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Wetting and interfacial chemistry in liquid metal-ceramic systems
Materials Science and Engineering, A 135 ( 1991 ) 83 88
83
Wetting and interfacial chemistry in liquid metal-ceramic systems N. Eustathopoulos, D. Chatain and L. Coudurier lnstitut National f~olyteehnique de Grenoble, Laboratoire de Therrnodynarnique et Physieo-Chimie M~tallurgiques', CNRS UA 29, ENSEEG, B.P. 75, 38402 St. Martin d'HOres Cedex (France)
Abstract Wetting data for liquid metal-ceramic systems are reviewed, with emphasis on experimental results obtained on oxides by the sessile drop technique. Examples are given in order to illustrate the different wetting behaviours of reactive and non-reactive pure metal-ceramic pairs. The effect of oxygen on the wettability is presented both when it acts as a dissolved element and when it causes the formation of an oxide film on the liquid metal. The influence of alloying elements is illustrated by numerous results and discussed using a thermodynamic model. Using some examples it is shown that the general behaviours holding for wettability in metal-oxide systems are also valid for the other families of ceramics.
1. Introduction
Technology working in the field of metal matrix
Wettability is usually characterized by the angle 0 formed at the line of contact of three phases, solid (S), liquid (L) and vapour (V). This angle is described by the fundamental equation of wetting, the Young-Dupr6 equation:
composites (MMCs) use calibrated particulate ceramic compacts to measure threshold pressure for liquid infiltration, a quantity which is proportional to the product aLv cos 0 [2]. 2. Pure metal-oxide interfaces
cos 0 = ~ W _ 1 (1) al~, This equation shows that the value of angle 0 for a given system results from the competition of two kinds of forces: cohesion forces of the liquid, which are proportional to the surface tension OLv of the liquid; adhesion forces developed between the liquid and solid phases and introduced into this equation by means of the quantity work W of adhesion, i.e. W= asv + OLv-OsL (2) At high temperatures the surface tension of the liquid and the contact angle are generally measured by the sessile drop method. Recently a tensiometric method adapted to the simultaneous measurement of the surface tension and the contact angle of a liquid in contact with a v e r tical rod or fibre was developed [1]. Finally, researchers from the Massachusetts Institute of 0921-5093/91/$3.50
Interfaces between pure metals (Me) and oxides (MO,,) are usually placed in two classes depending on the sign of the standard Gibbs free energy A Gr ° of the reaction between the two phases: Me + MO,, ~ MeO,,, + M (3) In non-reactive systems (AGr ~>>0) non-wetting or marginal wetting is generally observed (Table 1). The kinetics of wetting is very rapid, with a characteristic time of about 10- ~ s. The work of adhesion varies by almost an order of magnitude, for example between lead and nickel on monocrystalline alumina. The work of adhesion represents from 10% to 60% of the work W c of cohesion of the corresponding metal ( Wc = 2OLv) (Table 1). The absolute values of the temperature coefficients of 0 and W are very small but their signs are opposite, negative for d O / d T and posi© Elsevier Sequoia/Printed in The Netherlands
84 TABLE 1 Experimental data on the wettability of monocrystalline alumina by different non-reactive metals ]3]
We,p (mJ/m a
~e co
1000 Metal Au Pb Cu Si Ni
0
W
(deg)
(mJ m -2)
140 125 128 80 109
265 170 490 8.75 1200
Pa~
W/2 OLv
0,12 0.21 0.l 9 0.59 0.34
s~
,5o0 . . . . ~ u A ~ , ~ . in IGa lO6O
3obo
O I ,.J/,,,~
Fig. 2. Experimental values of the work of adhesion of different non-reactive metals on alumina as a function of the quantity Q defined by eqn. (4).
.
~
1004
3
t
. . . . .
~ i
TABLE 2 Contact angle and work of adhesion of copper on different oxides at T= 1423 K [5] . . . . .
