HIGH-DENSITY ALUMINA-ZIRCONIA SINTER AND PROCESS FOR ITS PRODUCTION

Abstract: A high-density alumina-zirconia sinter, which comprises more than 60 mol % to 99 mol % of α-alumina, 1 mol % to less than 40 mol % of zirconia containing at least 65 % of a tetra­ gonal crystal phase, and a transition metal oxide in an amount of 0.01-1% in terms of an atomic ratio of a transition metal to the sum of aluminum and zirconium, and which has a remarkably high sintering density, high strength, high toughness, and high hardness useful as mechanical mem­ bers, abrasion-resistant materials, cutting tools, etc. This sin­ ter can be obtained by dispersing α-alumina powder or its precursor powder and zirconia powder or its precursor in a solution or suspension of a transition metal compound, and sintering the powder obtained from the resulting dispersion. Particularly preferable starting powders include alumina­ zirconia composite powder obtained by mixing a dispersion of α-alumina powder in an aqueous solution of a zirconium salt with a precipitating agent, and calcining the obtained composite powder.

Description: The present invention relates to high density alumina zirconia ceramics consisting essentially of alumina and zirconia, and to a method of the production thereof. The high density alumina -zirconia ceramic body of the present invention is very hard and highly abrasion resistant and has high toughness, and therefore, can be expected to have a variety of applications such as for mechanical members, abrasion resistant material, and material for tools. Alumina containing ceramic material is a known material for cutting tools and has high abrasion resistance, but is disadvantageous because of easy pitching. Therefore, the improvement of toughness is a major object of development of the alumina ceramics. One known way of improving the toughness of alumina ceramics is that zirconia phases are finely dispersed in the alumina matrix so that the finely dispersed tetragonal zirconia phase can transform to the monoclinic phase under the stress field at the end of crack to transfer, and therefore the energy of crack transfer in the alumina ceramics will be absorbed. When the zirconia phase does not contain a stabilizing agent, the zirconia particle will maintain tetragonal and stable in the alumina matrix. The size of the particles needs to be less than the critical size (in the order of 0.5 µm). However, such a fine dispersion can not be produced in commercial size for the production of the ceramic body. A method was proposed in Japanese Patent publication No.54 -25908 for the production of such fine dispersions wherein partially stabilized zirconia is added to the ceramic formulation so as to maintain tetragonal even when the size of the zirconia is more than the critical particle size. In order to yield high density ceramics with a fine dispersion of zirconia in the ceramics, a higher sintering temperature such as a temperature higher than 1500 °C under atmospheric pressure, ordinarily a sintering temperature of higher than 1600 °C is necessary. When the lower sintering temperature is used for producing the high density ceramics, the higher pressure for sintering should be applied (HIP treatment). However, the sintering under higher pressure is not preferred and further, when the ceramics are sintered at the higher temperature under the atmospheric pressure, the grain growth of alumina and the grain growth of zirconia together with the transformation of the crystal to monoclinic phase may easily occur so as to decrease the strengthening with zirconia. In accordance with the present invention, the sintering ability of the ceramics is improved by the combination of alumina phase and zirconia phase, so that the grain growth of alumina and zirconia would be suppressed to result in the high density alumina zirconia ceramics with high breaking toughness and high abrasion resistance. It is one object of the present invention to produce ceramics having high density and controlled microstructure by suppressing the grain growth in the ceramics to have desired properties, for example oxygen ion conductivity, thermal stability, mechanical properties such as bending strength, tenacity and the like. There have been produced dense alumina zirconia ceramics having controlled microstructure by special moulding technique, and high pressure sintering technique such as the hot press technique or the HIP method. However, these methods need complicated operation and special installations, and therefore, the resultant product will be expensive. On the other hand, there has been proposed a process for the production of ceramics which comprises preparing the raw material powder