JPH066512B2 - High toughness silicon nitride sintered body and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silicon nitride (Si ₃ N ₄ ) sintered body and a method for producing the same, more specifically, Y ₂ O ₃ and CeO ₂ are solid-dissolved and mainly tetragonal. The present invention relates to a high-toughness silicon nitride sintered body having a built-in zirconia composed of crystals and having no deterioration in toughness and strength over time, and a method for producing the same.

[Conventional technology]

Since silicon nitride (Si ₃ N ₄ ) constitutes ceramics with low specific gravity, low thermal expansion, high elastic modulus and high heat resistance, it is expected to be applied as a high temperature structural material including gas turbine members as one of engineering ceramics. Has been done. However, since Si ₃ N ₄ is a hard-to-sinter material having a strong covalent bond, it is difficult to sinter densely like ordinary oxide ceramics. Therefore, Si ₃ N
The sintering method of No. ₄ is a reaction sintering method using reaction or M
Methods such as hot pressing or atmospheric pressure sintering have been carried out using oxides such as gO and Y ₂ O ₃ as a sintering aid.

In the reaction sintering method, there is almost no shrinkage during sintering and even complicated shapes can be sintered, but the density is low and the strength and corrosion resistance are poor. On the other hand, the hot pressing method and the atmospheric pressure sintering method can obtain a dense sintered body and have good strength and corrosion resistance, but since a sintering aid is added,
Deterioration of strength at high temperature is severe. Further, the characteristics of the sintered body, especially the high-temperature mechanical characteristics are deteriorated due to the change in the characteristics of the added sintering aid.

For example, it has been reported that the use of ZrO ₂ and Al ₂ O ₃ as a sintering aid has an effect on improving the sinterability (Ceramics Association Magazine 82 (12) 1976), but ZrO ₂
High-purity monoclinic ZrO ₂ is used as a raw material of, and deterioration of characteristics due to phase transformation has been confirmed from the crystal form of ZrO _{2 in} the obtained sintered body.

On the other hand, the zirconia sintered body undergoes a phase transition from a cubic crystal in a high temperature region to a tetragonal crystal to a monoclinic crystal, but at that time, the sintered body is broken due to a volume change. In order to eliminate this drawback, CaO, MgO, and Y ₂ O ₃ are usually used as a solid solution to partially stabilize or stabilize tetragonal or cubic crystal. In addition, it has been announced that partially stabilized zirconia in which a cubic metastable crystal is present in a matrix sintered body at room temperature exhibits high strength and toughness. This is because, when a mechanical external stress is applied, a phase transfer from a metastable tetragonal system to a room temperature stable phase, a monoclinic system, is induced and the stress is absorbed.

Various high-toughness ceramics sintered bodies that utilize such metastable tetragonal → monoclinic phase transformation have been reported, but in order to obtain a metastable tetragonal crystal at room temperature, it is necessary to use a non-stable matrix. Disperse the stable zirconia particles by making them extremely small, or use MgO, CaO, or
There is a method of using Y ₂ O _3, etc.
₂ O ₃ is used to exhibit high toughness and high strength.

Regarding the sintered body in which ZrO ₂ is dispersed in Si ₃ N ₄ ,
A method of producing a high toughness ceramic tool containing MgO and stabilized ZrO _{2 up} to 20 wt% and hot pressing in a non-oxidizing atmosphere has been proposed (Japanese Patent Laid-Open No. 55-26857), and a tool with few defects or cracks during use is proposed. providing.

Further, in order to improve fracture toughness, tetragonal Z stable at high temperature
When rO ₂ is subjected to stress, it is transformed into monoclinic ZrO _{2 which} is a stable phase at constant temperature, and a mechanism to relax the stress is utilized to propose a high-toughness ceramic tool that incorporates non-stabilized ZrO ₂ in the Si ₃ N ₄ substrate. (JP-A-57-71871).

Furthermore, for the purpose of improving mechanical strength and fracture toughness by uniformly dispersing ZrO ₃ particles in a sintered body of Si ₃ N ₄ , a zirconia oxide powder in which Y ₂ O ₃ was previously solid-dissolved was made into Si. A method of molding in addition to ₃ N ₄ powder and sintering in a nitrogen atmosphere has been disclosed (JP-A-58-20783).