14°/
. . . . . . . . . . 273
87a T(K)
1473
Oxide
0
W
AI203 Ti20~ TiO].14
128 1l 3 82 72
460 740 1460 1650
TiO~,.~,
Fig. 1. Work of adhesion and contact angle vs. temperature of gallium on (a) A1203 and (b) SiO 2 [4]. TABLE 3 Wettabilityof oxides by reactive metals
tive for d W/d T. As an example, a 1000 K rise in temperature causes a decrease in contact angle of gallium on sapphire or quartz substrates of only 5°-10 ° and an increase in W of only 50-100 mJ m - 2 (Fig. 1)[4]. At the present time, no model is capable of describing satisfactorily the bond between a metal and an ionocovalent oxide, materials which have very different electronic structures, It was, thus, for many years believed that the only possible interactions between these two types of materials were van der Waals interactions resulting from dispersion forces. However, values of W calculated for different metal-oxide systems by Naidich [5], using this assumption, vary by less than 50%, while experimental variations are as large as 500% (Table 1). More recently, Chatain et al. [6] proposed a model in which even for non-reactive systems adhesion would result from chemical interactions localized in the interface. The model leads to the equation C
W=
(
_ln
)
N1/3VMe2/3 A--H(~Me)"l- AHM3Me) =
CQ
(4)
,.Met01
Sn Ti AI
Oxide NiO MgO SiO2
T
0
AG R'
Refer-
(K)
(deg)
(kJ (molO) -I) -29 -25
ence
-100
[8]
1275 27 2000 0 1073 90
[5] [5]
where VMe is the molar volume of the liquid metal, AH ~o is the partial enthalpy of mixing at infinite dilution of oxygen and metal oxide M in the metal Me, C an empirical constant equal to 0.2 for refractory oxides [6, 7] and N the Avogadronumber. In Fig. 2, experimental values of W for different metals on alumina are plotted as a function of the quantity Q. The agreement appears to be rather good, especially if one takes into account that scattering of experimental W values is about 20% [3]. Equation (4) is valid for oxides which are electrical insulators. When metallic-like oxides, such as certain titanium oxides, are used, the interfacial bond becomes more metallic in character. W then increases strongly and 0 values lower than 90 ° are observed (Table 2).
85 In reactive metal-ceramic systems contact angles as low as a few degrees have been obtained (Table 3). In this case, the contact angle 0 is given by[8]
T, ....
cos 0 = c o s 0 °
~,,~
Ao,
A(,:,
Ol V
OI N
(5)
.....
~
where 0 ° is the angle in the absence of reaction at the interface and the two other terms represent the effect of the reaction on wetting, Actr takes into account the variation in interfacial tension due to the replacement of initial Me-OXI interface by a double interface: M e - O X 2 and OX2-OX ~. For example, this term would be very beneficial in the case of the Ti-MgO system where the metallic-like titanium monoxideThewould form at thetheinterface, last term, AGr, is Gibbs free energy released by the reaction at the interface. This irreversible contribution to the wetting driving force is effective only if the reaction is rapid and really localized near the triple line [9]. For example, this condition is not satisfied in the very reactive al-sio 2 system, where aluminium atoms, transported by either surface diffusion or evaporation-condensation, react with silica a long way in front of the triple line [9]. This explains why for this system large contact angles have been observed in spite of the very high reactivity (Table 3). 3. The effect of oxygen Depending on their affinity for oxygen, interactions of liquid metals with this element consist either in the formation of oxides (e.g. aluminium) or in the formation of a single metallic phase containing dissolved oxygen (e.g. silver). Results discussed here concern the Sn-AI203 system because tin presents both of these two forms of interactions depending on the temperature of the experiment. At a given oxygen partial pressure there is a corresponding temperature T~, below which tin is oxidized, while above it only dissolved oxygen is present in the melt. The contact angle vs. temperature curve obtained for this system has three ranges [10] (Fig. 3). ( 1 ) At low temperatures, very high 0 values are obtained owing to the tin oxide film covering the liquid metal. This behaviour is quite general: thin oxide films on liquid metals inhibit wet-
,; .: ;. ;, :,
,,
,0
~
Xo
~
*- -I°~Po, ,,.}
eo 10c
140
....
i~0
~--°, *~'I ~, i ~ ~ " ~ ~ ~a,~ ~ _ ~ - ' !
