CA1281340C - Zirconia ceramics and a process for production thereof - Google Patents

Abstract

ABSTRACT

In the present invention, there is provided an easy sintering particulate starting material for use in producing zirconia ceramic, which material is coated with transition metal compound(s) on the surface thereof;
by adding (a) fully stabilized zirconia powder, partially stabilized zirconia powder, or alumina containing partially stabilized zirconia powder, or (b) precursor to generate the zirconia powder through pyrolysis thereof, into a solution or dispersion of the transition metal compound(s) used as a sinter activator to form a suspension. The raw particulate material enables sintering at a lower temperature and suppression of grain growth in the ceramics, and therefore, can produce zirconia ceramics of high strength and high toughness even by firing at atmospheric pressure . In accordance with the present invention, a novel Y2O3 partially stabilized zirconia ceramic as well as a novel alumina containing partially stabilized zirconia ceramic can be produced by the above-mentioned method. The resultant Y2O3 partially stabilized zirconia ceramics have a Y2O3 content of from 1.3 mol.% to 2.0 mol.%, the content of the tetragonal phase being 65% or more, and is a high strength ceramic having a fracture toughness of more than 10 MN/m3/2, reaching 16 MN/m3/2. The resultant alumina containing partially stabilized zirconia ceramic has a novel composition containing transition metal ozide, and is a high strength ceramic having Vickers hardness of 1100 to 1600 Kg/mm2, and improved heat shock resistance. Particularly, when based on the above-mentioned Y2O3 partially stabilized zirconia, the fracture toughness will reach 18.5 MN/m3/2.

Description

~2~3~340 SPECIFICATIO~

Titla of Invention ZIRCONIA CERAMICS AND A PROCESS FOR PRODUCTIOM THEREOF

The present invention relates to zirconia ceramics including fully stsbilized zirconia, partially-stabilized zirconia and partially-stabilized zirconia containing alumina (hereinafter refer generally to "zirconia", unless otherwise specified).
More particularly, it relates to a process for the production of easy sintering raw material powder to be used for tbe production of æirconia ceramics as well as a method of the production of high density zirconia ceramic materials which comprises moulding the above raw particulate material, and, then sintering, and ~urther to Y203 partially stabilized zirconia ceramic material of a novel compos~tion as well as alumina containin~
partially stabilized zirconia ceramic material.
Two species of zirconia ceramic material are known, i.e. fully stabilized zirconia ceramics wherein all crystal phase of ZrO2 constitutin~
the ceramics is the cubic phase, and the cubic phase has been stabilized even at the lower temperature range, and partially stabilized zirconia ceramics wherein the crystal phase of ZrO2 constituting the ceramics is the tetragonal phase, and the tetragonal phase has been stabilized even at the lower temperature range.

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~2~3~340 As a method for stabilizing ths crystal phase of ZrO2 constituting the ceramics, a process has been widely used which comprises adding a stabiliæer such a~ CaO, ~gO, Y203, and/or CeOz to the raw matsrial powder or the ceramics in order to fully stabilize or to partially stabilize the crystal phase of ZrO2.
Y203 particularly is widely utilized as a stabiliz;ng agent for the production of partially stabilized zirconia ceramics having high strength because ceramics having excellent stability and good mechanical properties are obtained.
Fully stabilized zirconia ceramics have been used as solid electrolytic media, or as heat resistant materials for furnaces etc., because of excellent thermal stability.
On the other hand, partially stabilized zirconia ceramics have been called a phase transformation toughening type zirconia, wherein it is considered that if an external mechanical stress is applied to ths ceramics, the tetragonal phase of Zro2 constituting the ceramics will transform martensite-like into the monoclinic phase that is the stable phase at a lower temperature range, consequently the ceramics may have high tenacity because the fracture energy is absorbed by the phase transformation.
Accordingly, it is known that partially stabilized zirconia ceramics are functional ceramics having high strength and high tenacity, and it is expected to use them for structural material such as mechanical materials, abrasion resistant materials, and cutting materials, and the like.

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Among these stabilized zirconia ceramics, it is necessary to produce dense ceramics having high density and controlled microstructure by suppressing the grain growth in the ceramics to have objective ~unctions, for example, in the fully stabilized zirconia ceramics, oxygen ion conductivity, thermal stability, mechanical properties and the like, and in the partially stabilized zirconia ceramics, such objective functions as mechsnical properties such as bending strength, tenacity and the like.
Dense zirconia ceramics have been produced having controlled microstructure by special moulding techniques, and high pressure sintering techniques such as the hot press technique or HIP method. However, these methods require complicated operation and special installations, and therefore, the resultin~ product will be very expensive.
On the other hand, a process has been proposed for the production of ceramics which comprises preparing a raw mateial powder by using chemical techniques such as co-precipitation or the like and then sintering a moulding of the raw material powder at a comparsbly low temperature range.
However, it is known that in general the more finely divided particulate material has stronger cohesive ~orce. Therefore, it is difficult to produce ceramics having high density with hi~h reproducibility from the chemically-treated raw powder.
Further, the addition of sintering activator has been proposed, for example, Jbpanese Laid Open Gazette ~o. SH0 50-10351 describes a process for the production of ceramics comprising moulding and sintering a raw material powder which is obtained by adding aqueous ammonia to an aqueous solution ~28~L3~LO

containing water~soluble 7irconium salt, water-soluble salts of calcium, magnesium, yttrium and the like as a stabilizing agentts), and water-soluble salt of a transition metal as sintering activator, so as to precipitate the desired co-precipitated hydroxide containing the desired metals, then, drying and calcining thereof. The raw material powder can not provide satisfactory lower temperature sintering characteristics nor enough relative density of the ceramics.
In the above-mentioned process, aqueous ammonia is ussd for precipitation. Some transition metals will form ammine complexes with ammonia so that in practice aqueous ammonia can not be used in the cases of such transition metals.
In order to avoid this shortcoming, there is a process wherein the oxides of the transition metals are used in place of the water-soluble salts of the transition metals so as to disperse in the solution containing the other components wherein the hydroxides of the other metal components are co-precipitated together with oxides of the transition metals.
~ owever, in this process, the surfacs of the oxide of the transition metal is coated with the hydroxides of the other components. Therefore, it is difficult to impart satisfactory effect for the sintering activator by the transition metal in the ceramics in small amounts.
It was reported that the content of Y203 in the ceramics can be dacreased to 2.0 mol.% for Y203 partially stabilized zirconia ceramics so that a fracture toughness(RIc) of approximately 10 M~/m can bs obtained for the ceramics having high tenacity.

