JP2002068843A - Silicon nitride sintered body and method for producing the same - Google Patents

Abstract

(57) [Summary] PROBLEM TO BE SOLVED: To provide both high strength at high temperature and excellent oxidation resistance.
Provided is a silicon nitride-based sintered body. SOLUTION: The grain boundary phase is Lu_FourSi_TwoO₇N_TwoOf the crystalline phase of
Single phase, composition is Si_ThreeN _Four-SiO_Two-Lu_TwoO_ThreeOn the ternary state diagram
And point A: Si_ThreeN_Four, Point B: 28 mol% SiO_Two-72mol% Lu_TwoO _Three,
And C point: 16mol% SiO_Two-84mol% Lu_TwoO_ThreeA vertex with three points
The composition is that around or inside the square ABC.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride sintered body and a method for producing the same. More specifically, the invention of this application relates to a silicon nitride-based sintered body having both high strength and excellent oxidation resistance at a high temperature and usable as a material for structural parts of various machines and devices, and a manufacturing method for manufacturing the same. Things.

[0002]

2. Description of the Related Art A sintered body containing silicon nitride as a main component, that is, a silicon nitride-based sintered body is chemically stable at normal temperature and high temperature, has high mechanical strength, and is a sliding member such as a bearing. It is expected to be used for engine members such as turbocharger rotors.

In order to obtain a high-strength silicon nitride sintered body, an oxide is added to a silicon nitride powder as a sintering aid,
Baking at 1600 ° C. or higher, liquid phase sintering, and densification are performed. Magnesium oxide, aluminum oxide, and rare earth element oxides are known as oxides effective as sintering aids. Among them, magnesium oxide, aluminum oxide, and yttrium oxide are often used alone or as a mixture. During firing, these sintering aids react with silicon oxide, which is an oxide layer on the surface of the raw material at a high temperature, to generate a liquid phase. Sintering proceeds by diffusion of silicon nitride in the liquid phase thus generated. When cooled after sintering, the liquid phase partially crystallizes as oxides or oxynitrides, but mostly forms as a glass phase at grain boundaries. Therefore, the silicon nitride sintered body is generally composed of silicon nitride particles and a glass phase which is a grain boundary phase.

[0004]

When such a silicon nitride sintered body is used in a high-temperature environment of 1000 ° C. or more, the glass phase existing at the grain boundary softens and the strength rapidly decreases. Has been pointed out. Since the softening temperature is proportional to the melting temperature of the metal-Si-O system in the grain boundary phase, the degree of the strength reduction largely depends on the chemical composition of the grain boundary phase. Therefore, the high-temperature strength is higher when a mixture of aluminum oxide and yttrium oxide is added than when magnesium oxide is added as a sintering aid.

Recently, in order to improve the high-temperature strength of a silicon nitride sintered body, use of a mixture of a rare earth oxide and silicon oxide as a sintering aid has been studied.

For example, J. Am. Ceram. Soc. No. 75, 205
On page 0 (1992), there was reported a silicon nitride-based sintered body in which a high melting point Y ₂ Si ₂ O ₇ was precipitated at the grain boundaries by adding a yttrium oxide-silicon oxide sintering aid. ing. In this silicon nitride sintered body, nitrogen-containing apatite (N phase, Y ₁₀ Si ₇ O ₂₂ N ₄ ) or K phase (YSiO
₂ N) or a glass phase having a composition close thereto is a second grain boundary phase, second only to the Y ₂ Si ₂ O _7. The softening temperatures of the N and K phases are
Since the temperature is 1500 ° C. or lower, the high-temperature strength of the silicon nitride-based sintered body in which these phases or glass phases are formed as a grain boundary phase has not been sufficiently satisfactory. Furthermore, Si ₃ N ₄ -Y ₂ Si ₂ O _2-
In the phase diagram of the Si ₂ N ₂ O ternary system, the composition around and inside the triangle having the respective components as vertices was examined, but the high-temperature strength was not sufficient.

