Abstract:
A method for manufacture of a high toughness sintered body, characterized by sintering a shaped body of a mixed powder consisting essentially of from 40 to 70% by weight of a first component of powdered ZrO 2  containing at least one stabilizer selected from the group consisting of Y 2  O 3 , CaO, and MgO and having an average particle diameter of not more than 1μ and from 30 to 60% by weight of a second component of powdered α-Al 2  O 3  having an average particle diameter of not more than 1 μm, which mixed powder may also contain not more than 3% by weight of SiO 2 , not more than 0.5% by weight of Fe 2  O 3 , or not more than 0.5% by weight of TiO 2  in a combined proportion of not more than 3% by weight at a temperature in the range of from 1400° C. to 1600° C. under normal pressure thereby producing a sintered body wherein at least 90% by weight of ZrO 2  particles present therein are accounted for by tetragonal and cubic crystals, the ratio of said tetragonal crystals to said cubic crystals is at least 1:3, and the average crystal particle diameter of the entire sintered body is at least 3μ.

Description:
This is a continuation of application Ser. No. 408,086, filed Aug. 13, 1982, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to high toughness sintered bodies comprising at least one of ZrO 2  and HfO 2 , at least one of Al 2  O 3  and TiN, etc. 
     BACKGROUND OF THE INVENTION 
     Production of ceramic materials having improved flexural strength has been the subject of research by many investigators, because poor flexural strength is a most serious disadvantage of ceramic materials, and if ceramic materials having improved flexural strength can be developed, they can be effectively used in the fabrication of cutting tools, as synthetic bone materials, as parts for internal combustion engines, and so forth. 
     For example, Japanese Patent Application (OPI) No. 140762/80 (the term &#34;OPI&#34; as used herein refers to a &#34;published unexamined Japanese patent application&#34;) discloses &#34;zirconia-base cutting tool materials&#34; comprising ZrO 2  partially stabilized with oxides of Y, Ca, Mg, etc., in which the total fraction of tetragonal and cubic ZrO 2  is from 60 to 95% by weight. J. S. Reed et al., Ceramic Bulletin, Vol. 55, page 717 (1976) describes that high strength ZrO 2  sintered bodies can be obtained by sintering fine powdered ZrO 2  which is prepared by co-precipitating a mixture of ZrOCl 2  and YCl 3 , calcining the thus-formed powder, and stabilizing with Y 2  O 3 . 
     These ceramic materials, however, are not completely satisfactory in strength, and it has, therefore, been desired to further increase the strength, because it is expected that such improved ceramic materials would have a wider variety of uses. 
     SUMMARY OF THE INVENTION 
     As a result of extensive investigations to further increase the strength of such ceramic materials, it has been found that when Al 2  O 3 , TiN, or a combination thereof is added to form a solid solution in combination with ZrO 2  (&#34;ZrO 2  &#34; as used herein generally is intended also to refer to ZrO 2  wherein HfO 2  is substituted in part or in whole therefor) or is dispersed in ZrO 2 , the transformation temperature of ZrO 2  between the tetragonal ZrO 2  and the monoclinic ZrO 2  is lowered and grain growth of ZrO 2  is prevented, which increases the fraction of tetragonal ZrO 2 , the sliding resistance among ZrO 2  grains in the grain boundaries, and hardness, and, furthermore, increases the high temperature (up to 1200° C.) strength to as high as about 2 times the strength of ZrO 2  alone. 
     The present invention, therefore, relates to a high toughness sintered body consisting essentially of from 40 to 99.5% by weight Component A and from 0.5 to 60% by weight Component B, wherein the mean grain size of the sintered body is 3 microns or less, wherein: 
     Component A is partially stabilized ZrO 2  (containing, e.g., a stabilizer such as Y 2  O 3 , CaO, and MgO) in which the fraction of the tetragonal and cubic ZrO 2  are at least 90% by weight, and the ratio of the tetragonal ZrO 2  to the cubic ZrO 2  is at least 1/3; and 
     Component B is at least one of Al 2  O 3  and TiN, with impurities being 3% by weight or less SiO 2 , 0.5% by weight or less Fe 2  O 3 , and 0.5% by weight or less TiO 2 , provided that the total amount of impurities is 3% by weight or less (based on the total weight). 
