U.S. patent application number 08/829034 was filed with the patent office on 2003-03-13 for novel cermets and molten metal infiltration method and process for their fabrication.
Invention is credited to LANDINGHAM, RICHARD L..
Application Number | 20030050707 08/829034 |
Document ID | / |
Family ID | 25253360 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030050707 |
Kind Code |
A1 |
LANDINGHAM, RICHARD L. |
March 13, 2003 |
NOVEL CERMETS AND MOLTEN METAL INFILTRATION METHOD AND PROCESS FOR
THEIR FABRICATION
Abstract
Cermet comprising ceramic and metal components and a molten
metal infiltration method and process for fabrication thereof. The
light weight cermets having improved porosity, strength,
durability, toughness, elasticity fabricated from presintered
ceramic powder infiltrated with a molten metal or metal alloy.
Alumina titanium cermets biocompatible with the human body suitable
for bone and joint replacements.
Inventors: |
LANDINGHAM, RICHARD L.;
(LIVERMORE, CA) |
Correspondence
Address: |
THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
LAWRENCE LIVERMORE NATIONAL LABORATORY
PO BOX 808, L-703
LIVERMORE
CA
94551-0808
US
|
Family ID: |
25253360 |
Appl. No.: |
08/829034 |
Filed: |
March 31, 1997 |
Current U.S.
Class: |
623/23.51 ;
501/89; 623/901 |
Current CPC
Class: |
C04B 41/88 20130101;
C22C 1/1036 20130101; A61F 2310/00311 20130101; C04B 41/009
20130101; A61F 2310/00191 20130101; A61F 2310/00203 20130101; C04B
2111/00836 20130101; A61F 2310/00299 20130101; C22C 2001/1021
20130101; A61F 2/28 20130101; A61F 2/30 20130101; A61L 31/128
20130101; C04B 41/009 20130101; A61F 2310/00251 20130101; A61F
2310/00263 20130101; A61L 27/427 20130101; A61F 2310/00185
20130101; C04B 41/5133 20130101; C04B 41/009 20130101; C04B 41/009
20130101; C04B 41/5133 20130101; C04B 38/00 20130101; A61F
2002/30968 20130101; C04B 41/4523 20130101; C04B 35/00 20130101;
C04B 35/10 20130101 |
Class at
Publication: |
623/23.51 ;
623/901; 501/89 |
International
Class: |
A61F 002/28; C04B
035/52 |
Goverment Interests
[0001] The United States Government has rights in this invention
pursuant to Contract No. W-7405-ENG-48 between the U.S. Department
of Energy and the University of California, for the operation of
Lawrence Livermore National Laboratory.
Claims
What is claimed is:
1. A cermet comprising a ceramic powder presintered into a porous
ceramic matrix and infiltrated with a molten metal or metal
alloy.
2. The cermet of claim 1 wherein the ceramic powder is selected
from the group consisting of aluminum oxide, beryllium oxide,
zirconium oxide, hafnium oxide, vanadium oxide, aluminum nitride,
beryllium nitride, zirconium nitride, hafnium nitride, vanadium
nitride, aluminum boride, beryllium borate, zirconium boride,
hafnium boride, vanadium boride, aluminum silicate, zirconium
silicate, hafnium silicate, vanadium silicate, beryllium silicate
and a mixture thereof.
3. The cermet of claim 2 wherein the infiltrating metal or metal
alloy is selected from the group consisting of titanium, aluminum,
magnesium, nickel, lithium, copper, iron, silicon, manganese,
cobalt, molybdenum, niobium, zirconium, calcium and their
combination.
4. The cermet of claim 3 wherein the metal or metal alloy is
infiltrated into a ceramic powder presintered into a porous ceramic
matrix at a temperature higher than is the melting point of the
infiltrating metal or metal alloy.
5. The cermet of claim 4 wherein the ceramic powder is aluminum
oxide having particle sizes from about 10 to about 50 microns.
6. The cermet of claim 5 wherein the aluminum oxide has particle
sizes about 10 microns.
7. The cermet of claim 6 wherein the aluminum oxide is presintered
into the porous ceramic matrix at temperature from about
1450.degree. C. to about 1550.degree. C. and infiltrated with
molten titanium alloy at temperature lower than 1450.degree. C.
8. The cermet of claim 7 wherein the porous ceramic matrix has
about zero to about 50 degrees wetting angle.
9. The cermet of claim 8 wherein the particle sizes of the
presintered porous ceramic matrix are about the same as the
particle sizes of the ceramic powder used.
10. The cermet of claim 9 wherein the metal alloy is binary,
ternary or quaternary alloy selected from alloys shown in Table
1.
11. A bone implant fabricated from a cermet comprising a ceramic
powder presintered into a porous ceramic matrix and infiltrated
with a molten metal or metal alloy.
12. The implant of claim 11 wherein the ceramic powder is selected
from the group consisting of aluminum oxide, zirconium oxide,
hafnium oxide, vanadium oxide, aluminum nitride, zirconium nitride,
hafnium nitride, vanadium nitride, aluminum boride, zirconium
boride, hafnium boride, vanadium boride, aluminum silicate,
zirconium silicate, hafnium silicate, vanadium silicate, beryllium
oxide, beryllium nitride, beryllium borate, beryllium silicate and
a mixture thereof.
13. The implant of claim 12 wherein the infiltrating metal or metal
alloy is selected from the group consisting of titanium, aluminum,
magnesium, nickel, lithium, copper, iron, silicon, manganese,
cobalt, molybdenum, niobium, zirconium, calcium and their
combination.
14. The implant of claim 13 wherein the ceramic powder is pressed
into a near-net shape of the bone implant, presintered into a
porous ceramic matrix at a temperature higher than is the melting
point of the infiltrating metal or metal alloy and infiltrated with
the molten metal or metal alloy at temperature lower than the
sintering temperature of the ceramic powder.
15. The implant of claim 14 wherein the ceramic powder is aluminum
oxide having particle sizes from about 10 to about 50 microns
presintered into the porous ceramic matrix at temperature from
about 1450.degree. C. to about 1550.degree. C. and infiltrated with
molten titanium alloy at temperature lower than 1450.degree. C.
16. The implant of claim 15 wherein the porous ceramic matrix has
about zero to about 50 degrees wetting angle.
17. The implant of claim 16 wherein the particle sizes of the
aluminum oxide presintered into a porous ceramic matrix are about
the same as the particle sizes of aluminum oxide used.
18. The implant of claim 17 wherein the metal alloy is binary,
ternary or quaternary alloy selected from alloys shown in Table 1
having the melting point lower than 1450.degree. C.
19. A process for fabrication of cermets comprising of a ceramic
powder infiltrated with a molten metal or metal alloy, said process
comprising steps: (a) presintering a ceramic powder selected from
the group consisting of any one or a mixture of aluminum oxide,
aluminum nitride, aluminum boride, aluminum silicate, hafnium
oxide, hafnium nitride, hafnium boride, hafnium silicate, vanadium
oxide, vanadium nitride, vanadium boride, vanadium silicate,
beryllium oxide, beryllium nitride, beryllium borate and beryllium
silicate at its sintering temperature and proper atmospheres into a
porous ceramic matrix of a desired shape; (b) molten metal
infiltrating the matrix of step (a) with a metal selected from the
group consisting of titanium, nickel, magnesium, calcium, aluminum,
lithium, copper, iron, silicon, manganese, cobalt, molybdenum,
niobium, zirconium, and their combination wherein said selected
metal having a melting point lower than the sintering temperature
of the powder of step (a).
20. The process of claim 21 wherein additionally a wetting angle of
the metal or metal alloy is determined before the compact of step
(a) is infiltrated.
