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Wetting of B4C, TiC and graphite substrates by molten Mg
Materials Chemistry and Physics 130 (2011) 665–671
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Wetting of B4 C, TiC and graphite substrates by molten Mg Dan Zhang, Ping Shen ∗ , Laixin Shi, Qichuan Jiang Key Laboratory of Automobile Materials, Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 21 November 2010 Received in revised form 22 June 2011 Accepted 14 July 2011 Keywords: Wetting Evaporation Carbide Magnesium
a b s t r a c t The isotherm wetting of B4 C, TiC and graphite substrates by molten Mg was studied in a flowing Ar atmosphere at 973–1173 K using an improved sessile drop method. The initial contact angles are in the ranges of 95–87◦ , 74–60◦ and 142–124◦ , respectively, moderately depending on the temperature. All the systems are non-reactive in nature; however, the presence of impurity of free boron at the B4 C surface gave rise to the chemical reaction with molten Mg and thus promoted the wettability to a certain degree. A new method was proposed to evaluate the wetting behavior coupled with evaporation and chemical reaction. Furthermore, based on the comparison of the work of adhesion and cohesion, the bonding in the Mg/B4 C and Mg/TiC systems is presumably mainly chemical while that in the Mg/graphite system is physical. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbide ceramics such as TiC, B4 C and SiC together with graphite fibers are good candidates as reinforcement for pure Mg and its alloys due to their high strength and good chemical stability [1–4]. A popular method of producing these composites is to infiltrate the carbide particles or graphite fibers with liquid metals. As we know, when a pressureless infiltration is applied, the wettability of solid by liquid metal plays a crucial role. However, because of ready oxidation of Mg at relatively low temperatures (T ≤ 1073 K) and extensive evaporation at high temperatures (T > 1073 K), an accurate measurement of the wettability for Mg is rather difficult [5]. To our knowledge, so far, only limited work has concerned with the wetting of carbides and graphite by molten Mg. For example, Contreras et al. [6] studied the wetting and spreading of TiC by molten Mg at 1073–1273 K under a static Ar atmosphere using a sessile drop technique. They observed a large contact angle of ∼120◦ , being almost independent of temperature, and no reaction at the interface. However, an unusual change in contact diameter, i.e., a first increase and then decrease due to Mg deoxidation and extensive evaporation, was observed at 1273 K. In our previous study, we investigated the wetting and evaporation behaviors of molten Mg on SiC surfaces in a flowing Ar atmosphere at 973–1173 K using an improved sessile drop method [7], and found that the initial contact angles were between 83◦ and 76◦ , only mildly depending on the temperature. Shinozaki et al. [8] studied the wetting of porous graphite substrates with a relative open
∗ Corresponding author. Tel.: +86 431 85094699; fax: +86 431 85094699. E-mail address: [email protected] (P. Shen). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.07.040
porosity of 12 vol.% by molten magnesium at 973 K using both conventional and drop-extruding techniques. They reported initial contact angles of ∼125◦ , regardless of the experimental method. However, Shi et al. [9] reported a much smaller initial contact angle of approximately 74◦ for a molten Mg drop on the graphite substrate at 973 K by using a drop-extruding technique. The reason for the large discrepancy in the measured contact angles was not clarified. In addition, no work has been performed on the wettability in the Mg–B4 C system despite that B4 C is also an important reinforcement in the Mg-matrix composites [10,11]. The first and primary purpose of this study is to determine the wettability of the B4 C, TiC and graphite substrates by molten Mg at 973–1173 K. Secondly, we tried to assess the complex wetting behavior coupled with evaporation and chemical reaction. Finally, we evaluated the bonding characteristics in the above three systems together with the previously investigated Mg–SiC system. The results are expected to provide substantial guidance for the preparation of the Mg-matrix composites using liquid processing routes. 2. Experimental procedure The substrates used were hot-pressed polycrystalline B4 C, TiC and graphite plates. The B4 C plates were in a purity of ∼96.5 wt.% with a relative density over 98% and dimensions of 20 mm in diameter and 5 mm in thickness. The TiC plates were in a purity of 98.5 wt.% with a relative density over 95% and dimensions of 20 mm × 20 mm × 5 mm. The stoichiometry of C/Ti in the TiC substrates was determined to be ∼0.85, as described elsewhere [12]. The graphite plates were in a purity of over 99.99 wt.% with a total porosity of ∼19 vol.%, an open porosity of ∼16 vol.% and dimensions of 20 mm × 20 mm × 5 mm, respectively. Their surfaces were carefully polished using diamond pastes down to 0.5 m. The average roughness (Ra ) of the B4 C and TiC substrates was measured to be 50 nm and 65 nm, respectively, using a DEKTAK 6 M (Veeco Metrology Corp., USA) over a distance of 2 mm at a speed of 100 m s−1 , while that of the graphite substrates cannot be determined with a sufficient accuracy because of high porosity. The compositions at the polished B4 C
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and TiC surfaces were examined by X-ray diffraction (XRD, D/Max 2500PC, Rigaku Corp., Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA). The XRD examination did not detect any phase other than B4 C or TiC while the XPS analysis revealed the presence of some amount of impurities, mainly free boron and carbon at the B4 C surface, and TiO2 and carbon at the TiC surface. These impurities were more likely to come from the raw powders, particularly for C and B, as well as from the possible surface oxidation (such as the formation of TiO2 ) during the hot-press sintering process. The Mg samples were in a purity of 99.99 wt.% and cut into small cubes weighing about 52 ± 5 mg. An improved sessile drop method was used for the wetting experiment. The general feature of this method was described in detail elsewhere [13]. Before the experiment, both the substrate and the Mg specimen were ultrasonically cleaned in acetone. The substrate was then placed on an alumina supporter inside the chamber and adjusted to a horizontal position and the Mg specimen was stored in a stainless-steel tube outside the chamber. The stainless-steel tube connected an alumina tube (99.6 wt.% purity) with an inner diameter of 5 mm and an open hole of 1 mm in diameter at the bottom. The chamber was evacuated to a vacuum of about 8.5 × 10−4 Pa at room temperature. The substrate was then heated to 1373 K in vacuum at a rate of 20 K min−1 , holding for 10 min to remove surface absorbed impurities, and then cooled at 10 K min−1 to the desired testing temperature. The Ar gas (99.999% purity), purified by passing through a magnesium (99.9%) containing furnace at 673 K, a dehydrating tube filled with molecular sieves and finally an oxygen-absorption tube filled with high effective palladium-type agents to reduce water and oxygen levels, was introduced to the chamber. The pressure in the chamber was controlled to 0.11–0.12 MPa. After the temperature and the atmosphere had stabilized, the Mg specimen was inserted into the bottom of the alumina tube and kept for 30–50 s, decreasing with increasing temperature, for it to melt and to reduce the pre-deposition of Mg vapor on the substrate surface. The molten Mg was then forced out from the 1 mm hole and dropped on the substrate surface by a gradual decrease in the pressure inside the chamber. Simultaneously, the initial oxide covering the Mg surface was mechanically removed as the liquid passed through the hole. As soon as the drop detached from the tube, a high-resolution (1504 × 1000 pixels) photograph was taken and defined as the drop profile at zero time. Subsequent photographs were taken at certain intervals. Most of the experiments for the B4 C and TiC substrates were stopped until the Mg drops were completely disappeared. For the graphite substrates, however, the experiments were halted by shutting off the power after pinning of the triple line for a short time. The captured drop profiles were analyzed using drop-analysis software, from which contact angle, drop height and contact diameter were obtained. The microstructures at and near the triple junction of the exposed interface of the B4 C substrates were observed using a scanning electron microscope (SEM, Evo18, Carl Zeiss, Germany) equipped with an energy dispersive spectrometer (EDS), and the topography at that location was measured by the DEKTAK surface profilometer.
