U.S. patent number RE32,449 [Application Number 06/821,650] was granted by the patent office on 1987-06-30 for ceramic body of zirconium dioxide (zro.sub.2) and method for its preparation.
This patent grant is currently assigned to Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V. Invention is credited to Nils Claussen, Gunter Petzow, Manfred Ruhle.
United States Patent |
RE32,449 |
Claussen , et al. |
June 30, 1987 |
Ceramic body of zirconium dioxide (ZrO.sub.2) and method for its
preparation
Abstract
A ceramic body of zirconium dioxide containing, if desired,
aluminum oxide, nd partially stabilized with yttrium oxide and/or
one or more rare earth oxides (e.g., cerium dioxide) and/or
magnesium oxide and/or calcium oxide is partially stabilized with
0.5 to 5 mole-% of yttrium oxide and/or 5 to 12 mole-% of magnesium
oxide and/or calcium oxide and/or cerium dioxide or one or more
rare earth oxides, is 30 to 100% in the tetragonal lattice
modification and has in the surface region a content of yttrium
oxide, cerium dioxide, magnesium oxide, calcium oxide or rare earth
oxide that is 1 to 20 mole-% higher than the average content, such
that the body is coated with a thin, PSZ-like layer in a more
highly stabilized tetragonal or with a layer that is predominantly
in the cubic lattice form. For preparation, the surface of an
already sintered or only presolidified compact of partially
stabilized zirconium oxide is brought into intimate contact with
yttrium oxide, cerium dioxide, magnesium oxide, calcium oxide
and/or another rare earth powder or a zirconium dioxide powder
containing at least 12 mole-% of yttrium oxide and/or other
stabilizer oxides, and then annealed at 1000.degree. to
1600.degree. C. until a more highly stabilized tetragonal or
predominantly cubic surface layer of 0.1 to 200 micrometers
thickness and 2 to 20 mole-% higher content of yttrium oxide,
cerium dioxide, magnesium oxide, calcium oxide or rare earth oxide
has formed.
Inventors: |
Claussen; Nils (Leonberg,
DE), Ruhle; Manfred (Ditzingen, DE),
Petzow; Gunter (Leinfelden-Echterdingen, DE) |
Assignee: |
Max-Planck-Gesellschaft zur
Forderung der Wissenschaften e.V (Gottingen,
DE)
|
Family
ID: |
25811544 |
Appl.
No.: |
06/821,650 |
Filed: |
January 23, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
619939 |
Jun 12, 1984 |
04525464 |
Jun 25, 1985 |
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Foreign Application Priority Data
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Jun 16, 1983 [DE] |
|
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3321857 |
Dec 16, 1983 [DE] |
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3345659 |
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Current U.S.
Class: |
501/103; 264/662;
427/201; 428/701; 501/104; 501/105; 501/152 |
Current CPC
Class: |
A61L
27/047 (20130101); C04B 35/486 (20130101); C04B
35/4885 (20130101); C04B 41/009 (20130101); C04B
41/5042 (20130101); C04B 41/87 (20130101); C04B
41/5042 (20130101); C04B 41/4545 (20130101); C04B
41/4556 (20130101); C04B 41/009 (20130101); C04B
35/48 (20130101); C04B 2111/0025 (20130101); C04B
2111/00836 (20130101) |
Current International
Class: |
C04B
35/486 (20060101); C04B 41/50 (20060101); C04B
35/488 (20060101); C04B 41/45 (20060101); C04B
41/87 (20060101); C04B 035/48 (); C04B
035/50 () |
Field of
Search: |
;264/60
;501/103,104,105,152 |
References Cited
[Referenced By]
U.S. Patent Documents
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4067745 |
January 1978 |
Garvie et al. |
4298385 |
November 1981 |
Claussen et al. |
4322249 |
March 1982 |
Claussen et al. |
4354912 |
October 1982 |
Friese |
4419311 |
December 1983 |
Claussen et al. |
4421861 |
December 1983 |
Claussen et al. |
|
Foreign Patent Documents
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0067327 |
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Dec 1982 |
|
EP |
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2904069 |
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Jul 1980 |
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DE |
|
WO83/04247 |
|
Dec 1983 |
|
WO |
|
Primary Examiner: McCarthy; Helen M.
