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Wetting of polycrystalline SiC by molten Al and Al−Si alloys
Applied Surface Science 317 (2014) 140–146
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Wetting of polycrystalline SiC by molten Al and Al−Si alloys Xiao–Shuang Cong, Ping Shen ∗ , Yi Wang, Qichuan Jiang Key Laboratory of Automobile Materials (Ministry of Education), School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, P R China
a r t i c l e
i n f o
Article history: Received 24 March 2014 Received in revised form 22 July 2014 Accepted 11 August 2014 Available online 19 August 2014 Keywords: Wetting Interfaces Microstructures
a b s t r a c t The wetting of ␣-SiC by molten Al and Al–Si alloys was investigated using a dispensed sessile drop method in a high vacuum. In the Al–SiC system, representative wetting stages were identified. The liquid spreading was initially controlled by the deoxidation of the SiC surface and then by the formation of Al4 C3 at the interface. The intrinsic contact angle for molten Al on the polycrystalline ␣-SiC surface was suggested to be lower than 90◦ provided that the oxide films covering the Al and SiC surfaces were removed, i.e., the system is partial wetting in nature. An increase in the Si concentration in liquid Al weakened the interfacial reaction but improved the final wettability. The role of the Si addition on the wetting was presumably attributed to its segregation at the interface and the formation of strong chemical bonds with the SiC surface. © 2014 Elsevier B.V. All rights reserved.
1. Introduction SiC-reinforced aluminum-matrix composites are widely used in the industry due to their low density, high thermal conductivity, good mechanical properties, and low fabrication cost. In the fabrication of the SiC-reinforced Al-matrix composites, the wettability of SiC by liquid Al or Al-base alloys is of vital importance [1]. However, it is usually reported to be poor at relatively low temperatures (T < 1173 K). Increasing temperature would improve it but give rise to the formation of an Al4 C3 phase at the interface, which is brittle and prone to hydrolyze, and thus greatly deteriorates the property of the composites [2]. In order to produce the SiCp /Al composites with good performance, it is necessary to improve the wettability and simultaneously control the formation of Al4 C3 . The wettability of SiC by liquid Al has been studied by many researchers [1–8]. As indicated in Fig. 1, an increase in temperature significantly improves the wettability. Nevertheless, the reported contact angles are rather scattered presumably because of complex influencing factors such as atmosphere, different types of the substrates and measuring techniques. For instance, Laurent et al. [1,2] reported that the wettability of ␣-SiC single crystals by molten Al improved with increasing temperature as the contact angles decreased from 106◦ to 58◦ in a vacuum less than 5 × 10−5 Pa. They divided the entire wetting process into three stages and suggested that the first and second stages should be controlled by
∗ Corresponding author. Tel.: +86 431 8509 5326; fax: +86 431 8509 4699. E-mail addresses: [email protected], [email protected] (P. Shen). http://dx.doi.org/10.1016/j.apsusc.2014.08.055 0169-4332/© 2014 Elsevier B.V. All rights reserved.
deoxidation of the Al and SiC surfaces, respectively, and the formation of Al4 C3 at the interface should not play a significant role in promoting the wettability in this system. Similar work was done by Bao et al. [7] at 1273–1573 K in a vacuum of 10−3 Pa, but they reported much larger contact angles presumably due to poor vacuum. On the other hand, Han et al. [5] reported smaller contact angles in a poor vacuum (10−2 − 10−3 Pa) using reaction-bonded SiC substrates. The presence of 18 wt% free silicon in the substrates significantly reduced the contact angle (by a factor of about 2.8 as compared with the values for Al on the SiC substrates that were free of Si), and therefore, the measured small values do not represent for the true Al–SiC system. Other researchers reported much larger contact angles, as shown in Fig. 1, basically because of the presence of an oxide film at the Al drop surface, particularly at relatively low temperatures (T ≤ 1200 K). It is worth mentioning that the oxidation of the Al drop surface is almost inevitable, since from the viewpoint of thermodynamics the critical oxygen partial pressure in the furnace to keep Al clean should be very low, e.g., 10−44 Pa at 973 K. Only at a relatively high temperature (usually T > 1200 K) and in a high vacuum is the oxide film likely to be disrupted by a self-cleaning reaction (4Al(l) + Al2 O3(s) = 3Al2 O(g) ). The deoxidation temperature in practice depends to a great extent on the thickness of the oxide film covering the Al surface. Note that all the results shown in Fig. 1 were obtained using a conventional sessile drop method, in which the Al sample was preplaced on the SiC substrate and then the couple was heated together to the testing temperature, even though the detailed procedure was somewhat different in different studies. In this case, the oxide film covering the Al surface was relatively thick and difficult to eliminate. Therefore, the
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2. Experimental procedure
Fig. 1. Comparison of the collected quasi-equilibrium or equilibrium contact angles for the Al/SiC system as a function of temperature [1–8]. SC—single crystal SiC; S—sintered SiC; RB—reaction-bonded SiC.
wetting and spreading dynamics were greatly affected by this oxide film. In order to improve the wettability and inhibit the formation of Al4 C3 , alloying Al with Si is an effective method. However, the results concerning the influence of Si addition on the wettability of silicon carbide by liquid Al are quite inconsistent and even contradictory. For instance, Laurent et al. [1], in 1987, reported that the addition of 5–12 wt% Si to Al had a negligible effect on the wettability of ␣-SiC single crystals at 973–1173 K, while the addition of 18 wt% Si slightly improved it and inhibited the formation of Al4 C3 . But in a later study [2], they found that the addition of 20 wt% Si in Al hardly improved the wettability at temperatures lower than 1200 K. Therefore, they argued that the Si addition should not have a noticeable effect on the wettability in the Al/SiC system. In contrast, some researchers believed that the addition of Si should improve the wettability in the Al/SiC system. For instance, Choh and Oki [9] found that the addition of Si to Al decreased the incubation period of the wetting at temperatures between 1173 and 1373 K. Ferro and Derby [6] also claimed that the Si addition was beneficial to the wetting and the best wettability appeared for the Al–Si alloys close to the eutectic composition. Nevertheless, they attributed the good wettability in the Al–Si/SiC system to the dissolution and reconstruction of the SiC surface, which was questionable since the addition of Si to Al weakened or even inhibited the SiC dissolution. As exemplified, despite the fact that the wetting of SiC by Al and Al–Si alloys has been widely investigated, the intrinsic wettability and the role of Si are still ambiguous. The primary purpose of this study is thus to probe into these questions.