100
a~
'
i 1073
i
i 1273
i
i 14n
i
D
T, Fig. 3. Contact angle vs. temperature at constant partial pressure of oxygen (about 10 ~6 Pa) for tin on sapphire [10].
tability of ceramic surfaces by these metttls. A well-known example in the field of MMCs is alumininm [ 11]. (2) When the temperature reaches the value T~, the oxide film is dissolved in tin. A real metal-substrate interface is then established and the contact angle decreases sharply. However, the final value of 0 is even lower than the value for the pure tin on alumina. This is because enough dissolved oxygen is then present in the melt to cause this additional decrease in the contact angle. This means that oxygen is a solute active at metal-oxide interfaces. Actually, this element associates with the metal atoms to form, in the bulk liquid, clusters having a partially ionic character. These clusters can develop coulombian interactions with any ionic or ionocovalent ceramic and, as a consequence, adsorb strongly at metal-oxide interfaces[5]. (3) When the temperature is again increased, as the solubility of oxygen in tin decreases, oxygen desorbs from the interface. The contact angle then increases, passes through a local maximum and, at higher temperatures, the normal wetting behaviour of a pure metal on oxides is observed.
86
4. The effect of alloying elements
[O(deg)
A simple model was proposed to explain the effect of the non-reactive metallic solutes (B) (for which A Gk ° is positive) on the wettability of pure metal-oxide systems (A)[12]. In this model, the A-B alloy was assumed to be a regular solution and the liquid-,eapour surface and the solid-liquid interface were limited to a monolayer. Calculations of statistical thermodynamics led to the following expressions for the slopes of the surface tension of the liquid and the oxideliquid interfacial tension at infinite dilution of the B solute in the A matrix: (~x°R)x, ~0 - RT { 1 - exp( - ~T)}
u0 Sn-AI
)i,~ ~'~
T=I273K
0= ~,~ , (a)
(6)
i~
~..___.~---'-~~
. . . .
x
esL= ely-(
i
s
i
XAI
W(ml/m') Sn-AI
oo~
with ELv -- (OLv~- alv A)~ - m2
,
0,|
(7)
~ r
T=I27I3K /
T
[
WA)Q 80|
where ELV is the energy of adsorption in the surface monolayer of a B solute infinitely diluted in A metal. It is expressed as a function of the difference in surface tensions aLVB and OLVA of the B and A pure metals, the surface molar area f2 (~oc VMe2/3) of the B alloy, the molar exchange energy 2 of the AB solution estimated from enthalpy of mixing data, and a structural parameter m (m is 0.25 for liquid metals). In the expression for EsL an additional term appears, which is the difference between the work of adhesion of the A and B pure metals. It should be noted that negative values of E lead to very negative values of d o / d x while positive values of E lead to positive but very small values of do/dx. By introducing eqns. (6) into the expression for the work of adhesion (eqn. (2)) and the contact angle (eqn. (1)), the change in W and 0 values can be calculated as small quantities of B in A are added, This model was used in order to classify A-B alloy-oxide systems as a function of the relative values of the adsorption energies ELy and ESL. It has been shown that although it is possible to produce a decrease in the contact angle of a matrix A using solutes B modifying either the liquid-vapour or liquid-solid interfaces, very strong effects can be only obtained with solutes which modify the metal-ceramic interface, An example is the Sn-(AI-AI203) system at 1273 K. Additions of aluminium (OLV=815 mJ m - 2, W= 955 mJ m-2) to tin (OLv= 485 mJ m-2,
=0m 0
/,
,
,
,
I 0.s
,
,
~,AI '
(b) Fig. 4. (a) Contact angle isotherm of Sn-A1 alloys on sapphire [12]; (b) work of adhesion isotherm of Sn-A1 alloys on sapphire[12].
W = 220 mJ m -2) have a negligible effect on the surface tension of tin but induce a significant decrease in interfacial tension between tin and alumina and, as a consequence, a decrease in 0. When tin is the solute in aluminium, the surface tension of aluminium decreases but the interfacial tension between aluminium and alumina is not affected. Consequently, as it is less than 90 °, the contact angle decreases again (Fig. 4). However, it is clear from results of Fig. 4 that the surface tension effect, on the right, is rather weak in comparison with the interfacial tension effect, on the left. Let us compare now the effects on the wettability of alumina by copper of two solutes, one non-reactive (aluminium), the other reactive (titanium) (Fig. 5). By a simple adsorption process at the liquid side of the interface aluminium causes a
87
I
e.