3. Results and discussion 3.1. B4 C 3.1.1. Characterization of the B4 C substrates Fig. 1(a) shows the XPS spectrum of the B4 C substrate surface. C1s, B1s and O1s peaks were clearly identified. The binding energy of O1s at 532.0 eV may originate from the oxygen bonding with boron to form B2 O3 . The detailed information of the spectra of B1s and C1s was given in Fig. 1(b and c), respectively. The B1s peak can be deconvoluted into two peaks due to B–C and B–B bonds. The binding energy of the B–C pair is 187.5 eV [14] and that of B–B is 188.3 eV [15]. For the C1s spectrum, in addition to the C–C bond at the energy of 284.7 eV, the other two peaks at 282.3 eV and 287.7 eV correspond to the C–B [16] and C–O bonds, respectively. Clearly, the surface of the sintered B4 C substrate contains some impurities such as free boron and carbon. After the wetting experiment, the exposed interface of the substrate after complete disappearance of the Mg drop at 1173 K was also examined by XPS and the results are shown in Fig. 2. As indicated, in addition to the B1s, C1s and O1s peaks, two weak Mg 2p and Mg 2s peaks were also observed. Moreover, the primary B1s [Fig. 2(b)] peak has an energy shift of 0.7 eV, as compared with that in Fig. 1(b). This shift is presumably due to the formation of the B–Mg bond. Medvedeva et al. [17] and Wu et al. [18] have found that the formation of the B–Mg bond resulted in a shift of 0.3 eV and 0.6 eV toward lower energies with respect to that in pure B, respectively. The second peak at 192.1 eV corresponds to the B–O bond, suggesting the presence of oxidation of free boron or the
B4 C surface. The Mg 2p [Fig. 2(c)] has two different bonding states. The first peak with a binding energy of 49.5 eV corresponds to the Mg–B bond [19] and the second at 54.0 eV to the Mg–C–O bond [20], possibly resulting from the interaction between MgO and B4 C or impurity carbon. Based on these analyses, we conjecture the formation of MgB2 between molten Mg and the impurity boron at the B4 C surface according to the following reaction: Mg + 2B = MgB2 .
(1)
The changes in Gibbs free energy (G10 ) were between −80.9 kJ mol−1 and −75.7 kJ mol−1 at 973–1173 K [21], indicating that this reaction is favorable.
3.1.2. Wetting and evaporation behaviors Fig. 3(a) shows the variations in contact angle with time for the molten Mg drops on the B4 C surfaces during isothermal dwells at 973–1173 K. As indicated, the initial contact angles do not show a significant dependence on temperature, which are in the range of 95–87◦ . This result is somewhat similar to that found in the Mg–MgO system [5], which also shows a transition from nonwetting to partial wetting with increasing temperature. Fig. 3(b) shows the time-dependent variations in contact angle and drop characteristic sizes (base diameter, d and height, h) for the molten Mg drop on the B4 C surface at 1073 K. Clearly, the time variations in , d and h could be roughly divided into three stages: (I) an initial equilibrium stage (0 < t < 18 s), (II) a spreading stage (18 s ≤ t < 180 s) and (III) an advancing stop, yet the drop height continuously decreasing stage (t ≥ 180 s). The wetting in stage (I) can be regarded as in an initial equilibrium state since the contact angle and drop sizes remain almost constant. The initial contact angle ( 0 ) may be used for the characterization of the intrinsic wettability for the Mg–B4 C system if the molten Mg surface oxidation was insignificant and the effect of the impurities at the B4 C surface was neglected. In stage (II), the contact angle and drop height decrease while the contact diameter increases, indicating the spreading of the triple line. The spreading might be driven by the chemical reaction between Mg and impurity B, as indicated in Eq. (1). However, it persists only for several minutes and then stops, which may result from increasing surface roughness of the substrate especially at the triple junction, as illustrated in Fig. 4(a). On the other hand, the interfacial reaction together with the possible oxidation of Mg at the triple line leads to the formation of a noticeable ridge at the triple junction [Fig. 4(b)] and thus pins the triple line. Finally, in stage (III), the contact angle and drop height monotonically decrease while the contact diameter remains almost constant. Therefore, the decrease in the contact angle results mainly from diminishing drop volume or decreasing height because of the evaporation of Mg. It is worth mentioning that in the initial period of stage (II), the rapid decrease in the contact angle is mainly produced by the significant increase in the contact diameter as a result of the interfacial reaction, while in the later period of stage (II), the reduction in the contact angle could result from the combined effects of the chemical reaction and the Mg evaporation. In order to quantitatively estimate the respective contribution of these two factors at various stages, we adopted the method proposed by Kondoh et al. [22] through introducing a hypothetic contact angle, * , by adding the lost volume to the evaporating drop and assuming a spherical cap geometry of the drop. The former guarantees the constant drop volume, i.e., Vt = V0 during the entire hypothetic wetting process. In this context, the decrease in the contact angle is solely attributed to the effect of the chemical reaction, precluding the influence of the Mg evaporation.
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Fig. 1. (a) XPS spectrum of the as-polished B4 C substrate surface; (b and c) Details around the B1s peak [dashed rectangle in (a)] and the C1s peak [dashed circle in (a)] with corrections for the substrate electrical charge and the background.
Fig. 2. (a) XPS spectrum of the exposed B4 C surface at the triple junction after complete disappearance of the Mg drop due to evaporation at 1173 K; (b) B 1 s and (c) Mg 2p core level spectra and their corresponding fittings.
Fig. 3. (a) Variations in the contact angle with time for the Mg drops on the B4 C surfaces at isothermal temperatures between 973 K and 1173 K; (b) Variations in contact angle (), contact diameter (d) and drop height (h) with time at 1073 K. Similar behaviors were also observed at other temperatures.
Fig. 4. (a) Top-view microstructure and (b) surface topography at the triple junction of the B4 C surface after complete disappearance of the Mg drop at 1073 K.
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Based on the above assumptions, the volume (Vt ) of an evaporating drop at any moment can be written as [22] Vt = H 2
(3D/ sin − 2H) 6
(2)
where H is the drop height, D is the contact diameter, and is the contact angle. The hypothetic drop height, H* , and the radius of a sphere, r* , for a constant volume of the evaporating drop are given by the following equations [22]: (H ∗ )3 + r∗ =
3D2 H ∗ 6V0 − =0 4
(3)
H∗ . 3
(4)
V0 (H ∗ )2
+
Then, the hypothetic contact angle, * , could be estimated from [22]: ∗ = sin−1
D 2r ∗
.
