Biomedical Engineering Reference
In-Depth Information
4.3.4
Sintering and Firing
A sintering (or firing) procedure appears to be of a great importance
to manufacture bulk bioceramics with the required mechanical
properties. Usually, this stage is carried out according to controlled
temperature programs of electric furnaces in adjusted ambience of air
with necessary additional gasses; however, always at temperatures
below the melting points of the materials. The firing step can include
temporary holds at intermediate temperatures to burn out organic
binders [194-197]. The heating rate, sintering temperature and
holding time depend on the starting materials. For example, in the
case of HA, these values are in the ranges of 0.5-3°C/min, 1000-
1250°C and 2-5 h, respectively [241]. In the majority cases, sintering
allows a structure to retain its shape. However, this process might
be accompanied by a considerable degree of shrinkage [94, 242-
244], which must be accommodated in the fabrication process. For
instance, in the case of FA sintering, a linear shrinkage was found to
occur at ~715°C and the material reached its final density at ~890°C.
Above this value, grain growth became important and induced an
intra-granular porosity, which was responsible for density decrease.
At ~1180°C, a liquid phase was formed due to formation of a
binary eutectic between FA and fluorite contained in the powder as
impurity. This liquid phase further promoted the coarsening process
and induced formation of large pores at high temperatures [245].
The sintering mechanism is controlled by both surface and volume
diffusion at grain boundaries. In general, when solids are heated to
high temperatures, the constituent ions or atoms are driven to move
to fill up pores and open channels between the grains of powders, as
well as to compensate for the surface energy differences among their
convex and concave surfaces. At the initial stages, bottlenecks are
formed and grow among the particles (Fig. 4.3). Existing vacancies
tend to flow away from the surfaces of sharply curved necks; this
is an equivalent of a material flow towards the necks, which grow
as the voids shrink. Small contact areas among the particles expand
and, at the same time, a density of the compact increases and the
total void volume decreases. As the pores and open channels are
closed during a heat treatment, the particles become tightly bonded
together and density, strength and fatigue resistance of the sintered
object improve greatly. Grain-boundary diffusion was identified
as the dominant mechanism for densification [246]. Furthermore,
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