21. The process of claim 20 wherein the wetting angle for the metal
and ceramic compact interaction is zero or is not larger than about
60 degrees.
22. The process of claim 21 wherein the ceramic powder is
presintered under vacuum.
23. The process of claim 22 wherein the molten metal is infiltrated
evenly into the ceramic presintered compact.
24. The process of claim 23 wherein the ceramic powder is aluminum
oxide.
25. The process of claim 24 wherein the aluminum oxide powder has
particle sizes from about 10 .mu. to about 50 .mu..
26. The process of claim 25 wherein the sintering temperature is
between 1400.degree. C. and 1660.degree. C.
27. The process of claim 26 wherein the aluminum oxide has particle
sizes about 10 .mu..
28. The process of claim 27 wherein the sintering temperature is
about 1500.degree. C.
29. The process of claim 28 wherein the metal used for
molten-metal-infiltration is titanium alloy.
30. The process of claim 29 wherein the temperature used for
titanium alloy is lower than the sintering temperature of the
ceramic powder compact.
31. The process of claim 30 wherein the metal is the titanium alloy
selected from the group of binary, ternary and quaternary alloy
shown in Table 1.
32. The process of claim 31 wherein the titanium alloy contains
about 20-30% of nickel.
33. The process of claim 32 wherein the temperature for
molten-metal-infiltration is under 1500.degree. C.
34. The process of claim 33 wherein the ceramic powder additionally
contains an additive selected for the group consisting of calcium
oxide, magnesium oxide and sulfur present in an amount for about
0.01 to about 1%.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] This invention concerns novel cermet materials comprising
ceramic and metal components and a molten metal infiltration method
and process for fabrication thereof. The cermets of the invention
are fabricated from preformed ceramic powder infiltrated with a
molten metal or metal alloy. The novel cermets are useful for a
wide range of applications such as manufacture of surgical
instruments, cutting tools, engine parts, wear parts, etc. Their
properties, such as strength, durability, toughness, elasticity,
etc. can be designed according to their intended use.
[0004] Additionally, the invention concerns cermets, particularly
alumina-titanium cermets, having properties similar to bone. These
cermets are made of components biocompatible with the human body
and are suitable for bone and joint replacements.
[0005] 2. Background Art and Related Disclosures
[0006] While the material technology has substantially advanced in
recent years, providing many choices of materials having specific
properties, there is still a need for materials having selected
properties specifically suitable for specific purposes. These
materials typically need to be durable, hard to break, non-fragile,
non-brittle and yet elastic and reasonably light in weight.
Additionally, their fabrication should be economically feasible and
not overly laborious. Moreover, in order to meet requirements for
their specific Moreover, in order to meet requirements for their
specific use, many of these materials need to be able to be custom
designed.
[0007] Thus, it would be advantageous to have available a material
having all above named properties and a process for fabrication of
any such material where these properties could be easily varied and
changed depending on used components and process conditions and
design.
[0008] One example of the needed material having specific
properties is the material having weight, structure, strength and
other properties similar to and resembling bones or joints, which
material would be suitable for bone or joints replacement.
[0009] Medical advances of past several decades have substantially
extended life of the human population. The aging population,
however, faces a multiplicity of disorders which may limit its
quality of life. Among those disorders are osteoporosis, Paget's
disease of bone and joints, and arthritis. All these disorders may
cause limited mobility and often, particularly in the elderly, can
result in death due to resulting bone fractures. When not fatal,
these disorders still often require surgical bone or joint
replacement of hips, knees, elbows, etc.
[0010] The major problem associated with the bone replacement is a
lack of a suitable material which would have the same or similar
properties as bone but that would also be compatible with the human
body. The properties which the bone or joint replacement material
need to possess include light weight, porosity, strength,
durability, elasticity and, in order to prevent wear in joint areas
and to prevent or allow tissue attachment in other areas, as need
be, a possibility to be surface finished. Therefore, such material
must have approximately the same porosity, weight and structure and
must not be more fragile or more brittle than the normal bone.
[0011] Currently, several materials are known and medically
acceptable as implants. While these materials, namely alumina
ceramic (Al.sub.2O.sub.3) and titanium alloys containing 5%
titanium and 4% aluminum (Ti5-4) or 6% titanium and 4% aluminum
(Ti6-4), are acceptable as implant materials, alone or in
combination, neither has the desired properties to replace bone or
joints.
[0012] Thus, it would be advantageous to provide a biocompatible
material which would have the strength, durability, elasticity and
surface finish similar to the natural bone and which could also be
custom shaped in a relatively short time so that surgeons could
make necessary adjustments to the implant during the operation in
order to properly fit the patient.
[0013] Ceramic/metal combination materials, known as cermets, are
known. These cermets possess useful properties, such as toughness
and strength, and have been used for manufacturing lightweight
personnel armor, structural materials, cutting tools, radiation
resistant structures, insulation materials, impact, abrasive and
wear resistant structures, etc. However, none of the known cermets
possess properties which would make them suitable for bone or joint
replacement or for manufacture of other products requiring the
similar properties as bone implants. This is due to the fact that
the known processes for their preparation do not prevent ceramic
powder particles shrinking, enlargement or cluster formation during
their fabrication. These changes in particle sizes of the ceramic
powder result in an uneven and unpredictable porosity of the
resulting material.
[0014] A method for forming metal-filled ceramics is described in
U.S. Pat. No. 3,718,441. Cermets which are boron-carbide-aluminum
or boron-carbide-reactive metal composites are described in U.S.
Pat. No. 4,605,440 and infiltration processing of boron carbide,
boron and boride-reactive metal cermets is described in U.S. Pat.
No. 4,718,941. Cermets prepared by combustion synthesis and metal
infiltration are described in the U.S. Pat. No. 4,988,645. However,
none of the above produced cermets possess specific properties as
described above required for bone replacement or manufacture of
other products. This is primarily due to methods and/or materials
and conditions used for their fabrication.
[0015] There are two other methods currently known and used for
fabricating cermets. The first method involves cold press and
sintering (CPS). The second method involves hot pressing (HP) or
hot isostatic pressing (HIP).
[0016] Cermets, such as cermets prepared from titanium-aluminum
oxide (Ti--Al.sub.2O.sub.3) components, were prepared by the
conventional process of cold pressing and sintering of the blended
titanium (Ti) and aluminum oxide (Al.sub.2O.sub.3) powders.
Typically, the blended titanium-aluminum oxide powders are formed
into the desired shape, and submitted to a temperature at least as
high as the sintering temperature of the titanium-aluminum oxide
blend. This leads to a large shrinkage of more than 15% of the
aluminum oxide particles. Additionally, these shrunk particle
result in a grain growth and cluster formation of aluminum oxide
particles occurs. The high sintering temperature to which these
blends are submitted results in formation of a cermet containing
dense aluminum oxide areas unevenly distributed throughout the
matrix cermet interspaced with titanium filling-in voids between
these unevenly distributed areas. This is due to the aluminum oxide
particles sintering together into grains and clusters when
submitted to the high sintering temperatures allowing the titanium
migration only into the void sites between the growing grains of
aluminum oxide clusters.
[0017] The growth of aluminum oxide particles into grains and
clusters, therefore, does not allow an even distribution of the
molten titanium within the sintered ceramic particles but rather
results in molten titanium getting into voids between aluminum
oxide grains and forming larger metal areas.
[0018] The cold pressing and sintering method thus results in
cermets consisting of ceramic grains and clusters larger than 80
microns interspaced with unevenly distributed titanium areas larger
than the titanium powder particle size used in the starting powder
blend. Titanium thus forms distinct titanium islands within grains
and clusters of the aluminum oxide matrix. Because of this uneven
distribution due to the coarse dispersion of large titanium areas
in a coarse structure of aluminum oxide grains, the mechanical
properties of the formed cermets are poor and typically these
cermets are fragile, brittle and their porosity and weight is
uneven.
