II. évfolyam 3. szám 2001. július
[HUN] - Magyar cikk
Wettability of Alumina by Liquid Magnesium and Liquid AZ91 Alloy
Department of Science and Technology of
Metals, University of Erlangen-Nuremberg
The wettability of alumina by magnesium is a very important factor in producing metal matrix composites [1-7] or metallic foams [8-10] consisting of these two phases. As it is well known magnesium has high chemical activity to oxygen, so the surface of solid magnesium is always covered by a thin oxide layer with thickness constantly increasing with time (in contrast to aluminium). The contact angle in liquid magnesium alloy / ceramic systems is expected to be a function of the composition of the liquid metal and the substrate, the holding time, the atmosphere and the presence / thickness of the oxide layer formed at the interface between magnesium and the ceramic.
Liquid magnesium is very difficult to study experimentally partly due to its oxidation, and partly due to its heavy evaporation. As a consequence, only some studies [11-13] have been published on the contact angle of liquid magnesium on ceramics. Two of them [11, 12] deal with oxide ceramics, namely alumina. Their results are summarised in Fig. 1. In the present paper the wettability of alumina by commercial pure magnesium and liquid AZ91 magnesium-alloy are studied by a sessile drop method, in order to improve our understanding of the interfacial energies in these systems.
Fig. 1. Contact angle values measured in magnesium/Al2O3 system [11,12]
a. Fritze, 1995 : metal: QE22A (Mg + 2-3% Ag, 2% Nd+Pr, 0.4-1% Zr), ceramic: polycrystalline a-Al2O3, sessile drop method at 105 Pa in pure Ar,
b. Shi, 1999 : metal: Mg, purity: 99.97%, ceramic: polycrystalline Al2O3 (95.8% Al2O3, 2.8% SiO2, 1.35% MgO), sessile drop with capillary drop at 10-3 Pa, holding time: 3s
2. Prediction of the contact angle between pure magnesium and Al2O3
2.1. Prediction of the initial contact angle
The adhesion energy in the oxide-ceramics / non-reactive liquid metal systems is mainly based on ion-induced dipole interaction . At the very beginning of contact between the liquid magnesium and ceramic there is no chemical reaction between the two phases, so the ion-dipole adhesion energy model seems to be suitable to estimate the initial contact angle in this system. The physical term of the adhesion energy in liquid metal / oxide systems is based mainly on the interaction between the oxygen ions and the polarized (by the oxygen ions) atoms of the liquid metal. The adhesion energy is equal to the sum of the adhesion energy based on the London-dispersion interaction (Wa-a) and the energy based on ion-dipole interaction (Wi-a) :
c: surface atomic/ionic concentration (atom/m2) of metallic atoms
(Me) and oxygen ions (O),
The contact angle (Q ) can be calculated using the Young-Dupré equation:
where slg: surface tension of the liquid metal (J/m2).
Calculations are performed by Eqs. (1-4) using data given in Table 1. The calculated adhesion energy and contact angle values are summarised in Table 2. The calculated contact angle values do not significantly depend on temperature.
Table 1. Physical properties used for calculations
Table 2. Results of calculations of adhesion energy and contact angle in the Mg/Al2O3 system
2.2. Prediction of the final (equilibrium) contact angle
After the initial contact of the liquid Mg/Al2O3 interface, the following chemical reaction is expected to take place at the interface:
Al2O3 (s) + 3 Mg (l) = 3 MgO (s) + 2 Al (l) (5)
The standard Gibbs energy change accompanying this reaction varies between –117.1 kJ and –115.8 kJ in the temperature interval between 1,000 and 1,100 K . Although in addition to pure MgO also complex oxides are expected to form between alumina and magnesia , in our simplified model only the contact angle in the Mg/MgO system will be taken into account. Calculations are performed by the same Eq-s (1-4), as the Mg/MgO system is also a metallic/ionic ceramic system. The following initial data were used for calculations: cO = 1.53.1019 atom/m2, aoxide = 1.69.10-30 m3 , a = 1.75 , = -609 kJ/mol = -1.01.10-18 J/molecule .
