THE EFFECT OF SUBSTRATE VOLATILITY ON WETTING
KINETICS
S. Barzilai1, 2,, M. Lomberg1 , N. Froumin1, N. Frage1
1
Department of Material Engineering, Ben-Gurion University, Beer-Sheva, Israel
2
NRC-Negev, Beer-Sheva, Israel
Abstract. The wetting behavior of CaF2/Me systems was studied using the sessile
drop method in the temperature range of 823-1573K. The initial contact angles
indicate that Ge and Ga do not wet the substrate. However, different spreading
kinetics were observed during the experiments. For relatively high temperature, the
contact angle was increased with time in CaF2/Ge system, while the contact angle in
CaF2/Ga system decreases. It was suggested that these tendencies are related to the
differences in the vapor pressure values of each melt and of the substrate.
INTRODUCTION
Calcium fluoride is a stable compound with high heat of formation, has a relatively
high melting point and therefore can be a candidate material for liquid metals
container or crucible. Naidich et al. [1,2] studied the wetting behavior of CaF2/Me
systems, where Me= Sn, Cu, In, Ge, Ni or alloys containing V, Hf, Ti, Cr, Nb. It was
established that the investigated melts don't wet the CaF2 substrate. Moreover, a
"dewetting" phenomenon (increasing contact angle with time or temperature
increasing) was observed for some systems. This phenomenon was attributed to the
formation of volatile fluoride products, which sublimate from the interface and
prevent a contact between the drop and the substrate.
In order to in depth understand of the wetting and dewetting behavior in the CaF2/Me
systems, wetting experiments and thermodynamic analysis for the CaF2/Ga and
CaF2/Ge systems with different wetting behavior were performed.
EXPERIMENTAL
CaF2 powder (99.9% purity) was isostatically cold pressed under 240 MPa in a rubber
mold. The compacted samples were sintered in air furnace at 1273 K for 5 hours and
relative density about 94% was achieved. The substrates were polished down to 1
diamond paste. The samples were ultrasonically cleaned using acetone and ethanol
before the wetting experiments. Wetting experiments were carried out by the sessile
drop method in a vacuum furnace (10-3 Pa) in the 873-1573 K temperature range for
10-60 min. The liquid metals Ga and Ge were 99.99% purity.
RESULTS AND DISCUSSION
The wetting kinetics for the CaF2/Ga and CaF2/Ge systems were investigated in the
823-1543K and in the 1273-1573K temperature ranges, relatively. The experimental
results at 1298K for the CaF2/Ga system and at 1573K for the CaF2/Ge system are
shown in Fig.1. At these specific temperatures the initial wetting angle for the both
systems was close to 110 deg., however, different spreading kinetics of Ga and Ge
drops on the CaF2 substrate were observed. For the CaF2/Ga system the contact angle
decreases with time, while for the CaF2/Ge system the opposite behavior takes place.
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(a)
0
10
20
Contact Angle, deg
Contact Angle, deg
125
120
115
110
105
100
95
90
85
80
150
(b)
140
130
120
110
0
30
10
20
30
40
50
60
Time,min
Time, min
Fig. 1. Spreading kinetics for the CaF2/Ga (1298 K) (a) and CaF2/Ge (1573 K) (b)
systems
Contact angle,deg
A similar difference was observed for temperature dependencies of the contact angle
for these systems. The contact angle values, which were measured after 10 min of
contact for each temperature, are presented in Fig. 2.
150
140
130
120
110
100
90
80
70
Ga
Ge
800
1000
1200
1400
1600
Temperature, K
Fig. 2. Apparent contact angles in the CaF2/Ga and CaF2/Ge systems as a function of
temperature.
The contact angle decreasing with temperature is well known feature and generally is
attributed to the formation of a new interface layers or to changes of surface energy of
a melt [3]. The opposite phenomena are quite unique, particularly, for ceramic/metal
systems. The common definition of dewetting behavior is related to a liquid film,
which is unstable in contact with a solid substrate and transforms to a drop [4-7].
Another nature of dewetting was observed for Al2O3/Al system [6, 8, 9]. In this
system contact angle initially decreases due to significant evaporation of Al drop,
which is strongly "pinned" to the substrate [7]. At a certain moment the system is far
enough from equilibrium state, the drop "jumps up" in order to achieve its
equilibrium contact angle and an apparent dewetting is observed.
The reasons of the differences between spreading kinetics for the systems
investigated in the present study are well illustrated by Figs. 3 and 4. Significant
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reduction of the substrate thickness and only minor changes of the drop volume
during wetting experiment at 1573 K were detected for the CaF2/Ge system (Fig.3).
(a)
(b)
(c)
Fig. 3. Metal/ceramic interface in CaF2/Ge system at 1573 K after 10 min. (a), 35
min. (b) and 60 min. (c) of contact.
No changes of the substrate thickness were detected, for the CaF2/Ga system, while
the drop volume reduces significantly in the course of wetting experiment at 1298K.
(a)
(b)
(c)
Fig. 4. Metal/ceramic interface in CaF2/Ga system at 1298K after 2 min. (a), 16 min.
(b) and 24 min. (c) of contact.
It was suggested that these results are related to the vapor pressure values and rates of
sublimation of the melts and the substrate. In order to clarify this proposition a
thermodynamic analysis of solid-vapor and liquid-vapor equilibrium for the
investigated systems were preformed.
Fig. 5 exhibits the vapor pressure calculated for Ge, Ga and CaF2, using eq. 1 and 2.
In these equations X (L,S or G) refers to liquid, solid and gas states of the metals and
CaF2. Px refers to the equilibrium vapor pressure of X and G0 is the standard
evaporation Gibbs energy for reaction (1). The standard thermodynamic data were
extracted from [10].
(1)
X ( L or S)  X (G)
(2)
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 ΔG 0(1) 

PX  exp  
 RT 


1E-4
Vapor pressure, atm
Ga
1E-6
1E-8
Ge
CaF2
1E-10
1200 1300 1400 1500 1600
Temperature, K
Fig. 5. Vapor pressure of Ga, Ge and CaF2 as a function of temperature.
Ge vapor pressure and so its sublimation rate is lower than the same values for CaF2
substrate at 1573 K. The substrate sublimation takes place from the free surface and
the Ge drop prevents the homogeneous thickening of the substrate. As a result of the
metal and the ceramic sublimation a neck shape contact is formed and the apparent
wetting angle increases with a decreasing metal/ceramic contact area (Fig. 3).
Ga vapor pressure at 1298 K is much higher than that for the substrate, and the
vaporization rate of Ga is significantly higher than that for the CaF2. In this case due
to pinning of the drop to the substrate an altering of meta/ceramic contact area does
not occur, while the drop volume is remarkable reduces (Fig. 4). The apparent
wetting angles, which are measured for this system, are lower than these values at the
equilibrium conditions.
SUMMARY
The wettability of CaF2 substrate by Ga and Ge was investigated. The initial contact
angles indicate that both melts do not wet CaF2 ( >90º). Different spreading kinetics
were observed during the experiments. The contact angle in CaF2/Ge system
increased with time, while the contact angle in CaF2/Ga system decreased. This
feature was attributed to the difference between the evaporation rate of the metal
drop and the substrate. The experimental results are well accounted for by a
thermodynamic analysis of the investigated systems.
References
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Nonwettability Behavior of Solid Substrates in Contact with Chemical Active
Reach Ti-, Zr-, and Hf- liquid alloys", vol. 17, 1998.
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[3] N. Eustathopoulos, M.G. Nicholas and B. Devet “Wettability at High
Temperatures”, Pergamon Materials Series 1999.
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