U.S. patent application number 11/340999 was filed with the patent office on 2006-06-08 for method to prevent low temperature degradation of zirconia.
This patent application is currently assigned to Alfred E. Mann Foundation for Scientific Research. Invention is credited to Brian J. Lasater.
Application Number | 20060118035 11/340999 |
Document ID | / |
Family ID | 34941088 |
Filed Date | 2006-06-08 |
United States Patent
Application |
20060118035 |
Kind Code |
A1 |
Lasater; Brian J. |
June 8, 2006 |
Method to prevent low temperature degradation of zirconia
Abstract
The invention is directed to a method of producing the material
that is unaffected by the low-temperature degradation,
humidity-enhanced phase transformation typical of yttria-stabilized
zirconia in general, as well as of yttria-stabilized tetragonal
zirconia polycrystalline ceramic (Y-TZP). Because of the high
fracture toughness and high mechanical strength, this class of
materials is widely used, including as implants, such as for the
packaging material for small implantable neural-muscular sensors
and stimulators. The destructive phase transformation is eliminated
by converting the surface to stable cubic or T-prime zirconia by
post-densification thermal treatment in a cation-rich milieu.
Inventors: |
Lasater; Brian J.;
(Wenatchee, WA) |
Correspondence
Address: |
ALFRED E. MANN FOUNDATION FOR;SCIENTIFIC RESEARCH
PO BOX 905
SANTA CLARITA
CA
91380
US
|
Assignee: |
Alfred E. Mann Foundation for
Scientific Research
Santa Clarita
CA
|
Family ID: |
34941088 |
Appl. No.: |
11/340999 |
Filed: |
January 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10853922 |
May 25, 2004 |
|
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11340999 |
Jan 27, 2006 |
|
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Current U.S.
Class: |
117/4 ; 117/7;
117/9 |
Current CPC
Class: |
C04B 2235/3206 20130101;
C04B 2235/765 20130101; C04B 2235/6587 20130101; C04B 41/87
20130101; C04B 2235/662 20130101; C04B 41/5042 20130101; C04B
41/009 20130101; A61L 27/10 20130101; C04B 35/64 20130101; C04B
2235/3229 20130101; C04B 2111/00836 20130101; Y10T 428/12611
20150115; C04B 35/48 20130101; C04B 41/4556 20130101; C04B 41/4545
20130101; C04B 2235/3225 20130101; C04B 2235/785 20130101; C04B
35/486 20130101; C04B 2111/0025 20130101; C03C 4/0007 20130101;
C04B 41/009 20130101; C04B 41/5042 20130101; C04B 2235/3208
20130101; Y10T 428/12618 20150115 |
Class at
Publication: |
117/004 ;
117/007; 117/009 |
International
Class: |
C30B 1/00 20060101
C30B001/00; C30B 5/00 20060101 C30B005/00 |
Claims
1. A method of producing a stable tetragonal zirconia polycrystal
ceramic having an outer surface, comprising the steps of: selecting
a densified tetragonal zirconia polycrystalline ceramic; placing
said ceramic in a powder bed of a cation-rich material; and
converting said outer surface to a stable ceramic phase at a
controlled temperature, at a controlled pressure, and in a
controlled atmosphere to achieve an average grain size of less than
about 0.5 micron, thereby substantially eliminating low-temperature
degradation of said polycrystal ceramic.
2. The method according to claim 1, further comprising the step of
stabilizing said tetragonal zirconia polycrystal ceramic with
yttria.
3. The method according to claim 1, further comprising the step of
stabilizing said tetragonal zirconia polycrystal ceramic with three
mole percent of yttria.
4. The method according to claim 1, further comprising the step of
stabilizing said tetragonal zirconia polycrystal ceramic with at
least one oxide selected from calcia, magnesia, and ceria.
5. The method according to claim 1, comprising the step of
converting said outer surface at a controlled temperature of
1400.degree. C. to 1800.degree. C.
6. The method according to claim 1, comprising the step of
converting said outer surface at a controlled temperature of
1600.degree. C. to 1700.degree. C.
7. The method according to claim 6, comprising the step of
converting said outer surface at between about 1600.degree. C. to
1700.degree. C. for at least about one and less than about two
hours.