Attorney, Agent or Firm: Felfe & Lynch
Claims
We claim:
1. In a ceramic body of zirconium dioxide or zirconium dioxide
containing aluminum oxide, the improvement comprising said ceramic
body
being partially stabilized with 0.5 to 5 mole-% of yttrium oxide, 5
to 15 mole-% magnesium oxide, calcium oxide, 5 to 15 mole-% cerium
oxide, 5 to 15 mole-% of one or more rare earth dioxides or 5 to 15
mole-% of a combination thereof;
being 30 to 100% in the tetragonal lattice modification; and
having, in the surface region, a content of yttrium oxide, cerium
dioxide, magnesium oxide, calcium oxide or rare earth oxide that is
2 to 20 mole-% higher than the average content in said ceramic
body, such that the body is covered with a thin layer of a more
highly stabilized tetragonal lattice form or one predominantly in
the cubic lattice form.
2. The ceramic body of claim 1
partially stabilized with 0.5 to 5 mole-% yttrium oxide and having
a content of 2 to 20 mole % higher than the average, of yttrium
oxide in the surface region thereof.
3. A method for the preparation of a ceramic body which is
partially stabilized with 0.5 to 5 mole-% of yttrium oxide, 5 to 15
mole-% of magnesium oxide, calcium oxide and/or cerium oxide or one
or more rare earth dioxides; is 30 to 100% in the tetragonal
lattice modification; and has in the surface region a content of
yttrium oxide, cerium dioxide, magnesium oxide, calcium oxide or
rare earth oxide that is 2 to 20 mole-% higher than the average
content, such that the body is covered with a thin layer of a more
highly stabilized tetragonal lattice form of one predominantly in
the cubic lattice form, comprising the steps of
placing the surface of an already sintered or only presolidified
compact of partially stabilized zirconium dioxide in intimate
contact with yttrium oxide, cerium dioxide, magnesium oxide,
calcium oxide and/or other rare earth powder, or a zirconium
dioxide powder containing at least 12 mole-% yttrium oxide and/or
other stabilizer oxides, and then
annealing said sintered or presolidified compact at 1000.degree. to
1600.degree. C., to form a more highly stabilized tetragonal or
mainly cubic surface layer of 0.1 to 200 micrometers thickness
having a 2 to 20 mole-% higher content of yttrium oxide, cerium
dioxide, magnesium oxide, calcium oxide or rare earth oxide than
the average content in the ceramic body.
4. The method of claim 3, wherein the surface contact with yttrium
oxide, cerium dioxide, magnesium oxide, calcium oxide or rare earth
oxide comprises placing the ceramic body in a powder bed, spraying
or pressing a powder layer onto the ceramic body or treating the
ceramic body with a suspension of the powder.
5. The method of claim 3 wherein the ceramic body is a body
sintered at 1350.degree. to 1550.degree. C. and/or
hot-isostatically pressed and is formed of partially stabilized
zirconium dioxide.
6. The method of claim 3 wherein the ceramic body is an unsintered
compact of partially stabilized zirconium dioxide, and is sintered
at 1350.degree. to 1550.degree. C.
7. The method of claim 3 wherein the ceramic body is a zirconium
dioxide body, partially stabilized with 7 to 11 mole-% of magnesium
oxide, sintered at temperatures between 1690.degree. and
1800.degree. C., and thereafter annealed at temperatures between
1350.degree. and 1550.degree. C. in contact with yttrium oxide
and/or cerium oxide and/or other rare earth oxide powders for 1 to
5 hours.
8. The method of claim 3 wherein the ceramic body is a sintered
compact or green body containing 2 to 4 mole-% of yttrium oxide or
rare earth oxide as stabilizer.
