Investigation of Wetting Characteristics of Liquid Iron on Dense MgAlON-Based Ceramics by X-Ray Sessile Drop Technique Z.T. ZHANG, T. MATSUSHITA, W.C. LI, and S. SEETHARAMAN The wetting characteristics of liquid iron on dense MgAlON-based composite ceramics were investigated using X-ray sessile drop technique. The contact angles were measured on substrates of different composites as functions of temperature and varying partial pressures of oxygen. The results with pure argon gas showed that contact angles kept almost constant in the temperature range 1823 to 1873 K. The contact angle was found to show a slight increase with increasing boron nitride (BN) content in MgAlON-BN composites. These are attributed to the higher contact angle between BN substrate and liquid iron drop compared with that obtained for MgAlON substrate. When the COCO2-Ar gas mixtures were introduced into the system, the contact angle showed an initial quick decrease followed by a slow decrease and then a period of nearly constant contact angle at a given temperature corresponding to the steady-state condition. Even in this case, BN seemed to cause an increase in the equilibrium contact angle. The equilibrium contact angle was found to decrease with increasing temperature. XRD results indicated that the substrate was oxidized and the oxidation products combined with FeO formed by the oxidation of the iron drop to form FeAl2O4 and Mg1#xFexO. These were likely to form a ternary FeO-Al2O3-MgO slag or a quaternary slag by combining with B2O3. An interesting observation is that the iron drop moved away from the original site, probably due to the Marangoni effect.
I. INTRODUCTION
IN the modern steel industry, the reactions between refractories and molten iron are receiving great attention due to the refractories’ contamination resulting in nonmetallic inclusions.[1,2] These necessitate the development of new-generation refractories that can survive in extreme conditions—not only high temperatures and high heat fluxes, but also wear, chemical attack, and mechanical loads.[3] Such a development in turn requires a fundamental understanding of the reactions between the materials and molten iron. The present work is thus motivated. In recent years, magnesium aluminate spinel (MgAlON), a solid solution of Al2O3, MgO, and AlN, has attracted considerable attention for its wide applications in hostile environments, such as high-temperature-window materials and high-performance refractories, because of its favorable combination of mechanical (hardness) and optical properties.[4–9] A number of publications have appeared about the application of MgAlON as refractories. The results show that adding some MgAlON to carbon-bonded refractories can improve their resistance to slag and steel corrosion.[10–13] Boron nitride (BN) demonstrates excellent thermal shock resistance and machinability, in addition to its nonreactive nature.[14] In addition, their composites, MgAlON-BN, have been verified to have excellent mechanical and chemical properties Z.T. ZHANG, Graduate Student, T. MATSUSHITA, Doctor, and S. SEETHARAMAN, Professor, are with the Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. Contact e-mail:
[email protected] W.C. LI, Professor, is with the Department of Physical Chemistry, University of Science and Technology of Beijing, Beijing, People’s Republic of China. Manuscript submitted October 22, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
as well as thermal shock durability.[15–17] Thus it is expected that this composite could find applications as high-performance refractories that can be used, for example, in special refractory nozzles, tubes, and break rings for the continuous casting of steel. To our knowledge, no systematic study of the contact angle between MgAlON-BN composites and molten iron under different oxygen partial pressures has been carried out so far. The present paper presents the results of wetting characteristics of liquid iron on MgAlON-based ceramics under different oxygen partial pressures at a given temperature. SEM, XRD techniques were used to examine the interfacial parts. II. EXPERIMENTAL PROCEDURE The X-ray sessile drop technique was used. The oxygen partial pressures were controlled by using different ratios of CO, CO2, and Ar gases. The contact angles were measured as a function of time at a given PO2. A. Materials Pure iron rod with a 5-mm diameter supplied by Goodfellow Cambidge Ltd. was used. Table I presents the impurities of iron samples. The amount of oxygen was below analysis level. Sulfur and silicon concentrations were 3.4 and 16 ppm. Pure MgAlON and MgAlON-BN composites were synthesized by hot-pressing technique. The synthesis method has been described in detail elsewhere[15] and is briefly outlined here. Appropriate amounts of the fine powders of Al2O3, AlN, MgO, and BN, which were used to prepare MgAlON and MgAlON-BN composites, were mixed by ball milling in ethanol medium for 24 hours. The mixtures VOLUME 37B, JUNE 2006—421
Table I. The Compositions of the Iron Sample for Contact Angle Measurements Element Aluminium Boron Cobalt Chromium Copper Gallium Manganese Niobium Nickel Phosphorus Sulfur Silicon Titanium
Impurity Content (ppm) 3.3 0.6 9 2.4 0.7 1.5 0.7 0.9 3.7 1.4 3.4 16 8
Fig. 2—A schematic diagram of the X-ray sessile-drop unit.
