Biomedical Engineering Reference
In-Depth Information
which interface microstructure was controlled by the SiC/BN dual coating.
In particular, the BN coating provided the desired weak interface necessary
for high loading rate toughness, whereas the SiC overlayer provided a
barrier to diffusion and reaction at high temperatures. Both layers were
more stable in oxidizing atmospheres than either BN coatings alone or
previously studied carbon-rich interfaces (Brennan 1986, Cooper and Chung
1987).
1.6
Superplasticity characteristics of thermal shock
resistant ceramic nanocomposites
Superplasticity is defined as the ability of a polycrystalline material to
exhibit large elongations at high temperatures and relatively low stresses.
Today, from an engineering point of view, the term superplasticity is
ascribed to a polycrystalline material pulled out to very high tensile
elongations prior to failure with necking-free strain. This phenomenon is
usually found in many metals, alloys, intermetallics, composites and
ceramics (recently in high-temperature superconductor ceramics) when the
grain size is small enough: less than 10
m for
ceramics. When a polycrystalline material is deformed at high temperatures,
grain boundary sliding (GBS) takes place in two different ways.
μ
m for metals and less than 1
μ
.
Deformation takes place due to the flow of point defects in the
occurrence of GBS. This is called diffusional creep. In these cases, each
individual grain suffers almost the same deformation as that imposed on
the specimen and the grains that are nearest neighbours remain nearest
neighbours. This is termed Lifshitz GBS.
.
During GBS, deformation may be accompanied by intergranular slip
throughout adjacent grains, by localized slip adjacent to the boundaries
or by diffusional process of point defects. This is termed Rachinger
GBS.
The grains retain almost their original size and shape even after large
deformations when GBS is accommodated by some of the mechanisms
involving dislocation movement or diffusion of point defects. This GBS, as
the primary mechanism for deformation, is the basis for the high ductility
exhibited by some materials at high temperatures and therefore for their
structural superplastic behaviour. In many ceramic-related materials and
ceramic composites superplasticity is also said to occur even though the
polycrystal is deformed in compression, or in three-or four-point bending
conditions, as long as GBS is the primary deformation process (Chokshi
1993, Jimenez-Melendo et al. 1998, Nieh et al. 1991, 1997). The first
observation of superplasticity in a 3mol% yttria-stabilized tetragonal