[0019] The cermets produced by cold sintering are not suitable for
preparation of cermet materials which require that the material is
light, tough, durable non-fragile or non-brittle and has a uniform
porosity. These cermets do not have even distribution of titanium
within the aluminum oxide matrix and are, therefore, fragile and
subject to easy fracture. The shrinkage and grain growth occurring
during the cold pressing and sintering are clearly not acceptable
for bone replacement implant material which need to have a
consistently porous microstructure strengthened with metal.
[0020] Some improvement on this microstructure was achieved by
development of two subsequent methods utilizing the pressure during
the sintering, namely hot pressing (HP) or isostatic hot pressing
(HIP). This improvement consist of sintering of compositions of
titanium in aluminum oxide (5 to 60 vol/%) at lower temperatures
(1400-1600.degree. C.) and by applying pressure to the powder
preform while at these lower temperatures to force entry of
titanium in between the aluminum oxide grains.
[0021] Hot pressing of the titanium-aluminum oxide powder blend is
accomplished generally in a graphite die and punch assembly where
the pressure is applied to the powder inside the dies through
hydraulic force on the punches. The powders are heated to the
desired densification temperature (1400-1600.degree. C.) and the
applied pressure assists in the rapid (<1 hour) densification of
the powder.
[0022] Hot isostatic pressing is achieved by sealing the powder
blend in a metal, such as, for example, molybdenum, or in a glass
container and applying a gas pressure, generally argon or helium,
to the outside of this container while heating the gas to the
desired sintering temperature (1300-1500.degree. C.).
[0023] The cermet products obtained by hot pressing or hot
isostatic pressing have similar properties. Unfortunately, both
these methods still result in the shrinkage and in the grain growth
of the aluminum oxide-titanium powder blend and in an uneven
distribution of the metal through the aluminum oxide. This is due
to the same sintering problems observed during cold pressing and
sintering, where the sintering is performed at lower temperatures
and the process takes a longer time. Hot pressing or hot isostatic
pressing take shorter time as they are performed under high
pressure. Thus, while the finer microstructures than those obtained
during the cold pressing and sintering were obtained from hot
pressing or hot isostatic pressing, such processing did not result
in cermets having properties required for bone replacement. Even
under relatively high pressure (<30,000 psi) conditions, the
sintering process resulted in the growth of the titanium and
aluminum oxide grains larger than 60 microns and in large shrinkage
to achieve densification needed for bone replacement implant.
[0024] Therefore, the hot pressing improvement of the cold pressing
and sintering process still does not provide material having an
uniform distribution of metal throughout the ceramic matrix
suitable for bone replacement or for manufacture of other products
having similar requirements for material properties.
[0025] In an attempt to reduce the grain growth, special processing
of the ceramic metal blend was suggested using coating the aluminum
oxide particles with titanium metal. This was expected to allow the
HP or HIP processes to achieve densification of the powder at lower
temperatures and reduce grain growth. However, in order to be able
to be coated with sufficient titanium metal and still achieve a
dense product during hot pressing or hot isostatic pressing, the
aluminum oxide particles must be larger than 40 microns. This limit
prevents fabrication of cermets which would have no shrinkage, and
would have uniform distribution of the metal within the aluminum
oxide matrix assuring the strength, toughness and non-fragility of
the bone replacement implant.
[0026] Thus it would be very advantageous to have available a
material which would posses the above listed undesirable property
and a method and process for its preparation eliminating the above
listed disadvantages and problems.
[0027] It is therefore a primary objective of this invention to
provide a cermet of which properties can be designed to
specifically meet the requirements for its intended use. Due to the
improved processing, the new cermet is light, has an even porosity,
is strong, durable, elastic, and tough as well as non-fragile or
non-brittle. The new cermet can be prefabricated into a near-net
ceramic shape of the article to be used, and
molten-metal-infiltrated after the final shaping. Additionally, the
surface of the article can be surface finished in such a way that
it fits its use. The new cermet which can be made of components
fully compatible with the human body has properties similar to bone
and is able to withstand the pressures and weight to which the bone
in the body is constantly submitted without breaking. Additionally,
it can be made to fit the patient bones and joints and surface
finished to meet physiological functions of the replacement, such
as tissue attachment, lubrication, etc.
[0028] All patents, patent applications, and publications cited
herein are hereby incorporated by reference.
SUMMARY
[0029] One aspect of this invention is a new cermet comprising of a
ceramic powder presintered into a porous ceramic matrix and
infiltrated with a molten metal or metal alloy.
[0030] Another aspect of this invention is a cermet having
properties custom designed for its intended use.
[0031] Still yet another aspect of the current invention is a
cermet fabricated from a presintered ceramic powder selected from
the group consisting of aluminum oxide, zirconium oxide, hafnium
oxide, vanadium oxide, or combination of these ceramic powders and
infiltrated with a molten metal such as titanium, aluminum,
magnesium, nickel, lithium or calcium or alloys of these
metals.
[0032] Still yet another aspect of this invention is a cermet
fabricated from a ceramic selected from the group consisting of
aluminum, zirconium, hafnium and vanadium nitride, boride, or
silicate.
[0033] Yet another aspect of this invention is a method and a
process for fabricating cermets of the invention by presintering
and molten-metal-infiltration.
[0034] Another aspect of this invention is a new cermet which has
an evenly distributed porosity and light weight and is strong,
tough, durable, elastic, non-fragile and non-brittle.
[0035] Still yet another aspect of this invention is a cermet
comprising a porous ceramic matrix fabricated by presintering
ceramic powder of the same particle size limited to from about 10
to about 50 .mu., at a sintering temperature which is higher than
the melting point of the infiltrating metal and during sintering
does not result in shrinkage, grain growth and a formation of
voids, and by uniformly infiltrating the presintered ceramic matrix
with a molten metal or a metal alloy.
[0036] Still another aspect of the current invention is a new
cermet comprising a ceramic powder or a blend of ceramic powders
presintered into a porous ceramic matrix having a zero or a very
low wetting angle allowing quick and even molten metal infiltration
along the surface and into the porous ceramic matrix.
[0037] Another aspect of this invention is a cermet suitable for
manufacturing products such as surgical instruments, cutting tools,
wear parts, engine components or implants for bone replacement.
[0038] Still yet another aspect of the current invention is a
biocompatible titanium-aluminum oxide cermet material prefabricated
as an aluminum oxide near-net shape preform of the article to be
fabricated wherein said preform is custom finished by polishing,
grinding and/or machining to fit its intended use and subsequently
strengthened by an uniform infiltration of molten metal into a
porous ceramic matrix.
[0039] Another aspect of the current invention is a cermet bone
implant fabricated from a densely packed aluminum oxide ceramic by
presintering said densely packed ceramic powder having the same
particle sizes from about 10 to about 50 .mu. in a near-net shape
of bone preform matrix at a sintering temperature higher than the
melting point of infiltrating metal or metal alloy, said sintering
temperature preventing shrinkage, grain growth and formation of
intraparticle voids, said presintered ceramic matrix having a zero
or very low wetting angle, and uniformly infiltrating a molten
titanium or titanium containing alloy into said matrix.
[0040] Still yet another aspect of the current invention is a
biocompatible titanium-aluminum oxide cermet prefabricated in
near-net shape matrix preform of the bone or joint to be replaced
wherein said preform is custom finished by polishing, grinding
and/or machining to fit the patient's bone or joint, and
subsequently strengthened by an uniform infiltration of molten
metal into a porous ceramic matrix.
