirnov, and V. E. Listovnichii UDC 661.881:666

The temperature dependence of advancing and receding contact angles has been determined for tin and its alloys with silicon, aluminum, and iron on the surface of a solidified glass mass. The advancing and receding contact angles of melts containing iron differ from those of melts containing silicon and aluminum. The difference becomes more pronounced in the low-temperature range as the iron content in the melt increases.

Keywords: advancing and receding contact angle, glass mass, tin, silicon, aluminum, iron.

When polished sheet glass is formed on the surface of molten tin the glass mass passes through temperature zones from 1100 to 500°C in the tin bath. At the end of the bath, where the temperatures are lower, the glass ribbon is solid and the glass separates from the surface of the molten metal. Sometimes drops of metal are trapped by the bottom surface of the ribbon, thus lowering the production quality and causing an appreciable loss of metal. The process whereby individual drops of the melt break away and are captured by the surface of the glass is probabilistic and results in various surface defects: open and closed bubbles and other distortions of its geometry. That process, however, is also due to the influence of the wettability of the sheet glass surface by the tin melt.

To ascertain what role the surface properties at the boundary between the solid glass mass and the liquid tin play in the loss of metal melt carried away by the glass ribbon, we studied the temperature dependence of the contact angles (advancing and receding) of wetting of the surface of the solid glass mass by tin. For this purpose we added to the setup in [1] a device for bringing a wetted substrate to a drop of molten metal from above (Fig. 1). The hermetic seal of the setup was not affected. The advancing contact angle was determined with the melt in contact with the substrate as it moved downwards; the receding angle was found as the substrate moved upward, when the melt came into contact with the preliminarly wetted surface of the specimen.

1

2

34

5

Fig. 1. Device for determining the advancing and receding contact angles. 1) moveable rod; 2) specimen holder; 3) specimen of glass mass; 4) melt; 5) graphite dish.

Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5-6(425), pp. 82-86, May-June, 2002. Original article submitted March 16, 2001.

297

170160150140130

θ, deg

300 400 500 600 , Ct o300 400 500 600 700

1

2

a b

Fig. 2. Polythermal curves for wetting of a glass surface by liquid tin in a vacuum (a) and in nitrogen (b). Here and in Figs. 3, 4, and 5 the notation 1 and 2 denote the advancing and receding angles, respectively.

We used OVCh-000 tin, the additives being semiconducting single-crystal silicon, AV-000 aluminum, and B-3 carbonyl iron. Wetted substrates were prepared from heat-polished sheet glass containing (in %) 74.6 SiO2, 8.7 CaO,

15.7 Na2O, and 0.65 SO3, the remainder being Al2O3, Fe2O3, MgO, and Sb2O3. The metal melt was put in a dish made

of MPG-6 graphite. The experiments were carried out in a vacuum of 3⋅10−3 Pa and in special-purity nitrogen (oxygen content no more than 0.001% and the moisture content no more than 0.005 g/m3).

The polythermal curves obtained for wetting of a solid glass mass by liquid tin in a vacuum and in nitrogen (Fig. 2) have a narrow temperature range where the contact angles (wetting threshold) decrease abruptly. The wetting threshold on the polythermal curve obtained in nitrogen is shifted slightly into the region of lower temperatures. In the temperature interval from the melting point of tin to 600°C the advancing and receding angles are almost the same. At higher temperatures the receding angles are noticeably smaller than the advancing angles (that difference reaches 10 deg at 750°C in a vacuum and 24 deg at 660°C in nitrogen).

We have also studied how small additions of silicon, aluminum, and iron affect the temperature dependence of the arriving and receding angles of tin on the solidified surface of the glass mass. The polythermal curves were plotted for wetting of the glass surface by tin melts containing 0.09 and 0.35 mass% silicon (Fig. 3). The arriving and receding angles differ significantly at temperatures above 600°C. As the silicon content in the melt rises that difference is also

170

160

150

140

170

160

150

θ, deg

500 600 700 , Ct o

12

170

160

170

160

170

160

θ, deg

300 400 500 600 700 , Ct o

1

2

Fig. 3 Fig. 4

Fig. 3. Polythermal curves of the wetting of a glass surface by tin containing 0.09 (a) and 0.35% Si (b).

Fig. 4. Polythermal curves of wetting of a glass surface by tin containing 0.19 (a), 0.50 (b), and 0.75 mass% Al (c).

a

b

a

b

c

298

150140130120110

150140130120

160150140130120

θ, deg

300 400 500 600 700 , Ct o

1

2

Fig. 5. Polythermal curves of wetting of a glass surface by tin containing 0.01 (a), 0.05 (b), and 0.07 mass% Fe (c).

observed in the low-temperature region (the arriving and receding angles are the same in the temperature range 500-600°C for a melt containing 0.09 mass% silicon and differ sharply at 650°C; for a melt containing 0.35 mass% Si a noticeable difference is observed at 550°C, and that difference increases linearly as the temperature rises).