[Problems to be solved by the invention]

As described above, many Si ₃ N ₄ sintered bodies containing zirconia have been proposed, but no mention is made of deterioration with time of these sintered bodies. However, as long as zirconia is contained in the sintered body, the problem of deterioration of the sintered body over time is naturally expected.
This means that partially stabilized zirconia sintered bodies, especially tetragonal zirconia stabilized by Y ₂ O ₃ showing high strength and high toughness, are metastable tetragonal by heating at a relatively low temperature of 200 ° to 400 ° C. It is nodded by the transition from crystal to monoclinic. The volume expansion accompanied by this transition causes microcracks in the sintered body, and the mechanism of toughening does not work. In addition, deterioration of the toughness and strength of the sintered body over time is a serious problem. Particularly in the presence of water, the stability of the tetragonal crystal decreases, so that the deterioration with time is remarkable.

Therefore, even in a Si ₃ N ₄ sintered body containing zirconia whose matrix is Si ₃ N ₄ , when the contained zirconia is a non-stabilized zirconia (JP-A-57-71871), Of course, when the stabilizer is Y ₂ O ₃ (JP-A-58-20783), 2
Deterioration with time is extremely likely to occur at a relatively low temperature of 00 to 400 ° C.

Further, when MgO or CaO is used as a zirconia stabilizer, the strength and toughness of the obtained sintered body are not satisfactory. As described above, the conventional high toughness sintered body containing zirconia has limited applications as a structural material in terms of strength and durability.

The present invention has been made to solve such drawbacks of the conventional high toughness Si ₃ N ₄ sintered body, has a high strength, has little deterioration in strength and toughness with time, and has excellent thermal stability. The purpose of the present invention is to provide a high toughness Si ₃ N ₄ sintered body and to greatly expand the range of application as a Si ₃ N ₄ sintered body.

[Means for solving problems]

The high toughness Si ₃ N ₄ sintered body of the present invention is made of Y ₂ O ₃ and Ce.
O ₂ is solid-dissolved, and zirconia mainly composed of tetragonal crystals is added to 3
The gist is that the balance is 0% or less and the balance is Si ₃ N ₄ .

Si ₃ N ₄ of the present invention may be Al ₂ O ₃ or Al, if necessary.
One or two or more selected from N, BeO and MgO can be contained as a sintering aid in the range of 1 to 10 internal weight% of Si ₃ N ₄ .

In addition, Y ₂ O ₃ and CeO _{2 which} are solid-dissolved in zirconia are respectively Zr in three axes intersecting an equilateral triangle as shown in the drawing.
In the triangular coordinate showing the mol% of O ₂ , YO _1.5 , and CeO ₂ , the point A (ZrO ₂ 64.5 mol%, YO _1.5 35 mol
%, CeO ₂ 0.5 mol%) Point B (ZrO ₂ 94.5 mol%, YO _1.5 5 mol
%, CeO ₂ 0.5 mol%) Point C (ZrO ₂ 95 mol%, YO _1.5 2 mol%, C
eO ₂ 3 mol%) Point E (ZrO ₂ 60 mol%, YO _1.5 2 mol%, C
eO ₂ 37 mol%), and the composition can be within the range surrounded by the line connecting the four specific composition points.

The manufacturing method of the high toughness silicon nitride sintered body of the present invention is Y ₂ O.
₃ and CeO ₂ as a solid solution and mainly containing tetragonal zirconia in an amount of 30% by internal volume or less, and the balance being Si ₃ N ₄
It consists of powder, and if necessary Al as a sintering aid
A mixed powder containing one or more selected from ₂ O ₃ , AlN, BeO, and MgO in a range of 1 to 10 internal weight% of Si ₃ N ₄ is molded, and then sintered in a non-oxidizing atmosphere. The point is to do.

[Action]

The high toughness silicon nitride sintered body of the present invention is made of Y ₂ O ₃ and Ce.
Y ₂ O ₃ —CeO ₂ —ZrO ₂ system in which O ₂ is solid-solved is mainly tetragonal partially stabilized zirconia, and Si ₃ N
_Since it is dispersed in a matrix having ₄ as a substrate, it is more thermally unstable than the conventional temperature range (200-
Even after a long-term heat deterioration test at 400 ° C., there is almost no change, and extremely high strength is exhibited. This is Y ₂ O ₃ −
Zirconia compared to the ZrO _{2, Y 2 O 3 -CeO 2 -ZrO} 2 system is a sintered body made of tetragonal only in a wide composition range is understood than that obtained, which is stabilized by the addition of CeO ₂ Has a conventional crystal structure of Y ₂ O ₃
It is considered that this is because it is closer to the crystal structure of cubic crystal, which is a high temperature stable phase of zirconia, than the tetragonal zirconia stabilized by.