     According to a preferred embodiment of the invention, it has been found that when a co-precipitation method is employed to prepare a raw material consisting essentially of the components of ZrO 2  (and/or HfO 2 ), stabilizer, and Al 2  O 3  (and/or TiN) the resulting raw materials are dispersed more ideally, and by using the resulting raw materials, a sintered body can be obtained which has a uniform structure comprising fine grains, contains almost no micropores, and which has a strength as high as about 150 kg/mm 2  that could not be expected from conventional ceramic materials. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the high temperature strength of sintered bodies described in Example 1 and of a comparative specimen (No. R), i.e., commercially available partially stabilized zirconia sintered body manufactured by Corning Corp. 
     FIG. 2 shows the high temperature strength of sintered bodies described in Example 2. 
     FIG. 3 shows the high temperature strength of sintered bodies described in Example 3 and of the above-described comparative specimen (No. R). 
     FIG. 4 shows the high temperature strength of sintered bodies described in Example 4. 
     FIG. 5 shows the high temperature strength of sintered bodies described in Example 5 and of the above-described comparative specimen (No. R). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the high toughness sintered body of the invention, if the Al 2  O 3  or TiN content is less than 0.5% by weight, the effect of the addition of Al 2  O 3  or TiN is poor respectively, whereas if it is more than 60% by weight, ZrO 2  content is too low to give effect of strengthening and toughening due to ZrO 2  phase transformation. 
     Furthermore, the total fraction of the tetragonal ZrO 2  and the cubic ZrO 2  in the ZrO 2  should be at least 90% by weight thereof. When the fraction is less than 90% by weight, the toughness of the resulting sintered body is poor. It is also necessary that the ratio of the tetragonal ZrO 2  to the cubic ZrO 2  be at least 1/3. If the ratio is less than 1/3, the resulting sintered body has poor toughness. It is further required for the mean grain size of the sintered body to be 3 microns or less. If the mean grain size is more than 3 microns, the transformation from the tetragonal ZrO 2  to the monoclinic ZrO 2  will occur, resulting in a reduction in toughness. 
     The tolerable amounts of impurities are up to 3% by weight in the case of SiO 2  and up to 0.5% by weight each in the case of Fe 2  O 3  and TiO 2 , provided that the total amount of such impurities is 3% by weight or less. If the amount of each impurity or the total amount of impurities is more than the above-specified values, sintering properties are reduced, and only a sintered body having poor toughness can be obtained. 
     The same characteristics as above can also be obtained when part of all of the ZrO 2  is replaced by HfO 2 . 
     The following examples are given to illustrate the invention in greater detail. 
     EXAMPLE 1 
     To a monoclinic ZrO 2  having the characteristics shown in Table 1 were added Y 2  O 3 , CaO, or MgO as a stabilizer in the proportions shown in Table 2, and then fine particles of Al 2  O 3  having a mean particle size of 0.1 micron and a purity of 99.9% were added in proportions as shown in Table 2. The ingredients were then wet-mixed, and the resulting mixture was dried, powdered, press-molded, and sintered in an electric furnace in the air at 1,400° to 1,650° C. for 1 hour. After sintering, the thus-obtained sintered body was cut and ground to form a specimen of 4×8×25 mm. In this way, a series of specimens were produced. The phase composition and properties obtained from these specimens are shown in Table 2. In all the specimens, the mean grain size was less than 3 microns. When the sintering temperature was increased to higher temperatures than those shown in Table 2, the mean grain size was larger than 3 microns, and the strength was reduced. 
     As is shown clearly from Table 2, the addition of Al 2  O 3  inhibits the transformation from the tetragonal ZrO 2  to the monoclinic ZrO 2  and increases the fraction of the tetragonal ZrO 2 , improving the strength and toughness of the resulting sintered body. The hardness and high temperature flexural strength of some specimens were also measured. The results are shown in Table 3 and FIG. 1. It can be seen from the results that the hardness of the sintered body increased with increasing Al 2  O 3  content, for example, the hardness of specimen No. 33 with 60% Al 2  O 3  content was almost equal to that of Al 2  O 3  ceramics, and that the high temperature strength is markedly improved compared with the comparative partially stabilized zirconia sintered body which is commercially available from Corning Corp., U.S.A. (Specimen No. R). 
