Method for preparation of a functionally gradient material

The invention provides a method for in-situ powder metallurgy processing of a functionally gradient material (FGM) which uses a preceramic polymer binder system with the metal and/or ceramic powders used to produce the intermediate layers of the composite. The invention also provides a method for controlling shrinkage of the functionally gradient material during processing while still preserving the desired density of the intermediate layers by controlling the preceramic polymer binder content within the functionally gradient material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for producing a functionally gradient 
material and, more particularly, to a method for producing a functionally 
gradient material using a preceramic polymer binder, as well as to 
materials so produced. 
2. Description of the Prior Art 
A Functionally Gradient Material (FGM) is an anisotropic composite material 
and can be a metal-ceramic material. A gradient in composition and/or 
microstructure which results in a gradient in composite properties is 
deliberately introduced into the material. Depending upon the application 
of the FGM, the gradient can be created in a continuous or stepwise 
fashion. For example, FGM's are used in aerospace applications where a 
graded interlayer between a relatively low coefficient of thermal 
expansion ceramic and a relatively high coefficient of thermal expansion 
metal relieves thermal stress by comparison with that created when the 
metal and ceramic are directly bonded. Thus, crack formation is prevented. 
Typically, FGM's are prepared by (1) Chemical or Physical Vapor Deposition 
(CVD/PVD), conventional powder metallurgy processing, plasma spraying or 
Self Propagating High Temperature Synthesis (SHS). Powder metallurgy 
processing is the technique most commonly used for the preparation of 
composites from layers or plies having millimeter dimensions. 
Conventional powder metallurgy processing involves consolidation processes 
that typically require binders which must be removed during some point of 
the process. These binders include thermoplastic organic binders which are 
removed by heating of the green composite before sintering of the molded 
part. Thermolysis of the thermoplastic binder generates volatile 
byproducts which must be removed from the part. Removal of such volatile 
byproducts is costly and must be accomplished in a carefully controlled 
manner if cracking, deformation or bloating of the part is to be avoided. 
Also, shrinkage control of individual layers is an important issue, 
especially when top and bottom layers have large differences in density. 
Conventionally, this is done by blending powders of various particle sizes 
as described by Takemura et al., "Evaluation of Thermal and Mechanical 
Properties of Functionally Gradient Materials of ZrO.sub.2 -Ni System", 
Ceramic Transactions, 1993, 34, 271. 
Preceramic polymers are generally inorganic and organometallic polymers 
pyrolizable to yield one or more ceramic phases as a residue of the 
pyrolysis. It has been demonstrated, as described by Semen et al., "A 
Preceramic Polymer Route to Molded SiC Ceramic Parts", Ceram. Eng. Sci. 
Proc. 1991, 12, 1967, that preceramic polymers can be used successfully as 
binders in fabrication of shaped ceramic parts. Little evolution of 
gaseous byproducts is observed and parts having strengths of over 650 MPa 
have been fabricated. The use of preceramic polymers as binders for 
preparation of metal matrix composites has been reported in Yajima et al., 
"Heat-Resistant Fe-Cr Alloy with Polycarbosilane as Binder", Nature, 1976, 
264, 237, in Japanese Patent No. 04-128338 and in Seyferth et al., 
"Application of Preceramic Polymers in Powder Metallurgy: Their Use as 
Low-Loss Binders and for in Situ Formation of Dispersed Ceramic Phases in 
the Metal Matrix", Chem. Mater., 1994, 6, 10. 
Thus, there exists a need for a method of in-situ powder metallurgy 
processing of a functionally gradient material which avoids the use of 
those types of binders, which, when removed by thermolysis, generate 
volatile byproducts whose removal is relatively costly and often results 
in degradation of the properties of the material being formed. This method 
should also provide a means for controlling shrinkage of functionally 
gradient material during processing. 
SUMMARY OF THE INVENTION 
The invention provides a method for producing a functionally gradient 
material and includes the use of a preceramic polymer binder. 
