CA1313035C - Magnesia partially-stabilized zirconia ceramics and process for making the same - Google Patents

Abstract

ABSTRACT
A method for producing a magnesia partially-stabilized zirconia ceramics is provided. The method preferably consists of combining zirconium dioxide having a silica content of up to 0.5 weight percent with a magnesium-containing component. This mixture is then compacted, heated, and cooled at a controlled rate, without aging, to form said zirconia ceramics.

Description

3~5 MAGNESIA PARTIALLY-STABILIZED ZIRCONIA
CE~MICS AND PROCESS FOR MAKING TI~E SAME

Backqround of the Invention Methods of making partially-stabilized zirconia ceramics are known. For example, zirconias are known to be partially stabilized by calcia ~U.S. Patent No.
4,067,745, issued January 10, 1978, to Garvie et al.), yttria ~Canadian Patent No. 1,154j793, issued October 4, 1983, to Otagiri et al.), and magnesia (U.S. Patent No.
4,279,655, issued on July 21, 1981, to Garvie et al. and PCT Application No. PCT/AU83/00069, International Publi-cation No. WO 83/04247 filed on May 27, 1983, by Common-wealth Scientific and Industrial Research Organization).
The methods described therein are directed to the controlled formation of a specified microstructure and are reported to produce ceramics having good thermal shock resistance and strength.
None of these processes, however, has been found satisfactory for economically producing magnesia partially-stabilized zirconia ceramics. At least two problems can be identified in the known processes. One relates to counteracting the detrimental effects o~
silica impurities commonly ~ound. The second problem relates to the need for an aging step to control the formation of a specific microstructure.
The detrimental effects of silica on zirconias partially stabilized by magnesia are identified and explained in Drennan ~ Hannik, "Effect of SrO Additions on the Grain-Boundary Microstructure and Mechanical Properties of MagnPsia-Partially-Stabilized Zirconia,"
i J. Am. Ceram. Soc. 69, 541, 5q6 (1986). Accor~ing to this article, the accumulation of silica in the grain-boundary phases leads to the degradation of mechanical properties. In addition, the affinity of magnesium ~L 3 ~

oxides for silica results in the destabilization of zirconia ceramics.
Two methods of controlling these detrimental effects have been employed. One method involves the use of zirconia having a silica content of no greater than 0.03 weight percent as evidenced by U.S. Patent No.
4,279,655 to Garvie et al. PCT Application No.
PCT/~U83/00069 describes another method in which zirconias having a higher content of silica may be used if a metal oxide additive is included in the process.
Strontia, baria, rare earth oxides or a mixture thereof are used to form an insoluble zirconate, which is not combinable with magnesia, or to form a glass with silica. The need to use starting materials from which silica has been extracted and/or the need to use metal oxide additives for zirconias having a content of silica greater than 0~03 weight percent result in an expensive and time-consuming process. It would therefore to be advantageous to have a process for the preparation of partially-stabilîzed zirconia ceramics in which zirconium dioxide having a silica content of greater than 0.03% by weight can be used without the addition of metal oxide additives.
The second problem faced by existing processes relates to the requirement of an aging period to control microstructure formation in partially-stabilized zirconias. The term "aging" is used herein to refer to "isothermal holdl' (i.e., maintaining a particular soak temperature for a period of time during the cooling period before reaching ambient temperature) and "annealing" (i.e., maintaining a particular soak temperature for a period of time after reheating from ambient temperature~. U.S. Patent No~ 4,067,745 to Garvie et al. discusses the necessity of an aging period to ensure proper tetragonal domain size formation in 1.3~3~

