High strength zirconia ceramic

Abstract: A zirconia ceramic composition consisting essentially of ZrO₂ containing not more than 5.0 mol% Y₂O₃ as a stabilizer and further containing aluminum and magnesium compounds in a combined amount of from 1 to 30 weight% expressed in terms of Al₂O₃ and MgO relative to the amount of ZrO₂. From 0.5 to 12 mol% CeO₂; may also be present.

Description: The present invention relates to zirconia ceramics and is more particularly concerned with zirconia ceramic compositions superior in bending strength, acid resistance and thermal stability. Zirconia ceramics containing 1.5 -5.0 mol% yttria as a stablizer are each known in the art as a partially stabilized zirconia ceramic (PSZ) which has been developed as a high strength zirconia ceramic useful for mechanical structure materials. The partially stabilized zirconia ceramic is, however, still insufficient in bending strength. In Japanese Kokai Publication No. 60-226457 (a counterpart to EP-A-0 140 638), there has been proposed a method of producing a high strength zirconia ceramic the bending strength of which is higher than that of the partially stabilized zirconia ceramic. In this method, raw material fine powder with a specific primary mean particle diameter was mixed at a predetermined ratio and made by Hot Isostatic Pressing or sintered by uniaxial pressing to produce a ceramic composition consisting essentially of Zr0₂ containing 1.5 - 5.0 mol% Y₂O₃ and Al₂O₃-MgO as a stabilizer. During change of the ambient temperature from a high temperature to room temperature, the zirconia ceramic in the cubic structure form transforms into the tetragonal structure then to the monoclinic structure form with consequent volume changes. When the crystal structure changes on cooling from tetragonal to monoclinic, the zirconia ceramic is degradated due to a volume expansion thereof. For the purpose of preventing such degradation, there has been proposed a method of restraining the transformation of crystal stucture in ZrO₂ by solid solution of a stabilizer such as CaO, MgO, Y₂O₃ and the like. At present, Y₂O₃ is used as the stabilizer to produce a partially stabilized zirconia ceramic of high strength and fracture toughness in the tetragonal structure form at room temperature. Such partially stabilized zirconia ceramics are, however, unstable in crystal phase and transform into the monoclinic structure when heated at a relatively low temperature within a range of 200° - 400°C, resulting in deterioration in strength, fracture toughness and thermal stability thereof. To enhance the strength and thermal stability, a zirconia ceramic composition consisting essentially of ZrO₂ containing Y₂O₃ and Al₂O₃ has been proposed in Japanese Kokai Publication No. 58-32066, and a zirconia ceramic composition consisting essentially of ZrO₂ containing Y₂O₃, CeO₂ and Al₂O₃ has been further proposed in Japanese Kokai Publication No. 61-77665. In the former publication, a mixture of Al₂O₃ and MgO was suggested, but the mixture ratio of the oxides was not confirmed. In addition, it has been found that the zirconia ceramic composition proposed in the former publication is not useful to enhance the thermal stability, whereas the zirconia ceramic composition proposed in the latter publication is not useful to enhance the strength. The present invention has now found that the strength of partially stabilized zirconia ceramics is greatly influenced by the mixture ratio of the oxides Al₂O₃ and MgO and that if the mixture ratio of the oxides is out of a predetermined extent, the bending strength of the partially stabilized zirconia ceramics may not be increased. It is, therefore, a primary object of the present invention to provide a partially