The substrates used were pressureless—sintered polycrystalline ␣-SiC with a purity of 98.5 wt%, a density of 3.1 g/cm3 and dimensions of 20 × 5 mm2 . They were mechanically polished using diamond pastes with sizes down to 0.5 m to average roughness (Ra ) of 20 nm, as measured by DEKTAK 6 M (Veeco Metrology Corp. USA) over a distance of 2 mm. The presence of a thin SiO2 film on their surface was previously confirmed by X-ray photoelectron spectroscopy (XPS) examination [10]. The Al–Si alloys with Si concentrations of 7 and 12 wt% were prepared by arc-melting of high-purity Al (99.99 wt%) and Si (99.9999 wt%) in a Ti (99.8 wt%)gettered argon (99.999% purity) atmosphere. Wetting experiments were performed in a high vacuum (∼3 × 10−4 Pa) using a dispensed sessile drop method, in which liquid Al or Al–Si alloy was extruded through a narrow orifice at the bottom of an alumina tube (99.6 wt%) and then rested on the SiC surface. In this case, the initial oxide film covering the Al or Al–Si surface was mechanically removed and the contact angles as well as the spreading dynamics measured were more accurate. Detailed description of the experimental apparatus and procedure was given elsewhere [11]. The contact angle () and drop base diameter were directly measured from drop profiles using an axisymmetric drop shape analysis program. After the wetting experiments, in order to expose the interface beneath the drop and simultaneously to prevent the hydrolysis of Al4 C3 , the solidified Al and Al–Si drops were dissolved in saturated NaOH distilled-water solution after they were mechanically removed in most part. The interfacial microstructures were observed using a scanning electron microscope (SEM, Evo 18, Carl Zeiss, Germany) equipped with an energy dispersive spectrometer (EDS).
3. Results and discussion 3.1. Wetting in the Al/SiC system Fig. 2(a) shows the variations in the contact angle with time for molten Al on the SiC substrates during isothermal dwells at 973–1273 K. As indicated, the spreading was remarkably accelerated and the wettability improved with the increase in temperature. The initial contact angles, which represent the wettability of the oxidized SiC surface, i.e., a SiO2 film by liquid Al, decreased from 125◦ to 105◦ with increasing temperature, as more clearly shown in Fig. 2(b). The “pseudo–equilibrium” contact angles after wetting for 2 h (except at 1273 K) decreased from 95◦ to 57◦ . At T ≤ 1073 K, the contact angle remained constant after an initial rapid decrease, suggesting that the interfacial reaction between Al and SiC in this temperature range should not be very significant. However, increasing temperature to 1123 K led to a continuous
Fig. 2. Variations in (a) contact angle with time at different temperatures and (b) the initial and final contact angles with temperature for molten Al on the SiC substrates.
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Fig. 3. Representative wetting stages in the Al/SiC system at 1173 K.
decrease in the contact angle and a transition from non-wetting to wetting. At higher temperatures, the wetting was fast and the drop reached a final equilibrium state at a relatively short time. A general form of the variations in contact angle and drop base diameter is shown in Fig. 3. The wetting behavior can be characterized by three stages, similar to that reported by Laurent et al. [2]. However, Laurent et al. suggested that the first and second wetting stages should be controlled by deoxidation of the Al and SiC surfaces, respectively, and the third stage associated with the transition from the Al/SiC interface to an Al–Si/Al4 C3 interface. Note that the contact angle of 110◦ at the end of the first stage observed by Laurent et al. [2] is in good agreement with the initial contact angle of 109◦ measured in this study. Because of using the dispensed sessile drop technique, the oxide film covering the Al surface was removed during liquid extrusion. Namely, the first spreading stage observed in this study corresponds to the second stage suggested by Laurent et al. [2], which was controlled by the deoxidation of the SiC surface. In stage (II), the spreading kinetics, as shown by the blue lines in Fig. 4(a)−(c), seems to be reaction-controlled since the following model is well fit [12] cos e − cos d = (cos e − cos 0 ) exp(−kr t),
(1)
where e , d , and 0 are the equilibrium, dynamic, and initial contact angles in this stage, respectively, and kr is a dynamic constant, representing spreading rate in its physical meaning. Using the temperature-dependent spreading rates (i.e., the slopes of the fitting curves) and following the Arrhenius law, the activation energy in this linear spreading stage was calculated to be 302 kJ mol−1 , as shown in Fig. 4(d), close to the Si C bonding energy, which is 327 kJ mol−1 [9], implying that the spreading in stage (II) might have an inherent relation to the rupture of the Si C bonds in the SiC substrate. A similar conclusion was also drawn by Choh and Oki [9], who measured the wetting rates at different temperatures using a dip coverage method and calculated the wetting activation energy of 330 kJ mol−1 for the Al–SiC system. During the wetting, Al4 C3 formed readily at the interface because of very limited solubility of C in Al [13] while Si was displaced and then dissolved in liquid Al as the heterogeneous chemical reaction (4Al + 3SiC = Al4 C3 + 3[Si]) proceeded. Therefore, the wetting process in stage (II) was controlled by the formation of Al4 C3 at the interface. In stage (III), the spreading rate decreased dramatically and the change in the contact angle was limited. As indicated in Fig. 5, the central interface was covered by the Al4 C3 particles; whereas, at the triple line region, the distribution of the Al4 C3 phase was discrete due to short contact time (because the triple line was always moving during the isothermal dwell for 60 min). Therefore, we may infer that the formation of Al4 C3 promoted the wettability and the Al drop indeed spread on a SiC + Al4 C3 composite surface in stage (III) until it reached an equilibrium state, in which the contact angle was close to 55◦ (for Al on an Al4 C3 surface as reported by Ferro and Derby [6]). According to the above analysis, it is clear that the wetting in the Al–SiC system is influenced by the presence of the oxide film at the Al or/and SiC surface(s) in the early stage and then by the interfacial reaction after the disruption of the oxide film. These two stages are virtually overlapped in time and there is no clear boundary between them, making the cognition of the intrinsic wettability in the Al–SiC system rather difficult. As indicated in Fig. 1, a large discrepancy exists in the reported equilibrium contact angles. The large contact
Fig. 4. ln(cos e −cos d ) vs. time at temperatures (a) 1123 K, (b) 1173 K, and (c) 1273 K, respectively; (d) Arrhenius plot of lnkr against 104 /T for calculation of the activation energy of the spreading (Ea ) in the second stage
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Fig. 5. Exposed interfacial microstructures (a) at the triple line region and (b) at the central interface for molten Al on the SiC substrate after wetting at 1173 K for 60 min.
angles ( > 120◦ ) are obviously influenced by the oxidation of the Al or/and SiC surface(s), while the transition from non-wetting to wetting with increasing temperature is most likely due to the disruption of the oxide film, which in turn implies that the Al–SiC couple, if being free of surface oxidation, might be wettable in nature. Instead, Laurent et al. [2] suggested that the equilibrium contact angle for the Al–SiC system should be 55 ± 7◦ at 1150 K. They argued that the distribution of the Al4 C3 phase at the interface was discontinuous for the wetting performed at temperatures no higher than 1173 K, and consequently its formation should not have a significant contribution to the wettability. In our viewpoint, however, the small value (55 ± 7◦ ) is no longer for the Al/SiC system but for the Al/Al4 C3 or Al/(SiC + Al4 C3 ) system. Then, what is the intrinsic wettability for the Al–SiC system? We suggest that it might be characterized by the value at the moment transiting from stage (I) to stage (II). The wetting in stage (I) is obviously related to the reaction between Al and the SiC surface oxide film (SiO2 ). A more clear demonstration is given in Fig. 6. As indicated, the time variations in the contact angle in the early wetting stage are very similar in the Al/pre-oxidized SiC and Al/SiO2 (quartz) systems. However, after the contact angle reaches about 85◦ , the two curves diverge, implying the change in the wetting conditions. Thereafter, the contact angle in the Al–SiC system progressively decreases as a result of the interfacial reaction while that in the Al–SiO2 system remains almost constant (the contact angle is close to the equilibrium value of 85◦ for the Al–Al2 O3 system [14]). Regarding that the spreading in the second stage is reactioncontrolled (see Fig. 4), we can determine the characteristic initial contact angles ( ) using Eq. (1) for characterization of the intrinsic wettability. As shown in Fig. 4(a–c), are in the range of 84–78◦
Fig. 6. Comparison of the variations in the contact angles for pure Al on the SiC and SiO2 substrates at 1173 K.
