ZrO2-Al2O3 composite ceramic material and production method therefor

This is a continuation patent application of U.S. patent application Ser. No. 12/123,356 filed on May 19, 2008, now U.S. Pat. No. 7,928,028, which was a continuation-in-part application of U.S. patent application Ser. No. 11/085,039 filed on Mar. 22, 2005, now abandoned, which claims the priority of Japanese Patent Application No. 2004-085102 filed on Mar. 23, 2004, the entire contents of which are hereby incorporated by reference.

1. Field of the Invention

The present invention relates to a ZrO 2 —Al 2 O 3 composite ceramic material with excellent mechanical properties, and a method of producing the same.

2. Disclosure of the Prior Art

As compared with metal and plastic materials, ceramic materials demonstrate excellent hardness, wear resistance, heat resistance and corrosion resistance. As for practical use of the ceramic materials in various application fields, for example, biomaterial parts such as artificial joint and artificial tooth, medical equipments, cutting tools such as drill and surgical knife, mechanical parts for automobile, airplane and space craft used under severe conditions, it is desired to develop a ceramic material having further improved mechanical strength and toughness in a high level. In recent years, a zirconia (ZrO 2 )-alumina (Al 2 O 3 ) composite ceramic material has received attention as a potential candidate of the ceramic material.

For example, Japanese patent Early publication [kokai] No. 5-246760 discloses a ZrO 2 -based composite ceramic sintered body comprising a matrix phase of tetragonal ZrO 2 grains containing 5 to 30 mol % of CeO 2 and a dispersion phase of fine grains of at least one of selected from the group consisting of Al 2 O 3 , SiC, Si 3 N 4 and B 4 C, which are dispersed within the ZrO 2 grains and at grain boundaries of the matrix phase. By the presence of the dispersion phase, a grain growth of the matrix phase is prevented and a fine grained structure of the matrix phase is obtained, so that significant strengthening is achieved mainly due to a reduction in flaw size of the fracture origin.

In addition, U.S. Pat. No. 5,728,636 discloses a ZrO 2 -based ceramic material having high mechanical strength and toughness, which comprises a tetragonal ZrO 2 phase of ZrO 2 grains having an average grain size of 5 μm or less, and containing 8 to 12 mol % of CeO 2 and 0.05 to 4 mol % of TiO 2 as a stabilizer, and an Al 2 O 3 phase of Al 2 O 3 grains having an average grain size of 2 μm or less. In this ceramic material, the Al 2 O 3 grains are dispersed within the ZrO 2 grains at a dispersion ratio of 2% or more, which is defined as a ratio of the number of the Al 2 O 3 grains dispersed in the ZrO 2 grains relative to the number of the entire Al 2 O 3 grains dispersed in the ceramic material. In addition, by using the combination of CeO 2 and TiO 2 as the stabilizer, the grain growth of the ZrO 2 grains can be enhanced moderately, so that parts of the Al 2 O 3 grains are effectively dispersed within the ZrO 2 grains, and a critical stress of a stress induced transformation from the tetragonal ZrO 2 to monoclinic ZrO 2 is increased.

By the way, as one potential approach for improving wear resistance and hardness of this kind of the ceramic material, it is proposed to increase the additive amount of Al 2 O 3 . However, such an increase of the Al 2 O 3 amount generally leads to reductions in mechanical strength and toughness. In above cases, a preferred amount of Al 2 O 3 in the composite ceramic sintered body or the ceramic material is in a range of 0.5 to 50 vol %. When the Al 2 O 3 amount exceeds 50 vol %, Al 2 O 3 becomes the matrix phase, so that it is difficult to maintain a strengthening mechanism based on a stress induced phase transformation of ZrO 2 . Consequently, considerable reductions in mechanical strength and toughness may occur.

Thus, the previous ZrO 2 —Al 2 O 3 composite ceramic materials still have a problem to be solved for providing excellent wear resistance and hardness without causing reductions in mechanical strength and toughness under a larger amount of Al 2 O 3 .

Therefore, a primary concern of the present invention is to provide a ZrO 2 —Al 2 O 3 composite ceramic material having increased wear resistance and hardness, while maintaining a good balance between mechanical strength and toughness, under a larger amount of Al 2 O 3 than heretofore.

That is, the ZrO 2 —Al 2 O 3 composite ceramic material of the present invention comprises a ZrO 2 phase containing 10 to 12 mol % of CeO 2 as a stabilizer and composed of 90 vol % or more of tetragonal ZrO 2 , and an Al 2 O 3 phase. An amount of the Al 2 O 3 phase in the composite ceramic material is in a range of 20 to 70 vol %, and preferably 40 to 70 vol %. Al 2 O 3 grains each having a fine ZrO 2 grain therein are dispersed in the composite ceramic material. Some of the Al 2 O 3 grains each having the fine ZrO 2 grain therein are trapped within ZrO 2 grains to form composite grains. A first dispersion ratio defined as a ratio of the number of the Al 2 O 3 grains each having the fine ZrO 2 grain therein relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material is 10% or more, preferably 30% or more, and more preferably 50% or more. A second dispersion ratio defined as a ratio of the number of the Al 2 O 3 grains of the composite grains relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material is 0.3% or more, preferably 1% or more, and more preferably 1.5% or more.

According to the present invention, the Al 2 O 3 grains each having the fine ZrO 2 grain therein are dispersed at 10% or more of the first dispersion ratio in the ZrO 2 —Al 2 O 3 composite ceramic material. This is, by using a specific production method described later, an amount of so-called zirconia toughened alumina (ZTA) grains, which are formed by trapping fine tetragonal ZrO 2 grains within Al 2 O 3 grains, can be remarkably increased. This brings about an increase in critical stress required for causing stress-induced phase transformation from the tetragonal ZrO 2 to the monoclinic ZrO 2 . As a result, it is possible to reliably provide the composite ceramic material with excellent mechanical strength and toughness.

Without wishing to be bound by theory, it is presently believed that the remarkable improvement of mechanical properties in the present invention results from the following mechanism. As described above, the ZrO 2 —Al 2 O 3 composite ceramic material of the present invention is characterized in that the Al 2 O 3 grains having the fine (tetragonal) ZrO 2 grains therein are dispersed at a high dispersion ration (first dispersion ratio) in the ZrO 2 —Al 2 O 3 composite ceramic material. Since the Al 2 O 3 grains having the fine (tetragonal) ZrO 2 grains therein construct the zirconia toughened alumina (ZTA) grains, the toughness of the Al 2 O 3 grain is remarkably improved by the presence of the fine ZrO 2 grain. Moreover, when such a toughness-improved Al 2 O 3 grain is trapped within the (larger-sized) ZrO 2 grain, sub-grain boundaries are formed within the ZrO 2 grain. The formation of the sub-grain boundaries plays a role in dividing the (larger) ZrO 2 grain incorporating the toughness-improved Al 2 O 3 grain therein into imaginary more finer sized grains.

