JPS6048471B2 - Zirconia sintered body - Google Patents

Description

[Detailed description of the invention] (b) Industrial application field This invention relates to a zirconia sintered body.

Same Conventional Technology In a pure zirconia sintered body, the crystal structure of zirconia changes from monoclinic to tetragonal at around 1100°C, and further changes to cubic crystal structure at around 2400°C.

During the cooling process, the opposite transformation occurs, but when the tetragonal system transforms into the monoclinic system, a large volumetric expansion occurs, and the sintered body will be destroyed if it continues, so calcia and magnesia are added to zirconia. A sintered body made of zirconia having a cubic crystal structure, that is, a stabilized zirconia sintered body, is obtained by dissolving oxides such as , ittria, etc. as a stabilizer. However, since zirconia having a cubic crystal structure has a large coefficient of thermal expansion, the stabilized zirconia sintered body has very low thermal shock strength. On the other hand, there is also a report that thermal shock strength is improved when zirconia with a cubic crystal structure and zirconia with a tetragonal crystal structure coexist (Journal of American Ceramic Society, Vol. 56). , No. 6, No. 55
9-562, 1977).

f■→ Problems to be Solved by the Invention This invention was obtained as a result of various studies on sintered bodies made of coexisting zirconia having the above-mentioned cubic and tetragonal crystal structures. The purpose of the present invention is to provide a zirconia sintered body that has not only high thermal shock strength but also high mechanical strength.

(d) Means for Solving the Problems In order to achieve the above object, this invention is based on the coexistence of zirconia with a cubic crystal structure and zirconia with a tetragonal crystal structure. The zirconia sintered body is characterized in that the amount of zirconia having the crystal structure is 5 to 70 mol%, and the porosity calculated from the bulk density and the theoretical density is 2 to 10%.

To explain this invention in more detail, in the zirconia sintered body (hereinafter referred to as sintered body) of the present invention, zirconia includes calcia, ittria, itteribia, magnesia, lanthania, strontia, calcia, magnesia, magnesia, and ittria. , calcia magnesia,
Zirconia with a cubic crystal structure (hereinafter referred to as cubic zirconia) and zirconia with a tetragonal crystal structure (hereinafter referred to as tetragonal zirconia) are produced by dissolving oxides such as ittria as a stabilizer. ) coexist.

Furthermore, it contains 5 to 70 mol% of tetragonal zirconia. As a result, when the sintered body is subjected to thermal shock, the tetragonal zirconia transforms into zirconia with a monoclinic crystal structure (hereinafter referred to as monoclinic zirconia), and microcracks occur in or near the monoclinic zirconia. Thermal shock strength is improved by absorbing the destructive energy caused by thermal shock. Furthermore, when subjected to mechanical bending, tetragonal zirconia transforms into monoclinic zirconia, and the volume expansion accompanying this transformation forms a compressive stress field, which acts to reduce the elastic strain energy due to mechanical bending. Therefore, the mechanical strength of the sintered body is improved. The amount of tetragonal zirconia in the sintered body should be 5 to 70 mole percent, as mentioned above.

In other words, if the content of tetragonal zirconia is less than 5 mol%, even if it undergoes thermal shock or mechanical bending and transforms into monoclinic zirconia, the amount of transformation is too small to sufficiently absorb strain. Not only is it impossible to do so, but also it becomes impossible to prevent cracks from propagating if they occur in the sintered body. Moreover, if it exceeds 70 mol%, the amount of transformation will become too large, and the compressed region will spread throughout the sintered body, and the sintered body will easily break due to a slight thermal shock or mechanical force. It becomes like this. In this way, the sintered body. The amount of tetragonal zirconia in the range of 5 to 70 mol % is an essential requirement for achieving the object of the present invention, which is to obtain a sintered body with high thermal shock strength and mechanical strength. Here, the amount of tetragonal zirconia is calculated by the following formula from the intensity obtained by analyzing the sintered integral powder by X-ray diffraction and integrating the area of the diffraction pattern.

T=[(B-f-C)/(A+BfC)]×100 T: Amount of tetragonal zirconia (mol%) A: Diffraction intensity of cubic zirconia 400 plane B: Diffraction intensity of tetragonal zirconia 004 plane Diffraction intensity C: Tetragonal zirconia 22
Although it has a zero-plane diffraction intensity, in the present invention, monoclinic zirconia may further coexist in a range of 70 mol % or less.