(5)
Fig. 5(a) shows the changes in the hypothetic contact angle ( * ) and the actual contact angle () against time at 1073 K. As indicated, the variations in and * could be roughly divided into four phases. In phase I (0 < t < 18 s), both * and remain almost constant, suggesting that this is a short initial equilibrium phase. In phase II (18 s ≤ t < 30 s), an equal decreasing velocity in * and indicates that the rapid decrease in the contact angles is produced by the interfacial reaction while the contribution of the Mg evaporation could be neglected. In phase III (30 s ≤ t < 180 s), the decreasing velocity of is much larger than that of * , suggesting that in this phase, the decrease in the contact angle is not only driven by the chemical reaction but also by the Mg evaporation, and the role of the latter is more significant with the lapse of time. In phase IV (t ≥ 180 s), * remains almost constant because of the pinning of the triple line while decreases continuously and monotonically. The decrease in in this phase results solely from decreasing drop volume due to the Mg evaporation. In order to show more clearly the relative contributions of the spreading and the evaporation to the decrease in the contact angle at any moment, we use the following percentages, ( 0 − * )/( 0 − ) and ( * − )/( 0 − ), for the characterization. The results are shown in Fig. 5(b). Obviously, the relative contribution of the evaporation increases while that of the spreading decreases with increasing time. However, at t < 180 s, because ( 0 − * ) > ( * − ), the contribution of the spreading is relatively larger. When t = 180 s, ( 0 − * ) = ( * − ), i.e., the two factors play an equal role. At t > 180 s, ( * − ) > ( 0 − * ), indicating that the effect of the drop evaporation becomes dominant. Previously, we suggested that the wetting behavior in an inert system with an evaporating component should be characterized by combination of the changes in the contact angle, contact diameter and drop height [23]. However, for the systems with concomitant evaporation and reaction, the wetting behavior is better to be assessed by the changes in the hypothetic contact angle ( * ) and the actually measured contact angle (), as shown in Fig. 5(a). 3.2. TiC 3.2.1. Characterization of the as-polished TiC substrate Fig. 6(a) shows the XPS spectrum of the as-polished TiC substrate. Ti 2p, C1s and O1s peaks were clearly identified. Their detailed information is presented in Fig. 6(b–d). In the Ti 2p spectrum, the peaks with binding energies of 454.6 eV and 460.6 eV correspond to the Ti–C bond while the other one at 458.3 eV to the Ti–O bond, possibly coming from the slight oxidation of the TiC surface [24]. For the C1s spectrum, the primary peak at 284.6 eV corresponds to the C–C bond and the second one at 281.2 eV to the
C–Ti bond. For the O1s spectrum, the peak with the binding energy of 530.6 eV corresponds to the formation of the O–Ti bond. Based on the XPS analysis, we may conjecture that the TiC substrate surface was either slightly oxidized or covered by a thin film of TiO2 . 3.2.2. Wetting and evaporation behaviors Fig. 7(a) shows the variations in contact angle with time for the molten Mg drops on the TiC surfaces at 973–1173 K. As can be seen, the initial contact angles show an appreciable decrease with increasing temperature, which change from 74◦ at 973 K to 60◦ at 1173 K. It is apparent that these initial contact angles are much smaller than those reported by Contreras et al. [6], which are approximately 120◦ at temperatures 1073–1173 K, as we have mentioned before. The disparity in the results could be mainly attributed to the different sessile drop methods employed by us and by them. In this study, we used the improved sessile drop method, which favors minimizing the effect of the Mg surface oxidization by mechanically disrupting the oxide film and thus yields much smaller contact angles. On the other hand, the contact angles show a rapid decrease in the initial tens of seconds and then a progressive decrease with time until the Mg drop completely disappears through evaporation. For an illustration, Fig. 7(b) shows the representative variations in , d and h at 1073 K. Two characteristic stages can be distinguished: (I) an initial decrease in h and while an increase in d; (II) an almost linear decrease in h and while a nearly constant d, suggesting the pinning of the triple line. The decrease in the contact angle is more marked in stage (I) than in stage (II) because of the slight spreading, which is attributed to the reaction of the surface film, TiO2 , with molten Mg, i.e., 2Mg + TiO2 = 2MgO + Ti.
(6) (G60 )
The changes in the Gibbs free energy were between −224.6 kJ mol−1 and −213.7 kJ mol−1 at 973–1173 K [21]. This reaction-induced spreading was validated by Kondoh et al. [22] in their study on the wetting of oxidized Ti by molten Mg. The disruption of the oxide film, which was even very thin, substantially promoted the wetting. Similar situations were also observed for SiC and Si3 N4 substrates. Because of the coverage of a thin film of SiO2 , the wetting by molten Al or Si is significantly different from their cleaning surfaces [25,26]. On the other hand, the decreasing rate in stage (II) is larger than that observed by Contreras et al. [6] at the same temperature. In the latter, the contact angles decreased slowly at 1023 K and 1123 K but dramatically at 1173 K. Obviously, the oxide film on the Mg drop surface has a significant effect on the wetting behavior at low temperatures. However, even if the oxide film was present on the molten Mg surface, the evaporation of Mg was still in progress because of the incompact nature of the MgO film. A higher temperature increases the Mg vapor pressure, helps to disrupt and remove the oxide film, and thus promotes the wetting. In this sense, the non-wetting ( > 90◦ ) at temperatures between 1073 K and 1123 K and the transition from non-wetting to wetting with increasing temperature, as reported by Contreras et al. [6], were attributed to the presence of the oxide film on the Mg drop surface and then removal of the oxide film. As proved in this study, the Mg/TiC system is partial wetting (0◦ < < 90◦ ) in nature. 3.3. Graphite According to the annotation in the ASM binary alloy phase diagram for the Mg–C system [27], there is no Mg–C compounds at temperatures higher than 933 K and negligible solubility of C in Mg. Consequently, the wetting in this system is very simple and the experiments were stopped after a short time to alleviate the pollution of the chamber by the evaporation of Mg. Fig. 8 shows the variations in contact angle with time for the molten Mg drops on
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Fig. 5. (a) Variations in the hypothetic contact angle ( * ) and actual contact angle () with time at 1073 K; (b) Variation in the relative contributions of the spreading ( 0 − * ) and the evaporation ( * − ) to the total decrease in the contact angle ( 0 − ) with time at 1073 K.
Fig. 6. (a) XPS spectrum of the as-polished TiC substrate surface; (b–d) Ti 2p, C1s and O1s core level spectra and their corresponding fittings.
Fig. 7. (a) Variations in the contact angle with time for the Mg drops on the TiC surfaces at different temperatures; (b) Variations in contact angle (), contact diameter (d) and drop height (h) with time at 1073 K.
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Fig. 8. Variations in the contact angle with time for the Mg drops on the graphite surfaces at isothermal temperatures between 973 K and 1173 K.
Fig. 10. Variations in the measured initial contact angle with temperature for molten Mg on the TiC, SiC, B4 C and graphite substrate surfaces.
the graphite surfaces at 973–1173 K. The initial contact angles are between 142◦ and 124◦ , decreasing with increasing temperature, suggesting that the Mg/graphite system is non-wetting. In comparison, the initial contact angle at 973 K obtained in this study is larger than that reported by Shinozaki et al. [8], which is approximately 125◦ . We attributed this difference to the possible effect of the porosity in the graphite substrates. Supposing that the contact angle between the Mg drop and the Ar atmosphere in the open pores of the substrate surface is 180◦ , according to the Cassie equation [28],
for the drops with identical weights but much larger contact angles. Birdi et al. [29] have demonstrated that the rate of evaporation of sessile drops was dependent on the radius of the solid–liquid interface (i.e., contact radius), rather than on the liquid surface area. Fig. 9 shows the evaporation rates (dV/dt) of the Mg drops with almost identical weights on the TiC, B4 C and graphite surfaces at 1173 K. The smaller the initial contact angle, the larger the contact radius, and the rapider the evaporation. Consequently, the evaporation of the Mg drop, when it was resting on the TiC surface, was larger than that on the B4 C surface, which again than on the graphite surface. On the other hand, because of the high porosity of the graphite substrates, the atmosphere enclosed in the open pores may aggravate the oxidation of the Mg drop, especially at low temperatures, which can also decelerate the evaporation rate.
cos c = cos − f (cos + 1)
(7)
where f is the area percentage of the open pores in the substrate surface, c and are the apparent contact angle for the composite surface and the real contact angle for the dense graphite, respectively, we can deduce that the higher the porosity, the larger the apparent contact angle. On the other hand, the much smaller initial contact angle (74◦ at 973 K) reported by Shi et al. [9] is unbelievable. We speculate that their measurement might be largely affected by the serious pollution of the graphite surface by the Mg vapor. In their experiments, they pre-placed a magnesium ingot on a carbon supporter near the graphite substrate to reduce the oxygen partial pressure in the chamber. Under this circumstance, the Mg vapor would deposit on the graphite substrate surface during heating and thus lead to the much smaller contact angles. Another distinct feature in Fig. 8 is that the contact angle decreases with time at a much slower rate as compared with that in Figs. 4(a) and 7(a). The slower decreasing rate is primarily attributed to the smaller contact radius
Fig. 9. Variations in the drop volume, V, with time for the molten Mg drops with almost identical initial weights, m0 , on the graphite, B4 C and TiC substrates at 1173 K.