at 1123–1273 K, slightly decreasing with increasing temperature. Accordingly, we suggest that the clean Al–SiC couple should be partial wetting in nature. At low temperatures (T < 1123 K), however, non-wetting is usually observed because the interfacial reaction is rather mild and the surface oxide film cannot be effectively eliminated. 3.2. Effect of Si addition Fig. 7(a) and (b) show the variations in the contact angle with time for molten Al—7 wt% Si and Al–12 wt% Si alloys on the SiC substrates at 973–1273 K, respectively. A higher temperature significantly decreased the final contact angles and accelerated the spreading. Different wetting behaviors were observed at different temperatures for both Al–7 wt% Si and Al–12 wt% Si alloys. At 973 K, the contact angle decreased almost linearly with time for these two systems. At 1073–1173 K, the contact angle in the Al–7 wt% Si/SiC system experienced a hysteresis stage after rapid spreading; however, such behavior was not observed in the Al–12 wt% Si/SiC system. The wetting hysteresis is a complex phenomenon but could be qualitatively explained by a competition between a driving force and a retarding force. If the retarding force was larger than the driving force, the triple line would be pinned. Otherwise, the drop would keep spreading. Provided that the reaction was not fully inhibited, as indicated in Fig. 8, the retarding force resulted from the rough interface due to the presence of the reaction product Al4 C3 would be large enough to inhibit the movement of the triple line. Increasing temperature favors deoxidization by a self-cleaning reaction between Al and its oxide film in a high vacuum [2] and also accelerated the reaction between Al and SiC. As a consequence, the driving force is large enough to overcome the impedance. In this case, the contact angle hysteresis was inconspicuous. On the other hand, when the interfacial reaction was completely inhibited due to the addition of sufficient amount of Si to Al, as indicated in Fig. 9, the retarding force resulting from the interface roughness was quite small and thus the triple line could move freely up to the equilibrium state. Therefore, the contact angle hysteresis was not observed in the Al–12 wt% Si/SiC system. The equilibrium Si contents required to prevent the formation of Al4 C3 as a function of temperature has been investigated by many researchers [15–19] using both theoretical calculations and experimental method. Lee et al. [19] summarized the relevant results and made some corrections by considering both Al and Si activities in the alloys. They further drew an equilibrium Si-content profile on the Al–Si phase diagram as a function of temperature, as shown in Fig. 10. The experimentally determined equilibrium Si contents show a good agreement with their theoretical calculations in the
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Fig. 7. Variations in the contact angle with time at different temperatures for (a) Al–7 wt% Si and (b) Al–12 wt% Si alloys on the SiC substrates.
Al–Si/SiC system. In this work, we studied the interfacial reaction between two compositions of the Al–Si alloy and the SiC substrates at various wetting temperatures, and the results shown in Figs. 8–9 tally with the equilibrium Si-content profile given in Fig. 10. At higher temperatures (e.g., 1273 K), the liquid drop in both systems showed rapid spreading with the final equilibrium contact angles reaching 50◦ and 32◦ , respectively. Namely, the wettability is better than that in the Al/SiC system, suggesting that the addition of Si does promote the wettability of SiC by liquid Al. The reason for the role of Si remained unclear, as we have mentioned in the introduction. Ferro and Derby [6] attributed the good wettability in the Al–Si/SiC system to the dissolution and reconstruction of the ␣-SiC surface. However, in this study, it is obvious that no significant change occurred at the SiC surfaces after they were wetted by the Al–12 wt% Si alloy at T ≤ 1173 K for a long time (2 h or more), as indicated in Fig. 9. Therefore, new explanations are required to account for the role of Si on the wetting in this system. According to Young’s equation:
cos =
(sv − sl ) , lv
(2)
where sv and sl are the free energies of solid–vapor and solid–liquid interfaces, lv is liquid surface tension, and is equilibrium contact angle, respectively. The wetting improvement can result from the reduction in lv or/and sl . Because lv (Si) < lv (Al), it is reasonable to presume that Si would enrich at the drop surface and thus decrease surface tension of the liquid. Here, we assume that the liquid surface is completely covered by Si while sv and sl remain constant. The equilibrium contact angles for the Al/SiC and Al–Si/SiC systems could be written as cos Al =
(sv − sl ) lv(Al)
cos Al−Si =
(for the Al/SiC system),
(sv − sl ) lv(Si)
(for the Al − Si/SiC system).
(3) (4)
Using the values of Al = 81◦ , lv(Al) = 838 mN m−1 at 1173 K [20] and lv(Si) = 775 mN m−1 at its melting point [21], we obtain Al−Si = 80◦ (the surface tension of Si at 1173 K, if extrapolated from the melting point, will be even larger and so does Al−Si ), which is much larger than 34◦ as shown in Fig. 7(b), indicating that only the reduction in the liquid surface tension by the Si segregation should not be responsible for the small equilibrium contact angles. On the other hand, an alloying element (e.g., Si) can modify the wetting if it
Fig. 8. Interfacial microstructures at the exposed triple line regions for the Al–7 wt% Si/SiC samples tested at (a) 973 K, (b) 1023 K, (c) 1073 K, (d) 1123 K, (e) 1173 K, and (f) 1273 K after removal of the solidified drops. The wetting time is shown in Fig. 7(a) at respective temperature.
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Fig. 9. Interfacial microstructures at the exposed triple line regions for the Al–12 wt% Si/SiC samples tested at (a) 973 K, (b) 1023 K, (c) 1073 K, (d) 1123 K, (e) 1173 K, and (f) 1273 K after removal of the solidified drops. The wetting time is shown in Fig. 7(b) at respective temperature.
significantly decreases sl of the system by adsorption at the interface when the corresponding adsorption energy, Esl , is a large negative quantity. To estimate the adsorption energies at the liquid surface and liquid–solid interface, Elv and Esl , we use a thermodynamic model established by Li et al. [22]:
the exchange energy of the A1–Si alloy modeled as a regular solution equal to the enthalpy of mixing at infinite dilution of Si in Al Si and W Al are the work of adhesion for ( = −9 kJ mol−1 [23]), Wad ad pure Si and Al on the SiC substrates. Again, Sm can be evaluated using the following relationship [24]:
Elv = (lSiv − lAl v )Sm − m,
(5)
1 2 Sm = cN ⁄3 VSi ⁄3 ,
(6)
where c is a geometric factor (1.091 for a close-packed lattice), N is Avogadro’s number, and VSi is the molar volume of Si (VSi = 12.06 × 10−6 m3 mol−1 ). Therefore, Sm = 4.8 × 104 m2 mol−1 . Si and W Al can be calculated from Wad ad
Esl =
Al Elv + (Wad
Si − Wad )Sm ,
where Sm is the interface area occupied by a monolayer of liquid atoms, m is a structural parameter roughly equal to 1/4, is
Fig. 10. Equilibrium Si-content profile in the Al–Si/SiC system determined by Lee et al. [19] from both theoretical calculations and experimental measurements. Two compositions of the Al–Si alloys adopted in this study are superposed on this profile to show the reaction conditions at various wetting temperatures. The squares located in Region (I) mean that the formation of Al4 C3 is inhibited while those in Region (II) mean that the interfacial reaction can happen.
