Zirconia Porcelain and method of manufacturing the same

Abstract: Disclosed herein is a zirconia porcelain comprises 1.5-5 mol % of yttrium oxide, 1-5.5 mol % of cerium oxide and the balance being substantially zirconium oxide. Disclosed he­ rein also is a method of producing a zirconia porcelain, which comprises steps of preparing a mixed powder comprising 1.5-5 mol % of yttrium oxide, 1-5.5 mol % of cerium oxide and the balance being mainly zirconium oxide and having the average particle size of less than 1.5 µm when measured according to an air permeation method, shaping the mixed powder into a molding, and firing the molding thus obtained at 1,100°C-1,700°C. The use of both yttrium oxide and cerium oxide in the amounts specified provides considerable ther­ mal stability and long-term resistance to deterioration.

Description: The present invention relates to a zirconia porcelain and a method of manufacturing the same. Zirconia porcelains containing not more than 5 mol % of only yttrium oxide as a stabilizer have been widely used as high temperature structural materials, cutting tool materials and the like because of their high heat resistance, high strength and high toughness. In zirconia partially stabilized with yttrium oxide, the tetragonal zirconia which is stable at a high temperature is retained as a metastable phase at a low temperature by controlling the fine structure, and crack development due to the phase transformation to the monoclinic phase which is accompanied by the volume expansion is prevented so as to attain a high strength. The thus produced zirconia porcelain gradually undergoes the transformation from the tetragonal phase to the monoclinic phase during a long period of use under application of thermal stress and so on, so that the deterioration phenomenon such as shape change and crack development occurs to cause reduction in strength. The metastable tetragonal phase is a kind of super cooled state. It is considered that as the time during which the tetragonal phase is exposed to a temperature range of 200°C―1,000°C becomes longer, the nuclei of the monoclinic phase are formed increasingly, and the tetragonal phase is ultimately transformed to the monoclinic phase which is thermodynamically stable. Even under control of the fine structure, when the phase transformation temperature is not less than 200°C, the potential force to cause the transformation is large and the system is still extremely unstable from the standpoint of the thermodynamics because the metastable tetragonal phase is in an extremely super cooled state. Accordingly, there is a strong demand for zirconia porcelains having high strength and excellent thermal stability. It is therefore an object of the present invention to provide a zirconia porcelain with a high strength which has excellent thermal stability, suffers little or not at all from deterioration phenomena such as shape change, reduction in strength and the like even in long term use, and is excellent in durability. It is another object of the present invention to provide a method of manufacturing a zirconia porcelain having such properties. According to the present invention, there is provided a zirconia porcelain as set out in claim 1. According to another aspect of the invention there is provided a method of producing a zirconia porcelain, as set out in claim 3. It is mentioned that Chemical Abstracts, Vol. 87, No. 24 (12 December 1977), Abstract No. 188392c describes a zirconia porcelain containing 85 mol% ZrO 2 , 12 mol% CeO 2 and 3 mol% Y 2 O 3 . Specific resistivities are the only properties given. In the drawings:- Figure 1 is a composition diagram of a zirconia porcelain containing yttrium oxide and cerium oxide; Figure 2 is a diagram showing the relationship between the content of cerium oxide and the dimensional change rate before and after an electric furnace durability test; Figure 3 is a diagram showing the relationship between the content of a stabilizer and four point flexural strength; Figure 4 is a diagram showing the relationship between on the one hand the particle size of crystals in the zirconia porcelain and on the other hand the four point flexural strength and dimensional changes before and after the electric furnace durability test; Figure 5 is a diagram showing the relationship between the ratio of the sum of the content of tetragonal zirconium oxide and the content of cubic zirconium oxide to the content of monoclinic zirconium oxide in the zirconia porcelain and the dimensional change before and after the electric furnace durability test; Figure 6 is a diagram showing the relationship between the content