3.4. Comparison of the wettability and adhesion In a previous paper [23], we have suggested that the initial contact angle should be used to characterize the wettability in the systems with an evaporating component. In the present Mg/TiC and Mg/B4 C systems, although the initial contact angles could be affected by the impurities at the substrate surfaces, they are comparatively more reasonable than the advancing and receding contact angles since the latter are either affected by the interfacial reaction or by the evaporation. The interfacial reaction may lead to the intrinsic contact angle by removal of the oxide film, making molten Mg contact with the true substrate surface;
Fig. 11. The work of adhesion, Wad , for molten Mg on the TiC, SiC, B4 C and graphite substrates at different temperatures.
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(2) The chemical reaction between Mg and the impurity of free boron at the B4 C surface promotes the wettability in this system to a certain degree. (3) A simple model based on the compensation of the lost volume to the evaporating drop and the assumption of a spherical cap geometry could be used to estimate the hypothetic contact angle, precluding the effect of the Mg evaporation, and further to evaluate the complex wetting behavior coupled with the evaporation and chemical reaction. (4) The interfacial bonding in the Mg/TiC and Mg/B4 C systems is chemical while that in the Mg/graphite system is physical in nature. Acknowledgement
Fig. 12. Calculated values of Wad /Wc(l) for molten Mg on the TiC, SiC, B4 C and graphite substrates at different temperatures.
however, under the circumstance of concurrent evaporation, particularly at high temperatures, it is difficult to determine which is the closest one to the intrinsic contact angle since the intense evaporation could mask the effect of the reaction. Accordingly, here the initial contact angles for the above systems together with those for the previous Mg/SiC system [7] are gathered in Fig. 10 to qualitatively compare their wettability. Note that the initial contact angles vary only slightly with temperature but relatively largely with the substrate type. For the graphite substrates, the initial contact angles at various temperatures are larger than 90◦ , while for TiC and SiC, they are smaller than 90◦ . This result clearly shows that the former is non-wettable while the latter two are partially wettable by molten Mg at 973–1173 K. For the B4 C substrates, however, a transition from non-wetting to wetting occurs with the increase in the temperature. The work of adhesion, Wad , representing the extent of the bonding strength between a liquid and a solid, can be calculated using the Young–Dupre equation Wad = sg + lg − sl = lg (1 + cos )
(8)
where sg , lg and sl are the solid–gas, liquid–gas and solid–liquid interfacial free energies, respectively, and , here, is represented by the initial contact angle. The values of lg are obtained from Ref. [30]. The calculated results of Wad are given in Fig. 11, which are in the sequence of Wad (TiC) > Wad (SiC) > Wad (B4 C) > Wad (graphite). In addition, if one takes the empirical relationship of Wad /Wc(l) ≤ 0.2 (where Wc(l) is the cohesion of liquid and is equal to 2lg ) as a typical characteristic of physical interaction, while Wad /Wc(l) > 0.2 as chemical interaction [31], one may infer from Fig. 12 that the interaction between Mg and the B4 C, SiC and TiC surfaces is chemical while that between Mg and graphite is physical. 4. Conclusions (1) The initial contact angles of molten Mg on the B4 C, TiC and graphite substrates at 973–1173 K are in the ranges of 95–87◦ , 74–60◦ and 142–124◦ , respectively, decreasing with increasing temperature. Accordingly, the Mg/graphite system is nonwetting, the Mg/TiC system is partial wetting and the Mg/B4 C system changes from non-wetting to wetting with increasing temperature.
This work is supported by National Natural Science Foundation of China (no. 50871045), the Key Project of Chinese Ministry of Education (no. 108043) and Graduate Innovation Fund of Jilin University (no. 20111047). We appreciate the anonymous reviewers for their critical reading of the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16]
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
A. Contreras, V.H. Lopez, E. Bedolla, Scripta Mater. 51 (2004) 249–253. V. Kevorkijan, S.D. Skapin, MJoM 15 (2009) 3–18. R.A. Saravanan, M.K. Surappa, Mater. Sci. Eng. A 276 (2000) 108–116. C. Korner, W. Schaff, M. Ottmuller, R.F. Singer, Adv. Eng. Mater. 2 (2000) 327–337. P. Shen, D. Zhang, Q.L. Lin, L.X. Shi, Q.C. Jiang, Mater. Chem. Phys. 122 (2010) 290–294. A. Contreras, C.A. Leon, R.A.L. Drew, E. Bedolla, Scripta Mater. 48 (2003) 1625–1630. D. Zhang, P. Shen, L.X. Shi, Q.L. Lin, Q.C. Jiang, Appl. Surf. Sci. 256 (2010) 7043–7047. N. Shinozaki, J. Morita, K. Wasai, J. Japan Inst. Met. 55 (2005) 310–314. W. Shi, M. Kobasi, T. Choh, J. Japan Inst. Met. 64 (2000) 335–338. V. Kevorkijan, S.D. Skapin, J. Adv. Mater. 42 (2010) 35–47. W. Ke, E.H. Han, Y.F. Han, K. Kainer, A.A. Luo, Mater. Sci. Forum 488–489 (2005) 741–744. P. Shen, Q.L. Lin, Q.C. Jiang, H. Fujii, K. Nogi, J. Mater. Res. 24 (2009) 2420–2427. P. Shen, H. Fujii, T. Matsumoto, K. Nogi, J. Am. Ceram. Soc. 87 (2004) 2151–2159. W. Cermignani, T.E. Paulson, C. Onneby, C.G. Pantano, Carbon 33 (1995) 367–374. C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble Jr., NIST X-ray Photoelectron Spectroscopy Database, Version 3. 5, National Institute of Standards and Technology, Gaithersburg, 2003. T. Hu, L. Steihl, W. Rafaniello, T. Fawcett, D.D. Hawn, J.G. Mashall, S.J. Rozeveld, C.L. Putzig, J.H. Blackson, W. Cermignani, M.G. Robinson, Thin Solid Films 332 (1998) 80–86. N.I. Medvedeva, L.D. Finkel’shteˇin, S.N. Shamin, I.I. Lyakhovskaya, É.Z. Kurmaev, Phys. Solid State 46 (2004) 1994–1997. L.J. Wu, Y.M. Zhu, T. Vogt, H.B. Su, J.W. Davenport, Phys. Rev. B 69 (2004) 064501. S.F. Wang, Y.L. Zhou, Y.B. Zhu, Q. Zhang, S.Y. Dai, K. Xie, Z.H. Chen, H.B. Lu, G.Z. Yang, Phys. C 390 (2003) 1–5. A. Talapatra, S.K. Bandyopadhyay, P. Sen, P. Barat, S. Mukherjee, Phys. C 419 (2005) 141–147. I. Barin, Thermochemical Data of Pure Substances, 3rd ed., Wiley-VCH Verlag GmbH, Weinheim, Germany, 1995. K. Kondoh, M. Kawakami, H. Imai, J. Umeda, H. Fujii, Acta Mater. 58 (2010) 606–614. D. Zhang, L.X. Shi, P. Shen, Q.L. Lin, Q.C. Jiang, Appl. Phys. A 98 (2010) 601–607. A. Mari, P. Aubert, F. Mercier, H. Khodja, C. Berthier, P. Houdy, Surf. Coat. Technol. 194 (2005) 190–195. V. Laurent, C. Rado, N. Eustathopoulos, Mater. Sci. Eng. A 205 (1996) 1–8. B. Drevet, R. Voytovych, R. Israel, N. Eustathopoulos, J. Eur. Ceram. Soc. 29 (2009) 2363–2367. T.B. Massalski, Binary Alloy Phase Diagrams (CD-ROM), ASM International, 1996. A.B.D. Cassie, Discuss. Faraday Soc. 3 (1948) 11–16. K.S. Birdi, D.T. Wu, A. Winter, J. Phys. Chem. 93 (1989) 3702–3703. B.J. Keene, Int. Mater. Rev. 38 (1993) 157–192. N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Elsevier Science, Oxford, UK, 1999.
