Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 311

contributing to the scatter in contact angle measurements, as discussed in [131], and
in the following Section E.
Other interactions can occur at the solid-vapour and solid-liquid interfaces,
mostly due to the reduction or evaporation of surface oxides, or due to the dis-
solution of the boride into the liquid metal, as suggested by SIMS measurements
[132].
Already some 40 years ago, a large amount of work was done on the high-
temperature wettability and reactivity (in particular oxidation) of transition metal
borides. In an extensive study [133] it was found that non-transition metals do not
wet Group IV-VI metal diborides. The same authors [134] found that the diborides
of group IV metals are more 'inert' in contact with liquid Fe and Ni than those
of V and VI groups, which show very low contact angles often arriving to com-
plete spreading. However, a better wetting behaviour was reported for Ni on TiB 2
(θ 46 ◦ ) and ZrB 2 (θ 54 ◦ ) by Tumanov et al. [135] under vacuum conditions = =

(10 − 3 Pa).
Dissolution and chemical reactions have been found to occur in these systems,
where infiltration of Ni in the solid ZrB 2 has been observed together with the dis-
solution of the diboride into the molten Ni drop, leaving, upon solidification, well
recrystallized diboride crystals; the presence of a Ni x B phase has also been consis-
tently detected in the liquid drop [136].
In another study [137], Ag has been found to wet TiB 2 and ZrB 2 only at high
temperatures, the contact angle decreasing from 125 ◦ for TiB 2 and 115 ◦ for ZrB 2
at 1100 ◦ Cto92 ◦ and 74 ◦ , respectively, at 1600 ◦ C, authorising as an extrapolation
to foresee a good wetting behaviour also in the case of HfB 2 .
More recently, Voytovich et al. [138] addressed the study of the role of Ni in
modifying the wettability of Au on ZrB 2 . It was shown that the Au-40at%Ni alloy
had different interfacial reactivity with the substrate, leading to an 'equilibrium'
contact angle θ 25 ◦ and θ< 10 ◦ at 1170 ◦ C and 980 ◦ C respectively. This 'ab-

normal' behaviour (i.e., increasing wetting with decreasing T ) was attributed to
dissolutive wetting at 1170 ◦ C and compound formation (Ni 2 B) at the interface at
980 ◦ C.
In the recent years, our Team has undertaken a systematic study, by the sessile-
drop technique, of wettability, reactivity and interfacial properties of Group IV
diborides, both from the basic (wettability, interfacial tension) and the application
(joining) points of view [36, 94, 132, 139-141] in two main temperature ranges,
i.e., at around 1100 ◦ C and 1500 ◦ C.
Wetting of ZrB 2 by Cu, Ag and Au . A series of wetting experiments for Zr di-
boride in contact with liquid Ag, Cu and Au have been performed under high
vacuum (10 − 3 Pa) in the presence of a Zr getter [141] to keep low the oxygen
partial pressure (10 − 26 Pa at 1100 ◦ C).
These tests have shown that, at T

=

1 . 05 T m , at the end of the spreading process,
the contact angles of Ag, Cu and Au decrease in substantial way ( θ

=

153 ◦ ;80 ◦ and =

34 ◦ respectively). For Cu and Au a kinetics exists, leading to a stationary situation