Specifically, the method includes steps of providing a substrate; 
providing a top layer, providing a substrate material powder having the 
same composition as the substrate; providing a preceramic polymer binder 
pyrolyzable to yield a ceramic; mixing the substrate material powder with 
the preceramic polymer binder to form a substrate material 
powder/preceramic polymer binder mixture characterized by a substrate 
material/ceramic ratio; forming the intermediate layer by applying the 
substrate material powder/preceramic polymer binder mixture to the 
substrate and applying the top layer to form a top layer/intermediate 
layer/substrate composite body characterized by the substrate 
material/ceramic ratio and heating the top layer/intermediate 
layer/substrate composite body so that a functionally gradient material is 
produced. 
According to another aspect of the invention, the steps already described 
are performed; however, top layer material powder having the same 
composition as the top layer is mixed with the preceramic polymer binder 
to form the intermediate layer. 
According to yet another aspect of the invention, a method is provided for 
producing a functionally gradient material which in addition to the 
above-described steps further includes steps of providing a top layer 
material powder; mixing the top layer material powder with the substrate 
material powder and preceramic polymer binder to form a substrate material 
powder/preceramic polymer binder/top layer material powder mixture 
characterized by a substrate material/top layer material/ceramic 
composition and forming an intermediate layer from this mixture. 
It is an object of the invention to provide a method for in-situ powder 
metallurgy FGM processing which uses a preceramic polymer binder system 
characterized by minimal evolution of gaseous byproducts during pyrolysis 
and which forms a desirable residue, one or more ceramic phases, as a 
result of pyrolysis. 
It is a further object of the invention to provide a functionally gradient 
material including ceramic phases finely dispersed in the metal matrix to 
result in composites having greater hardness, strength and oxidation 
resistance than a bulk metal. 
It is yet another object of the invention to provide a functionally 
gradient material processing method which controls shrinkage of the 
functionally gradient material during processing, while still preserving 
the desired density of these layers. 
Other and further objects, features and advantages of the present invention 
will be readily apparent to those skilled in the art upon reading the 
description of the preferred embodiments which follows.

DETAILED DESCRIPTION OF THE INVENTION 
A functionally gradient material can be prepared by the application of an 
intermediate layer or stepwise layer-by-layer application of a plurality 
of intermediate layers which vary stepwise in composition on the substrate 
and, finally, applying a top layer to the topmost intermediate layer to 
form the top layer/intermediate layer or layers/substrate composite body 
which is a green body at this stage of the process. The intermediate layer 
or layers can include substrate material powder mixed with preceramic 
polymer binder and can be characterized by a substrate material/ceramic 
ratio calculated based upon the expected yield of ceramic residue from the 
preceramic polymer binder upon pyrolysis which, thus, remains fixed 
through step (8) of heating. The top layer/intermediate layer or 
layers/substrate composite body is then heated to produce the functionally 
gradient material. 
The foregoing method can be used, for example, to produce a functionally 
gradient material having a metal substrate, a metal/ceramic intermediate 
layer and ceramic top layer or to produce a symmetrical functionally 
gradient material wherein the top layer and substrate are the same 
material which can be a ceramic or a metal or a mixture of a ceramic and a 
metal. 
A functionally gradient material can also be produced by performing the 
same steps as described above; however, in step (6) forming an 
intermediate layer or layers from a mixture of top layer material powder 
and preceramic polymer binder characterized by a top layer material 
powder/ceramic ratio calculated based upon the expected yield of ceramic 
residue from the preceramic polymer binder upon pyrolysis and, thus, 
remains fixed through step (8) of heating. This method can be used, for 
example, to produce a functionally gradient material having a ceramic top 
layer and a ceramic substrate of the same composition as the preceramic 
polymer binder ceramic residue, but of a different composition than that 
of the ceramic top layer. 
Finally, a functionally gradient material can be produced by performing the 
already described steps, but in step (6) forming an intermediate layer or 
layers from a mixture including substrate material powder, preceramic 
polymer binder and top layer material powder. This method can be used, for 
example, to fabricate a functionally gradient material wherein the 
substrate is a metal, the top layer is a ceramic and the preceramic 
polymer binder is a preceramic polymer binder which has a residue of the 
same composition as the top layer and top layer material powder. For such 
a functionally gradient material, varying the relative amounts of top 
layer material powder and preceramic polymer binder can be one way of 
controlling shrinkage of the top layer/intermediate layer or 
layers/substrate green body during heating, as will be described in 
greater detail elsewhere in this patent application. 
Articles produced according to the method of the invention are also 
provided. 