Thus, by achieving a remarkably high dispersion ratio of the ZTA grains, i.e., 10% or more of the first dispersion ratio, it is possible to provide the ZrO 2 —Al 2 O 3 composite ceramic material having excellent wear resistance and hardness, while maintaining the good balance between mechanical strength and toughness, even under a larger amount of 40 to 70 vol % of the Al 2 O 3 phase in the composite ceramic material.

In the above composite ceramic material, when ZrO 2 grains each having an Al 2 O 3 grain therein are dispersed in the composite ceramic material, and a third dispersion ratio defined as a ratio of the number of the Al 2 O 3 grains trapped within the ZrO 2 grains relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material is 1.5% or more, it is preferred that the second dispersion ratio “R2” satisfies the following condition:

R 2≧α× R 3× R 1/100

wherein “R1” is the first dispersion ratio, “R3” is the third dispersion ratio, and “α” is a coefficient determined within a range of 0.8 to 1 by taking account of measurement errors.

A further concern of the present invention is to provide a method of producing the ZrO 2 —Al 2 O 3 composite ceramic material described above. That is, this method comprises the steps of:

mixing a first powder for providing the ZrO 2 phase with a second powder for providing the Al 2 O 3 phase such that an amount of the Al 2 O 3 phase in the composite ceramic material is in a range of 20 to 70 vol %;

molding a resultant mixture in a desired shape to obtain 3 green compact; and sintering said green compact in an oxygen-containing atmosphere,

wherein the second powder has Al 2 O 3 particles each having a fine ZrO 2 particle therein, and

a ratio of the number of the Al 2 CO 3 particles each having the fine ZrO 2 particle therein in the second powder is 5% or more.

According to the production method of the present invention, a high dispersion ratio of the ZTA grains, i.e., 10% or more of the first dispersion ratio can be achieved by using the second powder with 5% or more of the Al 2 O 3 , particles each having a fine ZrO 2 particle therein, and trapping fine ZrO 2 grains within Al 2 O 3 grains by help of grain growth during the sintering process.

As a preferred preparation process of the second powder in the above method, it comprises the step of adding a ZrO 2 powder to at least one selected from a θ-Al 2 O 3 powder and a γ-Al 2 O 2 powder having, a specific surface of 50 to 400 m 2 /g to obtain a mixed powder. In addition, it is preferred that the preparation process comprises the steps of adding a ZrO 2 powder to one of an aqueous solution of an aluminum salt and an organic solution of an aluminum alkoxide, hydrolyzing a resultant mixture to obtain a precipitate, and drying the precipitate. Alternatively, it is preferred that the preparation process comprises the steps of adding an aqueous solution of a zirconium salt to one of an aqueous solution of an aluminum salt and an organic solution of an aluminum alkoxide, hydrolyzing a resultant mixture to obtain a precipitate, and drying the precipitate. In theses preparation processes, it is preferred to calcine the mixed powder or the precipitate in an oxygen containing atmosphere it a temperature of 800° C. to 1300° C.

These and further purposes and advantages of the present invention will be clearly understood from the following detail explanation and examples of the invention.

The FIGURE is a SEM photograph showing a composite grain dispersed in a ZrO 2 —Al 2 O 3 composite ceramic material of the present invention.

The ZrO 2 phase of the ZrO 2 —Al 2 O 3 composite ceramic material of the present invention is composed of 90 vol % or more of tetragonal ZrO 2 . To obtain such a large amount of tetragonal ZrO 2 , the ZrO 2 phase contains 10 to 12 mol % of CeO 2 as a stabilizer. When the CeO 2 amount is less than 10 mol %, an amount of monoclinic ZrO 2 relatively increases, so that cracks may easily occurs in the composite ceramic material. On the other hand, when the CeO 2 amount exceeds 12 mol %, cubic ZrO 2 of a high-temperature stable phase begins to appear, so that there is a fear that the mechanical strength and toughness can not be sufficiently improved by the stress induced phase transformation of tetragonal ZrO 2 to monoclinic ZrO 2 . Preferably, the zirconia phase is composed of 90 vol % or more of tetragonal ZrO 2 and the balance of monoclinic ZrO 2 .

The composite ceramic material of the present invention is essential to contain 20 to 70 vol %, preferably 40 to 70 vol %, and more preferably more than 50% and less than 70% of the Al 2 O 3 phase. When the Al 2 O 3 amount is less than 20 vol %, the wear resistance and the mechanical strength of the composite ceramic material can not be sufficiently improved. On the other hand, when the Al 2 O 3 amount exceeds 70 vol %, considerable reductions in mechanical strength and toughness occur. When the Al 2 O 3 amount is in the range of 40 to 70 vol %, it is possible to provide a high-reliability ceramic material having good balance between the mechanical strength and toughness in a high level.

One of the important features of the composite ceramic material of the present invention is that Al 2 O 3 grains each having a fine ZrO 2 grain therein are uniformly dispersed in the composite ceramic material at a much higher dispersion ratio than heretofore by using an Al 2 O 3 powder containing a composite powder in a production method described below for the composite ceramic material of the present invention. In this regard, the composite powder is composed of Al 2 O 3 particles each having a fine ZrO 2 particle therein. An amount used of the composite powder is determined such that a ratio of the number of the Al 2 O 3 particles each having the fine ZrO 2 particle therein in the Al 2 O 3 powder is 5% or more, preferably 20% or more, and more preferably 35% or more. In addition, a ratio of the number of the Al 2 O 3 grains each having, the fine ZrO 2 grain therein relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material is defined as a first dispersion ratio. In the present invention, the first dispersion ratio is 10% or more, preferably 30% or more, and more preferably 50% or more. A mechanism of improving mechanical properties of the composite ceramic material resulting from an increase in the first dispersion ratio of the Al 2 O 3 grains each having the fine ZrO 2 grain therein in the composite ceramic material is explained later in detail.

