Elsevier

Applied Surface Science

Volume 377, 30 July 2016, Pages 340-348
Applied Surface Science

Reactive wetting of amorphous silica by molten Al–Mg alloys and their interfacial structures

https://doi.org/10.1016/j.apsusc.2016.03.162Get rights and content

Highlights

  • The wettability improves with increasing Mg concentration and temperature.

  • Reaction product zone consists of layered structures relating with Mg concentration.

  • Formation of MgAl2O4 and MgO at the interface does not promote the wettability.

  • Formation of Mg2Si plays a dominant role in promoting the wettability.

  • Anomalous recession of the triple line was mainly due to diminishing Mg in the alloy.

Abstract

The reactive wetting of amorphous silica substrates by molten Al–Mg alloys over a wide composition range was studied using a dispensed sessile drop method in a flowing Ar atmosphere. The effects of the nominal Mg concentration and temperature on the wetting and interfacial microstructures were discussed. The initial contact angle for pure Al on the SiO2 surface was 115° while that for pure Mg was 35° at 1073 K. For the Al–Mg alloy drop, it decreased with increasing nominal Mg concentration. The reaction zone was characterized by layered structures, whose formation was primarily controlled by the variation in the alloy concentration due to the evaporation of Mg and the interfacial reaction from the viewpoint of thermodynamics as well as by the penetration or diffusion of Mg, Al and Si from the viewpoint of kinetics. In addition, the effects of the reaction and the evaporation of Mg on the movement of the triple line were examined. The spreading of the Al–Mg alloy on the SiO2 surface was mainly attributed to the formation of Mg2Si at the interface and the recession of the triple line to the diminishing Mg concentration in the alloy.

Introduction

Silica (SiO2) is one of the most widely used reactants in preparation of Al–Mg matrix composites since it can readily react with the Al–Mg alloys to form in-situ oxide reinforcement [1], [2]. Also, a SiO2 layer is naturally present or artificially introduced at the SiC or Si3N4 surface, which is presumed to modify the wettability and interfacial bonds between the reinforcement and the Al–Mg alloys [3], [4], [5], [6]. For example, Lee et al. [4] exposed the SiC particles at temperatures above 1073 K in air to yield an amorphous SiO2 layer and incorporated them into an Al–Mg alloy. The pre-oxidation treatment could efficiently prevent SiC from being attacked by the Al matrix, thus avoiding the formation of a brittle and hydrolysable Al4C3 phase. Nevertheless, the intrinsic wettability of SiO2 by the Al–Mg alloys has seldom been investigated despite the fact that it plays a crucial role in the preparation of these composites since liquid metals or alloys usually contact the SiO2 layer prior to the SiC or Si3N4 particles.

Instead, the wettability of SiO2 by pure Al has been investigated by several researchers [7], [8], [9], [10]. For instance, Laurent et al. [8] investigated the wetting of SiO2 and oxidized SiC by molten Al at 933–1173 K under a vacuum of 10−4–10−5 Pa using a sessile drop method and an immersion-emersion tensiometric technique. They found that SiO2 acted as an oxygen source which caused the oxidation of liquid Al. As a consequence, the apparent contact angle at 973 K was very large (above 150°). At higher temperatures (above 1073 K), the deoxidation of Al allowed a real interface to be established between the solid and the liquid. However, the silica layer on SiC did not help the incorporation of particles or the infiltration of fibres by liquid aluminium since the strong reactivity between Al and SiO2 cannot improve the wetting. One of our authors [10] previously also investigated the wetting of SiO2 by molten Al using a dispensed sessile drop method and found that the initial contact angles were larger than 90°. However, the chemical reaction changed the system from non-wetting to partial wetting due to the formation of an (Al–Si)–Al2O3 interface.

Regarding the fact that the SiC or Si3N4 surface is usually covered by a silica layer grown naturally or artificially, which was reported to improve the wetting by Al–based alloys containing Mg, we suppose that the Mg in the alloys might play a significant role in promoting the wettability. Therefore, we performed a comprehensive study on the wetting of SiO2 by Al–Mg alloys over a wide composition range to clarify this question. The results are expected to be valuable to the development of the Al–Mg matrix composites using naturally or artificially oxidized SiC or Si3N4 as reinforcement or using SiO2 as reaction agent.

Section snippets

Experimental procedure

The substrates used were high-purity (99.99 wt%) amorphous silica glass in dimensions of 20 × 20 × 2 mm3 and one side of their surfaces was polished with a roughness of less than 10 nm. The Al–Mg alloys with the nominal desired compositions were prepared in-situ by direct melting of high-purity Al (99.99 wt%) and Mg (99.99 wt%) pieces during the wetting test.

The wetting was performed using a dispensed sessile drop method, as described in detail elsewhere [11]. The SiO2 substrate and the metals were

Wetting behavior

Fig. 1 shows the variation in the contact angle (θ) and drop base diameter (D) for different concentrations of the Al–Mg alloys on the SiO2 surfaces at 1073 K. As can be seen, the alloy composition has a significant effect on the wetting behavior. For pure Al, the wetting behavior shows a spreading stage and a final apparently stable stage. In the spreading stage, the drop base diameter progressively increased while the contact angle decreased. In the stable stage, both the diameter and contact

Interfacial reaction and microstructures

According to the aforementioned results, the primary reaction products in the Al–Mg/SiO2 system include MgO, MgAl2O4, Al2O3, Mg2Si and Si. Therefore, we presume that the following reactions might occur during the wetting process:4[Al]+3SiO2(s)2Al2O3(s)+3[Si],[Mg]+2[Al]+2SiO2(s)MgAl2O4(s)+2[Si],2[Mg]+SiO2(s)2MgO(s)+[Si],2[Mg]+[Si]Mg2Si(s),[Si]Si(s).The brackets indicate that the elements were in the liquid solution. We first consider Reactions (1)–(3). The changes in the standard Gibbs free

Conclusions

  • (1)

    The wettability of SiO2 by molten Al–Mg alloys improved with increasing nominal Mg concentration and temperature.

  • (2)

    With the reduction in the Mg concentration in the alloy, the primary oxide reaction product changed form MgO, MgAl2O4 to Al2O3 in turn, while the precipitates involving Si changed from Mg2Si to pure Si.

  • (3)

    The spreading of the Al–Mg alloy with a high Mg concentration on the SiO2 surface should be primarily attributed to the formation of wettable Mg2Si, while for pure Al to the formation

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 51571099), National Basic Research Program of China (973 program) (No. 2012CB619600), Project Funded by China Postdoctoral Science Foundation (No. 2015M570929) and the Fundamental Research Funds for the Central Universities (FRF-TP-15-005A2).

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