Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 312

Figure 7. Kinetics of wetting of Ag, Au and Cu on ZrB 2 at T = 1 . 05 T m [K].

only after a long time (35-60 minutes) whereas Ag maintains a very high non-
wetting angle for all the experimental time (Fig. 7).
In the case of Cu and Au, interactions should occur at the interface, most prob-
ably due to the reduction or evaporation of surface oxides, and to a limited disso-
lution of the boride into the liquid metal, as suggested by SIMS measurements and
thermodynamic calculations. In particular, boron can segregate to the liquid metal
surface, suggesting a surfactant action. Recent surface tension measurements on the
Cu-B system have shown that this effect is of the order of 10% [142].
In particular, gold not only wets well the ZrB 2 but also can penetrate along the
solid grains, as shown in Fig. 8.
Another series of experiments were aimed at elucidating the role of the addition
of active metal elements on the wetting kinetics of ZrB 2 by Ag.
As shown in Fig. 9, while pure Ag does not show any evolution of θ with time,
a sharp decrease in contact angle is obtained by adding an 'active' element, Ti, Zr
orHf,tothemoltenAgmatrix.
However, the extent of this effect is different for the various metals. Zr is the most
effective in promoting wettability of the boride. The Work of Adhesion, computed
using the Young-Dupré eq. (2) (Table 1, where the alloys surface tensions have been
computed by the Quasi-Chemical Solution Model [143, 144]), reaches quite high
values in these systems, despite the fact that the added elements raise the liquid alloy
surface tension with respect to the matrix, confirming that the relevant phenomenon
which promotes wetting is the segregation of the active element to the solid-liquid
interface. The term ( σ lv cos θ ) represents the amount ('adhesion tension') by which
the solid surface tension is decreased by the solid-liquid interactions to equal the
interfacial tension value ( σ sv −

σ lv cos θ = σ sl ).