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Applied Surface Science 256 (2010) 7043–7047

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Applied Surface Science

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etting and evaporation behaviors of molten Mg on partiallyxidized SiC substrates

an Zhang, Ping Shen ∗, Laixin Shi, Qiaoli Lin, Qichuan Jiangey Laboratory of Automobile Materials of Ministry of Education, Department of Materials Science and Engineering,

ilin University, No. 5988 Renmin Street, Changchun 130025, PR China

r t i c l e i n f o

rticle history:eceived 6 January 2010eceived in revised form 29 March 2010

a b s t r a c t

The wetting and evaporation behaviors of molten Mg drops on pressureless-sintered SiC surfaces werestudied in a flowing Ar atmosphere at 973–1173 K by an improved sessile drop method. The initial contactangles are between 83◦ and 76◦, only mildly depending on temperature. The formation of a ridge at the

ccepted 7 May 2010vailable online 1 June 2010

eywords:etting

triple junction as a result of reaction between molten Mg and the SiO2 film on the SiC surface pins the tripleline and leads to a constant contact diameter mode during the entire evaporation process. Moreover, thediffusion coefficients of the Mg vapor at different temperatures were evaluated based on a simple model.

vaporationgiffusion

. Introduction

Magnesium and its alloys are potential materials for advancedtructural applications such as in automobile and aerospace indus-ries with significant advantages of low density, high specifictrength and high stiffness [1]. However, their small elastic mod-lus, low strength at elevated temperatures and poor wear andorrosion resistance hamper their applications. Development of Mgatrix composites has been proven to be a promising way to over-

ome these shortcomings. In the past decades, considerable effortsave been made to produce the magnesium matrix compositeseinforced with various ceramic particles or fibers such as SiC [2,3],4C [4], TiC [5] and TiB2 [6]. For instance, Saravanan and Surappa [7]

ntroduced 30 vol.% SiCp with a particle size of 20 �m into molteng by way of a melt-stir and casting route. They reported that the

tiffness and ultimate tensile strength increased by 40% and 30%,espectively, for the composite compared with the unreinforcedatrix. Moreover, the Mg–matrix composite showed a wear rate

f about two orders of magnitude lower than pure Mg. Luo [2]eported that the pure Mg reinforced with SiC particles exhibited anncrease in yield strength of over 56%. On the other hand, becausef lack of solubility of C in Mg and no formation of any Mg carbide,

iC is chemically stable in the Mg melt. Therefore, SiC is a goodandidate of reinforcement for Mg and its alloys.

It is well known that the wettability of ceramics by molten met-ls plays a crucial role in preparation of metal–matrix composites

∗ Corresponding author. Tel.: +86 431 85094699; fax: +86 431 85094699.E-mail address: shenping@jlu.edu.cn (P. Shen).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.05.022

© 2010 Elsevier B.V. All rights reserved.

using a liquid casting or infiltration route [8,9]. However, becauseof ready oxidation of Mg at relatively low temperatures and exten-sive evaporation at high temperatures, an accurate measurementof the wettability for Mg becomes rather difficult. To our knowl-edge, so far, only limited work has been performed on this issue forMg and its alloys [10–14]. For instance, Contreras et al. [12] studiedthe wetting of TiC by molten Mg at 1073–1173 K under static Aratmosphere using a sessile drop technique. They reported an initialcontact angle of ∼120◦, being almost independent of temperature,and no reaction at the interface. However, a spreading of the tripleline was observed at 1173 K in the early wetting period (10–15 min),followed by a subsequent reduction in the drop base radius due tohigh evaporation of Mg. They further suggested that the degree ofwetting in the Mg–TiC system should be only the result of chemicalequilibrium achieved by the mutual saturation of the free valencesof the contacting surfaces, giving relative weak interfacial bond-ing. As we have indicated, SiC is an important reinforcement forthe preparation of the magnesium matrix composites. However,no study has been concerned on the wetting of the SiC substrateby molten Mg, making the present investigation necessary. Weexpect that such knowledge would not only provide guidancefor the preparation of the SiC-reinforced Mg–matrix compositesbut also enrich the understanding of the fundamental aspects ofwetting.