The composite ceramic material of the present invention is also characterized in that composite grains, each of which has a structure that an Al 2 O 3 grain having a fine (tetragonal) ZrO 2 grain therein is trapped within a (larger-sized) ZrO 2 grain, as shown in the FIGURE, are aggressively dispersed in the composite ceramic material. In the present specification, a ratio of the number of the Al 2 O 3 grains of the composite grains relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material is defined as a second dispersion ratio. The second dispersion ratio is 0.3% or more. When the second dispersion ratio is less than 0.3%, the formation amount of the zirconia toughened alumina (ZTA) in the composite ceramic material decreases, so that there is a fear that the effect of improving the mechanical strength and toughness is not sufficiently obtained as increasing, Al 2 O 3 content. In other words, as the second dispersion ratio is 0.3% or more, preferably 1% or more, and more preferably 1.5% or more, higher improvements of both mechanical strength and toughness of the composite ceramic material can be achieved.

In addition, when ZrO 2 grains each having, an Al 2 O 3 grain therein are dispersed in the composite ceramic material, a ratio of the number of Al 2 O 3 grains trapped within ZrO 2 grains relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material is defined as a third dispersion ratio. In the present invention, it is preferred that the third dispersion ratio is 1.5% or more. When the third dispersion ratio is less than 1.5%, an effect of dividing the ZrO 2 grains into more finer sized grains by the formation of sub-grain boundaries may become insufficient, so that a reduction in strength easily occurs as increasing Al 2 O 3 content. An upper limit of the third dispersion ratio is not restricted. In a theoretical sense, as the third dispersion ratio increases, the mechanical properties of the composite ceramic material can be further improved. The number of Al 2 O 3 grains each existing in the composite grain is included in the number of the Al 2 O 3 grains dispersed in the ZrO 2 grains.

In the present invention, it is preferred that the second dispersion ratio “R2” satisfies the following condition:

R 2≧α× R 3× R 1/100

wherein “R1” is the first dispersion ratio, “R3” is the third dispersion ratio, and “α” is a coefficient determined within a range of 0.8 to 1 by taking account of measurement errors. Thus, the composite ceramic material of the present invention is further characterized in that the formation amount of the composite grains increases as the first dispersion ratio and/or the third dispersion ratio becomes larger.

The size of the fine ZrO 2 grain of the composite grain is not restricted on the assumption that the fine ZrO 2 grain can be trapped within the Al 2 O 3 grain. For example, it is preferred that fine tetragonal ZrO 2 grains having an average grain size of several ten nanometers are trapped within the Al 2 O 3 grains. The number of the fine ZrO 2 grains each trapped within the Al 2 O 3 grain of the composite grain is included in the number of the ZrO 2 grains dispersed in the Al 2 O 3 grains.

It is preferred that the Al 2 O 3 grains of the composite ceramic material has an average grain size of 0.1 to 0.5 μm. When the average grain size exceeds 0.5 μm, it becomes difficult to disperse the Al 2 O 3 grains within the ZrO 2 grains at the above third dispersion ratio. On the other hand, when the average grain size is less than 0.1 μm, it is difficult to obtain a highly dense sintered body of the composite ceramic material by pressureless sintering.

The size of the ZrO 2 grain of the composite grain is determined such that the Al 2 O 3 grain having the fine ZrO 2 grain therein is trapped within the ZrO 2 grain. However, when the size of the ZrO 2 grain is excessively large, it may lead to a reduction in strength of the composite ceramic material. From this viewpoint, it is preferred that an average grain size of the ZrO 2 grains of the composite ceramic material is in a range of 0.1 to 1 μm. This average grain size is based on the ZrO 2 grains other than the fine ZrO 2 grains trapped within the Al 2 O 3 grains. When the average grain size exceeds 1 μm, reductions in wear resistance and mechanical strength may occur. On the other hand, when the average grain size is less than 0.1 μm, it becomes difficult to obtain a highly dense sintered body of the composite ceramic material by pressureless sintering.

By the way, in the case of a conventional composite ceramic material with a simply mixed structure of ZrO 2 and Al 2 O 3 grains having an average grain size of several micron levels, when the Al 2 O 3 amount exceeds 30 vol %, the toughening mechanism based on the stress induced phase transformation of tetragonal ZrO 2 to monoclinic ZrO 2 is not a dominant factor of the composite ceramic material, so that there is a tendency that the mechanical strength and toughness gradually decrease. In addition, when the Al 2 O 3 amount exceeds 50 vol %, it means that the matrix phase of the composite ceramic material is provided by the Al 2 O 3 phase. This leads to a considerable deterioration of the mechanical properties of the conventional composite ceramic material.

However, according to the ZrO 2 —Al 2 O 3 composite ceramic material of the present invention, the fine ZrO 2 grains each trapped within the Al 2 O 3 grain and the Al 2 O 3 grains each trapped within the ZrO 2 grain contribute to promote piling up dislocations and form the sub-grain boundaries within the crystal grains, so that the wear resistance and mechanical strength of the composite ceramic material can be remarkably improved. In particular, when the Al 2 O 3 amount is in the range of 40 to 70 vol %, fine tetragonal ZrO 2 grains are uniformly dispersed in the Al 2 O 3 grains to form the zirconia toughened alumina (ZTA) structure, so that the Al 2 O 3 grains are remarkably reinforced. In other words, even when the Al 2 O 3 amount exceeds 50 vol %, high mechanical strength and toughness can be maintained by the formation of a fine crystal-grain structure effectively reinforced by the tetragonal ZrO 2 grains. From these reasons, the ZrO 2 —Al 2 O 3 composite ceramic material of the present invention obtained under the Al 2 O 3 content larger than 50 vol % where the matrix phase is the Al 2 O 3 phase exhibits excellent mechanical strength and toughness substantially comparable to the former ZrO 2 —Al 2 O 3 ceramic material where the matrix phase is the ZrO 2 phase.

Without wishing to be bound by theory, it is presently believed that the mechanical properties of the composite ceramic material of the present invention are improved by the following mechanism. That is, when the composite grains are dispersed in the composite ceramic material, each of which has the structure that the Al 2 O 3 grain containing the fine tetragonal ZrO 2 grain therein is trapped within the tetragonal ZrO 2 grain, a residual stress field is locally generated around each of the fine tetragonal ZrO 2 grains within the Al 2 O 3 grains and around each of the Al 2 O 3 grains within the tetragonal ZrO 2 grains by a difference in thermal expansion coefficient between Al 2 O 3 and ZrO 2 during a cooling procedure after sintering. By the influence of the residual stress field, lots of dislocations easily occur within the respective crystal grains. The dislocations are then piled up with each other, and finally the sub-grain boundaries are formed within the Al 2 O 3 and ZrO 2 grains, respectively. The sub-grain boundaries provide the finer-grained structure, which has the capability of increasing a critical stress required for causing the stress-induced phase transformation from the tetragonal ZrO 2 to the monoclinic ZrO 2 . As a result, the composite ceramic material of the present invention demonstrates excellent wear resistance and hardness as well as high mechanical strength and toughness.