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Wetting of B4 C, TiC and graphite substrates by molten Mg Dan Zhang, Ping Shen ∗ , Laixin Shi, Qichuan Jiang Key Laboratory of Automobile Materials, Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 21 November 2010 Received in revised form 22 June 2011 Accepted 14 July 2011 Keywords: Wetting Evaporation Carbide Magnesium
a b s t r a c t The isotherm wetting of B4 C, TiC and graphite substrates by molten Mg was studied in a flowing Ar atmosphere at 973–1173 K using an improved sessile drop method. The initial contact angles are in the ranges of 95–87◦ , 74–60◦ and 142–124◦ , respectively, moderately depending on the temperature. All the systems are non-reactive in nature; however, the presence of impurity of free boron at the B4 C surface gave rise to the chemical reaction with molten Mg and thus promoted the wettability to a certain degree. A new method was proposed to evaluate the wetting behavior coupled with evaporation and chemical reaction. Furthermore, based on the comparison of the work of adhesion and cohesion, the bonding in the Mg/B4 C and Mg/TiC systems is presumably mainly chemical while that in the Mg/graphite system is physical. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Carbide ceramics such as TiC, B4 C and SiC together with graphite fibers are good candidates as reinforcement for pure Mg and its alloys due to their high strength and good chemical stability [1–4]. A popular method of producing these composites is to infiltrate the carbide particles or graphite fibers with liquid metals. As we know, when a pressureless infiltration is applied, the wettability of solid by liquid metal plays a crucial role. However, because of ready oxidation of Mg at relatively low temperatures (T ≤ 1073 K) and extensive evaporation at high temperatures (T > 1073 K), an accurate measurement of the wettability for Mg is rather difficult [5]. To our knowledge, so far, only limited work has concerned with the wetting of carbides and graphite by molten Mg. For example, Contreras et al. [6] studied the wetting and spreading of TiC by molten Mg at 1073–1273 K under a static Ar atmosphere using a sessile drop technique. They observed a large contact angle of ∼120◦ , being almost independent of temperature, and no reaction at the interface. However, an unusual change in contact diameter, i.e., a first increase and then decrease due to Mg deoxidation and extensive evaporation, was observed at 1273 K. In our previous study, we investigated the wetting and evaporation behaviors of molten Mg on SiC surfaces in a flowing Ar atmosphere at 973–1173 K using an improved sessile drop method [7], and found that the initial contact angles were between 83◦ and 76◦ , only mildly depending on the temperature. Shinozaki et al. [8] studied the wetting of porous graphite substrates with a relative open
∗ Corresponding author. Tel.: +86 431 85094699; fax: +86 431 85094699. E-mail address: [email protected] (P. Shen). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.07.040
porosity of 12 vol.% by molten magnesium at 973 K using both conventional and drop-extruding techniques. They reported initial contact angles of ∼125◦ , regardless of the experimental method. However, Shi et al. [9] reported a much smaller initial contact angle of approximately 74◦ for a molten Mg drop on the graphite substrate at 973 K by using a drop-extruding technique. The reason for the large discrepancy in the measured contact angles was not clarified. In addition, no work has been performed on the wettability in the Mg–B4 C system despite that B4 C is also an important reinforcement in the Mg-matrix composites [10,11]. The first and primary purpose of this study is to determine the wettability of the B4 C, TiC and graphite substrates by molten Mg at 973–1173 K. Secondly, we tried to assess the complex wetting behavior coupled with evaporation and chemical reaction. Finally, we evaluated the bonding characteristics in the above three systems together with the previously investigated Mg–SiC system. The results are expected to provide substantial guidance for the preparation of the Mg-matrix composites using liquid processing routes. 2. Experimental procedure The substrates used were hot-pressed polycrystalline B4 C, TiC and graphite plates. The B4 C plates were in a purity of ∼96.5 wt.% with a relative density over 98% and dimensions of 20 mm in diameter and 5 mm in thickness. The TiC plates were in a purity of 98.5 wt.% with a relative density over 95% and dimensions of 20 mm × 20 mm × 5 mm. The stoichiometry of C/Ti in the TiC substrates was determined to be ∼0.85, as described elsewhere [12]. The graphite plates were in a purity of over 99.99 wt.% with a total porosity of ∼19 vol.%, an open porosity of ∼16 vol.% and dimensions of 20 mm × 20 mm × 5 mm, respectively. Their surfaces were carefully polished using diamond pastes down to 0.5 m. The average roughness (Ra ) of the B4 C and TiC substrates was measured to be 50 nm and 65 nm, respectively, using a DEKTAK 6 M (Veeco Metrology Corp., USA) over a distance of 2 mm at a speed of 100 m s−1 , while that of the graphite substrates cannot be determined with a sufficient accuracy because of high porosity. The compositions at the polished B4 C
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and TiC surfaces were examined by X-ray diffraction (XRD, D/Max 2500PC, Rigaku Corp., Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA). The XRD examination did not detect any phase other than B4 C or TiC while the XPS analysis revealed the presence of some amount of impurities, mainly free boron and carbon at the B4 C surface, and TiO2 and carbon at the TiC surface. These impurities were more likely to come from the raw powders, particularly for C and B, as well as from the possible surface oxidation (such as the formation of TiO2 ) during the hot-press sintering process. The Mg samples were in a purity of 99.99 wt.% and cut into small cubes weighing about 52 ± 5 mg. An improved sessile drop method was used for the wetting experiment. The general feature of this method was described in detail elsewhere [13]. Before the experiment, both the substrate and the Mg specimen were ultrasonically cleaned in acetone. The substrate was then placed on an alumina supporter inside the chamber and adjusted to a horizontal position and the Mg specimen was stored in a stainless-steel tube outside the chamber. The stainless-steel tube connected an alumina tube (99.6 wt.% purity) with an inner diameter of 5 mm and an open hole of 1 mm in diameter at the bottom. The chamber was evacuated to a vacuum of about 8.5 × 10−4 Pa at room temperature. The substrate was then heated to 1373 K in vacuum at a rate of 20 K min−1 , holding for 10 min to remove surface absorbed impurities, and then cooled at 10 K min−1 to the desired testing temperature. The Ar gas (99.999% purity), purified by passing through a magnesium (99.9%) containing furnace at 673 K, a dehydrating tube filled with molecular sieves and finally an oxygen-absorption tube filled with high effective palladium-type agents to reduce water and oxygen levels, was introduced to the chamber. The pressure in the chamber was controlled to 0.11–0.12 MPa. After the temperature and the atmosphere had stabilized, the Mg specimen was inserted into the bottom of the alumina tube and kept for 30–50 s, decreasing with increasing temperature, for it to melt and to reduce the pre-deposition of Mg vapor on the substrate surface. The molten Mg was then forced out from the 1 mm hole and dropped on the substrate surface by a gradual decrease in the pressure inside the chamber. Simultaneously, the initial oxide covering the Mg surface was mechanically removed as the liquid passed through the hole. As soon as the drop detached from the tube, a high-resolution (1504 × 1000 pixels) photograph was taken and defined as the drop profile at zero time. Subsequent photographs were taken at certain intervals. Most of the experiments for the B4 C and TiC substrates were stopped until the Mg drops were completely disappeared. For the graphite substrates, however, the experiments were halted by shutting off the power after pinning of the triple line for a short time. The captured drop profiles were analyzed using drop-analysis software, from which contact angle, drop height and contact diameter were obtained. The microstructures at and near the triple junction of the exposed interface of the B4 C substrates were observed using a scanning electron microscope (SEM, Evo18, Carl Zeiss, Germany) equipped with an energy dispersive spectrometer (EDS), and the topography at that location was measured by the DEKTAK surface profilometer.