150
T=1&23K
1
0
deg
150
W_500
Ni Pd \
!, W_~1100
\ 1oo
0
I
Cr / C
T = 1523 K
\
N
0
-
I J 0,20 XAll XTi I
i W_~2500 0,~0
\
50
{a) -4._
liquid ~ull~i [u
solid
A[203
~
,, At
, ' I.=!
0!1
0.12
01.3
I X~r 0.4.
Fig.6. Variation in the contact angle of NiPd-Cr alloys on vitreous carbon as a function of the molar fraction
At203 [TiO,~J.J; u, Cu Ti
of chromium.
i
Fig. 5. (a) Effect of additions of aluminium [12] and titanium [13] on the contact angle of copper on alumina. (b) Schematic
description of the solid-liquid interface. "FABLE 4 Contact angle and work of adhesion of copper at T~ 1373 K
on different carbides Substrate
0
(deg) B4C
Cr3C:
136
44 18
Mo~C
TiC .... TiC0 .... TiC0~,
120 50 0
W (mJ m z.) 365
Reference [51
2235 2535 650 2100 2600
[13]
decrease in the contact angle from 130 ° to 80 ° and an increase in the work of adhesion by a factor of 2 [12]. Titanium leads first to an increase in W by adsorption at the liquid side of the interface, as for aluminium, and then, to an additional increase in W by formation of the metallic-like TiO at the solid side of the interface [5, 13]. It is this additional effect which explains the very low 0 values observed for CuTi alloys (Fig. 5). 5. Generalization to the other families of ceramics
The
general
tendencies
prevailing
For example, the contact angle of copper on a covalent carbide such as B4C is very high (Table 4), while it is equal only to a few tens of degrees for metallicqike carbides such as chromium or molybdenum carbides. At the same time, the work of adhesion increases by almost one order of magnitude. A similar change in 0 and W occurs when the nearly covalent titanium carbide becomes more and more metallic in character, as its carbon content decreases (Table 4). Finally, the curve of Fig. 6, describing the variation in the contact angle of N i P d - C r alloys on vitreous carbon as a function of the molar fraction o f chromium, clearly shows two wetting transitions: the first is due to simple chromium adsorption at the interface while the second is due to the formation, by reaction between chromium and carbon, of the wettable Cr~Cr 2 at the interface I14].
References 1 1. Rivollet, D. Chatain and N. Eustathopoulos, J. Mater. Sci., 25 (1990) 3179. 2 T. R. Fletcher, J. A. Cornie and K. C. Russell, in S. G. Fishman and A. K. Dhingra (eds.), Cast Reinforced Metal Composites, ASM International, Metals Park, OH, 1988,
p. 21.
for
metal-oxide systems are also valid for the other families of ceramics.
3 D. Chatain, 1. Rivollet and N. Eustathopoulos, J. ('him. Phys.. 83 (1986) 561. 4 Ju. v. Naidich and Ju. N. Chuvashov, J. Mater. Sci., 18
(1983) 2071. 5 Yu. v. Naidich, Prog. Surf Membr, Sci., 14 ( 1981 ) 353
88 6 D. Chatain, I. Rivollet and N. Eustathopoulos, J. Chim. Phys., 84 (1987) 201. 7 R. Sangiorgi, M. L. Muolo, D. Chatain and N. Eustathopoulos, J. Am. Ceram. Soc., 71 (1988) 742. 8 V. Laurent, D. Chatain and N. Eustathopoulos, Mater. Sci. Eng., A132 (1990) 000. 9 V. Laurent, Thesis, INP Grenoble, 1988. 10 I. Rivollet, D. Chatain and N. Eustathopoulos, Acta Metall., 35 (1987) 835.
11 N. Eustathopoulos and L. Coudufier, Ann. Chim., 10 (1985) 1. 12 J. G. Li, L. Coudurier and N. Eustathopoulos, J. Mater. Sci., 24 (1989) 1109. 13 R. Standing and M. Nicholas, J. Mater. Sci., 13 (1978) 1509. 14 P. Kritsalis, L. Coudurier, C. Parayre and N. Eustathopoulos, Acta Metallurgica and Materialia, to be published.
