Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 325

Another, more advanced possibility, is to work with a furnace where both the
solid ceramic and metal samples are introduced, separately, into the pre-heated
chamber and put into contact only when the metal is fully melted. This can be
done using either the dispensed drop or the transferred drop methods. In the first
one, a ceramic syringe is used from which a molten drop can be squeezed out by
a piston or by applying to the molten reservoir a small gas overpressure. This op-
timized method has the additional, important, advantage of 'cleaning' the liquid
metal surface of the oxides which could have formed prior to melting and to allow
multiple operations to be made ' in situ ' at the experimental temperature [212, 213],
as clearly shown in Fig. 20.
In the transferred drop method, the substrate to be tested is lowered until it just
touches a liquid drop, lying on substrate which must be as much as possible inert.
Using these methods, the spreading can be analysed from the very beginning of
the process without any prior interaction between the liquid and the substrate during
the heating up as in the conventional sessile-drop method.
A further remark: as, quite usually, wetting experiments are performed on poly-
crystalline surfaces, and this fact, as shown in Section C, can lead to hystere-
sis/pinning effects, it is sometimes possible to utilize micrometer-size drops formed
on top of single grains to obtain reliable measurements of contact angles (mi-
crowetting), assuming the solidification does not alter sensibly the contact angle.
In Fig. 21, a tiny Au drop, formed on top a single boride grain, allowed an evalua-
tion of the 'true' contact angle on a monocrystalline ZrB 2 surface, its value of about
θ 35 ◦ is very close to the 'macroscopic' angle measured at the experimental tem-

perature ( θ

=

2 ◦ ).
After solidification, the weight of the specimen must be measured to check
for the presence of evaporation phenomena, and the top surface of solidified ses-
sile drop/ceramic couples examined by optical and scanning electron microscopy
(SEM) coupled with EDS analysis. Next, the specimens are cross-sectioned and
polished for structural characterization by optical microscopy (OM) and Scanning
Electron Microscopy (SEM), by Energy Dispersion Analysis (EDS) or by Wave-
length Dispersion Analysis (WDS) technique, in the presence of light elements (B,
C in particular). More sophisticated techniques such as Focused Ion Beam (FIB),
Transmission Electron Microscopy (TEM), nano-indentation techniques should be
used if the properties of the real metal-ceramic interface has to be studied in a fine
way.
To summarize, and in close agreement with the recommendations given in Ref.
[131], the following steps should be observed, for reliable wetting and contact angle
measurements:

34 ◦ ± =

1. The chemical composition of the solid and liquid components should be explic-
itly given.
2. The wetting experiments must be conducted in controlled furnace conditions
obtained either by high vacuum or, if reactive gases (oxygen, sulfur, nitrogen,