3. Results and discussion 3.1. B4 C 3.1.1. Characterization of the B4 C substrates Fig. 1(a) shows the XPS spectrum of the B4 C substrate surface. C1s, B1s and O1s peaks were clearly identified. The binding energy of O1s at 532.0 eV may originate from the oxygen bonding with boron to form B2 O3 . The detailed information of the spectra of B1s and C1s was given in Fig. 1(b and c), respectively. The B1s peak can be deconvoluted into two peaks due to B–C and B–B bonds. The binding energy of the B–C pair is 187.5 eV [14] and that of B–B is 188.3 eV [15]. For the C1s spectrum, in addition to the C–C bond at the energy of 284.7 eV, the other two peaks at 282.3 eV and 287.7 eV correspond to the C–B [16] and C–O bonds, respectively. Clearly, the surface of the sintered B4 C substrate contains some impurities such as free boron and carbon. After the wetting experiment, the exposed interface of the substrate after complete disappearance of the Mg drop at 1173 K was also examined by XPS and the results are shown in Fig. 2. As indicated, in addition to the B1s, C1s and O1s peaks, two weak Mg 2p and Mg 2s peaks were also observed. Moreover, the primary B1s [Fig. 2(b)] peak has an energy shift of 0.7 eV, as compared with that in Fig. 1(b). This shift is presumably due to the formation of the B–Mg bond. Medvedeva et al. [17] and Wu et al. [18] have found that the formation of the B–Mg bond resulted in a shift of 0.3 eV and 0.6 eV toward lower energies with respect to that in pure B, respectively. The second peak at 192.1 eV corresponds to the B–O bond, suggesting the presence of oxidation of free boron or the
B4 C surface. The Mg 2p [Fig. 2(c)] has two different bonding states. The first peak with a binding energy of 49.5 eV corresponds to the Mg–B bond [19] and the second at 54.0 eV to the Mg–C–O bond [20], possibly resulting from the interaction between MgO and B4 C or impurity carbon. Based on these analyses, we conjecture the formation of MgB2 between molten Mg and the impurity boron at the B4 C surface according to the following reaction: Mg + 2B = MgB2 .
(1)
The changes in Gibbs free energy (G10 ) were between −80.9 kJ mol−1 and −75.7 kJ mol−1 at 973–1173 K [21], indicating that this reaction is favorable.
3.1.2. Wetting and evaporation behaviors Fig. 3(a) shows the variations in contact angle with time for the molten Mg drops on the B4 C surfaces during isothermal dwells at 973–1173 K. As indicated, the initial contact angles do not show a significant dependence on temperature, which are in the range of 95–87◦ . This result is somewhat similar to that found in the Mg–MgO system [5], which also shows a transition from nonwetting to partial wetting with increasing temperature. Fig. 3(b) shows the time-dependent variations in contact angle and drop characteristic sizes (base diameter, d and height, h) for the molten Mg drop on the B4 C surface at 1073 K. Clearly, the time variations in , d and h could be roughly divided into three stages: (I) an initial equilibrium stage (0 < t < 18 s), (II) a spreading stage (18 s ≤ t < 180 s) and (III) an advancing stop, yet the drop height continuously decreasing stage (t ≥ 180 s). The wetting in stage (I) can be regarded as in an initial equilibrium state since the contact angle and drop sizes remain almost constant. The initial contact angle ( 0 ) may be used for the characterization of the intrinsic wettability for the Mg–B4 C system if the molten Mg surface oxidation was insignificant and the effect of the impurities at the B4 C surface was neglected. In stage (II), the contact angle and drop height decrease while the contact diameter increases, indicating the spreading of the triple line. The spreading might be driven by the chemical reaction between Mg and impurity B, as indicated in Eq. (1). However, it persists only for several minutes and then stops, which may result from increasing surface roughness of the substrate especially at the triple junction, as illustrated in Fig. 4(a). On the other hand, the interfacial reaction together with the possible oxidation of Mg at the triple line leads to the formation of a noticeable ridge at the triple junction [Fig. 4(b)] and thus pins the triple line. Finally, in stage (III), the contact angle and drop height monotonically decrease while the contact diameter remains almost constant. Therefore, the decrease in the contact angle results mainly from diminishing drop volume or decreasing height because of the evaporation of Mg. It is worth mentioning that in the initial period of stage (II), the rapid decrease in the contact angle is mainly produced by the significant increase in the contact diameter as a result of the interfacial reaction, while in the later period of stage (II), the reduction in the contact angle could result from the combined effects of the chemical reaction and the Mg evaporation. In order to quantitatively estimate the respective contribution of these two factors at various stages, we adopted the method proposed by Kondoh et al. [22] through introducing a hypothetic contact angle, * , by adding the lost volume to the evaporating drop and assuming a spherical cap geometry of the drop. The former guarantees the constant drop volume, i.e., Vt = V0 during the entire hypothetic wetting process. In this context, the decrease in the contact angle is solely attributed to the effect of the chemical reaction, precluding the influence of the Mg evaporation.
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Fig. 1. (a) XPS spectrum of the as-polished B4 C substrate surface; (b and c) Details around the B1s peak [dashed rectangle in (a)] and the C1s peak [dashed circle in (a)] with corrections for the substrate electrical charge and the background.
Fig. 2. (a) XPS spectrum of the exposed B4 C surface at the triple junction after complete disappearance of the Mg drop due to evaporation at 1173 K; (b) B 1 s and (c) Mg 2p core level spectra and their corresponding fittings.
Fig. 3. (a) Variations in the contact angle with time for the Mg drops on the B4 C surfaces at isothermal temperatures between 973 K and 1173 K; (b) Variations in contact angle (), contact diameter (d) and drop height (h) with time at 1073 K. Similar behaviors were also observed at other temperatures.
Fig. 4. (a) Top-view microstructure and (b) surface topography at the triple junction of the B4 C surface after complete disappearance of the Mg drop at 1073 K.
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Based on the above assumptions, the volume (Vt ) of an evaporating drop at any moment can be written as [22] Vt = H 2
(3D/ sin − 2H) 6
(2)
where H is the drop height, D is the contact diameter, and is the contact angle. The hypothetic drop height, H* , and the radius of a sphere, r* , for a constant volume of the evaporating drop are given by the following equations [22]: (H ∗ )3 + r∗ =
3D2 H ∗ 6V0 − =0 4
(3)
H∗ . 3
(4)
V0 (H ∗ )2
+
Then, the hypothetic contact angle, * , could be estimated from [22]: ∗ = sin−1
D 2r ∗
.