of the cubic phase and the four point flexural strength; and Figure 7 is a diagram showing the relationship between the sintering temperature and the four point flexural strength or the dimensional change before and after the electric furnace durability test. The invention will be illustrated in detail with reference to the attached drawings as follows: The zirconia porcelain according to the present invention is characterized by containing both yttrium oxide and cerium oxide in specified amounts. The content of yttrium oxide in the porcelain is 2―3 mol%, while that of the cerium oxide is 2―4 mol%. These ranges are shown in Figure 1 at E-F-G-H. The average particle size of the zirconia crystals in the zirconia porcelain is not larger than 10 µm, more preferably not larger than 2 µm. As to the crystalline phases, the sum of the content of tetragonal zirconium oxide and the content of cubic zirconium oxide in the zirconia porcelain according to the invention is not less than three times that of monoclinic zirconium oxide in terms of the volume fraction. The content of cubic zirconium oxide in the zirconia porcelain is less than the sum of the content of tetragonal zirconium oxide and the content of monoclinic zirconium oxide in terms of the volume fraction. A method of manufacturing the zirconia porcelain according to the present invention is characterized by steps of shaping a molding from a mixed powder comprising preferably 2―3 mol%, of yttrium oxide, 2―4 mol% of cerium oxide, and the balance being mainly zirconium oxide which mixed powder has the average particle size of less than I.S. µm as measure according to the air permeation method, and firing the molding at a temperature of 1,100°C―1,700°C, more preferably 1,200°C―1,500°C, in such a manner that the specified crystal size and the specified ratios of the crystal types are obtained. That is to say, the present invention relates to the zirconia porcelain and the method of manufacturing the same in which tetragonal zirconium oxide is stabilized through the addition of yttrium oxide and cerium oxide in the respectively specified amounts, whereby the deterioration phenomena such as shape change, reduction in the strength and the like which are caused by the phase transformation from the tetragonal phase to the monoclinic phase under heating can be suppressed to the extremely low level. The present invention further relates to the zirconia porcelain and a manufacturing method thereof in which yttrium oxide and cerium oxide are contained in the respectively specified amounts, the average crystal particle size in the porcelain is suppressed to the specified value, or the content of cubic zirconium oxide is set at less than the specified value, whereby an extremely high strength is imparted upon the porcelain, and the average crystal particle size in the porcelain is set at the specified value, or the content of the monoclinic zirconium oxide is set less than the specified value, whereby the phase transformation from tetragonal zirconium oxide to monoclinic zirconium oxide can be more effectively restrained, so that the shape change and the reduction in the strength can be suppressed to the minimum for a long period of time. According to the present invention, in order that the porcelain may be high in strength, excellent in thermal stability and low in shape change with lapse of time, it is extremely important the porcelain contains as stabilizer 2―3 mol% of yttrium oxide and 2―4 mol% of cerium oxide. The reasons for these limitations will be explained below. Figure 2 shows the relationship between the contents of yttrium oxide and cerium oxide and the changes in shape before and after the electric furnace durability test, in which a solid line is the relationship between the content of cerium oxide and the dimensional change in the case of the content of yttrium oxide being 2 mol%, while a dotted line is that between the content of cerium oxide and the dimensional change in the case of the content of yttrium oxide being 3 mol%. From this diagram, it is seen that when cerium oxide is present in an amount of not less than 2 mol%, the dimensional change is zero. The measurement of the dimensional change before and after heat treatment in the electric furnace durability test is performed in the following manner. A zirconia porcelain is processed into a 3 mmx4 mmx40 mm shape according to the specification of JIS R 1601 "Fine ceramics flexural strength test method" by using a diamond cutter and a diamond grindstone. Then, it is placed into an electric fumace and thermally treated in air at 250°C for 5,000 hours. The change in length of