(7)
Si Wad = lSiv (1 + cos 1 ),
(8)
Al Wad = lAl v (1 + cos 2 ),
(9)
where 1 is the contact angle for the Si/␣-SiC system ( 1 = 38◦ [25]), and 2 is the intrinsic contact angle for the Al/SiC system ( 2 = 81◦ at 1173 K), respectively. As a consequence, we obtained Elv = −0.774 kJ mol−1 and Esl = −20.94 kJ mol−1 . The negligible negative value of Elv indicates that the addition of Si to A1 could have a reduction of the liquid surface tension but in a rather small quantity; whereas, a relatively large negative value of Esl suggests that Si should segregate at the solid–liquid interface to considerably reduce the interfacial free energy and thus improve the wetting. It is worth pointing out that Laurent et al. [2] argued that the adsorption of Si at the solid–liquid interface was insignificant based on their estimation of a negligible value of Esl . However, unfortunately, they used a contact angle of 60◦ for the Al/SiC system in their Al . This value, in our viewcalculation of the work of adhesion, Wad point, represents for the Al/Al4 C3 or Al/(Al4 C3 + SiC) system rather than for the Al/SiC system. The intrinsic contact angles for the Al/SiC system are likely to be 84–78◦ at 1123–1273 K, as we have argued before. Finally, the segregation of Si at the solid–liquid interface and its role in promoting the wetting have been reported by several researchers [26,27]. For instance, Drevet et al. [26] investigated the wettability in the Au–Si/SiC system and attributed the decrease in
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the contact angle from 138 ± 2◦ for pure Au to 13 ± 2◦ for Au–45 wt% Si to the adsorption of Si at the Au/SiC interface. They argued that in solid silicon, bonds were covalent but at melting, molten Si became a metallic conductor (delocalized bond). Thus, a strong Au–Si/SiC interface could be developed due to the formation of strong chemical bonds localized at the interface. Using ab-initio calculations, Sabisch et al. [28] suggested that Si should adsorb at the ¯ SiC surface because both (0001)-Si and (0001)-C faces covered by layers of chemisorbed Si had smaller energy than the ideal (0001)¯ Si and (0001)-C faces. Based on these viewpoints, we presume that Si could segregate at the Al–SiC interface and form strong chemical bonds with the SiC surface, which plays a significant role in promoting the final wettability. 4. Conclusions (1) Wetting of polycrystalline SiC by molten Al was initially controlled by the deoxidation of the SiC surface and then by the formation of Al4 C3 at the interface. The intrinsic contact angles for the clean Al–SiC couples were presumed to be 84–78◦ at 1123–1273 K, slightly decreasing with increasing temperature, suggesting that the system is partial wetting in nature. Much larger contact angles reported in literature result mainly from the oxidation at either the Al or SiC surface or both their surfaces. (2) Formation of Al4 C3 at the interface greatly promotes the wettability but it is not a necessary condition. In the case of the interfacial reaction being completely inhibited due to the addition of sufficient amount of Si to Al, the wettability could be substantially improved as well. The mechanism is presumably attributed to the segregation of Si at the interface and the formation of strong chemical bonds with the SiC surface. Acknowledgments This work is supported by National Basic Research Program of China (973 Program) (No.2012CB619600) and National Natural Science Foundation of China (No. 51271085). References [1] V. Laurent, D. Chatain, N. Eustathopoulos, Wettability of SiC by aluminum and Al–Si alloys, J. Mater. Sci. 22 (1987) 244–250. [2] V. Laurent, C. Rado, N. Eustathopoulos, Wetting kinetics and bonding of Al and Al alloys on ␣-SiC, Mater. Sci. Eng. A. 205 (1996) 1–8. [3] W. Köhler, Investigations into the wetting of Al2 O3 and SiC crystals by aluminum and aluminum alloys, Aluminum 51 (1975) 443–447. [4] M. Shimbo, Wettability of silicon carbide by aluminum, copper and silver, J Mater. Sci. Lett. 8 (1989) 663–666.
[5] D.S. Han, H. Jones, H.V. Atkinson, The wettability of silicon carbide by liquid aluminum: the effect of free silicon in the carbide and of magnesium, silicon and copper alloy additions to the aluminum, J. Mater. Sci. 28 (1993) 2654–2658. [6] A.C. Ferro, B. Derby, Wetting behavior in the Al–Si/SiC System: interface reactions and solubility effects, Acta Metall. Mater. 43 (1995) 3061–3073. [7] S. Bao, A. Kvithyld, T.A. Engh, M. Tangstad, Wetting of pure aluminum on graphite, SiC, and Al2 O3 in aluminium filtration, Trans. Nonferrous Met. Soc. China 22 (2012) 1930–1938. [8] E. Candan, Effect of alloying elements to aluminium on the wettability of Al/SiC system, Turkish J. Eng. Env. Sci. 26 (2002) 1–5. [9] T. Choh, T. Oki, Wettability of SiC to aluminum and aluminum alloys, Mater. Sci. Technol. 3 (1987) 378–385. [10] D. Zhang, P. Shen, L.X. Shi, Q.L. Lin, Q.C. Jiang, Wetting and evaporation behaviors of molten Mg on partially oxidized SiC substrates, Appl. Surf. Sci. 256 (2010) 7043–7047. [11] L.Y. Zhang, P. Shen, Y. Qi, Q.C. Jiang, Wettability in reactive Sn-base alloy/Ni-base metallic glass systems, Appl. Surf. Sci. 276 (2013) 424–432. [12] O. Dezellus, F. Hodaj, N. Eustathopoulos, Chemical reaction-limited spreading: the triple line velocity versus contact angle relation, Acta Mater. 50 (2002) 4741–4753. [13] G. Carotenuto, A. Gallo, L. Nicolais, Degradation of SiC particles in aluminumbased composites, J. Mater. Sci. 29 (1994) 4967–4974. [14] P. Shen, H. Fujii, T. Matsumoto, K. Nogi, The influence of surface structure on wetting of ␣-Al2 O3 by aluminum in a reduced atmosphere, Acta Mater. 51 (2003) 4897–4906. [15] J. Narciso, C. García-Cordovilla, E. Louis, Reactivity of thermally oxidized and unoxidized SiC particulates with aluminium–silicon alloys, Mater. Sci. Eng. B. 15 (1992) 148–155. [16] D.J. Lloyd, H. Lagace, A. Mcleod, P.L. Morris, Microstructural aspects of aluminum–silicon carbide particulate composites produced by a casting method, Mater. Sci. Eng. A107 (1989) 73–80. [17] T. Iseki, T. Kameda, T. Maruyama, Interfacial reactions between SiC and aluminum during joining, J. Mater. Sci. 19 (1984) 1692–1698. [18] H. Nakae, K. Yamamoto, K. Sato, Measurement of wetting of graphite by Al and Al–Si alloys using meniscography, Mater. Trans. JIM 32 (1991) 531–538. [19] J.C. Lee, S.B. Park, H.K. Seok, C.S. Oh, H.I. Lee, Prediction of Si contents to suppress the interfacial reaction in the SiCp/2014 Al composite, Acta Mater. 46 (1998) 2635–2643. [20] J.M. Molina, R. Voytovych, E. Louis, N. Eustathopoulos, The surface tension of liquid aluminum in high vacuum: the role of surface condition, Int. J. Adhes. 