stabilized ziroconia ceramic superior in bending strength, acid resistance and thermal stability on a basis of determination of an optimal mixture ratio between the oxides. According to the present invention, there is provided a process for the preparation of a high strength zirconia ceramic composition, which process comprises the pressureless sintering of a composition consisting essentially of a compound of ZrO₂ containing from 0.5 to 5.0 mol% Y₂O₃ as a stabilizer when CeO₂ is also present in the composition and from 1.5 to 5.0 mol% Y₂O₃ as a stabilizer when there is no CeO₂ present, and aluminium and magnesium compounds in a combined amount of from 1 to 5 weight % expressed in terms of Al₂O₃ and MgO relative to the amount of ZrO₂, the aluminium : magnesium molar ratio expressed in terms of Al₂O₃ and Mgo being from 35 to 45 : 65 to 55, from 60 to 75 : 40 to 25 or from 85 to 99 : 15 to 1. In cases where the zirconia ceramic composition is produced without CeO₂, the ingredient amount of Y₂O₃ is 1.5 - 5.0 mol% relative to the compound of ZrO₂. In cases where the zirconia ceramic composition is produced with CeO₂, the ingredient amount of Y₂O₃ is 0.5 - 5.0 mol% relative to the compound of ZrO₂, and the ingredient amount of CeO₂ has been determined to be 0.5 - 12.0 mol% relative to the compound of ZrO₂, provided that the total amount of Y₂O₃ and CeO₂ has been determined to be 1.0 - 15 mol%. It is preferable that the zirconia ceramic composition contains 1.5 - 5.0 mol% Y₂O₃ (or 1.5 - 3.5 mol% Y₂O₃ and 2 - 5 mol% CeO₂) as a stabilizer. With respect to the aluminum and magnesium contents in the composition, the molar ratio (Al₂O₃/MgO) between the aluminum and magnesium contents is selected from the following ranges (a) - (c) in terms of Al₂O₃ and MgO. (a) 35 - 45/65 - 55 (b) 60 - 75/40 - 25 (c) 85 - 99/15 - 1 Preparation of raw materials for the zirconia ceramic composition can be made by a method of mixing oxide powder of alumina-magnesia system or spinel powder with zirconia powder, a method of mixing each powder of alumina and magnesia with zirconia powder, a method of obtaining powder from aqueous solutions of ions of zirconium, yttrium, aluminum, magnesia and the like by means of wet mixing method. Preferably, the following two methods are adapted to preparation of the raw materials for the zirconia ceramic composition. In a primary preparation method, either mixed powder of Al₂O₃-MgO or aqueous salt solutions of Al₂O₃-MgO is added to mixed powder of ZrO₂-Y₂O₃ or ZrO₂-Y₂O₃-CeO₂. In a secondary preparation method, either powder or aqueous salt solution of MgO is added to mixed powder of ZrO₂-Y₂O₃-Al₂O₃ or ZrO₂-Y₂O₃-CeO₂-Al₂O₃. In these preparation methods, ZrO₂-Y₂O₃, ZrO₂-Y₂O₃-CeO₂, ZrO₂-Y₂O₃-Al₂O₃, ZrO₂-Y₂O₃-CeO₂-Al₂O₃ each may be prepared in the form of mixed powder thereof or may be prepared by calcination of mixed powders thereof. Although 0.5 - 3.0 wt% Hfo₂ is inevitably contained in the zirconia raw material, a part of ZrO₂ may be substituted for HfO₂ in the zirconia ceramic composition of the present invention. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, the accompanying drawings, in which:- Fig. 1 is a graph illustrating bending strength and acid resistance of the zirconia ceramic composition in relation to molar ratios of Al₂O₃ and MgO in the case that the mixture ratio of aluminum and magnesium contents to the whole contents of the zirconia ceramic composition in terms of Al₂O₃ and MgO was determined in a constant value; Fig. 2 is a graph illustrating bending strength and acid resistance of the zirconia ceramic composition in relation to weight ratios of Al₂O₃ and MgO to the whole contents of the zirconia ceramic composition in the case that the molar ratio