Note: Descriptions are shown in the official language in which they were submitted.
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8LIP CA8TING PROCE88 AND APPARATU8
FOR PRODUCING GRADED MAT~T~T8
Field of the Invention
This invention relates to a modified slip casting process
enabling the manufacture of cast products having graded
properties throughout the wall thickness of the products,
and in particular with continuous gradients of those
properties. The invention also encompasses an apparatus
for carrying out the process.
Background of the Invention
In conventional slip casting, also known as colloidal
filtration, molds are filled with slip, i.e. a suspension
of fine solid particles in a liquid phase and the latter is
removed from the suspension through the walls of the mold
leaving the suspended particles behind on the walls. Fresh
slip may be added to the mold to replenish the slip that
has been removed and ensure that the mold remains full.
For solid core casting, this process continues until the
part is solid. For hollow core casting, the process
continues until the desired wall thickness is achieved, at
which point the slip remaining in the mold is poured or
drained from the mold. In conventional casting, molds made
of plaster of Paris are commonly used. The driving force
for casting is the capillary pressure within the network of
pore channels in the walls of the mold, the pore channels
being of a size smaller than the suspended particles of the
slip. As a cast layer of the filtered-out particles builds
up on the mold walls, the cast layer itself acts as the
filter and the particles continue to deposit.
Various factors affect the rate at which the process
proceeds. The properties of the mold, of the suspension
(slip) and of the cast layer are all important. For the
normal casting of well-behaved slips of constant
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composition, the thickness of the cast layer is
proportional to the square root of time. Various
t~-hn;ques have been used to increase the casting rate, for
example pressure casting wherein a pressure is applied on
the suspension, or vacuum casting in which the exterior of
the mold is subjected to a vacuum. In the conventional
casting, as mentioned above, the composition,
microstrùcture and related properties of the cast layer and
the resulting part (following the drying and thermal
treatment of the cast layer) are uniform throughout.
Graded materials having a gradient in composition and, in
some cases, in the microstructure (porosity content, grain
size) hold potential for achieving higher performance
levels than similar monolithic and composite materials in
which the various phases are uniformly distributed. These
graded materials are normally most appropriately utilized
in applications for which the property requirements at
opposite faces of a component differ. However, even in
situations where both faces are subjected to similar
conditions in service, compositional gradients may be used
to enhance the performance. For example, symmetrical
grading from both surfaces to the interior can be used to
engineer materials containing residual compressive stresses
at the surface. Such materials could have improved
mechanical strength.
Various methods have been used to produce layered and
graded bodies including: tape casting/lamination, see P.
Boch et al., J. Am. Ceram. Soc., 69 (8) C-191-C-192 (1986);
compaction of graded powders, see R.A. Cutler et al., J.
Am. Ceram. Soc., 70 (10) 714-18 (1987), infiltration, see
B.R. Marple et al., J. Mater. Sci., 28, 4637-43 (1993);
sequential casting, see J. Requena et al. in Ceramic
Transactions, Functionally Gradient Materials, Vol. 34, pp.
203-10, American Ceramic Society, Ed. J.B. Holt et al.,
1993; electrophoretic deposition, see P. Sarkar et al.,
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J.Am. Ceram. Soc., 75(10) 2907-909 (1992); and
sedimentation-slip casting, see J. Chu et al., J. Ceram.
Soc. Jpn., 101(7) 818-20 (1993). In some cases, these
processes are suited for the production of only very simple
geometries or limited to producing layered materials having
a stepwise change in composition. The abrupt interface
present between the zones in some layered or laminated
materials may have a very positive effect on the behaviour
of the material (for instance on the crack propagation).
However, such interfaces are sometimes undesirable as may
be the case where a difference in the coefficient of
thermal expansion across the boundary leads to cracking.
In these cases, a continuously graded material having a
smooth transition in composition through the body may be
preferred.
Accordingly, it is the object of the present invention to
provide a process useful for making parts, usually ceramic
parts or elements, with graded properties, rather than
constant or abruptly changing properties, across the
thickness of the part.
It is another object of the invention to provide an
apparatus for making parts, especially ceramic parts, with
graded properties as explained above.
It is still another object of the invention to provide a
process and apparatus for making parts with continuously
graded properties using the principle of slip casting.
It is yet another object of the invention to provide a
controllable process and apparatus for making slip cast
parts having predetermined properties.
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8ummary of the Invention
According to the invention, there is provided a process for
making porous bodies of particulate material with graded
S properties across the thickness of the bodies, the process
comprising the steps of:
a) providing at least one source of a slip
comprising the particulate material,
b) passing the slip substantially continuously
through a filtering mould having an inlet and an outlet in
conditions suitable for the formation of a predetermined
layer of the particulate material within said mould, and
c) separating said layer from said mould.