The substrate can be a "bulk material or a solid" which as used herein in 
the specification and following claims refers to an integral body such as 
a layer or sheet or other shaped body. The substrate can also be provided 
in the form of powder which is sintered or otherwise reacts in situ to 
form a bulk substrate material. The powder can be mixed with a preceramic 
polymer binder which pyrolyzes to yield a ceramic. 
Similarly, the top layer can be provided as a bulk material or a bulk 
material top layer can be formed in situ from a powder starting material. 
The powder starting material can be mixed with preceramic polymer binder 
which pyrolyzes to yield a ceramic and which can, in some cases, serve as 
a sintering aid for the top layer material powder. When ceramic thin films 
are desired for the top layer, the preceramic polymer binder may be used 
alone so that the layer is formed in situ by pyrolysis of the preceramic 
polymer binder. Usually, the thickness of such layers is on the order of a 
few nanometers. 
A functionally gradient material having a ceramic top layer and a metal 
substrate or having a ceramic top layer and ceramic substrate of different 
composition from the ceramic top layer or having a ceramic top layer and 
ceramic substrate of the same composition or having a metal substrate and 
metal top layer of the same composition can be prepared according to the 
method of the invention. The method is broadly applicable to most of the 
transition metals, main group metals, such as aluminum, f-orbital inner 
transition metals, metalloids and alloys formed from these elements as 
well as to most ceramics. 
The metal substrate can be a transition metal such as Sc, Ti, V, Cr, Mn, 
Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, 
Re, Os, Ir, Pt, Au, or Hg or mixtures thereof. The metal substrate can 
also be an f-orbital inner transition metal such as Ce, Pr, Nd, Pm, Sm, 
Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th or U or mixtures thereof. Finally, 
the metal substrate can be a main group metal such as A1, Pb, Sn, Bi or Sb 
or mixtures thereof. The substrate can also be a metalloid such as B, C, 
Si or Ge or mixtures thereof. Carbon can be in an amorphous, graphite or 
diamond form. Most of the foregoing elements and their alloys are readily 
commercially available in powder form. Alloys can also be formed in situ 
by combining powders of the elemental constituents of the alloy. 
The method is also broadly applicable to preparation of a functionally 
gradient material including a ceramic layer of almost any ceramic 
including non-oxide non-metal ceramics, non-oxide metal ceramics, oxide 
non-metal ceramics and metal oxide ceramics. Non-oxide non-metal ceramics 
can include materials such as silicon carbide (SiC), silicon nitride 
(Si.sub.3 N.sub.4), silicon carbonitride, silicon oxycarbide, silicon 
oxynitride boron carbide (B.sub.4 C), boron nitride (BN), silicon boride 
(B.sub.4 Si), or aluminum boride (AlB). Non-oxide metal ceramics can 
include f-orbital inner transition metal carbides, transition metal 
carbides, such as titanium carbide (TiC) or zirconium carbide (ZrC), or 
transition metal borides, such as titanium boride (TiB.sub.2). Oxide 
non-metal ceramics can include metalloid oxides like silica (SiO.sub.2). 
Metal oxide ceramics can include materials such as zirconia (ZrO.sub.2) 
and alumina (Al.sub.2 O.sub.3). The method can also be used with 
transition metal silicide ceramics and diamond. 
Many non-oxide preceramic polymer binders can be used in the method of the 
invention. The preceramic polymer binder is selected so that it is 
chemically compatible with the powder with which it is to be combined to 
form a layer or layers of the functionally gradient material. As used 
herein, the term "chemical compatibility" indicates that when a particular 
preceramic polymer binder and powder are used together, the resulting 
functionally gradient material exhibits a desired microstructure as well 
as mechanical and physical properties. Preceramic polymer binders which 
upon pyrolysis yield a ceramic residue which has a residue mass of at 
least 50% of the starting mass of preceramic polymer binder are suitable 
for use in the method. Generally, an oxide precursor, i.e., a precursor 
which produces an oxide upon pyrolysis, can be combined with an oxide 
powder to yield satisfactory results. A non-oxide precursor, i.e., a 
precursor which produces a non-oxide upon pyrolysis, can be combined with 
a non-oxide powder to yield satisfactory results. 