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This means that partially stabilized zirconia ceramics containing Y2O3 in an amount ranging nearly to 2 mol.70 can evidence relatively high tenacity and strength.
F.F. Lange reported in Journal of ~aterials Science 17, 240-246 (1982) that "Thers is a ~ritical limit of t~e particle size of the tetragonal phase respectively to Y203 content, and when the size exceeds the critical limit, the tetragonal phase can not be present. Though the critical particle size is more than 1 m at a Y203 content of 3 mol.70, it decreases to the order of 0.2 ~m at a content of 2 molar ~".
As described before, the reduction of Y203 content in the Y203 partially stabilized zirconia ceramics is important in view of the high tenacity of the ceramics, and can be attained by suppressing the gro~th of crystal grains in the ceramics.
However, the suppression of the growth of crystal grains in the ceramics to control the size of the crystal grains to the order of equal to or less than 0.2~ m is extremely difficult by the prior art process for the production of the ceramics.
The characteristics of sintering at a lower temperature such that the grain size is controlled to approximately 0.2~ m or less cannot be attained even by using the raw material powder prepared by co-precipitation.
Therefore, the Y203 content in the Y203 partially stabilized zirconia ceramics has a lower limit of about 2 mol.%. A high strength Y203 partially stabilized zirconia ceramics having a Y2O3 content oÇ less than 2 mol.% is not known. Partially stabilized zirconia with a ~23 content approximating 2 mol.~ has the problem o heat deterioration and therefore, the prior art Y2O3 partially stabilized zirconia ordinarily uses a range o about 3 L3~
nol.% for the Y2O3 content.
As described above, the known partially stabilized zirconia is not satisfactory from the point of view of improving the mechanical strength ~nd stability, and therefore, ceramics wi~h higher tenacity and stren~th have been highly desired.
Japanese Patent Laid Open application Gazette Mo. SHO 60-86073 (1985) discloses a method to improve the mechanical properties of partially stabilized ~irconia ceramics by the addition of alumina to the composition of the ceramics.
However, such known partially stabilized zirconia ceramics containing alumina require special sinterin~ techniques such as the HIP process, and therefore, the ceramics products will be extremely expensive as described before.
The present invention provides in one aspect a process for the production of a raw material powder with characteristics of sintering at a lower temperature, which can be used for the production of dense zirconia ceramics as well as a process for the production of dense zirconia ceramics with controlled crystal structure.
ThP invention also provides in a particular embodiment a Y2O3 partially stabilized zirconia ceramics with high fracture tenacity, having a novel composition as well as a method for the production of the same.

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The present invention still further provides in another embodiment partially stabilized zirconia ceramics containin~ alumina as well as a process for the production of the same.
The present invention comprises a process for the productlon of an easy sintering raw material powder to be used for the production of zirconia ceramics which process comprises adding powder principally containin~
zirconium compound(s) together with stabiLizing agents, into a solution or slurry containing at least one species of transition metal co~pound(s) to form a suspension, then, removin~ the solvent from the slurry and dryin~ to obtain a powder product, and also a process for the production of high density zirconia ceramic, which comprises mouldin~ and sinterin~ the raw material powder prepared by the above-mentioned process.
The present invention also provides a Y203 partially stabilized zirconia ceramics bein~ characterized by a Y203 content in a ran~e of greater than or squal to 1.3 mol. percent and less than 2.0 mol. percent, the content of tetra~onal phase bein~ 6570 or more; and an alumina containing partially stabilized zirconia ceramics being characterized by the following ranges of composition: 99 to 40 mol. percent partially stabilized zirconia; 1 to 60 mol. percent ~-alumina; and transition metal oxide in an amount of 0.01 to 1 percent of the atomic ratio of transition metsls to the combination of Zr plus Al, which are produced by the above-mentioned process.
The transition metal compound(s) for this invention may be the oxides of at least one metal selected from the ~roup consistin~ of ~n, Fe, Co, ~i, Cu, and Zn as well as compound(s) which ~enerate the above-mentioned metal oxides by thermal decomposition.

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As to the transition metal compounds, there may be used inor~anic compounds such as oxides, hydroxides, nitrates, chlorides and the like of the above-mentioned metals; organic acid salts such as oxalates, acetates, propionates, higher fatty acid salt and the like of the above-mentioned metals; and organic metal compounds such as alkoxide compounds, chelate compounds and the like of the matals, if ,either soluble or insoluble in the solvents used. So1vent-soluble compounds are preferred.
The stabilizing agent for this imvention may include Y203, CaO, Mg~ or CeO2, as well as yttrium compounds, calcium coumpounds, magnesium compound, or cerium compounds to generate respectively ~2~3~ CaO, MgO, or CeO2, on thermal decomposition.
In the present in~ention, the powder principally containg zirconium compound(s) includes:
(a) fully stabilized zirconia powder, partially stabilized zirconia powder or stabilized zirconia powder containing alumina, and (b) precursor powders which generate fully stabilized zirconia, partially stabilized zirconia or alumina containing partially stabilized zirconia by thermal decomposition, all of which can be stabilized by adding the above-mentioned stabilizing agents.
The powder principally containing zirconium compounds can be the above-mentioned powder containing the stabilizing agents, which ara obtained by conventional processes such as the oxide method, the co-precipitation method, the hydrolysis method, the pyrolysis method and the like.

... , . , -1~8~34 Particularly, it is preferable to use a precursor powder which is obtained by drying the co-precipitated hydroxides or mixed carbonates prepared by adding as a precipitating agent, aqueous ammonia or ammonium carbonate to the mixed solution containing water-soluble zirconium compounds, and water-soluble yttrium compounds, water-soluble magnesium compounds, water-soluble calcium compounds, or water-soluble cerium compounds, and if desired, alumina powder, or water-soluble aluminium compounds.
In accordance with this invention, the powder principally containing zirconium compounds is added to the solution or slurry containing the transition metal compound, and ~hen the raw material powder is obtained by removing solvent from the suspended slurry and drying the residue.
The solvent used for dissolving or suspending the transition metal compound may be water and/or organic solvents, and the organic solvents are preferred because of convenient removal of the solvent by evaporation and because of less evaporation energy on drying.
The usable organic solvents are not limited, but the use of highly viscous solvents is not preferred because homogeneous suspension of the powder principally containing zirconium compounds and containing the transition metal compounds is difficult and further, removal and drying out of the solvent is then difficult. Preferably, lower alcohols as for example methanol, ethanol, propanol, buthanol and the like can be used.
The unit operation for suspension of the powder principally containing zirconium compounds into the solution or slurry containing the ~-~z~