9. The method of claim 4 wherein the ceramic body is sintered at
1350.degree. to 1550.degree. C. and/or hot-isostatically pressed
and is formed of partially stabilized zirconium dioxide.
10. The method of claim 4 wherein the ceramic body is unsintered
compact of partially stabilized zirconium dioxide, sintered at
1350.degree. to 1550.degree. C.
11. The method of claim 4 wherein the ceramic body is a zirconium
dioxide body, partially stabilized with 7 to 11 mole-% of magnesium
oxide, sintered at temperatures between 1690.degree. and
1800.degree. C., and thereafter annealed at temperatures between
1350.degree. and 1550.degree. C. in contact with yttrium oxide
and/or cerium oxide and/or other rare earth oxide powders for 1 to
5 hours.
12. The method of claim 11 wherein yttrium oxide, cerium oxide or a
combination thereof, is used.
13. The method of claim 4 wherein the ceramic body is a sintered
compact or green body containing 2 to 4 mole-% of yttrium oxide or
rare earth oxide as stabilizer.
Description
Finely granular zirconium dioxide bodies partially stabilized with
yttrium oxide, cerium dioxide and/or other rare earth oxides, and
coarsely granular zirconium dioxide bodies partially stabilized
with magnesium oxide or calcium oxide pertain to the
polycrystalline ceramics which have the highest strengths and
resistance to fracture which have been measured up to now. The
chief reason for this lies in the tension-induced transformation of
the tetragonal lattice modification to the monoclinic
room-temperature modification. For example, bodies containing
yttrium oxide are sintered, hot-pressed or hot-isostatically
pressed ("hipped") usually with an yttrium oxide content between 1
and 6 mole-%, either in the tetragonal monophasic field or in the
cubic/tetragonal two-phase region, at temperatures between
1400.degree. and 1550.degree. C. Their structure then consists of a
fine-grained (0.1-1.0 micrometer), tetragonal content (up to 100%)
and somewhat coarser, cubic grains (1-10 micrometers) (3.5-6.0
mole-% for high yttrium oxide contents). To increase the hardness
and modulus of elasticity the bodies can contain aluminum oxide in
larger amounts.
Zirconium oxide bodies containing magnesium oxide or calcium oxide
are usually sintered in the cubic monophasic region at temperatures
between 1690.degree. and 1800.degree. C.; they are therefore more
coarse-grained (50 to 70 micrometers).
The decisive disadvantage of these super-strong ceramic bodies,
especially those containing yttrium oxide, is that they lose their
strength drastically after relatively long heat treatment at
temperatures between 200.degree. and 550.degree. in air; this loss
of strength is greatly accelerated with increasing atmospheric
humidity or high steam pressure (O. T. Masaki, K. Kobayashi, Proc.
Ann. Meeting Jap. Ceram. Soc. 1981). Even in warm, aqueous
solutions, degradation of the bodies can occur. The reason for this
is not yet understood. It is assumed, however, that the mechanical
tensions of the tetragonal bodies are removed by tension crack
corrosion at the grain boundaries, and thus a transformation to the
monoclinic form occurs, or that other diffusion-controlled
mechanisms cause martensite nuclei to form at the surface and thus
initiate the transformation that ultimately results in the
destruction of the bodies.
This decisive disadvantage makes the new class of the so-called TZP
ceramics (TZP: Tetragonal Zirconia Polycrystals. A bibliography on
TZP ceramics is contained in the book, "Science and Technology of
Zirconia II", Advances in Ceramics, Vol. 11, 1984) suitable for use
in air only for application temperatures up to about 200.degree.
C., although such ceramics would offer substantial advantages for
use in internal combustion engines. This phenomenon would also be
disadvantageous for use as a bioceramic (hip joint
replacement).
Conventional zirconium dioxide ceramics partially stabilized with
magnesium oxide (Mg-PSZ), when exposed to heat for long periods at
somewhat higher temperatures (700.degree. to 1000.degree. C., have
a similar disadvantage. On account of the rapid diffusion or
vaporization rate of magnesium oxide, surface degradation occurs,
especially in a slightly reducing atmosphere.
