MATERIALS
FIGURE 2: Processing of primary ceramic powders.
temperature experienced. Particularly
useful is the ability to have a single
z-axis component in terms of “
desirability.” This single entity can be
calculated from any number of target
end properties, which are assigned
a particular weighting according to
their importance.
FED can also be useful for precipitation processes. Knowledge of the
surface charge (zeta potential) versus
pH of the phase being precipitated,
and the pH at which
precipitation takes place, can affect the
particle shape and particle size of the
powder that is created after drying/heating.
Dry powders
Powder suspension
Plastic deformable feedstock
Shaped component
Granulate
Add water, additives
Cast
Spray dry De-water
Extrude, injection mould Press
(uniaxial,
CIP, HIP)
Sintered end product
Suspension rheology
In terms of using primary ceramic or glass
powders to create a secondary powder or
shaped product, processing usually follows
one of three generic routes (Figure 2). It will
be appreciated from Figure 2 that, although
components are ultimately pressed from
granulate powder or extruded from a high
solids paste, processing almost always
initially proceeds via a fluid suspension.
Controlling the rheology of a given suspension is, therefore, important if yields
associated with the next process step are to
be high. In addition, there are implications
in terms of the final microstructure and,
therefore, in-service properties. For example, retaining a good fluid rheology prior to
spray drying whilst maximising the solids
content in the fluid suspension will ensure
granulates are more homogeneous. This in
turn leads, for example, to more consistent
coatings on metal implants or pressed components that are free from voids that affect
strength or electrical properties.
The rheological behaviour of aqueous
suspensions is governed by the extent to
which the suspended particles want to come
together (agglomerate) or stay apart. For
aqueous suspensions, this is dictated by the
surface charge, that is, the zeta potential of
powder particles. A low charge encourages
agglomeration and viscous suspensions. A
high charge (positive or negative) ensures
de-agglomeration and low viscosity. Figure
3 indicates that different ceramic powders
have different zeta potential versus pH
plots. Where different powders are to be
mixed together, this information is invalu-
able for ensuring that the correct conditions
exist so that the powders retain homogene-
ity with each other. Where unacceptable
zeta potential conditions exist, altering pH
and conductivity, employing surfactants
to change the surface charge, and paying
attention to the order of powder/additive
that is introduced to suspensions will
optimise suspension behaviour.
result in changes in pH, conductivity
and so forth that alter the suspension rheology as a function of time.
Zeta potential measurements made
as a function of time can provide an
insight into this unwanted behaviour prior to processing. Conversely,
ion leaching can be beneficial once
implants are placed in the body. In
this instance, zeta potential measurements made on calcined powders in
simulated body fluid can be used to
predict possible osteoblast–osteoclast
behaviour following implantation.
A sound knowledge of powder
suspension behaviour is also a valuable
tool in R&D. For example, nano-yttria
stabilised zirconias have the potential to
deliver greater longevity to zirconia hip
replacements through enhanced resistance
to hydrothermal ageing. Novel sintering to
retain the nano-structure in the final component is important, yet so is the ability to
maximise the solids content in suspensions
prior to granulation. Research is currently
being conducted to demonstrate the importance of re-optimising granulates contain-
Application examples
Zeta potential has been applied successfully in a number of medical devices. For
example, rheological quality control based
on initial zeta potential versus pH
measurements has ensured that spray drying
leads to a consistent ceramic granulate for
coating metal implants.
In the field of investment casting of
implants, faults associated with poor
control of ceramic slurries that are used
to fabricate cores and shells have been
overcome. An example of this is the avoidance of pimpling in cast alloys arising
from unwanted air bubbles present in the
ceramic shell.
Finally, leaching of inorganic ions from
suspended bioactive glass powders can
alter rheological behaviour versus time
prior to processing. This can be important
in the fabrication of bone replacement
components via powder suspensions; any
glass powder has the ability to lose ions
to the water it is suspended in. This can
A demand for mobility
and quality of life
means that there
are huge pressures
to create bespoke
products with
enhanced longevity
ing nano-zirconia against production
demands associated with ease of die filling,
ease of pressing and avoiding die sticking.
Suspension control is crucial to this proc-
ess. Other areas of ceramic research involv-
ing the optimisation of powder suspensions
include the following:
■ Granulate development for additive
layer manufacture where, for example, ink
jet printing of a fluid, possibly a fine pow-
der suspension, is used to “glue” granulate
powder particles together layer by layer