transition metal compounds can be a simple agitating operation resultin~ in a satisfactory mixture, but when a grinding and mixing operation such as milling is used, a complete mixing effect is more certain.
The removal of solvent(s) and drying are carried out by conventional evaporation methods, but when the transition metal compound is insoluble in water or organic solvent(s), or when the precipitation has been obtained by applying a precipitating a~ent to the solution containing the soluble transition metal compounds, the solvent can be removed by filtration.
Further, spray drying can be used to treat efficiently and effectively the material powder on a large scale.
The resulting raw material powder can be used for the productlon of ceramics as it is, but, it can be calcined at a temperature in the range Prom 300 to 1200C for further treatment.
In the raw material powder obtained by the above-mentioned process, the transition metal compounds uniformly adhere and/or coat the surface of the raw material powder, so that it functions effectively as a sintering activator.
The raw material powder will produce dense ceramics by the easy sintering through firing at atmospheric pressure at the relatively lower temperature in the range of 1100 to 1500C.
The atomic ratio of transition metal to Zr, or to the combination of Zr plus Al, when the raw material powder contains alumina, may be 0.01 to 1.0 percent, preferably, 0.01 to 0.5 percent.
When the atomic ratio of the transition metal is less than 0.01 percent, the effect as sintering activator is insufPicient. Further, when it is more than 1.0 percent, the properties of the resultant ceramics will be affected and a range o~ ~reater than 1.0 percent should be avoided.

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In accordance with the present invention, the zirconia ceramics are produced by moulding the raw material powder obtained by the above process, and then by sintering the moulded body.
The moulding may be press mouldin~ by using conventional moulding techniques, but it is preferable to further apply a hydrostatic compression after low pressure moulding, so as to improve the sintered density and the mechanical strength of the finished ceramics.
The sintering may be any of the known methods, and by atmospheric sintering under atmospheric pressure, the object of sintering is sufficiently improved.
Generally, the sintering temperature can be in the range of from 1100 to 1700C.
In order to control the growth of crystal grains in the ceramics for the production of dense ceramics, particularly high strength partially stabilized zirconia ceramics, a lower sintering temperature is better, and therefore, the range of 1200 to 1500C is preferable.
In accordance with such method, since the raw material powder has good sintering characteristics, the use of the atmospheric firing at a lower temperature can easily produce dense ceramics having relative density in relation to the theoretical density of more than 99qO.
In accordance with the present invention~ Y2O3 partially stabilized zirconia ceramics havinG a tetragonal phase content of 65% or more and high tenacity and high strength can be obtained by limiting the raw material powder to a powder of Y203 partially stabilized zirconia or ~.~8~3~) precursor powder which produces Y203 partially stabilized zirconia on thermal decomposition, in which said powder has a crystal particle size of 400 or less and B~T specific surface of 2 m /g or more.
In the production of the ceramics, when the particle si~e of the powder principally containing zirconia compounds e~ceeds 400 A, or when the ~ET specific surface is less than 2 m /g, the sinter activating effect by the transition metal compound(s) is insuf~icient and then ceramics of high enou~h density can not be obtained by atmospheric sintering at the lower temperature.
The atomic ratio of the transition metal to Zr in the production of the ceramics may be in a range of from 0.01 to 1.0%, preferably 0.01 to 0.57O.
The sintering temperature is preferably not more than 1400C.
In accordance with the present invention, by the above-mentioned process for the production of the ceramics, novel Y203 partially stabilized zirconia ceramics having a Y203 content of 1.3 mol.~ or more, but less than 2.0 mol.~ and having a tetragonal phase content of 6570 or more can be produced.
The Y2O3 content is based on the total combination of Y203 plus ZrO2 in the ceramics.
The ceramics products have sintered density of at least 5.8 g/cm , preferably more than 5.9 g~cm , and more preferably 6.0 g~cm or more, and fracture tenacity value (KIc) in the range of from 10 MN~m to 16 MN~m and therefore, are of high density, of high tenacity and of hi~h strength.

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The size of crystal grains in the ceramics may be 0.5 m or less, preferably 0.3 ~m or less, and the content of the tetragonal phase in the ceramics is 65qo or more.
When the Y203 content is less than 1.3%, the content of the monoclinic phase will increase and the tetragonal phase content of 65% or more is maintained with difficulty. On the other hand, ceramics having a Y203 content of 2.0 mol.% or more, have been known, and the fracture tenacity value theraof can not be more than 10 MN/m When the size of the crystal graLns in the ceramics exceeds 0.5~ m, it is extremely difficult to keep the content of the tetragonal phase 65~ or more. In addition, when the grain size is 0.3 ~m or less, the stability of the ceramics under heat stress will be improved and the mechanical strength of the ceramics will be stabilized.
Further, when the sinterin~ temperature exceeds 1400C, the grain growth in the ceramics will be activated, and the grain size becomes more than 0.5 ~m, and then, only the ceramics having a relatively higher content of monoclinic phase will be produced, and further cracks may be caused during firing.
The atomic ratio of the transition metal to '~r in the particulate raw material powder may range from 0.01 to 1.0~. When the atomic ratio exceeds 1.0%, the characteristics of the ceramics will be undesirably affected.

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~Z8~34~) In accordance with the present invention, the novel alumina containing partially stabiliæed zirconia ceramics comprising the composition of 99 to 40 mol.~ partially stabilized zirconia, 1 to 60 mol.7J~-alumina, and transition metal oxide havin~ the atomic ratio thereof to the combination of Zr plus A1 ranging from 0.01 to 170 can be produced.
The resultant ceramics have high tenacity and high hardness, in that the fracture tenacity is of the order of 18.5 U~Jm ~ and the Vickers hardness reaches to 1600 kg/mm3, as well as excellent heat shock resistance.
In accordance with the process of the invention, the powder principally containing the zirconium compound material which can be used for the production of the ceramics may be (a) a mixed powder comprising partia~ly stabilized zirconia of 99 to ~0 mol.%
and alumina of 1 to 60 mol.~; or (b) precursor powdsr to produce the above powder on thermal decomposition~
and the above powder is added to a solution or slurry containing the transition metal compounds to form a suspension, then followed by removing the solvent therefrom and drying to obtain a raw material powder having the transition metal in an atomic ratio of the transition metal(s) to the combination of Zr plus Al ranging from 0.01 to 1%, and then, the material is moulded and sintered, resulting in the desired ceramics.
The ceramics may be Y203 partially stabilized zirconia ha~ing a Y2O3 content in the range of from 1.3 to 4 mol.~, or Y2O3 partially stabilized zirconia wherein a part or all of the Y2O3 stabilizer is substituted by CaO, MgO, or CeO2, and the content of the stabilizer is 0.01 to 12 mol.%