calcia partially-stabilized zirconias to obtain the desired mechanical properties. This reference emphasizes the difficulty of controlling microstructure formation using magnesia because the kinetics of precipitation is ten times faster in MgO-ZrO2 systems than in CaO-ZrO2 systems. HPnce, quality control of magnesia partially-stabilized zirconias is more difficult.
PCT Application No. PCT/AU83~00069 describes a process for making magnesia partially-stabilized zirconias using metal oxide additives. This reference discloses the use of an isothermal hold to control the extent to which tetragonal precipitates are transformed in the cubic grain matrix. It also discloses that by controlling the transformation of tetragonal precipitates to the monoclinic form, the amount of grain matrix monoclinic zirconia in the final product can be controlled. Consequently, this reference teaches that j~ an aging process is required when substantial tetragonal precipitates are desired in the final product. It would therefore be advantageous to have a process that eliminates the expensive and time-consuming aging step.
The present invention has advantagPously been found to overcome the above-identified problems of the known processas. The process of the present invention surprisingly produced stronger ceramics compared to a process using zirconia having a silica content of up to about 0.03 weight percent. A further advantage of the present invention is the elimination of the aging step thought nece~sary to produce magnesia partially-stabilized zirconias having a metastable tetragonal phase microstructure within a cubic grain phase.

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SummarY of the Inventio In one embodiment, the present invention comprises : a process for making a magnesia partially-stabilized ~irconia ceramic which comprises the steps of: (1) forming a first mixture by combining zirconium dioxide powder, having a silica content of up to about 0.5 percent by weight, with a magnesium-containing component consisting of magnesium oxide, a magnesium oxide forming material or mixtures thereof, said magnesium component lo being present in an amount to provide a magnesium oxide content of about 2.6 to about 3.8 weight percent of said zirconia ceramic; ~2) compacting said first mixture to form a compacted mixture; (3) heating the compacted mixture to a soak temperature in the range of about 1675C and about 1800C to form a sintered ceramic; (4) cooling the sintered ceramic continuously to a temperature in the range of about 800C and 1400C at a rate such that tetragonal precipitates are formed and substantially maintained in the tetragonal phase throughout a constant cooling period; and (5) further cooling said sintered ceramic to ambient temperature ;~ without aging to provide said zirconia ceramic material.
In another embodiment, the instant invention involves a process for making magnesia partially-~25 stabilized æirconia comprises the steps of: (1) forming :~a first mixture by mixing zirconium dioxide powder~
having a silica content of up to about 0.5 weight percent, and a magnesium-containing component consisting of magnesium oxide, a magnesium oxide forming material or mixture thereof, said magnesium component being present in an amount to provide a magnesiu~ oxide content from about 2.6 to about 3.8 weight percent of the zirconia ceramic material; (2) calcining said first mixture to form a calcined mixture; (3) compacting said calcined mixture to a desired shepe to form a compacted :: .

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mixture: (4) heating said compacted mixture to a soak temperature in the range of about 1675C and about 1800C to form a sintered ceramic; (5) cooling said sintered ceramic continuously to a temperature in the range of about 800C and about 1400C at a rate between about 200C and about 1000C per hour: and (6) further cooling said sintered ceramic to ambient temperature without aging to provide said zirconia ceramic material.
In a further embodiment, the instant invention involves a process for making magnesia partially-stabilized zirconia comprises the steps of: (1) mixing zirconium dioxide powder, having a silica content of up to about 0.5 weight percent, and a magnesium-containing material consisting of magnesium oxide, a magnesium oxide forming material or mixtures thereof, said .
magnesium component being present in an amount to provide a magnesium oxide content from about 2.G to about 3.8 weight percent of the zirconia ceramic material; ~2) calcining said first mixture to form a . 20 calcined mixture: (3) milling said calcined mixture to form a milled mixture; (4~ drying said milled mixture to form a dried mixture; (5) compacting said dried mixture to form a compacted mixture; (6~ heating said compacted mixture to a soak temperature in the range of about 25 1675C and about 1800C to form a sintered ceramic; (7) cooling said sintered ceramic continuously to a temperature in the range of about 800C to about 1400C
at a rate between about 200C and about 1000C per hour;
and further cooling said sintered ceramic to ambient temperature without aging.
In a still further embodiment, magnesia partially-stabilized zirconia ceramics are provided by any of the . processes described above having a flexural strength ; greater than about 350 MPa, a critical stress intensity .