Referring to the SEM photograph of the FIGURE, the structure of the composite ceramic material of the present invention is more concretely explained. This SEM photograph shows that the above-described composite grain exists in a uniformly mixed structure of normal tetragonal ZrO 2 grains not having Al 2 O 3 grains therein, and normal α-Al 2 O 3 grains not having ZrO 2 grains therein. In addition, it shows that an Al 2 O 3 grain containing a fine ZrO 2 grain therein and Al 2 O 3 grains not containing the fine ZrO 2 grain therein are dispersed within the large ZrO 2 grain constructing this composite grain. Moreover, it shows that an Al 2 O 3 grain containing a fine ZrO 2 grain therein other than the composite grain exists in the composite ceramic material. The number of fine ZrO 2 grains in the single Al 2 O 3 grain and the number of Al 2 O 3 grains in the single ZrO 2 grain are not restricted. For example, a plurality of fine ZrO 2 grains may be trapped in the single Al 2 O 3 grain, or a plurality of Al 2 O 3 grains may be trapped in the single ZrO 2 grain.

As a preferred embodiment of the present invention, the zirconia phase may contain another stabilizer such as MgO, CaO, TiO 2 and/or Y 2 O 3 in addition to CeO 2 . For example, it is preferred to use 0.01 to 1 mol % of TiO 2 and/or 0.01 to 0.5 mol % of CaO with respect to the total amount of the zirconia phase in addition to 10 to 12 mol % of CeO 2 . In this case, the grain growth of the zirconia phase is enhanced to a moderate degree by the addition of TiO 2 , so that Al 2 O 3 grains can be easily dispersed within the ZrO 2 grains. In addition, it is possible to increase a critical stress of the stress induced phase transformation. When the additive amount of TiO 2 is less than 0.01 mol %, the effect of enhancing the grain growth of the zirconia phase may be not obtained sufficiently. On the other hand, when the additive amount of TiO 2 exceeds 1 mol %, abnormal grain growth of the zirconia phase easily occurs, so that the mechanical strength and the wear resistance of the composite ceramic material may deteriorate.

On the other hand, the addition of CaO prevents the abnormal grain growth of the zirconia phase to improve the balance between the mechanical strength and toughness. In particular, it is effective to obtain the composite ceramic material having excellent wear resistance and mechanical strength. When the additive amount of CaO is less than 0.01 mol %, the effect of preventing the abnormal grain growth of the zirconia phase may be not obtained sufficiently. On the other hand, when the additive amount of CaO exceeds 0.5 mol %, cubic zirconia beings to appear in the zirconia phase, so that it becomes difficult to obtain the zirconia phase composed of 90 vol % or more of tetragonal ZrO 2 . The generation of cubic zirconia easily leads to an increase in average grain size of the zirconia phase. In such a case, deterioration in mechanical strength, toughness and wear resistance may occur. The zirconia phase may contain a small amount of impurities. For example, it is desired that the amount of the impurities is 0.5 mol % or less with respect to the total amount of the zirconia phase.

The composite ceramic material of the present invention is preferably used in applications requiring excellent wear resistance expected by increasing the Al 2 O 3 content, while maintaining mechanical strength and toughness substantially equal to the previous ZrO 2 —Al 2 O 3 ceramic materials. For example, it is preferred to use the composite ceramic material of the present invention for an artificial joint described in the international patent application WO02/11780. That is, when a joint portion of the artificial joint is provided by a sliding contact between the composite ceramic material and polyethylene, it is possible to reduce a wear amount of polyethylene. In addition, when the joint portion of the artificial joint is formed by a sliding contact between the composite ceramic materials, particularly improved wear resistance can be achieved. Thus, by use of the composite ceramic material of the present invention, it is possible to obtain the artificial joint having the capability of stably providing a smooth joint motion for an extended time period under severe conditions in a living body.

Next, a method of producing the ZrO 2 —Al 2 O 3 composite ceramic material of the present invention is explained. The production method comprises the steps of mixing a first power for providing the ZrO 2 phase with a second powder for providing the Al 2 O 3 phase such that an amount of the Al 2 O 3 phase in the composite ceramic material is in a range of 20 to 70 vol %, molding a resultant mixture in a desired shape to obtain a green compact, and sintering the green compact at a sintering temperature in an oxygen-containing atmosphere. The production method of the present invention is characterized in that the second powder contains the composite powder described above, which is composed of the Al 2 O 3 particles each having the fine ZrO 2 particle therein, and a ratio of the number of the Al 2 O 3 particles each having the fine ZrO 2 particle therein in the second powder is 5% or more.

To obtain the ZrO 2 phase composed of 90 vol % or more of tetragonal ZrO 2 , the first powder is prepared such that the ZrO 2 phase contains 10 to 12 mol % of CeO 2 as a stabilizer. In addition, as the first powder, it is preferred to use a tetragonal ZrO 2 powder containing a required amount of TiO 2 and/or CaO in addition to CeO 2 . A preparation process of the first powder is not restricted. For example, the following process is recommended.

That is, a cerium containing compound such as cerium salts is added to an aqueous solution of a zirconium salt. If necessary, an aqueous solution of a titanium salt and/or a calcium salt, or an organic solution of a titanium or calcium alkoxide as a titanium containing compound or a calcium containing compound may be added. Then, hydrolysis is performed by adding an alkali aqueous solution such as aqueous ammonia to a resultant mixture to obtain a precipitate. The precipitate is dried, calcined in the oxygen-containing atmosphere, e.g., in the air, and then pulverized by means of wet ball milling to obtain the tetragonal ZrO 2 powder having a desired particle distribution.

In the case of using the tetragonal ZrO 2 powder, it is preferred that the ZrO 2 powder has a specific surface area of 10 to 20 m 2 /g to obtain the green compact of a sufficient green density. Such a green compact can be easily sintered by pressureless sintering. When the specific surface area is less than 10 m 2 /g, it becomes difficult to obtain the ZrO 2 phase having an average grain size of 1 μm or less after sintering. On the other hand, when the specific surface area exceeds 20 m 2 /g, the bulk density considerably decreases, so that handling of the first powder becomes difficult. As a result, there is a fear that the green compact cannot be densely sintered by pressureless sintering.