8. The method according to claim 1, comprising the step of
converting said outer surface to cubic phase.
9. The method according to claim 1, comprising the step of
converting said outer surface to T-prime phase.
10. The method according to claim 1, comprising the step of coating
said outer surface with a hermetic coating.
11. The method according to claim 1, comprising the step of
thermally treating said stabilized tetragonal zirconia polycrystal
ceramic in a bed of cation-rich material.
12. The method according to claim 11, comprising the step of
thermally treating said stabilized tetragonal zirconia polycrystal
ceramic in a powder bed of cation-rich material.
13. The method according to claim 11, comprising the step of
thermally treating said stabilized tetragonal zirconia polycrystal
ceramic in a bed of yttria-rich material.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of co-pending U.S. patent
application Ser. No. 10/853,922, filed May 25, 2004.
FIELD OF THE INVENTION
[0002] This invention relates to a method of increasing the useful
life of an yttria-stabilized zirconia structure when implanted in
living tissue.
BACKGROUND OF THE INVENTION
[0003] One widely employed bioceramic is alumina, which is
considered bioinert. The search for an ideal bioceramic has
included alumina, hydroxyapatite, calcium phosphate, and other
ceramics. The first use of aluminas for implants in orthopedics and
dentistry was in the 1960's. They were later employed in hip
prostheses as early as 1970. Since those early days the quality and
performance of aluminas have improved. High-purity, high-density,
fine-grained aluminas are currently used for a wide range of
medical applications, e.g. dental implants, middle ear implants,
and hip or knee prostheses.
[0004] Although the aluminas currently available perform
satisfactorily, a further improvement in strength and toughness
would increase the safety factor and may extend usage to higher
stressed components. A proposed candidate to add to this list is
stabilized-zirconia, because of its potential advantages over
alumina of a lower Young's modulus, higher strength, and higher
fracture toughness. Another advantage of stabilized-zirconia is
low-wear residue and low coefficient of friction. Because, zirconia
undergoes a destructive phase change at between 1000 and
1100.degree. C., changing from monoclinic to tetragonal, phase
stabilization admixtures of calcia, magnesia, ceria, yttria, or the
like are required.
[0005] Tetragonal zirconia polycrystalline ceramic, commonly known
as TZP, which typically contains 3 mole percent yttria, coupled
with the small size of the particles, results in the metastable
tetragonal state at room temperature. Under the action of a stress
field in the vicinity of a crack, the metastable particles
transform, with a 3% to 4% volume increase, by a shear-type
reaction, to the monoclinic phase. Crack propagation is retarded by
the transforming particles at the crack tip and by the compressive
back stress on the crack walls behind the tip, due to volume
expansion associated with transformation to the monoclinic
phase.
[0006] The well-known transformation toughening mechanism is
operative in zirconia ceramics whose composition and production are
optimized such that most of the grains have the tetragonal crystal
structure. These TZP ceramics, most notably their mechanical
properties in air at room temperature, are superior to those of
zirconia-toughened aluminas and to other classes of zirconias.
While the biocompatibility of TZP ceramic has not been fully
assessed, it has been preliminarily investigated.
[0007] For example, in one study by Thompson and Rawlings [see I.
Thompson and R. D. Rawlings, "Mechanical Behavior of Zirconia and
Zirconia-Toughened Alumina in a Simulated Body Environment,"
Biomaterials, 11 [7] 505-08 (1990)]. The result was that TZP
demonstrated a significant strength decrement when aged for long
periods in Ringer's solution and was therefore unsuitable as
implant material.
[0008] Drummond [see J. L. Drummond, J. Amer. Ceram. Soc., 72 [4]
675-76 (1989)] reported that yttria-stabilized zirconia
demonstrated low-temperature degradation at 37.degree. C. with a
significant decrement in strength in as short a period as 140 to
302 days in deionized water, saline, or Ringer's solution. He also
reports on similar observation by others, where yttria-doped
zirconia demonstrated a strength decrement in water vapor, room
temperature water, Ringer's solution, hot water, boiling water, and
post-in vivo aging.
