Method for making composite articles that include complex internal geometry

A process is described for making a composite article without shrinkage, particularly of ceramic and metal wherein the article includes complex internal surfaces or cavities. The process requires forming an insert body that includes an external surface that corresponds to an internal cavity of the article. The insert body consists of a material having a melting temperature less than that of the article. The process further requires forming a porous compact about the insert body wherein the compact is formed into the substantially the net shape of the article. The compact is made of a material that is wetted by liquid insert material and has a sintering temperature greater than the wetting temperature of the insert material. The process further requires heating the article to a temperature such that the inserts substantially melts and infiltrates the porous compact forming the finished composite article. The process produces products including complex internal surfaces without the necessity of costly and sometimes technically difficult internal machining operations.

BACKGROUND OF THE INVENTION 
This invention relates to densified composite articles, such as those made 
of ceramic-metal compositions, and methods of forming such articles. More 
particularly, the invention relates to such composite articles that 
include hollows or cavities of complex geometry and methods of formation 
whereby machining of internal surface is not required. 
Making composite articles of ceramic-metal, metal-metal or ceramic-glass 
materials often involves formation of a porous compact of the materials 
followed by a densification process such as, for example, sintering or hot 
isostatic pressing. Densification processes typically result in 
considerable shrinkage of the porous compact as it increases from 50-60 
volume percent to 80-100 volume percent of complete or theoretical 
density. Densification processes also tend to distort article dimensions 
and result in many reject pieces that do not have the desired finished or 
net shape of the article. 
The difficulty of achieving a dimensionally predictable and acceptable 
finished composite article of desired net shape is particularly acute for 
articles that must include an internal surface or cavity or hollow 
portion, particularly if the cavity comprises complex internal geometry. 
Such articles might comprise, for example, a tube, a tube shape having a 
variable internal diameter or a hollow ball. 
In the past, cavities in composite articles often have been made by boring 
out a capillary or passageway in the sintered article, followed by 
machining where variable diameters or other complex surfaces are required. 
Articles having completely enclosed cavities or hollows are typically made 
by forming the articles in separate sections followed by joining or 
welding the sections together to form the whole. 
These prior art techniques are expensive and often technically difficult, 
particularly where internal working space is limited. Where the materials 
of which the articles are made are very hard, such as for many ceramics, 
even simple borings may become exceedingly difficult and expensive. 
As an alternative to sintering or hot pressing, ceramic-metal composites 
may be densified to form by infiltrating the metal into a porous ceramic 
compact. Thus, Stibbs et al. in U.S. Pat. No. 3,749,571 describes 
infiltrating silicon into a boron carbide compact. While a density of 99 
percent of theoretical density was said to be achieved, a necessary 
sintering step still results in shrinkage of the article. Gazza et al. in 
U.S. Pat. No. 3,864,154 describe making a solid composite, such as a 
simple disk, of various ceramicmetal materials by surrounding a ceramic 
compact with powdered metal followed by heating until molten metal 
impregnates the ceramic skelton. Landingham in U.S. Pat. No. 3,718,441 
achieves a degree of densification of a beryllium oxide compact of simple 
cylindrical geometry by reducing oxide films present on metal powders that 
are said to prevent metal from wetting the ceramic during sintering 
operations. Excess metal must then be machined off. 
The focus of this earlier work was simply to achieve densification. The 
articles of interest were not complexly shaped articles having internal 
cavities. Thus, neither internal shrinkage nor internal machining to net 
shape was of significant concern. 
Where more reactive combinations of materials, such as B.sub.4 C and A1, 
are of interest and as infiltration process is employed, one often must 
balance the infiltration process with kinetics of potential chemical 
reactions that form ceramic phases that may interfere with densification 
by blocking infiltration channels in the porous compact. In order to 
achieve infiltration and, consequently, desired densification, Pyzik et 
al. in U.S. Pat. No. 4,707,770 reduce reaction rates of a B.sub.4 C-A1 
system by thermally treating B.sub.4 C at about 1800.degree. C. prior to 
metal infiltration of the porous compact. Halverson et al in U.S. Pat. No. 
4,718,941 employ a chemical treatment over a prolonged time period to 
allow infiltration. These material treatment techniques, of course, add 
cost to the composite article. Most of these densification techniques also 
involve later sintering steps that cause significant shrinkage of the 
final products. Consequently, heretofore, infiltration has achieved no 
advantage over hot pressing in producing net shape composities. 