In the present invention, to uniformly disperse the composite grains in the composite ceramic material, the second powder contains at least 5% of the composite powder composed of the Al 2 O 3 particles each having the fine ZrO 2 particle therein. For example, a required amount of the first powder is mixed with an Al 2 O 3 powder to obtain a mixed powder, and then a resultant mixed powder is calcined in an oxygen containing atmosphere at a temperature of 800° C. to 1300° C., and preferably 1000° C. to 1200° C. to obtain the second powder containing the composite powder. In this case, it is preferred that the Al 2 O 3 powder is at least one selected from a θ-Al 2 O 3 powder and a γ-Al 2 O 3 powder having a specific surface area of 50 to 400 m 2 /g. The specific surface area of this Al 2 O 3 powder is much greater than the specific surface of the first powder. In other words, since the Al 2 O 3 powder used to prepare the composite powder is much finer than the first powder, the above-described mixed powder comprises ZrO 2 particles surrounded with ultra-fine Al 2 O 3 particles.

Next, a phase transformation of θ-Al 2 O 3 and/or γ-Al 2 O 3 of the mixed powder to α-Al 2 O 3 occurs during the calcining procedure. At this time, the ZrO 2 particles in the mixed powder are trapped within α-Al 2 O 3 particles each having an increased particle size caused by the phase transformation. The thus obtained composite powder is excellent in moldability as compared with the case of using the θ-Al 2 O 3 or γ-Al 2 O 3 powder. In addition, there is an advantage that the average grain size of the Al 2 O 3 particle dispersed in the composite ceramic material can be easily controlled in the range of 0.1 to 0.5 μm.

It is preferred that the composite powder obtained by the above preparation process is mainly composed of α-Al 2 O 3 particles having an average grain size of 0.3 μm or less, each of which has the fine ZrO 2 particle therein. However, an amount of α-Al 2 O 3 in the composite powder is not restricted. That is, it is enough that a part of θ-Al 2 O 3 and/or γ-Al 2 O 3 is transformed to α-Al 2 O 3 by the calcining procedure, and allowed to be mixed condition of θ-Al 2 O 3 and/or γ-Al 2 O 3 and α-Al 2 O 3 . In the present invention, the ratio of the number of the Al 2 O 3 particles of the each having the fine ZrO 2 particle therein in the second powder is 5% or more, preferably 25% or more, and more preferably 35% or more.

As another preferred process of preparing the second powder, a ZrO 2 powder is added to an aqueous solution of an aluminum salt or an organic solution of an aluminum alkoxide. A resultant mixture is hydrolyzed to obtain a precipitate, and then the precipitate is dried. The dried precipitate is calcined in an oxygen containing atmosphere at a temperature of from 800° C. to 1300° C., and then pulverized by means of wet ball milling to obtain the second powder having a desired particle distribution. In the above method, an aqueous solution of a zirconium salt may be used in stead of the ZrO 2 powder.

In the case of preparing the composite powder as the second powder, a mixing ratio of Al 2 O 3 and ZrO 2 is not restricted. To efficiently obtain the α-Al 2 O 3 particles each containing a fine ZrO 2 particle therein, it is preferred that a volume ratio of Al 2 O 3 :ZrO 2 in the composite powder is in a range of 95:5 to 50:50. When the value of ZrO 2 in this volume ratio is less than 5, it is difficult to obtain sufficient amounts of the α-Al 2 O 3 particles each containing a fine ZrO 2 particle therein by the calcining procedure. Consequently, the formation amount of the composite grains in the composite ceramic material decreases. On the other hand, when the value of ZrO 2 in this volume ratio is more than 50, an agglomeration of the ZrO 2 particles may occur. When the above volume ratio is within the range of 90:10 to 60:40, it is possible to more efficiently obtain the α-Al 2 O 3 particles each containing a fine ZrO 2 particle therein, thereby providing a high-quality composite powder suitable for producing the composite ceramic material of the present invention.

If necessary, a HIP treatment may be performed in an oxygen-containing atmosphere after sintering. To obtain effects of the HIP treatment at the maximum, it is preferred that the sintered body of the composite ceramic material obtained by the pressureless sintering has a relative density of 95% or more. A concentration of oxygen in the oxygen-containing atmosphere is not restricted. A mixture gas of oxygen and an inert gas such as argon may be used. In this case, it is preferred that the concentration of oxygen is approximately 5 vol % or more with respect to a total volume of the mixture gas.

The present invention is concretely explained below according to preferred examples. These Examples are illustrative only, and therefore the present invention is not limited to them.

Examples 1 to 6 and Comparative Examples 1 to 3

A ZrO 2 —Al 2 O 3 composite ceramic material of each of Examples 1 to 6 and Comparative Examples 1 to 3 was produced by the following method. That is, as a first powder for providing a ZrO 2 phase of the composite ceramic material, a tetragonal ZrO 2 powder having a specific surface area of 15 m 2 /g and containing 11 mol % of CeO 2 , 0.05 mol % of TiO 2 , and 0.16 mol % of CaO was used. On the other hand, a second powder for providing an Al 2 O 3 phase of the composite ceramic material was prepared by using a γ-Al 2 O 3 powder having a specific surface area of 300 m 2 /g and a part of the tetragonal ZrO 2 powder. A mixture ratio by volume of the γ-Al 2 O 3 powder and the tetragonal ZrO 2 powder is 70:30.

That is, required amounts of the γ-Al 2 O 3 powder and the tetragonal ZrO 2 powder were ball-milled in an ethanol solvent for 24 hours, and then dried to obtain a mixed powder. Subsequently, the mixed powder was calcined at 1000° C. in the air for 2 hours. The thus obtained calcined powder was further ball-milled in an ethanol solvent for 24 hours, and then dried to obtain an Al 2 O 3 powder containing a composite powder (i.e., Al 2 O 3 particles each having a ZrO 2 particle therein) as the second powder for providing the Al 2 O 3 phase. From SEM observation, a ratio of the composite powder in the thus obtained Al 2 O 3 powder, i.e., a ratio of the number of the Al 2 O 3 particles each having a ZrO 2 particle therein relative to the entire Al 2 O 3 particles of the thus obtained Al 2 O 3 powder is about 28.3%.

The remaining tetragonal ZrO 2 powder was mixed with the Al 2 O 3 powder containing the composite powder such that an Al 2 O 3 amount in the composite ceramic material is in a range of 10 to 80 vol %, as listed in Table 1. A resultant mixture was ball-milled in an ethanol solvent for 24 hours, and then dried to obtain a powder for sintering. In Comparative Example 1, the Al 2 O 3 content is zero.

The thus obtained powder for sintering was molded at the pressure of 10 MPa by uniaxial pressing to obtain a disk-shaped green compact having a diameter of about 68 mm. After a CIP (cold isostatic pressing) treatment was performed to the green compact at the pressure of 147 MPa, the green compact was sintered at the sintering temperature of 1440° C. for 3 hours in the air by pressureless sintering to obtain a sintering body.