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factor greater than about 5 MPam~/2, and a Weibull modulus greater than about 10.

Description of the Preferred Embodiments It has now been found that magnesia partially-stabilized zirconias can be made using zirconia having a higher silica content than previously thought possible without the use of metal oxide additives. It has also been found that surprisingly an aging step thought to be necessary to control the formation of the desired microstructure can be eliminated.
The present invention further relates to the production of magnesia partially-stabilized zirconia ceramics capable of being used for a variety of purposes, including, for example, hot metal extrusion dies, wire drawing capstans, paper machine foils, projectiles and wear-resistant and/or corrosion-resistant articles.
In a preferred method of preparing the instant mag-nesia partially-stabilized zirconia, zirconium dioxide powder is combined with a magnesium-containing component which provides a magnesium oxide level of about 2.6 to about 3.8 weight percent of the final ceramic. The zir-conium dioxide powder consists essentially of up to about 0.5 weight percent silica. The process is particularly preferred with zirconium dioxide having a silica content of between about 0.05 weight percent and 0.35 weight percent. The zirconium dioxide powder can further comprise up to about 40 weight percent hafnia.
The zirconium dioxide powder can also contain Tio2~
Fe2O3 and S03 in amounts normally found in commercially available zirconium dioxide raw materials, e.g., up to about 0.2 weight percent Tio2, up to about 0.06 weight percent Fe203, and up to about 0.3 welght percent S03.

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Magnesium-containing materials useful as the magnesium component in the instant invention include magnesium oxide, a magnesium oxide forming material such as magnesium oxalate, magnesium acetate, magnesium hydroxide, magnesium carbonate, and mixtures of these materials either with or without magnesium oxlde. The hydrates of any of these magnesium compounds can also be used.
According to the present invention, extra silica-scavenging metal oxides having an affinity for silicaare not added to the zirconium dioxide-magnesium mixture. The terms "metal oxide" or "metal oxides" are used herein to refer to materials which can react or interact with silica when heated and the terms specifically exclude magnesium oxide and magnesium oxide forming materials. Such silica-scavenging metal oxides which are used in prior art processes include, for example, strontium oxide, barium oxide/ rare earth oxides and mixtures thereof. The effect of such metal oxide additives in the microstructure formation of magnesia partially-stabilized zirconias is described in Drennan & Hanneck, "Effect of SrO Additions on the Grain-Boundary Microstructure and Mechanical Properties of Magnesia-Partially-Stabilized Zirconia," J. M. Ceram.
Soc., 69, 541-46 (1~861-The zirconium dioxide powder and the magnesium-conta;ning material are combined by methods commonly used in the art, such as with a ball-mill, a twin-cone blender, or a V-blender. These materials are normally ; 30 combined at ambient temperature.
The resulting powder mixture can be calcined or milled directly. Calcination of the powder mixture is preferred for ease of handling the subsequent process steps and is accomplished at a temperature between about 800C and about 1700C, preferably between about 1000C

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and about 1600C, and more preferably about 1200C and about 1500C. ~s is known to those skilled in the ~ramic art, the calcination time can be adjusted to ensure that the bulk of the powdered mixture will reach the desired calcination temperature. The calcination time is normally between about 4 and 12 hours, preferably about 6 to about 10, and more preferably about 8 hours.
Milling can be accomplished by conventional dry-milling or wet-milling processes, until the median particle size is prefPrably less than about 3.0 microns, and more preferably between about 1.2 and about 2.6 microns. If the mixture is wet-milled, water and standard commercially a~ailable deflocculants are added to the milling slurry to form a flowable, high-solids content slurry normally having less than about 90~
solids. The deflocculants are added as necessary and in amounts sufficient to suspend particles and to lower the viscosity of the wet-milled mixture. Commercially ; 20 available anti-foaming agents, commonly used for this purpose and known to those skilled in the art, are preferably added to the milling slurry to minimize foamin~. The dry-milled or wet-milled mixture can be transferred to a ~lurry tank.
Organic binders can be added to the wet-milling slurry or to the dry-milled mixture. Such binders can be added, as required, to allow formation of a compact green body having suf~icient strength to allow formation to the desired shape.
Materials commonly used as organic binders in cera-mics include resins such as poly(vinyl butyral1, poly (ethylene glycol3, poly(ethylene oxide), poly(vinyl alcohol), methyl cellulose, vinyl acetate latex, para-finic hydrocarbons, poly(N, N'-ethylene-Bis-Stearamide), polymeric quinoline, potato starch, aqueous 1311 3~3~