Surprisingly, it has now been found--and the invention is based on
it--that in sintered specimens which have been heat treated in a
milieu rich in yttrium oxide, cerium oxide, magnesium oxide or
calcium oxide, e.g., a powder bed of yttrium oxide or magnesium
oxide, this degradation process does not occur, or occurs to a
lesser degree.
The invention is therefore based on the problem of reducing or
eliminating the above-described loss of strength or surface
degradation in ceramic bodies of zirconium dioxide.
The problem is solved by a ceramic body partially stabilized with
yttrium oxide and/or cerium oxide and/or one or more rare earth
oxides and/or magnesium oxide and possibly containing aluminum
oxide, which is characterized by being partially stabilized with
0.5 to 5 mole-% of yttrium oxide and/or 2 to 12 mole-% of magnesium
oxide and/or calcium oxide and one or more rare earth oxides (e.g.,
cerium oxide), is 30 to 100% in the tetragonal lattice
modification, and has in the surface region a content of yttrium
oxide or rare earth oxide and/or magnesium oxide and/or calcium
oxide, such that the body is covered by a thin layer that is mostly
in the cubic lattice form or in a more highly stabilized tetragonal
lattice form. It is obvious that a partially stabilized cubic layer
can be transformed by tempering (peak aging) at temperatures
commonly used in PSZ (1100.degree.-1420.degree. C.) to a PSZ-like
layer (i.e., cubic grains having tetragonal segregations).
The term, "thin surface layer," in the sense of the invention, is
to be understood to mean a layer with a thickness of 0.1 to 200
micrometers, preferably 0.3 to 30 micrometers. The body on the
basis of zirconium dioxide in accordance with the invention is
prepared by firing it in a milieu which is rich in yttrium oxide,
cerium oxide, magnesium oxide, calcium oxide and/or rare earth
oxides. The invention is explained hereinbelow on the basis of the
use of yttrium oxide, cerium oxide, magnesium oxide and calcium
oxide. It is to be understood, however, that it applies likewise to
other rare earth oxides. This surface stabilization or annealing is
likewise advantageous for conventional zirconia ceramics partially
stabilized with magnesium oxide or calcium oxide.
To prepare the ceramic body in accordance with the invention, it is
possible to set out either from the finished sintered or hipped
ceramic or from a green body presolidified at relatively low
temperature (e.g., room temperature). The ceramic or the green body
is now provided with a surface of yttrium oxide, cerium oxide,
magnesium oxide, calcium oxide, etc., either in the form of a
pressed-on layer of powder or of a slip containing yttrium oxide or
magnesium oxide, which can be sprayed on, for example, or applied
in the form of a bath for impregnating the surface. The bodies thus
treated are then fired or sintered at temperatures between
1000.degree. and 1600.degree. C., the length of the treatment being
able to be between about 10 minutes and about 100 hours. The
desired surface stabilization is also achieved to special advantage
by firing or sintering the ceramic or green body in a powder bed of
yttrium oxide and/or cerium oxide and/or magnesium oxide and/or
calcium oxide. Those conditions are preferred in which the desired
diffusion is achieved in the shortest possible time, while at the
same time achieving a PSZ-like layer.
For the preparation of the ceramic body itself, the body can be
performed either by mixing the oxides, or by wet chemical methods
such as sol gel, coprecipitation, spray reaction of aqueous
solutions, or from fine, homogeneous powders obtained by fusion and
prealloyed with yttrium oxide, cerium oxide, magnesium oxide and/or
calcium oxide, and then sintering or hipping, or sintered and then
hipped, at temperatures generally between 1350.degree. and
1550.degree. C. The finished ceramic is then, as mentioned above,
coated with yttrium oxide, cerium oxide, magnesium oxide, calcium
oxide, etc., or fired in a corresponding powder bed, until the
surface layer enriched with yttrium, cerium, magnesium, or calcium
oxide etc. is formed.