With respect to each of Examples 1 to 6 and Comparative Examples 1 to 3, the sintered body has a relative density of more than 99%. By X-ray diffraction analysis, it was confirmed that the ZrO 2 phase of the respective sintered body is composed of 90 vol % or more of tetragonal ZrO 2 and the balance of monoclinic ZrO 2 . In addition, from SEM (scanning electron microscope) and TEM (transmission electron microscope) observations, it was confirmed that Al 2 O 3 grains each having a fine ZrO 2 grain therein are dispersed in the sintered body of each of Examples 1 to 6 and Comparative Examples 2 and 3, and some of the Al 2 O 3 grains each having the fine ZrO 2 grain therein are trapped within ZrO 2 grains to form composite grains.

In addition, a first dispersion ratio (W1), which is defined as a ratio of the number of the Al 2 O 3 grains each having the fine ZrO 2 grain therein relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material, a second dispersion ratio (W2), which is defined as a ratio of the number of Al 2 O 3 grains of the composite grains relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material, and a third dispersion ratio (W3), which is defined as a ratio of the number of the Al 2 O 3 grains trapped within the ZrO 2 grains relative to the number of the entire Al 2 O 3 grains dispersed in the composite ceramic material, are listed in Table 2.

The first to third dispersion ratios (W1, W2, W3) were determined by the following method. First, a sample for observation was prepared by polishing the sintered body and performing a heat treatment to the polished surface. Then, the SEM observation of the sample or the TEM observation of the sintered body was performed to count the number (S1) of entire Al 2 O 3 grains existing within a view field (e.g., it is preferred to select an appropriate observation magnification such that at least 1000 grains exist in the view field), the number (n1) of Al 2 O 3 grains each having the fine ZrO 2 grain therein in the same view field, the number (n2) of Al 2 O 3 grains of the composite grains in the same view field, and the number (n3) of Al 2 O 3 grains trapped within the ZrO 2 grains in the same view field. By substituting these numbers to the following equations, the first to third dispersion ratios were calculated. Results are shown in Tables 2.

W 1[%]=( n 1/ S 1)×100

W 2[%]=( n 2/ S 1)×100

W 3[%]=( n 3/ S 1)×100

In addition, with respect to each of Examples 1 to 6 and Comparative Examples 1 to 3, average grain sizes of ZrO 2 grains and Al 2 O 3 grains of the sintered body were measured by the TEM/SEM observation. To evaluate mechanical properties of the composite ceramic material, test specimens having the dimensions of 4 mm×3 mm×40 mm were prepared from the sintered body, and then 3-point bending strength and fracture toughness were measured at room temperature. The fracture toughness was measured by the IF method. Results are listed in Tables 1 and 2.

Moreover, to evaluate wear resistance of the composite ceramic material, a pin-on-disc test was performed in the presence of distilled water as a lubricant. The pin and the disc are made of the same composite ceramic material. The pin is provided with a cylinder solid having a diameter of 5 min and a length of 15 mm, and a circular cone having an apical angle of 30° and formed on a top of the cylinder solid. The top end of the circular cone has a flat mirror area with a diameter of 1.5 mm, which is used as a sliding surface. A surface roughness of this sliding surface is smaller than 0.005 μm Ra.

On the other hand, the disc has a diameter of 50 mm and a thickness of 8 mm. A sliding surface of the disc to be made contact with the pin is a mirror polished surface having a surface roughness smaller than 0.005 μm Ra. After the pin was placed on a circumference having a radius of 22 mm from the disc center on the disc, the pin-on-disc test was performed at a disc rotational speed of 60 mm/sec. A load applied to the pin is 60N, and a sliding distance is constant (25 km). Since the diameter of the top end of the pin is 1.5 mm, an initial friction pressure on the top end of the pin is 33 MPa. The pin-on-disc test was repeated three times under the same condition. An average value of the tests was adopted as data.

A reduction in weight of the pin was measured after the test, and a wear factor (Wf) was calculated by the following equation.

Wf =( W 1− W 2)/ P·L·ρ

Where,

Wf: wear factor (mm3/Nm)

W1: dry weight (g) of pin before test

W2: dry weight (g) of pin after test

P: load (N)

L: sliding distance (m)

ρ: density (g/mm3) of test specimen

In addition, Vickers hardness of the composite ceramic material was measured. Measurement results of the wear factor and the hardness are shown in Table 2.

As understood from results of Tables 1 and 2, the sintered bodies of Examples 1 to 6 containing 20 to 70 vol % of Al 2 O 3 have the first dispersion ratio (W1) greater than 39%, the second dispersion ration (W2) greater than 0.5%, and the third dispersion ratio (W3) greater than 1.5%. In addition, these sintered bodies demonstrate excellent mechanical properties of a bending strength greater than 1200 MPa and a fracture toughness higher than 10.0 MPa·m 1/2 .

On the other hand, since the sintered body of Comparative Example 1 does not contain the Al 2 O 3 phase, it has excellent fracture toughness. However, the bending strength of the sintered body is considerably low. In the sintered body of Comparative Example 2, excellent fracture toughness is obtained, and the first to third dispersion ratios comparable to them of the sintered body of Example 1. However, the average grain size (=1.35 μm) of the ZrO 2 grains of Comparative Example 2 is much greater than the average grain size (=0.43 μm) of the ZrO 2 grains of Example 1. This suggests that the grain growth of the ZrO 2 grains can not be sufficiently inhibited by using such a small amount of Al 2 O 3 . As a result, the sintered body of Comparative Example 2 has a relatively low mechanical strength, and a tendency of increasing variations in mechanical properties. Thus, it is difficult to provide the composite ceramic material that is excellent in both of strength and toughness. With respect to the sintered body of Comparative Example 3, since the excessive amount of Al 2 O 3 was used, both of strength and toughness considerably lowered. In addition, the number of the Al 2 O 3 grains trapped within the ZrO 2 grains, i.e., the third dispersion ratio (W3) is extremely low. This suggests that the composite grains can not be efficiently dispersed in the composite ceramic material in the case of using such a large amount of Al 2 O 3 .

As described above, the concern of the present invention is to provide the ceramic material having excellent wear resistance and hardness, while maintaining high strength and toughness under the larger amount of Al 2 O 3 than heretofore. The results of Table 2 show that both of wear resistance and hardness can be highly improved when the Al 2 O 3 content is in the range of 20 to 70 vol %. On the contrary, the sintered body of Comparative Example 2 shows a deterioration in wear resistance due to the deficiency of Al 2 O 3 and the increase in average grain size of the ZrO 2 grains. In addition, the sintered body of Comparative Example 3 shows poor mechanical strength and toughness as well as the deterioration in wear resistance due to the excessive amount of Al 2 O 3 in the composite ceramic material.