83
Wetting and interfacial chemistry in liquid metal-ceramic systems N. Eustathopoulos, D. Chatain and L. Coudurier lnstitut National f~olyteehnique de Grenoble, Laboratoire de Therrnodynarnique et Physieo-Chimie M~tallurgiques', CNRS UA 29, ENSEEG, B.P. 75, 38402 St. Martin d'HOres Cedex (France)
Abstract Wetting data for liquid metal-ceramic systems are reviewed, with emphasis on experimental results obtained on oxides by the sessile drop technique. Examples are given in order to illustrate the different wetting behaviours of reactive and non-reactive pure metal-ceramic pairs. The effect of oxygen on the wettability is presented both when it acts as a dissolved element and when it causes the formation of an oxide film on the liquid metal. The influence of alloying elements is illustrated by numerous results and discussed using a thermodynamic model. Using some examples it is shown that the general behaviours holding for wettability in metal-oxide systems are also valid for the other families of ceramics.
1. Introduction
Technology working in the field of metal matrix
Wettability is usually characterized by the angle 0 formed at the line of contact of three phases, solid (S), liquid (L) and vapour (V). This angle is described by the fundamental equation of wetting, the Young-Dupr6 equation:
composites (MMCs) use calibrated particulate ceramic compacts to measure threshold pressure for liquid infiltration, a quantity which is proportional to the product aLv cos 0 [2]. 2. Pure metal-oxide interfaces
cos 0 = ~ W _ 1 (1) al~, This equation shows that the value of angle 0 for a given system results from the competition of two kinds of forces: cohesion forces of the liquid, which are proportional to the surface tension OLv of the liquid; adhesion forces developed between the liquid and solid phases and introduced into this equation by means of the quantity work W of adhesion, i.e. W= asv + OLv-OsL (2) At high temperatures the surface tension of the liquid and the contact angle are generally measured by the sessile drop method. Recently a tensiometric method adapted to the simultaneous measurement of the surface tension and the contact angle of a liquid in contact with a v e r tical rod or fibre was developed [1]. Finally, researchers from the Massachusetts Institute of 0921-5093/91/$3.50
Interfaces between pure metals (Me) and oxides (MO,,) are usually placed in two classes depending on the sign of the standard Gibbs free energy A Gr ° of the reaction between the two phases: Me + MO,, ~ MeO,,, + M (3) In non-reactive systems (AGr ~>>0) non-wetting or marginal wetting is generally observed (Table 1). The kinetics of wetting is very rapid, with a characteristic time of about 10- ~ s. The work of adhesion varies by almost an order of magnitude, for example between lead and nickel on monocrystalline alumina. The work of adhesion represents from 10% to 60% of the work W c of cohesion of the corresponding metal ( Wc = 2OLv) (Table 1). The absolute values of the temperature coefficients of 0 and W are very small but their signs are opposite, negative for d O / d T and posi© Elsevier Sequoia/Printed in The Netherlands
84 TABLE 1 Experimental data on the wettability of monocrystalline alumina by different non-reactive metals ]3]
We,p (mJ/m a
~e co
1000 Metal Au Pb Cu Si Ni
0
W
(deg)
(mJ m -2)
140 125 128 80 109
265 170 490 8.75 1200
Pa~
W/2 OLv
0,12 0.21 0.l 9 0.59 0.34
s~
,5o0 . . . . ~ u A ~ , ~ . in IGa lO6O
3obo
O I ,.J/,,,~
Fig. 2. Experimental values of the work of adhesion of different non-reactive metals on alumina as a function of the quantity Q defined by eqn. (4).
.
~
1004
3
t
. . . . .
~ i
TABLE 2 Contact angle and work of adhesion of copper on different oxides at T= 1423 K [5] . . . . .