     
                       TABLE 1______________________________________Crystal System     MonoclinicSpecific Surface Area              25 m.sup.2 /gChemical Analytical ValuesZrO.sub.2 (incl. HfO.sub.2)              99% or more (containing              3 to 5% HfO.sub.2)SiO.sub.2          0.5%CaO                0.06%Fe.sub.2 O.sub.3   0.1%TiO.sub.2          0.25%______________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________Composition     Stabilizer                       Crystal System of ZrO.sub.2     for ZrO.sub.2             Sintering   Flexural     Mono-                                          Tetra-Specimen Al.sub.2 O.sub.3        Amount             Temperature                    Density                         Strength                                K.sub.IC                                      clinic                                          gonal                                              CubicNo.   (wt. %)     Type        (mol %)             (°C.)                    (g/cm.sup.3)                         (kg/mm.sup.2)                                (kg/mm.sup.3/2)                                      (wt %)                                          (wt %)                                              (wt %)                                                  Remarks__________________________________________________________________________ 1    0.1 Y.sub.2 O.sub.3        4    1,600  5.72 35.1    9.3  24  41  35  Comparison 2    0.5 &#34;  &#34;    &#34;      5.80 70.5   21.0  9   58  33  Present                                                  Invention 3    1.5 &#34;  &#34;    &#34;      5.83 72.4   22.5  4   64  32  Present                                                  Invention 4     3  &#34;  1    1,500  *    --     --    95   0   5  Comparison 5    &#34;   &#34;  1.5  1,400  5.76 35.7   18.0  43  49   8  &#34; 6    &#34;   &#34;  2    1,500  5.97 95.9   44.9  5   83  12  Present                                                  Invention 7    &#34;   &#34;  2.5  &#34;      5.95 91.7   37.8  4   77  19  Present                                                  Invention 8    &#34;   &#34;  3    1,600  5.94 82.3   30.2  3   73  24  Present                                                  Invention 9    &#34;   &#34;  4    &#34;      5.88 84.1   23.8  1   67  32  Present                                                  Invention10    &#34;   &#34;  6    &#34;      5.75 44.6   15.0  0   28  72  Present                                                  Invention11    &#34;   &#34;  8    &#34;      5.67 31.2   13.0  0    0  100 Comparison12    &#34;   MgO        7    1,500  5.75 70.0   21.0  3   61  36  Present                                                  Invention13    &#34;   CaO        6    &#34;      5.78 65.0   19.0  4   55  41  Present                                                  Invention14    10  Y.sub.2 O.sub.3        1    &#34;      *    --     --    96   0   4  Comparison15    &#34;   &#34;  2    &#34;      5.71 99.4   38.8  4   85  11  Present                                                  Invention16    &#34;   &#34;  3    &#34;      5.69 87.2   32.5  2   76  22  Present                                                  Invention17    &#34;   &#34;  4    &#34;      5.64 85.3   30.0  1   70  29  Present                                                  Invention18    &#34;   MgO        7    &#34;      5.53 72.5   29.1  0   68  32  Present                                                  Invention19    &#34;   CaO        6    &#34;      5.55 67.4   27.3  0   63  37  Present                                                  Invention20    20  Y.sub.2 O.sub.3        1    &#34;      5.45 102.2  33.6  9   87   4  Present                                                  Invention21    &#34;   &#34;  2    &#34;      5.43 110.8  35.0  0   90  10  Present                                                  Invention22    &#34;   &#34;  3    &#34;      5.41 95.1   30.9  0   81  19  Present                                                  Invention23    &#34;   &#34;  4    &#34;      5.37 93.0   28.2  0   74  26  Present                                                  Invention24    &#34;   MgO        7    &#34;      5.28 79.1   27.3  0   71  29  Present                                                  Invention25    &#34;   CaO        6    &#34;      5.30 73.5   26.2  0   67  33  Present                                                  Invention26    40  Y.sub.2 O.sub.3        1    &#34;      4.97 107.3  27.7  0   97   3  Present                                                  Invention27    &#34;   &#34;  2    &#34;      4.96 121.3  26.8  0   93   7  Present                                                  Invention28    &#34;   &#34;  3    &#34;      4.94 106.4  25.5  0   86  14  Present                                                  Invention29    &#34;   &#34;  4    &#34;      4.92 104.1  24.5  0   81  19  Present                                                  Invention30    &#34;   MgO        7    &#34;      4.87 88.5   23.9  0   78  22  Present                                                  Invention31    &#34;   CaO        6    &#34;      4.88 82.2   23.4  0   75  25  Present                                                  Invention32    60  Y.sub.2 O.sub.3        1    1,600  4.61 82.5   22.0  0   97   3  Present                                                  Invention33    &#34;   &#34;  2    &#34;      4.59 75.0   21.8  0   94   6  Present                                                  Invention34    &#34;   &#34;  3    &#34;      4.59 65.8   21.2  0   88  12  Present                                                  Invention35    &#34;   &#34;  4    &#34;      4.58 64.4   20.9  0   84  16  Present                                                  Invention36    &#34;   MgO        7    &#34;      4.55 54.7   20.7  0   82  18  Present                                                  Invention37    &#34;   CaO        6    &#34;      4.55 50.8   20.6  0   79  21  Present                                                  Invention38    70  Y.sub.2 O.sub.3        2    &#34;      4.43 43.3   11.5  0   94   6  Comparison__________________________________________________________________________ *disintegration 
    
     
                       TABLE 3______________________________________         Specimen No.         6    15      21     27   33______________________________________Amount of Al.sub.2 O.sub.3 (wt %)           3      10      20   40   60Hardness        83.5   84.0    85.1 86.5 87.7______________________________________ 
    
     Note: 
     Measurement of Physical Properties 
     (1) The flexural strength was measured according to JIS B4104-1970, and an average value of five specimens is indicated. 