by using chemical technique such as co -precipitation or the like and then sintering the obtained moulded raw material powder at the relatively low temperature range. However, it is known that in general the more finely divided particulate material has the stronger cohesive force. Therefore, it is difficult to produce ceramics having high density with high reproduciblity from the chemically treated raw powder. Further, there has been proposed the addition of sintering activator, for example, Japanese patent publication JP-A-50130510 describes a process for the production of ceramics comprising moulding and sintering a raw material powder which is obtained by adding aqueous ammonia to an aqueous solution containing water soluble zirconium salt, water soluble salts of calcium, magnesium, yttrium and the like as a stabilizing agent(s), and water soluble salt of the transition metal for sintering activator, so as to precipitate the desired co-precipitated hydroxide containing the desired metals, then, drying and calcining thereof. The raw material powder can not provide satisfactory lower temperature sintering characteristics nor a satisfactory density of the ceramics. In the above mentioned process, aqueous ammonia is used for precipitation. Some of transition metals will form an amine complex with ammonia so that in practice aqueous ammonia can not be used in cases of such transition metals. In order to avoid this shortcoming, there is a process in that the oxides of the transition metals are used in place of the water soluble salt of the transition metals so as to disperse in the the mixed solution containing the other components whereas the hydroxides of the other metal components are co-precipitated together with oxides of the transition metals. EP-A-0 214 291 (filed 03.07.85 and published 18.03.87) forms part of the state of art pursuant to Article 54(3) EPC. This document discloses partially stabilized zirconia ceramics containing alumina and minor amounts of transition metal oxides as well as processes for the production of said ceramics. Ceramics containing 1-60 mol% alumina and 99-40 mol% zirconia are mentioned. EP-A-0 119 028 discloses a sintered aluminus material having high hardness and toughness, said material consisting of Al₂O₃ and up to 60 % Y₂O₃-stabilized ZrO₂. ZrO₂ is present in the tetragonal form. The particle size of the ZrO₂ in the ceramic (after sintering) is 0.3-0.4 µm. The sintering temperature used in the process for production of this known ceramic material is higher than 1500°C. According to the last-mentioned document it is desirable to control the sintering temperature strictly within the range of 1500-1700°C. If the sintering temperature is below said range, the reaction between ZrO₂ and Y₂O₃ may not be effected in a satisfactory way. According to the last-mentioned document transition metal may be present as an impurity in the ceramic material. F. F. Lange reported in Journal of Materials Science 17 , 240-246(1982) that "There is critical limit of the particle size of tetragonal phase respectively to Y₂O₃ content, and when the size exceeds the critical limit , the tetragonal phase can not be present. Though the critical particle size is more than 1 µm at the Y₂O₃ content of 3 mol. %, it decreases to in order of 0.2 µm at the content of 2 molar %." However, the suppressing of the growth of crystal grains in the ceramics, to control the size of the crystal grains to 0.2 µm or less is extremely difficult by the prior art process for the production of the ceramics. The characteristics of sintering at the lower temperature such that the grain size be controlled at approximately 0.2 µm or less cannot be attained even by using the raw material powder prepared by the co-precipitation. As described above, the known alumina zirconia ceramic material is not satisfactory in view of improving the mechanical strength and stability, and therefore, ceramics with higher tenacity and strength are highly desired. It is an object of the present invention to provide a process for the production of the raw material powder with the characteristics of sintering at the lower temperature, which can be used for the production of high density alumina zirconia ceramics as well as a process for the production of high density alumina zirconia ceramics with controlled crystal structure. It is another object of the invention to provide a method of the production of alumina zirconia ceramics with high fracture tenacity which comprise sintering the specified raw powder of alumina and zirconia at relatively lower temperature under the atmospheric pressure. The present invention comprises a high density ceramic body, containing 60 - 99 mol% α-A1203 