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The content of the stabilizin~ agent such as Y203 is based on the total amount of Zro2 and the presumed oxide for the stabilizing agent.
In the case of Y203 partially stabilized zirconia, when the Y203 content is less than 1.~ molar70, the ratio of monoclinic phase will increase even in the presence of Al203, and then it is difficult to keep the amount of tetragonal phase to 65% or ~nore. When the Y203 content exceeds 4 mol.%, the fracture tenacity va'Lue of the ceramics will decrease.
When the Al203 content in the ceramics is less than 1 mol.%, high enough hardness can not be obtained. Further, when tbe Al203 content is more than 60 mol.%, it is difficult to produce ceramics having high enough density.
When the atomic ratio of the transition metal to the combination of Zr plus A1 in the ceramics is less than 0.01~, ceramics with high enough density can not be obtained. Further, when such atomic ratio is more than 1 mol.%, the sintering chara~teristics of the ceramics will be degraded.
The grain size of zirconia in the ceramics may be 2~ m or less, and preferably 0.5~ m or less. The content of the tetragonal phase in the ceramics may be 65~ or more, and preferably 8070 or more. The ~rain size of Al203 in the ceramics may be 4 ~m or less, and preferably 2~ m or less.
The resultant alumina containing partially stabilized zirconia ceramics has extremely hi~h hardness such as Vickers hardness in the range of from 1100 to 1600 Kg/mm , and particularly excellent heat resistan~e such as a bending strength of 85 Kg/mm or more, even after heat treatment at 200C
for 1000 hours.

' -15-~L~8~L3~0 In the production of the raw material powder, the powder principally containing zirconium compounds may be: a mixture of partially stabilized zirconia powder having ~rain size of ~00 A or less, and BET specific surface of 2 m /g or more; or a precursor powder which generates partially stabilized zirconia on thermal decomposition; and-4-alumina powder having Brain size of 1.0~ m or less, and BET s~ecific surface of 2 m /g or more9 or a precursor powder ~hich generates alumina on thermal decomposition.
When partially stabilized zirconia powder or precursor powder has a grain size of more than 400 A, or when the BET specific surface thereof is less than 2 M /g, the sinter activating effect of the transition metal wilL
be decreased so that atmospheric sinterine at less than 1500C can not produce ceramics with high enough density.
The invention is illustrated by the following examples but these are not limiting to the scope of the invention.

Example 1: Production of Easy Sintering Raw Material Powder and Ceramics (1) Production of Raw Material Powder for production of ceramics.

Sample (1-1) To a solution containing ZrOCl2, and YC13 in a ratio of Y~03(Y2O3+ZrO2)=0.03 by oxide molar base calculation, was added aqueous ammonia to regulate the pH of the solution in order to produce co-precipitation of the combined hydroxides. The resultant co-precipitated L3~0 hydroxides were filtered and dried, and then the precursor powder of Y203 partially stabilized zirconia was obtained. By calcining a part of the obtained precursor powder at 800C for one hour, a powder of Y203 partially stabilized zirconia was obtained. For the transition metal compounds, the following compounds were dissolved or dispersed in ethanol to prepare, respectively, a solution or slurry of the transition metal compounds.

Solution Slurry Mn:MN(CH3CH00)2 4H2o MnO2 pe Fe(N03)2 9H2 ~e(OH)3 Co:Co(CH3Coo)2 4H2o CoO
Ni:Ni(No3)26H2o N'i(O'~)2 Cu:Cu(CH3CO0)2 CuO
Zn:Zn( CH3CO0)2.2H20 ZnO

To the prepared solution or slurry of the transition metal compounds, the partially stabilized zirconia powder was added to form a suspension, then, the ethanol distilled off and the residue dried to obtain a raw material powder for use in the production of Y203 partially stabilized zirconia ceramics.

Sample (1-2) In accordance with the same conditions for the production of Sample (1-1) except for using a precursor powder for Y~03 partially stabilized 34~

zirconia in placs of the powder of Y203 partially stabilized zirconia, the adhering treat~ent of the transition metal compound(s) was carried out, and then, the resultant material was calcined at 800C for one hour, yielding Y203 partially stabilized zirconia raw material powder for the production of zirconia ceramics.

Sample (1-3) The pH of a solution containing ZrOCl2 and CaCl2 in the ratio of CaO/(CaO+ZrO2)=0.12 by oxide molar calculation for the oxides was adjusted by adding aqueous ammonia to co-pracipitate mixed hydroxidas. The co-precipitated mixed hydroxides were filtered, dried and then calcined at 800C for one hour to obtain CaO fully stabilized zirconia powder.
The prepared CaO fully stabilized zirconia powder was treated under the same conditions as for Sample (1-1) with transition metal compounds, obtaining CaO fully stabilized zirconia raw particulate material (1-3) for the production of the zirconia ceramics.

Sample (1-4) The pH of a solution containing ZrOCl2 and MgCl2 in the ratio of MgO/(MgO+ZrO2)=0.081 by molar calculation for the oxides was adjusted by adding aqueous ammonia to co-precipitate mixed hydroxides. The co-precipitated mixed hydroxides were filtered, dried and then calcined at 800C for one hour to obtain M~O partially stabilized zirconia powder.

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The prepared M~O partially stabilized zirconia powder was treated under the same conditions as for Sample (1-1) with the transition metal compounds, obtainin~ MgO partially stabilized zirconia raw material powder (1-4) for the production of zirconia cer~mics.

Sample (1~5) A solution containing ZrOC12 and CeC14 in the ratio o~
CeO2~(CeO2+ZrO2)=0.08 by oxide molar calc~lation for the oxides was adjusted by adding aqueous ammonia to precipitate mixed hydroxides. The precipitated mixed hydroxides were filtered, dried and then calcined at 800C
for one hour to obtain CeO2 partially stabilized zirconia powder.
The prepared CeO2 partially stabilized zirconia powder was treated under the ssme conditions as for Sample (1-1) with the transition metal compounds obtainin~ CeO2 partially stabilized zirconia raw particulate material (1-5) for the production of zirconia ceramics.

Reference Sample (Cl-1) To the starting aqueous solution used for the production of the precursor powder as in Sample (1-1), was added the transition metal compound and then it was treated to precipitate mixed hydroxides containin~ the transition metal compound. The precipitated mixed hydroxides were filtered, and dried, obtaining raw particulate material tCl-1) for reference.

~8~4C9 Reference Sample (C2-2) The treatment for preparation of Sample (1-1) was carried out but omitting the deposition of the transition metal compound, in order to prepare Y203 partially stabilized zirconia precursor powder which was referred as raw material powder (Cl-2) for reference (hereinafter referred to as "untreated powder").