DEFINITIONS
[0041] As used herein :
[0042] "Cermet" means a material comprising a metal or a metal
alloy and a ceramic powder or a mixture of ceramic powders. Cermet
is fabricated from the ceramic powder selected from a group of
compounds represented and exemplarized by the titanium-aluminum
oxide system. Other systems, such as and including zirconium,
hafnium, beryllium, vanadium oxides, nitrates, silicates or
borides, etc., in combination with a metal, such as titanium,
aluminum, magnesium, nickel, lithium, calcium, or their alloys are
equally suitable for fabrication of cermets of the invention. In
addition to these named systems, any other suitable alloy system
meeting general conditions for processing of the cermets of the
invention may also be advantageously used to fabricate these
cermets using the molten-metal-infiltration method and process and
are intended to be within the scope of the invention.
[0043] "Preform" means a cermet material prefabricated in near-net
shape preform of the article to be molten infiltrated. Preform may
be custom finished by polishing, grinding, machining, etc., to fit
its intended use.
[0044] "Molten metal infiltration" means a method wherein the
molten metal or an alloy is infiltrated into a porous ceramic
matrix formed by presintering of the aluminum oxide or other
ceramic powder under temperatures which are higher than the melting
point temperature of the infiltrating metal and wherein the
infiltration is typically performed under a protective environment
such as under vacuum, or in argon, helium, hydrogen or nitrogen
atmosphere.
[0045] "Alloy" means a combination of two or more metals. Such
combination changes the properties, such as melting point, of the
infiltrating metals.
[0046] "Uniform metal infiltration" means a metal evenly
distributed within the porous ceramic matrix of the
prefabricate.
[0047] "Sintering" or "presintering" means fusing ceramic powder
particles into a bonded mass using heating at a temperature above
the melting point of the infiltrating metal or metal alloy and
optionally a pressure.
[0048] "Densification" means increasing density of the ceramic
powder by pressure and/or sintering.
[0049] "Wetting angle" means a contact angle as defined and
described in J. Am. Ceramic Soc. 54:332 (1971). The wetting angle
is measured in a sessile drop unit.
[0050] "Wetting" means any process in which an interface between
solid phase and liquid phase is formed. When the two phases
interface, these two phases are not in chemical equilibrium. During
these non-equilibrium conditions, the interfacial energies and
contact angle are continuously changing. This process continues
until the system reaches a state of chemical equilibrium. The
wetting phenomenon can be expressed as a contact or wetting angle
measurement.
[0051] "Void" means empty areas and spaces unevenly distributed
within the matrix formed of ceramic powder grains and clusters. The
formation of these voids within the cermet matrix are undesirable
as they contribute to the material's fragility and brittleness.
BRIEF DESCRIPTION OF DRAWINGS
[0052] FIG. 1 is a comparative graph showing the effect of
increasing the aluminum oxide particle size on its lowest sintering
temperature.
[0053] FIG. 2 is a graph showing improved wetting of an alloy
consisting of titanium and nickel.
DETAILED DESCRIPTION OF THE INVENTION
[0054] This invention concerns novel cermet materials of which
properties such as weight, porosity, strength, toughness,
flexibility and elasticity can be custom designed for the intended
use or for the use of the intended manufactured product. The new
cermets are light in weight, durable, strong, tough, non-fragile
and non-brittle and can be made of biocompatible components with
human or animal body and are, therefore, suitable for manufacture
of bone replacement implants, surgical instruments, cutting tools,
wear parts, engine components, etc. The invention also concerns a
method and a process for preparation of these cermets.
[0055] The new cermet material is a mixture of a ceramic, such as
for example, aluminum oxide, zirconium oxide, hafnium oxide,
beryllium oxide, vanadium oxide, boron carbide, aluminum nitride,
zirconium nitride, hafnium nitride, vanadium nitride, aluminum
boride, zirconium boride, hafnium boride, vanadium boride, aluminum
silicate, zirconium silicate, hafnium silicate, vanadium silicate
powders or their mixtures, in combination with a metal such as
titanium, aluminum, magnesium, nickel, lithium, calcium, or other
suitable metals, or their alloys, etc.
[0056] Briefly, the cermet of the invention is fabricated in a
two-step process from a ceramic powder and from an appropriate
metal or metal alloy.
[0057] The first step of the process provides a porous ceramic
matrix formed of the preformed ceramic powder. Due to a novel
process for preparation of these cermets, the sizes of the ceramic
powder particles are stable, do not shrink or grow into large
grains or clusters after preforming. When submitted to infiltration
with the molten metal or alloy, the preform remains the same as the
initial preform because there is little or no shrinkage.
[0058] The stability of the ceramic preformed powder is one of the
novel aspects of the invention distinguishing the invention from
the other known or previously described processes. All other
processes, as discussed in the background section, result in
ceramic powder particles growing during sintering into larger
grains and clusters, in void formation, and in an uneven and
uncontrollable porosity of the ceramics. This growth typically
continues during the molten metal step. Thus, most of the
previously described cermets result in ceramic-metal materials
which are brittle and fragile, have unpredictable weight and metal
enforcement and are in general unsuitable for fabrication of bone
implants or other products of manufacture requiring the uniform
distribution of the infiltrating metal through the porous ceramic
matrix.
[0059] The second step of the process, namely a molten metal
infiltration, allows an even and uniform infiltration of the
uniform porous preform ceramic matrix with a molten metal or a
suitable metal alloy under conditions which prevent further
sintering of the ceramic preform or grains. The first process step
thus involves partial sintering of the ceramic powder particles
into the porous preform matrix having preferably a premolded shape
of the intended product. This partial sintering results in a light
fusion of ceramic powder particles at higher temperatures than
those of the molten metal or alloy to be used for metal
infiltration. During this latter step, the ceramic particle sizes
remain stable. The processing pressure and difference between the
temperature used for sintering in step one and temperature
conditions used instep two for metal infiltration prevent the
further particle growth during the molten metal infiltration. These
conditions further assure that during the molten-metal-infiltration
at lower temperatures than those used for ceramic powder sintering,
the molten metal is evenly and uniformly distributed throughout the
porous preform ceramic matrix. Because the ceramic matrix is made
of the ceramic powder particles of the same size lightly sintered
together, molten metal is easily infiltrated into this ceramic
matrix.
[0060] The molten metal infiltration into the presintered ceramic
matrixes under the process conditions of the invention, therefore,
results in unexpectedly improved mechanical and physical properties
such as high fracture toughness, increased strength and wear
resistance making the cermets of the invention suitable for
manufacturing products having specific requirements as to the
weight, porosity, strength, durability, toughness, flexibility,
elasticity and variability of design a surface finish of cermet
products.
[0061] A primary advantage of the invention is that by changing the
particle sizes of the ceramic powder, pressure and temperature
conditions, and by selecting a suitable metal or metal alloy for
metal infiltration, the properties of the resulting cermet can be
easily designed, modified or changed. Two examples of cermets with
widely variable properties are the system of (1) tungsten
carbide-cobalt (WC--Co) and (2) titanium carbide molybdenum nickel
(TiC--Mo--Ni) cermets. These cermets are used in the commercial
field as various grades such as for example C-1, C-2, C-3, etc.,
depending on the amount of Co in the WC matrix containing 1% Co, 2%
Co, 3% Co, etc., respectively. The higher the amount of Co content,
the stronger and tougher the cermet, but at the expense of
decreasing hardness and wear properties. The similar effect is
observed for the TiC--Mo--Ni system. The application of such
cermets determines which grade cermet is best for the intended
purpose and illustrates versatility of the invention.