When monoclinic zirconia of 70 mol% or less coexists,
The gaps between the grain boundaries of monoclinic zirconia absorb strain caused by thermal shock, and microcracks are formed in these areas to prevent crack propagation, further improving the thermal shock strength of the sintered body.

In addition, when coexisting monoclinic zirconia in addition to tetragonal zirconia, the tetragonal zirconia is
It is preferable to set it to 0 mol%. Furthermore, the amount of monoclinic zirconia is determined by the following equation using the X-ray diffraction method, as in the case of the tetragonal zirconia described above. M=[(E+F)
/(D+E+F)]×100 M: Amount of monoclinic zirconia (mol%) D: Diffraction intensity of cubic zirconia 111 plane E: Diffraction intensity of monoclinic zirconia 111 plane F: Monoclinic zirconia The diffraction intensity of the 111 plane The amount of cubic zirconia is determined by the following formula from the amounts of tetragonal and monoclinic zirconia determined by the method described above.

C=100-T-M However, C: amount of cubic zirconia (mol %) In the sintered body of this invention, the porosity must be 2 to 10%.

Here, the porosity is defined by the formula P=[1-(bulk density/theoretical density)]×100, where P: porosity (%).

That is, if the porosity is less than 2%, the propagation speed of fracture energy due to thermal shock becomes high, and high thermal shock strength cannot be obtained. On the other hand, when the porosity exceeds 10%, the presence of pores causes stress concentration in that area, so the sintered body breaks down from the pore area, and cracks occur around the pores. As a result, a sintered body with high mechanical strength cannot be obtained because the propagation speed becomes faster. The sintered body of this invention can be manufactured by various methods.

For example, while referring to the phase diagram, zirconia powder and stabilizer powder are mixed in the desired ratio, simulated firing and pulverization are repeated to produce a raw material powder, and then a well-known molding method, rubber press method, etc. After molding it into a shape using
Calcinate at 800°C and then slowly age at a rate of 200 to 500°C/hour, rapidly cool at a rate of 1000 to 3000°C/hour, or hold at a temperature of 1200 to 1500°C for several hours after firing, and then It can be produced by slow cooling or rapid cooling. However, the firing temperature varies depending on the type of stabilizer, and when magnesia or calcia is used as a stabilizer, the firing temperature is 1700 to 180.
The temperature is preferably 0°C, and when itria is used, it is preferably 1500 to 1600°C. The amount of tetragonal zirconia and porosity in the sintered body vary depending on the purity and particle size of the zirconia powder and stabilizer used, the type and amount of the stabilizer, firing conditions, cooling conditions, etc. , these are carefully selected during manufacturing. As mentioned above, the sintered body of the present invention has high thermal shock strength and mechanical strength, so it can be used in crucibles, cutting tools, extrusion or wire drawing dies, tandate nozzles, various protection tubes, etc.
It is suitable as a constituent material for electrodes or insulating walls for magnetohydrodynamic generators (RvlHD generators).

In addition, since it exhibits oxygen ion conductivity at high temperatures,
It is suitable as a constituent material for so-called metallurgical oxygen sensors that measure the oxygen concentration in molten steel, and combustion control sensors for internal combustion engines, gas stoves, and the like. (e) Effects of the invention Next, the main effects of the invention will be explained in detail based on examples.

Example Using zirconia powder, ittria powder, and magnesia powder with an average particle size of 0.1μ and a purity of 99.9%, a total of H types of zirconia-yttria-based and zirconia-magnesia-based sintered bodies shown in the table were prepared. I made it.

That is, a mixture of zirconia powder and ittria powder was mixed so that the amount of ittria powder was 4 mol %, and the mixture and magnesia powder were mixed so that the amount was 8 mol %, and then fired at about 1000°C, and then The mixture was pulverized in a pot mill for about 24 hours, and this calcining and pulverization were repeated twice to obtain two types of raw material powders.

・Next, the hot breath method is used to obtain NO. ．． l~4
A sintered body was made.

That is, after filling a graphite die with zirconia-yttria-based raw material powder, it was fired at the temperature and time shown in the table while applying a pressure of about 225k9Id, and then cooled under the conditions shown in the table to obtain NO. 1 to 4 sintered bodies were manufactured. Also, NO. For the sintered bodies Nos. 5 to 14, zirconia-ittria-based or zirconia-magnesia-based raw material powders were used, 1% polyvinyl alcohol was added as a binder to these raw material powders, and plate-shaped bodies were formed by a rubber press method. Thereafter, the molded body was fired at the temperature and time shown in the table and cooled to produce a molded body. Next, the H type sintered body was cut and polished to a length of 3
4m771) Width 4wun) Thickness 377T77! A sample was prepared. Next, the amount of cubic and tetragonal zirconia, porosity, thermal shock strength, and bending strength as an index of mechanical strength were measured for the above-mentioned type H samples.

