Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 310

D. Examples of Interacting Systems at High Temperature
Ceramic/metal joining is of great technological significance because, through this
process, the individual characteristics of the two types of materials can be used
to produce new components with improved performances. Liquid phase bonding
processes, including brazing and Transient Liquid Phase bonding (TLPB) tech-
nique [82-88], are the most promising techniques for joining ceramics to metals.
Recently, a revival of metal-ceramic bonding via glass- and glass-ceramic phases
appeared in the literature, due to their thermal expansion characteristics, oxidation
resistance, easy of processing [89, 90]. Therefore, investigations on wetting, spread-
ing and interfacial behaviour in metal/ceramic [1, 19-21, 36, 91-96] or metal/metal
systems [97-101], joining processes and joint performances [102-113] are essen-
tial.
In the following, the attention will be focused on two classes of materials, borides
and carbides, less extensively studied with respect to oxides: they are classified
within the class of Ultra High Temperature Ceramics (UHTC's), of particular inter-
est for advanced applications, which are extensively studied also in our Laboratory.
Transition metals ceramic diborides, such as hafnium, titanium and zirconium
diborides, as well as silicon carbide, are members of a family of materials with
extremely high melting temperatures, high thermal conductivity, good electrical
properties, excellent thermal shock resistance, high hardness and chemical inert-
ness. These materials are promising candidates for use in high performance appli-
cations, where high temperatures, high thermal fluxes and severe surface stresses
are involved [114, 115]. The unique combination of their properties makes them
suitable for the extreme chemical and thermal environments associated with hyper-
sonic flight, atmospheric re-entry, and rocket propulsion [116, 117]. Applications
include nuclear plants [118, 119], refractory linings [120], electrodes [121, 122],
microelectronics [123] and cutting tools [124].
In the following, a few examples are shown, reporting an overview of old and
more recent results on the wettability and joining of these materials.
1. Transition Metals Diborides
Due to the high melting point of Ti-, Zr- and Hf-diborides (3216 ◦ C <T m <
3380 ◦ C), the main technological barrier to the production of these materials is the
sintering stage. As, generally, using pure borides the final microstructures are coarse
due to the residual porosity, higher densities are achieved by pressure-assisted sin-
tering procedures at temperatures approaching 2100-2300 ◦ C with the addition of
appropriate sintering aids [125-129]. On the other hand, sintering aids modify not
only the ceramic structure, but can also form new phases, grain-boundary precip-
itates and new solid solutions: in all cases they change the nature of the ceramic
body, so that the contact angle measurements should be referred to the new system,
which is often hard both to characterize or to reproduce [130]. Indeed, especially
in the case of diborides, residual porosity together with oxide grains are commonly
found, resulting in physical and chemical heterogeneity of substrate surface, both