Abstract:
A process for the production of high-density zirconia-based ceramics having high fracture toughness and strength suitable for use as nonconductive weld/guide pins, engine components and wear resistant parts. The process consists of mixing zirconia (with formula ZrO 2 ) yttria (Y 2 O 3 ) and ceria (CeO 2 ) in proper proportions to produce partially stabilized zirconia (PSZ) and then doping the PSZ with chromium oxide (Cr 2 O 3 ) to enhance the mechanical strength of the resultant body. The resultant sintered body has fracture toughness in excess of 15 MPa.m 1/2 , Vicker&#39;s Hardness in excess of 8.5 GPa and flexural strength of 1150 MPa.

Description:
FIELD OF INVENTION 
       [0001]    The invention relates to doped partially stabilized zirconia ceramics that exhibit high strength, high fracture toughness and excellent resistance to wear, and a method of producing the same. More particularly, the invention relates to the simultaneous addition of yttria (Y 2 O 3 ), ceria (CeO 2 ) and chromia (Cr 2 O 3 ) to partially stabilized zirconia (ZrO 2 ) in order to create a microstructure possessing high fracture toughness and strength. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    It is well known that zirconia ceramics exist in three different structural forms: cubic form, tetragonal form and monoclinic form, depending on the temperature. It is also known that the addition of stabilizing agents such as yttria (Y 2 O 3 ) and ceria (CeO 2 ) to zirconia serves to stabilize the tetragonal phase to room temperature and impart desirable mechanical properties. Although it is accepted that both yttria and ceria are useful as stabilizing agents, their role is quite different. For example, the addition of CeO 2  to zirconia polycrystals results in a very high fracture toughness but low fracture strength and hardness as described in Journal of Materials Science, Vol. 20, 1985, pp 1178-1184. To compensate the disadvantage of lower strength and hardness in ceria-doped partially stabilized zirconia, alumina (Al 2 O 3 ) ceramics were added to zirconia as described in Advances in ceramics, 24, science and Technology of zirconia III, The American Ceramic Society, Westerville, Ohio, 1988, pp 721-728. In this work, it has been shown that the strength reached values of up to 900 MPa but the fracture toughness dropped from 20 MPa to 5.5 MPa with increasing alumina content. To further improve the fracture strength, the ceria stabilized zirconia was doped with TiO 2  which is known to dissolve in tetragonal zirconia and acts as a stabilizing agent in a similar manner to Y 2 O 3  and CeO 2 , as described in Ceramics International 24, 1998, pp 497-506. Similarly, small amounts of tantalum oxide (Ta 2 O 5 ) dissolved in yttria stabilized zirconia was found to increase both fracture toughness and bending strength as described in U.S. Pat. No. 4,886,768. Yttria was most frequently used to stabilize zirconia and also to impart higher strength to the resultant body. To enhance the strength and thermal stability, a zirconia ceramic composition consisting essentially of zirconia containing yttria, ceria and alumina has been proposed in Japanese Patent No. 61-77665 and a zirconia ceramic composition consisting essentially of yttria, magnesia (MgO) and alumina has been proposed in U.S. Pat. No. 4,820,667. The U.S. Pat. No. 4,820,667 also states that the strength of partially stabilized zirconia ceramics is greatly influenced by the ratio of oxides in the mixture and if the mixture ratio is outside of the predetermined range, the bending strength of the partially stabilized zirconia ceramics may not be increased. For example, the highest bending strength was achieved in samples containing 3 mol % yttria, 0 mol % ceria and alumina/magnesia ratio of 90/10. However, no data on the effect of the composition on fracture toughness of the sintered body was reported in the former publication. It has been generally accepted that the zirconia ceramic compositions containing yttria as a stabilizing agent are not useful to enhance fracture toughness, whereas the zirconia ceramic compositions containing ceria as a stabilizing agent are not useful to enhance the strength. It is therefore desirable to provide a zirconia ceramic and a process for producing the same, that has a bending strength equal or higher than that of zirconia ceramic stabilized with yttria and fracture toughness at the level of equal or higher than that of zirconia stabilized with ceria. 
         [0003]    In the present invention it was found that if yttria, ceria and chromia are added to zirconia in a proper proportion, both high bending strength and high fracture toughness can be achieved in PSZ. It is, therefore, a primary objective of the present invention to provide a partially stabilized zirconia ceramic superior in bending strength and fracture toughness on the basis of determining an optimum concentration of yttria, ceria and chromia (Cr 2 O 3 ). 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is directed in overcoming the problems set forth above. Briefly summarized, according to one aspect of the present invention there is provided a process for producing a zirconia ceramic possessing bending strength of over 1150 MPa and fracture toughness of over 15 MPa.m 1/2 . According to another aspect, the present invention provides zirconia ceramics that contain yttria from about 1 mol. % to about 2.5 mol. %, ceria from about 4 mol. % to about 7 mol. %, and chromia from about 0.01 mol. % to about 0.035 mol. % and that average crystal grain size constituting the ceramics is less than 0.5 μm, and at least 80% of the crystal grains are tetragonal grains The first step of the process of the invention comprises mixing of the stabilizing agents and a dopant in the presence of water soluble binder and dispersants, drying the mix at 110° C. for 6-8 hours and compacting the powder under the pressure of 150 MPa. In order to achieve high green density, the compacting is carried out for a time sufficient to compact the powder to form a green part having a density in the range of about 3.1 g/cm 3  to about 3.6 g/cm 3 . A preferred density is about 3.4 g/cm 3 . The second step of the process comprises of: sintering the green part by heating the green part from room temperature at the heating ramp in the range of about 1° C./min. to about 2° C./min. to a sintering temperature T 1  in the range of about 1450° C. to about 1580° C.; maintaining the sintering temperature for about 2 hours; cooling the sintered part from the sintering temperature at the cooling ramp in the range of from about 6° C./min to about 10° C./min. 
         [0005]    The zirconia ceramic produced by the process of the invention consists essentially of tetragonal phase crystal grain over the specific range of dopant level. The invention overcomes the disadvantages of the prior art by providing a microstructure consisting essentially of tetragonal phase over a certain range of additive level, having the bending strength of over 1150 MPa and fracture toughness of over 15 MPa.m 1/2 , improved wear resistance and service lifetime, and is useful in applications such as weld/guide pins. 
     
