Vapor phase redistribution in multi-component systems

A process for preparing ceramic-metal composites without melting the metal is disclosed. A compact or green body is made from a ceramic and a metal, and the compact is sealed in a vacuum in a container such as a glass envelope. The compact is then heated to a temperature below the melting point of the metal, but high enough so that the vapor pressure of the metal is significant, and the metal redistributes through the ceramic by evaporation and condensation. The composite thereby forms a body having ceramic particles uniformly coated by the metal. Products formed by the process and fabrication of a B.sub.4 C/Cr composite are also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to multi-component systems, especially ceramic-metal 
composites and to methods of producing these systems, especially such 
composites. The invention provides a technique for redistributing one 
material, such as a metal, evenly throughout another material, such as a 
ceramic. This technique permits fabrication of materials, such as 
composites, that were formerly unattainable or attainable only at great 
cost. 
2. Description of Related Art 
Ceramic materials may be combined with metals to form composites having 
exceptional hardness, strength and fracture toughness. These materials are 
useful for cutting tools, structural materials, and armor. 
Preferably, the composites are at least 50% ceramic material, and obtaining 
satisfactory metal distribution throughout a primarily ceramic body has 
proven very difficult. The metal distribution is important in adding 
toughness and in obtaining void-free densification. 
In general, processes for impregnating a ceramic material with metal have 
followed two pathways. In the first pathway, a ceramic body is preformed 
and then coated with a metal. The metal is then driven into the ceramic 
body using various techniques. The second pathway involves forming a 
"green" body out of a mixture of ceramic and metal and then attempting to 
redistribute the two components within the body 
Various prior art techniques for either driving the metal into the ceramic 
or redistributing the metal in a ceramic-metal composite include: hot 
pressing, hot isostatic pressing and explosive compaction. Each of these 
techniques, however, is dependent upon the quality of metal distribution 
between the particles of the ceramic phase. Thus the techniques are 
limited by the type of ceramic-metal composites that can be formed. 
Another technique for preparing a ceramic-metal composite involves heating 
the composite to a temperature higher than that of the melting point of 
the metal to encourage metal migration in the composite due to capillary 
action and surface tension. Often, this procedure is carried out in a 
vacuum or in the presence of an inert gas to ensure against chemical 
reaction or contamination of the liquid phase. These processes are 
exemplified by U.S. Pat. No. 4,605,440 to Halverson et al. (Halverson I) 
and U.S. Pat. No. 4,718,941 to Halverson et al. (Halverson II). 
In Halverson I, a ceramic-metal "green body" is formed and then heated to a 
temperature from about 1050.degree. C. to about 1250.degree. C. for 2 to 
10 minutes in an inert atmosphere or vacuum to obtain "wetting" of the 
ceramic by the metal. Halverson I is careful to point out that mass 
transfer between the ceramic and metal takes place during this process, 
and warns that prolonged heating affects the ultimate product due to 
inter-phase contamination. 
In Halverson II, a ceramic "sponge" is impregnated with metal after 
preforming and chemical treatment to change the surface chemistry of the 
ceramic to permit wetting by the molten metal. As in Halverson I, 
impregnation takes place using capillary action and surface tension. 
Unfortunately, the molten metal wetting technique of the Halverson patents 
is limited to those ceramic-metal systems that do not have either a high 
metal melting point or unacceptable wetting properties. What is needed in 
the art is a technique that can both provide for a broad range of 
ceramic-metal composites, avoid extensive phase intermingling during 
redistribution and provide a uniform distribution of metals within a 
ceramic-metal composite. 
SUMMARY OF THE INVENTION 
The invention provides a method for making a ceramic-metal composite with 
little phase intermingling and without the limitation that the metal wet 
the surface of the ceramic. In the method of the invention, a ceramic and 
a metal are mixed in proportion corresponding to the desired final product 
and formed into a green body. The green body is then encapsulated in a 
glass preform or in a glaze at elevated temperature and in vacuum. The 
temperature of the green body is then raised to a point at which the vapor 
pressure of the metal is significant. The metal evaporates and recondenses 
on the ceramic material. Ceramic particles in the composite thus become 
enveloped in a shell of metal, ensuring that the ceramic-metal composite 
will have outstanding properties and will not suffer from undue phase 
intermingling as a result of redistribution. The ceramic-metal composite 
is then ready for final treatment, for example by hot pressing or hot 
isostatic pressing. 
The invention also comprises ceramic-metal composites made using the method 
of the invention. 