the sample between before and after the thermal treatment is determined by means of a micrometer. Figure 3 shows the relationship between the content of yttrium oxide or cerium oxide and the flexural strength in which a solid line shows the relationship between the content of cerium oxide and the flexural strength in the case of yttrium oxide content of 2 mol%, and a dotted line shows the relationship between the content of yttrium oxide and the flexural strength in the case of cerium oxide content of 2 mol%. It is seen from Figure 3 that the flexural strength is partienlarly high in the composition ranges of claim 1. As mentioned above, the reason why when the cerium oxide is used together with yttrium oxide as stabilizer, thermal stability is drastically increased while high strength is maintained is considered to be that when yttrium atoms are diffused following the diffusion of cerium atoms into zirconium oxide, the location of yttrium and cerium in the lattice of zirconium oxide serves to relatively reduce the free energy for the production of the tetragonal phase and in turn increase that for the production of the monoclinic phase. For this reason, it is considered that the thermodynamically stable range of the tetragonal phase is extended to a lower temperature side in the case of the zirconia porcelain of the invention as compared with the zirconia porcelain containing yttrium oxide alone as the stabilizer. As shown in Figure 4, the influence upon the strength and the shape change before and after the electric fumace durability test by the average particle size of the zirconia crystals in the porcelain becomes conspicuous when it exceeds 10 µm. According to the zirconia porcelain of the invention, the thermal stability and the high strength of not less than 490 MPa (50 kg/mm 2 ) can be maintained at the particle size up to 5 times as large as the critical particle size of 2 µm in the conventional zirconia porcelain partially stabilized with yttrium oxide but containing no cerium oxide. As shown in Figure 4, when the average particle size is 2 µm or less in the zirconia porcelain of the invention, high values are obtained with respect to the thermal stability and the strength. When the average particle size is over 10 µm, the production rate of monoclinic zirconium oxide is increased and fine cracks are formed, so the strength is abruptly lowered and the dimensional change becomes larger. The reason why the phase transformation from the tetragonal phase to the monoclinic phase of the zirconium oxide crystals is hard to take place when the crystal particle size is small is considered to be based on that the interfacial energy between the tetragonal phase and the crystal interfacial phase is lower than that between the monoclinic phase and the crystal interfacial phase, and that the tetragonal phase is more stable in the fine particles in which the interfacial energy more largely contributes to the energy of the whole system. The average particle size of the crystals in the zirconia porcelain is measured in the following method. A number, η, of particles present in a specified area S in which 50 or more of the particles are contained is counted in an electron micrograph of a sintered body a mirror-finished surface of which is etched with hydrofluoric acid, the average area per one particle, S/η, is defined as s, and the diameter d of a circle of an area equal to the s is calculated from the formula d=(4s/π) ½ or √4s/π. The values of d are determined with respect to three view positions of the sample sample, and the average value thereof is multiplied by √3/2 (a correction coefficient for correcting some crystal particles being not cut at the largest diameter portion ) to obtain the average particle size. The number η of the particles is denned as sum of a number of the particles completely included in the specified area S and a half of a number of the particles cut by the boundary of the area S. Figure 5 shows the relationship between the ratio of the sum of the content of tetragonal zirconium oxide and the content of cubic zirconium oxide to the content of monoclinic zirconium oxide in the zirconia porcelain and the dimensional change in the electric fumace durability test. It is seen from Figure 5 that when the summed content of tetragonal zirconium oxide and cubic zirconium oxide is in a range of less than 3 times the content of monoclinic zirconium oxide, the phase transformation from tetragonal zirconium oxide to monoclinic zirconium oxide occurs strongly in the electric furnace durability test, so that the dimension is changed due to the volume expansion of the whole