Reference Sample (Cl-3) "Untreated powder" of CaO fully stabilized zirconia produced by the process for production of Sample (1-3) was used as raw powder (Cl-3) for reference.

Reference Sample (Cl-4) "Untreated powder" of NgO partially stabilized zirconia powder prepared by omitting the deposition of the transition metal com~ound from the preparation process for Sample (1-4) was used as raw powder (Cl-4) for reference.

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Reference Sample (C1-5) "Untreated powder" of CeO2 partially stabilized zirconia powder prepared by omitting the deposition of the transition metal co~pound from the preparation method of Sample (1-5) was used as raw powder (C1-5) for reference.

(2) Production of Ceramics The raw material powder as before obtained was pressure molded under 2 pressure of 200 Kg/cm , and then hydrostatic compression applied to the product mouldings under a pressure of 2 ton/cm2, and mouldin~s having the desired shape were obtained. The obtained mouldings were fired under atmospheric pressure at the given temperature for three hours, producing zirconia ceramics.

(4) Evaluation Test The density of the resultant ceramics was measured and the three point bending test of a portion thereof based on JIS (Japan Industrial Standard) R 1601 (1981) was carried out.
Table 1 indicates the results of the following tests: on raw material powder; Samples (1-1), (1-2) and reference Samples (Cl-1)-(C1-2), in atomic ratio of the transition metal to Zr, the density and relative density . 1~

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to the theoretical density of the resultant Y2O3 partially stabilized zirconia ceramics produced from each raw material, and bending strength (average from 5 points) of the resultant ceramics.
Table 2 indicates the results of the tests on raw material powder;
(Samples (1-3~, (1-4), (1-5) and reference Samples (Cl-3) to (Cl-5), in atomic ratio of the transition metal to Zr, the density and relative density to the theoretical density of the resultant Y2O3 partially stabilized zirconia ceramics produced from each ra~l material, and bending strength ~average from 5 points) of the resultant ceramics.
The theoretical density of the ceramics are as follows:
Y203 partially stabilized zirconia ceramics: 6.10 ~/cm CaO fully stabilized zirconia ceramics: 5.68 g/cm MgO partially stabilized zirconia ceramics: 5.80 g/cm CeO2 partially stabilized zirconia ceramics: 6.23 g/cm In the following tables, (A) means that the transition metal compound(s) was adhered by the solution method, and (B) means that the transition metal compound(s) was adhered by the slurry method.

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raw material 1200 C riring 1300 C riring transition metal sintered relative bend sintered relative bcnd adhere adhered density density strength density density strength method atom % g/cm3 % kg/mm2 g/cm3 % kg~mm2 _____________________________________ ._________________ _________________ __ 1-2 Mn (A) 0.3 5.90 96.7 - ____5.98 98.0 ----1-1 Mn (A) 0.05 5.90 96.7 ____6.00 98.11 ----Mn ~A) 0.1 5.95 97.5 ---- 6.01 9805 ----Mn (A) 0.2 5.97 97.9 ---- 6.0Z 98.7 _~__ Mn (A) 3 5.96 97.6 ____6.02 98.7 100 Mn (A) 1.0 5.94 97.4 ---- 6.02 98.7 ____ Mn (8) 0.05 5.82 95.4 ---- 5.93 97.2 ____ Mn (8) 0.1 5.88 96.7 ---- 5 97 97 9 ~~~~
Mn (8) 0.2 5.90 96.7 ---- 5.99 98.~ ____ Mn (B) 3 5.91 96.9 956.00 98.4 100 Mn (8) 1.0 5.90 96.~ ____6.00 o8.4 ____ ____________________________________________________________________________ Fe (A) 0.05 5.72 93.9 ____5.92 97 0 ____ Fe (A) 0.1 5.73 93.9 ---- 5.94 97.4 ----Fe (A) 0.2 5.84 95.7 ____6.02 98.7 ___ Fe (A) 0.3 5.84 95.7 ___6.03 98.9 101 Fe (8) 0.3 5.80 95.1 845.98 98.0 98 ____________________________________________________________________________ Co (A) 5 5.89 96.6 ---- 5.99 98.2 ----Co (A) 0.1 5.95 97.5 ---- 6.oo 98.4 ___ Co (A3 0.2 5.89 96.6 ---- 6.02 98.~ ____ Co (A) 0.3 5.87 96.2 ---- 5.02 98.7 100 I Co (13) 0.3 5.~1l95.7 885.98 ~8.(~ 9~
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raw matcrial 1200 C ril~ing 1300'C riring transition metal sintered relative l~end sintcred relative bend adhere adhered density density stren~th density density Strength method atom ~ g/cm3 % ~g/mm2 g/cm3 % kg/mm2 ______________________________________ _ __ ____ ____ ________ ________ 1-1 Ni lA) 5 5.8996.6 90 5.99 98.2 90 Ni (A) 0.1 5.9297.0 90 5.99 98.2 95 Ni (A) 0.2 5.9998.2 93 6.02 98.7 98 Ni (A) 0.3 6.0298.7 96 6.o7 99.5 110 Ni (A) 1.0 6.0198.5 86 6.07 99.5 90 Ni (B) 0.3 5.9497.4 9 5.98 98.o 98 ____________________________________________________________________________ 1-2 Zn (A) 0.3 5.9096.7 85 6.02 98.7 100 1-1 Zn (A) ~5 5.8896~4 80 6.05 99.2 105 Zn (A) 0.1 5.9096.7 80 6.07 99.5 113 ., Zn tA) 0.2 5.9998.2 go 6.o8 99.7 115 Zn (A) 0.3 5~9998.2 90 6.o, 99.5 115 Zn (A) 1.0 5.9898.o 7~ 5.07 99.5 90 Zn (B) 0.3 5-9497 4 87 6.01 98.5 100 ____________________________________________________________________________ 1-2 Cu (A) 0.3 6~0198.5 88 6.o3 98.9 99 1-1 Cu (A) 0.05 5.9697.7 92 6.o5 99.2 108 Cu (A) 0.1 5.9998.2 95 6.07 99.5 115 Cu (A) 0.2 6.0298.7 99 6.09 99.8 120 Cu (A) 0.3 6.0599.2 ln5 6.o7 99.5 113 Cu (A) 1.0 6.o599.2 90 6.05 99.2 go Cu (B) 0.05 5.9297.0 ____ G.ol 98.5 ___ Cu (B) 0.1 5.9697.7 ---- 6.ol 98.5 ____ Cu (~) 0.2 5.9~~/.7 ---- fi.~)2 98.

