Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CE RAMI C COMPOS I TI ON
Background of the Invention
Sintered ceramic compositions composed
primarily of non-metallic refractory materials have
proven advantageous where high temperature resistance,
breakage strength, good wear and allied properties are
required. Typical instances of use include construction
materials and machining tools.
Many such compositions are predicated upon a
predominant base material which is aluminum oxide (A1203).
Aluminum oxide is both abundant and imbued with many of
the properties desired for these uses. Unfortunately, it
also possesses various de~iciencies, such as brittleness.
As a result, it has frequently been alloyed with other
components in an effort to improve over-all ceramic
properties.
Among the other sinterable and mechan:ically
resistant materials known to be suitable for this same
purpose are many metal carbides, nitrides, borides and
other oxides. Some of these materials have been used
with aluminum oxide to produce composite compositions
having hybrid properties.
A standard of the cutting tool industry, for
example, involves aluminum oxide-titanium carbide com-
positions. These ceramics offer many more desirable
physical properties. Unfortunately, they can generally
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be produced only by hot-pressing procedures. This draw-
back has contributed substantially to the seareh for other,
less difficult to produce, compositions.
Aluminum oxide-based compositions containing
zirconium oxide are also known. In German
OEfenlegungsschrift No. 2,549,652, it is indicated that
aluminum and zirconium oxiae compositions, in which the
zirconium oxide was transformed from tetragonal to the
less dense monoclinic form to create microfissures in
the ultimate ceramic article, are desirable. The micro-
fissures are said to increase fracture resistance by
allowing absorption of physical stress.
In U.S. Patent No. 4,218,253 issued August
19, 1980, another such composition is described. ~here,
particles of tetragonal zirconium oxide are incorporated
within a base material containing aluminum oxide.
Instead of possessing microfissures to enhance fracture
resistance, however, the resultant ceramic composition
is said to improve binding strength by an ability to
undergo stress-induced plastic deformation involving
conversion of the zirconium oxide to monoclinic form
or phase.
Ceramic compositions containing aluminum
oxide, titanium nitride and tungsten carbide are also
known. In U.S. Patent No. 4,204,873 issued May 27, 1980
such a composition is disclosed, but said composition
can be produced only by hot-pressing. U.S. Patent No.
3,652,304 issued March 28, 1972 discloses other
nitride-oxide refractories which are produced by ho-t-
pressing.
Notwi-thstanding the properties of the foregoing
compositions, further improvement remains desirable. In
many instances, the improvement in the properties has been
accompanied by impairment of others. Enhancement of
properties without such a drawback therefore remains a
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highly desirable object.
Improvement in production techniques has also
been desired. Most such compositions can be manufactured
only by such complicated techniques as hot-pressing. This
drawback greatly increases their price.
Introduction to the Invention
It has been discovered that ceramic compositions
comprising an essentially homogeneous admixture of an
aluminum oxide base material with certain other re~rac-
tories including zirconium oxide, titanium oxide, hafniumoxide, titanium nitride, zirconium nitride, and tungsten
or molybdenum carbide provide substantial improvements
in product properties and/or facilitate their production.
More particularly, it has been discovered that
the synergistic addition of tunsten carbide and zirconium
oxide -- both with or without titanium nitride -- to
aluminum oxide provides heightened hardness and strength.
These properties result in a substantial increase in
wear resistance for the ultimate ceramic articles.
Further, many of these novel articles are
capable of simplified production. Instead of hot-pressing
the initial powdered component admixture, pressureless
sintering is possible. Thus, conventional cold-press
and subsequent sintering proced~tres~ which reduce cost
and necessary equipment, may be employed.
Description of the Invention
The ceramic compositions of the present
invention are generally composed predominantly of
aluminum oxide. They desirably contain as much as 50
to 90% aluminum oxide by total weight.
