CA1318111C - Method of making a stress resistant, partially stabilized zirconia ceramic - Google Patents

Method of making a stress resistant, partially stabilized zirconia ceramic

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Publication number
CA1318111C
CA1318111C CA000563140A CA563140A CA1318111C CA 1318111 C CA1318111 C CA 1318111C CA 000563140 A CA000563140 A CA 000563140A CA 563140 A CA563140 A CA 563140A CA 1318111 C CA1318111 C CA 1318111C
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Canada
Prior art keywords
article
monoclinic
hours
phase
tetragonal
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CA000563140A
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French (fr)
Inventor
Morton Edwin Milberg
Donald James Cassidy
Roy Leonard Vanalsten
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Ford Motor Company of Canada Ltd
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Ford Motor Company of Canada Ltd
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    • CCHEMISTRY; METALLURGY
    • C04CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04BLIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/01Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
    • C04B35/48Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on zirconium or hafnium oxides, zirconates, zircon or hafnates
    • C04B35/486Fine ceramics

Abstract

Abstract of the Disclosure A method is disclosed for making an aging-resistant, partially stabilized zirconia ceramic, which method is not totally dependent upon grain size control, can function to eliminate substantially any monoclinic phase that results from machining such zirconia article, and employs a novel heat treatment to do so.
The method particularly comprises: (a) sintering such article at a temperature and for a period of time to produce a densified product having little or no monoclinic crystal phase when cooled to room temperature (i.e. 2550-2650F for .5-4 hours); (b) subjecting the cooled sintered article a machining operation that results in the formation of monoclinic crystal phase on the surface of the part; and (c) heating said treated article to the temperature range of 1800-1950°F
(982-1066°C) to re-transform monoclinic crystal structure to tetragonal crystal.

Description

1 - ~ 3 ~

METHOD OF MAKING A STRESS RESISTANT, PARTIAL~Y STABILI2ED ZIRCONIA CERAMIC

Ba~kground of the Invention Te~hnical Field The invention relates to the art of making partially stabilized zirconia with yktria and, more particularly, to the tschnology o eliminating the inherent aging degradation of such ceramic.

Description o~ the Prior Ar~
It is well known in the art that transformation toughened, partially stabilized zirconia (PSZ) ceramics, in which the stabilizer is yttria, have a major defect which comprises deterioration with elapse of time in the temperature range of 200-300C; the tetragonal phase zirconia in the surace layer, typical of the toughened mat~rial, reverts to the stable monoclinic form and the strength is substantially diminished. The deterioration is e~acerbated by water. A number of e~planations of this phenomenon have been offered but general agreement has not been achieved.
Although dealing with partially stabilized zirconia solid electrolytes, which are not transformation toughened, U.S. patents 4,360,598 and 4,370,393 do address the aeterioration problem stated above. Since the materials are not toughened, the stable phases present at room temperature are monoclinic and cubic.
These non-toughened materials are produced by firing at temperatures of at or above 1500C, at which temperature the stable phases are tetragonal and cubic or purely tetragonal, depending on the yttria content. With - 2 ~ ~3~

prolonged heating, grain growth will occur; on cooling, the tetragonal phase converts to the monoclinic or monoclinic and cubic phases, and the tetragonal to monoclinic transformation is accompanied by a volume e~pansion of about 4%. According to the aforementioned patents, this e~pansion on cooling due to crystal tran~formation leads to mechanical stress at the grain boundaries which, in turn, leads to cracking and deterioration when the mat:erial is held at temperatures of 200-300C for extended periods. These patents further state that the problsm can be overcome or reduced if the grain size is less than 2 ~, so that fine grained materials are protected from deterioration.
Transformation-toughened PSZ ceramics, containing Y203 as a stabilizer, will contain only tetragonal phase zirconia at room temperature if the Y2O3 content is less than about 4 mol percent, and a mi~ture of tetragonal and cubic phases with more than about 4 mol percent Y20~. The high temperature tetragonal phase is retained at room temperature by firing a very reactive high surface area coprecipitated ZrO2-Y2O3 powder, in the temperature range of 1400-1500C, so as to obtain a fine-grained ceramic body. Since this material contains no apparent monoclinic phase, it appears at Eirst glance as if the deterioration mechanism, of crystal transformation stress, suggested by U.S. patents 4,360,598 and 4,370,393, cannot be applicable. However, in order to form a useful item from a fired PSZ body~ machining of some sort is usually required and this operation causes the transformation of some of the tetragonal zirconia in the machined surfaces to the monoclinic form, so that stress associated with the formation of the monoclinic phase again presents the problem of low temperature aging deterioration.

