Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 300

E. Experimental Techniques for High Temperature Wettability Measurements . ........... 322

F.References................................................ 328

A. Introduction
1. Characteristics of High Temperature Wetting
A large number of industrial processes, extremely relevant for the world economy,
are governed, to a great extent, by interfacial interactions. Among them, those in-
volving the interaction of solid bodies with molten phases at high temperatures
pertain to metallurgy, glass making, electronics, aerospace, joining processes and
so on. In all these categories, the control of the interactions of molten metals with
higher melting point metals and ceramics is the critical step to assure good quality
of products and reliability in their performances. These characteristic interactions
can be classified, from a general point of view, under the term 'wetting phenomena'.
Wetting phenomena at high temperatures are characterized by a number of pro-
cesses which are specific of this peculiar, aggressive, environment [1]. At variance
with 'room temperature' processes, where water or organic liquids are mainly in-
volved, at high temperatures the atomic mobility is high; diffusion processes are
very active, and reactivity between the different phases in presence can put into
play a large amount of chemical energy. Thus, even if capillarity still plays an im-
portant role, its impact on the final equilibrium configuration of the system under
study cannot be foreseen only on the basis of the Young and Young-Dupré equa-
tions (Eqs (1) and (2)).

cos θ = (σ SV − σ SL )/σ LV , (1)

cos θ), (2)
where σ SV , σ SL , σ LV and θ represent, as usual, the solid-vapour, solid-liquid,
liquid-vapour surface tensions and θ the contact angle at the triple line (TL); W a
is the thermodynamic work of adhesion. It should be noted that Eq. (1) is strictly
valid only if the solid surface can be considered 'infinitely' rigid, or if the atomic
mobility is not sufficient to rearrange the surface structure.
The main processes which can take place at the various interfaces can be de-
scribed following the schematic representation shown in Fig. 1.
2. Exchange Phenomena at Solid-Liquid-Vapour Interfaces
As, usually, the solid-liquid interactions are studied by the sessile drop method,
reference is made here to this special configuration. The first point to be underlined
is that the solid and the liquid phases are 'immersed' in a specific environment,
namely 'high vacuum' or an atmosphere whose chemical composition and flow
conditions can affect to a high extent the liquid bulk composition and all the three
solid-vapour, solid-liquid and liquid-vapour interfacial tensions.
The role of the solid-vapour interface is twofold: (a) it can interact with the
vapour phase (containing both gaseous elements and metal vapours) giving rise to

W a = σ LV ( 1 +