The properites of composites have improved as the ability to produce near 
fully densified and pore-free articles has progressed. It is now possible 
to realize the potentials of combining the properties of the materials, 
such as ceramics and metals. It is now desirable to focus upon producing 
densified composite articles of a geometric complexity that meet the 
functional demands of suitable applications that are now evident from the 
improved properties of the composite materials. There is a need to produce 
composite articles that are dimensionally precise yet include complex 
geometry, such as internal cavities and the like. It is desirable to 
produce such articles in a fully densified, "net shape" form, that is, 
without significant shrinkage and, thus, completely finished without the 
need for further shaping, such as by machining. 
SUMMARY OF THE INVENTION 
The present invention is a substantially shrinkage-free process for making 
a composite article of ceramic-metal, metal-metal, ceramic-glass materials 
or the like, where the article include complex internal surfaces, such as 
a bore, a partially enclosed cavity or even a totally enclosed cavity. The 
process achieves densities above 99 percent of theoretical by means of an 
infiltration technique which surprisingly does not require thermal or 
chemical pretreatment of reactive ceramic-metal materials, such as B.sub.4 
C-A1. The infiltrated articles is heat treated at less than sintering 
temperatures such that article shrinkage is substantially eliminated. 
Thus, distorted internal geometry and internal machining, typical of the 
prior art, are substantially eliminated in the production of net shape, 
densified ceramic-metal and the like composites. Further, the process in 
an infiltration process that avoids the need to pre-treat materials in 
order to achive successful inflitration. 
The process requires forming an insert body that includes an external 
surface that corresponds to an internal surface or cavity of the article. 
The insert body consists of a material that has a wetting temperature less 
than that of the finished article. The process further requires forming a 
porous compact about the insert body wherein the compact is formed into 
substantially the net shape of the desired article. The compact is made of 
a material that is wetted by the insert material and has a sintering 
temperature greater than the wetting temperature of the insert material. 
The process then requires heating the assembled porous compact and insert 
body to the insert material wetting temperature such that the insert 
substantially melts and the insert material infiltrates into the porous 
compact, forming the finished compact article. 
The invention includes articles made by the above-described process, 
particularly those articles having internal surfaces that are 
substantially enclosed, and hollow articles. Such an article is, for 
example, a ceramic-metal composite that is a hollow ball or the like. 
In a preferred process of the invention, the insert body material is a 
metal or glass and the porous compact material is a ceramic or metal. The 
compact-insert materials may be, for example, A1B.sub.12 -A1, B.sub.4 
C-Si, SiC-Si, SiB.sub.6 -A1, SiB.sub.4, B.sub.4 C-Mg, TiB.sub.2 -Ni, 
A1.sub.2 0.sub.3 -A1-Mg, and TiB.sub.2 A1. Ceramic-glass systems include, 
for example, A1.sub.2 0.sub.3 -(Si0.sub.2 -B.sub.2 0.sub.3 glass), 
Si.sub.3 N.sub.4 -(Si0.sub.2 -Mg0 glass), Si.sub.3 N.sub.4 -(Si0.sub.2 
-Mg0-Y.sub.2 0.sub.3 -Ca0 glass), Si.sub.3 N.sub.4 -(Si0.sub.2 -Y.sub.2 
0.sub.3 glass) and Si.sub.3 N.sub.4 (Si0.sub.2 -A1.sub.2 0Metal-metal 
systems include, for example, Ti-Mg or W-Cu. 
In a preferred embodiment of the invention, the process includes an insert 
body material that is a metal and a porous compact material that is a 
ceramic. Most preferred, are chemically reactive systems, such as B.sub.4 
C-A1 or B.sub.4 C-A1 alloys, that react at elevated temperature. In these 
chemically reactive systems the metal component, after infiltration, can 
be depleted to form ceramic phases that modify article properties such as 
hardness and wear resistance. 
The process of the invention may be utilized to produce articles in which 
the insert material infiltrates and fills all of the pores of the porous 
compact. Alternatively, only a portion of the pores of the porous compact 
or only pores adjacent internal surfaces of the compact might be filled. 
Thus, the process of the invention allows substantial flexibility for 
tailoring composite properties to local adverse conditions such as 
corrosion, wear or the like.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process of the invention is particularly useful for forming composite 
articles that have complex internal geometry, such as partially or totally 
enclosed cavities or surfaces. Articles having intricately shaped passages 
or capillaries or hollow portions or hollow balls may be made, for 
example. The process permits the formation of hollow net-shape articles in 
one piece. In contrast to the prior art, wherein ceramic-metal composites 
typically undergo considerable shrinkage through sintering. 