between aluminum and magnesium contents was determined in a constant value; Fig. 3 is a graph illustrating bending strength of the zirconia ceramic composition in relation to a mixed amount (mol%) of Y₂O₃ in the composition; Fig. 4 is a graph illustrating bending strength of the zirconia ceramic composition in relation to a mixed amount (mol%) of CeO₂ in the composition; Fig. 5 is a graph illustrating bending strength of the zirconia ceramic composition in relation to a mixed amount (wt%) of aluminum and magnesium contents in terms of Al₂O₃ and MgO in the composition; Fig. 6 is a graph illustrating bending strength of the zirconia ceramic composition in relation to a molar ratio of Al₂O₃ and MgO in the composition; and Fig. 7 is a graph illustrating the rate of transformation from cubi structure into tetragonal structure and to monoclinic structure in relation to a mixed amount (mol%) of CeO₂ and Y₂O₃ in the zirconia ceramic composition. Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the drawings. EXAMPLE 1 A compound of ZrO₂-Y₂O₃ was precipitated by hydrolysis of aqueous salt solutions thereof. The precipitated compound was calcined at 900°C to obtain zirconia powder of particle diameter less than 1 µm. The zirconia powder was added with powder of Al₂O₃ and MgO, ground by a pot mill and dried by spray to obtain a raw material consisting essentially of ZrO₂-Y₂O₃-Al₂O₃-MgO. The raw material was preformed under pressure of 19,6MPa (200 kg/cm²) and shaped by Cold Isostatic Pressing into a rectangular plate of 60 x 60 x 8 (mm) under pressure of 294,3MPa (3 ton/cm²). The rectangular plate was fired by pressureless sintering at about 1400°C for 5 hours and cut into a plurality of bar specimens each size of which is 3 x 4 x 40 (mm). Thus, the bar specimens were tested as follows. (1) The bending strengths were measured by 4-point bending test with 10mm upper span and 30mm lower span at a cross-head speed 0.5mm/min using the bar specimens. (2) For measurement of the acid resistances, the bar specimens were put into solution of 36 wt% HCl in a sealed container and retained in the solution at 150°C for 200 hours. Thereafter, the weight of bar specimens was measured to calculate reduction of the weight per a unit area (mg/cm²). The test results of the bar specimens are shown in Tables 1 and 2 attached hereto and illustrated in Figs. 1 and 2. The test results of No. 1 - No. 27 shown in Tables 1 and 2 were obtained in the case that a molar ratio between Al₂O₃ and MgO or an added amount of Al₂O₃ and MgO was varied in a condition where the molar ratio of a stabilizer Y₂O₃ to the compound ZrO₂ was determined in a constant value (3/97). The data of Fig. 1 was obtained by variation of the molar ratio between Al₂O₃ and MgO in a condition where the added amount of Al₂O₃ and MgO was determined in a constant value (2 wt%). The data of Fig. 2 was obtained by variation of the added amount of Al₂O₃ and MgO in a condition where the molar ratio between Al₂O₃ and MgO was determined in a constant value (40/60, 70/30, 90/10). As is illustrated in Figs. 1 and 2, the acid resistance of the bar specimens added with Al₂O₃ and MgO was noticeably enhanced in the case that the molar ratio between Al₂O₃ and MgO was determined more than 30/60, and the bending strength of the bar specimens was increased up to a peak value in the case that the molar ratio between Al₂O₃ and MgO was determined to be 40/60, 70/30 and 90/10, respectively. Particularly, the bending strength of the bar specimens was noticeably increased in the case that the molar ratio between Al₂O₃ and MgO was determined to be 35 - 45/65 - 55, 60 - 75/40 - 25, 85 - 99/15 - 1, respectively. In addition, the bending strength was increased in the case that the