27 (2007) 394–401. [21] B.J. Keene, Review of data for the surface tension of pure metals, Inter. Mater. Rev. 38 (1993) 157–192. [22] J.G. Li, L. Coudurier, N. Eustathopoulos, Work of adhesion and contact angle isotherm of binary alloys on ionocovalent oxides, J. Mater. Sci. 24 (1989) 1109–1116. [23] N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at High Temperatures, Pergamon, Elsevier Science, Oxford, UK, 1999. ¯ ␣[24] P. Shen, H. Fujii, K. Nogi, Wetting adhesion and adsorption in Al–Si/(0112) alumina system at 1723 K, Mater. Trans. 45 (2004) 2857–2863. [25] J.V. Naidich, The wettability of solids by liquid metals, in: D.A. Cadenhead, J.F. Danielli (Eds.), Progress in Surface and Membrane Science, Academic Press, NY, 1981, p. 353. [26] B. Drevet, S. Kalogeropoulou, N. Eustathopoulos, Wettability and interfacial bonding in Au–Si/SiC system, Acta Metall. 41 (1993) 3119–3126. [27] C. Rado, S. Kalogeropoulou, N. Eustathopoulos, Wetting and adhesion in metalsilicon carbide systems: the effect of surface polarity of SiC, Scripta Mater. 42 (2000) 203–208. [28] M. Sabisch, P. Kruger, J. Pollmann, Ab-initio calculations of structural and electronic properties of 6H-SiC (0001) surfaces, Phys. Rev. B: Condens. Matter. 55 (1997) 10561–10570.
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Wetting of polycrystalline SiC by molten Al and Al−Si alloys Xiao–Shuang Cong, Ping Shen ∗ , Yi Wang, Qichuan Jiang Key Laboratory of Automobile Materials (Ministry of Education), School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, P R China
a r t i c l e
i n f o
Article history: Received 24 March 2014 Received in revised form 22 July 2014 Accepted 11 August 2014 Available online 19 August 2014 Keywords: Wetting Interfaces Microstructures
a b s t r a c t The wetting of ␣-SiC by molten Al and Al–Si alloys was investigated using a dispensed sessile drop method in a high vacuum. In the Al–SiC system, representative wetting stages were identified. The liquid spreading was initially controlled by the deoxidation of the SiC surface and then by the formation of Al4 C3 at the interface. The intrinsic contact angle for molten Al on the polycrystalline ␣-SiC surface was suggested to be lower than 90◦ provided that the oxide films covering the Al and SiC surfaces were removed, i.e., the system is partial wetting in nature. An increase in the Si concentration in liquid Al weakened the interfacial reaction but improved the final wettability. The role of the Si addition on the wetting was presumably attributed to its segregation at the interface and the formation of strong chemical bonds with the SiC surface. © 2014 Elsevier B.V. All rights reserved.
1. Introduction SiC-reinforced aluminum-matrix composites are widely used in the industry due to their low density, high thermal conductivity, good mechanical properties, and low fabrication cost. In the fabrication of the SiC-reinforced Al-matrix composites, the wettability of SiC by liquid Al or Al-base alloys is of vital importance [1]. However, it is usually reported to be poor at relatively low temperatures (T < 1173 K). Increasing temperature would improve it but give rise to the formation of an Al4 C3 phase at the interface, which is brittle and prone to hydrolyze, and thus greatly deteriorates the property of the composites [2]. In order to produce the SiCp /Al composites with good performance, it is necessary to improve the wettability and simultaneously control the formation of Al4 C3 . The wettability of SiC by liquid Al has been studied by many researchers [1–8]. As indicated in Fig. 1, an increase in temperature significantly improves the wettability. Nevertheless, the reported contact angles are rather scattered presumably because of complex influencing factors such as atmosphere, different types of the substrates and measuring techniques. For instance, Laurent et al. [1,2] reported that the wettability of ␣-SiC single crystals by molten Al improved with increasing temperature as the contact angles decreased from 106◦ to 58◦ in a vacuum less than 5 × 10−5 Pa. They divided the entire wetting process into three stages and suggested that the first and second stages should be controlled by
∗ Corresponding author. Tel.: +86 431 8509 5326; fax: +86 431 8509 4699. E-mail addresses: [email protected], [email protected] (P. Shen). http://dx.doi.org/10.1016/j.apsusc.2014.08.055 0169-4332/© 2014 Elsevier B.V. All rights reserved.
deoxidation of the Al and SiC surfaces, respectively, and the formation of Al4 C3 at the interface should not play a significant role in promoting the wettability in this system. Similar work was done by Bao et al. [7] at 1273–1573 K in a vacuum of 10−3 Pa, but they reported much larger contact angles presumably due to poor vacuum. On the other hand, Han et al. [5] reported smaller contact angles in a poor vacuum (10−2 − 10−3 Pa) using reaction-bonded SiC substrates. The presence of 18 wt% free silicon in the substrates significantly reduced the contact angle (by a factor of about 2.8 as compared with the values for Al on the SiC substrates that were free of Si), and therefore, the measured small values do not represent for the true Al–SiC system. Other researchers reported much larger contact angles, as shown in Fig. 1, basically because of the presence of an oxide film at the Al drop surface, particularly at relatively low temperatures (T ≤ 1200 K). It is worth mentioning that the oxidation of the Al drop surface is almost inevitable, since from the viewpoint of thermodynamics the critical oxygen partial pressure in the furnace to keep Al clean should be very low, e.g., 10−44 Pa at 973 K. Only at a relatively high temperature (usually T > 1200 K) and in a high vacuum is the oxide film likely to be disrupted by a self-cleaning reaction (4Al(l) + Al2 O3(s) = 3Al2 O(g) ). The deoxidation temperature in practice depends to a great extent on the thickness of the oxide film covering the Al surface. Note that all the results shown in Fig. 1 were obtained using a conventional sessile drop method, in which the Al sample was preplaced on the SiC substrate and then the couple was heated together to the testing temperature, even though the detailed procedure was somewhat different in different studies. In this case, the oxide film covering the Al surface was relatively thick and difficult to eliminate. Therefore, the
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2. Experimental procedure
Fig. 1. Comparison of the collected quasi-equilibrium or equilibrium contact angles for the Al/SiC system as a function of temperature [1–8]. SC—single crystal SiC; S—sintered SiC; RB—reaction-bonded SiC.