[0009] TZP components suffer a decrement in strength properties
after exposure for only a few days to humid environments. This
degradation of mechanical properties occurs when moisture is
present in any form, for example, as humidity or as a soaking
solution for the TZP component. TZP components have been observed
to spontaneously fall apart after times as short as a few weeks in
room temperature water. This is of particular importance in
living-tissue implanted devices that contain components made of
this class of material. Long-term implantation of devices that
contain yttria-stabilized (or partially-stabilized) zirconia
components is not feasible with available materials.
[0010] One approach to preventing the low-temperature degradation
of zirconia that was doped with 3 mole percent yttria is presented
by Chung, et al. [see T. Chung, H. Song, G. Kim, and D. Kim,
"Microstructure and Phase Stability of Yttria-Doped Tetragonal
Zirconia Polycrystals Heat Treated in Nitrogen Atmosphere," J. Am.
Ceram. Soc., 80 [10] 2607-12 (1997)]. The TZP sintered material was
held for 2 hours at 1600.degree. or 1700.degree. C. in flowing
nitrogen gas.
[0011] Analysis showed that the resulting surface consisted of
cubic grains with tetragonal precipitates, while the interior was
only slightly affected by the nitrogen exposure. Chung reported
that low-temperature degradation was prevented because degradation
of TZP started at the surface, which is protected from degradation
by the stable cubic phase.
[0012] An alternate material and an easy to apply method of
producing stable material to prevent the detrimental
low-temperature phase change are needed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 presents a schematic representation of a ceramic
component being thermally treated.
[0014] FIG. 2 presents a schematic representation of a combustion
chemical vapor deposition process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0015] A broadly applicable material and method of producing the
material begins with the densified as-sintered, post-hot pressed,
or hot isostatically pressed tetragonal zirconia polycrystalline
ceramic (TZP) material that has been made by processes that are
known to one skilled in the art, containing about 3 mole percent of
yttria, which is subsequently thermally processed to convert the
surface to a stable phase of cubic or T-prime zirconia (zirconium
oxide) phase. The density of the TZP material is in the range of 90
to 100% of theoretical density, and preferably at least 98% dense.
It is well established that the cubic and T-prime phases of
zirconia are stable in moist environments and are not subject to
the deleterious low-temperature degradation failure mechanism that
plagues TZP materials.
[0016] As presented in FIG. 1, a thermal treatment apparatus 2 is
utilized to form the low-temperature resistant material. A
densified ceramic component 4 comprised of TZP is placed in a
containment vessel 6. A cation-rich bed 8 preferably comprised of a
powder material surrounds the ceramic component 4.
[0017] The thermal treatment apparatus 2 is preferably placed in a
furnace at one atmosphere pressure in an air environment and held
at between 800.degree. C. to 1500.degree. C. for 15 to 90 minutes,
and more preferably at 1100.degree. C. to 1200.degree. C. for 30 to
45 minutes. In alternative embodiments, the atmosphere may be an
inert atmosphere, such as argon, or a reducing or vacuum
atmosphere.
[0018] The cation-rich bed 8 is preferably substantially comprised
of yttria, ceria, magnesia, or calcia. It is believed that the
cation diffuses into the surface of the ceramic component and
increases the molar percentage of stabilizing oxide from
approximately 3 mole percent to about 10 mole percent. The
conversion layer is preferably 0.1 to 10 microns deep, and more
preferably 4 to 7 microns deep.
[0019] In an alternate embodiment, the ceramic component 4 is
coated with 500 to 15,000 angstroms, and more preferably 5,000 to
10,000 angstroms, of a cation-rich layer of yttria, ceria,
magnesia, or calcia. The coating is applied by methods known to one
skilled in the art, such as chemical vapor deposition, physical
vapor deposition, electron beam evaporation, ion beam assisted
deposition, ion implantation, plasma spraying, sol-gel processing,
or metallic plating followed by post-deposition oxidation or
diffusion.
[0020] In yet another alternate embodiment, the thermal treatment
apparatus 2 is operated absent the cation-rich bed 8, while the
ceramic component 4 has been coated with the cation-rich layer,
thus achieving the stable surface conversion to cubic or T-prime
zirconia.