1 3 ~ 5~
An article entitled "Degradation Durin~ Aging of Transformation-Toughened ZrO2-Y20~ Materials at 250C~, by Lange et al, appearing in the Journal of the American ~eramic oçiet~, Vo:l. 69, pp. 237-240 (1986~, deals directly with the use of low temperature aging deterioration in transformation-toughened ZrO2 Y203 materials and suggests that ~ater leaches yttrium out of tetragonal PSZ grains, leadi.ng to their transformation to the monoclinic form accompanied by e~pansion which leads to stress and, i~ the monoclinic grains are above a critical size, microcracking. In any event, the connection between the deterioration and the presence of the monoclinic crystalline phase is generally invoked.
That the deterioration mechanism suggested by Lange et al further requires the involvement of water is not a drawback, since water vapor is always present in ambient atmospheres.
Even i~ process steps are taken to ensure the inhibition o~ the transformation of tetragonal crystal to the monoclinic crystal, such as sugge~ted in U.S. Patents 4,370,393 and 4,360,598, there remains the stimulation of the monoclinic phase by machining of such sintered material. That is to say, the mat~rial may be cooled to room temperature by observing certain sintering conditions so that the tetragonal crystal phase predominates even at room temperature, accompanied only by sm~ll se~ds of monoclinic crystal.
The important problem that remains is that of eliminating the monoclinic phase from a partially stabili~ed zirconia material after sintering whether stimulated by machining or by improper sintering ~echniques.

Summary of the InventiQn This invention provides a method of imparting ..
~ , .

1 3 ~

aging~resistance to a fabricated article of zirconium oxide and yttrium oxid~, which method is not totally dependent upon grain size control, can function to substantially eliminate any monoclinic phase and/or monoclinic seed that results from machining or improper sintexing such zirconia article, and employs a combination of a shortened sintering treatment and novel heat treatment to do so.
The method particularly comprises (a) sintering such article at a temperature and for a time period to produce a densified product having little or no monoclinic crystal phase when cooled to room temperature tpreferably 2550 to 2650F (1399 to 1454C) for 0.5 to 4 hours); (b) subjecting the si~tered article to a machining operation that results in the formation of monoclinic crystal phase at the surface of the article; and (c) heating the machined article to the temperature range of 1800 to 1950F (982 to 1066C) for 4 to 30 hours to substantially transform all monoclinic crystal structure to tetragonal crystal and/or remove substantially all seeds for monoclinic phase growth.

Detailed Des~riP~ion and Best_Mode Partially stabilized zirconia ceramîcs, which contain yttria as a stabilizi~g agent, are those that contain less than 7.3 mol percent of Y2O3. The forming method comprises: (a) preparing an article from zirconium o~ide and a yttrium compound which is sintered at a temperature in the range of 2550-2650F
(1399-1454C) for a shortened period of time to produce a densified product having little or no monoclinic phase when cooled to room temperature; (h3 subjecting the cooled sintered article to a machining operation that results in the ormation of monoclinic crystal phase on the surface of the part; and (c) heating the machined article to the temperature range of 1800-1950F

....
5 ~ 3 ~

(982-1065C) for 4-30 hours to transform any of said stimulated monoclinic phase to a tetragonal crystal structure.

Chemis~rv To prepare a partially-stabilized zirconia body, a coprecipitated powder mi~ture o~ ZrO2 and Y2O3 is used with less than 7.3 mol percent Y2O3 and preferably with Y2O3 present in amounts ranging from 2-5 mol percent. The average particle size of such coprecipitated material is typically 250 ang~troms and has a purity of greater than 99.3~ ZrO2/Y203.
Major impurities present in such powder material include o~ides of silicon, iron, and sodium in the range of 0.2-0.3%. The coprecipitated powder has a very high specific surface area of approximately 18 m2/g.

Fabri~a~ion The prepared powder mixture is formed into an article, preferably by isostatic pressing, under a pressure of 105-315 MPa. Alternatively, the powder mi~ture may be formed as a slurry and slip cast according to conventional slip cast techniques r~quiring water removal via low temperature drying. For a teaching as to this technique see U.S. patent 4,067,943, Sinterina The formed article is sintered in air in the temperature range of 2550-2650F (1399-1454C) for a period of less than 10 hours, preferably 0.5-4 hours, to provide a resulting density equal to or yreater than 97%
of theoretical. Conventional sintering periods normally require 20 or more hours. Upon cooling to room temperature at a rate generally obtained with air "