When up to 0.75 mass% Al is introduced into the tin melt the arriving and receding angles on the surface studied are the same (Fig. 4). The polythermal curves do not have a wetting threshold. Evidently the melt is prevented from interacting with the glass surface by the aluminum oxide film that forms on the surface of the melt as a result of its reacting with the oxygen in the nitrogen medium (as indicated above the nitrogen contained up to 0.001% oxygen). In the entire temperature range studied the contact angles remain large (170-160 deg).

Iron additives have an unusual influence on the contact angles of wetting of the glass surface. Even at an iron content of a mere 0.012 mass% Fe in the tin melt the advancing and receding angles differ substantially from each other over the entire temperature studied (Fig. 5). The wetting threshold shifts by nearly 550°C in comparison with pure tin. The shape of polythermal curves of the arriving angle is similar to that of polythermal curves of the receding angles. As the iron content in the melt increases to 0.047 and 0.072 mass% that similarity is disrupted, whereupon the differences between the advancing and receding angles increase in the range of low temperatures. It is noteworthy that for melts containing 0.047 and 0.072 mass% Fe the receding angles depend linearly on the temperature. At 400°C the receding angles drop from 130 deg for melts with 0.012 mass% Fe to 120 deg for a melt with 0.072 mass% Fe.

To estimate the size and mass of tin drops that can be carried by the glass ribbon from the bath containing the melt, we consider the diagram of the vertical section of a drop of melt hanging on the surface (Fig. 6), on condition that that surface is horizontal. The radius R of the drop is small enough so that the effect of gravitation can be ignored in the given approximation, i.e., we assume that the drop is spherical and the contact angle θ corresponds to the conditions of recession. The volume V of the drop in this approximation is

( ) ( ) .3/2cos3coscos

3322∫θ−

−+θ−θπ=−π=

R

R

RdZZRV (1)

The condition for the surface of the glass to suspend a drop of maximum radius Rm is determined by the equilibrium of the gravitational forces F1 acting on the drop,

F1 = Vm⋅ρ⋅g (2)

a

b

c

299

R 0 θ

Z

-Z

M kg 10m, -3

R m 10m, -3

1.5

1.0

0.5

7.5

5.0

2.5

100 140 180θ, deg

Fig. 6. Diagram of the vertical section of a drop of melt on the bottom surface of the glass ribbon.

Fig. 7. Dependence of critical radius and mass of tin drop on contact angle at 700°C.

(here ρ is the density of the melt and g is the free fall acceleration), and the vertical component F2 of the resultant

capillary force along the wetting perimeter [2]

F2=2πRm⋅σ⋅sin2θ, (3)

where σ is the surface tension of the melt. Setting F1 equal to F2, we find the radius of the drop of critical size,

Rm = α 6 ⋅sinθ/ ,2cos3cos3 +θ−θ (4)

and its volume

Vm = 15.39α3⋅sin3θ/ ,2cos3cos3 +θ−θ (5)

where α3 = σ/ρ⋅g is the capillary constant. The results of calculations from Eqs. (4) and (5) to find the radius Rm of the drop of critical size as well as its

mass Mm = Vm⋅ρ for 90° < θ < 180° are given on Fig. 7. The nature of the curve Mm = ƒ(θ) indicates that as θ decreases

in the interval 180° > θ > 90° the mass of tin that can be captured as a drop by the bottom surface of the glass ribbon from the bath of melt increases sharply.

An admixture of iron introduced into the tin melt, in contrast to silicon and aluminum additives, abruptly lowers the temperature at which the receding angle is 120 deg and, therefore, according to the aforementioned calculations the capture of drops of large mass by the bottom surface of the glass becomes more probable.

The recommendation for lowering the level of tin entrainment from the bath of melt in the form of drops of metal on the bottom surface of the glass ribbon thus is to lower the temperature of the section of the bath where the glass ribbon breaks away from the surface of the metal melt and to carefully monitor the iron impurity content in the melt. REFERENCES

1. V. I. Nizhenko and Yu. I. Smirnov, “Setup for determining the surface properties and density of melts with specimens fed semi-automatically into the heating zone,” in: Methods of Investigation and Properties of the Phase Boundary of Contacting Phases [in Russian], Nauk. Dumka, Kiev (1977).

2. J. J. Brennan and J. A. Pask, “Effect of nature of surfaces on wetting of sapphire by liquid aluminium,” J. Amer. Ceram. Soc., 51, No. 10, 569 (1968).