Preferably, at least two sources of dissimilar slips
are provided, and the process comprises the step of mixing
the slips before passing the resulting stream through the
mould.
There is also provided an apparatus for carrying out the
process of the invention, the apparatus comprising:
at least one source of a slip,
a filtering mould having an inlet and an outlet, and
supply means associated with said at least one source
of slip and the inlet of the mold for passing said slip
substantially continuously through said mold from the inlet
to the outlet thereof.
In a preferred aspect of the invention, the apparatus
comprises:
at least two sources of dissimilar slips,
mixing means associated with said sources of slips and
said mold for mixing said slips in a predetermined ratio to
obtain a combined stream thereof,
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a filtering mold having an inlet and an outlet, and
supply means associated with said mixing means for
passing said combined stream substantially continuously
through said mold.
The mold may be stationary or movable, e.g. rotatable.
The process can be carried out under increased pressure.
Brief Description of the Drawings
The invention will be described in more detail by way of
the following description to be taken in conjunction with
the accompanying drawings, in which
Fig. la is a schematic of an exemplary apparatus of
the present invention, with a stationary mold,
Fig. lb is a schematic of another exemplary apparatus
of the invention, with a rotatable mold,
Fig. lc is a schematic of still another exemplary
apparatus of the invention with inlet and outlet in the
same region of the mold;
Fig. 2 illustrates an effect of the flow rate of slip
through the mold on the thickness of the cast layer,
Fig. 3 illustrates the concentration profiles obtained
in a trial run No. 1 of the process of the invention,
Fig. 4a illustrates the microstructure at various
locations within the composite obtained in a trial run (No.
2) of the process,
Fig. 4b shows the concentration profiles obtained in
the trial run No. 2, and
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Fig. 5 shows the concentration profiles obtained in
the trial run No. 3.
Detailed Description of the Invention
Referring to Fig. la, an apparatus for graded slip casting
has a filtering mold 10 (referred to hereinafter as a mold)
defining a cavity 12. The mold 10 has an inlet 14 at its
lowermost portion and an outlet 16 at its uppermost
portion. This arrangement ensures that a liquid suspension
passing through the mold fills practically its entire
volume, unless very complicated shapes are involved.
Two slip reservoirs 18 and 20 and tubing 21 are provided to
supply selected slips 22 and 24 respectively to a mixing
device exemplified by a static mixer 26. The flow ratio of
respective slips 22 and 24 can be controlled by adjustable
pumps/flowmeters 28 and 30 installed on the tubing 21 so
that each pump/flowmeter controls the flow of a separate
slip stream. The flowmeters are controlled by a computer-
driven control unit 32 which processes a control signal
according to a predetermined set of conditions and passes
the control signal to the flowmeters 28 and 30.
The mixer 26 is connected to the inlet 14 of the mold 10
via tubing 34. The outlet 16 of the mold is connected via
outlet tubing 36 with an overflow collector 38. The tubing
34 is equipped with a drain 40. An optional pressure
regulator 41 is installed on the outlet tubing 36 to
illustrate the applicability of the apparatus described for
graded pressure casting. A cast layer 42 of relatively
uniform thickness is shown as formed within the cavity 12.
An alternative embodiment of the apparatus of the invention
is illustrated in Fig. lb in which like reference numerals
correspond to like or similar elements as in Fig. la. The
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mold 10 is positioned so that the flow between the inlet 14
and the outlet 16 of the mold is essentially or
approximately horizontal. The mold is rotatably mounted on
a roller 43. Slip rings 44 are provided to allow for a
sealed flow of the slip into the mold 10 and of the excess
slip from the mold 10 through a stationary tubing 46 into
an overflow collector 38.
It is not essential that the inlet of the mold in the
stationary mold version of the invention be positioned at
the lowermost portion of the mold. As shown in Fig. lc,
both the inlet and outlet of the mold can be situated at
the same end of the mold, or in the same portion thereof.
Arrows in Fig. lc indicate the flow of slip between the
inlet and the outlet of the mold. While Figure lc shows a
stationary mold, the apparatus of Fig. lc can be adapted to
handle a rotatable mold if slip rings 48 are used. It is
also possible to employ pressure casting with the apparatus
of Fig. lc, similarly as in the case of the embodiments of
Figs la and lb, if appropriate means for increasing the
pressure of the incoming slip stream are incorporated into
the system.