acrylic emulsions, poly(ethylene glycol) resins of molecular weight from about 7,000 to about 20,000 and poly (ethylene oxide) resins of molecular weight from about l0,000 to about 300,000. Mixtures of ~uch binders can advantageously be used. The amount of hinder needed depends on the method of formation and the particular binder used and can be readily determined by those skilled in the art. Ordinarily the level of binder is between about 0.1 and about 7 weight percent of the calcined mixture.
- The wet-milled mixture is subsequently dried. A
convenient method of drying is by spray drying.
Commercially available devices generally known to those skilled in the art can be used to dry the mixture until the moisture content is no greater than about 0.5 weight percent water. The preferred particle size varies depending on the method of compaction. All screen designations given below are based on the Tyler Equivalent (TE) mesh sizes. For dry pressing, the particle distribution after drying is preferably in the range of about 20 percent less than TE 48 mesh and greater than T~ 80 mesh, about 20 percent less than TE
200 mesh, with the remainder within this range. For isostatic pressing, the particle distribution is preferably about 10 percent less than TE 48 mesh and greater than TE 80 mesh, 40 percent less than TE 200 mesh, with the remainder within this range.
The dried powder mixture can then be formed into a compact of the desired shape by standard commercial processes, such a dry pressing, slip casting, injection molding, extrusion, or preferably by isostatically pressing the powder at a pressure abo~e about 2,000 psi (13789.6 kPa~, more preferably above about 20,000 psi - (137396 kPa). The compact can then be mechanically 3S formed to the desired shape by use of a lathe, by _g_ 3~35 milling or by other methods generally known to those skilled in the art.
The formed compact is then heated from ambient temperature at a rate of between about 25C per hour and about 250C per hour, preferably between about 50C and about 100C per hour, to a soak temperature of between about 1675C and about 1800C, preferably between about 1700C and about 1750C to form a sintered ceramic. The soak temperature is maintained for between about 1 and about 10 hours, and preferably about 2 to 6 hours. As is known to those skilled in the art, the preferred soak time depends on the size of the compact; i.e., for laxger compacts, a longer soak time is preferred to ensure the formation of a substantially single phase cubic structure and to obtain the preferred density of above about 5.3 grams per cubic centimeter (g/cc).
The sintered ceramic is then cooled without aging at a rate between about 200C and about 1000C per hour, preferably about 250C to about 500C per hour, to a temperature between about 800C and 1400C, preferably between about 800C and about 1100C. This cooling rate can be accomplished at a more rapid cooling rate followed by a slower cooling rate to provide an average cooling rate within the above-stated ranges. This cooling rate must be controlled to the stated preferred temperatures so that tetragonal phases precipitate and are maintained without substantially transforming into the monoclinic phase. If the compact is cooled too slowly from about 1720C, it is presently believed that tetragonal phases transform to the monoclinic form resulting in loss of strength. In contrast, if the compact is allowed to cool too rapidly, a weaker c~ramic is produced because it is believed that the tetragonal precipitates are too small to transform into the monoclinic phase upon applied stress.