     (2) The fracture toughness was measured according to ASTM Special Technical Publication No. 410; i.e., a specimen having a width of 4 mm, a thickness of 5 mm, and a length of 25 mm was provided with a notch having a depth of 0.5 mm and a width of 0.15 mm, and was measured by a three-point bending test with a span as 20 mm. An average value of five specimens is indicated. 
     (3) The hardness was measured by the use of a Rockwell Super Fischal hardness tester at a load of 45 kg. 
     (4) The crystal system was analyzed by X-ray diffraction using Geiger Flex Model RAD-γA manufactured by Rigaku Denki Co., Ltd. In the first place, by X-ray diffraction of a specimen which had been mirror-polished with a 15μ diamond paste, the integrated strength Im of each of the (111) plane and the (111) plane of monoclinic ZrO 2 , the integrated strength It of the (111) plane of tetragonal ZrO 2 , and the integrated strength Ic of the (111) plane of cubic ZrO 2  were measured, and the fraction of monoclinic ZrO 2  was determined by the ratio of Im/(Im+It+Ic). Then the sintered body was ground until all particles could pass through a 325 mesh screen and the ground particles were analyzed by X-ray diffraction under the same conditions as above to measure the integrated strength I&#39;m of monoclinic ZrO 2  and the integrated strength I&#39;c of cubic ZrO 2 . In this case, it is considered that the residual tetragonal ZrO 2  in the sintered body is subjected to mechanical stress by the above-described pulverization and undergoes a transformation into monoclinic ZrO 2 . Therefore, the fraction of cubic ZrO 2  is determined by the ratio of I&#39;c/(I&#39;m+I&#39;c) and then the fraction of tetragonal ZrO 2  is determined. 
     EXAMPLE 2 
     An aqueous solution of zirconium oxychloride and an aqueous solution of yttrium chloride were mixed, co-precipitated, and calcined at 800° to prepare a powder consisting of ZrO 2  and Y 2  O 3 . The characteristics of the powder are shown in Table 4. To the co-precipitated powder was added Al 2  O 3  powder having a mean particle size of 0.1 μm and a purity of 99.9% in the proportions shown in Table 5. Using the powder, a sintered body was produced in the same manner as in Example 1. The results are shown in Table 5. The high temperature strength was measured in the same manner as in Example 1, and the results are shown in FIG. 2. It can be seen from the results that even when the co-precipitated ZrO 2  powder is used, the addition of Al 2  O 3  provides a great effect, as was the case in Example 1. 