As described above, according to the present invention, metastable tetragonal zirconia dispersed in a Si ₃ N ₄ matrix, which is conventionally thermally unstable, particularly tetragonal zirconia stabilized by Y ₂ O ₃ is used. The thermal stability is further improved by simultaneous addition of CeO ₂ , which is a method of controlling the composition of zirconia, and is improved so as not to be thermally deteriorated.

In this thermally more stable tetragonal zirconia _{Y 2 O} 3 -CeO 2 -ZrO 2 system with the addition of CeO ₂ that does not exhibit deterioration, because of metastable tetragonal near ZrO ₂ particles When stress is concentrated, it transforms into a monoclinic crystal, which is a stable phase at low temperature, and has a function of relaxing stress. Therefore, the high toughness silicon nitride sintered body of the present invention has high strength and high toughness as well as excellent high temperature strength. In addition, ZrO ₂
The addition of S suppresses grain growth of Si ₃ N ₄ during sintering and contributes to improvement of hardness and wear resistance due to improvement of sinterability. When fine particles of zirconia in which Y ₂ O ₃ and CeO ₂ are solid-dissolved in this way are uniformly dispersed in the Si ₃ N ₄ grain boundary, an excellent silicon nitride sintered body is obtained.

However, Y ₂ O ₃ , CeO ₂ , ZrO ₂ powder,
When Si ₃ N ₄ powder is mixed with a usual mixing method using a ball mill or the like, a difference in specific gravity or segregation due to aggregation occurs, resulting in non-homogeneous composition and microscopically Y ₂ O ₃ , CeO ₂ and Si ₃ N ₄ form a matrix forming region, and ZrO ₂ and Si ₃ N ₄ form a matrix forming region. Due to the grain growth of _{2 and} the like, zirconia in the silicon nitride sintered body is likely to become monoclinic, the thermal stability becomes low, and at the same time, the improvement of the mechanical properties of the silicon nitride sintered body becomes satisfactory. Hateful.

On the other hand, when using ZrO ₂ powder in which a necessary amount of Y ₂ O ₃ and CeO ₂ are dissolved in advance, ZrO ₂ fine particles are uniformly dispersed in a tetragonal crystal form in the silicon nitride sintered body. be able to. The present invention has been completed based on such new findings, and is to mix ZrO ₂ mainly composed of tetragonal crystals in which Y ₂ O ₃ and CeO ₂ are solid-solved with Si ₃ N ₄ , A part of Y ₂ O ₃ and CeO ₂ solid-dissolved in ZrO ₂ diffuses into Si ₃ N ₄ during the sintering process and also promotes sintering, and Si ₃ N ₄ which is a matrix.
The grain boundaries of the particles and the zirconia particles that are the dispersed phase are strengthened,
Therefore, it is possible to obtain a high toughness silicon nitride sintered body which is excellent in mechanical strength and fracture toughness at normal temperature and high temperature and does not deteriorate with time.

In the present invention, zirconia containing Y ₂ O ₃ and CeO ₂ as a solid solution is used. The compounding amounts of the three components Y ₂ O ₃ , CeO ₂ and ZrO ₂ must be within the range surrounded by the line connecting points A, B, C and E in the triangular coordinates as shown in the drawing. . Within this range, the stability of the tetragonal crystal is high and the heat resistance is excellent, and outside this range, the heat resistance is significantly reduced and the mechanical properties are also poor.

That is, YO is more than point A (YO _1.5 35 mol%).
_{When a} large amount of _1.5 is contained, the zirconia particles dispersed in Si ₃ N ₄ are mostly cubic crystals, and improvement in mechanical properties such as fracture toughness of the Si ₃ N ₄ sintered body cannot be recognized. Point B
When the amount of YO _1.5 is less than (YO _1.5 5 mol%), the progress of sintering of Si ₃ N ₄ is delayed and the heat resistance is lost. Point C (YO _1.5 2mol%, CeO ₂ 3m
The heat resistance is also poor when YO _1.5 and CeO ₂ are less than ol%). In addition, point E (CeO ₂ 37m
If the content of CeO ₂ is larger than that of ol%), sufficient mechanical strength cannot be obtained.