The embodiment of Fig. lc is particularly useful to produce
parts with one end closed.
The exact configuration and orientation of the mold during
casting and during subsequent draining will depend on the
geometry of the mold and the position of the inlet and
outlet ports. It is important that, during the casting
step, the slip have access to all sections of the mold and
that no pockets of air become trapped in unvented portions
of the mold. For draining, care must be taken to ensure
that all the excess slip can be drained from the mold.
This is true regardless of which of the three
configurations shown in Figs. la, lb and lc is being used.
To ensure that draining is complete it may be necessary to
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tilt or completely invert the mold during the draining
step. This may be done manually or by mechanical means.
To facilitate movement of the mold, flexible tubing and
quick disconnect fittings can be used.
Although only two reservoirs are shown in Figs. la - lc,
the number is not limited and depends on the desired
properties of the product.
As in conventional slip casting, the molds can be
fabricated from the plaster of Paris, porous plastic or any
other material of a porosity such as to enable the
filtering of the liquid phase while preventing the entry of
the solid phase (suspended particles). For the purpose of
the present application, the term mold will be used to
designate such filtering molds.
The slips may be prepared using water or another suitable
liquid, e.g. ethanol or methanol, or their mixture, as the
liquid phase. A wide range of materials can be used as the
solid phase of the slip, including ceramics, metals and
plastics. The slips can consist of particles of a size
smaller than 0.1 ~m and as large as 100 ~m. In fact, the
factor controlling the suitability of any particular
material and its particle size distribution for the process
of the invention, as for conventional slip casting, is the
stability of the suspension. Because of the continuous
movement of the suspension in the mold and the continuous
stirring in the mixer plus optional stirring in the
reservoir(s), the stability of the suspension (slip) is
less an issue in the process of the invention than in the
conventional casting where the slip remains undisturbed in
the mold for a period of time and settling can be a
problem. Of course, the lower limit of the particle size
depends on the porosity of the mold walls.
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_
Many of the same considerations which apply to the
preparation of slips for traditional slip casting are still
applicable for graded casting. In aqueous suspensions, the
pH is often used to aid in controlling the stability of the
suspension. Small amounts of other chemicals are sometimes
added. These include, dispersants to aid in dispersing the
particles in the suspension, anti-foam agents to assist in
removing air and eliminate bubbles so that defects in the
form of entrapped gas do not appear in the cast body, and
binders which may be used to give more strength to the
green part (i.e. the cast part before densification). The
solids loading (amount of solid phase in the suspension)
can vary over a wide range. There is really no lower limit
other than that dictated by the requirements of the cast
part. Very dilute suspensions of less than 1~ by volume
can be used. The upper bound will be determined, in part,
by the stability of the suspension. At the present time,
a practical upper bound for many systems is approximately
50% by volume. However, the present invention can be used
with more highly loaded systems if desired. The solid
phase in these suspensions may have a form of particles,
platelets, whiskers or fibers. For practical reasons, the
density of the particles should be higher than the liquid
in which they are suspended or appropriate measures taken
to ensure that they stay in suspension and do not float to
the top.
The embodiment of Fig. lb, with a horizontal or
approximately horizontal mold, is particularly useful for
the production of parts having complex geometries with
sections in which pockets of stagnant slip could develop.
This design can also serve to limit the amount of slip
required for the fabrication process since, unlike in the
vertical mold version of Fig. la, it is no longer necessary
to completely fill the mold. This can be quite important
when large parts are being fabricated.
2 1 2 ~ 8 6 3
The rotating mold embodiment can be combined with
centrifugal casting wherein a mold is filled with slip and
spun in order to increase the gravitational forces. For
that purpose, of course, the slip must be contained in the
cavity with closing means, not shown.
The flow rate from~ the particular reservoirs and the
overall flow rate through the mold is controlled during
casting. There is no Iower limit on the flow rate and, in
fact, it could be completely stopped if a layer of constant
composition were being cast. The upper limit of the flow
rate can be determined experimentally by monitoring the
effect of the flow rate on casting. At high flow rates,
the movement of the slip will interfere with the casting
process and as the flow is increased to still higher
levels, erosion of the cast layer may occur. The flow rate
at which these processes occur depends on the materials
being cast. It has been shown for aqueous alumina slips
having a pH of 5 and containing 38.6 vol. % solids having
a median particle size of 0.3 ~m that linear flow rates of
61 cm/min had no effect on the casting rate. Higher flow
rates were not tested so it is not yet known at which point
problems may arise. Linear flow rates of 60 cm/min are
higher than will be required for many applications. For a
given application, the flow rate which will be required
will be dictated by the change in composition or
microstructure which is being produced across the sample.