2. Experimental procedure

The substrates used here were pressureless-sintered polycrys-talline SiC with a purity of ∼98.5 wt.%, a density of 3.10 g cm−3 anddimensions of 20 mm in diameter and 5 mm in height. The sur-

7 ace Science 256 (2010) 7043–7047

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aces were polished using diamond pastes down to 0.5 �m to anverage roughness (Ra) of less than 20 nm, as measured by DEK-AK 6 M (Veeco Metrology Corp., USA) over a distance of 2 mm at apeed of 100 �m/s. The compositions and their chemical states athe polished substrate surface were examined by X-ray diffractionXRD, D/Max 2500PC, Rigaku Corp., Tokyo, Japan) and X-ray photo-lectron spectroscopy (XPS, Thermo ESACLAB 250, USA), revealinghe presence of a SiO2 film. The Mg specimens, with a purityf 99.99 wt.%, were in the form of small cubes weighing about0–50 mg. The substrate and the Mg specimen were separately

mmersed in acetone and ultrasonically cleaned for three times.The wetting experiments were performed in a controlled flow-

ng Ar (flow rate 0.5 l min−1) atmosphere at a constant pressure of.12 MPa using an improved sessile drop method, as described inetail elsewhere [15]. The SiC substrate was placed horizontally

n a stainless-steel chamber while the Mg specimen was stored inmetal tube outside the chamber. The metal tube connected the

lumina dropping tube (99.6 wt.% purity) with an inner diameterf 5 mm and an open hole of 1 mm in diameter at the bottom. Ithould be specifically indicated that the inner surface of the alu-ina tube was pre-attacked by molten Mg to alleviate their reaction

fter the Mg specimen was inserted and then melted at the bottomf the tube. The chamber was first evacuated to a vacuum of about× 10−4 Pa at room temperature and then heated to 1373 K at a

ate of 20 K/min, holding for 10 min to remove surface absorbedmpurities, and then cooled at 10 K/min to the desired testing tem-erature. The Ar gas (99.999% purity), purified by passing throughmagnesium (99.9%) furnace at 673 K, a dehydrating tube filledith molecular sieves and finally an oxygen-absorption tube filledith high effective palladium-type agents to reduce water and oxy-

en levels, was introduced to the chamber up to 0.12 MPa. Afterhe temperature and the atmosphere were stabilized, both the gasnlet and outlet were closed, and the Mg specimen was immediatelynserted into the bottom of the alumina tube and kept for 30–50 sdecreasing with increasing temperature in order to reduce the pre-eposition of Mg vapor on the substrate surface). The molten Mgas forced out from the 1 mm hole and dropped on the SiC sur-

ace using a contact-dropping (i.e. non-impingement) mode [15]y a gradual decrease in the pressure inside the chamber. Then, thetage supporting the SiC substrate was slightly lowered down forhe Mg melt to detach from the bottom of the alumina tube. Simul-aneously, high-resolution (1504 × 1000 pixel) photographs wereaken at a maximum speed of 2 frames/s to capture the initial droprofiles. The moment at which the melt separated from the tubeas defined as the start time (t = 0) for the wetting.

