Wetting, grain boundaries

Kleebe, H.-J., (2002). Comparison between SEM andTEM imaging techniques to determine grain-boundary wetting in ceramic poly crystals , J. Am. Ceram. Soc., 85 (1), 43—48. 

No wetting of pure Mg at 850-950 °C in vacuum or air 90-150° [482, 484] Wetting, 10° at melting point [484] Reaction of the oxides with the glassy grain boundary phase... 

However, when HRTEM was employed on the SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual intergranular films was not detected even at the triple junctions. Hence, Kleebe concluded that SEM imaging and Fresnel fringe TEM imaging alone do not enable a safe conclusion to be drawn about interface wetting in ceramic polycrystals. 

Belousov, V.V., (2004), Wetting of grain boundaries in ceramic materials , Colloid Journal, 66 (2) 121-127. 

E., (2000), Adsorption and wetting mechanisms at ceramic grain boundaries , Ceramic Transactions, 118, 427 -44. 

Figure 1.13. Formation of a wetting ridge at the triple line (a) by a mechanism similar to grain-boundary grooving (b).

Saiz et al. (1998) considered that in the case of a triple line, the L/V surface can play the role of a grain boundary and the wetting ridge can move either by bulk or surface (or interface) diffusion of solid atoms (Figure 2.14). They treated the case of surface diffusion with n = 4, taking into account the difference of diffusivities at the S/V surfaces and S/L interfaces. In their experiments with Cu and Ni droplets on AI2O3 surfaces (see Section 1.2.4), Saiz et al. maintained the... 

Figure 3.2. Grain-boundary grooves formed on polycrystalline Ni surface (grain diameter of the order of 100 /mi) in two hours at 900CC. For wetting liquids, these grooves can act as capillaries. The profile was obtained by high-resolution optical profilometry.

Figure5.4.1) Contact angle versus time for a eutectic (Ag-Cu) dropon polycrystalline W at 900°C in a high vacuum. Before the experiment, the W substrate was heat-treated in high vacuum at 1100°C for 2 h. Despite this treatment, the surface remained oxidised and a slow spreading, controlled by W deoxidation, was observed. 2) The same without prior heat treatment of W. In this case segregation of O at the W surface, by fast grain-boundary diffusion, prevents deoxidation of the substrate, resulting in non-wetting behaviour. From Lorrain (1996).

The Cr3C2 is wettable (see Table 7.10) and as a result, low wetting contact angles can be achieved by Cu-Cr alloys (Figure 7.18). However, this reaction layer is not protective and Cu penetrates the grain boundaries of the SiC substrate far from the interface to react and form graphite (Landry and Eustathopoulos unpublished, Xiao and Derby 1998). 

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