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The sintered ceramic is then cooled to ambient temperature without aging. The cooling rate at this point has not been found to be critical. Any means known in the art can be used, including, for example, furnace cooling, subjecting the sintered material to room temperature, or shutting off all rate-controlling instruments. The term "without aging" is used herein to mean that the sintered ceramic is not subjected to an isothermal hold or an annealing process i.e., the sintered ceramic is continuously cooled to ambient temperature and is not reheated to annealing conditions.
Subsequent surface finishing can be performed if desired.
The present invention also relates to magnesia partially-stabilized 2irconia ceramics produced by the above-described process. These ceramics are characterized as having a flexural strength preferably greater than about 350 MPa, more preferably greater than about 600 MPa, a critical stress intensity factor preferably greater than about 5 MPaml/2, more preferably greater than about lO MPam1/2, and a Weibull modulus greater than about lO, and more preferably greater than about 18.
The following examples are intended by way of ~- 25 illustration and not by way of limitation.

A ceramic article was prepared using zirconium dioxide powder reported to contain 99 percent zirconium dioxide plus hafnium dioxide in which the hafnium dioxide accounted for approximately 2 weight percent of - the total, about 0.2 weight percent sio2, about 0.15 weight percent Tio2, about 0.02 weight percant Fe203, and about 0.25 weight percent S03. The material was also reported to have about 0.30 weight parcent loss on ~3~ 3~

ignition at 1400C, a tamped bulk d~nsity of 2.4 g/cm3, an average particle size of 14 microns, and a specific surface area of between 2 and 4 m2/g. The zirconium dioxide powder was mixed with reagent grade magnesium carbonate in proportions such that the effective magnesium oxide content upon firing comprised 3.0 weight percent of the mixture. The mixed powders were calcined at about 1440C for about 8 hours. The calcined mixture ; was wet-milled to provided an average particle size of about 1.5 micrometer. An organic binder was added to the wet-milled slurry in an amount of approximately 1.7 weight percent based upon the dry calcined mixture. The binder consisted of a 20,000 molecular weight polytethy-lene glycol) resin. The resulting slurry was spray dried to form a powder. The resulting powder was isostatically pressed at about 20 kpsi to form a compact having the approximate dimensions of 0.25 inches (") x 0.125" x 2.0" and weighing about 5.91 grams. The compact was then formed on a lathe to the desired shape.
~he formed compact was fired by heating from ambient temperature at a rate of about 100C per hour to about 1720C. This temperature was maintained for approximately 4 hours after which the sintered article was cooled at an average rate of about 400C per hour to 1000C. The sintered article was then furnace cooled to room temperature and subsequently ~inished to the desired configuration by diamond grinding. The resulting ceramic product exhibited a flexural strength of 610 MPa, a critical stress intensity factor of ll MPaml/2, and had a density factor of 5.77 g/cc.

~ EXAMPLE 2 ; A ceramic article was prepared using zirconium dioxide powder reported to contain 99 percent zirconium dioxide plus hafnium dioxide in which the hafnium ~3~3~

dioxide accounted for approximately 2 weight percent of the total, about 0.2 weight percent SiO2, about 0.15 weight percent Tio2~ about 0.02 weight percent Fe2O3, and about 0.25 weight percent SO3. The material was al~o reported to have about 0.30 weight percent loss on ignition at 1400C, a tamped bulk density of ? . 4 g/cm3, an average particle size of 14 microns, and a specific surface area of between 2 and 4 m2/g. The zirconium dioxide powder was mixed with reagent grade magnesium carbonate in proportions such that the effective magnesium oxide content of the final ceramic comprised 3.0 weight percent of the mixture. The mixed powders were calcined at about 1440C for about 8 hours. The calcined mixture was wet-milled to provided median particle size of about 1.5 microns. An organic b~nder was added to the wet-milled slurry in an amount of approximately 1.7 weight percent based upon the dry calcined mixture. The binder consisted of a poly(ethy-~ lene glycol) resin of molecular weight between about ; 20 15,000 and about 20,000. The resulting slurry was spray dried to form a powder. The resulting powder was isostatically pressed at about 20 kpsi to form a compact having the approximate dimensions of 0.25" x 0.125" x 2.0" and weighing about 5.88 grams. The ~ompact was then formed on a lathe to the desired shape. The formed compact was fired by heating from ambient temperature at a rate of about 100C and about 150C per hour to about 1720C. This temperature was maintained for approximately 2 hours after which the sintered article was cooled at an average rate of about 400C per hour to 1000C. From room temperature the sintered article was reheated at a rate of about 100C per hour to a soak temperature of 1100C. This temperature was maintained for approximately 2 hours, after which the annealed article was cooled to room temperature. The final ~' '"' : , ,; :, ~.