Most of the experiments were terminated until the Mg drops

ere completely disappeared in order to obtain the comprehen-

ive wetting behaviors under evaporation and only limited runsere halted in the halfway by shutting off the power to pre-

erve the drops for interfacial microstructural observation. Thehamber and the shielding reflectors were carefully cleaned after

ig. 2. (a) XPS spectrum of the as-polished SiC substrate surface; (b and c) Details arounith corrections for substrate electrical charge and background. The shadow in (c) corres

Fig. 1. XRD pattern of the as-sintered SiC substrate surface.

each experiment so as to mitigate the influence of the Mg vapordeposited inside the chamber on the next run of the experiment.The captured drop profiles were analyzed using drop-analysis soft-ware, from which contact angle, drop height and contact diameterwere obtained. The cross-sectional microstructures were observedusing a scanning electron microscope (SEM, Evo18, Carl Zeiss, Ger-many) equipped with an energy dispersive spectrometer (EDS).The phases at the exposed triple junction were examined by X-ray micro-diffraction (XRD, D8 Discover with GADDS, Bruker AXS,Karlsruhe, Germany) using an 800-�m beam diameter and thetopography at that location was measured by the DEKTAK surfaceprofilometer.

3. Results and discussion

3.1. Characterization of the as-sintered SiC substrates

Fig. 1 shows the XRD pattern of the as-sintered SiC substratesurface. It can be seen that, in addition to the SiC peaks, there is aminute peak corresponding to SiO2. Fig. 2 shows the XPS analysisof the SiC surface. In addition to C 1s, Si 2s and Si 2p, an O 1s linewith the peak at 535.0 eV was also observed, but it may come fromthe adsorption of oxygen molecules on the SiC surface. In order toconfirm whether the SiC surface was oxidized or not, the bindingenergies of the Si 2p and O 1s were further analyzed with highaccuracy. Detailed information around these two peaks with cor-rections for substrate electrical charge and background is given inFigs. 2(b) and (c). The former demonstrates that the Si 2p, in prac-

tice, corresponds to two states, i.e. the primary peak of 102.3 eV toSi4+ in the bulk of SiC [16] and a position shift of 1.2 eV to that inSiO2. The shift towards a higher value of 1.2 eV observed here isgenerally consistent with the fact that the oxidized state of Si has apeak shift of 1.7–2.7 eV higher than that in the bulk SiC [17]. Also,

d the Si 2p peak [dashed rectangle in (a)] and the O 1s peak [dashed circle in (a)]ponds to the binding energy range for the adsorption of oxygen on the SiC surface.

D. Zhang et al. / Applied Surface Science 256 (2010) 7043–7047 7045

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diffusion was observed at the interface in the scope of microme-ter dimension, suggesting that the reaction layer, if present, shouldbe very thin. However, XRD analysis (Fig. 7) revealed the presenceof the minute phases of Mg2Si, MgO and Si at the triple junctionafter the Mg drop was completely disappeared through evapora-

ig. 3. Variations in contact angle with time for the Mg drops on the partially oxi-ized SiC surfaces at isothermal temperatures between 973 K and 1173 K in Artmosphere.

he binding energy is completely different from that in the Si–Siond, which is in the range of 98.7–99.7 eV. The latter suggests thathe peak of the O 1s with the binding energy of 534.25 eV corre-ponds to the formation of the Si–O chemical bond rather than tohe simple adsorption of the oxygen molecules on the SiC surface18,19]. Based on these analyses, we conjecture that the as-receivediC substrate surface was partially oxidized during the pressurelessintering process.

.2. Wetting and evaporation behaviors

Fig. 3 shows the variations in contact angle with time for theolten Mg drops on the SiC surfaces during isothermal dwells at

73–1173 K. The initial contact angles do not show a significantependence on temperature, which are generally between 83◦ and6◦, slightly decreasing with increasing temperature. These initialontact angles are much smaller than those reported by Contrerast al. [12] for the Mg–TiC system, which are approximately 120◦ atemperatures 1073–1173 K, as we have mentioned before. We pre-ume that the divergence in the initial contact angles is attributedo the difference in the sessile drop methods adopted by us and byhem or the oxygen partial pressure in the atmospheres rather thano the systems. In fact, Mg is very easy to be oxidized and a large con-act angle is usually encountered when the drop surface is coveredy an oxide film. A transition from non-wetting to wetting is onlybserved after the eventual break-up of the film as a result of Mgvaporation, particularly at elevated temperatures. This might behe case in the experiments of Contreras et al. [12]. However, in theresent study, the oxide film was initially mechanically removedy squeezing the Mg liquid through the small hole at the bottomf the alumina tube, giving a clean drop surface and thus a muchmaller contact angle. Similar results were also demonstrated in ourrevious study on the wetting of �-Al2O3 by molten Mg using theessile drop methods with and without squeezing technique [20].he initial contact angles are ∼20◦ smaller by way of the squeezingreatment.

