Wettability Of SiC Single Crystal And Carbon Nanotubes By Molten Aluminum And Their Interfacial Microstructures

The urgent need of high-performance and lightweight materials gives rise to extensiveresearches on the traditional SiC-reinforced and the emerging carbon nanotubes-reinforcedaluminum matrix composites. As known, the wettability and interfacial chemistry betweenmatrix and reinforcement are crucial factors for the fabrication of the composites especiallywhen a liquid processing route is involved. Despite the fact that the wetting behavior in theAl/SiC system has been studied by many researchers, the results are very scattered.Moreover, their viewpoints on the wettability, spreading dynamics and reactivity in thissystem are quite inconsistent and even contradictory. As for the Al/carbon nanotubes (CNTs)system, up to date, there is rare research on the wettability in this system. Therefore, in thispaper, we investigated the wetting of SiC and CNTs by molten Al using an advancedsessile-drop testing equipment and method. The main results obtained are as follows:(1) The surface polarity of SiC has a remarkable influence on the wettability andreactivity in the Al/SiC system. The chemical stability of the C-terminated surface is muchstronger than that of the Si-terminated surface when they are in contact with molten Al,which may be related to the difference in their surface structures. The interfacial reactionbetween molten Al and the Si-terminated surface is rather rapid. The spreading is initiallycontrolled by the deoxidization of the substrate surface and then by the formation of Al4C3at the interface. The final equilibrium is established for an Al[–Si]([Si] was generated frominterfacial reaction) drop on the continuous Al4C3reaction layer with a stable contact angleof56±1o. However, the deoxidization and the interfacial reaction are sluggish on theC-terminated surface. The final quasi-equilibrium is for an Al[–Si] drop on the compositesurface consisting of Al4C3and C-terminated SiC. Nevertheless, the final wettability ismuch better than that on the Si-terminated surface.(2) The intrinsic wettability between clean Al and the clean SiC surface should be fairlygood, especially for the C-terminated surface. The formation of Al4C3at the interface does not essentially promote the wettability. The traditional viewpoint of the poor wettability inthis system and its promotion by the formation of Al4C3is largely influenced by thepresence of the oxide films at the Al or/and SiC surface(s) as well as by the rapid formationof the Al4C3phase, which inhibits the establishment of an intimate Al/SiC interface.(3) The crystal structure of SiC has insignificant effect on the wettability. There is noremarkable difference in the spreading dynamics and the final wettability between the4Hand6H SiC single crystals. In addition, the spreading dynamics for molten Al on theSi-terminated SiC and the polycrystalline SiC are very similar. And their equilibrium statesare the same despite that the morphologies of the interfacial reaction layers are different.(4) The addition of Si to Al has a remarkable influence on the wetting behavior. For theSi-terminated surface, the final wettability is closely related to the concentration of Si (XSi),which determines the characteristic of the liquid/solid interface. When XSiis lower than thecritical value, the wettability is hardly affected. When XSiis higher than the critical value,the final wettability is greatly improved with the interfacial reaction being almostcompletely inhibited. However, for the C-terminated surface, the wettability is alwaysdeteriorated. The higher the XSi, the worse the final wettability.(5) The wettability between carbon nanotubes (CNTs) and Al is very poor, even worsethan that of the Al/graphite system. The intrinsic contact angle should be larger than140o.Since the chemical stability of CNTs is higher than that of graphite, the spreading formolten Al on CNTs substrate is much slower. The reactivity between CNTs and Al is closelyrelated to the structural integrality of CNTs. The high-crystalline tube wall is almost inert toliquid Al even at a temperature as high as1173K, while the chemical stability of theregions with structural defects is much lower. The open tips tend to dissolve slowly intomolten Al at973K and the structure will not be severely damaged within a short time (10min). However, at higer temperatures (T≥1073K), the dissolution of the open tips is veryquick. Besides, the notches on the tube wall and the regions with high curvatures are easilyto be eroded and then fracture, which accelerates the destruction of the structure of CNTs.