When a stabilizer-rich coating is applied to a green body, the body
is commonly preformed at a low pressure, say of about 100 MPa, and
then pressed again at higher pressure, e.g., 200 to 650 MPa. In
most cases, however, the preferred method is the sintering of the
pressed body or the firing of a finish-sintered and processed body
in a powder bed containing magnesium oxide or yttrium oxide and/or
cerium oxide.
The ceramic bodies of the invention, in comparison to specimens
prepared under otherwise equal conditions but without the
above-described surface treatment, in a treatment for accelerated
aging, consisting of four hours of firing at temperatures between
250.degree. C. and 400.degree. C. at steam pressures of 4 to 15
bar, show scarcely any effect.
In X-ray examination, in the case of the ceramic bodies of the
invention, only the cubic and tetragonal reflections of the body
subjected to the accelerated aging are detectable after this
treatment, while the specimens used for comparison show strong
monoclinic reflections which are an indication of incipient
degradation. The best results were obtained when the thin surface
layer was produced by firing the ready-sintered samples in
magnesium oxide, yttrium oxide, cerium oxide or calcium oxide
powder, or by treatment with yttrium oxide powder or a zirconium
powder containing at least 12 mole-% of yttrium oxide, the surface
layer being pressed onto the zirconium oxide compacts stabilized by
a small addition (0.5 to 5, preferably 2 to 4 mole-%) of yttrium
oxide, or being applied as an aqueous suspension of powder and
sintered. But, no matter how the surface layer is produced, an
important condition for the achievement of the protective action of
the stabilizer-containing coating is very close contact with the
surface of the zirconium oxide specimen to be heated or
sintered.
The thin, generally 0.5 to 30 micrometers deep, stabilizer-rich
zirconium oxide surface layer which is formed by the treatment of
the invention, appears to constitute a protection against long-term
thermal disintegration. This layer can also contain aluminum oxide
for fining the grain. Presumably other rare earth oxides produce a
similarly positive effect, as previously mentioned. On account of
the extremely slow diffusion of yttrium oxide into zirconium oxide
at temperatures below 1000.degree. C., this layer represents
primarily a thermally stable protection for TZP ceramics, but also
for conventional zirconium oxide partially stabilized with
magnesium or calcium (Mg-, Ca-PSZ).
The following examples further explain the invention.
EXAMPLE 1
Samples of reaction-sprayed powders (EDS powders: Evaporation
Decomposition of Solutions, Am. Ceram. Soc. Bull 50 (1977) 1023)
which contained 2 mole-% of yttrium oxide and 1.5 vol.-% of
aluminum oxide, and had been ground for 4 hours in water in an
attrition mill with alumina balls containing silica and spray
dried, were isostatically pressed at 630 MPa and sintered in air
for 2 hours at 1450.degree. C. The X-ray reflections thereafter
indicated a predominantly tetragonal structure (grain size approx.
0.4 micrometers). Flexural test samples indicate, in the polished
surface state, a strength of 920 MPa (type I) and, after 36 h of
firing at 1350.degree. C. in an yttria powder bed, a strength of
810 MPa (type II). After all of the samples were cooked in the
autoclave at 400.degree. C. for 4 hours at 4 bar steam pressure,
the strength of type I was only 420 MPa, while type II showed a
strength of 740 MPa.
EXAMPLE 2
Samples of a powder which was prepared and treated as in Example 1,
but contained only 2 mole-% of yttrium oxide by volume, were formed
as in Example 1. An aqueous suspension of yttria powder was applied
to the cylindrical compacts and some of it penetrated into the
surface pores; then the coated compacts (type I) were sintered at
1500.degree. C. for 2 hours, and then subjected to the autoclaving
described in Example 1, together with identical samples with no
coating (type II). After this treatment, type I showed only
tetragonal and cubic X-ray reflections, but type II showed
tetragonal and large monoclinic X-ray reflections which indicates
the thermal degradation of the uncoated samples.