In order to make the present invention more effective, the compounding amounts of the above three components are represented by a point F (ZrO ₂ 69.5 mol%, YO _1.5 30 mol) in a triangular coordinate as shown in the drawing.
%, CeO _{2 1.5} mol%) Point G (ZrO ₂ 92.5 mol%, YO _1.5 6 mol
%, CeO ₂ 0.5 mol%) Point H (ZrO ₂ 94 mol%, YO _1.5 3 mol%, C
eO ₂ 3 mol%) Point I (ZrO ₂ 93 mol%, YO _1.5 2 mol%, C
eO ₂ 5 mol%) Point J (ZrO ₂ 66 mol%, YO _1.5 2 mol%, C
eO ₂ 32 mol%) is preferably selected within the range by a solid line connecting the _two . In addition, Y ₂ O ₃
A part of Nd ₂ O ₃ , Yb ₂ O ₃ , La ₂ O ₃ , Er ₂
It is also possible to substitute it with a rare earth metal oxide such as O ₃ .

In the present invention, the reason why the addition amount of zirconia containing Y ₂ O ₃ and CeO ₂ of the above composition contained in the silicon nitride sintered body as a solid solution is limited to 30 internal volume% or less is This is because the excellent characteristics of Si ₃ N ₄ itself cannot be maintained. Therefore, the amount of zirconia added is preferably in the range of 30 internal volume% or less, and more preferably in the range of 3 to 25 internal volume%.

In the present invention, Si ₃ N ₄ powder is mixed with Al as a sintering aid.
One or more selected from ₂ O ₃ , AlN, BeO, and MgO are added, because the addition of these sintering aids makes it possible to obtain a more dense sintered body. The total amount of addition is 1 to 10% by weight based on the Si ₃ N ₄ powder because the effect of densification cannot be sufficiently obtained when the content is 1% or less. This is because the amount of the glass phase is increased and the high temperature mechanical properties of the Si ₃ N ₄ sintered body deteriorate. A more dense sintered body can be obtained by preferably adding in the range of 2 to 8 internal weight%.

The method of the present invention is usually carried out as follows. Y ₂ O
₃ and CeO ₂ as a solid solution ZrO ₂ powder and Si ₃ N ₄ powder, and if necessary, Al ₂ O ₃ , AlN, BeO,
After pulverizing and mixing one or more kinds of powders selected from MgO, a molding aid such as polyvinyl alcohol is added and molded into a predetermined shape, in a non-oxidizing atmosphere such as a nitrogen atmosphere, Firing is carried out under normal pressure or pressure at 1650 to 1850 ° C., preferably 1670 to 1730 ° C. for 0.2 to 5 hours, preferably 0.5 to 2 hours. Or 1600-185
Hot pressing is performed at 0 ° C. in a non-oxidizing atmosphere at a pressure of 100 kg / cm ² or more.

Further, the composition of the present invention has a relative density of 95 at an appropriate temperature condition.
% Non-breathable sintered body is obtained, and then, in a hot isostatic press, gas pressure of 1000 kg / cm ² or more, 110
If sintering is performed for 0.5 hours or more under a temperature condition of 0 ° C. or more and 1800 ° C. or less, it is possible to obtain a high-performance fine particle density sintered body which is almost completely densified.

The Si ₃ N ₄ powder may contain either α phase or β phase, but preferably contains 50% by weight or more of α phase,
The impurities are Ca, Fe and the like in an amount of 3% by weight or less, preferably 1% by weight or less, and an average particle diameter of 5 μm or less, preferably 1
The BET specific surface area is 1 μm or less and 1 to 50 m ² / g, preferably 5 to 30 m ² / g. The ZrO ₂ powder is preferably a solid solution of Y ₂ O ₃ and CeO ₂ having an average primary crystal particle size of 0.5 μm or less, preferably 0.2 μm or less.