[0062] 1. Ceramic Powder Matrix and Its Fabrication
[0063] The cermet of the invention typically comprises at least two
major components, namely a ceramic and a metal or a metal alloy.
Ceramic is a single ceramic powder or a mixture of several powders
and the metal is a single metal or an alloy of two or more metals.
Additionally, additives improving the properties of the cermet or
the processing conditions may be optionally added to the ceramic
powder or to the metal or metal alloy. Additives such as, for
example, calcium oxide, magnesium oxide, sulfur or any such
compound are added to the ceramic powder to reduce grain growth,
shrinkage, cluster formation, etc., during sintering. Additives
such as nickel, magnesium, calcium, aluminum, lithium and any such
elements are added to a primary metal, such as for example
titanium, to lower the melting point of the metal or alloy and in
this way to lower molten metal infiltration temperature and
increase wettability. Other additives, such as boron, carbon,
nitrogen, silicon, etc., can also improve strength and other
properties.
[0064] The first component of the cermet of this invention is a
ceramic powder, preferably an oxide metal such as aluminum oxide,
comprising spherical and/or smooth surface particles having a low
surface area to retard sintering during the
molten-metal-infiltration process. This powder is presintered into
preforms having an evenly distributed porosity. Ceramic powder
particles can be packed into a shape and fused at their contact
points by controlling the temperature, pressure, and/or protective
environment during sintering. The protective environment includes
but is not limited to hydrogen, argon, helium, nitrogen atmosphere
or vacuum, etc.
[0065] Typically, the preforming of the ceramic powder into a
near-net shape is done by any process suitable therefor such as
hydraulic pressing in a die, in a rubber or plastic mold, slip
casting a slurry, extrusion pressing, injection molding, etc.
[0066] The atmosphere may, for example, include a vapor that
assists the particle fusion as silicon oxide (SiO) or can deposit
aluminum oxide at the surface of the aluminum oxide particles by a
chemical vapor deposition process (CVD). CVD is accomplished by
using aluminum chloride (AlCl.sub.4) in moist hydrogen atmosphere
according to the equation
AlCl.sub.4+H.sub.2+H.sub.2O.fwdarw.Al.sub.2O.sub.3+HCl. The
Al.sub.2O.sub.3 particles are bonded with the CVD Al.sub.2O.sub.3
at their contact points.
[0067] The preforming or presintering step in the process for
cermets fabrication is based on properties of the ceramic powder or
a mixture of two or more ceramic powders. These properties include
the size of the ceramic powder particles, their surface,
composition, and the sintering temperature.
[0068] Selecting the appropriate ceramic powder or the mixture of
powders typically involves testing of the particle size vis-a-vis
the powder's sintering temperature, as illustrated in FIG. 1.
Subsequently to such testing, the ceramic powder or its mixture,
its particle size and the temperature are selected which assure
that following the presintering the ceramic particles remain of the
same size, that they do not shrink, grow or form clusters and voids
within the matrix.
[0069] In order to determine the optimal sizes of the ceramic
particles as well as the temperature suitable for the ceramic
matrix formation, the aluminum oxide particles of various sizes
were submitted to sintering under increasing temperature. Results
are seen in FIG. 1.
[0070] FIG. 1 shows the effect of increasing the aluminum oxide
particle size on its initial sintering temperature. Particles
having increased size have reduced surface area, and as seen in
FIG. 1, these particles require higher sintering temperature. The
sintering temperature of very small aluminum oxide particles
between 0.2 and 1 .mu. was found to be around 1250-1300.degree. C.
The sintering temperature of aluminum oxide particles having sizes
from 5 to 20 .mu. was found to be from 1400.degree. C. to
1600.degree. C. These temperatures are below the melting point
1668.degree. C. of titanium. Therefore, the aluminum oxide powder
having the 5-20 .mu. particle sizes is not suitable to be
presintered into the cermet matrix according to the invention
unless the metal or metal alloy used for molten metal infiltration
has the melting point lower than 1400.degree. C. to 1600.degree. C.
temperature. The ceramic preform can be improved depending on the
exact particle size and temperature used for sintering as well as
on the presence of additives or impurities which retard shrinkage
of the ceramic preform at the infiltration temperature.
[0071] According to the FIG. 1, the sintering temperature of
aluminum oxide powder having particle sizes about 50 .mu. is about
1650.degree. C., which is still below 1668.degree. C. melting point
of pure titanium. In this case, therefore, the pure titanium would
not be suitable for molten metal infiltration unless combined with
another metal into an alloy having the temperature lower than
1650.degree. C. so that metal alloy of the metal combination would
have a temperature lower than 1650.degree. C. and would not cause
aluminum oxide sintering.
[0072] The sintering temperature of particles above 50 .mu. is
around 1700.degree. C. which would fit within the general
conditions of the method of the invention. These particles were
found to have limited usefulness for preparation of the ceramic
matrix according to the invention, as these particles would fuse
together at 1700.degree. C. When infiltrated at this temperature,
their relatively large size would prevent shrinking and grain
growth. This would result in an even and uniform distribution of
the metal through the matrix having, however, relatively large
microstructure.
[0073] When the ceramic powder or a blend of several powders
contain naturally or artificially added additives or impurities,
such as sulfur, calcium oxide, magnesium oxide, etc., in amounts
between about 0.01 to about 1%, such small additions or presence of
these impurities at the surface of the ceramic powder particles
helps retard sintering and grain growth of smaller particles.
[0074] Although the actual size of the selected ceramic powder
particles depends on the metal or metal alloy used, the preferred
particle sizes of the ceramic powder used for formation of the
ceramic porous matrix have particles between about 10 and 50 .mu..
The most preferred particles sizes for aluminum oxide powder are
about 10 .mu.. At this size, and at about 10.sup.-6 torr pressure,
the sintering temperature is around 1500.degree. C.
[0075] In practice, when the appropriate combination of a ceramic
powder with a selected metal or metal alloy is contemplated,
ceramic powders suitable for preparation of cermets of the
invention selected from the group consisting of aluminum,
zirconium, hafnium, beryllium or vanadium of oxides, carbides,
nitrates, silicates, borides, etc., or their mixture are submitted
to testing and their individual sintering temperature is determined
as described in Example 1. The density of each compacted ceramic
matrix is measured and its sintering temperature is determined. The
preferred density of the ceramic matrix is about 50 to about 70,
most preferably about 60%. Preferred porosity of the ceramic matrix
is about 30 to about 50, most preferably 40%. Because of their
different physical and chemical properties, the sintering
temperature for each individual ceramic powder or a mixture of two
or more powders is different. However, the principle of
determination of the optimal sintering temperature vis-a-vis the
particle size is the same as for aluminum oxide described
herein.
[0076] The powder particle size selected for each cermet is based
on the sintering temperature of that particle size and such
sintering temperature must be above the melting point of the
selected metal or metal alloy used for molten metal
infiltration.
[0077] Thus, when the cermet is designed, the suitable ceramic
powder is selected, for example the one which is physiologically
acceptable, then the sintering temperature of the various particle
sizes of each ceramic powder or a blend of two or more powders is
determined and then the most optimal particle size is selected
depending on the melting point of the metal or the alloy selected
to be used for infiltration.
[0078] The selected particles are then cast into their near-net
shape, presintered at their sintering temperature into a desired
shape prefabricate forming the stable porous ceramic matrix not
subject to any significant sintering during the
molten-metal-infiltration process. Significant sintering during the
infiltration process would close off the connecting porous channels
and infiltration would stop.
[0079] The density and porosity of the ceramic matrix is designed
based on the intended use of the cermet.
[0080] 2. A Molten Metal Infiltration Method
[0081] The cermets of the invention are preferably prepared by the
molten-metal-infiltration method.