In addition, in a ternary system of Si ₃ N ₄ —SiO ₂ —RE ₂ O ₃ (RE: rare earth element), Japanese Patent Application Laid-Open No. 4-15466 discloses a J phase shown in the phase diagram of FIG. JP-A-4-43972 discloses that (RE ₄ Si ₂ O ₇ N ₂ ), N phase and K phase are precipitated at the grain boundary.
Precipitating the J phase and the rare earth nitride at the grain boundary,
Japanese Patent Application Laid-Open No. H4-229465 reports that an S phase (RE ₂ SiO ₅ ) is precipitated at grain boundaries. Further, JP-A-8-48
No. 565 reports that a J phase or two phases of a J phase and an S phase in the phase diagram shown in FIG. 5 are precipitated as a grain boundary phase.

However, in any case, since a large amount of rare earth oxide is added for controlling the composition, the high temperature strength is improved to some extent, while the creep resistance and
There is a new problem that the oxidation resistance is reduced. Further, these documents do not report any difference in the effect of addition between rare earth elements.

The present invention has been made in view of the above circumstances, and provides a silicon nitride sintered body having both high strength and excellent oxidation resistance at a high temperature, and a production method for producing the same. It is intended to be.

[0010]

Means for Solving the Problems The inventors of the present invention pay attention to the phase diagram and sinterability difference between rare earth elements,
By selecting lutetium (Lu) as a rare earth element and removing oxygen impurities in the silicon nitride powder as a starting material, the J phase (Lu ₄ Si ₂ O ₇ N ₂ ) can be formed at the grain boundary even with the addition of a small amount of Lu oxide. The inventors succeeded in precipitation and found that a silicon nitride sintered body having not only high temperature strength but also oxidation resistance can be obtained.

The phase diagram of the ternary Si ₃ N ₄ —SiO ₂ —RE ₂ O ₃ is shown in FIG.
And two types shown in FIG. The type shown in FIG. 4 has a K phase and an N phase, and rare earth elements having a large ionic radius such as yttrium (Y) belong to this. On the other hand, the type shown in FIG.
Publications include ytterbium (Yb), thulium (T
m), lutetium (Lu) has been reported. When the inventors of the present application examined the disparity between rare earth elements in detail, Lu also belongs to the type shown in FIG. 5, and among them, only Lu is almost completely crystallized at the grain boundary. It turned out to be.

Incidentally, the chemical composition of the J phase in the ternary system of Si ₃ N ₄ —SiO ₂ —Lu ₂ O ₃ is Lu ₄ Si ₂ O ₇ N _{2 as} described above.
The composition is Si ₃ N ₄ : SiO ₂ : Lu ₂ O ₃ = 1: 1: 4 (molar ratio). Therefore, when firing the silicon nitride sintered body,
In order to completely crystallize the generated liquid phase, it is necessary to add Lu ₂ O ₃ four times as much as SiO _{2 in} the composition. Assuming that the raw material silicon nitride powder contains 1.5% by weight of oxygen as an impurity (an oxygen content of this level or higher is often found), when converted to SiO ₂ ,
This corresponds to approximately 3% by weight (6 mol%). From this fact, it is necessary to add 24 mol% or more of Lu ₂ O ₃ in order to make the grain boundary phase a single phase of J phase. This facilitates the progress and causes a decrease in oxidation resistance as described above. If the grain boundary phase is not a single phase but a mixed phase with another phase, the heat resistance is reduced. In the silicon nitride based sintered body reported so far, the grain boundary is Lu ₄ S
It is not a single phase of i ₂ O ₇ N ₂ but is a mixture with all other phases.

Therefore, the inventors of the invention of the present application disclose
By reducing the oxygen content of the raw material, sintering aid
Lu added as_TwoO_ThreeTo reduce the amount of Lu added during firing._Four
Si_TwoO ₇N_TwoTo form a liquid phase having the composition_FourSi_TwoO₇N_Twoof
Realized silicon nitride sintered body with a substantially single phase grain boundary phase
It was done.

Here, being substantially single-phase means that
This means that Lu ₄ Si ₂ O ₇ N ₂ crystallizes completely or almost completely, with no other phases present or a slight identification of other phases in the microstructure.

The invention of this application is based on the above findings,
It is completed.