Since the zirconia contained in the high toughness silicon nitride sintered body of the present invention is a partially stabilized zirconia mainly composed of tetragonal crystals, it exhibits high strength and high toughness. Originally, the tetragonal crystal is a metastable phase, so when the sample surface is ground, the crystal lattice is distorted and partly transferred to the monoclinic crystal, and the residual compressive stress of the surface layer contributes to the strengthening of the sintered body. . The extent of this enhancement is
It also depends on the surface roughness due to grinding and the grain size of the sintered body.
Therefore, the zirconia mainly composed of tetragonal crystal according to the present invention means zirconia containing at least 20% or more of tetragonal crystal in a mirror state in the measurement of the crystal phase by X-ray diffraction. If the tetragonal system is 20% or less, the toughness is low, so it is necessary that the tetragonal system is contained in 20% or more.

ZrO ₂ contained in the silicon nitride of the present invention shows completely the same characteristics even if at least one part thereof is replaced with HfO ₂ .

〔Example〕

Examples of the present invention will be described in detail below to clarify the effects of the present invention.

Example 1 Average particle size 0.7 μm, BET specific surface area 8.5 m ² / g.
Si ₃ N ₄ powder containing 92% α-Si ₃ N ₄ , and
ZrO ₂ powder having a BET specific surface area of 25 m ² / g and an average particle size of 1.5 μm based on the average particle size of 0.02 μm crystalline primary particles in which Y ₂ O ₃ and CeO ₂ were previously solid-dissolved in the proportions shown in Table 1.
Formulated _Al 2 _O 3, AlN of ～0.3Myuemu, BeO, each sintering aids MgO in proportions shown in Table 1 or Table 2, were performed 48 hours wet powder mixture using a modified alcohol. After drying this powder, 2.0 ton / cm ²
After being isotropically molded at a pressure of 1700 ° C., additive-free sintering was performed at 1700 ° C. for 1 hour in an atmospheric pressure nitrogen atmosphere.

The bulk density of the obtained sintered body was measured and 3 × 4 × 40
The sample was cut and polished to mm, and the anti-segregation strength and fracture toughness were measured and shown in Table 1 or 2.

As for the electro-deposition strength, in accordance with JIS standard, using a 3 x 4 x 40 mm sample piece, span 30 mm, crosshead speed 0.5 mm
The average value of 10 pieces was shown by 3-point bending of / min.

Fracture toughness is determined by the single edge notch beam (Single
e Edge Noched Beam) method, the same shape as the bending strength measurement sample, processing accuracy of 3 × 40 of the sample
Using the mm surface as the tension surface, cut a groove with a width of 0.1 mm, a depth of 1 mm and a length of 3 mm with a diamond cutter,
Span 30mm, Crosshead speed 0.5mm / min
It is the value measured in.

Sample No. 1 in Table 1 1-No. 7 is ZrO _{2 to be} used
15m each of YO _1.5 and CeO ₂ dissolved in powder
It is fixed to ol% and 8 mol%, and the addition amount of ZrO ₂ is 0 to
FIG. 1 shows a first invention example of the present invention in which Si ₃ N ₄ powder is added and mixed while gradually increasing it to 40% by volume step by step and a comparative example thereof. Sample No. which is a comparative example containing no ZrO ₂ . 1
Then, the sintering did not proceed, and the density and the cohesive strength were extremely low. In addition, Comparative Example No. _{1 in} which ZrO ₂ was added in the composition range of the present invention or more. In No. 7, both bending strength and fracture toughness deteriorate.
On the other hand, the case of No. 2 to No. In No. 6, it was revealed that the density, the segregation strength, and the fracture toughness show high values.

Sample No. No. 8 is a comparative example using ZrO ₂ containing no stabilizer, but both the density and the segregation strength were low, and the zirconia present in the sintered body was monoclinic. In addition, Comparative Example No. No. 9 is a mixture of ZrO ₂ powder and Y ₂ O ₃ that do not contain a stabilizer in advance and is mixed and used with Si ₃ N ₄ powder, but the fracture toughness and the segregation strength are not satisfactory, and the sintered body It contained a considerable amount of monoclinic zirconia. From this result, in the present invention, Y ₂ O
_It was found that it is effective to add ₃ and CeO ₂ as a solid solution and mainly consist of tetragonal crystals, and to add zirconia to Si ₃ N ₄ and perform mixed sintering.