1 - 40 mol% ZrO2 having an average crystal grain size of less than 1 µm and a content of tetragonal phase of more than 65% 0,2 - 1 mol% (based on the sum of Al and Zr) of transition metal oxides. When the content of zirconia is less than 1 mole %, the high toughness of the resultant ceramics can not be obtained. Further, when the content of zirconia exceeds 40 mole %, the hardness of the resulting ceramics will be decreased. The high density alumina zirconia ceramic body of the present invention can be produced in accordance with the following method. Zirconia powder having the grain size of less than 30 nm (300 A) and BET relative surface area of more than 8 m²/g or the precursor to generate the zirconia powder by heat decomposition, and α-alumina powder having the grain size of less than 1.0 µm and the BET relative surface area of more than 5 m²/g, or the precursor to generate the α-alumina powder by heat decomposition are mixed in a solution or a dispersion containing compound(s) of at least one metal selected from the group consisting of Mn, Fe, Co, Ni, Cu, and Zn and then removing the solvent and drying to yield the combined powder. The resultant powder is moulded in a shape, and then fired under the room pressure at the temperature of lower than 1500 °C to sinter. The resultant sintered ceramic body is obtained. The stabilizing agent, when a partially stabilized zirconia is used in this invention, may be Y₂O₃, CaO, MgO or CeO₂, as well as yttrium compounds, calcium compounds, magnesium compounds, or cerium compounds to generate respectively Y₂O₃, CaO, MgO, or CeO₂, by its thermal decomposition. Normally, Y₂O₃ is used as a stabilizing agent, and the amount to be added is less than 3 mole % in case of Y₂O₃, and less than 12 mole % in case of MgO or CaO, and the amount to be added to the ceramics composition is less than 14 mole % in case of CeO₂. If the content of the stabilizing agent exceeds the above limits, the toughness of the resulting ceramics will be lowered. The used zirconia powder should have the grain size of less than 30 nm (300 A) and the BET specific surface area of more than 8 m²/g, which may be prepared by any process. Further any precursor to generate such powder of the above mentioned properties upon heat decomposition or pyrolysis can be used in the present invention. Any of α-alumina of the above mentioned properties to satisfy the above explained condition or the precursor to generate such α-alumina can be used without any special limitation. In accordance with the present invention, the raw material is produced as a combined deposit or co -precipitation of zirconium compound and alumina, from aqueous dispersion containing zirconium salt (or zirconium salt and a stabilizing agent) and alumina powder and precipitating agent, and then the deposit is calcined to prepare the combined powder of alumina and zirconia. This combined powder is especially prefered for the production of the ceramic bodies of the invention. In the method of the invention, alumina powder is added to an aqueous solution of zirconium salt (or an aqueous solution of zirconium and stabilizing agent) to prepare the aqueous dispersion. Then, the resulting dispersion is added to aqueous ammonia and agitated and the precipitation occurs. Alternatively, an aqueous solution of zirconium salt (or an aqueous solution of zirconium salt and stabilizing agent) is added to a dispersion of alumina powder and aqueous ammonia and agitated and then, the precipitation occurs to prepare the raw material for the production of the alumina zirconia ceramics of the present invention. The zirconium salt is soluble in water, and the regulation of the pH will produce the hydrated oxide. The resulting oxide may produce zirconia by calcining. Such salt may include oxychloride, oxynitrate, oxyacetate, oxysulfate and the like. In the production of the ceramics, when the particle size of the raw material zirconia powder exceeds 30 nm (300 Å), or when the BET specific surface of the zirconia powder is less than 8 m²/g, or when the particle size of the raw material α-alumina exceeds 1.0 µm, or when the BET specific surface of the α-alumina is less than 5 m²/g, the high density of the resulting ceramics can not be obtained only by sintering at the temperature of below 1500 °C . In the other words, a satisfactory high density can not be obtained by the atmospheric sintering at the lower temperature. The atomic ratio of the transition metal to the combination of Al and Zr in the production of the ceramics may be in a range of from 0.01 % to 1.0 %, preferably 0.01 % to 0.5 % . If the ratio exceeds 1.0 %, the sintering properties are affected. And if the ratio is below 0.01%, a