Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 321

Figure 17. Contact angle (and relative drop dimensions) as a function of time. Spreading kinetics
of Ni-56Si on SiC: (a) at three temperatures in static Ar

5%H 2 ; (b) at 1200 ◦ C in static Ar

+
5%H 2 ; (c) at three temperatures in a vacuum and (d) at 1350 ◦ C in a vacuum, showing the effects
of temperature and atmosphere on the wettability and spreading process (reprinted from [191] with
permission).

+

10 − 3 (deg/s),
respectively. As shown in Fig. 17a and c, the spreading processes show some differ-
ences between the tests made in a gas mixture (Ar

10 − 1 ,2 . 7 10 − 2 and 3 . 7 stages fall in three ranges, namely, ∼ 2 . 9 × × ×

5%H 2 ) and under a vacuum.
In vacuum the spreading arrives at the equilibrium in 500 seconds at 1200 ◦ Cand
1350 ◦ C (Fig. 17c and d), however, at the lower temperature (1100 ◦ C) the spread-
ing does not arrive at the equilibrium after the fast spreading and keeps going at a
low rate for

+

50 min (Fig. 17c). These facts can be seen as a confirmation of the
mechanisms of 'surface cleaning' explained by reactions (12)-(14).
Figure 18 shows the SEM micrographs of cross-sectioned Ni-Si/SiC couples at
two temperatures. It can be seen that the solid-liquid interface in the Ni-56Si/SiC
system is smooth and clean without any precipitates also in the region close to the
triple line. Thus, from the applications point of view, it should be noted that the
Ni-Si alloys possess the best wettability ( θ

∼

20-30 ◦ ) and the greater potential for
joining SiC ceramics. However, as shown also in Fig. 18b, some cracks are present
at the interfaces and in the bulk phases due to the presence of the brittle silicides

≈