Table _2, Y 2 _{O 3} and MgO to CeO ₂ as in addition to the sintering aid of ZrO ₂ powder dissolved consisting tetragonal, Me
It is a second invention example in which O, Al ₂ O ₃ , AlN and the like are added. In Table 2, No. 10-No. In the case of No. 17, YO _1.5 and CeO ₂ which are solid-solved in the ZrO ₂ powder to be used are fixed at 15 mol% and 8 mol%, respectively, and the addition amount of the ZrO ₂ powder is gradually increased up to 40% by volume and M
It is a mixture of Si ₃ N ₄ powder added and mixed with a sintering aid selected from gO, BeO, Al ₂ O ₃ and AlN.
No. 1, which is an example of the present invention. 10-No. In No. 16, it was confirmed that the density of the sintered body was improved by the addition of the sintering aid as compared with Table 1 and that both the segregation strength and the fracture toughness were high. On the other hand, Comparative Example No. containing ZrO ₂ in the composition range of the present invention or more. In No. 17, satisfactory characteristics were not obtained.

Example 2 Si ₃ N ₄ powder having the same average particle size, BET specific surface area, and α-Si ₃ N ₄ as used in Example 1, and Y
₂ O ₃ and CeO ₂ were dissolved beforehand in the proportions shown in Table 3,
ZrO ₂ powder having a BET specific surface area of 25 m ² / g and an average particle diameter of 0.02 μm crystal primary particles was prepared in a ratio shown in Table 3 and wet-milled and mixed with a modified alcohol for 48 hours. And 2.0 ton / c after drying this powder
After being isotropically molded at a pressure of m ² , it was pressureless sintered at 1700 ° C. for 1 hour in an atmospheric pressure nitrogen atmosphere.

The bulk density, the segregation strength and the fracture toughness of the obtained sintered body were measured in the same manner as in Example 1. In addition, Zr on the surface of the sintered body
After measuring the crystal phase of O ₂ , a thermal deterioration test is carried out by holding the sintered body in an electric furnace at 300 ° C. for 3000 hours, and the crystal phase and the segregation strength of the surface of the sintered body after the thermal deterioration test are measured. did. The measured results are shown in Table 3.

The quantitative measurement of the crystal phase was performed by the X-ray diffraction method. That is, the integrated intensity I _{M of the} monoclinic (111) face and the (111) face of the sample piece mirror-polished with diamond paste, and the tetragonal (111) face and the cubic (11)
1) From the integrated intensities I _T and I _{C of the} plane, the amount of monoclinic crystal is Was determined by the formula. The amount of monoclinic crystals after the heat deterioration test was obtained by this. Next, the sintered body is pulverized to 5 μm or less,
Monoclinic ZrO ₂ and cubic ZrO under the same conditions by X-ray diffraction
₂ product component intensity _I ^M _*, was determined _I ^{C *.} That is, the tetragonal ZrO ₂ existing in the sintered body during the pulverization process
Are all considered to be transformed into monoclinic ZrO ₂ by mechanical stress. Therefore, the cubic amount is The tetragonal amount was then determined.

In this embodiment, the amounts of ZrO ₂ powder and Si ₃ N ₄ powder are kept constant, and YO _1.5 and C dissolved in the ZrO ₂ powder are dissolved.
The composition of eO ₂ is increased step by step.

In Table 3, No. 18-No. In No. 21, YO _1.5 was set to 2 mol% and the number of moles of CeO ₂ was gradually increased. A comparative example in which the amount of CeO ₂ added is small and is outside the composition range of the present invention is No. No. 18 has insufficient bending strength, and the strength deteriorates significantly after the heat deterioration test.

On the contrary, in Comparative Example No. 7 in which the amount of CeO ₂ added was too large to fall outside the composition range of the present invention. No. 21 has little heat deterioration,
Sufficient values cannot be obtained for bending strength and fracture toughness. No. of CeO ₂ amount is the composition range of the present invention, on the other hand
19 and No. In No. 20, bending strength and fracture toughness were obtained at desired values, and it was revealed by a heat deterioration test that the crystal phase did not change and the strength did not deteriorate.