If bodies are being cast in which the change in composition
through the thickness must be steep, then, to increase the
response time of the process, more dilute suspensions can
be used which will have lower casting rates and therefore
require lower flow rates to achieve the same sensitivity to
changes in slip composition at the flow meters as
suspensions having higher concentrations. This approach
should alleviate most flow-rate related problems.
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The flow rates from the various reservoirs and hence the
overall flow rate through the mold can be controlled by
computer-driven pumps or, as in the embodiments described
herein, pumps/flowmeters. The flow rates which are
required are dictated by the slip composition, the casting
rates and the desired composition profile through the cast
sample. In each case, the composition of the slips or
suspensions in the reservoirs and the profile being sought
will be known; however, the casting rates of the various
slips must be determined and as well, it must be determined
how the casting rate changes with composition as the
various suspensions are mixed. Using this information, a
computer can be programmed to control the flow rates from
the reservoirs to produce a desired profile. For example,
in the simplest case, if two suspensions A and B were being
cast and if both the slips separately and combined in any
ratio had the same casting constant, then to produce a
linear profile through the thickness of the sample from
100% A to 100% B, the composition of the slip flowing
through the mold would need to be changed in a parabolic
fashion, i.e.
t (1)
f
V~ = ( tt)2 (2)
where VA and VH are, respectively, the volume fraction of
slip A and slip B in the fluid stream entering the mold, t
is the time at any point and t~ is the total time required
to achieve the desired thickness.
Because of a continuous flow of slurry through the mold,
the slip must be collected as it exits the mold. At the
end of the cast, the composition of this composite slurry
11
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can be readily determined by noting the amount of slurry
that has flowed through each pump, by measuring the volume
of slurry used from each reservoir or by chemical analysis
of the composite suspension. This new combined slurry
(slip) can then be reused in subsequent casting trials
either to produce homogeneous composite bodies by
conventional slip casting or used in an existing reservoir
for graded casting.
Following the casting step, treatment of the cast bodies is
similar to that normally used in slip casting. Bodies are
left in the mold for several minutes to hours, depending on
the material and the size of the component, to dry. The
mold is then disassembled and the part removed. It is left
for further drying either under ambient conditions
(relative humidity and temperature) or under controlled
conditions. This drying is normally completed in an
overnight period but for very large parts it may take days.
Following drying, the part undergoes a thermal treatment
which will depend on the composition of the cast body and
the desired properties of the fired body. For samples
containing an organic component such as a binder, this
thermal treatment will normally include a heating step
(often 25C-600C) during which the heating rate is very
slow (5-20C/h). It may be carried out in an atmosphere of
air or in other environments such as nitrogen, argon or
under vacuum. Heat treatment at higher temperatures will
again depend on the material. For example, for oxide
ceramics, if a dense final product is desired, samples will
typically be heated in air to a temperature in the range of
1400C-1700C. Heating rates vary but are normally 1C/min-
10C/min.
ExDerimental
Slips were prepared using 3-mole%-Y203-stabilized ZrO2 (TZ-
3YS, Tosoh, Tokyo, Japan) and Al203(RC-HP DBM, Reynolds
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Metals Co., Richmond, VA). The percentage of solids in the
alumina, zirconia and composite slips was maintained at
38.6~ by volume and a pH value of 5 was typical for all
slips. Molds were produced by mixing water and plaster of
Paris (No. 1 Pottery Plaster, United States Gypsum Co.,
Voorhees, NJ) in a weight ratio of water to plaster of
73:100 (consistency of 73).
The excess slip that flowed from the mold was collected and
reused in subsequent casting trials after determining its
composition. Following casting, the cast bodies were
removed from the mold, allowed to dry overnight at room
temperature and sintered at 1500C for 2 hours.
Sintered samples were sectioned and polished for
microstructural and compositional analysis. To accent the
grain junctions, samples were thermally etched by heat
treatment at 1450C for 15 min. Analyses were performed
using a sc~nn;ng electron microscope (JSM-6100, JEOL Ltd.,
Tokyo, Japan) equipped with an energy dispersive X-ray
(EDX) analysis system. Images of the microstructure were
obtained using the secondary electron signal or in the
mixed mode, using a combined secondary electron/back-
scattered electron signal. To determine the compositional
change across samples, EDX analysis was performed at 100 ~m
intervals over the surface. For each point, an area of
approximately 40 ~m x 50 ~m was analyzed.