A composite article of the invention comprises two or more solid phases of 
ceramic or metal. The process if particularly useful for forming 
ceramic-metal composites. Different metals forming a metal-metal 
composite, as well as ceramic-glass systems may be combined to produce 
composite articles of the invention. 
The process of the invention, in concept, requires forming an insert body 
of relatively low melting point material wherein the body has an external 
surface that is the net shape of the desired cavity of the composite 
article. A porous compact is then formed about the inert body. The 
assembled compact and insert body are heated to the melting point of the 
insert body material such that the low melting point material infiltrates 
into the porous compact. Once the insert material is fully depleted or 
absorbed into the compact, the article is complete and includes the 
desired net shape cavity. 
Criteria for selecting suitable materials for the process of the invention, 
aside from the requirements of the functioning of the article or of the 
system in which the composite will function, first of all require that the 
materials have different melting temperatures. A second criterion is that 
the lower melting temperature solids phase must "wet" the other material, 
at a wetting temperature that is below the sintering temperature of the 
higher melting temperature other material. The wetting temperature is 
equal to substantially the melting point of the lower melting temperature 
material. That is, when the lower melting material reaches its liquid 
state, it is characterized by a low contact angle of less than about 
45.degree., preferably less than about 10.degree., on the phase remaining 
solid. (See Halverson, et al., U.S. Pat. No. 4,615,440 and Pyzik et al., 
U.S. Pat. No. 4,702,770 for discussions of wetting). The essential result 
required is than when the materials are in contact one with the other and 
heated to the wetting temperature of the lower melting temperature 
component, said component will wet and infiltrate a porous compact object 
made of a higher melting point material, before the higher melting 
temperature component begins to shrink and pores close. 
A third, but optional, criterion for ceramic-metals systems is that the 
system be chemically reactive. Such a system is particularly versatile 
because, after infiltration, a metal phase may be depleted to form various 
ceramic phases. Proper selection of conditions produces certain ceramic 
phases, the selection of which allows tailoring of physical properties to 
particular use requirements. For example, properties such as hardness or 
wear resistance may be improved by continued heating of the composite 
article to appropriate temperatures, causing desired reactions and ceramic 
phases to form. 
Typically, the insert body is constructed of metal or glass with the 
intention that the insert materials will infiltrate into a ceramic or 
metal porous compact. The insert body is made by any convenient 
conventional process such as, for a metal, by casting or machining. The 
insert may be a solid or hollow object but must have an external surface 
that corresponds to or is the mirror image of the desired cavity or 
internal surface of the finished composite article. For example, a 
suitable metal insert might be a solid cylinder having external surfaces 
machined to the desired finished cavity shape and dimensions. 
The volume of material of which the insert is made is limited to that 
amount of material desired to be absorbed into the porous compact. Where 
the insert body is too small to provide a sufficient volume of material, a 
series of conduits or the like may be extended into the cavity to add 
material during the infiltration process. 
Making the porous compact of ceramic or metal material about the insert 
body is likewise accomplished by conventional techniques. The ceramic 
materials are typically appropriately sized and formed into a homogeneous 
mixture with a binder. The material may then be consolidated by some 
convenient method such as assembling the insert and ceramic material in an 
appropriately shaped die and subjecting the assembly to cold isostatic 
pressing or the like. 
The density of the resulting porous compact will depend upon the ability of 
the ceramic particles to pack and usually is on the order of about 50-70 
volume percent of theoretical full density. The particle size and form of 
ceramic particles will affect porous compact or green body characteristics 
as well as finished product qualities. For example, if the ceramic 
component is in the form of whiskers, a density of 40 to 50 percent may be 
obtained. The finished composite will have a high whisker content, which 
is very difficult to achieve by applying other techniques. Other physical 
forms of the ceramic material such as particulates, platelets, fibers, 
chopped fibers, or the like, may be selected to achieve particular desired 
results. 
The assembly, consisting of the metal or glass insert surrounded by the 
porous ceramic or metal compact is heated to the wetting temperature at 
which the insert material substantially melts and infiltrates the porous 
compact. It is a requirement of the system that the sintering temperature 
of the porous compact be higher than the wetting or melting temperature of 
the insert. It is desired that the process be conducted below those 
temperatures at which there is significant pore closing of the porous 
body. The intent is that all pores and their interconnecting channels of 
the porous compact remain open during the infiltration process so that 
they may fill with metal producing a desired net shape article, without 
the shrinkage which occurs at sinterint temperatures. As the wetting 
temperature is achieved, the insert material liquifies and is drawn into 
the porous body by means of capillary action of the compact pores. The 
infiltration continues until all of the insert body material is absorbed, 
leaving behind a cavity having the desired net shape. The heating and 
infiltrating step may be conducted under vacuum, inert gas or air as the 
system requires. 