     
                       TABLE 4______________________________________Amount of Y.sub.2 O.sub.3           2 mol %     3 mol %Crystal System  Tetragonal  TetragonalSpecific Surface Area           32 m.sup.2 /g                       34 m.sup.2 /gChemical Analytical ValuesZrO.sub.2 (incl. HfO.sub.2)           95% (containing                       93.7% (containing           3 to 5% HfO.sub.2)                       3 to 5% HfO.sub.2)Y.sub.2 O.sub.3 4.04%       5.30%CaO             0.09%       0.06%Na.sub.2 O      0.05%       0.05%______________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________Composition    Stabilizer                      Crystal System of ZrO.sub.2    for ZrO.sub.2            Sintering   Flexural    Mono-                                        Tetra-SpecimenAl.sub.2 O.sub.3       Amount            Temperature                   Density                        Strength                              K.sub.IC                                    clinic                                        gonal                                            CubicNo.  (wt %)    Type       (mol %)            (°C.)                   (g/cm.sup.3)                        (kg/mm.sup.2)                              (kg/mm.sup.3/2)                                    (wt %)                                        (wt %)                                            (wt %)                                                Remarks__________________________________________________________________________101  20  Y.sub.2 O.sub.3       2    1,500  5.51 112.5 35.4  0   94  6   Present                                                Invention102  &#34;   &#34;  3    &#34;      5.48 96.7  31.2  0   87  13  Present                                                Invention103  40  &#34;  2    &#34;      5.03 124.0 27.3  0   97  3   Present                                                Invention104  &#34;   &#34;  3    &#34;      5.01 108.1 25.9  0   90  10  Present                                                Invention105  60  &#34;  2    1,600  4.63 78.3  22.1  0   97  3   Present                                                Invention106  &#34;   &#34;  3    &#34;      4.62 70.1  21.5  0   91  9   Present                                                Invention__________________________________________________________________________ 
    
     EXAMPLE 3 
     To a monoclinic ZrO 2  having the characteristics shown in Table 1 above were added Y 2  O 3 , Cao, or MgO as a stabilizer in the proportions shown in Table 6, and then fine particles of TiN having a mean particle size of 0.1 micron and a purity of 99.9% were added in proportions as shown in Table 6. The ingredients were then wet-mixed, and the resulting mixture was dried, powdered, press-molded, and sintered in an electric furnace in N 2  atmosphere at 1,400° to 1,650° C. for 1 hour. After sintering, the thus-obtained sintered body was cut and ground to form a specimen of 4×8×25 mm. In this way, a series of specimens were produced. The phase composition and properties obtained from these specimens are shown in Table 6. In all the specimens, the mean grain size was less than 3 microns. When the sintering temperature was increased to higher temperature than those shown in Table 6, the mean grain size was larger than 3 microns, and the strength was reduced. 
     As is shown clearly from Table 6, the addition of TiN inhibits the transformation from the tetragonal ZrO 2  to the monoclinic ZrO 2  and increases the fraction of the tetragonal ZrO 2 , improving the strength and toughness of the resulting sintered body. The hardness and high temperature flexural strength of some specimens were also measured. The results are shown in Table 7 and FIG. 3. It can be seen from the results that the addition of TiN greatly increases the hardness and strength, and in particular, the high temperature strength is markedly improved compared with the comparative partially stabilized zirconia sintered body which is commercially available from Corning Corp., U.S.A. (Specimen No. R). 
     
                                           TABLE 6__________________________________________________________________________Composition     Stabilizer                       Crystal System of ZrO.sub.2     for ZrO.sub.2             Sintering   Flexural     Mono-                                          Tetra-Specimen TiN    Amount             Temperature                    Density                         Strength                                K.sub.IC                                      clinic                                          gonal                                              CubicNo.   (wt %)     Type        (mol %)             (°C.)                    (g/cm.sup.3)                         (kg/mm.sup.2)                                (kg/mm.sup.3/2)                                      (wt %)                                          (wt %)                                              (wt %)                                                  Remarks__________________________________________________________________________T1    0.1 Y.sub.2 O.sub.3        4    1,600  5.74 33.4   10.8  27  38  35  ComparisonT2    0.5 &#34;  &#34;    &#34;      5.83 67.0   29.5  10  57  33  Present                                                  InventionT3    1.5 &#34;  &#34;    &#34;      5.86 68.8   33.8  6   63  31  Present                                                  InventionT4     3  &#34;  1    1,400  *    --     --    96   0   4  ComparisonT5    &#34;   &#34;  1.5  &#34;      5.79 33.9   19.3  44  47   9  &#34;T6    &#34;   &#34;  2    1,500  6.01 91.1   