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raw matcrial _ 1200 C riring 1300 C riring transition mctal sintered relative bend sintered relative bend adhere adhered density density strength density density strength method atom % g/cm3 Z kg/mm2 g/cm3 S kg/mm2 ___________________ ________________________________________________________ C1-1Mn (A) 0.3 5.3384.4 ---- 5-5090.2 ----Mn (B) 0,3 5.25 86.138 5.50 90.265 Fe (A) 0.3 5-38 88.2 ---- 5.7 93-4~~~~
Fe (B) 0.3 5.30 86.943 5.55 91.068 Zn (A) 0.3 5-38 88.2113 5.60 91.852 Zn tB) 0.3 5.28 86.640 5.48 89.869 Cu (B) 0.3 5.30 86.942 5.52 90.560 C1-2 ---- ---- 5.28 86.640 5.48 89.869 raw material 1200'C firing 1300 C firing transitioo metal sintered relative bend sintered relative bend adhere adhered density density strength density density strength method atom % ~/cm3 ~ kg/mm2 g/cm3 ~ _g/mm2 ____________________________________________________________________________ El-3 Mn (A) 0.3 5.24 95.5 __~ 5.56 97 9 ____ Fe (A) 0.3 5.56 96.1 ---- 5-63 99.1 ____ Co (A) 0.3 5.14 95.5 ____ 5.61 98.8 ----Ni (A) 0.3 5.23 97,4 ____ 5.67 99.8 ____ Zn (A) 0,3 5.23 97.1 ---- 5.57 98.1 ----Cu (A) 0.3 5~55 97~7 ~~~~ 5.67 99.8 ____ _______________________________________________________________________._____ 1-4 Mn (A) 0.3 5~73 98.8 ---- ---- -___ ____ Fe (A) 0.3 5-76 99.3 ---- ---- ____ ____ Co (A) 3 5.69 98.1 ---- ---- ____ _ __ Ni (A) ~3 5.69 98.1 ---- ---- -___ ____ ~n [A) 0.3 5.7i 98.4 ---- ---- ____ __ _ Cu (A) '3 5.69 98.1 ---- ---~ ~~~~ ~~~~
____________________________________________________________________________ 1-5 Mn (A) 0.3 6.20 99.5 ---- 6.21 99.7 ----Ni (A) 0.3 6.18 9~.1 ____ 6.20 99.5 ----Cu (A) 0.3 6.22 99.8 -___ 6.22 99.8 ____ ____________________________________________________________________________ C1-3 ---- ---- 4.85 85.4 ---- 5.30 93.8 ____ C1-4 ---- ---- 5.43 93.6 ---- ---- -___ ____ Cl-5 _--- -_.-- 5.80 93.1 ____ 5.98 96.0 ----~313~

Example 2: Y203 Partially Stabilized Zirconia Ceramics and Its Production (1) Preparation of raw particulate material The same procedure as that for the production of Sample (1-1) in Example 1 was carried out but chan~ing the mixture ratio of ZrOC12 and YC13 to produce co-precipitates of hydroxide.

The obtained co-precipitates of hydroxide were treated under the same conditions as those of examyle 1 to produce partially stabilized zirconia powders with different Y203 contsnts.

Sample (2-1) The resultant partially stabilized zirconia powders were treated with the solution used in example 1 under similar conditions to produce raw particulate material for the production of ceramics.

Sample (2-2) The dried co-precipitates of hydroxide were treated in a similar way to that for sample (2-1) and further, calcined at 800C for one hour to produce the raw material powder (2-2) for the production of ceramics.

.

3~

Sample (2-3) Partially stabilized zirconia powders as before produced were treated with the slurry used in example 1 under similar conditions to produce raw material powder (2-3).

(2) Production of ceramics Raw materials (2-l) to ~2-3) were moulded under similar conditions to that of Example 1 and then fired at atmospheric pressure for 3 hours at a given temperature to result in Y203 partially stabilized zirconia ceramics.
For reference, omitting the treatment with the transition metal compounds, raw materials wherein the Y203 content was less than 1.3 mol.%, the particle size was greater than 400 A, and the atomic ratio of the transition metal to Zr was greater than 1.0~ were used to mould and produce sintered ceramics. The firing temperature was 1500 C.

(3~ Characteristics of Partially Stabilized zirconia Powder and The Ceramics Hade Therefrom The following characteristics were meas~red on partially stabilized zirconia powder and ceramics obtained in the above items (1) and (2).
Table 3 shows the characteristics of raw material powder (Z-l) and the ceramics produced from that material.

~2~ 4~

Table 4 shows the characteristics of raw material powder (2-2) and (2-3~ and the ceramics produced from those materials.
(A) Size of partially stabilized zirconia particle: D
The size D can be calculated from the wiclth at the half value of the peak of X ray diffraction by the following Schellar's formula:

D=o.9 ~/s cos ~ ~: the wave l~ngth Or X ray B: the width at the halr value Or the dirfraction peak ~: the dirfractionangle (B) BET relative surface area of partially stabilized zirconia powder was measured by using micromeritics (machine manufactured by Shimazu Works).

(C) Fracture tenarity of partially stabilized zirconia caramics: KIc was measured by VicXers indent test.
The Vickers indenter was pressed against the polished surface of the samples, and the resultin~ indentation size and the resultin~ len~th of the generated cracX were measurad and RIc was calculated from the followin~
formula which ~iihara et al proposed. The applied indentation load was 50 k~f.

(~Ic~/Hal/2)(H/e~)O 4 o o3s (1~ )-1/2 ~: restraint moduras H: Vickers hardness E: modulus Or elasticity a: halr Yaluc Or dia~onal Icn~th Or indcllt.ltion 3~

(D) Bending strength of partially stabilized zirconia ceramics was measured in accordance with JIS R 1601 (lg81) rule.
A sample of 3x4x40 mm in size was used, and the measurement was carried out on a span length of 30 mm under crosshead speed of 0.5 mm/min. and the value was determined by average from five samples.

(E) Content of tetragonal phase in the partially stabili~ed zirconia ceramics.
The surface of the sample was polished by a diamond slurry containing 3 m size of diamond particles, and then, X ray diffraction measurement was carried out on that surface followed by the calculation of the following formula.