Fig. 2 illustrates the results of tests conducted to
determine the effect of flow through the mold on the
quality of the cast. The tests were carried out using an
Al203 slip and no attempt was made to change the composition
during casting. The total casting time was the same for
each flow rate. The results indicate that, for the
conditions employed in these trials, the flow did not
affect the final thickness of the cast layer. It should be
noted that at the highest flow rate tested (a linear flow
2 1 2 4 8 6 3
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rate at the wall of 61 cm/min or a volume flow rate of 300cm3/min), a sufficient amount of slip was entering the mold
to completely replace the existing slip every 7 seconds.
These results are important for two reasons. Firstly, in
order to ensure a rapid response to changes in
concentration, particularly with concentrated slips and
fast casting rates, a sufficiently high flow rate will be
required. The highest flow rates tested in this work are
lo probably faster than would normally be required to ensure
sensitivity of the process to changes in slip composition.
Secondly, because in some cases molds will be used in which
the cross-section may vary from point to point, it is
important that the change in linear flow rate which will
occur in such molds not result in differences in wall
thickness in the different sections. For cylindrical
cavities, a decrease by a factor of two in the diameter of
the circular cross-section will result in a four-fold
increase in the linear flow rate. Therefore, mold design
will play an important role in determining the conditions
which are used for casting.
It is important to note that as the flow rate is increased
to higher values, one would expect to reach a point where
the casting process would be affected and the casting rate
reduced. Due to the abrasive nature of ceramic slips, if
the flow rate were increased still further, it is
reasonable to predict that the flowing slip would begin to
erode some of the previously cast layer. In cases where
such problems arise, steps could be taken to reduce the
casting rate by, for example, changing the properties
(reduced level of porosity, slower ab~orption rate) of the
mold or reducing the solids content in the slip. With such
changes, lower flow rates could be used and the sensitivity
of the response to changes in slip composition maintained.
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The conditions used for three of the casting trials carried
out during the test program are presented in Table 1.
These casting trials were conducted using a linear change
in the composition of the slip flowing through the mold
during the 8 minute grading portion of the casting. If
both slips and the resulting mixture had identical casting
properties, such a change would have been expected to
produce a parabolic composition profile since the thickness
of the cast layer is proportional to the square root of the
casting time.
Results of the EDX analysis of the samples produced in
these three trials are presented in Figs. 3, 4b and 5
respectively. These results indicate a relatively smooth
change in composition through the wall of the component.
In all three trials, the composition of the inner surface
of the cast body was very close to the final composition of
the slip entering the mold. This indicates that changes in
slip composition at the flow meters were rapidly reflected
in the composition of the cast layer.
Micrographs of one of the three sintered samples (Run No.
2 of Table 1) are presented in Fig. 4a. In these
micrographs,
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N N N
I ~ _ tO~
~ ~ _
C ~ tn to~ I~
E u~
t`~ t t
._
.~ ~
~ ~ '' ,~ tO" ,~ o~
C~ O
U~
o
~D _ 8 o 8 o 8 o
Q C ~ ~`7 ~
~n
.~ ~ N N N r
Q âe o ~ O ~ ~ C
q~ ~ _ tOt~ O ttO~
m
U~ N
O O
1~1
c m ~ m C m
~ O~
D 6
212~8~3
alumina appears as the darker phase. The location noted on each
micrograph represents the distance from the outside surface of
the sample. The results show that the smaller zirconia grains
are relatively well distributed within the alumina matrix;
however, in some cases small clusters or groups of zirconia
grains are present. This may indicate that the zirconia has not
been completely dispersed in the original slip. In general,
however, the micrographs confirm that one of the objects of the
invention, i.e. the production of continuously graded elements,
with a relatively smooth transition in composition across the
bodies, has been achieved.
While only alumina-zirconia composite samples were produced in
the tests, it is quite reasonable to expect, based on the
scientific principles, that a variety of other graded composites
can be obtained using the process and apparatus of the invention.
The essential feature of the process of the invention is the
continuous flow of the slip or a mixture of slips through a mold.
Certain advantages can be realized even with a single source of
a slip and continuous flow of the slip through the mold, for
example if the composition of the single slip is changed over
time. For example, slips may be utilized containing agents which
undergo a time-dependent chemical reaction which affects the
state of dispersion of the particles in the slip and hence the
microstructure of the cast layer. The apparatus of the invention
clearly lends itself to such an arrangement.
The apparatus and process of the invention allow for the
production of composite elements of desired gradients, either
continuous or step-wise.
Various modifications of the above-described embodiments may
occur to those versed in the art, and such modifications are
intended to form part of the invention which is only limited by
the scope of the appended claims.