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ceramic material produced by this method ~xhibited a flexural strength of 365 MPa, a critical stress intensity ~actor of 7 MPaml/2 and had a density factor f 5.74 g/cc.

A ceramic material was prepared in the same manner as Example 2, except that (1) about 0.25 weight percent SrO was added to ~he zirconium dioxide-magnesium carbonate mixture and (2) no aging (i.e., no annealing) was performed. The final ceramic material produced by this method exhibited a flexural strength of 470 MPa and had a densi~y factor of 5.74 g/cc.

; A ceramic material was prepared in the ~ame manner as Example 2, except that the sio2 content of the zirconium dioxide powder was 0.01 weight percent. The final ceramic material made by this method exhibited a Elexural strength of 520 MPa and had a density of 5.5 g/cc.

A ceramic material was prepared in the same manner as Example 3 except that the sintered article was ; reheated from room temperature at a rate of about 100C
per hour to an aginq temperature of 1100C. This temperature was maintained ~or approximately 2 hours after which the aged article was cooled to room temperature. The ceramic article exhibited a flexural strenqth of 410 MPa and had a density of 5.76 g/cc.

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TABLE l Flexural Critical Strength Intensity ~actor Density 5 Example No. (MPa) _ _~MPaml/2) ~/cc) l 6~0 11 5 77 3 470 -- 5.73 : lo 4 520 - 5 55 As shown in Table 1, the ceramic product prepared by one process of the instant invention, as described in Example 1, exhibited superior properties over other processes described in Examples 2-50 :Example 2 shows the effect of an aging period on such properties, while Example 3 describes the effect of adding a silica-scavenging metal oxide. In view o~ the prior art, Example 4 surprisingly shows that use of a zirconium dioxide powder having a very low silica content produced worse results than use of a zirconium powder having a 0.2 weight percent silica. Finally, Example 5 describes a process that uses a metal oxide additive and aging which also produced a weaker and less dense ceramic compared with the ceramic produced by the present invention.

~: Although the preferred embodiments have been ~: described by way of illustration and example, a number ~:~ of variations and modifications of the invention, as :~ 35 known to those skilled in the art, can be practiced :within the scope of the present invention as limited only by the appended claims.

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Claims (23)

1. A method for making magnesia partially-stabilized zirconia ceramic materials, comprising the steps of:
forming a first mixture by mixing zirconium dioxide powder having a silica content of up to about 0.5 percent by weight and a magnesium component consisting of magnesium oxide, a magnesium oxide forming material or mixtures thereof said magnesium component being present in an amount to provide a magnesium oxide content of about 2.6 to about 3.8 weight percent of said zirconia ceramic;
compacting said first mixture to form a compacted mixture;
heating said compacted mixture to a soak tempera-ture in the range of about 1675°C and about 1800°C to form a sintered ceramic:
cooling said sintered ceramic continuously to a temperature in the range of about 800°C and about 1400°C
at a rate to precipitate and maintain tetragonal phases without substantially transforming said phases into monoclinic forms; and further cooling said sintered ceramic to ambient temperature without aging to provide said zirconia ceramic material.

2. The method of Claim 1, wherein said magnesium oxide forming material is selected from the group con-sisting of magnesium oxalate, magnesium acetate, magnesium hydroxide, magnesium carbonate, hydrates thereof or mixtures thereof.

3. The method of Claim 1, wherein said silica content comprises up to about 0.35 weight percent of said zirconium dioxide powder.