Examples 7 to 21

A ZrO 2 —Al 2 O 3 composite ceramic materials of each of Examples 7 to 21 was produced by the following method. That is, as listed in table 3, a tetragonal ZrO 2 powder having a specific surface area of 15 m 2 /g and containing a CeO 2 amount of 10 to 12 mol % or containing the CeO 2 amount of 10 to 12 mol %, TiO 2 amount of 0.02 to 1 mol %, and a CaO amount of 0.02 to 0.5 mol % was used as a first powder for providing a ZrO 2 phase of the composite ceramic material.

On the other hand, a second powder for providing an Al 2 O 3 phase of the composite ceramic material was prepared by the following process. That is, a part of the above-described tetragonal ZrO 2 powder was added to a hydrochloric acid solution of aluminum chloride (AlCl 3 ) such that a mixture ratio by volume of Al 2 O 3 :ZrO 2 is 70:30. Next, an aqueous solution of sodium hydroxide was added to a resultant mixed solution, and hydrolyzed to obtain a precipitate. The precipitate was washed with water, and then dried. Next, the dried precipitate was calcined at 1000° C. in the air for 2 hours. The thus obtained calcined powder was ball-milled in an ethanol solvent for 24 hours, and then dried to obtain an Al 2 O 3 powder containing a composite powder. From SEM observation, a ratio of the composite powder in the thus obtained Al 2 O 3 powder, i.e., a ratio of the number of the Al 2 O 3 particles each having a ZrO 2 particle therein relative to the entire Al 2 O 3 particles of the thus obtained Al 2 O 3 powder is about 39.8%.

The remaining tetragonal ZrO 2 powder was mixed with the Al 2 O 3 powder containing the composite powder such that an Al 2 O 3 amount in the composite ceramic material is 40 vol %. A resultant mixture was ball-milled in an ethanol solvent for 24 hours, and then dried to obtain a powder for sintering. The powder for sintering was molded into a required shape by uniaxial pressing, and then sintered by pressureless sintering to obtain a sintered body, as in the case of Example 1.

With respect to each of Examples 7 to 21, the sintered body has a relative density of more than 99%. By X-ray diffraction analysis, it was confirmed that the ZrO 2 phase of the respective sintered body is composed of 90 vol % or more of tetragonal ZrO 2 and the balance of monoclinic ZrO 2 . In addition, from SEM and TEM observations, it was confirmed that Al 2 O 3 grains each having a fine ZrO 2 grain therein are dispersed in the sintered body of each of Examples 7 to 21, and some of the Al 2 O 3 grains each having the fine ZrO 2 grain therein are trapped within ZrO 2 grains to form composite grains. In addition, according to the same method as Example 1, the first to third dispersion ratios were determined with respect to each of Examples 7 to 21. Results are shown in Table 4.

In addition, with respect to each of Examples 7 to 21, average grain sizes of the ZrO 2 grains and the Al 2 O 3 grains of the sintered body were measured by the SEM/TEM observation. The average grain size of the ZrO 2 grains is in a range of 0.2 to 0.5 and the average grain size of the Al 2 O 3 grains is 0.3 μm or less. To evaluate mechanical properties of the composite ceramic material, test specimens having dimensions of 4 mm×3 mm×40 mm were prepared from the sintered body, and 3-point bending strength and fracture toughness were measured at room temperature. The fracture toughness was measured by the IF method. Results are shown in Tables 3 and 4.

The results of Tables 3 and 4 suggest that the bending strength can be further improved by using slight amounts of TiO 2 and CaO in addition to CeO 2 as the stabilizer, without deteriorating the fracture toughness.

Examples 22 to 27

A ZrO 2 —Al 2 O 3 composite ceramic materials of each of Examples 22 to 27 was produced by the following method. That is, as a first powder for providing a ZrO 2 phase of the composite ceramic material, a tetragonal ZrO 2 powder having a specific surface of 15 m 2 /g and containing 11 mol % of CeO 2 , 0.05 mol % of TiO 2 , and 0.13 mol % of CaO was used. On the other hand, a second powder for providing an Al 2 O 3 phase of the composite ceramic material was prepared by using a θ-Al 2 O 3 powder having a specific surface of 100 m 2 /g and a part of the above-described tetragonal ZrO 2 powder. A mixture ratio by volume of the θ-Al 2 O 3 powder and the tetragonal ZrO 2 powder was changed in a range of 95:5 to 50:50, as shown in Table 5.

That is, required amounts of the θ-Al 2 O 3 powder and the above-described tetragonal ZrO 2 powder were ball-milled in an ethanol solvent for 24 hours, and then dried to obtain a mixed powder. Subsequently, the mixed powder was calcined at 1000° C. in the air for 2 hours. The thus obtained calcined powder was further ball-milled in an ethanol solvent for 24 hours, and then dried to obtain an Al 2 O 3 powder containing a composite powder. From SEM observation, a ratio of the composite powder in the thus obtained Al 2 O 3 powder, i.e., a ratio of the number of the Al 2 O 3 particles each having a ZrO 2 particle therein relative to the entire Al 2 O 3 particles of the thus obtained Al 2 O 3 powder was estimated. Results are listed as “composite power ratio” in TABLE 5.

The remaining tetragonal ZrO 2 powder was mixed with the Al 2 O 3 powder containing the composite powder such that an Al 2 O 3 amount in the composite ceramic material is 50 vol %. A resultant mixture was ball-milled in an ethanol solvent for 24 hours, and the dried to obtain a powder for sintering. The powder for sintering was molded into a required shape by uniaxial pressing, and then sintered by pressureless sintering to obtain a sintered body, as in the case of Example 1.

With respect to each of Examples 22 to 27, the sintered body has a relative density of more than 99%. By X-ray diffraction analysis, it was confirmed that the ZrO 2 phase of the respective sintered body is composed of 90 vol % or more of tetragonal ZrO 2 and the balance of monoclinic ZrO 2 . In addition, by SEM (scanning electron microscope) and TEM (transmission electron microscope) observations, it was confirmed that Al 2 O 3 grains each having a fine ZrO 2 grain therein are dispersed in the sintered body of each of Examples 22 to 27, and some of the Al 2 O 3 grains each having the fine ZrO 2 grain therein are trapped within ZrO 2 grains to form composite grains. In addition, according to the same method as Example 1, the first to third dispersion ratios were determined with respect to each of Examples 22 to 27. Results are shown in Table 6.