Fig. 1—XRD analysis of MgAlON-BN containing 30 vol pct BN.
obtained in this way were placed in a graphite mold covered with BN coating and sintered at 2073 K, at 20 MPa, in argon gas atmosphere for 2 hours. The XRD results confirmed the formation of pure MgAlON and MgAlON-BN composites, as can be seen in Figure 1. Then the samples were cut into 15 $ 15 $ 4-mm3 pieces, ground, and polished to a 3--m roughness. The CO, CO2, and argon ('5 ppm O2) gases used in the present experiment were supplied by AGA Gas AB, Stockholm. The argon gas was purified by passing through columns of silica gel and dehydrate (Mg(ClO4)2) to remove moisture, through ascarite to remove carbon dioxide, and through tube furnaces containing copper and magnesium at 773 K and 673 K respectively to remove residual oxygen. It was shown that the gas after purification had an oxygen partial pressure less than 10#13 Pa as measured by a calcia-stabilized zirconia galvanic cell mounted in the exit gas route. B. Apparatus and Procedure The X-ray sessile drop technique was used. The sessile drop apparatus consists of a high-temperature furnace and a Philips BV-26 mobile imaging system with an X-ray source of 40- to 105-kV power. A schematic diagram of the experimental assembly is shown in Figure 2. The furnace assembly (Thermal Technology Inc.), with a maximum temperature 422—VOLUME 37B, JUNE 2006
of 2300 K, was equipped with a graphite heating element. The furnace was fitted with an alumina reaction tube. The temperature of the furnace was controlled by a Eurotherm temperature regulator within &2 K. There were two parallel ports in the furnace assembly with quartz windows that allow the passage and detection of X-rays from the Philips BV X-ray unit. The imaging system consisted of a CCD camera with digital noise reduction. The image acquisition card enabled the recording of the X-ray image at a maximum rate of 30 frames per second. The system could be used under vacuum or inert gas or with gas mixtures. The pure iron rod with a mass of about 1 g was placed in the furnace. All handling was done with the aid of tweezers to avoid contamination. The upper surface of the sample was leveled carefully before the furnace assembly was closed. The system was evacuated to about 103 Pa. The reaction tube was then filled with argon gas for approximately 12 hours to flush the system completely. The furnace was heated to the preset temperature above the melting point. After this temperature was reached, the furnace was stabilized for about 0.5 hours. In the case of experiments under pure argon gas atmosphere, five different temperatures were used, and at each temperature the furnace was stabilized for 30 minutes. X-ray images were taken at each temperature in static mode using three different X-ray source powers. The flow rate of argon gas was 0.140 L/min. In the case of the experiments at different oxygen partial pressures, gas mixtures of CO-CO2-Ar with preset ratios were introduced into the system. The corresponding oxygen partial pressures are presented in Table II. Once the gas mixtures were introduced into the system, X-ray images were taken at regular intervals in video mode. At the end of the experimental run, CO and CO2 were stopped and the furnace was shut down. The substrate was taken out at room temperature and the surface was examined by X-ray diffraction. The substrate was then cut along the contact part METALLURGICAL AND MATERIALS TRANSACTIONS B
Table II. CO/CO2/Ar Ratios (The Number Corresponding to L/min) 0/0/0.14 0.112/0.028/0.14 0.084/0.056/0.14
Oxygen Partial Pressures and Corresponding Equilibrium Contact Angles at 1823 K
PO2
Contact Angle for MgAlON
Contact Angle for MgAlON-15 vol pct BN
Contact Angle for MgAlON-30 vol pct BN
1 $ 10#14 Pa 9.9 $ 10#4 Pa 1.5 $ 10#2 Pa
130 95 75
133 110 96
138 116 114
with iron drop. The cross-section was examined by scanning electron microscope (SEM) analysis. Some selected experiments were repeated to confirm the reproducibility of the results. The contact angle evaluation was carried out using the software package developed at the Department of Materials Science and Engineering, Carnegie Mellon University. The image-analysis part of this program was developed at the Division of Materials Process Science, Royal Institute of Technology, Stockholm.