No. 22-No. 24 is YO _1.5 mol%, C
The amount of eO ₂ added was gradually increased. Comparative example No. containing no CeO ₂ at all. In No. 22, the strength was significantly reduced after the heat deterioration test. On the other hand, in the composition range of the present invention, Ce
No. including O ₂ 23 and No. 23. No. 24 had high bending strength and fracture toughness, and it was confirmed by the heat deterioration test that the crystal phase did not transfer and the strength did not deteriorate.

No. 25-No. In No. 27, YO _1.5 was set to 10 mol% and the addition amount of CeO ₂ was changed similarly.
Comparative example No. in which CeO ₂ was not added. No. 25 has a high bending strength, but it is severely deteriorated by heat. However, in the case of No. ₂ containing CeO _{2 in} the composition range of the present invention. 26-No. Nos. 27 and 27 have the desired strength, and thermal deterioration does not occur. No. 28
And No. 29 contains CeO ₂ containing 15 mol% of YO _1.5
However, the bending strength and the fracture toughness are both high, and the crystal phase does not transfer and the strength does not deteriorate even in the heat deterioration test.

No. 30-No. 32 has 5 mol% CeO ₂ ,
Although YO _1.5 is an example containing more than 20 mol%, contains YO _1.5 in the composition range of the present invention No. 30 and No.
No. 31, which has the desired values for bending strength, fracture toughness, and bending strength after the heat deterioration test, has a large amount of YO _1.5 .
It became apparent that No. 32 could not obtain sufficient strength.

As is clear from the results shown in FIG. ₃ , a silicon nitride sintered body in which Y ₂ O ₃ and CeO ₂ within the composition range of the present invention are solid-dissolved, and zirconia is uniformly dispersed, the CeO ₂ component is Y ₂ O _3.
By coexisting with the components, it has high strength and retains high strength even after the heat deterioration test. Also, in the heat deterioration test, it was found that the zirconia on the surface of the sample was extremely stable without forming monoclinic crystals from tetragonal crystals.

(Example 3) A sintered body prepared by the method of Examples 1 and 2 was used.
The sample was held in an electric furnace at 0 ° C. for a predetermined time and a thermal deterioration test was conducted to measure the amount of monoclinic crystal on the surface of the sintered body sample.

In FIG. 2, No. 2 which is a comparative example using zirconia in which only Y ₂ O ₃ is solid-dissolved. 22 and No. 25 is 500
The amount of monoclinic crystals increased at a stretch with the retention of time, and at 1500 hours, 60% or more was transformed into monoclinic crystals. On the other hand, No. ₂ , which is an example of the present invention in which Y ₂ O ₃ and CeO ₂ having the composition of the present invention are solid-dissolved. 14, No. 26, No. 27, and No. Two
It was confirmed that No. 9 exhibited excellent thermal stability, with almost no transfer to monoclinic crystals.

〔The invention's effect〕

The high toughness silicon nitride sintered body and the manufacturing method thereof according to the present invention,
As described above, zirconia, which is mainly tetragonal in which Y ₂ O ₃ and CeO ₂ are previously solid-dissolved, is mixed, and Al ₂ O ₃ , AlN, BeO, and
One or more kinds of MgO are added and sintered in a non-oxidizing atmosphere, and the obtained sintered body is a conventional silicon nitride sintered body containing zirconia stabilized only by Y ₂ O _3. Compared with the above, since it is a high-grade denseness, it has excellent mechanical strength and toughness at normal temperature and high temperature, and further, the transfer of the crystal phase occurs even in the temperature range (200 ° to 400 ° C.) which is more unstable than before. And shows extremely excellent thermal stability. In addition, even in pressureless sintering, a sintered body having excellent density, mechanical strength and toughness can be industrially obtained at low cost, further expanding the application range of the silicon nitride sintered body.

As described above, the high toughness silicon nitride sintered body of the present invention which is further excellent in durability is, for example, a gas turbine engine part, a diesel engine part, a pump part, a material for a high temperature furnace, a mechanical seal, an abrasion resistant bearing, or a cutting tool. It has a great industrial value and can be used for other purposes.

[Brief description of drawings]

FIG. 1 is a triangular coordinate diagram showing the composition range of ZrO ₂ , YO _1.5 , and CeO ₂ , and FIG. 2 is a diagram showing the relationship between the time of the heat deterioration test of Example 3 and the amount of monoclinic crystals.