14°/
. . . . . . . . . . 273
87a T(K)
1473
Oxide
0
W
AI203 Ti20~ TiO].14
128 1l 3 82 72
460 740 1460 1650
TiO~,.~,
Fig. 1. Work of adhesion and contact angle vs. temperature of gallium on (a) A1203 and (b) SiO 2 [4]. TABLE 3 Wettabilityof oxides by reactive metals
tive for d W/d T. As an example, a 1000 K rise in temperature causes a decrease in contact angle of gallium on sapphire or quartz substrates of only 5°-10 ° and an increase in W of only 50-100 mJ m - 2 (Fig. 1)[4]. At the present time, no model is capable of describing satisfactorily the bond between a metal and an ionocovalent oxide, materials which have very different electronic structures, It was, thus, for many years believed that the only possible interactions between these two types of materials were van der Waals interactions resulting from dispersion forces. However, values of W calculated for different metal-oxide systems by Naidich [5], using this assumption, vary by less than 50%, while experimental variations are as large as 500% (Table 1). More recently, Chatain et al. [6] proposed a model in which even for non-reactive systems adhesion would result from chemical interactions localized in the interface. The model leads to the equation C
W=
(
_ln
)
N1/3VMe2/3 A--H(~Me)"l- AHM3Me) =
CQ
(4)
,.Met01
Sn Ti AI
Oxide NiO MgO SiO2
T
0
AG R'
Refer-
(K)
(deg)
(kJ (molO) -I) -29 -25
ence
-100
[8]
1275 27 2000 0 1073 90
[5] [5]
where VMe is the molar volume of the liquid metal, AH ~o is the partial enthalpy of mixing at infinite dilution of oxygen and metal oxide M in the metal Me, C an empirical constant equal to 0.2 for refractory oxides [6, 7] and N the Avogadronumber. In Fig. 2, experimental values of W for different metals on alumina are plotted as a function of the quantity Q. The agreement appears to be rather good, especially if one takes into account that scattering of experimental W values is about 20% [3]. Equation (4) is valid for oxides which are electrical insulators. When metallic-like oxides, such as certain titanium oxides, are used, the interfacial bond becomes more metallic in character. W then increases strongly and 0 values lower than 90 ° are observed (Table 2).
85 In reactive metal-ceramic systems contact angles as low as a few degrees have been obtained (Table 3). In this case, the contact angle 0 is given by[8]
T, ....
cos 0 = c o s 0 °
~,,~
Ao,
A(,:,
Ol V
OI N
(5)
.....
~
where 0 ° is the angle in the absence of reaction at the interface and the two other terms represent the effect of the reaction on wetting, Actr takes into account the variation in interfacial tension due to the replacement of initial Me-OXI interface by a double interface: M e - O X 2 and OX2-OX ~. For example, this term would be very beneficial in the case of the Ti-MgO system where the metallic-like titanium monoxideThewould form at thetheinterface, last term, AGr, is Gibbs free energy released by the reaction at the interface. This irreversible contribution to the wetting driving force is effective only if the reaction is rapid and really localized near the triple line [9]. For example, this condition is not satisfied in the very reactive al-sio 2 system, where aluminium atoms, transported by either surface diffusion or evaporation-condensation, react with silica a long way in front of the triple line [9]. This explains why for this system large contact angles have been observed in spite of the very high reactivity (Table 3). 3. The effect of oxygen Depending on their affinity for oxygen, interactions of liquid metals with this element consist either in the formation of oxides (e.g. aluminium) or in the formation of a single metallic phase containing dissolved oxygen (e.g. silver). Results discussed here concern the Sn-AI203 system because tin presents both of these two forms of interactions depending on the temperature of the experiment. At a given oxygen partial pressure there is a corresponding temperature T~, below which tin is oxidized, while above it only dissolved oxygen is present in the melt. The contact angle vs. temperature curve obtained for this system has three ranges [10] (Fig. 3). ( 1 ) At low temperatures, very high 0 values are obtained owing to the tin oxide film covering the liquid metal. This behaviour is quite general: thin oxide films on liquid metals inhibit wet-
,; .: ;. ;, :,
,,
,0
~
Xo
~
*- -I°~Po, ,,.}
eo 10c
140
....
i~0
~--°, *~'I ~, i ~ ~ " ~ ~ ~a,~ ~ _ ~ - ' !