III. RESULTS The contact angle measurements of pure iron were carried out on three different substrates—MgAlON, MgAlON15 vol pct BN, and MgAlON-30 vol pct BN—using three different oxygen partial pressures at two different temperatures. The partial pressures of oxygen were controlled by mixing Ar-CO-CO2 carefully. The calibration of the X-ray sessile drop technique was carried out by Kapilashrami et al.[18] and Jakobsson et al.[19] using Cu, Ni, and Fe, which confirmed the reliability of this technique. The results obtained for contact angle measurements under purified argon gas with different substrates showed that contact angles were almost constant in the temperature range 1823 to 1873 K (Figure 3). The points in the figure are the average values of numbers in two parallel experimental runs after 30 minutes of stabilization. The results obtained have a value similar to those reported with alumina substrate earlier in the present laboratory.[18] On the other hand, the contact angles of MgAlON-BN composites were higher than those for Al2O3. The error in the present experiment was estimated to be &4 degrees. The X-ray images of liquid iron drop on different substrates are presented in Figure 4. Figure 5 shows the variation of contact angles at various time intervals at 1823 K under oxygen partial pressures PO2 % 9.9 $ 10#4 Pa and PO2 % 1.5 $ 10#2 Pa, respectively. The zero time marks the onset of the gas mixture, and the gas mixture may need approximately 120 seconds to reach the liquid iron drop. The contact angle in this time interval remained constant (Figure 5). After this, the contact angle showed a drastic drop and later a slow decrease, reaching a steady value at last. The contact angles observed in the steady state corresponding to the variation PO2 value imposed are presented in Table II. Figure 6 shows the X-ray images of the iron drop at different time intervals on pure MgAlON and MgAlON-30 vol pct BN substrates. The change in contact angle of iron drop over MgAlON15 vol pct BN substrate as a function of time at a given oxygen partial pressure, PO2 % 9.9 $ 10#4 Pa, under two different temperatures, 1823 and 1873K, is shown in Figure 7. The METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 3—The contact angle between substrates and liquid iron at different temperatures.
contact angles decreased from 133 degrees to 110 and 103 degrees at 1823 and 1873 K, respectively. Figure 8 shows the variation of contact angles as a function of time at 1823 K at a given oxygen partial pressure, PO2 % 9.9 $ 10#4 Pa, which showed that the contact angles for composites containing 15 vol pct BN and 30 vol pct BN decreased with increasing time; the contact angle for MgAlON, however, remained constant during the experimental time. The results of XRD analysis of the surfaces of MgAlON30 vol pct BN composite is shown in Figure 9. Cross-sections of the samples were analyzed using SEM. Figures 10 and 11 show the photographs of cross-section of pure MgAlON as well as MgAlON-30 vol pct BN composite substrates, respectively, after the sessile drop experiments at 1823 K under different oxygen partial pressures. A product layer was observed on top of the substrate surfaces with slag droplets in contact with the iron drop. The slag droplets under low oxygen partial pressure were denser and thinner than those under high oxygen partial pressures. The thinness of the product layers for Figures 10(a) and (b) and Figure 11 were about 294 -m, 440 -m, and 367 -m, respectively. Energy dispersion spectroscopy (EDS) analysis was used to examine the element distribution of the product layers. The results revealed that the layers contained different ratios of alumina, iron, oxygen, and magnesium. IV. DISCUSSION Normally, the contact angle is expected to decrease with increasing temperature. However, the contact angles after 30 minutes of stabilization over three substrates remained VOLUME 37B, JUNE 2006—423
Fig. 4—The images of liquid iron over substrates—(a) pure MgAlON, (b) MgAlON-15 vol pct BN, and (c) MgAlON-30 vol pct BN—in purified argon atmosphere after 30 minutes.