4. The method of Claim 1, further comprising a step of milling said first mixture until a milled mixture is formed having a median particle size of less than about 3 microns.

5. The method of Claim 4, wherein said milling comprises wet-milling to form a wet-milled mixture.

6. The method of Claim 4, wherein an organic binder is added during said milling.

7. The method of Claim 6, wherein said organic binder is selected from the group consisting of poly (vinyl butyral), poly(ethylene glycol), poly(ethylene oxide), poly(vinyl alcohol), methyl cellulose, vinyl acetate latex, parafinic hydrocarbons, poly(N, N'-ethylene-Bis-Stearamide), polymeric quinoline, potato starch, aqueous acrylic emulsions, and mixtures thereof.

8. The method of Claim 5, further comprising drying said wet-milled mixture to form a dried mixture.

9. The method of Claim 8, wherein said drying step comprises spray drying.

10. The method of Claim 8, wherein said dried mixture has a moisture content of less than about 0.5 weight percent.

11. The method of Claim 1, wherein said compacted mixture is heated at a rate between about 25°C and about 250°C per hour to said soak temperature.

12. The method of Claim 1, wherein said compacted mixture is heated at a rate between about 50°C and 100°C
per hour to said soak temperature.

13. The method of Claim 1, wherein said soak temperature is held for a period sufficient to allow substantial formation of a single phase cubic micro-structure and to densify to at least about 5.3 g/cc.

14. The method of Claim 1, wherein said soak temperature is held for a period between about 1 hour and about 10 hours.

15. The method of Claim 1 wherein said first cooling step is at a cooling rate between about 200°C
and about 1000°C per hour.

16. A method of Claim 1 wherein said first cooling step is at a cooling rate between about 250°C
and about 500°C per hour.

17. A method for making magnesia partially-stabil-ized zirconia ceramic materials, comprising the steps of:
forming a first mixture by mixing zirconium dioxide powder, having a silica content of up to about 0.5 percent by weight, and a magnesium component consisting essentially of magnesium oxide, a magnesium oxide forming material or mixtures thereof said magnesium component being present in an amount to provide a magnesium oxide content of from about 2.6 to about 3.8 weight percent of said zirconia ceramic:
calcining said first mixture to form a calcined mixture;
milling said calcined mixture to form a milled mixture;
compacting said milled mixture to form a compacted mixture;
heating said compacted mixture to a soak tempera-ture in the range of between about 1675°C and about 1800°C to form a sintered ceramic;
cooling said sintered ceramic to a temperature in the range of between about 800°C and 1400°C at a rate of 200°C and about 1000°C per hour; and further cooling said sintered ceramic to ambient temperature without aging.

18. The method of Claim 17, further comprising a calcining at a temperature between about 800°C and about 1700°C for a period of between about 4 to about 12 hours.

19. The method of Claim 17, wherein said milling comprises wet-milling.

20. The method of Claim 19, further comprising drying said milled mixture.

21. The method of Claim 17, wherein an organic binder is added during said milling.

22. A magnesia partially-stabilized zirconia ceramic produced by a method comprising the steps of:
forming a first mixture by mixing zirconium dioxide powder having a silica content of up to about 0.5 percent by weight and a magnesium component consisting of magnesium oxide or a magnesium oxide forming material in proportions to provide a magnesium oxide content of said ceramic of about 2.6 to about 3,8 weight percent;
compacting said first mixture to form a compacted mixture:
heating said compacted mixture to a soak temperature in the range of about 1675°C and about 1800°C to form a sintered ceramic;
cooling said sintered ceramic to a temperature in the range of about 800°C and about 1400°C at a rate such that tetragonal phases precipitate and are maintained throughout the cooling period without substantially transforming into monoclinc forms; and further cooling said sintered ceramic to ambient temperature without aging to provide said zirconia ceramic.

23. A magnesia partially-stabilized zirconia ceramic of Claim 22 having a flexural strength greater than about 350 MPa, a critical stress intensity factor greater than about 5 MPam1/2, and Weibull modulus greater than about 10.