~ 3 ~

cooling, no monoclinic æro2 was detected by x-ray diffraction ~XRD), despite the fact that during cooling considerable time was spent in the temperature range of 302-752F (150-400C). Small amounts of cubic ZrO2 could have been present but undetected because of the overlap of diffraction peaks. Seeds of monoclinic crystal growth usually still remain, the latter being a phenomenon that will begin a crystal growth under subsequent favorable conditions.
Machinina The sintered art;cle is subjected to a machining operation that can stimulate the growth of the monoclinic crystal structure, particularly from the seeds present.
This occurs with machining of such articles to define a desired shape; machining is used in the broad sense of the movement of a cutting tool against the article in a manner to shear metal therefrom at a rate and feed tpat increases the local temperature of the article resulting in a stimulated phase transformation. The cutting acts as the equivalent of cracking which stimulates the growth of monoclinic crystal structure hy the phenomena outlined in the previous articles. It has been found that certain types of machining promote greater growth of the monoclinic phase than others. For instance, high speed deep roughing promotes growth of the monoclinic phase, while very fine machining or the extreme of fine machining, i.e., polishing, can actually remove some of the thin layer of monoclinic crystal produced by previous machining. The machined article thus may have varying concentrations of monoclinic crystal on the surface zones which haYe been subjected to such machining stress. If the sintered/machined article were to be utilized in such co~dition, and subjected to aging in i~s useful environment (aging i~ used here to mean the result of _ 7 ~

being in the temperature range of 200-300C for a period of time in e~cess of 20 hours), extensive monoclinic crystal growth would occur, resulting in severe loss in mechanical strength. It has also been found that water vapor accelerates the aging phenomenon in transormation toughened ZrO2-Y2O3 polycrystalline materials.

Héat Treatmçnt The machined article is then heated in air to the temperature range of lB00-1950F ~982-1066~C) for 4-30 hours to transforrn the monoclinic phase to tetragonal phase. Although other stabilization or heat treatment temperatures have not been explored, it is suspected that heat treatment temperatures lower than 1800F will produce desired results, but at a penalty of longer heat treatment times. Likewise, higher heat treatment temperatures could also be effective up to the point of an increase in crystal size above a critical size where tetraqonal crystals revert spontaneously to monoclinic at room temperature.

Aqinq Treatment When a conventionally sintered and machined article was subjected to prolonged e~posure at low temperatures, namely, 500F in ambient air, the fle~ural strength of the article was reduced from 121 to 40 kpsi, about 67% ~Table I, test 1). This strength was reduced to 10~ after aging for 180 days at 500F. (Aging this same material under high relative humidity conditions would have e~acerbated these losses in strength.) Using the sintering and heat treatment of this invention, the material will retain its fracture strength after 60 days;
and, even after 60 days in water vapor at such temperatures, will have lost none of its fracture strength (see Table 1, tests 1 and 2).

- 8 - 1 3 1 3 ~ ~ ~

Although the reason for the physical improvement is not fully understood, it is believed that the shortsned si~tering treatment and post-heat treatment not only re-transforms the monoclinic to tetragonal phase, but also inhibits the formation of further monoclinic phase by eliminating the ~presence of monoclinic seed crystals on the surface oE the machined article.

E~amples A series of e~amples was prepared to corroborate ths limits of the processing of this invention. In each example, samples were prepared from a reactive grade Zr2 powder material containing 3 mol perc~nt Y203. The powder was then isostatically pxessed under a pressure of about 315 MPa to form articles for sintering. The purity of the powder used in making ths compacts was 99.3% ZrO2/Y2O3. Test 1 was run without hiyh humidity aging conditions and test 2 did have high humidity during aging. Some of the samples were subjected to a sintering treatment characteristic of the prior art at high temperaturPs of about 2650F for e~tended periods of time of about 24 hours. Such material was machined into test bars and subjected to strength measurements before and after aging at 500F
(Table I, test 1, samples 1 a~d 2). Other samples were sintered for shorter time periods of 0.5 to 4 hours, machined into test bars, and heat treated at 1900F for 24 hours duration ~Table I, tests 1 and 2, samples 3-83.
Samples were subject to aging tests at 500F for a period 30 of 60 days (and in one case 180 days). In addition, the samples were aged, in test 2, in an atmosphere of high relative humidity at 500F. The high humidity atmosphere cQnsisted o air which had been saturated with water vapor at room temperature.
From Table I, it can be concluded that material 9 ~ 13.~ L 11 sintered and heat treated according to the teachings o this patent is far more resistant to the aging procsss of prolonged e~posure at 500F in both ambient air and high relative humidity conditions.