That is, the invention of this application is a silicon nitride-based sintered body composed of silicon nitride particles and a grain boundary phase, wherein the grain boundary phase is Lu ₄ S
The crystalline phase of i ₂ O ₇ N ₂ consists essentially of a single phase, and the composition of the silicon nitride sintered body is represented by A ₃ N ₄ —SiO ₂ —Lu ₂ O ₃ point: Si ₃ N _4, B _{point: 28mol% SiO 2 -72mol% Lu} 2 O 3, and point C: around or inside the triangle ABC which vertices three points _{16mol% SiO 2 -84mol% Lu 2} O 3 The present invention provides a silicon nitride-based sintered body (Claim 1) having the following composition:

The invention of this application is based on the silicon nitride sintered
2. The composition of the silicon nitride sintered body according to claim 1, wherein
Point D on triangle ABC: 99mol% Si_ThreeN_Four-0.28mol% SiO_Two-
0.72mol% Lu_TwoO_Three, Point E: 99mol% Si_ThreeN_Four-0.16mol% SiO_Two-0.84
mol% Lu_TwoO_Three, F point: 94mol% Si_ThreeN _Four-1.68mol% SiO_Two-4.32mol%
Lu_TwoO_Three, And G point: 94mol% Si_ThreeN_Four-0.96mol% SiO_Two-5.04mol%
Lu_TwoO_ThreeAround or inside the square DEFG with the four points of
Part of the composition (Claim 2), Lu_FourSi_TwoO₇N_Two2.5 to 1
Containing 0% by weight (Claim 3), elements other than Lu, Si, O and N
Is less than 1% by weight (claim 4),
90% by volume or more of the grain boundary phase existing at the grain boundary is Lu_FourSi_TwoO₇N_Two
Each having a crystal phase of
Provide as

Further, the invention of this application has an oxygen content of 1.0
1 to 12% by weight of lutetium oxide powder is added to and mixed with 1% to 12% by weight of lutetium oxide powder to silicon nitride powder of 1% by weight or less until the composition according to claim 1 or 2 is obtained at 1700 to 2200 ° C in a nitrogen atmosphere of 1 to 100 atm The present invention provides a method for producing a silicon nitride sintered body, characterized by firing.

Further, the invention of this application has an oxygen content of 1.5
1 to 12% by weight of lutetium oxide powder is added to and mixed with silicon nitride powder of 1% by weight or less and heated to 1600 ° C. or less in a nitrogen atmosphere of 1 atm or less prior to firing, and the oxygen content is reduced to 1%. Or volatilization of oxygen until the oxygen content of the composition described in 2 is reached, and then 1700 to 22 in a nitrogen atmosphere of 1 to 100 atm.
A method for producing a silicon nitride-based sintered body characterized by firing at 00 ° C. is provided.

According to the method for producing a silicon nitride sintered body of the invention according to claim 7, the invention of this application is characterized in that silicon powder is further added (claim 8) and the amount of silicon powder added is 1 to 10. % By weight (claim 9) is provided as a preferred embodiment.

Further, the invention of the present application provides a method for producing a silicon nitride-based sintered body, wherein 1 to 12% by weight of silicon powder is added to silicon powder.
Lutetium oxide powder is added and mixed, and then heated to 1500 ° C. or less in a nitrogen atmosphere to convert silicon to silicon nitride, and then at 1700 to 2200 ° C. in nitrogen at 1 to 100 atm. 2. A method for producing a silicon nitride-based sintered body, characterized by firing until the composition described in (2) is reached.
0).

With respect to the above-mentioned method for producing a silicon nitride sintered body, the invention of this application provides, as a preferred embodiment, that firing is performed by hot pressing (claim 11).

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, a silicon nitride sintered body of the present invention is a silicon nitride sintered body composed of silicon nitride particles and a grain boundary phase, and the grain boundary phase is Lu. ₄ Si ₂ O ₇
The composition of the silicon nitride sintered body substantially consists of a single phase of the N ₂ crystal phase, and the composition of the silicon nitride based sintered body is represented by the Si ₃ N ₄ —SiO ₂ —Lu ₂ O ₃ ternary system shown in FIG. Point A: Si ₃ N ₄ , Point B: 28 mol% SiO ₂ -72 mol% Lu ₂ O
₃ and C points: The composition around or inside the triangle ABC having three vertices of 16 mol% SiO ₂ -84 mol% Lu ₂ O ₃ . Side AB
In the composition closer to the SiO ₂ side, the S phase (Lu ₂ SiO ₅ ) is used as the crystal phase.
Precipitates, and as a result, the high-temperature strength decreases. Also, side A
In the composition on the Lu ₂ O ₃ side from C, melilite (L
u ₂ Si ₃ O ₃ N ₄ ) is produced, which reduces the oxidation resistance.