EXAMPLE 3
Samples from the powder of Example 1 were isostatically pressed at
a pressure of 100 MPa, and then sprayed with a suspension of 12
mole-% zirconia powder containing 12 mole-% of yttrium oxide
(coating thickness approx. 40 to 200 micrometers), then pressed
again isostatically at 630 MPa, and sintered as in Example 1. After
the autoclaving treatment (as in Example 1), no thermal degradation
of the surface could be detected.
EXAMPLE 4
Samples in accordance with Example 2 were coated with the same
suspension, but this time with the addition of 20% alumina by
volume, and otherwise treated as in Example 1. Here, again, no
degradation could be detected after the heat treatment in the
autoclave.
EXAMPLE 5
50 volume-percent of alumina powder (Pechinee Ugine Kuhlman, A6)
[was added] to the powder from Example 1 and ground in the
attrition mill as in Example 1. Isostatically pressed cylinders
(approx. 1.times.1 cm diameter) were sintered at 1500.degree. C.,
some with (type I) and some without (type II) a slip of 50 wt.-% of
yttrium oxide and 50 wt.-% of cerium oxide. Then type I contained
on the polished surface only tetragonal zirconium oxide plus
aluminum oxide (measured by X-ray analysis), while type II
additionally contained cubic forms. After autoclaving as in Example
8, with only 8 bar of steam pressure, the surface of type I had a
high content of monoclinic zirconium oxide, while type II showed no
measurable change.
EXAMPLE 6
A coprecipitated zirconia powder containing 2.2 mole-% of yttrium
oxide was pressed isostatically at 620 MPa; the samples were then
sintered in air for 2 hours at 1500.degree. C. The bodies thus
prepared contained exclusively tetragonal grains of an average size
of 0.4 micrometers (material type A). A similarly made commercial
material with 3 mole-% of yttrium oxide contained approximate 80%
of tetragonal grains (approximately 0.4 micrometers) and
approximately 20% cubic grains (about 5 micrometers) (material type
B).
Material types A and B were subjected to an autoclave test with a
steam pressure of 5 bar at 250.degree. C. for 2 hours, and both
types degrade greatly, i.e., show mostly monoclinic reflections at
the surface; type A was even completely decomposed.
Types A and B were then fired each for 2 hours in powder beds of
yttrium oxide, cerium oxide, titanium oxide, magnesium oxide and
calcium oxide, at different temperatures. The heat treatment
temperatures and the results of the autoclave test that followed
are listed in Table 1. From this it appears that, with the
exception of titanium oxide, all the other oxides have a positive
effect, especially at higher temperatures. A firing in a magnesium
oxide powder bed is effective even at relatively low temperatures
(1120.degree. C.).
Types A and B, in the form of unsintered compacts, were sintered
for 2 h at 1500.degree. C. in powder beds of yttrium oxide, cerium
oxide, calcium oxide and magnesium oxide (in air). The
above-described autoclave test again showed no surface
degradation.
TABLE 1 ______________________________________ Sintering in a
powder bed, followed by autoclave test for 2 hours, 5 bar steam
pressure, 250.degree. C. Powder bed Yttrium Cerium Titanium
Magnesium Calcium Sintering oxide oxide oxide oxide oxide tempera-
Type of Material ture .degree.C. A B A B A B A B A B
______________________________________ 1120 - - - - - - - - - o
1220 - o o o - - - o + + 1320 o + o + - - + + + + 1420 + + + + - o
+ + + + ______________________________________
EXAMPLE 7
A conventional zirconia partially stabilized with magnesia
(Mg-PSZ), containing 3.3 wt.-% of magnesium oxide, was subjected to
a solution anneal in air at 1700.degree. C. for 2 hours, followed
by rapid cooling to room temperature, and was then subjected to two
hours of sintering at 1420.degree. C. in yttria powder. While the
monoclinic content at the surface in the untreated (as-received)
sample increased, after 100 hours in a slightly reducing atmosphere
at 920.degree. C., from originally 15% to 32%, the monoclinic
content in the sample sintered in yttria was below the measurable
range, i.e., less than 4%.
* * * * *