The results of calculations are given in Table 3. From Tables 2-3 one can conclude that in the reactive Al2O3 system the initial contact angle of 106-107o is expected to decrease with time due to the formation of MgO at the interface, and in equilibrium it is expected to reach the value of 98-99o.
Table 3. Results of calculations of adhesion energy and contact angle in the Mg/MgO system
3. Experimental materials and procedure
The ceramic used was polycrystalline sintered a-alumina (of 99.9% purity), with a surface having a random crystallographic orientation. The metals were 99.9 % Mg and an AZ91 alloy (Mg + 9% Al, 1% Zn, 0.5% Mn).
Sessile drop technique was applied to determine the contact angle between solid alumina and the liquid metals. For the measurements of the contact angle a high-temperature vacuum furnace was used with Leitz optical system, with a CCD camera and a supporting computer system. The set-up (Fig.2.) consists of a horizontal cylindrical furnace situated in a vacuum chamber connected to vacuum pumps and argon gas. Photographs (Fig.3.) were made by digital camera and the contact angle values were measured from the enlarged (by approximately 20 times) photographs. The reproducibility of contact angle measurement was ± 2o for the same specimen and ± 5o for separate tests.
Fig. 2. The experimental set-up used for the wettability measurements
In the course of the experiment the whole system was evacuated up to the vacuum of 2.10-5 mbar at room temperature and was flushed three times with very pure (99,999 %) argon. Experiments were performed in vacuum of about 200 mbar, in argon residual atmosphere in order to avoid fast evaporation of magnesium and to decrease the role of the adsorbing oxygen, which could significantly alter the interfacial energies. The heating rate from room temperature to 700 or 800 oC was 35 K/min. The system was kept at this temperature until the majority of the original drop evaporated, and then cooled down naturally, under argon. The experiments lasted only about 5 minutes at 800 oC and about 50 minutes at 700 oC. This ratio of about 10 corresponds by the order of magnitude to the ratio of 4.6 between the vapour pressure values of Mg at these two temperatures .
Fig.3. Photographs of sessile magnesium drops on Al2O3 at 700 oC, 200 mbar
In order to break the natural oxide layer on the surface of liquid magnesium the “capillary” drop method was used. The liquid magnesium sample was melted in a quartz capillary covered by boron nitride and located at about 5 mm above the alumina substrate. When the system reached the desired temperature, a drop of the liquid alloy was pushed out of the capillary onto the surface of the substrate below, by some extra argon pressure. While the Mg drop was pressed through the capillary, its surface oxide film was broken and actually left behind on the capillary wall. Thus, fresh liquid magnesium – alumina interface was achieved by this technique.
The weight of metal sample was about 0.04 g, the alumina plate was in the form of square plates (10*10*2 mm). Before the experiments the ceramic substrates were ultrasonically cleaned in alcohol. Right before its introduction into the capillary the oxide layer covering the metallic sample was removed by SiC papers and than washed with alcohol. Thus, the magnesium used was coated with a fresh oxide layer with the thickness of about 20 - 50 nm .
4. Experimental results and discussion
Fig.4. shows the contact angle data as a function of contact time in the magnesium / Al2O3 systems at two different temperatures. One can see that in the case of t=0 the magnesium does not wet the alumina and at both 700 °C and 800 °C the initial contact angle values are the same, about 110° (± 5o). This value is in a very good agreement with the estimated data (Table 2.), based on our ion-induced dipole interaction model . This means that at short contact times only physical interactions are active in the system, with no effect of chemical interactions. Therefore, the experimentally measured contact angle values do not depend on temperature within the accuracy of the experiments (see Table 2).
Fig. 4. Contact angle as function of contact time in the magnesium / Al2O3 systems at different temperatures
With longer contact times chemical reactions take place at the liquid metal / ceramic interface, with the formation of MgO or MgO-Al2O3 complexes (see reaction 5). The evidence of chemical reactions was confirmed by the SEM analysis of the cross section of the cooled Mg/Al2O3 sample (Fig.5). In Fig.5 a thin reaction layer was observed at the interface between the macroscopic phases. Although its exact composition could not be identified, probably MgO and/or MgAl2O4 were formed (see reaction 5).