45.3  7   82  11  Present                                                  InventionT7    &#34;   &#34;  2.5  &#34;      5.99 87.1   42.4  5   76  19  Present                                                  InventionT8    &#34;   &#34;  3    1,600  5.97 79.9   41.5  4   73  23  Present                                                  InventionT9    &#34;   &#34;  4    &#34;      5.91 78.2   38.7  2   64  34  Present                                                  InventionT10   &#34;   &#34;  6    &#34;      5.79 42.4   16.1  0   25  75  Present                                                  InventionT11   &#34;   &#34;  8    &#34;      5.70 29.6   15.3  0    0  100 ComparisonT12   &#34;   MgO        7    1,500  5.78 66.5   37.1  4   59  37  Present                                                  InventionT13   &#34;   CaO        6    &#34;      5.80 61.8   35.2  5   53  42  Present                                                  InventionT14   10  Y.sub.2 O.sub.3        1    &#34;      *    --     --    97   0   3  ComparisonT15   &#34;   &#34;  2    &#34;      5.95 94.4   43.2  6   85   9  Present                                                  InventionT16   &#34;   &#34;  3    &#34;      5.91 82.8   40.7  3   76  21  Present                                                  InventionT17   &#34;   &#34;  4    &#34;      5.86 81.0   39.1  1   70  29  Present                                                  InventionT18   &#34;   MgO        7    &#34;      5.74 68.9   38.7  0   69  31  Present                                                  InventionT19   &#34;   CaO        6    &#34;      5.76 64.1   37.6  0   65  35  Present                                                  InventionT20   20  Y.sub.2 O.sub.3        1    &#34;      5.91 97.1   41.4  9   88   3  Present                                                  InventionT21   &#34;   &#34;  2    &#34;      5.89 105.3  41.1  5   87   8  Present                                                  InventionT22   &#34;   &#34;  3    &#34;      5.85 90.5   39.7  0   81  19  Present                                                  InventionT23   &#34;   &#34;  4    &#34;      5.81 88.4   37.7  0   73  27  Present                                                  InventionT24   &#34;   MgO        7    &#34;      5.71 75.1   36.8  0   70  30  Present                                                  InventionT25   &#34;   CaO        6    &#34;      5.72 69.8   35.8  0   66  34  Present                                                  InventionT26   40  Y.sub.2 O.sub.3        1    &#34;      5.71 101.4  37.8  0   98   2  Present                                                  InventionT27   &#34;   &#34;  2    &#34;      5.69 115.3  36.9  0   94   6  Present                                                  InventionT28   &#34;   &#34;  3    &#34;      5.67 101.2  35.5  0   87  13  Present                                                  InventionT29   &#34;   &#34;  4    &#34;      5.64 98.9   33.9  0   80  20  Present                                                  InventionT30   &#34;   MgO        7    &#34;      5.56 84.1   33.7  0   79  21  Present                                                  InventionT31   &#34;   CaO        6    &#34;      5.57 78.2   32.4  0   74  26  Present                                                  InventionT32   60  Y.sub.2 O.sub.3        1    1,600  5.59 78.4   29.7  0   98   2  Present                                                  InventionT33   &#34;   &#34;  2    &#34;      5.58 72.3   28.3  0   93   7  Present                                                  InventionT34   &#34;   &#34;  3    &#34;      5.56 62.9   27.1  0   89  11  Present                                                  InventionT35   &#34;   &#34;  4    &#34;      5.54 62.3   25.6  0   85  15  Present                                                  InventionT36   &#34;   MgO        7    &#34;      5.49 52.1   23.7  0   81  19  Present                                                  InventionT37   &#34;   CaO        6    &#34;      5.50 49.5   22.9  0   80  20  Present                                                  InventionT38   70  Y.sub.2 O.sub.3        2    &#34;      5.52 41.1   13.2  0   94   6  ComparisonT39   &#34;   MgO        7    &#34;      5.46 39.6   12.6  0   84  16  &#34;T40   &#34;   CaO        6    &#34;      5.47 38.4   10.8  0   79  21  &#34;__________________________________________________________________________ *disintegration 
    
     
                       TABLE 7______________________________________        Specimen No.        T6    T15    T21     T27  T33______________________________________Amount of TiN (wt %)          3       10     20    40   60Hardness       83.4    83.8   84.5  85.7 86.1______________________________________ 
    
     EXAMPLE 4 
     An aqueous solution of zirconium oxychloride and an aqueous solution of yttrium chloride were mixed, co-precipitated, and calcined at 800° C. to prepare a powder consisting of ZrO 2  and Y 2  O 3 . The characteristics of the powder are shown above in Table 4. To the co-precipitated powder was added TiN powder having a mean particle size of 0.1 μm and a purity of 99.9% in the proportions shown in Table 8. Using the powder, a sintered body was produced in the same manner as in Example 3. The results are shown in Table 8. The high temperature strength was measured in the same manner as in Example 1, and the results are shown in FIG. 4. It can be seen from the results that even when the co-precipitated ZrO 2  powder is used, the addition of TiN provides a great effect, as was the case with Example 3. 