Non-oxide preceramic polymer binders include organosilicon polymers that 
serve as pyrolyric precursors for silicon carbide, silicon nitride or 
silicon carbonitride, silicon oxycarbide, or silicon oxynitride. The 
properties, including the type and quantity of pyrolysis products, of 
these organosilicon polymers vary considerably. For example, pyrolysis in 
an inert atmosphere can result in formation of differing amounts of free 
carbon or free silicon in addition to the desired silicon-containing 
ceramic phase. Some polymers yield an excess of free carbon which can 
serve as a sintering aid when used with a silicon carbide filler. Other 
polymers pyrolyze to yield nearly stoichiometric silicon nitride and 
silicon carbide. The polymer can be selected depending upon the properties 
desired for a particular application. 
The non-oxide preceramic polymer binder can be a polysilane having the 
chemical formula (R.sup.1 R.sup.2 Si).sub.n wherein R.sup.1 and R.sup.2 
are organic substituents such as methyl, vinyl, aryl, phenyl, or allyl 
groups or hydrogen and having a molecular weight in the range of from 
about 500 to about 500,000 and more preferably from about 500 to 100,000 
relative to polystyrene. The polymer binder may be chemically 
cross-linked, depending on the identity of the R.sup.1 and R.sup.2 
substituents, with a catalyst such as an early transition metal compound 
catalyst, a late transition metal compound catalyst, a metalloid compound 
catalyst, an organic peroxide or azo compound catalyst, or ultraviolet or 
gamma irradiated. The polymer binder can be a poly(methylsilane) of 
composition [(CH.sub.3 SiH).sub.x (CH.sub.3 SiH.sub.2).sub.z (CH.sub.3 
Si).sub.y ].sub.n where x+y+z=1 x/z=17, and 0.30&lt;(x+z)&lt;0.95 as described 
by Seyferth et al., "Near-Stoichiometric Silicon Carbide from an 
Economical Polysilane Precursor", J. Am. Ceram. Soc., 1992, 75, 1300. When 
either the R.sup.1 or R.sup.2 substituent of the polysilane is hydrogen, 
use of catalytic amounts of antioxidants such as BHT 
(2,6-Di-tert-butyl-4-methylphenol) or IRGANOX 245.TM. 
(tri-ethylglycol-bis-3-(3-tert-butyl-4-hydroxy-5-methylphenyl) 
-3-propionate) for oxidation stability, is highly recommended as described 
by Bryson, "Polysilane-based Compositions, Especially for the 
Manufacturing of Silicon Carbide, Stabilization of the Compositions, and 
Manufacturing of Silicon Carbide", PCT International Application WO 
93/14164. 
The non-oxide preceramic polymer binder can be a polycarbosilane as 
described by Seyferth, "Polycarbosilanes: An Overview", p. 21 in Inorganic 
and Organometallic Polymers, ACS Symposium Series 360, Edited by M. 
Zeldin, K. J. Wynne and H. R. Allcock, American Chemical Society, 
Washington, DC, 1988. The polycarbosilane can have the chemical formula 
[(R.sup.1 R.sup.2 Si).sub.a (CH.sub.2).sub.b ].sub.n wherein R.sup.1 and 
R.sup.2 are methyl, vinyl, aryl, phenyl, or allyl groups or hydrogen, 
wherein a is in the range of from about 1 to about 4 and b is in the range 
of from about 1 to about 4 and having a molecular weight in the range of 
from about 200 to about 500,000 and more preferably in the range of from 
about 200 to about 100,000 relative to polystyrene. The structure can be 
of a more complicated type, e.g., the Nicalon.TM. polycarbosilane, as 
described by Yajima, "Special Heat-Resisting Materials from Organometallic 
Polymers", Am. Ceram. Bull., 1983, 62[8], 893, which has been suggested to 
contain alternate Si-C bonds arranged in cyclic, linear and crosslinked 
configurations. 
The non-oxide preceramic polymer binder can be a poly(vinylsilane) having 
the chemical formula [(CH.sub.2 CH) SiH.sub.3 ].sub.n having a molecular 
weight in the range of from about 200 to about 500,000, and more 
preferably in the range of from about 200 to 50,000 relative to 
polystyrene. 
