Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 319

ThepresenceofaSiO 2 layer at the SiC external surface inhibits wetting by liq-
uid metals and adhesion. Only the removal of this oxide layer allows wetting to
take place. As a consequence, the spreading kinetics is controlled by the kinetics of
removing of these wetting barriers on SiC surface, which can be due to the pres-
ence of a specific reducing agent such as Si, or to a sufficiently low oxygen partial
pressure. The related chemical reactions may be expressed as follows:
SiO 2 ( s ) + Si ( l ) → 2SiO ( g ) ↑ (12)

SiC ( s ) + 2SiO 2 ( s ) ⇔ 3SiO ( g ) ↑+ CO ( g ) ↑ (13)

1 / 2O 2 ( g ) (14)
Reaction (14) shows that low oxygen partial pressures shift the equilibrium towards
silicon monoxide and molecular oxygen formation.
Furthermore, the spreading arrives at equilibrium more slowly in the gas mixture
than under a vacuum, and sometimes it can even not be attained in the test time.
This can be explained by the fact that the reaction (12) and the forward reaction
(13) are slower in the gas mixture than under vacuum due to the presence of gaseous
products.
Thus, experimental or processing conditions should be chosen where this surface
layer can be eliminated. This can be obtained by fixing an oxygen partial pres-
sure PO 2 for which the transition between passive (formation of SiO 2 ) to active
(formation of SiO) oxidation takes place (in the case of an Ar atmosphere, with
PO 2 ≈

SiO 2 ( s ) ⇔ SiO ( g ) +

1100 ◦ C). But, on the other hand, the SiO 2 surface layer can also
be eliminated through reaction (12), by adding Si to the molten alloy. In both cases,
wetting is made to occur on a clean SiC surface.
At present, the techniques for joining SiC to itself or to metals for high temper-
ature applications include mainly brazing (including TLPB) [102-105, 172-174],
diffusion bonding [175-179], self-propagating high temperature synthesis (SHS)
welding [111, 180] and glass- or glass-ceramic sealing [89, 90].
In the case of (non-reactive pure metal

1Pa, T ≈

Si)/SiC systems, the good wetting
and adhesion on clean SiC surfaces is attributed to silicon chemisorption at the
metal/SiC interface with formation of strong covalent-like bonds between Si and
SiC, to the electronic properties of these alloys and to their interaction with SiC, as
well as the high adhesion energy of the pure metal itself on SiC. Therefore, many
researchers endeavoured to seek for some non-reactive joining processes of SiC
using binary or ternary compounds with high silicon content [181-187]. Actually,
many investigations showed that a series of binary silicon alloys, such as Ni-Si
[188-191], Co-Si [205], Fe-Si [192], Au-Si [193, 194], Ag-Si [195, 196] and Cu-
Si [197-199], have this good non-reactive wettability in contact with SiC.
In the (reactive pure metal

+

Si)/SiC systems [200-204], it has been shown that
the Si concentration in the liquid, reacting metal is a critical parameter in determin-
ing the solid-liquid behaviour. Indeed, pure metals like Ni, Co, Fe, etc. when put
in contact with SiC react with it and dissolve a large amount of Si and C into the
liquid phase. However, it has been found in the Ni-40Si/SiC [190], Co-72.5Si/SiC

+