1 t Tetragonal phase content (%) = - - x loO
(lll)tf(lll)m+(lll~m ~lll)t: tetrasonal (111) face diffraction intensity (lll)m: monoclinic tlll) case diffraction intensity (lll)~: monoclinic (lll) face diffraction intensity (iiijt diffraction pcaf~ inciudes CUDiC ~iil k diffraction pea~, but tne calculation was carried out presuming that that peak is entirely by tetragonal difrraction.
(F) Graln size in the partially stabilized zirconia cersmics The grain size was measured by observing the ~racture face of the ceramics through a scanning type electron microscope. It was confimed that all samples except the reference sam*les had grain sizes rangin~ from 0.1 to 3~ m.

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Example 3: Alumina Containing Zirconia Ceramics ancl ~ethod of Production of the Same (1) Preparation of raw material powder for production of ceramics Sample (3-1) The procedures of Example 1 to produce a powder principally containin~ zirconium compounds was repeated under the same conditions as in Example 1, except for chan~in~ the amount of YC13, M~C12, CaC12, and CeC13, to be added, to prepare partially stabilized zirconia raw particulate materials.
A solution of tha resultant partially stabilized zirconia material, alumina powder, and nitrates of each transit`ion metal compound in ethanol were milled, then solvent removed therefrom and the residue dried to obtain raw material powder for the production of ceramics.

Sampla (3-2~
The procedure for Sample (3-1) to prepsre a material principally containin~ zirconiu~ compound was repeated under similar condâtions except to add alumina powder to the mixed solution of zirconium compounds and stabilizer, to prepare partially stabili7ed zirconia powder containing alumina.

12~3~L34~3 The alumina containing partially stabilized zirconia particulate material and a solution of nitrate of each transition metal compound in sthanol were milled, followed by removal of solvent therefrom and drying to obtain tha raw material powder (3-2) for the production of ceramics.

Sample (3-3) The procedures for Sample (3-1) to prepare a material consisting essentially of zirconium compound were repeated under similar conditions except for the addition of AlC13 powder to the mixed solution of zirconium compound and stabilizing agents so as to form a homogeneous mixture, in order to prepare partially stabilized zirconia powder containing alumina.
The partially s~abilized zirconia powder containing alumina and a solution of the nitrate of each transition metal compound in Pthanol were milled followed by removal of solvent therefrom and drying to obtain raw material powder (3-3) for the production of ceramics.

(2) Production of ceramics The raw material powders ~3-1) to ~3-3) were moulded under similar conditions to that of Example 1 into a desired shape, then fired at a given temperature for 3 hours under atmospheric pressure, thus oStaining alumina containing partially stabilized zirconia ceramics.
For reference, using the raw material powder prepared without treatment by the transition metal compounds the same procedure was repeated under the same conditions to sinter, obtaining reference ceramics.
Further, in comparison with raw material (3-1), the ceramics were produced under the same conditions by using a raw material with 1.0~ of Y203 content.

(3) Characteristics of raw material powder and ceramics The same properties as those of Example 2 were measured for the raw particulate materials as prepared and the ceramics as produced.
Further, ~ickers hardness and bending strength after thermal treatment at 200C and for 100 hours, of the resultant ceramics were measured.
Table 5 shows the properties of Sample t3-1) and the ceramics made therefrom.

The content of the tetrazonal phase in all the ceramics except for Reference Samples was 95% or more.
It can be confirmed that the grain size of ZrO2 in the ceramics as produced of all Samples except the Reference Samples was 2 ~m or less, and the particle size of the A1203 was 4 ~m or less.
Table 6 shows the properties of Sample ~3-2) and the cersmics made therefrom.
The content of tetragonal phase in all the ceramics except for the Reference Samples was 95~ or more.
It can be confirmed that the ~rain size of ZrO2 in the ceramics as produced of all Samples except for the Reference Samples was 2~ m or less, and the particle size of the A1203 was 4~ m or less.
Table 7 shows the properties of Sample S3-3) snd the ceramics made therefrom.
The content of tetragonal phase in all the ceramics except for the Reference Sample was 95~ or more.
It can be confirmed that the grain size of ZrO2 in the ceramics as produced of all Samples except for the Reference Samples was 2 ~m or le~ss, and the particle size of the A12O3 ~as 4~ m or less.

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The first aspect of the invention in this application is a process for preparation of easy sintering raw material powder for use in the production of zirconia ceramic materials.
The said process, as set forth in the above-mentioned examples, is an extremely simple process wherein the transition matal is deposited on the powder consisting essentially of zirconium compound.
In the process of the invention, the transition metal compounds are deposited uniformly on the surface of the powder particles consisting essentially of zirconium compounds so that the sinter activating effect can be obtained even at a lesser amount, and, as set forth in the examples, the prepared particulate material can be easily sintered.
Accordingly, the said process can be accepted widely for the preparation of particulate raw materials for use in the production of fully stabilized zirconia ceramics, partially stabilized zirconia ceramics, alumina containing partially stabilized zirconia ceramics, and-special zirconia ceramics.
A second aspect of the invention is a method for the production of high density zirconia ceramics.
This method is characterized by using the easy sintering raw particulate material prepared as described.
In this method, as set forth in the above-mentioned examples, sintering at atmospheric pressure is possible, and any special equipment of operation is unnecessary to produce high enough density and a~ceptable enough mechanical properties of the desired specified zirconia ceramics.

L3~

In the preceding examples, only sintering at atmospheric pressure was used, but other techniques such as the hot press technique and the HIP method can be used to produce such high density zirconia ceramics.
A third aspect of the invention is a method for the production of high density and high toughness Y2~3 partially stabilized zirconia ceramics.
This method is characterized by limiting the starting material to a powder consisting essentially of zirconium compound.
In accordance with this method, the particulate raw material can have lower temperature sintering ability in addition to easy sinter, whereas the grain growth during sintering treatment of ceramics is restricted so as to produce high density and high tou~hnsss Y203 partially stabilized zirconia ceramics having a microstructure, and high tetragonal phase content.
A fourth aspect of the invention is a Y203 partially stabilized zirconia ceramics having a novel composition.
The said ceramics are characterized by having a Y203 content of greater than 1.3 mol.70, and less than 2.~ mol.~, and also the tetragonal phase content is greater than 65~.
The ceramics of the invention are expected to function as structural members of high density and high toughness.
A ifth aspect of the invention is alumina containing partially stabilized zirconia ceramics having a novel composition.
These ceramics have high density and high hardness imparted by the alumina content, as set forth in the above-mentioned examples, and, further, evidence high strength and excellent thermal stability.

Particularly when the Y203 partially stabilized zirconia ceramics of the invention incorporate alumina, high toughness can be imparted as set forth in the above examples. Therefore, such ceramics can be expected to be utilized as structural members of high hardness and high thermal stability such as cutting materials.
The present invention provides a process for the preparation of easy sintering raw particulate material and zirconia ceramics made thereÇrom that have high density, high strength, high toughness, high hardness and excellent thermal stability as well as an economical method for the production of the same. Therefore, the present invention can provide industrial significance.