[0082] Previous attempts to use molten-metal-infiltration for other
cermet systems have not been very successful, especially in the
oxide-metal systems, because at molten metal temperatures it
results in further sintering of the ceramic powder and the molten
metal infiltration into the ceramic powder is hindered by the loss
of interconnecting pores.
[0083] It has now been found that under proper conditions, molten
metal can be infiltrated into a powder preform of metal carbide,
metal oxide, metal nitride, metal boride, etc., and that under
these conditions a fully dense ceramic/metal cermet is
obtained.
[0084] As described above, the first condition for successful
fabrication of the ceramic/metal cermet is the use of sintering
temperature for presintering of the ceramic powder particles into
the porous ceramic matrix which temperature is higher than the
melting point of the infiltrating metal. The second condition is
the molten metal infiltration at the infiltration temperature which
is lower than the sintering temperature used in the first step.
This prevents further sintering of the ceramic powder particles
during the molten metal infiltration.
[0085] The lack of further sintering of ceramic particles allows
the molten metal to "wet" the surfaces of the ceramic within the
porous ceramic matrix and infiltrate into its porous structure.
Such infiltration does not result in shrinkage to achieve
densification, there is little to no sintering or grain growth of
the ceramic particles since the infiltration temperature is lower
than the sintering temperature of the ceramic powder. The high
wetting ability of the molten metal at these lower temperatures
allows rapid and uniform infiltration of the metal or metal alloy
into the ceramic matrix in less than 30 minutes.
[0086] Under the normal circumstances and using previously known
methods without current modifications and improvements according to
the invention, the relatively high melting temperature
(1668.degree. C.) of titanium would prevent the infiltration of
molten titanium into an aluminum oxide powder preform. This is
because the infiltration temperature for the molten titanium is
significantly higher than the sintering temperature of fine (10-50
u) aluminum oxide powder particles. Therefore, when the normal
melting point temperature of titanium is used, the aluminum oxide
particles sinter rapidly at this high (1668.degree. C.)
temperature, grow into the grains and seal off the open pores
between the particles and in this way prevents the uniform molten
titanium infiltration of the ceramic matrix.
[0087] In the molten metal infiltration step of the process, the
second component, namely a light-weight metal, such as titanium,
aluminum, manganese, nickel, calcium, lithium or their respective
alloy, is infiltrated into the porous ceramic matrix at the
temperature which is lower than the sintering temperature of the
used ceramic powder.
[0088] In practice, the metal or metal alloy is selected according
to its melting point temperature which must be lower than the
sintering temperature of the used ceramic powder. The selected
metal or metal alloy is melted at its melting point temperature and
then, as a molten metal, is brought in contact with the porous
ceramic matrix so that the metal wets the ceramic within the porous
ceramic matrix. The metal must wet the ceramic to infiltrate into
it or a pressure must be applied to the molten metal to force it
into the porous ceramic matrix.
[0089] Wetting of the ceramics assists in the molten metal
infiltration. In this respect, the wetting angle expressed in
degrees is a measure of the ceramics/metal interface. The wetting
angle of the molten metal depends on the ceramics, metal or metal
alloy composition, on ceramic and metal interaction, on
infiltration temperature, atmosphere, and pressure. The wetting
angle decreases with increased temperature. For the best
infiltration, the wetting angle is either zero or as close to zero
(.ltoreq.20.degree.) as possible. When the metal infiltration is
performed at a temperature having the 180 degrees wetting angle,
the metal stays at the point of infiltration and forms into spheres
of molten metal. This would prevent even and uniform metal
distribution throughout the whole ceramic matrix. When the metal is
infiltrated at a temperature having a zero wetting angle, the metal
spreads evenly throughout the ceramic matrix because the surface
energy allows it to spread more easily.
[0090] Dependence of the wetting angle on the temperature is
illustrated in FIG. 2 for two Ti--Ni alloys.
[0091] According to the invention, metal infiltration can be either
subsequent to the ceramic matrix formation or simultaneous with
formation of the matrix provided that the simultaneous infiltration
and matrix formation does not lead to the sintering and shrinkage
of the ceramic powder.
[0092] For the cermets of the invention, a uniform distribution of
the metal within the ceramic matrix is essential and necessary
since only when the metal is uniformly distributed throughout the
porous matrix, the cermet has properties required for bone
replacement or for manufacturing products which require it to be
evenly porous, light in weight, strong, tough, durable, elastic,
non-fragile and non-brittle. The porosity in the ceramic matrix can
be graded to give desired properties at various regions of the
cermet component.
[0093] The molten-metal-infiltration of the invention is performed
under as low temperature conditions as is possible in order to
prevent the sintering of the ceramic powder particles into large
grains or formation of clusters.
[0094] The ability of the metal, preferably titanium, to "wet" the
surface of the ceramic, preferably aluminum oxide, is
advantageously improved by substituting the metal with an alloy or
by additions of one or more other metals or metal alloys prior to
or during molten metal infiltration. In the case of titanium,
addition of a metal, such as nickel, copper, iron, manganese,
aluminum, silica, etc., in amounts of less than 45 weight percent
lowers the melting point of titanium and increases its
"wettability" onto aluminum oxide as seen in FIG. 2.
[0095] The suitable binary, ternary or quaternary metal alloys and
their melting points are listed in Table 1.
1TABLE 1 Titanium Alloys Melting Points Binary, Ternary and
Quaternary Alloys of Titanium (Ti) With Melting Points Below Pure
Ti (1668.degree. C.) Titanium Alloy Compositions Binary Alloys
Ternary Quaternary (Balance Alloys Melting Alloys Melting Ti)
(>60% Ti) Point (C.) (>60% Ti) Point (C.) 40% Cu 990
Mn--Cu--Si 800 30% Ni 955 Fe--Cu--Si 1300 40% Al 1455 Al--Cu--Mg
500 25% Fe 1085 Al--Fe--Si 1200 15% Si 1330 Al--Ni--Si 1200 40% Mn
1250 Fe--Ni--Si 1200 25% Co 1025 Al--Mn--Fe 1150 10% Si 1330
Al--Mo--Fe 1200 30% Cu--Co 1400 35% Mg--Cu 552 35% Mn--Cu 870 15%
Si--Cu 802 20% Fe--Cu 1450 35% Al--Cu 750 35% Mg--Al 450 35% Al--Fe
1165 15% Al--Ni 1385 12% Si--Al 577 10% Si--Ni 1152 12% Si--Co 1195
20% Si--Fe 1200 30% Co--Fe 1470 20% Mn--Fe 1450 30% Mo--Fe 1450 20%
Nb--Fe 1360 30% Ni--Fe 1450 15% Zr--Fe 1330
[0096] Table 1 provides a list of exemplary binary, ternary, and
quaternary titanium alloys that have melting points below the
melting point of pure titanium (1668.degree. C.).
[0097] As seen in Table 1, addition of 10 to 40% of a single metal
to titanium to form binary alloys results in substantial lowering
of the melting point below 1668.degree. C. For example, addition of
40% copper to balance titanium decreased the melting point of this
alloy to 990.degree. C., addition of 10% of silica lowered the
melting point of this binary alloy to 1330.degree. C. Ternary
alloys where the titanium is present in an amount higher than 60%
decrease the melting points to about one-third of the melting point
of the pure titanium. For example, addition of 35% of a
manganese-copper mixture results in decrease of the melting point
to 552.degree. C. and a mixture of 35% of manganese-aluminum
decreases the melting point to 450.degree. C. Quaternary alloys,
similarly, have lower melting points than pure titanium. All these
alloys and their combinations as well as alloys formed with other
metals are suitable for the infiltration of the ceramic matrix
prepared by the method of this invention and all these combinations
are intended to be within the scope of the invention.