    
     
       BRIEF DESCRIPTION OF TH FIGURES 
         [0006]      FIG. 1  is an X-ray diffraction curve before fracture (as received samples) for a zirconia ceramic of the invention doped with 6.5 mole percent CeO 2 , 1.8 mole percent Y 2 O 3 , 0.01 mol percent Cr 2 O 3  and 91.7% mol ZrO 2    
           [0007]      FIG. 2  shows polished and etched surface of a zirconia ceramic doped with CeO 2 , Y 2 O 3  and Cr 2 O 3 . 
           [0008]      FIG. 3  shows fracture surface of a zirconia ceramic doped with CeO 2 , Y 2 O 3  and Cr 2 O 3    
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    Most additives in ZrO 2  go into solid solution at the sintering temperature and affect the polytype of zirconia retained and hence affect the fracture toughness and bending strength. Such additives include CeO 2 , Y 2 O 3 , MgO and lanthanide oxides. Cr 2 O 3  does not go into solid solution with zirconia to a large extent and is an effective grain growth inhibitor. Fracture surface observations revealed that the addition of Cr 2 O 3  to zirconia does limit the grain growth resulting in higher hardness and strength. It is also found that the presence of Cr 2 O 3  does not have an adverse effect on fracture toughness resulting in zirconia ceramics having both high fracture toughness and high bending strength. By controlling the amount of CeO 2 , Y 2 O 3  and Cr 2 O 3 , partially stabilized zirconia ceramics can be made with strength of over 1150 MPa which is similar to yttria-doped tetragonal zirconia particle (Y-TZP) with the added benefit of higher fracture toughness that can be achieved in ceria-doped tetragonal zirconia (Ce-TZP). It has been discovered that a fracture toughness of over 15 MPa.m 1/2  and bending strength of over 1150 MPa can be achieved in partially stabilized zirconia provided that the level of Y 2 O 3  is kept below 2.5 mol. %, the level of CeO 2  is kept below 7 mol. % and the content of Cr 2 O 3  is kept below 0.035 mol. % but not less than 0.01 mol. %. It has also been discovered that only with the simultaneous addition of all three additives in proper proportion can both high fracture toughness and high strength be achieved. For the purpose of the present invention, the terms high toughness and high strength refer to values in excess of 15 MPa.m 12  and in excess of 1150 MPa, respectively. If the level of Y 2 O 3  is increased above about 2.5 mol. % the strength of the resultant zirconia will increase but the fracture toughness will decrease to less than 6-7 MPa.m 1/2 . Similarly, if the level of CeO 2  is increased to above 7 mol. % the fracture toughness will increase to above 15 MPa.m 1/2  but the bending strength will decrease below 800-900 MPa. In the present invention, the enhancement of fracture toughness coincides with optimum addition of cerium oxide and the enhancement of bending strength coincides with optimum addition of yttrium oxide and chromium oxide. High toughness and high strength can sometimes be achieved by adding whiskers or short fibers to a ceramic matrix, as has been demonstrated for SiC whisker-reinforced alumina. However, the problem with this composite is that the presence of whiskers in the matrix inhibits sintering and so far it was impossible to densify the composite to a level above 98% of theoretical density using pressureless sintering. 
         [0010]    Conventional powder processing techniques can be used to make high density ceramics. Although the additives (Y 2 O 3 , CeO 2  and Cr 2 O 3 ) can be used in the form of commercially available powders, co-precipitation or sol-gel processing can be used to synthesize the powders first before using them in that form as raw materials. 
         [0011]    When using conventional powder processing techniques, the selected reactants can be mixed by ball milling, vibratory milling, attrition milling, jet milling, high shear mixing or another suitable technique. The powder is then formed by pressing, injection molding, slip casting, extrusion, tape casting, or any other conventional method used for ceramic processing. 
         [0012]    The sintering is generally done at temperatures ranging between 1350° C. and 1750° C. in a heating furnace and held at the sintering temperature for several hours, for example 1-4 hours. The sintering atmosphere may be optionally chosen depending on the purpose. For example, air, oxygen, non-oxidizing atmosphere such as a vacuum, nitrogen, argon, or the like or first in the air and then in the non-oxidizing atmosphere can be used. 