Claims (21)

1. A process for the production of an easy-sintering raw material to be used for the production of zirconia ceramics which comprises adding powder principally containing zirconium compound(s) and containing stabilizing agent(s) into a solution or slurry containing at least one transition metal compound(s) and subsequently removing the solvent from the slurry and drying to obtain the raw material product.

2. The process for the production of an easy sintering raw material powder to be used for the production of zirconia ceramics according to claim 1, wherein the transition metal compound(s) are compound(s) of at least one metal selected from the group consisting of Mn, Fe, Co, Ni, Cu and Zn.

3. The process for the production of an easy sintering raw material powder to be used for the production of zirconia ceramics according to claim 1, wherein the stabilizing agent is Y2O3, CaO, MgO, CeO2, or yttrium compound, calcium compound, magnesium compound, or cerium compounds which by thermal decomposition produce Y2O3, CaO, MgO, or CeO2 respectively.

4. The process for the production of an easy sintering raw material powder to be used for the production of fully stabilized zirconia ceramics, partially stabilized zirconia ceramics, or alumina containing partially stabilized zirconia ceramics according to claim 1, wherein the powder principally containing zirconium compound(s) and containing stabilizing agent comprises;
(a) a fully stabilized zirconia powder, partially stabilized zirconia powder, or alumina containing partially stabilized zirconia powder, or (b) a precursor powder which on thermal decomposition produces fully stabilized zirconia, partially stabilized zirconia or partially stabilized zirconia powder containing alumina respectively.

5. The process for the production of an easy sintering raw material powder to be used for the production of zirconia ceramics according to claim 1, wherein the atomic ratio of the transition metal to zirconium, or to zirconium plus aluminum when alumina is present, in said raw material powder is in the range of 0.01 to 1.0%.

6. The process for the production of an easy sintering raw material powder to be used for the production of fully stabilized zirconia ceramics or partially stabilized zirconia ceramics according to claim 4, wherein the precursor powder, which on thermal decomposition produces fully stabilized zirconia, or partially stabilized zirconia, is co-precipitated from a solution containing zirconium compound(s) and stabilizing agent.

7. The process for the production of an easy sintering raw material powder to be used for the production of partially stabilized alumina containing zirconia ceramics according to claim 4, wherein the precursor powder, which on thermal decomposition produces alumina containing partially stabilized zirconia is co-precipitated from a solution or slurry containing:
(a) .alpha.-alumina powder or aluminium compounds which on thermal decomposition produce alumina, (b) zirconium compounds, and (c) a stabilizing agent.

8. A process for the production of high density zirconia ceramics which comprises adding a powder principally containing zirconium compounds and containing a stabilizing agent into a solution or slurry containing at least one transition metal compound(s) and then removing the solvent from ths slurry and drying the residue to produce raw material powder, and then moulding and sintering said raw material powder.

9. The process for the production of zirconia ceramics having high density according to claim 8, wherein sintering is conducted under atmospheric pressure at the temperature in a range from 1100°C to 1700°C.

10. A process for the production of Y2O3 partially stabilized zirconia ceramics having high density and high tenacity, which comprises suspending Y2O3 partially stabilized zirconia powder having a grain size of less than 400 A, and BET specific surface of 2 m2/g or more, or a precursor powder which on thermal decomposition produces said Y2O3 partially stabilized zirconia powder, into a solution or slurry containing at least one transition metal compound(s) and subsequently removing the solvent from the slurry and drying to produce raw material powder, and then moulding and sintering said raw material powder.

11. The process for the production of Y2O3 partially stabilized zirconia ceramics having high density and high tenacity, according to claim 10, wherein the transition metal compound(s) are compound(s) of at least one metal selected from the group consisting of Mn, Fe, Co, Ni, Cu and Zn.

12. The process for the production of Y2O3 partially stabilized zirconia ceramics having high density and high tenacity, according to claim 10, wherein the atomic ratio of the transition metal to zirconium in said Y2O3 partially stabilized zirconia ceramics is in the range of from 0.01% to 1.0%.

13. The process for the production of Y2O3 patially stabilized zirconia ceramics having high density and high tenacity, according to claim 10, wherein the precursor powder which produces, on thermal decomposition, Y2O3 partially stabilized zirconia powder is co-precipitated from a solution containing zirconium compound(s) and yttrium compound(s).

14. The process for the production of Y2O3 partially stabilized zirconia ceramics having high density and high tenacity, according to claim 10, wherein sintering is conducted at atmospheric pressure at 1500°C or lower temperature.

15. Y2O3 partially stabilized zirconia ceramics having high density and high tenacity, which are characterized by a Y2O3 content in the range of from 1.3 mol.% to 2.0 mol.%, the content of tetragonal phase being 65% or more.

16. Y2O3 partially stabilized zirconia ceramics having high density and high tenacity, according to claim 15, wherein the crystal grain size of the ZrO2 in the ceramics is 0.5 µm or less.

17. Y2O3 partially stabilized zirconia ceramics having high density and high tenacity, according to claim 15, wherein the density of said zirconia ceramics is 5.8 g/cm3 or more.

18. A process for the production of zirconia ceramics containing alumina, having the following ratio in the composition:
partially stabilized zirconia : 99 to 40 mol.%
alumina : 1 to 60 mol.%
transition metal oxide : the atomic ratio thereof to the combination of Zr and Al ranging from 0.01 to 1%.

which comprises; removing solvent from a slurry containing (a) partially stabilized zirconia powder or precursor powder which on thermal decomposition produces partially stabilized zirconia, (b) ?-alumina powder or precursor powder which on thermal decomposition produces ?-alumina, and (c) transition metal compound(s), and drying so as to produce raw material powder, and, then, moulding the raw material powder, and sintering said moulded material.

19. The process for the production of partially stabilized zirconia ceramics containing alumina, according to claim 18, wherein the transition metal compound(s) are a compound(s) of at least one.
metal selected from the group consisting of Mn, Fe, Co, Ni, kCu and Zn.

20. The process for the production of partially stabilized zirconia ceramics containing alumina, according to claim 18, wherein the raw material powder has a grain size of 400.ANG. or less, and BET specific surface of 2 m2/g or more, the .alpha.-alumina powder has a crystal particle size of 1.0 µm or less, and BET specific surface of 2 m2/g or more.

21. The process for the production of partially stabilized zirconia ceramics containing alumina, according to claim 18, wherein the sintering is carried out under atmospheric pressure at a temperature of 1500°C or lower.