[0098] When another metal or alloy thereof is selected to be used
for fabrication of the cermets of this invention, the process
illustrated in Table 1 is repeated, that is the melting points of
the single metal and its binary, ternary and quaternary alloys are
determined and matched with the selected ceramic powder so that the
sintering temperature is higher then the melting point of the metal
to be used.
[0099] The alloy of the invention which would be used for
fabrication of the bone replacement implant preferably contains
metal or metals which are approved for use in humans.
[0100] As discussed above, the wetting of the ceramic with metal is
desirable to obtain implant material for bone replacement.
Therefore, before the cermet of the invention is fabricated, the
wetting angle of the metal or alloy is also investigated and the
temperature at which there is highest wetting (i.e. lowest wetting
angle) is achieved. FIG. 2 shows improved wetting shown as
decreasing wetting angle of two alloys, namely the alloy comprised
of 30% nickel-titanium compared to 20% nickel-titanium alloy. As
seen in FIG. 2, at increasing infiltration temperatures from
800.degree. C. to 1600.degree. C., the wetting angle of the alloy
comprising increased amounts of nickel (30%Ni--Ti) was lower and
had improved wetting compared to the alloy comprising only 20% of
nickel (20% Ni--Ti). For the 20% Ni--Ti alloy, the wetting angle of
180 degrees was observed until the temperature reached 1200.degree.
C. Only after that temperature was reached, the wetting angle of
this alloy begun to decrease. At a temperature of about
1600.degree. C., the wetting angle of this alloy decreased to about
40 degrees. The 30% Ni--Ti alloy, on the other hand, has shown
increased wettability, i.e., decreasing wetting angle constantly
from 160 degrees at temperature 800.degree. C. to about 20 degrees
at temperature 1400.degree. C. and zero wetting angle at
temperature 1600.degree. C., that is well under melting point of
pure titanium. Thus, the wettability of the 30% Ni--Ti was
substantially improved against wettability of 20% Ni--Ti alloy and
can be used to infiltrate fine (10 micron) Al.sub.2O.sub.3
preforms.
[0101] 3. A Process for Cermet Fabrication
[0102] The specific conditions utilized in the process of the
invention for fabrication of the cermets of the invention are
described above. The process thus comprises a selection of one or
several blended ceramic powders of a specific particle sizes for
fabrication of a presintered porous ceramic matrix. The metal or
metal alloy to be used for a molten metal infiltration into a
ceramic preform is than selected by matching the sintering
temperature of the ceramic preform with the melting point and
wetting characteristics of the metal or alloy.
[0103] In the case of aluminum oxide and titanium used for
exemplarization of the process of the invention, a preformed matrix
of 10 u diameter aluminum oxide particles that sinter above
1500.degree. C. in one-half hour, as seen in FIG. 1, is infiltrated
with a titanium alloy comprising 30% of weight % of nickel at
1400.degree. C. in vacuum (106 torr). This infiltration is improved
further by the addition of 0.5% sulfur to the aluminum oxide
particles and by the addition of 10% silica to the 30% of Ni--Ti
alloy (Table 1).
[0104] The cermets of the invention have a wide compositional range
in that they can consist from about 5% to about 60% titanium or
other metal. The range and amount of the metal effects its
mechanical and physical properties. The moderate fracture toughness
at low titanium contents (5%-30%) gives high wear resistance and
low electrical conductivity for applications such as seal rings,
brake shoes, corrosion resistant valve seats, etc. While density is
increased with increasing titanium content, at these levels it is
still attractive for armor applications due to low cost and
multi-hit capability. When the metal content is increased above
25%, the properties shift to higher fracture toughness and possible
electrical conductivity for such applications as bone replacement,
for manufacturing MRI compatible surgical instruments and
electrical contacts with moderate wear resistance. These and any
other modifications of the process of the invention are intended to
be within the scope of the invention.
[0105] The preferred system of the invention for preparation of
cermets of the invention is the metal oxide system. The most
preferred of the oxide systems are the aluminum oxide infiltrated
with titanium, magnesium, lithium or their respective alloy.
[0106] The cermet of the invention is exemplarized and represented
by the titanium-aluminum oxide system but other systems such as any
combination of systems listed above is equally suitable for
preparation of the cermets using the molten-metal-infiltration as
long as the main premise of the invention, namely a use of a lower
molten metal infiltration temperature than the sintering
temperature of the ceramic powder, is present.
[0107] The procedure for fabrication of the new cermets described
herein is suitable for screening and developing other cermet
systems with promising properties specific to other applications
not specifically described herein. All these variations and
modifications are intended to be within the scope of this
invention.
[0108] The composition of these cermets can be advantageously
adjusted during fabrication by, for example, changing the
proportions of ceramic/metal, changing the composition of the
ceramic from a single material to a mixture of two or more
materials, and changing the prefabrication conditions such as
temperature, pressure and atmosphere.
[0109] In this invention, the obstacles previously encountered have
been overcome by reducing the sinterability of the metal oxide
and/or other systems, improving the wettability, and reducing the
melting point of the metal used to infiltrate.
[0110] 4. Cermets Suitable for Bone Replacement Implants
[0111] An additional advantage of this invention is that the
cermets of the invention are conveniently fabricated from the
components fully compatible with the human or mammal body and,
having improved properties as described above, are therefore
particularly suitable for fabrication of bone replacement implants.
Currently, both alumina and titanium alloys are acceptable and FDA
approved implant materials. These materials alone or in
combinations, fabricated by methods known in the art do not possess
the desired properties required for bond or joints replacement.
However, because of the new processing, the cermets of the
invention made of these two materials have similar properties as
bones or joints to be replaced and have, in addition to other
already listed properties, also an elasticity modulus and surface
finish which prevents wear in the joint area and allows an
attachment of the ligaments or other tissues.
[0112] The cermets according to the invention are composed of a
high packing density porous ceramic matrix with evenly distributed
ceramic particles presintered together and uniformly infiltrated
with the molten metal. The density and porosity of the ceramic
matrix is designed based on the intended use of the cermet. In case
of bone implant, both the density and porosity of the cermet
implant is similar to the porosity and density of the bone to be
replaced.
[0113] Typically, for a custom-fit implant which needs to be shaped
and finished during surgery, a porous prefabricate of the ceramic
powder, preferably aluminum oxide, is prepared in near-net shape of
the bone or joint before the operation. The method of the invention
allows surgeons, within a typical implant operating time, to fit a
specific bone replacement application, i.e. to fit the recipient's
bones or joints, before the infiltration of the matrix with molten
metal. Final shaping using machining, grinding, polishing and
surface finishing is then done under the surgeon's instruction. The
ceramic prefabricate can be modified to allow attachment of the
ligament or tissue, to prevent such attachment, or to allow or
disallow the joint lubrication. The surface of the prefabricate can
also be modified in various regions to improve its wear resistance
as a moving joint surface, its lubrication ability in a joint
surface and surface condition to improve tissue attachment. Only
after the implant has the shape that fits the patient, the molten
metal, preferably titanium alloy, is infiltrated into the porous
aluminum oxide ceramic pre-fabricate.
[0114] After the metal alloy infiltration, the surface of the
cermet bone implant can additionally and optionally be modified to
improve tissue attachment by, for example, removing the metal alloy
at the surface and expose only porous aluminum oxide in regions
where tissue attachment or bone growth is desired. Similarly, the
implant surface can be modified for improved hardness by exposing
that surface to a number of processes that harden the metal alloy
like nitrating, ion-implantation, coating, etc. Areas that require
self-lubrication for joint movement can be modified to have fine
pores at the surface to allow body lubricates to flow into the
joint region by removing the surface metal alloy after
infiltration. When alternate lubricants are developed and/or used,
these artificial lubricants are advantageously injected into these
porous networks to lubricate moving parts.