     
                                           TABLE 8__________________________________________________________________________Composition    Stabilizer                      Crystal System of ZrO.sub.2    for ZrO.sub.2            Sintering   Flexural    Mono-                                        Tetra-SpecimenTiN    Amount            Temperature                   Density                        Strength                              K.sub.IC                                    clinic                                        gonal                                            CubicNo.  (wt %)    Type       (mol %)            (°C.)                   (g/cm.sup.3)                        (kg/mm.sup.2)                              (kg/mm.sup.3/2)                                    (wt %)                                        (wt %)                                            (wt %)                                                Remarks__________________________________________________________________________T101 20  Y.sub.2 O.sub.3       2    1,500  5.95 106.9 41.5  6   89   5  Present                                                InventionT102 &#34;   &#34;  3    &#34;      5.91 91.9  41.1  0   87  13  Present                                                InventionT103 40  &#34;  2    &#34;      5.81 117.8 37.3  0   96   4  Present                                                InventionT104 &#34;   &#34;  3    &#34;      5.78 102.7 35.9  0   89  11  Present                                                InventionT105 60  &#34;  2    1,600  5.68 74.4  29.6  0   97   3  Present                                                InventionT106 &#34;   &#34;  3    &#34;      5.66 66.8  27.5  0   90  10  Present                                                Invention__________________________________________________________________________ 
    
     EXAMPLE 5 
     To a 1 mol% solution of zirconium oxychloride having a purity of 99.9% (wherein the ZrO 2  component contains 3 to 5% of HfO 2 ) were added yttrium chloride, magnesium chloride or calcium chloride as a stabilizer, all having a purity of 99.9%, and aluminum chloride having a purity of 99.9% so as to prepare a mixture having the composition shown in Table 9. They were uniformly mixed and then co-precipitated to obtain a hydroxide mixture. The hydroxide mixture thus prepared was dehydrated, dried, and calcined at 800° C. to obtain a starting powder having a mean particle size of 200 Å. The thus-obtained powder was press-molded at a pressure of 1.5 ton/cm 2 , and sintered in an electric furnace in the air at 1,400° to 1,650° C. for 1 hour. After sintering, the resulting sintered body was cut and ground to provide a specimen of 4×8×25 mm. In this way, a series of sintered bodies were produced. In all of the sintered bodies, the mean grain size was less than 3 microns. However, when the sintering temperature was increased to higher temperatures than those shown in Table 9, the mean grain sizes of resulting sintered bodies were larger than 3 microns, resulting in a reduction in strength. 
     As shown clearly from Table 9, the co-precipitation of Al 2  O 3  inhibits the transformation from the tetragonal ZrO 2  to the monoclinic ZrO 2  and increases the fraction of the tetragonal ZrO 2 , improving the strength and toughness. The hardness and high temperature flexural strength of some specimens were also measured. The results are shown in Table 10 and FIG. 5. It can be seen from the results that the hardness of the sintered body increased with increasing Al 2  O 3  content, for example, the hardness of the specimen No. P34 with 60% Al 2  O 3  content was almost equal to that of Al 2  O 3  ceramics, and that the high temperature strength markedly improved compared with the comparative partially stabilized zirconia sintered body commercially available from Corning Corp., U.S.A. (Specimen No. R). 