constant in this, as shown in Figure 3. This is likely to be the case in view of the small temperature range of 1823 to 1873 K. The substrate, MgAlON and BN, contained chemically combined nitrogen, which is likely to dissolve in the liquid iron, the reaction being MgAlON(s) → MgAl2O4(s) " N(Fe(l))
[1]
BN(s) → N(Fe(l)) " B(Fe(l))
[2]
It can be expected that the nitrogen concentration in the iron would increase due to Reactions [1] and [2]. As shown in Figure 8, the contact angles for composites containing 15 vol pct BN and 30 vol pct BN decreased with increasing time, corresponding to the process of dissolving of nitrogen into the molten liquid iron. These are in agreement with the observations by Zhu and Mukai.[20] Nitrogen is a surface-active element in the liquid iron. The interfacial tension between substrate and liquid iron drop (*sl) decreases with increasing nitrogen contents. The interfacial tensions between gas and liquid iron drop (*lg) as well as substrate(*sg) but remain constant under the experimental conditions, the result being a decrease of contact angle according to Young’s equation. The contact angles for pure MgAlON, however, remained almost constant in the present experiment, for which the nitrogen content in the composite was low (3.35 mass pct). This phenomenon is similar to Si3N4 and SiAlON, which decomposed by the dissolution of silicon and nitrogen into the molten steel at high temperature.[21] As shown in Figure 3, the contact angles increased with an increase in BN content, from 132 degrees for pure MgAlON to 139 degrees for MgAlON-30 vol pct BN composites. Since the contact angle between liquid steel with BN is larger than that for pure MgAlON, MgAlON-BN composites are likely to have larger contact angles than pure MgAlON. These have been explained previously[22,23] for nonwetting situations by applying the following equations developed for heterogeneous surfaces cos u % f1 cos u1 " f2 cos u2
[3]
where f1 and f2 % area fractions of MgAlON and BN at the surface, ,1 and ,2 % their respective contact angles, and , is the resulting contact angle. In addition, the introduction of BN would decrease the density from 3.88 g/cm3 for pure MgAlON to 3.22 g/cm3 424—VOLUME 37B, JUNE 2006
for MgAlON-30 vol pct BN. The theoretical density for the composite should be 3.397 g/cm3, calculated using composites rule.[16] In other words, the addition of BN leads to an increase of porosity. DeJonghe et al.[24] measured the contact angles by the immersion–emersion technique in high vacuum for various nonreactive metals using high-purity monocrystalline )–alumina cylinders. The experimental results showed that hysteresis results mainly from roughness. The upper umax and lower umin limits of hysteresis are r a related to the maximum slope amax of the surface profile a by the equations % uY " amax, umax a
umin % uY # amax Y
[4]
where ,Y % the accuracy contact angle, as can be seen from Figure 12 and Eq. [4]. For a wetting system (i.e., , ' 90 degrees) the contact angle between a liquid and a substrate will decrease with increasing roughness, while for a nonwetting system (i.e., , ( 90 degrees) the contact angle will increase with increasing surface roughness.[25] Thus it is reasonable to suppose that the increasing contact angles observed in the present work were induced by the increase in porosity. It is well known that oxygen is a strong surface-active element and the surface tension of liquid iron at constant temperature decreases with increasing oxygen content.[20,26–31] Under the present experimental condition, the surface of the liquid iron will receive different amounts of oxygen due to the difference in convection path (Figure 13). The shortest path for diffusion of oxygen is at the top of the liquid drop (i.e., region 1), and hence oxygen reaches region 1 first. According to the phase diagram for the system Fe-O (Figure 14), there is a two-phase region consisting of liquid iron and liquid oxide in the experimental temperature range.[27] This would result in the higher oxygen concentration in region 1 corresponding to lower surface tension; consequently, the oxygen gradient makes the oxygen diffuse from region 1 to region 2 by Maragoni flow. This would cause the surface velocities from region 1 to region 2 corresponding to the initial drop of contact angle labeled as stage 1 in Figure 5. Only during the initial period do the surface velocities play an important role in oxygen transfer. During later stages, the concentration gradients along the surface diminish and the surface velocities decrease accordingly. The diffusion in the metal from the gas–metal interface to the metal bulk becomes important for the oxygen transfer. Since the reaction rate between METALLURGICAL AND MATERIALS TRANSACTIONS B