The preceramic polymer binder can also be a polyorganosilazane of the type 
[(R.sup.1 R.sup.2 SiNH).sub.a (R.sup.1 SiN).sub.b ].sub.n wherein R.sup.1 
and R.sup.2 are methyl, vinyl, aryl, phenyl, or allyl groups or hydrogen, 
as described by Seyferth et al., U.S. Pat. No. 4,482,669. In the foregoing 
chemical formula, a is in the range of from about 0.3 to about 0.6, b is 
in the range of from about 0.3 to about 0.7 and having a molecular weight 
in the range of from about 500 to about 500,000 and more preferably in the 
range of from about 500 to about 50,000 relative to polystyrene. 
The preceramic polymer binder can be an organosiloxane polymer such as 
(R.sup.1 R.sup.2 SiO).sub.n or [(R.sup.1 R.sup.2 SiO).sub.a (R.sup.3 
SiO.sub.3/2b ].sub.n wherein R.sup.1, R.sup.2 and R.sup.3 can be a methyl, 
vinyl, aryl, phenyl or allyl group or hydrogen, a is in the range of from 
0 to 0.8, b is in the range of 0.2 to 0.5 and having a molecular weight of 
in the range of from about 300 to about 500,000 and more preferably in the 
range of from about 300 to about 50,000 relative to polystyrene. 
The preceramic polymer binder can be a boron nitride precursor such as, but 
not limited to, a decaborane(12)-diamine polymer, a fused ring type 
polyborazine, a linked ring type polyborazine or a 
poly(silazanylborazine). 
The preceramic polymer binder can also contain other elements such as, but 
not limited to, titanium, zirconium or aluminum chemically bonded within 
the polymer structure. 
The preceramic polymer binder should be present in the substrate material 
powder/preceramic polymer binder mixture at levels in the range of from 
about 2 wt. % to about 60 wt. %, more preferably in the range of from 
about 2 wt. % to about 40 wt. %, and most preferably in the range of from 
about 2 wt. % to about 20 wt. % preceramic polymer. A ceramic filler can 
also be added to the substrate material powder/preceramic polymer binder 
mixture to produce a desired substrate material/ceramic ratio. As used 
herein, a "ceramic filler" refers to a powder having the composition of 
any of the ceramic materials previously set forth. 
Steps (5) and (6) of mixing the substrate material powder with the 
preceramic binder to form a substrate material powder/preceramic polymer 
binder mixture characterized by a particular substrate material/ceramic 
ratio and of applying the substrate material powder/preceramic polymer 
binder to form an intermediate layer having a particular substrate 
material/ceramic ratio, respectively, can be repeated a plurality of 
times. In addition, a filler material having the top layer material 
composition can be incorporated into the mixture to form an intermediate 
layer or layers having a particular substrate material/top layer material 
ratio. It is noted that the top layer material powder can be a ceramic 
filler. With each repetition of steps (5) and (6), a new intermediate 
layer is applied to the intermediate layer previously formed to produce a 
top layer/intermediate layer/substrate composite body having a desired 
composition gradient. 
Since each individual layer of the plurality of intermediate layers has a 
different composition, each of the layers is also characterized by a 
different individual layer shrinkage rate, heating the top 
layer/intermediate layers/substrate composite body in step (8) can result 
in distortion, such as warpage of the top layer/intermediate 
layers/substrate composite body. Individual layer shrinkage can be 
controlled and, thus, distortion of the overall top layer/intermediate 
layers/substrate composite body can be prevented by varying the content of 
preceramic polymer binder such as by gradually increasing its amount from 
the amount, if any, present in the substrate layer to the amount present 
in the top layer. Shrinkage can likewise be controlled by keeping the 
substrate material/ceramic filler ratio constant in each individual layer 
and varying the amount of preceramic polymer binder; varying the substrate 
material powder/preceramic polymer binder ratio while keeping top layer 
material, which can be ceramic filler, content constant; varying the top 
layer material or ceramic filler/preceramic polymer binder ratio while 
keeping the substrate material content constant; or varying all three 
components, but having a gradual increase of preceramic polymer binder 
from the amount present in the substrate to that present in the top layer. 
In these ways, shrinkage can be controlled, while still preserving the 
desired density of the intermediate layers. 