[0021] In another embodiment, the thermally treated ceramic
component 2 having the stable cubic or T-prime surface layer is
coated with a hermetic coating to further assure that the ceramic
component 2 will remain stable and will not be subject to
low-temperature degradation. The hermetic coating is comprised of
known ceramic materials that are capable of forming a hermetic
coating, including silica, alumina, silicon nitride, zirconia,
silicon-oxynitride, aluminum oxynitride, silicon-aluminum
oxynitride, and ultra-nanocrystalline diamond thin film. These
ceramic coatings may be applied by combustion chemical vapor
deposition, physical vapor deposition, electron beam evaporation,
ion beam assisted deposition, ion implantation, or chemical vapor
deposition.
[0022] The coating can be deposited at room temperature for
combustion chemical vapor deposition, physical vapor deposition,
electron beam evaporation, or ion beam assisted deposition.
[0023] Chemical vapor deposition, known to those skilled in the
art, is performed in a high temperature furnace. The furnace is
heated to 800.degree. to 1300.degree. C. and chemical precursors
are placed into the furnace in gaseous form. The gases dissociate
in the heat and deposit on the substrate or part. Combustion
chemical vapor deposition, illustrated in FIG. 2, does not employ a
furnace and may be practiced in an air or inert gas milieu. Rather
a combustion gun 10 is used in which a fuel 14 flows and ignites a
very hot flame 16. Once the hot flame 16 is established, the
precursor chemicals 12 are injected into the flame 16 with a
similar end result as achieved with a classic chemical vapor
deposition process, where the precursor chemical 12 is deposited on
ceramic component 20.
[0024] Ion implantation is performed in a vacuum chamber with an
ion gun and source of materials to be implanted. For example, one
wanted to implant titanium into ZrO.sub.2, a titanium target would
be placed inside a vacuum chamber. Argon ions would bombard the
titanium target and knock atoms off and ions which would be
directed in a beam to impact the surface of the ZrO.sub.2. The
energy is sufficient to embed or implant the titanium atoms to a
depth of about 0.1 microns. Thus there is no coating, per se, to
flake off of the underlying substrate.
[0025] In addition to these hermetic ceramic coatings, other
coatings that are known to those skilled in the art may be applied
by known methods to create a hermetic coating on the TZP ceramic,
where the coating protects from low-temperature degradation by
virtue of keeping humidity and moisture isolated from the
vulnerable TZP ceramic. Such coatings include
polytetrafluoroethylene, silicone, or any biocompatible organic
coating, such as parylene or liquid crystal polymer. Parylene, for
example, is a vacuum deposited plastic film used to coat many types
of substrates. Parylene coatings provide excellent corrosion
resistance, barrier properties and exhibit superior dielectric
protection.
[0026] The resulting coating is preferably 500 to 15,000 angstroms
thick, and more preferably 5,000 to 10,000 angstroms thick.
[0027] In an alternate embodiment, the TZP ceramic is coated with a
hermetic coating without first converting the surface to the stable
cubic or T-prime phase.
[0028] An additional embodiment is to apply a glass or glass
ceramic coating to the TZP where the glass coating is hermetic to
moisture and is biocompatible. Examples of glass coating include
Cabal 17, Babal-1d, Srbal-1, or TIG-24, which are known to those
skilled in the art, see for example U.S. Pat. No. 5,021,307 to
Brow, et al., U.S. Pat. No. 5,104,738 to Brow, et al., U.S. Pat.
No. 5,648,302 to Brow, et al., and U.S. Pat. No. 5,693,580 to Brow,
et al., each of which is incorporated by reference herein in its
entirety. It is important that the selected glass or glass ceramic
coating have a coefficient of thermal expansion that matches that
of the TZP ceramic. See for example U.S. Pat. No. 4,414,282 to
McCollister, et al., U.S. Pat. No. 4,536,203 to Kramer, and U.S.
Pat. No. 5,820,989 to Reed, et al., each of which is incorporated
by reference herein in its entirety. The preferred material is
TIG-24.
[0029] The preferred deposition method for the glass or glass
ceramic is to apply the material by spraying, painting,
electrophoresis, physical vapor deposition, electron beam
evaporation, ion beam assisted deposition or similar known
processes for applying a thin hermetic coating to TZP ceramic.
[0030] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that, within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described.
* * * * *