Another significant feature in Fig. 3 is that the contact angleonotonically decreases with time until the Mg drop completely

isappeared through evaporation. However, it should be indicatedhat the decrease in the contact angle results from the diminishingrop volume or the decreasing drop height while the contact diam-ter remains almost constant during the entire wetting process, as

learly indicated in Fig. 4. Therefore, the contact angles in questionre virtually receding ones and their decrease does not represent anmprovement in the wettability. A higher temperature significantlyromotes the Mg evaporation and thus accelerates the decrease

n the contact angle. In comparison, the monotonically decreasing

Fig. 4. Variations in contact angle (�), contact diameter (d) and drop height (h) withtime for the Mg drop on the partially oxidized SiC surface at 1073 K. Similar variationbehaviors were also observed at other temperatures.

behavior observed in this system is somewhat different from that inthe Mg/ZrO2 [14] and Mg/Al2O3 [20] systems, in which the contactangle was found to decrease to a certain degree and then increasedue to the receding of the triple line.

The constant contact diameter implies the persistent pinning ofthe triple line, which seems to result from the increase in the sub-strate surface roughness, especially at the triple junction, as shownin Fig. 5. However, according to the DEKTAK measurements, theaverage roughness of the as-polished SiC surfaces was about 20 nm,much smaller than that of the Al2O3 substrates (Ra = 100–120 nm),on which the receding of the triple line and consequently theincrease in the contact angle were usually observed [20]. A reason-able explanation is that the increased roughness at the SiC surfacesresults from the interfacial reaction. As indicated in Section 3.1,the SiC surface was partially oxidized. As a result, the followingreactions may take place

2Mg + SiO2 = 2MgO + Si (1)

2Mg + Si = Mg2Si (2)

Thermodynamically, the changes in the standard Gibbs freeenergy, �G0, for reactions (1) and (2) at 1173 K are −245.3 kJ mol−1

and −63.5 kJ mol−1 [21], respectively, indicating that these reac-tions are favorable. Fig. 6 shows the cross-sectional microstructurefor the solidified Mg drop on the SiC surface after wetting at 973 Kfor 600 s. No appreciable reaction product but limited elemental

Fig. 5. Topography around the triple junction after disappearance of the Mg dropat 1173 K.

7046 D. Zhang et al. / Applied Surface Science 256 (2010) 7043–7047

Fig. 6. Cross-sectional microstructure at the Mg/SiC interface after wetting at 973 Kfor 600 s and elemental line-scanning spectra.

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Eq. (5) implies that the diffusion coefficients (D) of the Mg vapor

TP

ig. 7. XRD pattern of the phases at the exposed triple junction after completeisappearance of the Mg drop at 1173 K.

ion. The interfacial reaction together with the possible oxidation ofg at the triple line led to the formation of a noticeable ridge at the

riple junction (Fig. 5), thus pinning the triple line and giving rise tohe constant contact diameter mode. According to the viewpoint ofhanahan [22], when the contact diameter remains constant whilehe volume of the drop decreases due to evaporation, the drop tendso keep a spherical geometry to minimize its surface excess energy;amely, provided on an absolutely smooth and homogeneous sur-

ace, the drop would maintain a constant contact angle with amoothly receding contact line during evaporation. However, nat-ral surfaces are usually more or less rough and heterogeneous,nd thus the pinning of the triple line is commonly encountered. Inhis case, the drop deviates from the capillary equilibrium and pro-uces an excess free energy within the system. When it exceeds the

nergy barrier, de-pinning would occur. Following this viewpoint,t can be inferred that in the current Mg/oxidized SiC system, thenergy barrier due to the enhanced roughness at the triple junctionxceeds the excess free energy resulting from the decrease in the

able 1arameters used for calculation of diffusion coefficient.