Fig. 5. SEM of the cross-section of the Mg/Al2O3 specimen (after 1 hour contact at 700 °C)
From Fig.4 it seems that the contact angle decreases with no limit. Equilibrium (steady-state) contact angle was not even achieved after 50 minutes of contact time at 700 oC. At higher temperature the contact angle decreases more rapidly with time. The activation energy of spreading can be calculated as follows. The time, needed to achieve a given contact angle in the system (see Fig.4) is inversely proportional to the velocity of spreading. If the ratio of these velocities are found from Fig.4 at the two different temperatures, the activation energy of spreading can be calculated by the well-known equation. The activation energy of spreading as function of contact angle is shown in Fig.6. In Fig.6 two distinct regions can be observed. While spreading takes place from the original 110o to about 80o (see Fig.4), the activation energy changes approximately linearly, indicating that probably two processes are taking place at the same time, with a variable relative importance. However, when the contact angle drops below 80o (see Fig.4), the activation energy becomes constant (see Fig.6), indicating that the spreading process below 80o is controlled by one single process. The activation energy of this process is around 160 kJ/mol, being close to the heat of evaporation of Mg at this temperature (130 kJ/mol ). Therefore, the process of spreading of the Mg-drop below 80o is obviously due to the evaporation of the drop, and has nothing to do with the interfacial energies in the system. The dramatic decrease in the drop volume can also be observed in Fig.3, comparing the shape (volume) of freshly melted drop with that after 40 minutes of contact time.
The second process, being less and less important while the contact angle changes from 110o to 80o is obviously the chemical interaction between the phases (see reaction (5) and Figure 5). Therefore, the decrease in contact angle observed in Fig.4 is probably due to the following two reasons:
The question is, how to find the “equilibrium” contact angle of the drop, if the influence of the evaporation is excluded. This question was already addressed by Fujii and Nakae . However, the character of the processes in the system studied in  are different from those, studied in this paper. From our Fig.6, one conclusion is straightforward: the equilibrium contact angle in the system should be between 110o and 80o.
Fig.6. The activation energy of spreading in Mg/Al2O3 system as function of contact angle
In order to find the equilibrium value more precisely, the diameter of the drop as function of time is presented in Fig.7. From Fig.7 one can see that these graphs at both temperatures passes trough a maximum point, corresponding to 96o at 700 oC and to 97o at 800 oC. Also, the plateau can be observed in both systems, when the system changes from chemically controlled to the evaporation controlled mechanism. The beginning of this plateau is between 101o and 96o (average of 98o) at 700oC and at 97o at 800 oC. It should be noted that these two values are situated in the interval indicated by Fig.6 (80 – 110o), and also are in perfect agreement with our theoretical estimation (see Table 3): 99 and 98o at 700 oC and 800 oC, respectively. Therefore, the equilibrium contact angle in this paper will be determined as the contact angle, corresponding to the beginning of the plateau in the drop diameter – time diagram.
Hence, we can conclude from our experiments that the real contact angle, dictated by interfacial energies in the Mg/Al2O3 system changes from the initial 110o (corresponding to true Mg/Al2O3 interface) to the final 97-98o (corresponding to the Mg/MgO interface). Further experiments are needed in the Mg/MgO system to confirm the validity of this conclusion.
In Fig.8, the contact angle as function of contact time is shown in the liquid AZ91 alloy / Al2O3 system at 700 °C (for comparison the results for the Mg/Al2O3 at 700 oC presented in Fig.4 are repeated in Fig.8). One can see that the AZ91 liquid alloy wets somewhat better the alumina substrate compared to pure liquid Mg. The initial contact angle is found to be 98 ± 5° at 700 °C. The decrease is probably due to the alloying effect of Al, as the contact angle in the pure aluminium / alumina system is below 90° . The rate of contact angle decrease is almost identical in the two systems (see Fig.8), indicating that the mechanism of spreading is probably identical in the two similar systems. The beginning of the plateau in the drop diameter – contact time graph (see Fig.9) corresponds to 93o.