To facilitate production of the ultimate,
essentially homogeneous admixture of components in the
sintered compositions of this invention, the aluminum
oxide-base material is usually provided in an initial,
finely powdered form. Particles having a mean size of
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from about 0.01 to 1.0~ are particularly desirable for
this purpose. Commercial purity, 99.5%, aluminum oxide
which contains .05% to .1% magnesium oxide as a grain
growth inhibitor is also sufficient for this purpose.
The present compositions must also contain a
tungsten or molybdenum carbide modifier. For admixture
with the aluminum oxide ~prior to sintering), the
modifier(s) employed should also be finely powdered.
Thus, a mean particle size of from 0.01 to 1.0~ is
preferred. The carbide modifier can be present in
amounts up to 5% of the total composition weight.
The present compositions must also contain
tetragonal zirconium or hafnium oxide. Again, a mean
particle size of 0.01 to 1.0~l is preferred. The form
or phase of the zirconium oxide and/or hafnium oxide
present in the compositions is quite important. Mono-
clinic zirconium oxide and hafnium oxide (as opposed to
tetragonal) are the stable forms of these oxides at
ambient temperatures. The monoclinic structure also
possesses the greater volume of the two crystalline forms.
In provision of the initial zirconium oxide or
hafnium oxide powder and during subsequent treatment
(especially including sintering) care should be taken
that the oxide be in the desired crystalline form. The
ultimate product composition is preferably essentially
free of fissures, including the microfissures described
in the previously discussed German Offenlegungsschrift
No. 2,549,652. Therefore, the monoclinic form or phase
transformation described in that publication should be
avoided.
Some monoclinic zirconium or hafnium oxide may,
of course, be tolerated in the present composition. Trans-
formation of monoclinic crystals on cooling should, how-
ever, be controlled through conventional techniques to
ensure the foregoing objective. There are no such
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provisions for the titanium oxide powder as both mono-
clinic and tetragonal ~orms are stable at room
temperature.
Although the titanium oxide, hafnium oxide,
and/or zirconium oxide may be present in widely varient
proportions, normally they should not together amount
to more than 30%, most usually 10 - 20%, by total
composition weight. The remainder constitutes: aluminum
oxide up to 90%, most usually 50 to 75%; and zirconium
or titanium nitride up to 30%, most usually 1 to 15%,
and tungsten or molybdenum carbide up to 5%r most usually
2 to 3%, by total composition weight. These nitride and
carbide additions further increase wear resistance and
do so without adverse effect on sinterability or resultant
rupture strength.
In one preferred embodiment of the invention,
from 1 to 15% titanium nitride and from 10 to 15~
zirconium oxide with 2 to 3% tungsten carbide are
employed, with a remainder of from about 67 to 87~ aluminum
oxide by total weight. The initial powdered admixture
of these components may be subjected to cold-pressing and
then sintering procedures to produce the ceramic
compositions.
In another preferred embodiment, from 25 to 30%
by total weight of titanium nitride and 10 to 15%
zirconium oxide, with a remainder of from about 52 to
63% aluminum oxide by total weight, are employed. Further,
up to 5%, but preferably from 2 to 3%, tungsten carbide
is added. The high proportion of titanium nitride,
however, may make difficult the use of cold~pressing and
sintering procedures. Consequently, substituting
titanium oxide for part of the titanium nitride has the
advantage of facilitating the pressureless sintering
without adversely affecting the strength and hardness
of the final product. The titanium oxide can be
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substituted in amounts up to 50% of the amount of
titanium nitride by weight.
As set forth above, all these compositions can
be produced by hot-pressing oE an admixture of their
initial, powde~ed components. This conventional pro-
cedure normally involves subjecting the powdered admixture
to the combined conditions of a pressure of from 100 to
600 kg/cm2 and temperature of from 1500C to 2000C for
at least about 2 minutes. Because of the limitations
of this procedure, however, cold-pressing and sintering
is preferred. For this procedure, the powdered
admixture is first compressed under a pressure of from
1000 to 3000 kg/cm2 at ambient or mild temperature.
