Wetting and evaporation behaviors of molten Mg on partially oxidized SiC substrates
Introduction
Magnesium and its alloys are potential materials for advanced structural applications such as in automobile and aerospace industries with significant advantages of low density, high specific strength and high stiffness [1]. However, their small elastic modulus, low strength at elevated temperatures and poor wear and corrosion resistance hamper their applications. Development of Mg matrix composites has been proven to be a promising way to overcome these shortcomings. In the past decades, considerable efforts have been made to produce the magnesium matrix composites reinforced with various ceramic particles or fibers such as SiC [2], [3], B4C [4], TiC [5] and TiB2 [6]. For instance, Saravanan and Surappa [7] introduced 30 vol.% SiCp with a particle size of 20 μm into molten Mg by way of a melt-stir and casting route. They reported that the stiffness and ultimate tensile strength increased by 40% and 30%, respectively, for the composite compared with the unreinforced matrix. Moreover, the Mg–matrix composite showed a wear rate of about two orders of magnitude lower than pure Mg. Luo [2] reported that the pure Mg reinforced with SiC particles exhibited an increase in yield strength of over 56%. On the other hand, because of lack of solubility of C in Mg and no formation of any Mg carbide, SiC is chemically stable in the Mg melt. Therefore, SiC is a good candidate of reinforcement for Mg and its alloys.
It is well known that the wettability of ceramics by molten metals plays a crucial role in preparation of metal–matrix composites using a liquid casting or infiltration route [8], [9]. However, because of ready oxidation of Mg at relatively low temperatures and extensive evaporation at high temperatures, an accurate measurement of the wettability for Mg becomes rather difficult. To our knowledge, so far, only limited work has been performed on this issue for Mg and its alloys [10], [11], [12], [13], [14]. For instance, Contreras et al. [12] studied the wetting of TiC by molten Mg at 1073–1173 K under static Ar atmosphere using a sessile drop technique. They reported an initial contact angle of ∼120°, being almost independent of temperature, and no reaction at the interface. However, a spreading of the triple line was observed at 1173 K in the early wetting period (10–15 min), followed by a subsequent reduction in the drop base radius due to high evaporation of Mg. They further suggested that the degree of wetting in the Mg–TiC system should be only the result of chemical equilibrium achieved by the mutual saturation of the free valences of the contacting surfaces, giving relative weak interfacial bonding. As we have indicated, SiC is an important reinforcement for the preparation of the magnesium matrix composites. However, no study has been concerned on the wetting of the SiC substrate by molten Mg, making the present investigation necessary. We expect that such knowledge would not only provide guidance for the preparation of the SiC-reinforced Mg–matrix composites but also enrich the understanding of the fundamental aspects of wetting.
Section snippets
Experimental procedure
The substrates used here were pressureless-sintered polycrystalline SiC with a purity of ∼98.5 wt.%, a density of 3.10 g cm−3 and dimensions of 20 mm in diameter and 5 mm in height. The surfaces were polished using diamond pastes down to 0.5 μm to an average roughness (Ra) of less than 20 nm, as measured by DEKTAK 6 M (Veeco Metrology Corp., USA) over a distance of 2 mm at a speed of 100 μm/s. The compositions and their chemical states at the polished substrate surface were examined by X-ray diffraction
Characterization of the as-sintered SiC substrates
Fig. 1 shows the XRD pattern of the as-sintered SiC substrate surface. It can be seen that, in addition to the SiC peaks, there is a minute peak corresponding to SiO2. Fig. 2 shows the XPS analysis of the SiC surface. In addition to C 1s, Si 2s and Si 2p, an O 1s line with the peak at 535.0 eV was also observed, but it may come from the adsorption of oxygen molecules on the SiC surface. In order to confirm whether the SiC surface was oxidized or not, the binding energies of the Si 2p and O 1s
Conclusions
- (1)
For the system involving a volatile drop, the wettability is better to be characterized by the initial contact angle.
- (2)
The initial contact angles for molten Mg on the partially oxidized SiC surfaces at 973–1173 K are between 83° and 76°, mildly decreasing with increasing temperature, suggesting that the system is intrinsically partial wetting.
- (3)
The interfacial reaction between Mg and the SiO2 film on the SiC surface increases the roughness of the solid–liquid interface and leads to the formation of
Acknowledgements
This work is supported by National Natural Science Foundation of China (No. 50871045) and the Key Project of Chinese Ministry of Education (No. 108043).
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