wetting and spreading dynamics were greatly affected by this oxide film. In order to improve the wettability and inhibit the formation of Al4 C3 , alloying Al with Si is an effective method. However, the results concerning the influence of Si addition on the wettability of silicon carbide by liquid Al are quite inconsistent and even contradictory. For instance, Laurent et al. [1], in 1987, reported that the addition of 5–12 wt% Si to Al had a negligible effect on the wettability of ␣-SiC single crystals at 973–1173 K, while the addition of 18 wt% Si slightly improved it and inhibited the formation of Al4 C3 . But in a later study [2], they found that the addition of 20 wt% Si in Al hardly improved the wettability at temperatures lower than 1200 K. Therefore, they argued that the Si addition should not have a noticeable effect on the wettability in the Al/SiC system. In contrast, some researchers believed that the addition of Si should improve the wettability in the Al/SiC system. For instance, Choh and Oki [9] found that the addition of Si to Al decreased the incubation period of the wetting at temperatures between 1173 and 1373 K. Ferro and Derby [6] also claimed that the Si addition was beneficial to the wetting and the best wettability appeared for the Al–Si alloys close to the eutectic composition. Nevertheless, they attributed the good wettability in the Al–Si/SiC system to the dissolution and reconstruction of the SiC surface, which was questionable since the addition of Si to Al weakened or even inhibited the SiC dissolution. As exemplified, despite the fact that the wetting of SiC by Al and Al–Si alloys has been widely investigated, the intrinsic wettability and the role of Si are still ambiguous. The primary purpose of this study is thus to probe into these questions.
The substrates used were pressureless—sintered polycrystalline ␣-SiC with a purity of 98.5 wt%, a density of 3.1 g/cm3 and dimensions of 20 × 5 mm2 . They were mechanically polished using diamond pastes with sizes down to 0.5 m to average roughness (Ra ) of 20 nm, as measured by DEKTAK 6 M (Veeco Metrology Corp. USA) over a distance of 2 mm. The presence of a thin SiO2 film on their surface was previously confirmed by X-ray photoelectron spectroscopy (XPS) examination [10]. The Al–Si alloys with Si concentrations of 7 and 12 wt% were prepared by arc-melting of high-purity Al (99.99 wt%) and Si (99.9999 wt%) in a Ti (99.8 wt%)gettered argon (99.999% purity) atmosphere. Wetting experiments were performed in a high vacuum (∼3 × 10−4 Pa) using a dispensed sessile drop method, in which liquid Al or Al–Si alloy was extruded through a narrow orifice at the bottom of an alumina tube (99.6 wt%) and then rested on the SiC surface. In this case, the initial oxide film covering the Al or Al–Si surface was mechanically removed and the contact angles as well as the spreading dynamics measured were more accurate. Detailed description of the experimental apparatus and procedure was given elsewhere [11]. The contact angle () and drop base diameter were directly measured from drop profiles using an axisymmetric drop shape analysis program. After the wetting experiments, in order to expose the interface beneath the drop and simultaneously to prevent the hydrolysis of Al4 C3 , the solidified Al and Al–Si drops were dissolved in saturated NaOH distilled-water solution after they were mechanically removed in most part. The interfacial microstructures were observed using a scanning electron microscope (SEM, Evo 18, Carl Zeiss, Germany) equipped with an energy dispersive spectrometer (EDS).
3. Results and discussion 3.1. Wetting in the Al/SiC system Fig. 2(a) shows the variations in the contact angle with time for molten Al on the SiC substrates during isothermal dwells at 973–1273 K. As indicated, the spreading was remarkably accelerated and the wettability improved with the increase in temperature. The initial contact angles, which represent the wettability of the oxidized SiC surface, i.e., a SiO2 film by liquid Al, decreased from 125◦ to 105◦ with increasing temperature, as more clearly shown in Fig. 2(b). The “pseudo–equilibrium” contact angles after wetting for 2 h (except at 1273 K) decreased from 95◦ to 57◦ . At T ≤ 1073 K, the contact angle remained constant after an initial rapid decrease, suggesting that the interfacial reaction between Al and SiC in this temperature range should not be very significant. However, increasing temperature to 1123 K led to a continuous
Fig. 2. Variations in (a) contact angle with time at different temperatures and (b) the initial and final contact angles with temperature for molten Al on the SiC substrates.
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Fig. 3. Representative wetting stages in the Al/SiC system at 1173 K.
decrease in the contact angle and a transition from non-wetting to wetting. At higher temperatures, the wetting was fast and the drop reached a final equilibrium state at a relatively short time. A general form of the variations in contact angle and drop base diameter is shown in Fig. 3. The wetting behavior can be characterized by three stages, similar to that reported by Laurent et al. [2]. However, Laurent et al. suggested that the first and second wetting stages should be controlled by deoxidation of the Al and SiC surfaces, respectively, and the third stage associated with the transition from the Al/SiC interface to an Al–Si/Al4 C3 interface. Note that the contact angle of 110◦ at the end of the first stage observed by Laurent et al. [2] is in good agreement with the initial contact angle of 109◦ measured in this study. Because of using the dispensed sessile drop technique, the oxide film covering the Al surface was removed during liquid extrusion. Namely, the first spreading stage observed in this study corresponds to the second stage suggested by Laurent et al. [2], which was controlled by the deoxidation of the SiC surface. In stage (II), the spreading kinetics, as shown by the blue lines in Fig. 4(a)−(c), seems to be reaction-controlled since the following model is well fit [12] cos e − cos d = (cos e − cos 0 ) exp(−kr t),
(1)
where e , d , and 0 are the equilibrium, dynamic, and initial contact angles in this stage, respectively, and kr is a dynamic constant, representing spreading rate in its physical meaning. Using the temperature-dependent spreading rates (i.e., the slopes of the fitting curves) and following the Arrhenius law, the activation energy in this linear spreading stage was calculated to be 302 kJ mol−1 , as shown in Fig. 4(d), close to the Si C bonding energy, which is 327 kJ mol−1 [9], implying that the spreading in stage (II) might have an inherent relation to the rupture of the Si C bonds in the SiC substrate. A similar conclusion was also drawn by Choh and Oki [9], who measured the wetting rates at different temperatures using a dip coverage method and calculated the wetting activation energy of 330 kJ mol−1 for the Al–SiC system. During the wetting, Al4 C3 formed readily at the interface because of very limited solubility of C in Al [13] while Si was displaced and then dissolved in liquid Al as the heterogeneous chemical reaction (4Al + 3SiC = Al4 C3 + 3[Si]) proceeded. Therefore, the wetting process in stage (II) was controlled by the formation of Al4 C3 at the interface. In stage (III), the spreading rate decreased dramatically and the change in the contact angle was limited. As indicated in Fig. 5, the central interface was covered by the Al4 C3 particles; whereas, at the triple line region, the distribution of the Al4 C3 phase was discrete due to short contact time (because the triple line was always moving during the isothermal dwell for 60 min). Therefore, we may infer that the formation of Al4 C3 promoted the wettability and the Al drop indeed spread on a SiC + Al4 C3 composite surface in stage (III) until it reached an equilibrium state, in which the contact angle was close to 55◦ (for Al on an Al4 C3 surface as reported by Ferro and Derby [6]). According to the above analysis, it is clear that the wetting in the Al–SiC system is influenced by the presence of the oxide film at the Al or/and SiC surface(s) in the early stage and then by the interfacial reaction after the disruption of the oxide film. These two stages are virtually overlapped in time and there is no clear boundary between them, making the cognition of the intrinsic wettability in the Al–SiC system rather difficult. As indicated in Fig. 1, a large discrepancy exists in the reported equilibrium contact angles. The large contact
Fig. 4. ln(cos e −cos d ) vs. time at temperatures (a) 1123 K, (b) 1173 K, and (c) 1273 K, respectively; (d) Arrhenius plot of lnkr against 104 /T for calculation of the activation energy of the spreading (Ea ) in the second stage
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Fig. 5. Exposed interfacial microstructures (a) at the triple line region and (b) at the central interface for molten Al on the SiC substrate after wetting at 1173 K for 60 min.