Depending upon article geometry and volume of the insert body, additional 
amounts of liquid-phase material may be required to fill all of the voids 
in the porous body. Additional liquid may be introduced from external 
sources as necessary by whatever means convenient. Alternatively, the 
volume of the insert body may be limited such that the characteristics of 
the finished composite article will vary radially from the surface in 
contact with the insert. Limiting the volume of material available for 
infiltration allows forming a structure wherein some articvle property of 
interest is graded with respect to article geometry. 
Once infiltration is complete, the composite article may be cooled to room 
temperature. The finished article dimensions are substantially unchanged 
from the porous compact before infiltration and thus typically require no 
further processing. Articles that differ in dimensions by no more than 
0.002 inch (50.8 micrometers) or less from compact to finished article are 
typicaly achieved. 
The finished composite article of reactive systems may be subjected to 
further heat treatment in which the metal phase, for example, is reacted 
to form new ceramic phases. Such heat treatment processes often increase 
hardness and wear resistance in article surfaces. For example, a composite 
B.sub.4 C-A1 system cermet produced by the infiltration process of this 
invention is readily altered in microstructure by means of the optional 
additional heat treatment. 
As noted above, the process of the invention permits formation of composite 
articles that may include properties that vary from surface-to-surface to 
meet specific environmental or use requirements of each surface. For 
example, in a reactive system such as boron carbide and aluminum or 
aluminum alloys, a fully dense, multi-ceramic phase article may be 
produced having such variable properties. Such an article may include 
exterior surfaces of complex geometry as well as complex interior surfaces 
or cavities, each surface treated through either infiltration or metal or 
post-heat treatment to achieve desired surface properties. 
The interior surface of each an article is, for example, prepared by 
machining an aluminum or aluminum alloy cylinder to include an exterior 
surface that is the net shape of the desired interior surface or cavity of 
the desired composite article. The cylinder insert body is then bored or 
otherwise treated to limit the amount of metal available for infiltration 
to just that amount necessary to form a thin ceramic-metal layer adjacent 
the interior surface or cavity of the finished composite article. 
Particulate B.sub.4 C is combined with a binder and cold pressed in a die 
about the insert body, wherein the die includes complex surfaces 
constituting the desired exterior surfaces of the composite article. The 
porous compact-insert assembly is elevated in temperature to the wetting 
temperature of the aluminum or aluminum alloy insert and held at that 
temperature until the insert material has liquidified and infiltrated the 
interior surface of the porous compact. At that point, the partially 
infiltrated boron carbide compact could be elevated in temperature between 
700.degree. to 1200.degree. C. to react the aluminum metal with the boron 
carbide ceramic to form new ceramic phases having the desired high 
hardness qualities for the composite article interior surfaces. 
The partially infiltrated compact may then be treated to alter the exterior 
properties of the article by contacting the exterior with a second metal 
at its wetting temperature. After the second infiltration, the composite 
article is characterized by a hard ceramic interior and a tough 
ceramic-metal exterior. The metals, of course, may differ for each 
infiltration, depending upon desired finished qualities. The second 
infiltration could also be followed by a heat treatment where new ceramic 
phases in the exterior are also desired. 
The process of the invention has the capability of producing many unusual 
shapes. As noted above, hollow balls or other such hollow shapes may be 
made. It may be desirable to include materials or objects in a hollow that 
remain in place within the hollow or cavity after infiltration has been 
achieved. Such material might comprise a skeletal support for the cavity 
or simply a material that requires cermet encapsulation, for example. Such 
forms are achieved by including the material to be encapsulated, in their 
desired shape, as a component of the insert body. The materials to be left 
in the cavity after infiltration must not be wetted by the insert body 
material at infiltration temperature. Such non-wetting character may 
require a chemical surface treatment or the like of the materials to be 
left in the cavity. 
As noted above, prior workers found that infiltration of some ceramic-metal 
systems, such as B.sub.4 C-A1, were significantly improved where the 
ceramic phase is subjected to a thermal or chemical treatment. In U.S. 
Pat. No. 4,718,941 the problem was described as a requirement to remove 
oxide layers from the B.sub.4 C powder prior to infiltration. This process 
involved exceedingly lengthly treatment periods of about 10 days. It has 
been discovered that the oxide contamination problem that interferes with 
infiltration appears to be solved by the present process without the 
necessity of either prior art treatment. 