Temperature (K) Parameters

K � [25] (g/cm3) rb (cm) �0

973 1.028 1.577 0.2494 831023 1.024 1.564 0.2614 801073 1.051 1.551 0.2545 791123 0.987 1.538 0.2224 791173 1.010 1.525 0.2390 76

Fig. 8. Variations in the normalized drop volume, V/V0, with the normalized time,t/tlife for the Mg drops on the partially oxidized SiC surfaces during isothermal dwellsat 973–1173 K.

drop volume with the pinned triple line throughout the lifetime ofthe Mg drops.

3.3. Diffusion of Mg vapor

Fig. 8 shows the variations in the normalized drop volume, V/V0,with the normalized time, t/tlife, at different temperatures, givingthe following linear relationship

V

V0= 1 − K · t

tlife(3)

where V0 is the initial volume of the drop, tlife is the lifetime ofthe drop, and K is the slope. In fact, dV/dt = −KV0/tlife, meaningthe evaporation rate. Based on a diffusion-controlled evaporationmodel and an assumption of a spherical cap geometry of the drop,the linear slope, K, in Eq. (3) for the pinned drop under evaporationcould be expressed by the following equation [23]

K = 2�rbD�PM · sin �0 · tlife

�RT(1 + cos �0) · V0(4)

where D is the diffusion coefficient of the Mg vapor in Ar at absolutetemperature T, �0 is the initial contact angle, rb is the constant con-tact radius (using the initial value), � is the density of the liquid, M isthe molar mass of Mg, �P (�P = P0 − P∞) is the pressure differencebetween the saturation pressure P0 [24] at the drop surface and thevapor pressure P∞ far from the drop surface (assuming P∞ = 0), andR is the gas constant, respectively. Accordingly,

D = �RT · (1 + cos �0) · K · V0

2�rb · �PM · sin �0 · tlife. (5)

in Ar could be estimated from the evaporation rates at respectivetemperatures. The values of K approximated to 1, as deduced fromEq. (3) and Fig. 8. Other necessary parameters for the calculation aregiven in Table 1, yielding the values of D of the order of 10−6 cm2/s.

(◦) �P (Pa) tlife (s) V0 (cm3) D (cm2/s)

875.3 2376 0.03044 5.77 × 10−6

1953.1 1273 0.03133 5.19 × 10−6

4030.9 635 0.02966 5.33 × 10−6

7776.8 386 0.03381 5.77 × 10−6

14148.3 290 0.03016 3.92 × 10−6

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D. Zhang et al. / Applied Surf

. Conclusions

1) For the system involving a volatile drop, the wettability is betterto be characterized by the initial contact angle.

2) The initial contact angles for molten Mg on the partially oxi-dized SiC surfaces at 973–1173 K are between 83◦ and 76◦,mildly decreasing with increasing temperature, suggesting thatthe system is intrinsically partial wetting.

3) The interfacial reaction between Mg and the SiO2 film on theSiC surface increases the roughness of the solid–liquid inter-face and leads to the formation of a ridge at the triple junction,thus pinning the triple line and making the constant contactdiameter mode dominate the entire wetting and evaporationprocess.

4) The volume loss of the pinned Mg drop under evaporation isalmost linear with time and the diffusion coefficient of the Mgvapor could be estimated from the evaporation rate based on asimple model.

cknowledgements

This work is supported by National Natural Science Foundationf China (No. 50871045) and the Key Project of Chinese Ministry ofducation (No. 108043).

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