Elsevier

Carbon

Volume 159, 15 April 2020, Pages 561-569
Carbon

Wetting of graphite by molten Cu–xSn–yCr ternary alloys at 1373 K

https://doi.org/10.1016/j.carbon.2019.12.097Get rights and content

Highlights

  • Transition of reaction product coupled wettability of Cu-xSn-1Cr/Cgraphite at x∼ 35 at.%.

  • The formed precursor film was induced both by decreased surface tension and high affinity at interface.

  • Diffusion-limited reactive spreading model described the wetting behavior.

Abstract

This paper presents an experimental approach where the effect of Cr and Sn concentrations on the wetting of graphite by molten Cu-xSn-yCr ternary alloys at 1373 K. It shows that a small quantity of Cr addition (≤2 at.%) can improve the wettability significantly owing to the formation of chromium carbides at the interface. Sn addition (x ≤ 30 at.%) can further improve the wettability by decreasing the surface tension. However, higher Sn concentration (x ≥ 40 at.%) will cause the precipitation of main phase at the interface, varying from Cr7C3 to Cr3C2, which in turn will slightly deteriorate the wettability. The phase equilibrium transition point (x ≈ 35% in Cu–xSn–1Cr/Cgraphite) is responsible for wettability variation by the increase of Sn concentration. When the Sn concentration is ≥ 40 at.%, the spreading dynamics are satisfied with the assumption of the classical diffusion-limited spreading model. When the Sn concentration is ≤ 30 at.%, the transport route of Cr is from the Sn segregation layer on the drop surface to the triple line rather than from the internal diffusion of the bulk drop to the triple line, and then a precursor film with the specific width can be formed.

Introduction

Graphite, an advanced carbon material, has been applied in various fields [[1], [2], [3]] owing to its special properties, such as high melting point, electrical conductivity, and self-lubricity, and has thus triggered remarkable interest in fabrication of metallic matrix composites (MMCs). Moreover, owing to the high thermal/electrical conductivity and good plasticity of copper, Cu/Cgraphite composites are used as functional materials (i.e., heat sink materials) as well as structural engineering materials. However, in the production processes involving liquids, non-wetting and weak interfacial bonding between Cu and graphite pose enormous challenges to the successful fabrication of MMCs and their quality.

It has been established that there is poor compatibility between the highly stable covalent bonds of graphite and the metallic bonds of Cu as the electron exchange at the interface is negligible and the interactions between them are very weak, which may be regarded as Van der Waals interaction. Usually, active additions, i.e., carbide-forming elements, such as Cr, Ti, V, Zr, Hf, Si, and Al, are necessarily introduced into liquid Cu for resolving these problems [4,5]. The wetting of graphite by Cu alloys containing active elements has been studied deeply. According to the currently available literatures [[6], [7], [8]], Cr is one of the most effective active elements, that can improve the wettability significantly despite using very low concentrations (≤2 at.%). In our previous study related to the Cu–Cr/Cgraphite system [9], a large amount of scattered data was reported, which may be related to several factors, such as temperature, atmosphere, different types of substrates, and different reaction products at the interface. For instance, the final contact angle of 41° was reported by Devincent and Michal [10] in the wetting of graphite by molten Cu alloys containing with 0.61–1.22 at.% Cr at 1403 K in an Ar atmosphere (99.995% purity). Mortimer et al. [6,11] and Voitovitch et al. [12] reported the equilibrium contact angles of 41° ±4° under vacuum ∼ 10−2 Pa at 1418 K and ∼10−3 Pa with purified helium micro leaks at 1373 K, respectively, regardless of the Cr concentrations (0.5, 1.0, and 1.5 at.%). Equilibrium contact angle of 20° was reported by Nogi et al. [13] when Cu was alloyed with 0.038–1.34 at.% Cr at 1773 K in an Ar–5% H2 atmosphere. The authors favored high temperature for improving the wettability. Nevertheless, the final contact angles were not lower than 20°. We noted that Cr addition in a similar system (i.e., Sn–Cr/Cdiamond system) yielded approximately comparable results [14], i.e., the wettability can be improved significantly by the precipitation of chromium carbides at the interface. However, the reasons behind why different types of chromium carbides in the Cu–Cr/C system lead to the different wettability and spreading dynamics are still controversial. Devincent and Michal [10] as well as Mortimer and Nicholas [[7], [15]] considered Cr3C2 as the most stable phase in the wetting of graphite by molten Cu alloyed with 0.61–1.22 at.% Cr at 1403 K. Whereas, the results reported by Naidich and Kolesnichenko in 1967 [16] and Sobczak et al. [17] showed both Cr3C2 and Cr7C3 at the interface after wetting. Meanwhile, compared to Cu, liquid Sn can dissolve greater amount of Cr at 1173 K [18]. The increase of Sn in the Cu–Sn–Cr ternary alloys may result in further wettability improvement [19]. However, the manner in which these factors, are related to the wettability coupling reaction products at the interface should to be clarified.