[0115] The current invention provides a method utilizing these
and/or other physiologically acceptable ceramics and metals to
provide a cermet material that has the properties similar to bone
than any other material known and available today.
[0116] Another equally important requirement for the acceptable
bone or joint replacement materials is that they need to be
magnetic resonance (MR) compatible. The magnetic resonance
compatibility requires that the implants, instruments and tools
used during the operation must be non-magnetic so that the
implants, tools and instruments being used near or in the magnetic
field will not be attracted to the magnets. These implants,
instruments and tools must not effect the image of the magnetic
resonance image (MRI) so as to give false views.
[0117] The cermet materials of the invention are not magnetic and,
therefore, meet the requirement for magnetic resonance
compatibility. The degree of MRI compatibility depends on the
amount and effect any metal component in the cermet will have on
the image. Fine dispersing of non-magnetic metals like Ti in an
Al.sub.2O.sub.3 matrix are MRI compatible at 25% Ti. Cermets are
tested up to 5 Tesla in MRI units to determine their
compatibility.
Utility
[0118] The cermets of the current invention are new materials
having new and improved properties. These materials are suitable
for manufacturing products having specific requirements as to their
durability, toughness, fragility, brittleness, etc. They are
particularly suitable for implants as bone and joint
replacements.
[0119] The invention further provides options to customize the
cermet to fit its intended use.
[0120] In addition to the cermets used as bone and joint
replacements, the new cermets are suitable for fabrication of
armors for military applications due to increased hardness,
strength, and toughness. They have many advantages over cermets
previously known and over ceramics or metals alone. They have also
shown great potential as wear parts, cutting tools, surgical
instruments and engine components in civil or military
applications.
EXAMPLE 1
Preparation of Presintered Aluminum Oxide Matrix
[0121] This example describes the first step of the current method
for preparation and testing of presintered aluminum oxide porous
matrix. This procedure is suitable for determination of the optimal
sintering temperature of the ceramic powder and for selection of
its optimal particle size.
[0122] Five grams of aluminum oxide powder having particles sizes
0.2 .mu., 1 .mu., 5 .mu., 10 .mu., 20 .mu., 50 .mu., and 60 .mu.
each were packed into preform shapes and cold pressed at 15,000 psi
for 1 minute at room temperature to form a packed compact of the
ceramic powder. Each compact was submitted to an increasing
temperature inside a tungsten element furnace from 1200.degree. C.
up to 1700.degree. C., under vacuum. The temperature was
incrementally increased each 30 minutes by 200.degree. C. under
10.sup.-6 torr pressure. The density of each compact was measured
and its sintering temperature was noted from the onset of
shrinkage. The preferred compacts had about 40% porosity and 60%
density. The degree of sintering was determined for each particle
size.
[0123] At higher particle sizes, the temperatures needed for
sintering were much higher than temperatures needed for sintering
of powders having smaller particle sizes. The results are seen in
FIG. 1.
[0124] After determination of sintering temperature for each size
was performed, each compact of different particle size was
infiltrated with a molten titanium alloy in order to find out the
best and most practical particle size of aluminum oxide powder to
achieve even distribution of titanium. Compacts made of 0, 2 .mu.,
1 and 5 .mu. were found to have too high a density to permit a
thorough infiltration with the molten metal. Compacts made of
particle sizes higher than 60 .mu. required high sintering
temperatures but were relatively large grain size.
[0125] Molten metal infiltration of compacts made of 10-50 .mu.
particle sizes were found to be suitable for molten infiltration,
having a sintering temperature between 1500.degree. C. for 10 .mu.
particles, 1600.degree. C. temperature for 20 .mu. particles and
1660.degree. C. for 50 .mu. particles. The most preferred was 10
.mu. particles compact having the right porosity and density
allowing even and continuous molten-metal-infiltration with a
titanium alloy (Ti--Ni 30%).
[0126] Other ceramic powders, namely aluminum nitride, aluminum
boride, aluminum silicate, zirconium oxide, zirconium nitride,
zirconium boride, zirconium silicate, hafnium oxide, hafnium
nitride, hafnium boride, hafnium silicate, beryllium oxide,
beryllium nitride, beryllium boride, beryllium silicate, vanadium
oxide, vanadium nitride, vanadium boride, vanadium silicate, and
other such compounds are treated according to the procedure
described above, their individual sintering temperatures are
determined for compacts of powder having different particle
sizes.
EXAMPLE 2
Determination of Wettability of Titanium
[0127] This example illustrates a procedure used to determine
wettability and wetting angle of titanium infiltration into
aluminum oxide ceramic compacts in order to select conditions for
molten-metal-infiltration. This procedure suitable for
determination of the optimal wettability and wetting angle of the
infiltrating metal or metal alloy.
[0128] The ceramic compact obtained in Example 1 was covered with a
titanium-nickel alloy containing 20% or 30% of nickel. The ceramic
metal composite was heated from 800.degree. C. to 1600.degree. C.
in a vacuum under 10.sup.-6 torr. The temperature was selected on
the basis of the sintering temperature of the ceramic powder
composite. As the sintering temperature of aluminum oxide 10 .mu.
particle size compact was found to be around 1500.degree. C. and
the sintering temperature of aluminum oxide 50 .mu. particle size
compact was around 1660.degree. C., the temperature of the molten
titanium containing alloys was investigated so that only the alloys
having the melting point below the sintering temperature were
used.
[0129] The angle where ceramic/metal bonding was maximized was
achieved with 30% titanium nickel alloy at temperature 1600.degree.
C. but the wetting angle near to zero was achieved at 1500.degree.
C. temperature. This alloy therefore was suitable for
molten-metal-infiltration of aluminum oxide compact made of 10 .mu.
particles and also for compact made of larger than 10 .mu. particle
size.
[0130] The 20% titanium nickel alloy has about a 40 degree wetting
angle at 1600.degree. C. and it was found to be acceptable for
metal infiltration of compacts made of smaller or equal to 50 .mu.
particles. At 1500.degree. C. this alloy had about a 70 degree
wetting angle and it would not be very suitable for metal
infiltration of compacts made of about 10 .mu. particles.
[0131] The results discussed herein are seen in FIG. 2.
[0132] In this manner, wetting properties of other metals such as
magnesium, nickel, aluminum, calcium, lithium or their alloys in
any combination, are investigated in the same manner and a
determination is made of the most suitable paring of the ceramic
powder compact and the metal or metal alloy used for
molten-metal-infiltration.
EXAMPLE 3
Process for Fabrication of Cermets
[0133] This example illustrates a process for fabrication of
cermets of the invention having properties suitable to be used as
bone implants.
[0134] The presintered aluminum oxide compact of 10 .mu. particle
sizes of Example 1 is cast into the bone near shape form and
pressed together. The compact is than presintered at 1500.degree.
C. in the furnace heated in incremental increased temperatures for
30 minutes. The presintered porous ceramic matrix is brought in
contact with the melted titanium-nickel 30% alloy at a temperature
of around 1400 or 1450.degree. C., which temperature is lower than
the sintering temperature of the aluminum oxide. The contact is
made in the vacuum furnace at 10.sup.-6 torr pressure for about 10
minutes. Titanium-nickel 30% alloy infiltrated uniformly into the
ceramic matrix and good ceramic/metal interfacial bonding was
achieved.
[0135] The bone implant was then submitted to testing for strength,
fragility, brittleness and its metal infiltration was
microscopically checked. There were no void or metal accumulation
and the metal infiltration was even nd uniform throughout the whole
cermet structure.
* * * * *