It appears that oxide removal may be more critical for the metal surfaces 
than for the ceramic material. Thus, in the B4C-A1 system of interest, 
while it may be more difficult to remove oxide form A1 than it is to 
remove oxides from B4C, the problem is reduced to insignificance because 
the infiltration process of the invention utilizes the metal phase in the 
form of a relatively massive insert body. The A1 phase is in the form of a 
rod, a shaped block, a plate, or the like, not in particulate form. 
Consequently, the surface area of the metal and, hence, oxide content of 
the systems is at a reduced level and does not appear to interfere with 
infiltration. 
The following examples illustrate the invention. 
EXAMPLE 1 
An insert body of aluminum was made consisting of a rod machined to include 
helical threads on its cylindrical surfaces. A ceramic phase mixture was 
prepared by mixing boron carbide powder (ESK 1500, manufactured by 
Elektroschemeltzwerk Kempten of Munich, West Germany) with 3 percent by 
weight of a wax binder for 3 hours and then passing the mixture through a 
220 micrometers sieve. The ceramic powder mixture was placed in a rubber 
die, such that the powder surrounded the metal insert, and isostatically 
pressed at 45 ksi (310 MPa) for 1 minute. The pressure was then released 
and the porous compact-insert body assembly was removed from the rubber 
die and placed into a graphite furnace. A vacuum of 1-100 milliliters was 
applied at room temperature. The temperature was elevated at 10.degree. C. 
per minute to 1170.degree. C. and held for 1 hour. The article was then 
cooled down to room temperature under flowing argon. The properties of the 
resulting composite article included a fracture strength of 81 ksi (558 
MPa), a fracture toughness K.sub.IC of 7.9 MPa.m.sup.1/2 and a hardness of 
695 kg/mm.sup.2. The finished composite article was characterized by 
dimensions that differed by only 0.001 inch (25.4 micrometers) when 
compared to the size of the porous compact. The finished article was fully 
densified and included no detectable pores. 
A second article according to the above-described process was made and 
subjected to a heat treatment wherein the composite article was cooled 
down to 800.degree. C. and held for 10 hours. The properties of the 
composite article after heat treatment included a fracture strength of 77 
ksi (530 MPa), K.sub.IC of 5.8 MPa.m.sup.1/2 and a hardness of 1295 
kg/mm.sup.2. 
Carefully controlling the heating schedule as the infiltration temperature 
is approached improves the infiltration process of the invention. For the 
B4C-A1 system of interest, a heating rate of less than about 10.degree. 
C./min from 1000.degree. C. to 1100.degree. C. and less than about 
5.degree. C./min from 1100.degree. C. to a maximum desired temperature is 
particularly suitable. Heating at maximum temperature generally requires 
about 15 minutes plus 10 minutes for each 1 cm which the liquid phase 
needs to penetrate (where such distances do not exceed about 10 cm). 
Materials are fully dense and show excellent mechanical properties under 
these conditions. 
A series of composite articles, made in accord with the above-described 
process, include the ranges of properties shown in the following table. 
TABLE 
______________________________________ 
Properties Ranges for B.sub.4 C--Al Composition Articles 
Heat Treatment 
Fracture Fracture 
After Strength Toughness Hardness 
Infiltration 
[Ksi (MPa)] [MPa.multidot.m1/2] 
[kg/mm.sup.2)] 
______________________________________ 
None 70-90 (480-620) 
6-9 500-700 
800.degree. C. for 10 hours 
60-80 (414-550) 
5-7 1100-1400 
______________________________________ 
EXAMPLE 2 
An aluminum cylindrical rod 8.00 inches long and 0.50 inches in diameter 
(20.32 centimeters long and 1.27 centimeters in diameter) was selected as 
the insert body for preparation of a TiB.sub.2 -A1 tube. TiB.sub.2 powder, 
characterized by an average particle size of 7 micrometers and supplied by 
Union Carbide was mixed with a wax binder in rotary mixer and 
isostatically cold pressed at 60 ksi (420 MPa) around the aluminum rod 
insert. The TiB.sub.2 porous compact-A1 insert body assembly was then 
heated under vacuum to 1250.degree. C. and held at that temperature for 1 
hour. After cooling, the finished composite article was 99 percent of 
theoretical density and required no machining to meet the required 
dimensions of 0.50.+-.0.002 inches (1.270.+-.0.005 cm).