(5)
Fig. 5(a) shows the changes in the hypothetic contact angle ( * ) and the actual contact angle () against time at 1073 K. As indicated, the variations in and * could be roughly divided into four phases. In phase I (0 < t < 18 s), both * and remain almost constant, suggesting that this is a short initial equilibrium phase. In phase II (18 s ≤ t < 30 s), an equal decreasing velocity in * and indicates that the rapid decrease in the contact angles is produced by the interfacial reaction while the contribution of the Mg evaporation could be neglected. In phase III (30 s ≤ t < 180 s), the decreasing velocity of is much larger than that of * , suggesting that in this phase, the decrease in the contact angle is not only driven by the chemical reaction but also by the Mg evaporation, and the role of the latter is more significant with the lapse of time. In phase IV (t ≥ 180 s), * remains almost constant because of the pinning of the triple line while decreases continuously and monotonically. The decrease in in this phase results solely from decreasing drop volume due to the Mg evaporation. In order to show more clearly the relative contributions of the spreading and the evaporation to the decrease in the contact angle at any moment, we use the following percentages, ( 0 − * )/( 0 − ) and ( * − )/( 0 − ), for the characterization. The results are shown in Fig. 5(b). Obviously, the relative contribution of the evaporation increases while that of the spreading decreases with increasing time. However, at t < 180 s, because ( 0 − * ) > ( * − ), the contribution of the spreading is relatively larger. When t = 180 s, ( 0 − * ) = ( * − ), i.e., the two factors play an equal role. At t > 180 s, ( * − ) > ( 0 − * ), indicating that the effect of the drop evaporation becomes dominant. Previously, we suggested that the wetting behavior in an inert system with an evaporating component should be characterized by combination of the changes in the contact angle, contact diameter and drop height [23]. However, for the systems with concomitant evaporation and reaction, the wetting behavior is better to be assessed by the changes in the hypothetic contact angle ( * ) and the actually measured contact angle (), as shown in Fig. 5(a). 3.2. TiC 3.2.1. Characterization of the as-polished TiC substrate Fig. 6(a) shows the XPS spectrum of the as-polished TiC substrate. Ti 2p, C1s and O1s peaks were clearly identified. Their detailed information is presented in Fig. 6(b–d). In the Ti 2p spectrum, the peaks with binding energies of 454.6 eV and 460.6 eV correspond to the Ti–C bond while the other one at 458.3 eV to the Ti–O bond, possibly coming from the slight oxidation of the TiC surface [24]. For the C1s spectrum, the primary peak at 284.6 eV corresponds to the C–C bond and the second one at 281.2 eV to the
C–Ti bond. For the O1s spectrum, the peak with the binding energy of 530.6 eV corresponds to the formation of the O–Ti bond. Based on the XPS analysis, we may conjecture that the TiC substrate surface was either slightly oxidized or covered by a thin film of TiO2 . 3.2.2. Wetting and evaporation behaviors Fig. 7(a) shows the variations in contact angle with time for the molten Mg drops on the TiC surfaces at 973–1173 K. As can be seen, the initial contact angles show an appreciable decrease with increasing temperature, which change from 74◦ at 973 K to 60◦ at 1173 K. It is apparent that these initial contact angles are much smaller than those reported by Contreras et al. [6], which are approximately 120◦ at temperatures 1073–1173 K, as we have mentioned before. The disparity in the results could be mainly attributed to the different sessile drop methods employed by us and by them. In this study, we used the improved sessile drop method, which favors minimizing the effect of the Mg surface oxidization by mechanically disrupting the oxide film and thus yields much smaller contact angles. On the other hand, the contact angles show a rapid decrease in the initial tens of seconds and then a progressive decrease with time until the Mg drop completely disappears through evaporation. For an illustration, Fig. 7(b) shows the representative variations in , d and h at 1073 K. Two characteristic stages can be distinguished: (I) an initial decrease in h and while an increase in d; (II) an almost linear decrease in h and while a nearly constant d, suggesting the pinning of the triple line. The decrease in the contact angle is more marked in stage (I) than in stage (II) because of the slight spreading, which is attributed to the reaction of the surface film, TiO2 , with molten Mg, i.e., 2Mg + TiO2 = 2MgO + Ti.
(6) (G60 )
The changes in the Gibbs free energy were between −224.6 kJ mol−1 and −213.7 kJ mol−1 at 973–1173 K [21]. This reaction-induced spreading was validated by Kondoh et al. [22] in their study on the wetting of oxidized Ti by molten Mg. The disruption of the oxide film, which was even very thin, substantially promoted the wetting. Similar situations were also observed for SiC and Si3 N4 substrates. Because of the coverage of a thin film of SiO2 , the wetting by molten Al or Si is significantly different from their cleaning surfaces [25,26]. On the other hand, the decreasing rate in stage (II) is larger than that observed by Contreras et al. [6] at the same temperature. In the latter, the contact angles decreased slowly at 1023 K and 1123 K but dramatically at 1173 K. Obviously, the oxide film on the Mg drop surface has a significant effect on the wetting behavior at low temperatures. However, even if the oxide film was present on the molten Mg surface, the evaporation of Mg was still in progress because of the incompact nature of the MgO film. A higher temperature increases the Mg vapor pressure, helps to disrupt and remove the oxide film, and thus promotes the wetting. In this sense, the non-wetting ( > 90◦ ) at temperatures between 1073 K and 1123 K and the transition from non-wetting to wetting with increasing temperature, as reported by Contreras et al. [6], were attributed to the presence of the oxide film on the Mg drop surface and then removal of the oxide film. As proved in this study, the Mg/TiC system is partial wetting (0◦ < < 90◦ ) in nature. 3.3. Graphite According to the annotation in the ASM binary alloy phase diagram for the Mg–C system [27], there is no Mg–C compounds at temperatures higher than 933 K and negligible solubility of C in Mg. Consequently, the wetting in this system is very simple and the experiments were stopped after a short time to alleviate the pollution of the chamber by the evaporation of Mg. Fig. 8 shows the variations in contact angle with time for the molten Mg drops on
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Fig. 5. (a) Variations in the hypothetic contact angle ( * ) and actual contact angle () with time at 1073 K; (b) Variation in the relative contributions of the spreading ( 0 − * ) and the evaporation ( * − ) to the total decrease in the contact angle ( 0 − ) with time at 1073 K.
Fig. 6. (a) XPS spectrum of the as-polished TiC substrate surface; (b–d) Ti 2p, C1s and O1s core level spectra and their corresponding fittings.
Fig. 7. (a) Variations in the contact angle with time for the Mg drops on the TiC surfaces at different temperatures; (b) Variations in contact angle (), contact diameter (d) and drop height (h) with time at 1073 K.
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Fig. 8. Variations in the contact angle with time for the Mg drops on the graphite surfaces at isothermal temperatures between 973 K and 1173 K.
Fig. 10. Variations in the measured initial contact angle with temperature for molten Mg on the TiC, SiC, B4 C and graphite substrate surfaces.
the graphite surfaces at 973–1173 K. The initial contact angles are between 142◦ and 124◦ , decreasing with increasing temperature, suggesting that the Mg/graphite system is non-wetting. In comparison, the initial contact angle at 973 K obtained in this study is larger than that reported by Shinozaki et al. [8], which is approximately 125◦ . We attributed this difference to the possible effect of the porosity in the graphite substrates. Supposing that the contact angle between the Mg drop and the Ar atmosphere in the open pores of the substrate surface is 180◦ , according to the Cassie equation [28],
for the drops with identical weights but much larger contact angles. Birdi et al. [29] have demonstrated that the rate of evaporation of sessile drops was dependent on the radius of the solid–liquid interface (i.e., contact radius), rather than on the liquid surface area. Fig. 9 shows the evaporation rates (dV/dt) of the Mg drops with almost identical weights on the TiC, B4 C and graphite surfaces at 1173 K. The smaller the initial contact angle, the larger the contact radius, and the rapider the evaporation. Consequently, the evaporation of the Mg drop, when it was resting on the TiC surface, was larger than that on the B4 C surface, which again than on the graphite surface. On the other hand, because of the high porosity of the graphite substrates, the atmosphere enclosed in the open pores may aggravate the oxidation of the Mg drop, especially at low temperatures, which can also decelerate the evaporation rate.
cos c = cos − f (cos + 1)
(7)
where f is the area percentage of the open pores in the substrate surface, c and are the apparent contact angle for the composite surface and the real contact angle for the dense graphite, respectively, we can deduce that the higher the porosity, the larger the apparent contact angle. On the other hand, the much smaller initial contact angle (74◦ at 973 K) reported by Shi et al. [9] is unbelievable. We speculate that their measurement might be largely affected by the serious pollution of the graphite surface by the Mg vapor. In their experiments, they pre-placed a magnesium ingot on a carbon supporter near the graphite substrate to reduce the oxygen partial pressure in the chamber. Under this circumstance, the Mg vapor would deposit on the graphite substrate surface during heating and thus lead to the much smaller contact angles. Another distinct feature in Fig. 8 is that the contact angle decreases with time at a much slower rate as compared with that in Figs. 4(a) and 7(a). The slower decreasing rate is primarily attributed to the smaller contact radius
Fig. 9. Variations in the drop volume, V, with time for the molten Mg drops with almost identical initial weights, m0 , on the graphite, B4 C and TiC substrates at 1173 K.