Thereafter, the compact may be sintered (without
pressure) at a temperature of at least 1~00C, preferably
at from 1550C to 1800C, for from 30 minutes to ~
hours. During the sintering operation, the tungsten
or molybdenum carbide reacts with the oxide components
of the composition, increasing the sinterability of said
composition and transforming the tungsten/molybdenum
carbide to tungsten/molybdenum metal.
The following examples illustrate the present
invention:
EXAMPLE 1
Eight-six parts by weight of aluminum oxide
were mixed with 12 parts of zirconium oxide and 2 parts
of tungsten carbide. The mixture was wet-milled for a
period of 12 hours, paraffin wax was added as a pressing
aid, and pieces were pressed at room temperature using
30 1400 kg/cm2 pressure. The pieces were presintered at
500C for 30 minutes and sintered at 1600C for 1 hour
in vacuum. The sintered pieces had a density greater
than 99% of the theoretical and X-ray diffraction analysis
indicated the presence of A1203, ZrO2, and W. The ZrO2
was present predominantly in the tetragonal form, with
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less than 10% of the ZrO2 present in the monoclinic form.
Test bars were ground using diamond wheels, and
the modulus of rupture was determined in three-point
bending The average rupture strength was about 100,000
to 115,000 psi, and the hardness was measured to be 93.5
on the Rockwell "A" scale.
EXAMPLE 2
Severty-four parts by weight of aluminum
oxide were mixed with 12 parts zirconium oxide, 12 parts
titanium nitride and 2 parts tungsten carbide, and test
bars were prepared according to the method of Example 1.
The sintered pieces had a density greater than 99% of
theoretical, and X-ray diffraction analysis indicated
the presence of A1203, ZrO2, TiN, and W. The ZrO2
was present predominantly in the tetragonal form. The
average modulus of rupture was determined to be 100,000
to 115,000 psi, and the hardness was measured to be 9~.0
on the Rockwell "A" scale.
EXAMPLE 3
Fifty-seven parts by weight of aluminum oxide
were mixed with 12 parts zirconium oxide, 25 parts of
titanium nitride, 4 parts of titanium oxide, and 2 parts
of tungsten carbide. Test bars were prepared according
toExample 1 except that the sintering temperature was
raised to 1650C. The sintered pieces had a density
greater than 99% of theoretical, and X-ray diffraction
analysis indicated the presence of A1203, ZrO2, TiN and W.
The ZrO2 was present predominantly in the tetragonal form.
The average modulus of rupture was determined to be
30 100,000 to 110,000 psi, and the hardness was measured to be
93.5 on the Rockwell "A" scale.
EXAMPLE ~
Fifty-seven parts by weight of aluminum oxide
were mixed with 12 parts zirconium oxide, 29 parts of
35 titanium nitride and 2 parts of tungsten carbide. The
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mixture was wet-milled for a period of 12 hours and dried.
The powder was charged into a graphite die and hot-pressed
at a pressure of 200 kg/cm at a temperature of 1700C
for 30 minutes. Test bars were sliced out of the hot-
pressed piece using diamond wheels and ground using diamondwheels. The average modulus of rupture was determined
to be 125,000 psi, and the hardness was measured to
be 94.0 on the Rockwell "A" scale.
It should be understood that the foregoing
examples relate only to presently preferred embodiments
and that it is intended to cover all changes and
modifications of the examples of the invention chosen
herein for the purpose of the disclosure which do not
depart from the spirit and scope of the invention as
set forth in the appended claims. As an example, in
the compositions set forth in the Examples, hafnium
oxide may be substituted for zirconium oxide. In
addition, molybdenum carbide may be substituted for
tungsten carbide.
It is to be understood that changes may be
made in the particular described embodiments hereof
without departing from the scope of this invention.
