Abstract
Wettability between liquid Ag and graphene-coated SiC single crystal has been investigated by dispensed drop method at T = 970 °C under vacuum accompanied with subsecond recording of the drop/substrate images (100 frames per second) by high-speed high-resolution CCD camera. Non-contact heating method coupled with capillary purification of the Ag drop procedure has been applied. Scanning electron microscopy combined with EDS analysis and scanning probe microscopy combined with Raman spectroscopy techniques has been utilized for microstructure and surface characterization of samples before and after high-temperature wetting tests. Immediately after its detachment from the capillary, the Ag drop showed non-wetting behavior (θ > 90°) forming a high contact angle of θ = 114°. Surface characterization of the drop surface after wettability tests evidenced the presence of graphene and Si transferred from the substrate to the top of Ag drop. These findings suggest chemical interaction phenomena occurring at the interface. Locally, an intimate contact between liquid Ag and SiC substrate was allowed by the appearance of discontinuities in the graphene layer basically produced by thermomechanical stress. Local dissolution of carbon into liquid Ag and its reorganization (by segregation, nucleation and growth) as secondary graphene layer at the Ag surface was also observed.
Highlights
The use of graphene (CGn) as a reinforcement material in metal matrix composites (MMCs) raises hopes of scientists to obtain materials with outstanding functional properties such as high thermal conductivity coupled with low thermal expansion coefficient (Ref 1), useful for aeronautical, aerospace and automobile industries (Ref 1)
The analysis of images collected from the test (Fig. 3a-h) revealed that after the squeezing of the Ag drop and its gentle deposition on the CGn/SiC single crystal (SiCsc) substrate, a slight drop movement to the left edge of the substrate was provoked by the further rising of the capillary (Fig. 3f and g)
Increasing the temperature up to T = 1200 °C, a further decrease in the contact angle was observed with the final value of h = 66°. This value was obtained for silica-free SiC substrate, and it was associated with the reaction between Ag and SiC leading to the formation of graphite (Ref 32): SiCðsÞ ! ðSiÞ þ Cgr ðEq 2Þ. These findings suggest that freshly formed Si dissolves in liquid Ag, and this process, i.e., dissolutive wetting is responsible for wettability improvement in the Ag/SiC system
Summary
The use of graphene (CGn) as a reinforcement material in metal matrix composites (MMCs) raises hopes of scientists to obtain materials with outstanding functional properties such as high thermal conductivity coupled with low thermal expansion coefficient (Ref 1), useful for aeronautical, aerospace and automobile industries (Ref 1). It should be highlighted that the successful fabrication of Ag- and Cu-matrix composites with carbon reinforcements by liquid-assisted processes remains highly uncertain, despite the fact that the most common technological problems related to non-uniform distribution of the reinforcing phase in the metal matrix or weak bonding between the metal matrix and the reinforcement (due to lack of wetting and weak adhesion) have been largely solved using nanostructured carbon reinforcement having similar surface structure to graphene such as nanotubes. Only two reports are available in the literature on wetting behavior of different liquid metals [Sn (Ref 14), Cu (Ref 15)] on graphene. The preliminary studies performed by the authors at the Foundry Research Institute with liquid Sn on graphene-coated Cu substrates (Ref 14) did not give clear evidences of wetting transparency behavior in the Sn/ CGn/Cu system since, despite significant suppression of wetting kinetics, a strong chemical interaction accompanied by the formation of intermetallic phases (Cu3Sn, Cu5Sn6) at the interface were observed. It was concluded in (Ref 14) that Sn-transfer phenomenon
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