Although the invention is preferably used to make ceramic-metal composites, 
the invention may be used to prepare a composite of any two materials. If 
a first material has a higher vapor pressure at a given temperature than a 
second material, then the first material will redistribute at that 
temperature to cover the second material. If, on the other hand, the two 
materials have a similar vapor pressure at a given temperature, then the 
two materials will tend to form an alloy upon redistribution, and the 
consistency of the alloy will be affected by the intimacy of the mixing 
between the two materials. 
It is an object of the invention to provide materials such as ceramic-metal 
composites made by a new method that does not depend on wetting of the 
surface of one material, i.e., the ceramic by the other material, i.e., 
the metal and that does not cause undue phase intermingling. 
One advantage of the invention is that materials can now be made that were 
formerly impossible or prohibitively expensive to make. Additional objects 
and advantages of the invention will be apparent from the description 
provided below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The method of the invention comprises a technique for redistributing 
material in a two or more component mixture comprising the steps of: (a) 
mixing a first material and a second material; (b) placing the materials 
in a nonreactive environment; and (c) allowing the materials to 
redistribute at a temperature at which at least one of the materials has a 
significant vapor pressure. 
In the preferred embodiment, the invention comprises a method for preparing 
a ceramic metal composite. The method comprises the steps of (a) preparing 
a ceramic-metal green body; (b) sealing the ceramic-metal green body in 
vacuum; (c) raising the temperature of the green body to a point where the 
vapor pressure of the metal becomes significant to achieve redistribution 
of the metal; and (d) treating the green body to form the final desired 
product. 
Mixing the materials before redistribution is not strictly necessary to 
obtain some coating of one material by another. But, in the embodiment of 
two solid materials in powder form, the individual granules of material 
will be more likely to redistribute uniformly, if the particles are 
intimately mixed. 
In the preferred embodiment, a formed mixture, called a green body, may be 
formed in any conventional manner. Preferably, however, ceramic and metal 
powders are mixed together in appropriate proportions using liquid 
dispersion or ball milling or attrition milling. A powder compact is then 
formed from the powder mixture using slip casting, and cold pressing, cold 
isostatic pressing or any other conventional greenware preparation 
technique to make the compact a green body. 
Preferably, the mixture or green body should be substantially similar to 
the final desired product chemically and should have a ceramic grain size 
similar to the final product to ensure predictable and uniform 
characteristics of the final product. 
After forming a mixture of the materials to undergo redistribution, the 
mixture is isolated from reactive factors in the environment to prevent 
contamination. For example, in the preferred embodiment, some metal, like 
titanium, are highly reactive with oxygen and oxygen containing materials 
at elevated temperatures. If titanium is one of the materials, the mixture 
should be kept away from air and water. 
In the preferred embodiment, the green body is placed under vacuum or low 
pressure at an elevated temperature using a glass preform or a glazing 
operation. The glass or glazing isolates the green body and the vacuum 
from the atmosphere. In an alternative embodiment, an inert atmosphere, 
made up of materials that do not react with the materials being 
redistributed at the temperatures and pressure involved in the 
redistribution, may be used in place of vacuum or low pressure. Depending 
on the inert atmosphere used, however, the atmosphere may act to suppress 
the vapor pressure of the material being redistributed and thus may cause 
the redistribution to take place at a higher temperature. 
In the preferred embodiment, the green body may be placed in a glass 
preform or envelope in a vacuum furnace. The particular type of glass in 
the envelope or the particular type of glazing is not critical to the 
invention, so long as the preform or "envelope" or glazing is sufficiently 
strong to withstand the pressure differential between the atmosphere 
outside and the vacuum inside. The degree of vacuum is also not critical 
to the invention. The lower the pressure inside the envelope or glazing, 
however, the better the expected results. 
The material encapsulating the green body need not be glass, although glass 
is preferred. In an alternative embodiment, the material may be a shell of 
some other material, such as titanium. The encapsulating material should 
be sufficiently strong to withstand any pressure differential to which it 
is exposed, and the material should not significantly react with any of 
the materials being redistributed at the temperatures and pressures of the 
redistribution process. 
If the encapsulating material is glass, it preferably should not contain 
high levels of material that can react with the redistributing materials. 
For example, some of the composites that can be made using the method of 
this technique include titanium as one of the materials. At the 
temperatures and pressures of a metal-ceramic redistribution, titanium 
reacts with both oxygen and alkali oxides. Accordingly, the glass envelope 
preferably should be as free of dissolved oxygen and water as possible, 
and the glass should also preferably be a low alkali glass or a glass that 
contains an alkali of relatively low vapor pressure, such as lithium 
oxide. In the alternative, the green body may be covered with a small 
amount of titanium dust that can react with impurities in the glass and so 
protect the green body from potential degradation by the impurities. 
