Reactive wetting

In this section the additional features involved in reactive wetting will be introduced. The reader is referred to the excellent monograph by Eustathopoulos et al. [2] for an in-depth discussion of this subject. 

An additional factor that can affect reactive wetting is the formation of a new phase at the interface between the solid and liquid. If the new phase is wetted by the liquid, the contact angle will decrease. If the new phase is poorly wetted, the contact angle will increase. See Reference 

We will first discuss the case of simple dissolution of the solid in the liquid (Section 2.2.1) and then the case of formation at the interface of a 3D compound by reaction between the solid and the liquid (Section 2.2.2). 

Up to this point, we have dealt exclusively with passive liquids - liquids that do not alter the surface of the substrate. now consider the case of active liquids, of which there arc two broad categories  

those that etch the surface (such as a solution of nitric acid attacking a copper plate), 

those able to deposit some substance on the surface. 

We shall restrict our attention to the second type. Examples include the following  

Another example studied by Ondarguhu and Dominguez dos Santos involves a cholorosilanc in an organic solution. Here too a reaction occurs with OH groups on the surface. lu its simplest form, the reaction is 

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Figure 2.30. Schematic contact angle versus time curve during reactive wetting with formation of a...

Ti, Zr or Hf in Ag improve the wetting, and 0 drops to 20-80 l Therefore, the presence of Ti in AgCuTi brazes shall facilitate reactive wetting of ZrB2. Additionally, these brazes contain Cu which wets ZrBz (0 -80 at 1413K ">). 

When excessive reactive wetting of the preform leads to metal migration over particle surfaces ahead of the main reaction front, residual porosity becomes a ubiquitous feature of DM0 infiltration. The surface of a partially-infiltrated region then becomes sealed before the underlying pores are filled with the advancing composite. In the case of oxidation in air, a similar phenomenon can lead to the entrapment of nitrogen-containing pores. 

We now return to the case of reactive wetting. The capillary example described above is relatively straightforward to analyze. Unfortunately, it is quite a challenge to obtain good, clean surfaces (free of hysteresis) inside a capillary tube. That is the reason why running drops were first observed and studied on planar, horizontal surfaces. 

In dip coating processes it is essential that the Zn-Al alloy used to coat the steel substrate should wet the substrate. The temperature of the steel substrate is important, for as can be seen from Figure 39, the contact angle is >90 when the substrate is 300 C but the reactive wetting occurs when the temperature of the substrate is heated to 550 °C [67]. 

In this chapter the phenomenon of wetting has been described. The fundamental equations have been developed for the case of non-reactive wetting on ideal perfectly smooth surfaces. The complications introduced by non-ideal surfaces, including rough surfaces and heterogeneous surfaces have been described. Finally, the effects of reactive wetting have been introduced. 

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