satisfactory high density can not be obtained. The transition metal compound(s) in this invention may be oxide of at least one metal selected from the group consisting of Mn, Fe, Co , Ni, Cu, and Zn as well as the compound(s) to generate one of the above mentioned metal oxides by its thermal decomposition. As the transition metal compounds, there may be used inorganic compounds such as oxides, hydroxides, nitrates, chlorides and the like of the above mentioned metals; organic acid salts such as oxalates, acetates, propionates, higher fatty acid salt and the like of the above mentioned metals; and organic metal compounds such as alkoxide compounds, chelate compounds and the like of the metals, even if it is not only soluble but also insoluble to the used solvents. Preferable one is solvent soluble compounds. Removal and drying of solvent is carried out by the conventional evaporation method, but if the transition metal compound is not soluble in water nor organic solvent, or if the soluble compound is readily precipitated by using a precipitating agent, filtration can be used for removal of solvent. Further, a spray drying technique can be applied for effective and efficient production of volumous raw powder. While the combined powder obtained from the mixture of zirconia powder (or the precursor thereof), α-alumina powder (or the precursor thereof ), and the transition metal compound can be used as starting material for the ceramics. The powder is preferably calcined at the temperature of 400 °C to 1000 °C, particularly when the powder contains the precursor materials. The conventional moulding technique can be satisfactorily used to form the raw material in a shape, but preferably the hydrostatic compression should be applied after moulding under the lower pressure so as to improve the density and the mechanical strength of the final ceramic body. The sintering of the moulding may be carried out by any of the conventional methods, and the conventional sintering under the atmospheric pressure can be satisfactory. The ceramics obtained by the method of the invention have zirconia grain size in average of less than 1.0 µm, preferably less than 0.5 µm and the content of tetragonal phase of more than 65 %, preferably more than 80 %. The average size of alumina grain in the ceramics is less than 3 µm, preferably 2 µm or less. As mentioned before, the improvement of the toughness and abrasion resistance of the alumina ceramics need the uniform and fine dispersion of fine zirconia grains in the ceramics, but high temperature sintering of the ceramics is needed for obtaining the high density. If it is not sintered at the higher temperature, it will result in lowering of the hardness and toughness. In accordance with the present invention, firing in the presence of the transition metal compound(s) including Mn, Fe, Co, Cu, Ni or Zn as a metal species will improve the sintering ability of the material to result in the excellent high density alumina zirconia ceramics. The high density alumina zirconia ceramics of the present invention can maintain higher content of tetragonal phase, even when zirconia content is increased in the ceramics having the lower content of the stabilizing agent. Therefore, the resulting ceramics can have higher hardness and higher toughness. This fact indicates that the grains in the ceramics is suppressed, and therefore, the fine zirconia grains are well and uniformly dispersed. The transition metal compound(s) can suppress the grains of zirconia and alumina from growing in the ceramics, so that the resulting ceramics will be densified. On the other hand, ZrO₂ grains well dispersed in Al₂O₃ phase will suppress the grains of Al ₂ O ₃ growing as well as suppress the transformation of tetragonal phase to monoclinic phase. Therefore, the toughness of the resulting ceramics is significantly increased. These phenomenon functions synergically so that the toughness, hardness and abrasion resistance of the resulting ceramics are significantly improved to have 12 MN/m 3/2 of K IC . The best mode for carrying out the invention may be illustrated by the following examples. EXAMPLE 1 (1) Production of Raw Material Powder The pH of the combined solution containing ZrOCl₂, and YCl₃ was regulated to co-precipitate the deposit of the combined hydroxides. The resultant co-precipitation was calcined to yield the partially stabilized zirconia having the content of Y₂O₃ as shown in Table 1, and the characteristics of powder as shown in Table 1. The resultant powder, α-alumina having the characteristics as shown in Table 2, and the nitrate of the transition metal were dissolved in ethanol, and the resultant ethanolic solution is put in pot for milling, and mixed and ground and then ethanol was evaporated to dry thereby to yield the raw powder for the production of ceramic body which have the contents of transition metal compound as shown in Table 3. (2) Production of Ceramics By pressure moulding the raw material powder as before obtained, and then further hydrostatic compressing the obtained mouldings under pressure of 2 ton/cm², mouldings having the desired shape was obtained. The obtained mouldings was fired under atmospheric pressure at the given temperature as shown in Table 3, For reference, omitting the addition of the transition metal compounds, the raw materials wherein Y₂O₃ content is more than 3.0 mol.