porcelain, and when the strain is large, cracking is induced with decrease of strength. When the content of cubic zirconium oxide is over the sum of the content of tetragonal zirconium oxide and the content of monoclinic zirconium oxide in the zirconia porcelain, that is, the content of cubic zirconium oxide is over 50% by volume, as shown in Figure 6, the strength drops down to 490 MPa (50 kg/mm 2 ) or less. This is considered to be because the strength of cubic zirconium oxide particles is intrinsically low, and cubic zirconium oxide is likely to be grain-grown as compared with tetragonal zirconium oxide and monoclinic zirconium oxide, and in most cases the particles of cubic zirconium oxide grow to larger particles having a diameter of 5 times as large as that of the other zirconium oxide phases, so that it is likely to undergo a large stress concentration. The contents of the respective crystal phases were measured on the basis of the polygonal crystal quantitative analysis method by the X-ray diffraction. The integrated intensity of the X-ray diffraction peaks is hereinafter referred to briefly as the integrated intensity. Since the tetragonal phase and the cubic phase of zirconium oxide are near to each other in terms of the lattice constant, their peaks are very closely approached to each other at the low angle diffraction, so that accurate separation can not be made. Accordingly, the ratio between the integrated intensity of the monoclinic phase peak and the sum of the integrated intensity of the tetragonal phase peak and the cubic phase peak [monoclinic phase/(tetragonal phase+cubic phase)] is measured, and the ratio of the tetragonal phase and the cubic phase is measured from the integrated intensities of the tetragonal phase peak and the cubic phase peak at a high angle at which the tetragonal phase peak and the cubic phase peak are separated from each other. A specific method of calculation of the respective crystal phase of the zirconia on the basis of the X-ray diffraction is as follows: indexing of the diffraction peaks of tetragonal zirconium oxide was made according to the method of "JCPDS-Powder diffraction file" No. 17―923. (1) Measuring of the integrated intensity: Integrated intensity of a mixture of tetragonal phase <111>and cubic phase<111>:I T+C <111> Integrated intensity of monoclinic phase <11 1 > and <111>: 1 M <11 1 >, 1 M <111> Integrated intensity of tetragonal phase <004> and <400>: I T <004>, I T <400> Integrated intensity of cubic phase <400>: I c <004> (2) The contents of the respective crystal phases were obtained in terms of "% by weight" according to the following formulae: Monoclinic zirconium oxide (% by volume) M = I M <11 1 >+I M <111> I M <11 1 >+I M <111>+I T + C <111> × 100 Tetragonal zirconium oxide (% by weight) T=(100-M)× I T <004> +I T <400> I T <004> +I T <400> +I C <400> Cubic zirconium oxide (% by volume) C=100-M-T In this case, when it is revealed by the X-ray diffraction that there is a crystal phase containing zirconium oxide, for instance, zircon (SiO 2 · ZrO 2 ), other than the crystals of zirconium oxide, a calibration curve of the concentration of the crystals of this crystal phase is drawn with respect to the ratio between the crystal phase and zirconium oxide, and the content of tetragonal zirconium oxide may be measured therefrom. In the present invention, the term "zirconia porcelain" means the zirconia porcelain mainly using yttrium oxide and cerium oxide as the stabilizer for zirconium oxide in which not more than 5% by weight of zirconium oxide may be substituted by hafnium oxide. Further, not more than about 10 mol% of yttrium oxide or cerium oxide may be substituted by other rare earth element compound such as samarium oxide, calcium oxide, magnesium oxide, titanium oxide or the like. The zirconia porcelain may include not higher than 10% of a sintering aid such as silica, silicate, alumina, clay, zircon or the like with respect to the whole porcelain. Next, an example of a method of producing the zirconia porcelain according to the invention will be explained. First, a mixture containing as stabilizer yttrium compound of 2―3 mol% when calculated as yttrium oxide, and a cerium compound of 2―4 mol% when calculated as cerium oxide with the balance being mainly a zirconium compound including zirconium oxide is prepared. When these compounds are not the oxides or they are oxides from which a volatile compound is required to be removed, calcining is done at 500―1,200°C, and then the fine crushing is carried out by a crushing mixer such as ball mill, vibration mill, attrition mill or the like, such that the average particle