Reactive wetting of a metal/ceramic system
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Fusheng Pan College of Materials Science and Engineering, Chongqing University, Chongqing 400044, People’s Republic of China (Received 25 July 2001; accepted 5 February 2002)
The mechanism of reactive wetting of metal/ceramic was investigated on the basis of a detailed description of the thermodynamics of interfacial reactions. The interfacial reaction of the metal/ceramic system has been treated as the reaction between the surface phase of the metal matrix and that of the substrate. It is concluded that reactive wetting is governed not only by the term accounting for the intensity of interfacial chemical reactions but also by the term accounting for the physicochemical properties of the resulting interface. In some cases, only one of them contributes dominantly to wetting. The criteria for the choice of an alloying element to promote wetting should not only include its reactivity with the substrate but also its ability to favorably modify the metal/substrate interface.
I. INTRODUCTION
In many systems of practical interest the wetting of liquid metals on metallic or ceramic substrates is accompanied by reactions between the liquid and solid phases. There have been many investigations about the mechanisms of reactive wetting and the relationship between wettability and reactivity of the liquid metal and the ceramic substrate. Aksay et al.1,2 initially proposed that the main contribution to the driving force of wetting was the change in Gibbs free energy of interfacial reaction of metal/ ceramic. According to them, when a reaction occurs at an interface, the free energy change per unit area per unit time will enhance wetting and the Young equation should be corrected for this additional driving force. It means that an intense reaction is required to obtain a good wetting of a liquid on a solid.1–5 In supporting this wetting theory, McDonald5 and OH3 found a strong correlation between the alloying effect of the wettability and the free energy of formation of oxide phase of the alloying element. More reactive alloying elements were more effective in improving wettability. Mg was found the most effective of the alloying elements tested in promoting wetting of both SiC and B4C by Al alloys. Conversely, Cu and Si had little or no effect on wettability. However, some experimental results6–9 seem to violate the correlation between the wettability and the reactivity. In the Fe–Si/SiC6 and Al–Si/SiC7 systems, metal matrix alloys react strongly with silicon carbide but wet it moderately. While the additions of Si in a metal matrix J. Mater. Res., Vol. 17, No. 4, Apr 2002
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led to a strong reduction in reactivity, it resulted in the improvement of wetting. At this time, the reactivity and wettability vary in opposite directions. It is clear that these experimental results cannot be explained by the above wetting theory. According to their experimental results, Espie et al.6,9,10 proposed that the key factor in reactive wetting is not the intensity of chemical reactions but
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