     
                                           TABLE 9__________________________________________________________________________Composition     Stabilizer                       Crystal System of ZrO.sub.2     for ZrO.sub.2             Sintering   Flexural     Mono-                                          Tetra-Specimen Al.sub.2 O.sub.3        Amount             Temperature                    Density                         Strength                                K.sub.IC                                      clinic                                          gonal                                              CubicNo.   (wt %)     Type        (mol %)             (°C.)                    (g/cm.sup.3)                         (kg/mm.sup.2)                                (kg/mm.sup.3/2)                                      (wt %)                                          (wt %)                                              (wt %)                                                  Remarks__________________________________________________________________________P1     0  Y.sub.2 O.sub.3        4    1,600  6.04 60.7   18.9  8   57  35  ComparisonP2    0.1 &#34;  4    &#34;      6.04 63.5   20.1  3   63  34  &#34;P3    0.5 &#34;  4    &#34;      6.02 81.1   22.6  1   65  34  Present                                                  InventionP4    1.5 &#34;  4    &#34;      5.99 83.3   23.8  0   67  33  Present                                                  InventionP5     3  &#34;  1    1,400  *    --     --    96   0   4  ComparisonP6    &#34;   &#34;  1.5  &#34;      5.91 41.1   18.1  41  52   7  &#34;P7    &#34;   &#34;  2    1,500  6.00 110.3  47.1  4   85  11  Present                                                  InventionP8    &#34;   &#34;  2.5  &#34;      5.99 105.5  39.7  3   79  18  Present                                                  InventionP9    &#34;   &#34;  3    &#34;      5.97 96.7   31.7  1   76  23  Present                                                  InventionP10   &#34;   &#34;  4    1,600  5.95 94.6   25.0  0   68  32  Present                                                  InventionP11   &#34;   &#34;  6    &#34;      5.88 51.3   15.8  0   26  74  Present                                                  InventionP12   &#34;   &#34;  8    &#34;      5.76 35.7   13.7  0    0  100 ComparisonP13   &#34;   MgO        7    1,500  5.78 80.5   22.1  2   63  35  Present                                                  InventionP14   &#34;   CaO        6    &#34;      5.81 74.8   20.0  3   57  40  Present                                                  InventionP15   10  Y.sub.2 O.sub.3        1    &#34;      *    --     --    97   0   3  ComparisonP16   &#34;   &#34;  2    &#34;      5.79 117.3  41.5  2   89   9  Present                                                  InventionP17   &#34;   &#34;  3    &#34;      5.76 102.9  34.8  1   80  19  Present                                                  InventionP18   &#34;   &#34;  4    1,600  5.74 100.7  32.1  0   73  27  Present                                                  InventionP19   &#34;   MgO        7    1,500  5.56 85.6   31.1  0   71  29  Present                                                  InventionP20   &#34;   CaO        6    &#34;      5.58 79.5   29.2  0   67  33  Present                                                  InventionP21   20  Y.sub.2 O.sub.3        1    &#34;      5.52 120.6  36.6  5   93   2  Present                                                  InventionP22   &#34;   &#34;  2    &#34;      5.51 130.7  38.2  0   95   5  Present                                                  InventionP23   &#34;   &#34;  3    &#34;      5.49 112.2  33.7  0   89  11  Present                                                  InventionP24   &#34;   &#34;  4    1,600  5.48 109.7  30.7  0   78  22  Present                                                  InventionP25   &#34;   MgO        7    1,500  5.31 93.3   29.8  0   75  25  Present                                                  InventionP26   &#34;   CaO        6    &#34;      5.33 86.7   28.6  0   71  29  Present                                                  InventionP27   40  Y.sub.2 O.sub.3        1    &#34;      5.03 134.1  31.0  0   98   2  Present                                                  InventionP28   &#34;   &#34;  2    &#34;      5.03 151.6  30.1  0   96   4  Present                                                  InventionP29   &#34;   &#34;  3    &#34;      5.02 133.0  28.6  0   93   7  Present                                                  InventionP30   &#34;   &#34;  4    1,600  5.01 130.1  27.4  0   86  14  Present                                                  InventionP31   &#34;   MgO        7    1,500  4.89 110.6  26.8  0   81  19  Present                                                  InventionP32   &#34;   CaO        6    &#34;      4.90 102.8  26.2  0   79  21  Present                                                  InventionP33   60  Y.sub.2 O.sub.3        1    1,600  4.63 98.9   24.2  0   100  0  Present                                                  InventionP34   &#34;   &#34;  2    &#34;      4.63 90.2   23.9  0   98   2  Present                                                  InventionP35   &#34;   &#34;  3    &#34;      4.62 78.9   23.3  0   94   6  Present                                                  InventionP36   &#34;   &#34;  4    &#34;      4.62 77.3   23.1  0   89  11  Present                                                  InventionP37   &#34;   MgO        7    &#34;      4.57 65.6   22.8  0   85  15  Present                                                  InventionP38   &#34;   CaO        6    &#34;      4.58 61.0   22.7  0   81  19  Present                                                  InventionP39   70  Y.sub.2 O.sub.3        2    &#34;      4.45 49.8   12.7  0   98   2  ComparisonP40   &#34;   MgO        7    &#34;      4.39 48.0   11.5  0   86  14  &#34;P41   &#34;   CaO        6    &#34;      4.40 46.6   10.2  0   83  17  &#34;__________________________________________________________________________ *disintegration 
    
     
                       TABLE 10______________________________________         Specimen No.         P7   P16     P22    P28  P34______________________________________Amount of Al.sub.2 O.sub.3 (wt %)           3      10      20   40   60Hardness        83.6   84.2    85.1 86.7 87.8______________________________________ 
    
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.