added amount of Al₂O₃ and MgO was determined to be less than 20 wt%, preferably less than 10 wt%. The test results of No. 28 - No. 43 shown in Table 2 were obtained in the case that the molar ratio between Al₂O₃ and MgO was varied in a condition where the molar ratio of a stabilizer Y₂O₃ to the compound ZrO₂ was determined in a constant value (1.5/98.5, 2/98, 5/95) and where the added amount of Al₂O₃ and MgO was determined in a constant value (2 wt%, 5 wt%). The test results of No. 28 - No. 43 indicate that the bending strength and acid resistance of the bar specimens were enhanced in contrast with the bar specimen prepared without Al₂O₃ and MgO. From the test results described above, it will be understood that the zirconia ceramic composition of the present invention is superior in bending strength and acid resistance and useful as mechanical structure materials. In Example 1, Al₂O₃ and MgO were adapted to contain aluminum and magnesium contents into the compound of ZrO₂-Y₂O₃. The amount of the aluminum and magnesium contents is consistent with an amount of aluminum and magnesium contents in a ceramic composition ground in 44 µm after firing and measured by fluorescent X-ray analysis. For this reason, the amount of aluminum and magnesium contents in terms of Al₂O₃ and MgO is substantially consistent with the added amount of Al₂O₃ and MgO. In addition, it was found by fluorescent X-ray analysis that the following impurity is contained in the bar specimens. --less than 2.0 wt% SiO₂, less than 2.0 wt% TiO₂, less than 0.5 wt% CaO, less than 0.5 wt% K₂O, less than 0.5 wt% Na₂O and less than 3.0 wt% HfO₂-- EXAMPLE 2 A compound of ZrO₂-Y₂O₃-CeO₂ was precipitated by hydrolysis of aqueous slat solutions thereof. The precipitated compound was calcined at 900°C to obtain zirconia powder of particle diameter less than 1 µm. The zirconia powder was added with powder of Al₂O₃ and MgO, ground by a pot mill and dried by spray to obtain a raw material consisting essentially of ZrO₂-Y₂O₃-CeO₂-Al₂O₃-MgO. The raw material was preformed under pressure of 19,6MPa (200 kg/cm²) and shaped by Cold Isostatic Pressing into a rectangular plate of 60 x 60 x 8 (mm) under pressure of 294,3MPa (3 ton/cm²). The rectangular plate was fired by pressureless sintering at about 1400°C for 5 hours and cut into a plurality of bar specimens each size of which is 3 x 4 x 40 (mm). Thus, the bar specimens were tested as follows. (1) The bending strengths were measured by 4-point bending test with 10mm upper span and 30mm lower span at a cross-head speed 0.5mm/min using the bar specimens. (2) For measurement of the thermal deterioration, the bar specimens were put into hot water at 250°C under vapor pressure of 3,8MPa (39 kg/cm²) in an autoclave and treated by heat for 50 hours. Thereafter, the rate of transformation from cubic structure into tetragonal structure and to monoclinic structure in the bar specimens were calculated as follows. The bar specimens were mirror surface finished with diamond paste and applied to X-ray diffraction to measure integration intensity IM, IT, IC of diffraction peaks on the monoclinic crystal surface (ll l ), the tetragonal crystal surface (lll) and the cubic crystal surface thereby to calculate an amount of tetragonal and cubic contents V₀ = (IT + IC)/(IM + IT + IC) . The bar specimens after heat treatment were further applied to X-ray diffraction to calculate an amount of tetragonal and cubic contents (V₁) in the same manner as described above. Thus, the rate of transformation (%) = (V₀ - V₁)/V₀ x 100 was calculated on a basis of the calculated amounts of tetragonal and cubic contents V₀ and V₁. The test results of the bar specimens are shown in Tables 3-11 attached hereto and illustrated