Biomedical Engineering Reference
In-Depth Information
(similar to oil and water), which can be frozen in this state so that they
are preserved at room temperature, is truly amazing and permits us to
make useful inhomogeneous glasses. We called this phenomenon phase
separation in glasses.
The term ''ceramic'' is the name of another type of material. In
contrast to glasses, ceramics contain only a very small amount of a
glass phase. In scientific language, this amount is expressed as 1%
by volume (1 vol%). Ceramics are mainly composed of crystals. They
may be made of one or several types of crystals: scientists refer to
these as crystal phases. These crystals determine the main characteristics
of the ceramic. For example, ZrO
2
(zirconia) crystals produce very
strong and tough ceramic materials. The flexural strength (resistance
to bending) of zirconia ceramics is more than 1000MPa. This means
that 10 000 times atmospheric pressure must be applied before the
material bends and breaks (1 atm
0.1MPa). The fracture toughness of
the ceramic, which is symbolized by the abbreviation
K
IC
and describes
the ability of a material containing a crack to resist breaking, is about
4.5MPa
≈
m
1/2
. Apart from having to be strong and tough, ceramics
can also be required to have other characteristics like magnetism or
biocompatibility. Biocompatible ceramics have been specially developed
for use in the human body, for example, in the form of bearing surfaces
in artificial hips and knees and teeth.
Glass and ceramics make up a wide range of materials. The term
''glass-ceramics'' refers to a material that combines the characteristics
of both types of materials to produce an even better material [1]. This
improvement is made possible because the new material contains both
glass components (glass phases) and crystals (crystal phases). Glass-
ceramics are made by first designing and making a glass and then
heating it to high temperature. Some of the glass will crystallize to
become a glass-ceramic. This gives a very intimate relationship between
the glass and crystal phases, yielding impressive mechanical properties.
The special glass from which glass-ceramics are produced is called a
base glass. The remarkable thing about glass-ceramics is not the fact
that they are a successful mixture of glass and crystals, which can be
fired at temperatures around 1000
◦
C to create the final product, but
that the formation and growth of crystals can be controlled in a glass.
Simply put, glass-ceramics are materials that are composed of one
or more glass phases and one or more crystal phases, with the crystals
having been formed in a base glass.
This definition is very general and you will want to know exactly what
makes this process possible and why it was only discovered in 1959.
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