size when calculated in a form of sphere according to the air permeation method [FSSS (Fisher Sub-Sieve Sizer)] may not be larger than 1.5 µm, and shaped in a desired profile by a conventional shaping method such as rubber press, extrusion, slip casting and the like. Then, the molding is held at the maximum temperature which is within a range of 1,100°―1,7000C, preferably 1,200°C―1,500°C, for 30 minutes to 20 hours. In order to suppress the particle size of the zirconium oxide crystals in the zirconia porcelain to not larger than 10 µm, it is necessary that the average particle size of the powder constituting the molding is not larger than 1.5 µm, the content of the cerium oxide is not higher than 5.5 mol%, and the firing temperature is not higher than 1,700°C. That is, the particle size of the zirconium oxide in the zirconia porcelain increases with the increase in the average particle size of the particles constituting molding, the content of the cerium oxide, and the firing temperature, and further the holding time at the highest temperature as the case may be. The content of the cubic zirconium oxide particles increases with the increase in the content of yttrium oxide and the firing temperature. The content of the particles of monoclinic zirconium oxide on the surface of the tested member increases with the decrease in the content of cerium oxide, firing at a higher temperature, a slow temperature drop in the firing stage and the like. The reasons for the restrictions imposed upon the manufacturing method according to the present invention will be mentioned below. By suppressing the average particle size of the particles constituting the molding to 1.5 µm or less as measured in the air permeation method, the average particle size of the crystals of zirconium oxide in the zirconia porcelain is prevented from being increased under the influence of the cerium oxide which promotes the grain growth, so that the reduction in the strength, yttrium oxide and cerium oxide are well diffused into the zirconium oxide, and the phase transformation from tetragonal zirconium oxide to monoclinic zirconium oxide is made difficult. If the average particle size exceeds 1.5 µm, a higher sintering temperature unfavorably become necessary, and the particle size of the crystals of zirconium oxide becomes larger, leading to lower strength after densification. The sintered body treated at the sintering temperature of less than 1,100°C suffers from insufficient densification with a low strength, whereas when the sintering is carried out at a temperature higher than 1,700°C, the particles grow larger, cracks are developed by the stress concentration or the transformation from tetragonal zirconium oxide to monoclinic zirconium oxide, thereby lowering the strength. Due to the densification, large pores in the sintered body subjected to the sintering at a temperature range of 1,100°C to 1,700°C are reduced and the tetragonal phase in the sintered body is relatively stable, so that it is high in the strength with an excellent thermal stability. Particularly, since the sintered body sintered at the sintering temperature range of 1,200―1,500°C is fully densified, and its average particle size is not larger than 5 µm, it has a high strength and a large thermal stability, in Figure 7 is shown the dependency of the flexural strength and the dimensional change rate before and after the durability test in the electric furnace upon the sintering temperature. The invention will be explained more in detail with reference to the following examples. They are given for Illustration of the invention, but not to limit the scope of the invention. Zirconium oxide, yttrium oxide, cerium oxide or a compound or compounds thereof were combined to be in the compounding recipe shown in Table 1, and mixed together in a ball mill. The mixture was calcined at 500―1,200°C, wet crushed in the ball mill, and dried. The powder is press shaped, and sintered at 1,100°C―1,700°C for 1 hour―20 hours to obtain a zirconia porcelain according to the invention. The X-ray diffraction integrated intensity, the flexural strength, the average crystal particle size, and the dimensional change in the electric furnace durability test were measured for comparison with respect to the porcelain thus obtained. In the X-ray diffraction integrated intensity measurement, a pellet which was ground by using a diamond grindstone and buffed to a surface roughness of not larger than 0.8 s prescribed in JIS B 0601 was used as a sample. The integrated intensity of a specified diffraction peak was determined in the conditions of a tube voltage of 50 KV, a tube current