While particular examples of the invention have been illustrated and desc.ribed, it will be obvious to those skilled in the art that various changes and modifications may be made without daparting from the invention, and it is intended to cover in the appended claims all such changes and equivalents as fall within the true spirit and scope of the invention.

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Claims (12)

1. A method of imparting aging resistance to a fabricated article of zirconium oxide and yttrium oxide, comprising (a) sintering said article at a temperature and for a period of time to produce a densified product having little or no monoclinic crystal phase when cooled to room temperature;
(b) subjecting the sintered article to a machining operation that results in the formation of monoclinic crystal phase at the surface of the article;
and (c) heating said machined article to the temperature range of 1800-1950°F (982-1065°C) for 4-30 hours to substantially transform all monoclinic crystal structure to tetragonal crystal and/or remove substantially all feeds for monoclinic phase growth.
2. The method as in claim 1, in which said zirconia article contains less than 7.3 mol percent yttrium compound.
3. The method as in claim 1, in which said yttrium compound is Y2O3 present in an amount of 2-5 mol percent.
4. The method as in claim 1, in which said step (a) is carried out in the temperature range of 2550-2650°F (1399-1454°C) and for a period of time of .5-4 hours.
5. The method as in claim 1, in which said steps (a) and (c) are carried out in air.
6. The method as in claim 1, in which the machining of step (b) is an equivalent to cracking of the ceramic surface.
7. The method as in claim 1, in which the fabricated article is prepared from a powder mix having an average particle size of about 250 angstroms.
8. The method as in claim 1, in which said fabricated article is prepared by compacting a reactive powder of said zirconium oxide and yttrium compound, said compaction being carried out at a pressure in the range of about 105-315 MPa.
9. The method as in claim 1, in which said fabricated article is prepared by slip casting.
10. A sintered machined article comprised of zirconium oxide and 1-7.3 mol percent yttrium oxide, said article being comprised of a substantially tetragonal crystalline structure with surface zones of re-transformed monoclinic to tetragonal crystals.
11. The article in claim 10, in which the crystal structure has an average particle size of .5-2 microns.
12. A method of imparting aging resistance to a fabricated article of zirconium oxide and yttrium oxide, comprising:
(a) sintering said fabricated article at a temperature and for a period of time of less than 10 hours to produce a transformation-toughened yttria partially stabilized densified product having little or no monoclinic crystal phase when cooled to room temperature;
(b) heating said product to the temperature range of 1800-1950 for 4-30 hours to remove substantially all seeds for monoclinic phase growth and/or transform substantially all monoclinic crystal structure to tetragonal crystal; and following step (a) and prior to or subsequent to step (b), subjecting the product to a machining operation.
CA000563140A 1987-04-13 1988-03-31 Method of making a stress resistant, partially stabilized zirconia ceramic Expired - Fee Related CA1318111C (en)

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US07/037,654 US4866014A (en) 1987-04-13 1987-04-13 Method of making a stress resistant, partially stabilized zirconia ceramic
US037,654 1987-04-13

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EP0603818B1 (en) * 1992-12-22 1996-07-31 Eastman Kodak Company Method of preparation of zirconia articles having tetragonal cores and monoclinic cases
US6258233B1 (en) * 1995-07-13 2001-07-10 Denso Corporation Multilayered air-fuel ratio sensing element
US6174489B1 (en) * 1995-09-01 2001-01-16 Denso Corporation Method for manufacturing a gas sensor unit
US5932507A (en) * 1998-02-19 1999-08-03 Van Weeren; Remco Method for preventing low-temperature degradation of tetragonal zirconia containing materials
US7527761B2 (en) * 2004-12-15 2009-05-05 Coorstek, Inc. Preparation of yttria-stabilized zirconia reaction sintered products
US7833469B2 (en) * 2004-12-15 2010-11-16 Coorstek, Inc. Preparation of yttria-stabilized zirconia reaction sintered products
US20070007146A1 (en) * 2005-07-07 2007-01-11 Severn Trent Water Purification, Inc. Process for producing hypochlorite
FR2969145B1 (en) * 2010-12-16 2013-01-11 Saint Gobain Ct Recherches REFRACTORY PRODUCT HAVING A HIGH ZIRCONY CONTENT.
CN114105633B (en) * 2021-11-18 2023-04-07 长裕控股集团股份有限公司 Method for improving aging resistance of zirconia ceramic

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EP0287262A1 (en) 1988-10-19
US4866014A (en) 1989-09-12
JPS63260857A (en) 1988-10-27

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