%, that is, using the unpreferable raw material characteristics of alumina powder and/or zirconia powder, the ceramic body was produced. (3) Characteristics of Raw Material Powder and The Ceramics made therefrom . The following characteristics were measured on the raw material powder and the ceramics obtained above. The data measured on the characteristics are shown in Tables 1, 2 and 3. (A) Size of partially stabilized zirconia particle and α-alumina particle:D The size D can be calculated from the width at the half value of the peak of X ray diffraction by the following Schellar's formula: D=0.9λ/βcosϑ λ : the wave length of X ray β : the width at the half value of the diffraction peak ϑ : the diffraction angle (B) BET Specific surface area of partially stabilized zirconia particle and α-alumina particle was measured by using micromeritics (machine manufactured by Shimazu Works). (C) Breaking Toughness Strength of high density alumina zirconia ceramics ;K IC was measured by Vickers indent test. The Vickers indenter was pressed to the polished surface of the samples, and the resulting indentation size and the resulting length of the generated crack were measured and K I c was calculated from the following formula which Niihara et al proposed. The applied indentation load was 50 kgf. (K IC Φ/H a¹ ²)(H/EΦ ) 0.4 =0.129(C/a) -3/2 Φ: restraint moduras(-3) H: Vickers hardness E: modulus of elasticity. a: half value of diagonal length of indentation c: length of crack generated from indentation. (D) Bending strength of high density alumina zirconia ceramics ; was measured in accordance with JIS R 1601(1981) rule. The sample of 3×4×40 mm in size was used, and measurement was carried out on span length of 30 mm under crosshead speed of 0.5 mm/min, and the value was determined by average from five samples. (E) Content of tetragonal phase in the high density alumina zirconia ceramics The surface of the sample was polished by diamond slurry containing 3 µm in size of diamond particles, and then, X ray diffraction measurement was carried out on that surface followed by the calculation of the following formula. (111)t:tetragonal (111) face diffraction intensity (111)m:monoclinic (111) face diffraction intensity (11 1 )m:monoclinic (11 1 ) face diffraction intensity (111)t diffraction peak includes cubic (111)c diffraction peak, but the calculation was carried out presuming that that peak is entirely by tetragonal diffraction. (F) Grain size in the high density alumina zirconia ceramics . The grain size was measured by observing the fracture face of the ceramics through scanning type electron microscope. It was confirmed that all samples except of reference samples have content of tetragonal phase of ZrO₂ in the resultant ceramics ranging more than 95 %, the grain size of ZrO₂ ranging 0.3 to 0.8 µm,and the content of Al₂O₃ ranging 1 to 2 µm. EXAMPLE 2 Using acetate of transition metal, and methanol instead of ethanol, the procedures of Example 1 was repeated to obtain the raw material powder for the production of ceramic body, which contains transition metal compound. The resultant raw powder was shaped, calcined as in the procedure of Example 1, to yield the high density alumina zirconia ceramic body. The characteristics of the resultant ceramics were measured as in Example 1. It was confirmed that all samples except of reference samples have content of tetragonal phase of ZrO₂ in the resultant ceramics ranging more than 95 %, the grain size of ZrO₂ ranging 0.5 to 1.0 µm, and the content of Al₂O₃ ranging 1 to 3 µm. Example 3 A given amount of α-alumina powder (grain size;0.4 µm, Specific surface area;8 m²/g ) was added to an aqueous solution of ZrOCl₂ (or aqueous solution of ZrOCl₂ containing a stabilizing agent) and agitated by ball milling to form uniform dispersion. The resultant dispersion was then added to excess ammonia water at such rate that the pH thereof was maintained at more than 9, and agitated, and then the resultant precipitation was filtered, washed with water, and