Fig. 7. Drop contact-diameter (mm) as a function of contact time in the Mg / Al2O3 system (see Fig.4)
Fig.8. Contact angle as a function of contact time in pure liquid Mg/Al2O3 (top curve)and in the liquid AZ91 alloy/Al2O3 systems (bottom curve) at 700 °C
Fig.9. Drop diameter (mm) as a function of contact time in the AZ91 / Al2O3 system
Thus, based on Fig-s. 8-9 we can conclude that the real contact angle, dictated by interfacial energies in the AZ91/Al2O3 system changes from the initial 98o (corresponding to true AZ91/Al2O3 interface) to the final 93o (corresponding to the AZ91/MgO interface). Further experiments are needed in the AZ91/MgO system to confirm the validity of this conclusion.
This work has been performed at the Department of Science and Technology of Metals, University of Erlangen-Nuremberg, Germany. The author would like to thank her colleagues for their support. Dr. G.Kaptay of the University of Miskolc is acknowledged for his help in writing the discussion to this paper (Fig-s 6, 7, 9).
1.F.Dellanay, L. Froyen, A.Deruyttere: J. of Material Science 22 (1987) 1-16
2. A.Mortensen, I.Jin: Int. Mat. Rev., 37 (1992) 101-128
3. G. Kaptay: Materials Science Forum, vols. 215-216 (1996) 459-466.
4. G.Kaptay: in: “High Temperature Capillarity”, ed. by N.Eustathopoulos and N.Sobczak,
Foundry Research Institute, Cracow, Poland., 1998, pp. 388-393
5. Mortensen A.: Comprehensive Composite Materials, Ed. by A.K.C. Zweben et al., Vol. 3,
2000, Elsevier Science Ltd., pp.521-554
6. C.Körner, W.Schaff, M.Ottmüller, R.F.Singer: Adv. Eng. Mats, 2 (2000) 327-337
7. G.Kaptay: Metall. Mater. Trans. A., 32A (2001) 993-1006.
8. S.W.Ip, Y.Wang, J.M.Toguri, Canadian Metallurgical Quarterly, 38 (1999) 81
9. C.Körner, R.F.Singer: in: “Proc. Int. Conf. Metal Foams and Porous Metal Structures”,
Eds: J.Banhart, M.Ashby, N.Fleck, MIT-Publishing Bremen (1999), 91-96.
10. G.Kaptay: same as , pp. 141-145.
11.C. Fritze, G. Nientit: J. Mat. Sci. Letter 14 (1995) 464-466
12.W. Shi, M. Kobashi, T. Choh: Z. Metallkd. 90 (1999) 294-298
13. G. Levi, M. Bamberger, W.D: Kaplan: Acta Mater. 47 (1999) 3927-3934
14.G. Kaptay, E. Bader: Ion-dipole adhesion energy model for wettability of oxide ceramics by non-reactive liquid metals, to be published in Transactions of Joining and Welding Research Institute, 2001
15.G.B. Bokij: Krisztallokhimiia, Nauka, Moskva, 1971
16.CRC Handbook of Chemistry and Physics, Ed. By D.R.Lide, CRC Press, 1993-94
17.J. Emsley: The Elements, Clarendon Press, Oxford, 1989
18.G. Kaptay, E. Bader, L. Bolyan: Materials Science Forum, Vols. 329-330 (2000) 151-156
19.I. Barin: Thermochemical Data of Pure Substances, WHC, Weinheim, 1993
20.J.H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu: Corrosion Sci, 39 (1997) 1397-1414
21.H.Fujii, N,Nakae: Acta mater, 44 (1996) 3567-3573
22.E. Saiz, A.P. Tomsia, R.M. Cannon: Scripta Mater. 44 (2001) 159-164