Biomedical Engineering Reference
In-Depth Information
and spraying [229], as well as to form ceramic sheets from slurries
tape casting [124, 216, 230, 231] doctor blade [232] and colander
methods might be employed [194-197, 233]. A combination of
several techniques is also possible [216, 234]. Furthermore, some of
these processes might be performed under the magnetic field, which
helps crystal aligning [235-238].
Powders are usually pressed damp in metal dies or dry in
lubricated dies at pressures high enough to form sufficiently strong
structures to hold together until they are sintered. An organic binder
such as polyvinyl alcohol helps to bind the powder together [200].
Drying at about 100°C is a critical step in preparing damp-formed
pieces for firing. Too much or too little water in the compacts can
lead to blowing apart the ware on heating or crumbling, respectively.
The binder is removed by heating in air to oxidize the organic phases
to carbon dioxide and water [194-197].
Furthermore, forming and shaping of any ceramic products
require a proper selection of the raw materials in terms of particle
sizes and size distribution. Namely, tough and strong bioceramics
consist of pure, fine and homogeneous microstructures. To attain
this, pure powders with small average size and high surface area
must be used as the starting sources. However, for maximum packing
and least shrinkage after firing, mixing of ~70% coarse and ~30%
fine powders have been suggested [197]. Mixing is usually carried
out in a ball mill for uniformity of properties and reaction during
subsequent firing. Mechanical die forming or sometimes extrusion
through a die orifice can be used to produce a fixed cross-section.
Drying involves removal of water and subsequent shrinkage of the
product. However, due to local variations in water content, warping
and even cracks may be developed during drying. Dry pressing and
hydrostatic molding can minimize these problems [197]. Afterwards,
the manufactured green samples are sintered.
Finally, to produce the accurate shaping, necessary for the fine
design of bioceramics, machine finishing might be essential [163, 194,
239]. Unfortunately, cutting tools developed for metals are usually
useless for bioceramics due to their fragility; therefore, grinding and
polishing appear to be the convenient finishing techniques [163,
194]. Furthermore, the surface of bioceramics might be modified by
various additional treatments [240].
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