angles ( > 120◦ ) are obviously influenced by the oxidation of the Al or/and SiC surface(s), while the transition from non-wetting to wetting with increasing temperature is most likely due to the disruption of the oxide film, which in turn implies that the Al–SiC couple, if being free of surface oxidation, might be wettable in nature. Instead, Laurent et al. [2] suggested that the equilibrium contact angle for the Al–SiC system should be 55 ± 7◦ at 1150 K. They argued that the distribution of the Al4 C3 phase at the interface was discontinuous for the wetting performed at temperatures no higher than 1173 K, and consequently its formation should not have a significant contribution to the wettability. In our viewpoint, however, the small value (55 ± 7◦ ) is no longer for the Al/SiC system but for the Al/Al4 C3 or Al/(SiC + Al4 C3 ) system. Then, what is the intrinsic wettability for the Al–SiC system? We suggest that it might be characterized by the value at the moment transiting from stage (I) to stage (II). The wetting in stage (I) is obviously related to the reaction between Al and the SiC surface oxide film (SiO2 ). A more clear demonstration is given in Fig. 6. As indicated, the time variations in the contact angle in the early wetting stage are very similar in the Al/pre-oxidized SiC and Al/SiO2 (quartz) systems. However, after the contact angle reaches about 85◦ , the two curves diverge, implying the change in the wetting conditions. Thereafter, the contact angle in the Al–SiC system progressively decreases as a result of the interfacial reaction while that in the Al–SiO2 system remains almost constant (the contact angle is close to the equilibrium value of 85◦ for the Al–Al2 O3 system [14]). Regarding that the spreading in the second stage is reactioncontrolled (see Fig. 4), we can determine the characteristic initial contact angles ( ) using Eq. (1) for characterization of the intrinsic wettability. As shown in Fig. 4(a–c), are in the range of 84–78◦
Fig. 6. Comparison of the variations in the contact angles for pure Al on the SiC and SiO2 substrates at 1173 K.
at 1123–1273 K, slightly decreasing with increasing temperature. Accordingly, we suggest that the clean Al–SiC couple should be partial wetting in nature. At low temperatures (T < 1123 K), however, non-wetting is usually observed because the interfacial reaction is rather mild and the surface oxide film cannot be effectively eliminated. 3.2. Effect of Si addition Fig. 7(a) and (b) show the variations in the contact angle with time for molten Al—7 wt% Si and Al–12 wt% Si alloys on the SiC substrates at 973–1273 K, respectively. A higher temperature significantly decreased the final contact angles and accelerated the spreading. Different wetting behaviors were observed at different temperatures for both Al–7 wt% Si and Al–12 wt% Si alloys. At 973 K, the contact angle decreased almost linearly with time for these two systems. At 1073–1173 K, the contact angle in the Al–7 wt% Si/SiC system experienced a hysteresis stage after rapid spreading; however, such behavior was not observed in the Al–12 wt% Si/SiC system. The wetting hysteresis is a complex phenomenon but could be qualitatively explained by a competition between a driving force and a retarding force. If the retarding force was larger than the driving force, the triple line would be pinned. Otherwise, the drop would keep spreading. Provided that the reaction was not fully inhibited, as indicated in Fig. 8, the retarding force resulted from the rough interface due to the presence of the reaction product Al4 C3 would be large enough to inhibit the movement of the triple line. Increasing temperature favors deoxidization by a self-cleaning reaction between Al and its oxide film in a high vacuum [2] and also accelerated the reaction between Al and SiC. As a consequence, the driving force is large enough to overcome the impedance. In this case, the contact angle hysteresis was inconspicuous. On the other hand, when the interfacial reaction was completely inhibited due to the addition of sufficient amount of Si to Al, as indicated in Fig. 9, the retarding force resulting from the interface roughness was quite small and thus the triple line could move freely up to the equilibrium state. Therefore, the contact angle hysteresis was not observed in the Al–12 wt% Si/SiC system. The equilibrium Si contents required to prevent the formation of Al4 C3 as a function of temperature has been investigated by many researchers [15–19] using both theoretical calculations and experimental method. Lee et al. [19] summarized the relevant results and made some corrections by considering both Al and Si activities in the alloys. They further drew an equilibrium Si-content profile on the Al–Si phase diagram as a function of temperature, as shown in Fig. 10. The experimentally determined equilibrium Si contents show a good agreement with their theoretical calculations in the
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Fig. 7. Variations in the contact angle with time at different temperatures for (a) Al–7 wt% Si and (b) Al–12 wt% Si alloys on the SiC substrates.
Al–Si/SiC system. In this work, we studied the interfacial reaction between two compositions of the Al–Si alloy and the SiC substrates at various wetting temperatures, and the results shown in Figs. 8–9 tally with the equilibrium Si-content profile given in Fig. 10. At higher temperatures (e.g., 1273 K), the liquid drop in both systems showed rapid spreading with the final equilibrium contact angles reaching 50◦ and 32◦ , respectively. Namely, the wettability is better than that in the Al/SiC system, suggesting that the addition of Si does promote the wettability of SiC by liquid Al. The reason for the role of Si remained unclear, as we have mentioned in the introduction. Ferro and Derby [6] attributed the good wettability in the Al–Si/SiC system to the dissolution and reconstruction of the ␣-SiC surface. However, in this study, it is obvious that no significant change occurred at the SiC surfaces after they were wetted by the Al–12 wt% Si alloy at T ≤ 1173 K for a long time (2 h or more), as indicated in Fig. 9. Therefore, new explanations are required to account for the role of Si on the wetting in this system. According to Young’s equation:
cos =
(sv − sl ) , lv
(2)
where sv and sl are the free energies of solid–vapor and solid–liquid interfaces, lv is liquid surface tension, and is equilibrium contact angle, respectively. The wetting improvement can result from the reduction in lv or/and sl . Because lv (Si) < lv (Al), it is reasonable to presume that Si would enrich at the drop surface and thus decrease surface tension of the liquid. Here, we assume that the liquid surface is completely covered by Si while sv and sl remain constant. The equilibrium contact angles for the Al/SiC and Al–Si/SiC systems could be written as cos Al =
(sv − sl ) lv(Al)
cos Al−Si =
(for the Al/SiC system),
(sv − sl ) lv(Si)
(for the Al − Si/SiC system).
