Superplasticity In Ceramics
(santonu)
If you drop a cup it breaks! The first thing we get in our mind about
ceramics is fragile. In technical worlds they are know as brittle
materials, but imagine a ceramic elongating like a rubber band! It is
possible for some ceramics and they are called superplastic ceramics.
When a ceramic material exhibits more than 100 % strain at higher
temperature (typically > 0.5 Tm, where Tm is the melting point of
that material) the system is called a superplastic system. Among
various oxide and non oxide ceramics only a few exhibits
superplasticity. There are few criteria that are required to be
fullfilled for a ceramics to be structurally superlastic. (1) The
material should possess a very fine grain size (1673 K). This material
in the sintered condition possesses a very fine grain size (~0.3 µm).
The grain growth rate also remains extremely sluggish till 1873 K.
Usually grain-boundary energy is low in these systems. 3 mol % yttria
stabilized tetragonal zirconia (3YTZ) was found to exhibit an
elongation of 800 % at 1773 K under tensile stress. It is also possible
to get high elongation through grain boundary modifications (adding a
liquid phase or using a two phase materials). Silica doped 3YTZ,
alumina zirconia two phase composites, Si3N4 doped SiC are examples of
such systems. It was possible to achieve an elongation more that 2000 %
under tensile stress by engineering materials structural properties.
Supperpalstic deformation takes place by grain boundary sliding. The
grain reorients and slides past each other results in a total
elongation. As the grains slides, there is a chance of formation of
cavities, so the system requires an accommodation mechanism also
operating. Mechanism which drives these accommodation are actually
controls the deformation rate. In ceramics there are various studies on
the deformation mechanism. These mechanisms depends on the systems,
temperature, stress and also grain sizes. Grain boundary and lattice
diffusion, dislocation mobility, interfaces reactions, in case of
liquid phase doped system, solute precipitations are known mechanisms
which control the deformation.
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