dried to yield the combined deposit compound of alumina and zirconium compound. Then, the resultant powder and nitrate of transition metal were dissolved in ethanol solution and put in a milling pot, and mixed, ground and then ethanol was evaporated to be dry so that the raw material powder was obtained for the production of the ceramic body. The resultant grain size of ZrO₂ in the combined powder was 200 Å. The resultant raw powder was shaped, and calcined as in the procedure, to yield the high density alumina zirconia ceramic body. The characteristics of the resultant were measured as in Example 1. The result of the measurement are shown in Table 3. It was confirmed that all samples have content of tetragonal phase of ZrO₂ in the resultant ceramics ranging more than 95 %, the grain size of ZrO₂ ranging 0.5 to 1.0 µm, and the content of Al₂O₃ ranging 1 to 3 µm. The high density alumina zirconia ceramic body of the present invention is as shown in Examples, highly dense in sintered density, and has excellent breaking toughness value(K IC ), and high bending strength and hardness. In accordance with the present invention, a high density alumina zirconia ceramic body can be produced by using finely divided zirconia powder and a sinter -accelerating effect by transition metal compound(s). In the inventive ceramics, zirconia crystal is restrained to grow so that crystal phase of zirconia particle in the ceramics enables to maintain the tetragonal phase of zirconia in the ceramics even when the content of partially stabilized zirconia is increased, and even when the content of the stabilizing agent is reduced. Further, the crystal growth of alumina in the ceramics is suppressed. By these two suppression of the crystal growth the resultant ceramic body is improved so that the breaking toughness value is increased to 12 MN/m 3/2 . Further, the resultant ceramics has improved bending strength, and the improved hardness. Therefore, the ceramic body of the present invention is expected to be used as a strong structural ceramic member such as abrasion resistant material, and mechanical member, and is useful in application such as cutting tool, and can be used for high performance material. In view of the foregoing, the present invention provides alumina zirconia ceramics with high density, high strength, high hardness and excellent toughness, and a method of production thereof. Therefore, the present invention can provide industrial significance.

Claims: A high density ceramic body, containing: 60 - 99 mol% α-Al2O3 1 - 40 mol% ZrO2 having an average crystal grain size of less than 1 µm and a content of tetragonal phase of more than 65% 0.2 - 1 mol% (based on the sum of Al and Zr) of transition metal oxides. The ceramic body as claimed in claim 1, wherein the transition metal oxide comprises oxide(s) of at least one metal selected from the group consisting of Mn, Fe, Co, Ni, Cu and Zn. The ceramic body as claimed in claim 1, wherein crystal size in average of Al₂O₃ is less than 3 µm. The ceramic body as claimed in claim 1, wherein zirconia is partially stabilized zirconia with Y₃O₃. The ceramic body as claimed in claim 4, wherein the content of Y₂O₃ in the partially stabilized zirconia is less than 3 mole %. A process for the production of a high density alumina - zirconia ceramic body, as claimed in claim 1, comprising removing solvent from the combined dispersion liquid comprising (a) α-alumina powder, or the precursor to generate α-alumina powder by heat decomposition. (b) zirconia powder, or the precursor to generate zirconia powder by heat decomposition and, (c) solution or suspension containing transition metal compound(s), and if necessary, drying the dispersion to produce powder material forming in a certain shape after heat decomposition, and sintering the material. The process as claimed in claim 6, wherein the crystal size of the partially stabilized zirconia powder is less than 30 nm (300 Å), BET specific surface area of the partially stabilized zirconia powder is more than 8 m²/g, and the crystal size of α-alumina powder is less than 1.0 µm, and the BET specific surface area is more than 5 m²/g. The process as claimed in claim 6, wherein alumina - zirconium combined powder produced by calcining the combined precipitation of alumina and zirconium compound, which is obtained from aqueous dispersion containing zirconium compound salt or zirconium salt and stabilizing agent therefor, alumina powder and precipitating agent is used as α-alumina powder and zirconia powder.

Inventors: KIMURA, NOBUO, MORISHITA, JUNICHI, OKAMURA, HIROMICHI
Applicants: NIPPON SODA CO., LTD.
Application year: 1986
Main classification: C04B35
IPC codes: C04B35