For example, a green top layer/intermediate layer/substrate composite body 
having a ceramic top layer and a metal substrate, i.e., bottom layer, will 
develop convex curvature upon pyrolysis as the metal bottom layer 
undergoes greater shrinkage than the ceramic top layer, if all 
intermediate layers have constant preceramic polymer binder concentration. 
However, gradually increasing the amount of binder from none present in 
the metal substrate layer to the amount present in the top ceramic layer 
will cause greater shrinkage to occur in the ceramic-rich layers, thereby 
keeping shrinkage uniform throughout the functionally gradient material 
and avoiding formation of convex curvature. 
FIG. 1 shows a preferred method for applying the substrate material 
powder/preceramic polymer binder mixture to a substrate and for applying 
successive intermediate layers of varying substrate material powder/top 
layer material powder (ceramic filler)/preceramic polymer binder ratio 
with a gradient in composition from substrate material-rich to top 
layer-material rich. In FIG. 1a, substrate powder 10 is loaded into piston 
assembly 12. As shown in more detail in FIG. 2, piston assembly 12 further 
includes bottom plunger 14 and top plunger 16 slidably mounted within 
support member 18 and roller 20. Roller 20 is then translated laterally in 
the direction given by arrow 22 over powder 10 as roller 20 is itself 
rotated in the direction given by arrow 24. This counter-rotation and 
translation of roller 20 results in smooth powder layer 26 shown in FIG. 
1b. Although not shown in FIG. 1, a solid bulk material, such as a metal, 
substrate can be provided to form the first layer. Lower plunger 14 is 
then retracted downward as shown in FIG 1c, so that an additional powder 
layer richer in top layer material can be applied to surface 28 of smooth 
powder layer 26 which is poorer in top layer material content than is the 
new layer being applied thereto. When an assembly of intermediate layers 
30 has been built up on bottom plunger 14, top plunger 16 is brought into 
contact with support member 18 as shown in FIG. 1d and uniaxial pressure 
is applied in the directions given by arrows 32 in FIG. 1e. A strong, 
coherent functionally gradient material 34, having metal rich surface 36 
and ceramic rich surface 38 is produced once additional steps of 
isopressing and pyrolysis are completed. 
Step (8) of heating can include a first pyrolysis at a first pyrolysis 
temperature to pyrolyze the preceramic polymer binder to obtain a ceramic 
product which is followed by heating the ceramic product to a reaction 
temperature higher than the pyrolysis temperature at which the solid-state 
reaction of the ceramic product with the substrate material powder occurs. 
In a preferred embodiment of the invention, the functionally gradient 
material is a copper/silicon carbide functionally gradient material, the 
substrate is copper powder, the substrate material powder is copper powder 
and the preceramic polymer binder is polycarbosilane. 
In another preferred embodiment of the invention, the functionally gradient 
material is a copper/silicon carbide functionally gradient material, the 
substrate is bulk copper metal, the substrate material powder is copper 
powder and the preceramic polymer binder is polycarbosilane. 
In yet another preferred embodiment of the invention, the functionally 
gradient material is an aluminum/silicon carbide functionally gradient 
material, the substrate is aluminum powder combined with preceramic 
polymer binder, the substrate material powder is aluminum powder and the 
preceramic polymer binder is poly(methylsilane). 
In order to further illustrate the method of the present invention and the 
characteristics of articles produced according to that method, the 
following examples are provided. The particular compounds and processing 
conditions utilized in the examples are meant to be illustrative of the 
present invention and not limiting thereto. 
EXAMPLE 1 
The following example is provided to show how a Cu/SiC functionally 
gradient material can be prepared using a powder substrate. 
Powder mixes were prepared having the compositions given in Table 1 which 
follows. 
TABLE 1 
______________________________________ 
SiC 
(&lt;1 micron Nicalon .TM. Precursor 
Cu Powder 
particle size) 
mole % (-325 mesh) 
Mix 
mole % based on Si 
based on Si mole % No. 