Claims (8)

[Claims] 1. A high toughness silicon nitride, characterized in that it contains Y ₂ O ₃ and CeO ₂ as a solid solution and mainly contains tetragonal zirconia in an internal volume ratio of 30% or less, and the balance is Si ₃ N _4. Sintered body. 2. Zirconia is contained in Y ₂ O ₃ , C
As shown in the attached drawing, in the triangular coordinates in which mol% of ZrO ₂ , YO _1.5 , and CeO ₂ are respectively displayed on the three axes where eO ₂ intersects with an equilateral triangle, point A (ZrO ₂ 64.5 mol%, YO _1.5 35 mol
%, CeO ₂ 0.5 mol%) Point B (ZrO ₂ 94.5 mol%, YO _1.5 5 mol
%, CeO ₂ 0.5 mol%) Point C (ZrO ₂ 95 mol%, YO _1.5 2 mol%, C
eO ₂ 3 mol%) Point E (ZrO ₂ 60 mol%, YO _1.5 2 mol%, C
The high toughness silicon nitride sintered body according to claim 1, which has a composition within a range surrounded by a line connecting four specific composition points represented by (eO ₂ 37 mol%). 3. A high toughness silicon nitride according to claim 1 or 2, wherein the monoclinic amount of zirconia contained in the sintered body after being kept in air at 300 ° C. for 3000 hours is 30% or less. Sintered body. 4. A zirconia solution containing Y ₂ O ₃ and CeO ₂ as a solid solution and mainly consisting of tetragonal crystals in an amount of 30% or less in terms of internal volume, and one or more selected from Al ₂ O ₃ , A1N, BeO and MgO. 1 to ₃ of Si ₃ N ₄ using two or more as a sintering aid
A high toughness silicon nitride sintered body, characterized in that it is contained within a range of 10% by weight and the balance is Si ₃ N ₄ . 5. Zirconia is contained in Y ₂ O ₃ , C
As shown in the attached drawing, in the triangular coordinates in which mol% of ZrO ₂ , YO _1.5 , and CeO ₂ are respectively displayed on the three axes where eO ₂ intersects with an equilateral triangle, point A (ZrO ₂ 64.5 mol%, YO _1.5 35 mol
%, CeO ₂ 0.5 mol%) Point B (ZrO ₂ 94.5 mol%, YO _1.5 5 mol
%, CeO ₂ 0.5 mol%) Point C (ZrO ₂ 95 mol%, YO _1.5 2 mol%, C
eO ₂ 3 mol%) Point E (ZrO ₂ 60 mol%, YO _1.5 2 mol%, C
The high toughness silicon nitride sintered body according to claim 4, which has a composition within a range surrounded by a line connecting four specific composition points represented by eO ₂ 37 mol%). 6. The high toughness silicon nitride according to claim 4 or 5, wherein the amount of zirconia monoclinic crystal contained in the sintered body after being kept at 300 ° C. in the atmosphere for 3000 hours is 30% or less. Sintered body. 7. A zirconia powder having a solid solution of Y ₂ O ₃ and CeO ₂ and mainly composed of tetragonal crystals is mixed in an amount of 30% by volume or less and the balance is Si ₃ N ₄ powder, and the mixture is molded in a non-oxidizing atmosphere. A high toughness silicon nitride sintered body characterized by sintering and a method for producing the same. 8. _Y 2 _{O 3} and zirconia powder consisting mainly tetragonal solid solution CeO ₂ is, _Y 2 O contained therein
₃ and CeO ₂ are present on the three axes intersecting the equilateral triangle as shown in the attached drawings, and ZrO ₂ , YO _1.5 , and CeO ₂ mol, respectively.
%, The point A (ZrO ₂ 64.5 mol%, YO _1.5 35 mol
%, CeO ₂ 0.5 mol%) Point B (ZrO ₂ 94.5 mol%, YO _1.5 5 mol
%, CeO ₂ 0.5 mol%) Point C (ZrO ₂ 95 mol%, YO _1.5 2 mol%, C
eO ₂ 3 mol%) Point E (ZrO ₂ 60 mol%, YO _1.5 2 mol%, C
The manufacturing method of the high toughness silicon nitride sintered body according to claim 7, which has a composition within a range surrounded by a line connecting the four specific composition points represented by (eO ₂ 37 mol%).