oxygen atom and liquid iron is higher than the rate of oxygen diffusion, the rate-determining step of oxygen transfer will be the diffusion of oxygen through the metal. This is likely to result in the slow decrease of surface tension corresponding to the slow decrease of contact angle (labeled as stage 2 in Figure 5). Finally, the droplet becomes saturated with oxygen, corresponding to the given oxygen partial pressure. It attains the equilibrium contact angle (labeled as stage 3). The different values of contact angle in a steady state at 1873 K compared with those at 1823 K were probably due to different oxygen concentration in liquid iron drop. At a given oxygen partial pressure, the oxygen concentration can be calculated by Sievert’s law [O]Fe % K 1PO2
Fig. 5—Contact angles variation between substrates and liquid iron at 1823 K as function of time under different oxygen partial pressures. (a) pure MgAlON; (b) MgAlON-15 vol pct BN; (c) MgAlON-30 vol pct BN. METALLURGICAL AND MATERIALS TRANSACTIONS B
[5]
where K % an equilibrium constant. The value of K increases with increasing temperature. This results in the high oxygen concentration at 1873 K, leading to the decrease of contact angle in a steady state. The substrates would be oxidized when the gas atmosphere was changed from pure argon gas to the PO2 values imposed.[28] The oxidation products of substrate are Al2O3, B2O3, MgAl2O4, and (Al2O3)9(B2O3)2. B2O3 could not be detected by XRD, and oxidation products would combine FeO to form FeAl2O4 and Mg1#xFexO at the experimental temperature. This was verified by XRD analysis, shown in Figure 9. The products would form a ternary FeO-MgO-Al2O3 or a quaternary slag by combining with B2O3. The above result is in agreement with the results of SEM-EDS analyses. The photographs of the cross-sections of the pure MgAlON substrate at two oxygen partial pressures are shown in Figure 10. As can be seen, no obvious slag layer but a discontinuous slag part was observed at lower oxygen partial pressure, PO2 % 9.9 $ 10#4 Pa, shown in Figure 10(a). EDS results showed that liquid iron penetrated into the substrate though the grain boundary. One slag layer followed by the discontinuous slag part, however, was observed when the oxygen partial pressure was 1.5 $ 10#2 Pa, shown in Figure 10(b). Even in this case, EDS analysis was used to examine the element distribution of the reaction layers. The results revealed that iron was distributed in all the product layers. The slag layers contained different ratios of alumina, iron, oxygen, and magnesium. The large particles found in Figure 10(b), labeled as 1 and 2, were alumina. In the periphery of these particles, there was a layer of hercynite, as found by EDS analyses. The reaction layer in this case was about 440 -m. Figure 11 shows the SEM photographs of the MgAlON-30 vol BN pct substrate under PO2 % 1.5 $ 10#2 Pa. The BN in composites was oxidized to generate liquid B2O3 at the experimental temperature.[29] As the oxidation proceeded, some of the B2O3 evaporated and part of the formed B2O3 combined with Al2O3 to yield the compound corresponding to (Al2O3)9(B2O3)2, as revealed by XRD analysis. The remaining liquid B2O3 would form a protective oxidation layer, preventing oxygen diffusion and consequently a decrease in the oxidation rate. The liquid phase, however, was likely to provide the path for liquid iron to sediment through product layer due to gravity, as shown in Figure 11. This is likely to be why the reaction layer of substrate containing 30 vol pct BN was very dense compared with that for pure MgAlON as the substrate. The contact angle would VOLUME 37B, JUNE 2006—425
Fig. 6—(a) X-ray images of the sessile drop over MgAlON substrate at 1823 K at different time intervals and an oxygen partial pressure of 9.9 $ 10#4 Pa. (b) X-ray images of the sessile drop over MgAlON-30 vol pct BN substrate at 1823 K at different time intervals and an oxygen partial pressure of 1.5 $ 10#2 Pa.