(3) (4)
Using the values of Al = 81◦ , lv(Al) = 838 mN m−1 at 1173 K [20] and lv(Si) = 775 mN m−1 at its melting point [21], we obtain Al−Si = 80◦ (the surface tension of Si at 1173 K, if extrapolated from the melting point, will be even larger and so does Al−Si ), which is much larger than 34◦ as shown in Fig. 7(b), indicating that only the reduction in the liquid surface tension by the Si segregation should not be responsible for the small equilibrium contact angles. On the other hand, an alloying element (e.g., Si) can modify the wetting if it
Fig. 8. Interfacial microstructures at the exposed triple line regions for the Al–7 wt% Si/SiC samples tested at (a) 973 K, (b) 1023 K, (c) 1073 K, (d) 1123 K, (e) 1173 K, and (f) 1273 K after removal of the solidified drops. The wetting time is shown in Fig. 7(a) at respective temperature.
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Fig. 9. Interfacial microstructures at the exposed triple line regions for the Al–12 wt% Si/SiC samples tested at (a) 973 K, (b) 1023 K, (c) 1073 K, (d) 1123 K, (e) 1173 K, and (f) 1273 K after removal of the solidified drops. The wetting time is shown in Fig. 7(b) at respective temperature.
significantly decreases sl of the system by adsorption at the interface when the corresponding adsorption energy, Esl , is a large negative quantity. To estimate the adsorption energies at the liquid surface and liquid–solid interface, Elv and Esl , we use a thermodynamic model established by Li et al. [22]:
the exchange energy of the A1–Si alloy modeled as a regular solution equal to the enthalpy of mixing at infinite dilution of Si in Al Si and W Al are the work of adhesion for ( = −9 kJ mol−1 [23]), Wad ad pure Si and Al on the SiC substrates. Again, Sm can be evaluated using the following relationship [24]:
Elv = (lSiv − lAl v )Sm − m,
(5)
1 2 Sm = cN ⁄3 VSi ⁄3 ,
(6)
where c is a geometric factor (1.091 for a close-packed lattice), N is Avogadro’s number, and VSi is the molar volume of Si (VSi = 12.06 × 10−6 m3 mol−1 ). Therefore, Sm = 4.8 × 104 m2 mol−1 . Si and W Al can be calculated from Wad ad
Esl =
Al Elv + (Wad
Si − Wad )Sm ,
where Sm is the interface area occupied by a monolayer of liquid atoms, m is a structural parameter roughly equal to 1/4, is
Fig. 10. Equilibrium Si-content profile in the Al–Si/SiC system determined by Lee et al. [19] from both theoretical calculations and experimental measurements. Two compositions of the Al–Si alloys adopted in this study are superposed on this profile to show the reaction conditions at various wetting temperatures. The squares located in Region (I) mean that the formation of Al4 C3 is inhibited while those in Region (II) mean that the interfacial reaction can happen.
(7)
Si Wad = lSiv (1 + cos 1 ),
(8)
Al Wad = lAl v (1 + cos 2 ),
(9)
where 1 is the contact angle for the Si/␣-SiC system ( 1 = 38◦ [25]), and 2 is the intrinsic contact angle for the Al/SiC system ( 2 = 81◦ at 1173 K), respectively. As a consequence, we obtained Elv = −0.774 kJ mol−1 and Esl = −20.94 kJ mol−1 . The negligible negative value of Elv indicates that the addition of Si to A1 could have a reduction of the liquid surface tension but in a rather small quantity; whereas, a relatively large negative value of Esl suggests that Si should segregate at the solid–liquid interface to considerably reduce the interfacial free energy and thus improve the wetting. It is worth pointing out that Laurent et al. [2] argued that the adsorption of Si at the solid–liquid interface was insignificant based on their estimation of a negligible value of Esl . However, unfortunately, they used a contact angle of 60◦ for the Al/SiC system in their Al . This value, in our viewcalculation of the work of adhesion, Wad point, represents for the Al/Al4 C3 or Al/(Al4 C3 + SiC) system rather than for the Al/SiC system. The intrinsic contact angles for the Al/SiC system are likely to be 84–78◦ at 1123–1273 K, as we have argued before. Finally, the segregation of Si at the solid–liquid interface and its role in promoting the wetting have been reported by several researchers [26,27]. For instance, Drevet et al. [26] investigated the wettability in the Au–Si/SiC system and attributed the decrease in
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the contact angle from 138 ± 2◦ for pure Au to 13 ± 2◦ for Au–45 wt% Si to the adsorption of Si at the Au/SiC interface. They argued that in solid silicon, bonds were covalent but at melting, molten Si became a metallic conductor (delocalized bond). Thus, a strong Au–Si/SiC interface could be developed due to the formation of strong chemical bonds localized at the interface. Using ab-initio calculations, Sabisch et al. [28] suggested that Si should adsorb at the ¯ SiC surface because both (0001)-Si and (0001)-C faces covered by layers of chemisorbed Si had smaller energy than the ideal (0001)¯ Si and (0001)-C faces. Based on these viewpoints, we presume that Si could segregate at the Al–SiC interface and form strong chemical bonds with the SiC surface, which plays a significant role in promoting the final wettability. 4. Conclusions (1) Wetting of polycrystalline SiC by molten Al was initially controlled by the deoxidation of the SiC surface and then by the formation of Al4 C3 at the interface. The intrinsic contact angles for the clean Al–SiC couples were presumed to be 84–78◦ at 1123–1273 K, slightly decreasing with increasing temperature, suggesting that the system is partial wetting in nature. Much larger contact angles reported in literature result mainly from the oxidation at either the Al or SiC surface or both their surfaces. (2) Formation of Al4 C3 at the interface greatly promotes the wettability but it is not a necessary condition. In the case of the interfacial reaction being completely inhibited due to the addition of sufficient amount of Si to Al, the wettability could be substantially improved as well. The mechanism is presumably attributed to the segregation of Si at the interface and the formation of strong chemical bonds with the SiC surface. Acknowledgments This work is supported by National Basic Research Program of China (973 Program) (No.2012CB619600) and National Natural Science Foundation of China (No. 51271085). References [1] V. Laurent, D. Chatain, N. Eustathopoulos, Wettability of SiC by aluminum and Al–Si alloys, J. Mater. Sci. 22 (1987) 244–250. [2] V. Laurent, C. Rado, N. Eustathopoulos, Wetting kinetics and bonding of Al and Al alloys on ␣-SiC, Mater. Sci. Eng. A. 205 (1996) 1–8. [3] W. Köhler, Investigations into the wetting of Al2 O3 and SiC crystals by aluminum and aluminum alloys, Aluminum 51 (1975) 443–447. [4] M. Shimbo, Wettability of silicon carbide by aluminum, copper and silver, J Mater. Sci. Lett. 8 (1989) 663–666.
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