Biomedical Engineering Reference
In-Depth Information
phase diagram. This type of cast object is not often used
commercially because the large shrinkage cavity and large
grains produced during cooling make the material weak
and subject to environmental attack.
If the MO and SiO 2 powders are first formed into the
shape of the desired object and fired at a temperature T 3 ,
a liquid-phase sintered structure will result (path 3).
Before firing, the composition will contain approximately
10-40% porosity, depending upon the forming process
used. A liquid will be formed first at grain boundaries at
the eutectic temperature, T 2 . The liquid will penetrate
between the grains, filling the pores, and will draw the
grains together by capillary attraction. These effects de-
crease the volume of the powdered compact. Since the
mass remains unchanged and is only rearranged, an in-
creased density results. Should the compact be heated
for a sufficient length of time, the liquid content can be
predicted from the phase diagram. However, in most
ceramic processes, liquid formation does not usually
proceed to equilibrium owing to the slowness of the re-
action and the expense of long-term heat treatments.
The microstructure resulting from liquid-phase
sintering, or vitrification as it is commonly called, will
consist of small grains from the original powder compact
surrounded by a liquid phase. As the compact is cooled
from T 3 to T 2 , the liquid phase will crystallize into a fine-
grained matrix surrounding the original grains. If the
liquid contains a sufficient concentration of network
formers, it can be quenched into a glassy matrix sur-
rounding the original grains.
A powder compact can be densified without the
presence of a liquid phase by a process called solid-state
sintering. This is the process usually used for
manufacturing alumina and dense HA bioceramics.
Under the driving force of surface energy gradients, atoms
diffuse to areas of contact between particles. The material
may be transported by either grain boundary diffusion,
volume diffusion, creep, or any combination of these,
depending upon the temperature or material involved.
Because long-range migration of atoms is necessary,
sintering temperatures are usually in excess of one-half of
the melting point of the material: T > T L /2 (path 4).
The atoms move so as to fill up the pores and open
channels between the grains of the powder. As the pores
and open channels are closed during the heat treatment,
the crystals become tightly bonded together, and the
density, strength, and fatigue resistance of the object
improve greatly. The microstructure of a material that is
prepared by sintering consists of crystals bonded to-
gether by ionic-covalent bonds with a very small amount
of remaining porosity.
The relative rate of densification during solid-state
sintering is slower than that of liquid-phase sintering
because material transport is slower in a solid than in
a liquid. However, it is possible to solid-state sinter
individual component materials such as pure oxides since
liquid development is not necessary. Consequently, when
high purity and uniform fine-grained microstructures are
required (e.g., for bioceramics) solid-state sintering is
essential.
The fifth class of microstructures is called glass-
ceramics because the object starts as a glass and ends up as
a polycrystalline ceramic. This is accomplished by first
quenching a melt to form the glass object. The glass is
transformed into a glass-ceramic in two steps. First, the
glass is heat treated at a temperature range of 500-700 C
(path 5a) to produce a large concentration of nuclei from
which crystals can grow. When sufficient nuclei are
present to ensure that a fine-grained structure will be
obtained, the temperature of the object is raised to a range
of 600-900 C, which promotes crystal growth (path 5b).
Crystals grow from the nuclei until they impinge and up
to 100% crystallization is achieved. The resulting micro-
structure is nonporous and contains fine-grained, ran-
domly oriented crystals that may or may not correspond
to the equilibrium crystal phases predicted by the phase
diagram. There may also be a residual glassy matrix,
depending on the duration of the ceraming heat treat-
ment. When phase separation occurs (composition B in
Fig. 3.2.10-3 ), a nonporous, phase-separated, glass-in-
glass microstructure can be produced. Crystallization of
phase-separated glasses results in very complex micro-
structures. Glass-ceramics can also be made by pressing
powders and a grain boundary glassy phase (Kokubo,
1993). For additional details on the processing of ce-
ramics, see Reed (1988) or Onoda and Hench (1978), and
for processing of glass-ceramics, see McMillan (1979).
Nearly inert crystalline ceramics
High-density, high-purity ( > 199.5%) alumina is used in
load-bearing hip prostheses and dental implants because
of its excellent corrosion resistance, good bio-
compatibility, high wear resistance, and high strength
(Christel et al. , 1988; Hulbert, 1993; Hulbert et al. ,
1987; Miller et al. , 1996). Although some dental im-
plants are single-crystal sapphires (McKinney and
Lemons, 1985), most Al 2 O 3 devices are very fine-grained
polycrystalline < a-Al 2 O 3 produced by pressing and
sintering at T ΒΌ 1600-1700 C. A very small amount of
MgO ( < 0.5%) is used to aid sintering and limit grain
growth during sintering.
Strength, fatigue resistance, and fracture toughness of
polycrystalline < a-Al 2 O 3 are a function of grain size and
percentage of sintering aid (i.e., purity). Al 2 O 3 with an
average grain size of < 4 m m and > 99.7% purity exhibits
good flexural strength and excellent compressive strength.
These and other physical properties are summarized in
Table 3.2.10-4 , along with the International Standards
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