Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 309

particular, it has been shown [79] that two non-stoichiometric titanium diboride sub-
strates (TiB 1 . 9 and TiB 1 . 95 ) show surprisingly good wetting in contact with liquid
Cu and Au. This feature was attributed to a departure of titanium diborides com-
position from its stoichiometry. A thermodynamic analysis of the Me-Ti-B system
has shown that an extremely limited boride dissolution takes place at the TiB x /Me
interface while the ratio of B/Ti in the interacting TiB x layers could change sub-
stantially, improving wetting. For these reasons, it was suggested that the scatter
of the experimental results of the wetting experiments using titanium diboride sub-
strates reported in the literature could be attributed to the uncertainty regarding the
substrate composition. Other interesting systems are represented by metal/oxide
samples. In a dedicated study [80] the influence of stoichiometry on the wetting
of zirconia by metals was investigated. The wetting behaviour of stoichiometric
(white) and of ipo-stoichiometric (black) zirconia by different metal melts was stud-
ied. The results show that black zirconia is wetted by inert (Cu, Sn, Ag, Au, Pd, Pt,
Cu-Ga, Pd-Rh) and by low active (Al, Cu-5Zr, (Cu-17.5Ga)-10Ti) melts better
than the white one, while reactive melts have contact angles on black and on white
substrates close to each other. Moreover, the ipo-stoichiometrization of the substrate
can occur not only by annealing under either a vacuum or reducing atmospheres but
also in presence of active metals in the melts.
The same problem has been also investigated by means of theoretical models. In
one case [81], the wetting of sapphire by aluminium was simulated using a poten-
tial applying a combination of the Embedded Atom Method (EAM) and Coulomb
potentials on a system of atoms with defined electronegativity. In the simulations of
high-temperature wetting, the formation of an oxygen-deficient reaction layer be-
tween the liquid and the substrate was observed. The driving force for the creation
of this layer has been attributed to the partial oxidation of the metallic aluminium,
which results in partial reduction of the aluminium ions in the substrate and diffu-
sion of oxygen from the substrate to the reaction layer. This change in stoichiometry
at the metal-ceramic interface may influence diffusivities in the surface of the ce-
ramic and may therefore have an important effect on the kinetics of the evolution of
surface features observed experimentally during reactive wetting.
In a recent study [32], ab-initio Density Functional Theory (DFT) calculations
have been applied to a Ag(111)/sapphire interface with a surface oxygen concentra-
tion which is intermediate between the Al-terminated stoichiometric interface and
the O-rich, O-terminated interface. It has been shown that, in a large range of oxy-
gen partial pressures in the gas phase, the Al-terminated/O-rich-Ag(111) interface
is more stable with respect to the other two interfaces. On the basis of the electronic
structure of the two most important configurations, a mechanism has been pro-
posed to explain the gradual increase in adhesion observed experimentally as due
to an increasing amount of oxygen at the interface. Such oxygen does not represent
a particular termination of the alumina surface, instead probably diffuses through
the molten Ag drop from the surrounding atmosphere.