In addition, with respect to each of Examples 22 to 27, average grain sizes of ZrO 2 grains and Al 2 O 3 grains of the sintered body were measured by the SEM/TEM observation. The average grain size of the ZrO 2 grains is in a range of 0.2 to 0.3 μm, and the average grain size of the Al 2 O 3 grains is 0.3 μm or less. To evaluate mechanical properties of the composite ceramic material, test specimens having dimensions of 4 mm×3 mm×40 mm were prepared from the sintered body, and 3-point bending strength and fracture toughness were measured at room temperature. The fracture toughness was measured by the IF method. Results are shown in Tables 5 and 6.

The results of Tables 5 and 6 suggest that when the mixture ratio of Al 2 O 3 and tetragonal ZrO 2 in the composite powder is in the range of 95:5 to 50:50, and particularly 90:10 to 60:40, the ZrO 2 grains can be efficiently trapped within the Al 2 O 3 grains. Furthermore, by selecting an appropriate mixture ratio of Al 2 O 3 and tetragonal ZrO 2 in the composite powder, it is possible to obtain the composite ceramic material having a further improved strength, while keeping the toughness constant.

Examples 28 to 32 and Comparative Examples 4 to 8

A ZrO 2 —Al 2 O 3 composite ceramic material of each of Examples 28 to 32 was produced by the following method. That is, as a first powder for providing a ZrO 2 phase of the composite ceramic material, a tetragonal ZrO 2 powder having a specific surface area of 15 m 2 /g and containing 11 mol % of CeO 2 and 0.02 mol % of Y 2 O 3 was used. On the other hand, a second powder for providing an Al 2 O 3 phase of the composite ceramic material was prepared by using a spherical γ-Al 2 O 3 powder having a specific surface area of 100 m 2 /g and a part of the tetragonal ZrO 2 powder. A mixture ratio by volume of the γ-Al 2 O 3 powder and the tetragonal ZrO 2 powder is 50:50.

That is, required amounts of the spherical γ-Al 2 O 3 powder and the tetragonal ZrO 2 powder were ball-milled in an ethanol solvent for 24 hours, and then dried to obtain a mixed powder. Subsequently, the mixed powder was calcined at 1000° C. in the air for 2 hours. The thus obtained calcined powder was further ball-milled in an ethanol solvent for 24 hours, and then dried to obtain an Al 2 O 3 powder containing a composite powder. From SEM observation, a ratio of the composite powder in the thus obtained Al 2 O 3 powder, i.e., a ratio of the number of the Al 2 O 3 particles each having a ZrO 2 particle therein relative to the entire Al 2 O 3 particles of the thus obtained Al 2 O 3 powder is about 62%.

The remaining tetragonal ZrO 2 powder was mixed with the Al 2 O 3 powder containing the composite powder such that an Al 2 O 3 amount in the composite ceramic material is in a range of 30 to 70 vol %, as listed in Table 7. A resultant mixture was ball-milled in an ethanol solvent for 24 hours, and then dried to obtain a powder for sintering.

The obtained powder for sintering was molded at the pressure of 10 MPa by uniaxial pressing to obtain a disk-shaped green compact having a diameter of about 68 mm. After a CIP (cold isostatic pressing) treatment was performed to the green compact at the pressure of 147 MPa, the green compact was sintered at the sintering temperature of 1440° C. for 3 hours in the air by pressureless sintering to obtain a sintering body.

On the other hand, a ZrO 2 —Al 2 O 3 composite ceramic material of Comparative Example 4 was produced by the same method as Examples 28 except that the composite powder was added such that the content of the Al 2 O 3 phase in the composite ceramic material is 80 vol %. In addition, ZrO 2 —Al 2 O 3 composite ceramic materials of Comparative Examples 5 to 8 were produced by the same methods as Examples 29 to 32, respectively, except that a commercially available α-Al 2 O 3 powder having an average particle size of 0.2 μm was used in place of the Al 2 O 3 powder containing the composite powder.

With respect to each of Examples 28 to 32 and Comparative Examples 4 to 8, the sintered body has a relative density of more than 99%. By X-ray diffraction analysis, it was confirmed that the ZrO 2 phase of the respective sintered body is composed of 90 vol % or more of tetragonal ZrO 2 and the balance of monoclinic ZrO 2 . In addition, from SEM (scanning electron microscope) and TEM (transmission electron microscope) observations, it was confirmed that Al 2 O 3 grains each having a fine ZrO 2 grain therein are dispersed in the sintered body of each of Examples 28 to 32 and Comparative Example 4, and some of the Al 2 O 3 grains each having the fine ZrO 2 grain therein are trapped within ZrO 2 grains to form composite grains. In addition, according to the same method as Example 1, the first to third dispersion ratios were determined with respect to each of Examples 28 to 32 and Comparative Examples 4 to 8. Results are shown in Table 8.

As understood from results of Tables 7 and 8, the sintered bodies of Examples 28 to 32 containing 30 to 70 vol % of Al 2 O 3 have the first dispersion ratio (W1) greater than 88%, the second dispersion ration (W2) greater than 1.5%, and the third dispersion ratio (W3) of 1.8% or more. In addition, these sintered bodies demonstrate excellent mechanical properties of a bending strength greater than 1400 MPa and a fracture toughness higher than 12 MPa·m 1/2 .

On the other hand, in Comparative Example 4, the bending strength and the fracture toughness of the sintered body deteriorated due to the excessive amount (80 vol %) of the Al 2 O 3 phase, despite an increase in the first dispersion ratio. In Comparative Examples 5 to 8, since the sintered bodies were produced without using the composite power, the first and second dispersion ratios considerably lowered, and deteriorations in bending strength and fracture toughness of the sintered body occurred. Thus, it becomes difficult to obtain the composite ceramic material having excellent bending strength and fracture toughness.

As understood from the above Examples, the ZrO 2 —Al 2 O 3 composite ceramic material of the present invention is characterized in that Al 2 O 3 grains each having a fine ZrO 2 grain therein are dispersed in the composite ceramic material at a higher dispersion ratio than heretofore, and some of the Al 2 O 3 grains each having the fine ZrO 2 grain therein are trapped within larger-sized ZrO 2 grains to form composite grains. The formation of such a nanocomposite structure provides further improvements in wear resistance, hardness, strength and toughness of the ZrO 2 —Al 2 O 3 ceramic material under a larger amount of Al 2 O 3 than heretofore.

Therefore, the composite ceramic material of the present invention is expected to be preferably utilized in various application fields, for example, parts for industrial machine such as ferrule for optical fiber connector, bearings and dies, cutting tools such as scissor and saw blade, stationery goods, chemical goods such as mechanical seal and milling media, goods for sport, medical equipments such as surgical knife, biomaterial parts such as artificial joint, artificial bone, artificial dental root, abutment and crown.