100
a~
'
i 1073
i
i 1273
i
i 14n
i
D
T, Fig. 3. Contact angle vs. temperature at constant partial pressure of oxygen (about 10 ~6 Pa) for tin on sapphire [10].
tability of ceramic surfaces by these metttls. A well-known example in the field of MMCs is alumininm [ 11]. (2) When the temperature reaches the value T~, the oxide film is dissolved in tin. A real metal-substrate interface is then established and the contact angle decreases sharply. However, the final value of 0 is even lower than the value for the pure tin on alumina. This is because enough dissolved oxygen is then present in the melt to cause this additional decrease in the contact angle. This means that oxygen is a solute active at metal-oxide interfaces. Actually, this element associates with the metal atoms to form, in the bulk liquid, clusters having a partially ionic character. These clusters can develop coulombian interactions with any ionic or ionocovalent ceramic and, as a consequence, adsorb strongly at metal-oxide interfaces[5]. (3) When the temperature is again increased, as the solubility of oxygen in tin decreases, oxygen desorbs from the interface. The contact angle then increases, passes through a local maximum and, at higher temperatures, the normal wetting behaviour of a pure metal on oxides is observed.
86
4. The effect of alloying elements
[O(deg)
A simple model was proposed to explain the effect of the non-reactive metallic solutes (B) (for which A Gk ° is positive) on the wettability of pure metal-oxide systems (A)[12]. In this model, the A-B alloy was assumed to be a regular solution and the liquid-,eapour surface and the solid-liquid interface were limited to a monolayer. Calculations of statistical thermodynamics led to the following expressions for the slopes of the surface tension of the liquid and the oxideliquid interfacial tension at infinite dilution of the B solute in the A matrix: (~x°R)x, ~0 - RT { 1 - exp( - ~T)}
u0 Sn-AI
)i,~ ~'~
T=I273K
0= ~,~ , (a)
(6)
i~
~..___.~---'-~~
. . . .
x
esL= ely-(
i
s
i
XAI
W(ml/m') Sn-AI
oo~
with ELv -- (OLv~- alv A)~ - m2
,
0,|
(7)
~ r
T=I27I3K /
T
[
WA)Q 80|
where ELV is the energy of adsorption in the surface monolayer of a B solute infinitely diluted in A metal. It is expressed as a function of the difference in surface tensions aLVB and OLVA of the B and A pure metals, the surface molar area f2 (~oc VMe2/3) of the B alloy, the molar exchange energy 2 of the AB solution estimated from enthalpy of mixing data, and a structural parameter m (m is 0.25 for liquid metals). In the expression for EsL an additional term appears, which is the difference between the work of adhesion of the A and B pure metals. It should be noted that negative values of E lead to very negative values of d o / d x while positive values of E lead to positive but very small values of do/dx. By introducing eqns. (6) into the expression for the work of adhesion (eqn. (2)) and the contact angle (eqn. (1)), the change in W and 0 values can be calculated as small quantities of B in A are added, This model was used in order to classify A-B alloy-oxide systems as a function of the relative values of the adsorption energies ELy and ESL. It has been shown that although it is possible to produce a decrease in the contact angle of a matrix A using solutes B modifying either the liquid-vapour or liquid-solid interfaces, very strong effects can be only obtained with solutes which modify the metal-ceramic interface, An example is the Sn-(AI-AI203) system at 1273 K. Additions of aluminium (OLV=815 mJ m - 2, W= 955 mJ m-2) to tin (OLv= 485 mJ m-2,
=0m 0
/,
,
,
,
I 0.s
,
,
~,AI '
(b) Fig. 4. (a) Contact angle isotherm of Sn-A1 alloys on sapphire [12]; (b) work of adhesion isotherm of Sn-A1 alloys on sapphire[12].
W = 220 mJ m -2) have a negligible effect on the surface tension of tin but induce a significant decrease in interfacial tension between tin and alumina and, as a consequence, a decrease in 0. When tin is the solute in aluminium, the surface tension of aluminium decreases but the interfacial tension between aluminium and alumina is not affected. Consequently, as it is less than 90 °, the contact angle decreases again (Fig. 4). However, it is clear from results of Fig. 4 that the surface tension effect, on the right, is rather weak in comparison with the interfacial tension effect, on the left. Let us compare now the effects on the wettability of alumina by copper of two solutes, one non-reactive (aluminium), the other reactive (titanium) (Fig. 5). By a simple adsorption process at the liquid side of the interface aluminium causes a
87
I
e.