of 80 mA, a scanning speed of 0.25°/min. in a rotating anode (Cu target) type X-ray diffraction device, and the amounts of the respective crystal phases were quantitatively determined according to the polygonal measuring method. The flexural strength was measured according to a four point flexural test at an upper span of 10 mm, a lower span 30 mm, and a cross head speed of 0.5 mm/min by using a strength measuring sample of 3 mm ×4 mm ×40 mm according to the prescription of JIS R 1601 which was prepared from a rectangular plate by means of a diamond cutter and a diamond grindstone. The dimensional change in the electric fumace durability test was determined by placing the sample in an electric fumace, performing a thermal treatment by holding it in air at 250°C for 5,000 hours, and then measuring the length after the thermal treatment by a micrometer, and finally the dimensional change was determined from the measured lengths before and after the thermal treatment. As seen from Table 1, it was confirmed that the zirconia porcelain according to the present invention has a strength of not less than 490 MPa (50 kg/mm 2 ), a dimensional change of less than 100 ppm (parts per million) in the electric furnace durability test, high strength and an excellent thermal stabile without deterioration in a long period of use. In Table 1, Examples 3,4 and 7 to 12 fall within the scope of the present invention, while the other Examples are shown as Reference Examples. Since the zirconia porcelain according to the present invention can have high strength and excellent thermal stability by reason of the presence of the specific amounts of yttrium oxide and cerium oxide, the porcelain can be suitable for parts which are subjected to mechanical and thermal stress such as thermal stress, thermal impact stress, repeated stress, repeated thermal stress and the like, for instance, engine parts such as engine cylinder liner, piston cap cylinder head, valve, valve guide, exhaust port, rocker arm tip sub-combustion chamber, tappet, cam, bearing and the like, an oxygen sensor, parts exposed to chemicals such as acid, or alkali and the like, for instance, rotor for an acidproof pump, seal member, cutting tool such as scissors, knife, kitchen knife, or the like, in a wide range of the industrial material, and thus extremely useful industrially.

Claims: A zirconia porcelain containing yttrium oxide and cerium oxide, characterized in that the amount of yttrium oxide is 2-3 mol% and the amount of cerium oxide is 2-4 mol%, and that the average particle size of the zirconia crystals is not larger than 10 µm, and wherein the sum of the content of tetragonal zirconium oxide and the content of cubic zirconium oxide is not less than 3 times in terms of volume fraction the content of monoclinic zirconium oxide, and the content of cubic zirconium oxide is less than the sum of the content of tetragonal zirconium oxide and the content of monoclinic zirconium oxide in terms of the volume fraction. A zirconia porcelain according to claim 1, wherein the average particle size of the zirconia crystals is not larger than 2 µm. A method of producing a zirconia porcelain, which comprises the steps of preparing a mixed powder comprising zirconium oxide, yttrium oxide and cerium oxide, shaping the mixed powder into a molding and firing the molding thus obtained, characterized in that said powder contains 2-3 mol% of yttrium oxide, 2-4 mol% of cerium oxide, the balance being mainly zirconium oxide and has an average particle size of less then 1.5 µm when measured according to an air permeation method and in that the firing takes place at 1,100°C-1,700°C in a manner such that the average particle size of the zirconia crystals of the zirconia porcelain is not larger than 10 µm, the sum of the content of tetragonal zirconium oxide and the content of cubic zirconium oxide is not less than 3 times in terms of volume fraction the content of monoclinic zirconium oxide, and the content of cubic zirconium oxide is less than the sum of the content of tetragonal zirconium oxide and the content of monoclinic zirconium oxide in terms of the volume fraction. A method according to claim 3 wherein the firing temperature is 1,200°C-1,500°C. A method according to claim 3 or claim 4 wherein a powder obtained by finely crushing oxides of zirconium, yttrium and cerium or the pyrolyzed product of compounds yielding these oxides on pyrolysis is used as the mixed powder.

Inventors: IMANISHI YUICHIRO, MATSUI MINORU, IMANISHI, YUICHIRO, MATSUI, MINORU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1984
Main classification: C04B35
IPC codes: H01M8, C04B35