______________________________________ 
90 10 0 1 
50 10 40 2 
20 10 70 3 
10 10 80 4 
0 10 90 5 
0 5 95 6 
0 0 100 7 
______________________________________ 
Dry mixtures of powders having Mix Nos. 1-7 were each suspended in toluene 
with sonic activation using a Branson 3200 apparatus operating between 45 
KHz and 56 KHz to maintain dispersion of the particles and to maintain 
cleanliness of the surfaces of the metal particles. After overnight sonic 
activation, the solvent was removed in vacuum and the powders were dried, 
pulverized in a mortar and pestle and passed through a 270 mesh sieve to 
assure a uniform particle size distribution. Nicalon.TM. X9-6348 precursor 
is the precursor to Nicalon198 fibers and is a commercially available 
polycarbosilane distributed by Dow Corning Co. and manufactured by Nippon 
Carbon Co. Ltd., Japan, characterized by a molecular weight relative to 
polystyrene of in the range of from about 1,000 to about 275,000 and from 
about 1,000 to about 400,000 when cross-linked. The powders were then 
compacted and uniaxially compressed at 13.5 kpsi for 1 hour and 
isostatically pressed at 40 kpsi for 10 minutes to form a green 
functionally gradient body according to the steps shown in FIG. 1. The 
green functionally gradient material body was then pyrolyzed in a furnace 
under an Ar atmosphere according to the following firing schedule: ramp I: 
5.degree./min.; dwell I: 2 hours, 250.degree. C.; ramp II: 5.degree./min.; 
dwell II: 3 hour, 900.degree. C. with the green functionally gradient 
material body kept under a 200 g uniaxial load. 
FIG. 3 shows Cu/SiC functionally gradient material 40 produced according to 
the foregoing example. In the photomicrograph, layers 42, 44, 46, 48, 50, 
52 and 54, respectively, have compositions corresponding to Mix Nos. 1, 2, 
3, 4, 5, 6 and 7. 
EXAMPLE 2 
The following example is provided to show how a Cu/SiC functionally 
gradient material can be prepared using a solid, bulk copper metal 
substrate. 
Powder mixes were prepared having the compositions given in Table 2 which 
follows. 
TABLE 2 
______________________________________ 
SiC Cu Powder 
(-600 mesh) Nicalon .TM. Precursor 
(-325 mesh) 
Mix 
mole % based on Si 
mole % based on Si 
mole % No. 
______________________________________ 
90 (with 3 wt. % B 
10 0 1 
for sintering) 
0 10 90 2 
0 5 95 3 
______________________________________ 
The mixes were prepared using the same procedure as already described in 
Example 1. The powders were then applied in the sequence first Mix No. 2 
followed by Mix No. 1 on a solid Oxygen Free Copper (OFC) grade solid, 
bulk copper plate having dimensions 1.25.times.3.78.times.0.15 cm. The 
surface of the copper plate was previously oxidized by extensive polishing 
on 4000 grit SiC paper in air using a water coolant. The compaction, 
pressing and firling procedures used were the same as those already 
described in Example 1. 
FIG. 4 shows Cu/SiC functionally gradient material 60 produced according to 
the foregoing example. In the photomicrograph, layer 62 has the same 
composition as Mix No. 1 except for containing only 2 wt. % B and layer 64 
is solid copper. 
FIG. 5 shows Cu/SiC functionally gradient material 65 produced according to 
the foregoing example. In the photomicrograph, layer 66 has the same 
composition as Mix No. 1, layer 67 has the same composition as Mix No. 2 
and layer 68 is solid copper. 
EXAMPLE 3 
The following example is provided to show how an Al/SiC functionally 
gradient material can be prepared using an aluminum powder substrate. 
Powder mixes were prepared having the compositions given in Table 3 which 
follows. 
TABLE 3 
______________________________________ 
SiC (&lt;1 micron Al Powder 
particle size) 
Poly(methylsilane) 
(-325 mesh) 
Mix 
mole % based on Si 
mole % based on Si 
mole % No. 
______________________________________ 
90 10 (Nicalon .TM. 
0 1 
precursor) 
50 10 40 2 
20 10 70 3 
10 10 80 4 
0 10 90 5 
0 0 100 6 
______________________________________ 
Powder mixing, compaction and compressing into the green functionally 
gradient material body were performed using the procedures already 
described in Example 1. The poly(methylsilane) was prepared as described 
in Seyferth et al., "Near-Stoichiometric Silicon Carbide from an 
Economical Polysilane Precursor", J. Am. Ceram. Soc. 1992 , 75, 1300 which 
is incorporated herein by reference. Pyrolysis was conducted using the 
following firing program: ramp I: 5.degree./min.; dwell I: 2 hours, 
250.degree. C.; ramp II: 5.degree./min.; dwell II: 3 hours, 550.degree. C. 
with final pyrolysis at 1000.degree. C. 