         [0013]    Furthermore, the matrix consists of a tetragonal crystal structure which contains yttrria and ceria as stabilizers and chromia as a grain growth inhibitor. The obtained zirconia of this invention is of fine grain with equiaxed grains between 0.1 and 3 μm, preferably below 0.5 μm in diameter which provides very high strength without lowering the fracture toughness below 15 MPa.m 1/2 . 
         [0014]    As explained in detail thereabove, this invention provides zirconia ceramics which contains CeO 2  and Y 2 O 3  as stabilizers, and Cr 2 O 3  as a grain growth inhibitor, having very small average grain size, at the level below about 3 μm and preferably below about 0.5 μm, of the tetragonal crystal phase, which is superior to CeO 2 -containing zirconia ceramic in bending strength and markedly superior to Y 2 O 3 -containing zirconia ceramics in fracture toughness and in bending strength as compared with those containing CeO 2  and Y 2 O 3  individually and not in combination. The zirconia ceramics of this invention are useful for applications as weld/guide pins, engine parts, extrusion and drawing dies, ball for boll point pens, mechanical seals, and solid electrolyte materials such as oxygen sensors. 
       Examples 1-4 
       [0015]    Zirconia powders comprising 1 to 2.5 mol. % Y 2 O 3 , 3 to 7 mol. % CeO 2 , 0.01 to 0.0.035 mol. % Cr 2 O 3  and 89.5 mol. % to 90.5 mol. % ZrO 2 , were mixed for 6 hours in a plastic jar with methanol as the vehicle and zirconia balls as the milling media. The slurry was dried in a dryer at 75° C. for 12 hours. The powder mixture was dry screened to −40 mesh before uniaxial pressing at 150 MPa, followed by cold isostatic pressing at 250 MPa. The rectangular shape specimens (35×16×8 mm) were sintered in air at temperatures in the range from 1450° C. to 1650° C. for 1 to 4 hours. 
         [0016]    The rate of heating was as follows: 0.5° C./min to 500° C., 1° C./min from 500° C. to sintering temperature. Fracture toughness was determined by the indentation method on the polished surfaces and the fracture strength was determined by four-point bending from the fracture of the bend specimens at cross head speed of 0.5 mm/min. Properties of sintered samples containing 6 mol. % ceria and various levels of ytria are given in Table 1. 
         [0017]    Detailed scanning microscopy and X-ray diffraction analysis revealed the presence of 13.63 vol. % monoclinic phase with the remaining phase being the tetragonal phase. The results of Table 1 also show that the resultant zirconia body has fine grain structure and the highest bending strength is achieved with material having lowest mean particle size. 
       Examples 5-8 
       [0018]    A series of tests were performed in which starting powders were prepared from the co-precipitation of the aqueous solution of zirconium oxi-chloride (ZrOCl 2 ), yttrium nitrate (YNO 3 ) 3 , cerium nitrate and chromium nitrate. The composition of the powder was the same as in examples 1-4. The obtained precipitates were dried and transformed to the oxides by calcination. The as synthesized powder was milled to particle sizes below 1 μm, followed by drying, crushing and sieving through the sieve −40 mesh size. The rectangular shape bars (35×16×8 mm) were isostatically pressed and sintered in air at temperatures in the range from 1450° C. to 1650° C. for 1 to 4 hours. Mechanical properties of the sintered samples as a function of ceria content are presented in Table 2. The level of ytria was kept at the level of 2 mol. %. The highest fracture toughness was obtained in samples having the highest mole percent of ceria additive. 
       Examples 9-11 
       [0019]    In this set of samples, the initial powder was prepared from the co-precipitation of aqueous solutions of zirconia, yttria, ceria and chromia. The concentrations of ceria was kept at 6 mol. % and yttria at 2 mol. %. The chromia content was varied from 0.2 to 0.9 mol. %. The obtained precipitates were dried, screened, milled, pressed and sintered in the same manner as in examples 1-4. The resultant mechanical property data is presented in Table 3. The results in Table 3 show that as the amount of chromia is increased, the bending strength of the sintered body is increased. 
         [0020]    The process and product of this invention are explained in detailed in the proceeding examples which are illustrative only. Those skilled in the art will recognize that there are numerous modifications and variations and that the present invention is not limited to such examples. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Fracture 
                   