The measurement results are shown in the table. The amounts of cubic and tetragonal zirconia were measured by the above-mentioned X-ray diffraction method. Also,
The porosity was calculated by first determining the bulk density of the sintered body from the volume and weight, and using the above-mentioned formula from this bulk density and the theoretical density. Furthermore, thermal shock strength can be measured at any temperature T
After heating to x'C, drop it into water at T'C and quench it.
Next, the bending strength is measured by a well-known three-point bending test method, and the heating temperature Tx°C at which the bending strength begins to decrease is read as the critical temperature Tc'C, and the difference between this critical temperature Tc'C and the above T'C is determined. The difference Tc-T'C was used as an index. The conditions for this bending strength measurement are span length 30Tn! FL
, the load application acceleration was 1 wrm for 1 minute. In addition, bending strength, which is an index of mechanical strength, was determined by the well-known three-point bending test method.
The measurement was carried out under the conditions of a span length of 3 cynrffL and a load application acceleration of 1 Wf1I. From the above table, it can be seen that NO. 2 contains cubic zirconia, the amount of tetragonal zirconia is in the range of 5 to 70%, and the porosity is in the range of 2 to 10%. 3
, 7, 8, 11 and 12, that is, the sintered bodies of the present invention, both thermal shock strength and mechanical strength are significantly higher than those that do not meet the above requirements.

Furthermore, even if it contains cubic zirconia and the amount of tetragonal zirconia is within the range of 5 to 70 mol% (even if the porosity is in the range of 2 to 10%, it will have high thermal shock strength and mechanical strength). It can be seen that a sintered body having both strength and strength cannot be obtained.

For example, NO. Samples 2 and 3 are both sintered bodies containing cubic zirconia, and the amount of tetragonal zirconia is within the range of 5 to 70 mol % as specified in the present invention. However, the porosity is NO. Sample 2 is 1.
5%, which is lower than 2%, while NO. That of sample No. 3 is 3.5%, which is within the range of 2 to 10% defined in this invention. Comparing the two, NO. The bending strength of sample No. 2 was 1050 MPa, and the bending strength of sample No. 2 was 1050 MPa. ．． 3 samples,
In other words, it is about 5% higher than the 1000 MPa of the sintered body of the present invention, which seems good at first glance. However, the thermal shock strength is NO. ．． It is only about 61% of that of sample No. 3. In addition, it contains cubic zirconia and has a porosity in the range of 2 to 10%, but even if the amount of tetragonal zirconia is not in the range of 5 to 70 mol%, it also has high thermal shock strength and orange A sintered body with mechanical strength cannot be obtained.

For example, NO. Samples 12 and 13 are both sintered bodies containing cubic zirconia and having porosity within the range of 2 to 10% defined in the present invention. However, the amount of tetragonal zirconia is NO. The sample of l2 is 11
mol% is within the range of 5 to 70 mol% defined in this invention, while NO. That of the l3 sample is 3 mol%, lower than 5%. When comparing the bending strength of both, NO. The sample No. 13 has a pressure of 250 MPa. l
This is only about 35% of the value of 720 MPa of the sample No. 2, that is, the sintered body of the present invention. N
O. The thermal shock strength of sample 13 is NO. It is about 4% higher than that of the l2 sample. However, as mentioned above,
No. The bending strength of sample 13 is NO. It is only about 35% of that of the 12 sample. Thus, in addition to containing cubic zirconia, the amount of tetragonal zirconia in the range of 5 to 70 mol% and the porosity in the range of 2 to 10% result in high thermal shock strength. These are essential requirements for obtaining a zirconia sintered body having mechanical strength, and even if any of these requirements are lacking, a sintered body as good as the present invention cannot be obtained.

Claims (1)

[Claims] 1 Zirconia with a cubic crystal structure and zirconia with a tetragonal crystal structure coexist, and the amount of zirconia with a tetragonal crystal structure is 5 to 70 mol%, and the bulk density and Porosity calculated from theoretical density is 2-1
A zirconia sintered body characterized by being 0%.