FIG. 6 shows A1/SiC functionally gradient material 70 produced according to 
the foregoing example and pyrolyzed to 550.degree. C. In the 
photomicrograph, layers 72, 74, 76, and 78 correspond, respectively, to 
Mix Nos. 1, 2, 4, and 5, respectively. 
FIG. 7 shows Al/SiC functionally gradient material 80 produced according to 
the foregoing example and pyrolyzed to 1000.degree. C. In the 
photomicrograph, layers 82, 84, 86, and 88 correspond, respectively, to 
Mix Nos. 1, 2, 4, and 5, respectively. 
FIG. 8 shows a cross-section of an Al/SiC "sandwich" functionally gradient 
material 90 produced according to the foregoing example and pyrolyzed to 
550.degree. C. In the photomicrograph, layers 92, 94, 96, 98, 100, 101 and 
102 correspond, respectively, to Mix Nos. 6, 5, 4, 3, 4, 5 and 6. 
FIG. 9 shows a cross-section of an A1/SiC "sandwich" functionally gradient 
material 110 produced according to the foregoing example and pyrolyzed to 
1000.degree. C. In the photomicrograph, layers 112, 114, 116, 118, 120, 
122 and 124 correspond, respectively, to Mix Nos. 1, 2, 4, 5, 4, 2 and 1. 
EXAMPLE 4 
The following example is provided to show how shrinkage of a green 
functionally gradient material can be controlled during firing by 
controlling the amount of preceramic polymer binder mixed with powder to 
form the intermediate layers. 
Blends of SiC of less than one micron particle size and cross-linked 
Nicalon.TM. precursor having varying Nicalon.TM. precursor contents were 
compacted uniaxially at 90 kpsi for one hour and isostatically pressed at 
40 kpsi for 10 minutes. The resulting green bodies of varying Nicalon.TM. 
precursor contents were pyrolyzed in Ar for 3 hours at 1500.degree. C . 
FIG. 10 is a plot of % volume shrinkage measured by volume displacement of 
isopropyl alcohol observed as a function of Nicalon.TM. precursor content 
and indicates that higher Nicalon.TM. precursor contents result in greater 
shrinkage. Shrinkage control was demonstrated by using varying ratios of 
precross-linked Nicalon.TM. polycarbosilane/silicon carbide and substrate 
material/toplayer material ratio; along with identical pressing and 
heating conditions for each sample. Linear behavior of % volume shrinkage 
rs. wt. % of the preceramic polymer binder was observed as shown in FIG. 
10. For all Nicalon.TM. precursor contents, minimal density change 
compared with volume shrinkage was observed. The average density of the 
final ceramic was 2.58 g/cc. 
The cross-linked Nicalon.TM. was prepared by refluxing the as-received 
polymer in hexane with 0.5 mole % of Co.sub.2 (CO).sub.8 catalyst compound 
for 16 hours. 
______________________________________ 
SiC (&lt;1 micron 
Cu powder 
particle size) 
(-325 
Nicalon .TM. Precursor 
mole % mesh) Mix 
mole % based on Si 
based on Si mole % No. 
______________________________________ 
31 69 0 1 
24 62 14 2 
19 52 29 3 
14 36 50 4 
8 20 72 5 
0 0 100 6 
______________________________________ 
FIG. 11 is an optical micrograph at 14.times.magnification and shows a 
cross-section of Cu/SiC "sandwich" functionally gradient material 130 
produced according to the foregoing example and pyrolyzed to 900.degree. 
C. The Cu/SiC "sandwich" is made up of layers corresponding to the 
following Mix Nos. of Table 4: 1, 2, 3, 4, 5, 6, 5, 4, 3, 2 and 1. 
FIG. 12 is an optical micrograph at 14.times.magnification and shows a 
cross-section of a Cu/SiC "sandwich" functionally gradient material 140 
produced according to the foregoing example and pyrolyzed to 900.degree. 
C. The sandwich includes layers corresponding to the following Mix Nos. as 
given in Table 1 from Example 1: 1, 2, 3, 4, 5, 7, 5, 4, 3, 2 and 1.