After sealing the envelope or the glazing so that the vacuum may be 
maintained, the pressure outside the envelope may be returned to one 
atmosphere or higher. One atmosphere is preferred, for simplicity, but a 
higher or lower pressure may be used depending on the particular 
application of the invention, the strength of the envelope or glaze and 
the environment in which the process takes place. 
The ceramic-metal composite green body is then heated to a temperature at 
which the vapor pressure of the metal becomes significant. The exact 
temperature at which the vapor pressure becomes "significant" will depend 
upon the nature of the metal in the composite and the desired speed of the 
process. If the green body is only heated to a relatively low temperature, 
then the redistribution of the metal in the green body will take a longer 
time, and a higher temperature will provide a more rapid reaction. An 
upper limit to the temperature is the melting point of the metal. Molten 
metal redistributes within the ceramic using a different mechanism. It 
will be apparent to those skilled in the art that the degree of vacuum, 
the vapor pressure and the temperature are interrelated. One skilled in 
the art will be aware that the selection of these conditions will be 
affected by the ceramic and metal. 
The metal phase of the composite will redistribute through the composite by 
evaporation and subsequent condensation on the ceramic, and the ceramic 
particles become enveloped in a shell of metal. As a result, even 
distribution of the metal on the ceramic matrix can be obtained. Since the 
temperature is typically lower than a liquid phase redistribution, the 
tendency to form unwanted phases within the composite during 
redistribution is reduced. Since the temperature is lower than the melting 
point of the metal, this invention makes it possible to make many 
desirable metal-ceramic composites that could not be made or could only be 
made at great expense, since the melting point of some metals is so high 
that the chosen ceramic could be damaged or destroyed at or below the 
melting temperature of the metal. 
The last step in the process is formation of the final product from the 
green body. This step is accomplished by subjecting the redistributed 
composite to a finishing technique such as hot pressing or hot isostatic 
pressing. Optionally, the glass envelope or glaze may be removed before 
forming the final product, but this step in not necessary to prepare the 
finished product. 
The invention is illustrated in the following example, but the example does 
not serve to limit the invention to the particular combination shown. 
EXAMPLE 1 
PREATION OF B.sub.4 C/Cr CERAMIC/METAL COMPOSITE 
Preparation of a B.sub.4 C/Cr composite under conventional procedures is 
very difficult because the melting point of Cr is about 1900.degree. C. 
and the melt will react with B.sub.4 C at that temperature to form 
undesired phases. 
A 60% B.sub.4 C 40% Cr (by volume) powder compact was prepared by mixing 
starting powders in an alumina crucible. A pellet was pressed from the 
starting powder mixture in a WC/Co die under vacuum and then the pellet 
was placed inside of a glass preform. The preform and pellet were then put 
into a vacuum furnace. After outgassing at a temperature below 800.degree. 
C., the sample was sealed in glass under vacuum at 825.degree. C. 
The vacuum furnace was then cooled down and the encapsulated pellet was 
removed and placed in a high temperature oven and heat treated at 
1200.degree. C. for 30 minutes. The pellet was then removed from the oven, 
cooled and allowed to sit for two weeks. Upon examination, the Cr was very 
uniformly distributed within the pellet. 
EXAMPLE 2 
PREATION OF B.sub.4 C/Ti CERAMIC/METAL COMPOSITE 
Eight grams of a mixture of 60 weight percent B.sub.4 C (1 micron) and 40 
weight percent Ti (1-5 microns) was mixed with 4 ml water, and formed into 
a slip. The slip was ultrasonically dismembrated to break down 
agglomerates. The slip was then cast into a 2 piece gypsum mold, yielding 
a strong green body. 
The green body was placed into a glass envelope and heated to 515.degree. 
C. in a 10 mtorr vacuum, where it remained for two hours. The temperature 
was then raised to 900.degree. C. and the envelope was left for 30 
minutes. The vacuum valve of the vacuum furnace was then closed and Argon 
gas was introduced into the vacuum furnace up to 1 atmosphere. The 
temperature was then raised to 1545.degree. C. and the glass envelope 
remained exposed to the elevated temperature for 30 minutes. The envelope 
was then permitted to cool in the oven after the heat was turned off. 
The cooled sample was placed in a hot press and heated slowly to 
1000.degree. C. The sample was hot isostatically pressed in glass at 4000 
psi for 30 minutes. The sample was then hot ejected at 700.degree. C., and 
cut on slow speed diamond saw. The sample was then polished and measured 
for hardness using the Knoop hardness test. A hardness of 1000 was 
measured, and SEM analysis showed very little porosity in the sample 
(about 2 microns). 
Aluminum oxide and copper have also been formed into a composite using this 
technique. 
It will be understood by those of skill in the art that various 
modifications and alterations may be made to the invention without 
departing from the scope and spirit thereof.