150
T=1&23K
1
0
deg
150
W_500
Ni Pd \
!, W_~1100
\ 1oo
0
I
Cr / C
T = 1523 K
\
N
0
-
I J 0,20 XAll XTi I
i W_~2500 0,~0
\
50
{a) -4._
liquid ~ull~i [u
solid
A[203
~
,, At
, ' I.=!
0!1
0.12
01.3
I X~r 0.4.
Fig.6. Variation in the contact angle of NiPd-Cr alloys on vitreous carbon as a function of the molar fraction
At203 [TiO,~J.J; u, Cu Ti
of chromium.
i
Fig. 5. (a) Effect of additions of aluminium [12] and titanium [13] on the contact angle of copper on alumina. (b) Schematic
description of the solid-liquid interface. "FABLE 4 Contact angle and work of adhesion of copper at T~ 1373 K
on different carbides Substrate
0
(deg) B4C
Cr3C:
136
44 18
Mo~C
TiC .... TiC0 .... TiC0~,
120 50 0
W (mJ m z.) 365
Reference [51
2235 2535 650 2100 2600
[13]
decrease in the contact angle from 130 ° to 80 ° and an increase in the work of adhesion by a factor of 2 [12]. Titanium leads first to an increase in W by adsorption at the liquid side of the interface, as for aluminium, and then, to an additional increase in W by formation of the metallic-like TiO at the solid side of the interface [5, 13]. It is this additional effect which explains the very low 0 values observed for CuTi alloys (Fig. 5). 5. Generalization to the other families of ceramics
The
general
tendencies
prevailing
For example, the contact angle of copper on a covalent carbide such as B4C is very high (Table 4), while it is equal only to a few tens of degrees for metallicqike carbides such as chromium or molybdenum carbides. At the same time, the work of adhesion increases by almost one order of magnitude. A similar change in 0 and W occurs when the nearly covalent titanium carbide becomes more and more metallic in character, as its carbon content decreases (Table 4). Finally, the curve of Fig. 6, describing the variation in the contact angle of N i P d - C r alloys on vitreous carbon as a function of the molar fraction o f chromium, clearly shows two wetting transitions: the first is due to simple chromium adsorption at the interface while the second is due to the formation, by reaction between chromium and carbon, of the wettable Cr~Cr 2 at the interface I14].
References 1 1. Rivollet, D. Chatain and N. Eustathopoulos, J. Mater. Sci., 25 (1990) 3179. 2 T. R. Fletcher, J. A. Cornie and K. C. Russell, in S. G. Fishman and A. K. Dhingra (eds.), Cast Reinforced Metal Composites, ASM International, Metals Park, OH, 1988,
p. 21.
for
metal-oxide systems are also valid for the other families of ceramics.
3 D. Chatain, 1. Rivollet and N. Eustathopoulos, J. ('him. Phys.. 83 (1986) 561. 4 Ju. v. Naidich and Ju. N. Chuvashov, J. Mater. Sci., 18
(1983) 2071. 5 Yu. v. Naidich, Prog. Surf Membr, Sci., 14 ( 1981 ) 353
88 6 D. Chatain, I. Rivollet and N. Eustathopoulos, J. Chim. Phys., 84 (1987) 201. 7 R. Sangiorgi, M. L. Muolo, D. Chatain and N. Eustathopoulos, J. Am. Ceram. Soc., 71 (1988) 742. 8 V. Laurent, D. Chatain and N. Eustathopoulos, Mater. Sci. Eng., A132 (1990) 000. 9 V. Laurent, Thesis, INP Grenoble, 1988. 10 I. Rivollet, D. Chatain and N. Eustathopoulos, Acta Metall., 35 (1987) 835.
11 N. Eustathopoulos and L. Coudufier, Ann. Chim., 10 (1985) 1. 12 J. G. Li, L. Coudurier and N. Eustathopoulos, J. Mater. Sci., 24 (1989) 1109. 13 R. Standing and M. Nicholas, J. Mater. Sci., 13 (1978) 1509. 14 P. Kritsalis, L. Coudurier, C. Parayre and N. Eustathopoulos, Acta Metallurgica and Materialia, to be published.