In this work, the effect of Cr and Sn concentrations on the wetting of graphite by molten Cu–xSn–yCr ternary alloys at 1373 K was studied. Moreover, interfacial structures were characterized by the discussion of interfacial equilibrium and spreading dynamics. It is expected that the results of this study can assist in the development of high-performance copper/carbon composites.

Section snippets

Experimental procedure

The substrates used are graphite with purity >99.9%, ash content <40 ppm, density of ∼1.85 g/cm3 (∼13 vol% total porosity and ∼10 vol% open porosity), and dimensions of ∼20 × 20 × 5 mm3. The roughness (Ra) of polished graphite substrate was ∼278 nm over a tested distance 4 mm. The metallic materials are Cu foil with a purity >99.999%, Sn foil with a purity ≥99.99%, Cr powder with a purity >99.99% and the diameter of the particles ∼300–450 μm. Cu or Sn foil was used and parceled with designed

Results

The work in this study compared to the work done by Yang et al. [9] is shown in Fig. 1(a), i.e., the variations in the contact angles with time in the isothermal wetting of Cu–1Cr/Cgraphite at 1373 K. Although most data are consistent with the reported results, the initial contact angle has a slight deviation. The initial contact angle was ∼143° in this work as compared to 127° in Yang’s work. The alloying process may be responsible for the deviation of the initial contact angle. We know that

Wettability and infiltration

Wettability of graphite by molten Cu with Cr addition can be improved significantly and has been widely studied. However, even without the addition of Sn, certain issues cannot be understood. Similar Cu–Cr/C systems have different wettability characteristics. In fact, the main reason behind the different results can be ascribed to the precipitation of reaction products with different wettability characteristics. In a similar ternary alloy Cu–Ga–Cr/Cgraphite system [22], the lower contact angles

Conclusions

The wetting of graphite by the molten Cu–xSn–yCr ternary alloys was studied with a variation of Sn and Cr concentrations.

  • 1.

    Keeping Sn concentration at 20 at.% in the Cu–20Sn–yCr ternary alloy, the variation of Cr concentration in the range of 0.5–2 at.% had little effect on wettability and spreading dynamics. The precipitated Cr7C3 at the interface and decrease in surface tension dur to Sn together determined the final wettability.

  • 2.

    Keeping Cr concentration at 1 at.% in the Cu–xSn–1Cr ternary alloy

Declaration of competing interest

The authors have declared that no conflict of interest exists.

Acknowledgement

This work was supported by National Natural Science Foundation of China (Nos. 51665031 and 51675256), the Program of Innovation Groups of Basic Research of Gansu Province, No. 17JR5RA107, the Foundation of Collaborative Innovation Teams in College of Gansu Province, No. 2017C-07, Provincial Fund for Distinguished Young Scientists (No. 1506RJDA087). The authors are grateful to Dr. Ci for providing them with chromium powder.

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