3.4. Comparison of the wettability and adhesion In a previous paper [23], we have suggested that the initial contact angle should be used to characterize the wettability in the systems with an evaporating component. In the present Mg/TiC and Mg/B4 C systems, although the initial contact angles could be affected by the impurities at the substrate surfaces, they are comparatively more reasonable than the advancing and receding contact angles since the latter are either affected by the interfacial reaction or by the evaporation. The interfacial reaction may lead to the intrinsic contact angle by removal of the oxide film, making molten Mg contact with the true substrate surface;
Fig. 11. The work of adhesion, Wad , for molten Mg on the TiC, SiC, B4 C and graphite substrates at different temperatures.
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(2) The chemical reaction between Mg and the impurity of free boron at the B4 C surface promotes the wettability in this system to a certain degree. (3) A simple model based on the compensation of the lost volume to the evaporating drop and the assumption of a spherical cap geometry could be used to estimate the hypothetic contact angle, precluding the effect of the Mg evaporation, and further to evaluate the complex wetting behavior coupled with the evaporation and chemical reaction. (4) The interfacial bonding in the Mg/TiC and Mg/B4 C systems is chemical while that in the Mg/graphite system is physical in nature. Acknowledgement
Fig. 12. Calculated values of Wad /Wc(l) for molten Mg on the TiC, SiC, B4 C and graphite substrates at different temperatures.
however, under the circumstance of concurrent evaporation, particularly at high temperatures, it is difficult to determine which is the closest one to the intrinsic contact angle since the intense evaporation could mask the effect of the reaction. Accordingly, here the initial contact angles for the above systems together with those for the previous Mg/SiC system [7] are gathered in Fig. 10 to qualitatively compare their wettability. Note that the initial contact angles vary only slightly with temperature but relatively largely with the substrate type. For the graphite substrates, the initial contact angles at various temperatures are larger than 90◦ , while for TiC and SiC, they are smaller than 90◦ . This result clearly shows that the former is non-wettable while the latter two are partially wettable by molten Mg at 973–1173 K. For the B4 C substrates, however, a transition from non-wetting to wetting occurs with the increase in the temperature. The work of adhesion, Wad , representing the extent of the bonding strength between a liquid and a solid, can be calculated using the Young–Dupre equation Wad = sg + lg − sl = lg (1 + cos )
(8)
where sg , lg and sl are the solid–gas, liquid–gas and solid–liquid interfacial free energies, respectively, and , here, is represented by the initial contact angle. The values of lg are obtained from Ref. [30]. The calculated results of Wad are given in Fig. 11, which are in the sequence of Wad (TiC) > Wad (SiC) > Wad (B4 C) > Wad (graphite). In addition, if one takes the empirical relationship of Wad /Wc(l) ≤ 0.2 (where Wc(l) is the cohesion of liquid and is equal to 2lg ) as a typical characteristic of physical interaction, while Wad /Wc(l) > 0.2 as chemical interaction [31], one may infer from Fig. 12 that the interaction between Mg and the B4 C, SiC and TiC surfaces is chemical while that between Mg and graphite is physical. 4. Conclusions (1) The initial contact angles of molten Mg on the B4 C, TiC and graphite substrates at 973–1173 K are in the ranges of 95–87◦ , 74–60◦ and 142–124◦ , respectively, decreasing with increasing temperature. Accordingly, the Mg/graphite system is nonwetting, the Mg/TiC system is partial wetting and the Mg/B4 C system changes from non-wetting to wetting with increasing temperature.
This work is supported by National Natural Science Foundation of China (no. 50871045), the Key Project of Chinese Ministry of Education (no. 108043) and Graduate Innovation Fund of Jilin University (no. 20111047). We appreciate the anonymous reviewers for their critical reading of the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16]
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
A. Contreras, V.H. Lopez, E. Bedolla, Scripta Mater. 51 (2004) 249–253. V. Kevorkijan, S.D. Skapin, MJoM 15 (2009) 3–18. R.A. Saravanan, M.K. Surappa, Mater. Sci. Eng. A 276 (2000) 108–116. C. Korner, W. Schaff, M. Ottmuller, R.F. Singer, Adv. Eng. Mater. 2 (2000) 327–337. P. Shen, D. Zhang, Q.L. Lin, L.X. Shi, Q.C. Jiang, Mater. Chem. Phys. 122 (2010) 290–294. A. Contreras, C.A. Leon, R.A.L. Drew, E. Bedolla, Scripta Mater. 48 (2003) 1625–1630. D. Zhang, P. Shen, L.X. Shi, Q.L. Lin, Q.C. Jiang, Appl. Surf. Sci. 256 (2010) 7043–7047. N. Shinozaki, J. Morita, K. Wasai, J. Japan Inst. Met. 55 (2005) 310–314. W. Shi, M. Kobasi, T. Choh, J. Japan Inst. Met. 64 (2000) 335–338. V. Kevorkijan, S.D. Skapin, J. Adv. Mater. 42 (2010) 35–47. W. Ke, E.H. Han, Y.F. Han, K. Kainer, A.A. Luo, Mater. Sci. Forum 488–489 (2005) 741–744. P. Shen, Q.L. Lin, Q.C. Jiang, H. Fujii, K. Nogi, J. Mater. Res. 24 (2009) 2420–2427. P. Shen, H. Fujii, T. Matsumoto, K. Nogi, J. Am. Ceram. Soc. 87 (2004) 2151–2159. W. Cermignani, T.E. Paulson, C. Onneby, C.G. Pantano, Carbon 33 (1995) 367–374. C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble Jr., NIST X-ray Photoelectron Spectroscopy Database, Version 3. 5, National Institute of Standards and Technology, Gaithersburg, 2003. T. Hu, L. Steihl, W. Rafaniello, T. Fawcett, D.D. Hawn, J.G. Mashall, S.J. Rozeveld, C.L. Putzig, J.H. Blackson, W. Cermignani, M.G. Robinson, Thin Solid Films 332 (1998) 80–86. N.I. Medvedeva, L.D. Finkel’shteˇin, S.N. Shamin, I.I. Lyakhovskaya, É.Z. Kurmaev, Phys. Solid State 46 (2004) 1994–1997. L.J. Wu, Y.M. Zhu, T. Vogt, H.B. Su, J.W. Davenport, Phys. Rev. B 69 (2004) 064501. S.F. Wang, Y.L. Zhou, Y.B. Zhu, Q. Zhang, S.Y. Dai, K. Xie, Z.H. Chen, H.B. Lu, G.Z. Yang, Phys. C 390 (2003) 1–5. A. Talapatra, S.K. Bandyopadhyay, P. Sen, P. Barat, S. Mukherjee, Phys. C 419 (2005) 141–147. I. Barin, Thermochemical Data of Pure Substances, 3rd ed., Wiley-VCH Verlag GmbH, Weinheim, Germany, 1995. K. Kondoh, M. Kawakami, H. Imai, J. Umeda, H. Fujii, Acta Mater. 58 (2010) 606–614. D. Zhang, L.X. Shi, P. Shen, Q.L. Lin, Q.C. Jiang, Appl. Phys. A 98 (2010) 601–607. A. Mari, P. Aubert, F. Mercier, H. Khodja, C. Berthier, P. Houdy, Surf. Coat. Technol. 194 (2005) 190–195. V. Laurent, C. Rado, N. Eustathopoulos, Mater. Sci. Eng. A 205 (1996) 1–8. B. Drevet, R. Voytovych, R. Israel, N. Eustathopoulos, J. Eur. Ceram. Soc. 29 (2009) 2363–2367. T.B. Massalski, Binary Alloy Phase Diagrams (CD-ROM), ASM International, 1996. A.B.D. Cassie, Discuss. Faraday Soc. 3 (1948) 11–16. K.S. Birdi, D.T. Wu, A. Winter, J. Phys. Chem. 93 (1989) 3702–3703. B.J. Keene, Int. Mater. Rev. 38 (1993) 157–192. N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Elsevier Science, Oxford, UK, 1999.