in Figs. 3-7. In the test results, the value of 6.86 x 10⁵kPa (70 Kgf/mm²) was defined as a standard value for determination of the bending strength, and the value of 25% was defined as a standard value for determination of the thermal stability. In Fig. 3, the specimens in eight groups A - H are plotted near or below the standard value 6.86 x 10⁵kPa (70 kgf/mm²). The specimens in groups A - D were prepared to contain less than 82-85 mol% ZrO₂ and more than 15 mol% Y₂O₃ and CeO₂, the specimens in groups E and F to contain 85 mol% ZrO₂ and 15 mol% Y₂O₃ and CeO₂, the specimens in group G to contain 5 mol% Y₂O₃, and the specimens in group H to contain 7 mol% Y₂O₃. In Fig. 4 there is illustrated a relationship between a mixed amount of CeO₂ (mol%) in a zirconia ceramic composition containing 3 mol% Y₂O₃ and 2 wt% Al₂O₃ and MgO and bending strength of the same ceramic composition. In this illustration, it was found that the bending strengths are decreased in accordance with an increase of the mixed amount of CeO₂ and noticeably decreased by mixture of CeO₂ more than 12 mol%. From the test results described above, it was confirmed that a high strength zirconia cermaic composition can be obtained by a compound of ZrO₂ containing 0.5 - 5 mol% Y₂O₃, 0.5 - 12 mol% CeO₂, Al₂O₃ and MgO, provided that the total amount of Y₂O₃ and CeO₂ should be determined to be 1.0 - 15 mol%. In Fig. 5 there is illustrated a relationship between a mixed amount (wt%) of Al₂O₃ and MgO in a zirconia ceramic composition containing 3 mol% Y₂O₃ and 2 mol% CeO₂ and bending strength of the same ceramic composition. In this case, the mixed amount (wt%) of aluminum and magnesium contents was determined in terms of Al₂O₃ and MgO. In this illustration, it was found that the bending strengths are decreased in accordance with an increase of the mixed amount of Al₂O₃ and MgO and noticeably decreased by mixture of Al₂O₃ and MgO more than 30 wt%. In Fig. 6 there is illustrated a relationship between a molar ratio Al₂O₃/MgO in a zirconia ceramic composition containing 3 mol% Y₂O₃, 2 mol% CeO₂ and 2 wt% Al₂O₃ and MgO and bending strength of the same ceramic composition. In this illustration, it was found that each peak of the bending strengths is obtained by determiation of the molar ratio Al₂O₃/MgO in 40/60, 70/30 or 90/10. Preferably, the molar ratio Al₂O₃/MgO should be determined in a extent of 35 - 45/65 - 55, 60 - 75/40 - 25 or 85 - 99/15 - 1. From the test results, it was confirmed that a high strength zirconia ceramic composition can be obtained by a compound of ZrO₂ containing less than 30 wt% Al₂O₃ and MgO at the molar ratio listed below. (a) 35 - 45/65 - 55 (b) 60 - 75/40 - 25 (c) 85 - 99/15 - 1 As is understood from the data of Figs. 3-5, it was confirmed that the bending strength is noticeably increased in a zirconia ceramic composition containing 2 mol% Y₂O₃, less than 5 mol% CeO₂, 1 - 5 mol% Al₂O₃ and MgO. In Fig. 7 there is illustrated a relationship between mixed amounts (mol%) of Y₂O₃ and CeO₂ in a zirconia ceramic composition containing 2.0 wt% at a molar ratio 90/10 and the rate of transformation of the tetragonal structure and cubic structure in the ceramic composition. From the data of Fig. 7, it was confirmed that the rate of transformation is decreased in accordance with an increase of the mixed amounts of Y₂O₃ and CeO₂. This means that the thermal stability of the zirconia ceramic composition is greatly enhanced by an increase of the mixed amounts of Y₂O₃ and CeO₂. EXAMPLE 3 Powders of ZrO₂-Y₂O₃ and ZrO₂-Y₂O₃-CeO₂ were prepared in the same manner as described in Example 1 added with aluminum and magnesium contents in the form of Al(OH)₃, AlCl₃, Al(NO₃)₃, Al₂(C₂O₄)₃ or the like and Mg(OH)₂, MgCl₂, Mg(NO₃)₂, MgC₂O₄, MgCO₃ or the like, ground by a pot mill and dried by spray to obtain a raw material consisting essentially of ZrO₂-Y₂O₃ and the aluminum and magnesium contents and a raw material consisting essentially of ZrO₂-Y₂O₃-CeO₂ and the aluminum and magnesium contents. Using the raw materials, various bar specimens were prepared in the same manner as described in Example 1 and tested in the same manner as described in Example 1. The test results of the bar specimens are shown in Tables 12-15 attached hereto. From the data of Tables 12-15, it was confirmed that the use of aluminum and magnesium contents in the form of the above-described compounds is effective to obtain the same results as those in use of Al₂O₃-MgO. EXAMPLE 4 (1) Sol-solution was prepared by hydrolysis of aqeous solution containing zirconium oxychloride added with yttrium chloride or yttrium chloride and cerium chloride. The sol-solution was added with Al(OH)₃ and Mg(OH)₂, treated by heat, ground and dried in the same manner as described in Example 1 to obtain a raw material. (2) The sol-solution was added with Al(OH)₃ in such a manner that a molar ratio to MgO is determined to be 90/10 in terms of Al₂O₃. Powder obtained by heat treatment of the sol-solution was added with powder of MgO, ground and dried in the same manner as described in Example 1 to obtain a raw material. Using the raw materials, various bar specimens were prepared in the same manner as described in Example 1 and tested in the same manner as described in Example 1. The test results of the bar specimens are shown in Table 16 attached hereto. From the data of Table 16, it was confirmed that the use of zirconium, yttrium and cerium ions in the form of aqueous salt solution is effective to obtain the same results as those in use of precipitated ZrO₂-Y₂O₃ or ZrO₂-Y₂O₃-CeO₂.

Claims: A process for the preparation of a sintered zirconia ceramic composition, which process comprises the pressureless sintering of a composition consisting essentially of ZrO₂ containing from 0.5 to 5.0 mol% Y₂O₃ as a stabilizer when CeO₂ is also present in the composition and from 1.5 to 5 mol% Y₂O₃ as a stabilizer when there is no CeO₂ present, and aluminium and magnesium compounds in a combined amount of from 1 to 5 weight % expressed in terms of Al₂O₃ and MgO relative to the amount of ZrO₂, the aluminium : magnesium molar ratio expressed in terms of Al₂O₃ and MgO being from 35 to 45 : 65 to 55, from 60 to 75 : 40 to 25 or from 85 to 99 : 15 to 1. A process according to claim 1, wherein the composition comprises from 1.5 to 5 mol% Y₂O₃. A process for the preparation of a sintered zirconia ceramic composition, which process comprises the pressureless sintering of a composition consisting essentially of ZrO₂ containing, as a stabilizer, 0.5 to 5 mol% Y₂O₃ and 0.5 to 12 mol% CeO₂, such that the total amount of Y₂O₃ and CeO₂ is from 1.0 to 15 mol%, and further containing aluminium and magnesium compounds in a combined amount of from 1 to 5 weight% expressed in terms of Al₂O₃ and MgO relative to the amount of ZrO₂, the aluminum : magnesium molar ratio expressed in terms of Al₂O₃ and MgO, being from 35 to 45 : 65 to 55, from 60 to 75 : 40 to 25 or from 85 to 99 : 15 to 1. A process according to any one of claims 1 to 3 wherein the aluminium compound in the composition is Al₂O₃, Al(OH)₃, AlCl₃, Al(NO₃)₃ or Al₂(C₂O₄)₃. A process according to any one of claims 1 to 4 wherein the magnesium compound in the composition is MgO, Mg(OH)₂, MgCl₂, Mg(NO₃)₂, MgC₂O₄ or MgCO₃.

Inventors: FUKUTA KEN, IWABUCHI MUNEYUKI, TSUNEKAWA KYOSUKE, FUKUTA, KEN, IWABUCHI, MUNEYUKI, TSUNEKAWA, KYOSUKE
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1987
Main classification: C04B35
IPC codes: C04B35