                 Mean  
                   
               
               
                   
                 toughness 
                 Bending 
                 particle 
                 Yttria 
               
               
                   
                 K IC , 
                 strength 
                 size 
                 content 
               
               
                 Sample 
                 MPa · m 1/2   
                 MPa 
                 μm 
                 Mol. % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 No. 1 
                 6.6 
                 1150 
                 0.35 
                 1.0 
               
               
                 No. 2 
                 12.0 
                 850 
                 0.7 
                 2.5 
               
               
                 No. 3 
                 15.7 
                 1100 
                 0.50 
                 1.5 
               
               
                 No. 4 
                 16.0 
                 900 
                 0.7 
                 2.0 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Fracture 
                   
                 Mean 
                   
               
               
                   
                   
                 toughness 
                 Bending 
                 particle 
                 Ceria 
               
               
                   
                   
                 K IC ,  
                 strength  
                 size 
                 content 
               
               
                   
                 Sample 
                 MPa · m 1/2   
                 MPa 
                 μm 
                 Mol. % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 No. 5 
                 6.5 
                 1180 
                 0.30 
                 3 
               
               
                   
                 No. 6 
                 13.0 
                 900 
                 1.0 
                 4 
               
               
                   
                 No. 7 
                 14.7 
                 1100 
                 0.35 
                 6 
               
               
                   
                 No. 8 
                 15.0 
                 1100 
                 0.5 
                 7 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Fracture 
                   
                   
                 Mean  
               
               
                   
                 toughness 
                 Bending 
                 Chromia 
                 particle 
               
               
                   
                 K IC ,  
                 strength 
                 content 
                 size 
               
               
                 Sample 
                 MPa · m 1/2   
                 MPa 
                 Mol. % 
                 μm 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 No. 9  
                 6.0 
                 1220 
                 0.01 
                 0.2 
               
               
                 No. 10 
                 12.5 
                 1100 
                 0.02 
                 0.5 
               
               
                 No. 11 
                 15.9 
                 980 
                 0.03 
                 0.9