increase with increasing BN content. The thickness of the reaction layer in this case was only 294 -m, compared to 440 -m with the pure MgAlON. Soon after the quick decrease of contact angle as noted, the liquid drop started rolling from the original site, probably due to the Marangoni effect. The movement of the liquid iron drop did not exhibit uniform velocity but was instantaneous. As the liquid iron drop reached a new place 426—VOLUME 37B, JUNE 2006
induced by the initial Marangoni effect, part of the bottom of the liquid drop would still be in contact with the original substrate; the other, however, would come into contact with new substrate surface. This resulted in a new interfacial tension gradient along the liquid–solid interface and hence induced new Marangoni flow. Kapilashrami measured the contact angles between alumina and molten iron and observed a similar phenomenon.[18] METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 7—Change in contact angle with time at 1823 K and 1873 K at an oxygen partial pressure of 9.9 $ 10#4 Pa.
Fig. 8—Time dependence of contact angles as a function of time at 1823 K at an oxygen partial pressure of 9.9 $ 10#4 Pa.
Fig. 9—XRD analysis of surface of MgAlON-30 vol pct BN composites after experiment at an oxygen pressure of 1.5 $ 10#2 Pa. METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 10—The cross-section of pure MgAlON substrate after the sessile drop experiments at 1823 K and different oxygen partial pressures; the treat time was about 300 seconds. (a) PO2 % 9.9 $ 10#4 Pa. (b) PO2 % 1.5 $ 10#2 Pa.
Fig. 11—The cross-section of MgAlON-30 vol pct BN substrate after the sessile drop experiments at 1823 K and oxygen partial pressure of PO2 % 1.5 $ 10#2 Pa. VOLUME 37B, JUNE 2006—427
V. CONCLUSION
min Fig. 12—Maximum advancing (umax a ) and minimum receding (ur ) contact angles on a rough surface.
This paper presents the results of an investigation of reaction phenomenon between molten liquid iron containing oxygen and MgAlON-based composite containing different BN content using the X-ray sessile drop technique. The results show that the contact angles remain almost constant under purified argon gas. BN was found to increase the contact angle from 132 degrees for pure MgAlON to 135 and 139 degrees for composites containing 15 vol pct BN and 30 vol pct BN. It can be expected that the nitrogen concentration in the iron would be higher than pure iron. When the CO-CO2-Ar gas mixtures were introduced into the system, it was found that there was an initial quick decrease in the contact angle, followed by a gradual decrease. These contributed to the Marangoni effect initially due to the gradient along the gas–liquid interface, followed by the gradient along the liquid–solid interface. The oxidation products combined with the FeO formed by oxidation of Fe to build a ternary or quaternary FeO-Al2O3-MgO(-B2O3) slag, as shown by XRD analysis and SEM (EDS) analysis.
ACKNOWLEDGMENTS The authors thank Dr. Era Kapilashrami and Dr. Lidong Teng for technical support and valuable discussion during the experiment. Special thanks to Professor Kusuhiro Mukai for his constructive discussions. The samples preparation was financially supported by National Nature Science Foundation of China for No. 50332010, No. 50272010, and No. 50372004. Fig. 13—Marangoni convection induced by oxygen on the surface of liquid iron. +C and +S are the surface tension of the liquid iron in the center and surrounding of alumina crucible, respectively.
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Fig. 14—Phase diagram for the system Fe-O. 428—VOLUME 37B, JUNE 2006
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