The composition of the silicon nitride sintered body can be confirmed by chemical analysis. More specifically, Si, Lu and other metal elements are pulverized by sintering and then decomposed by heating in a mixed solution of hydrofluoric acid and nitric acid, using a high frequency induction emission spectrometer (ICP). The content in the sintered body is quantified. Oxygen and nitrogen can be quantified using a gas analysis technique. That is, the sintered body is thermally decomposed together with tin and carbon as a combustion aid, and can be quantified by the concentrations of nitrogen and carbon monoxide in the decomposition gas.

The silicon nitride sintered body according to the invention of the present application has the above composition, and has a point D on the triangle ABC of 99 mol% Si ₃ N ₄ −.
0.28 mol% SiO ₂ -0.72 mol% Lu ₂ O ₃ , point E: 99 mol% Si ₃ N ₄ -0.16
mol% SiO ₂ -0.84mol% Lu ₂ O ₃ , F point: 94mol% Si ₃ N ₄ -1.68mol%
SiO ₂ -4.32 mol% Lu ₂ O ₃ and point G: 94 mol% Si ₃ N ₄ -0.96 mol%
Square DEFG with four vertices of SiO ₂ -5.04mol% Lu ₂ O ₃
By making the composition around or inside, the oxidation resistance is further improved. In the composition on the Si ₃ N ₄ side from the side DE, the liquid phase component is small, and the composition is slightly inferior in densification. In the composition on the Lu ₂ O ₃ side from the side FG, the amount of the sintering aid is large, which is reflected in the oxidation resistance.

By sintering in the above composition range, the J phase (Lu ₄ Si ₂ O ₇ N ₂ ) precipitates as a crystal phase and substantially as a single phase at grain boundaries.

Lu ₄ Si ₂ O ₇ N _{2 in the} silicon nitride sintered body
Is suitably 2.5 to 10% by weight. If the content of Lu ₄ Si ₂ O ₇ N ₂ is less than 2.5% by weight, the sinterability is reduced and the densification does not easily proceed. If it exceeds 10% by weight, the oxidation resistance decreases. This J phase is composed of, for example, β-Si ₃ N ₄ and Lu ₄ Si ₂
A calibration curve is prepared from a powder mixture with O ₇ N _2, and quantification can be performed from the peak height by X-ray diffraction.

Further, silicon nitride sintered bodies include Lu, Si,
When the content of elements other than O and N is 1% by weight or less, heat resistance is particularly excellent. When the content of elements other than Lu, Si, O, and N exceeds 1% by weight, the heat resistance slightly decreases. The heat resistance can be improved also by the microstructure of the silicon nitride sintered body. That is, by setting 90% by volume or more of the grain boundary phase existing in the multi-grain boundary as shown in FIG. 3 as the crystal phase of Lu ₄ Si ₂ O ₇ N ₂ , particularly excellent heat resistance is realized. Is done. The remainder of the grain boundary phase present at the multi-grain grain boundary is usually Si-ON or
It is an amorphous phase having a Lu-Si-ON composition. If the amorphous phase exceeds 10% by volume, heat resistance is reduced. The quantification of the Lu ₄ Si ₂ O ₇ N ₂ crystal phase in the microstructure can be performed, for example, by cutting a flake from a sintered body and observing a multi-particle boundary using a transmission electron microscope (TEM). .

As described above, the method for producing a silicon nitride sintered body of the invention of this application is mainly characterized in that the oxygen content in the starting material is reduced, but silicon nitride is used as the raw material powder. In this case, any of α-type, β-type, amorphous, or a mixture of two or more of these can be used. The size, distribution, shape, purity, etc. of the particles are not particularly limited. On the other hand, in order to obtain a sintered body having high strength at both room temperature and high temperature, a powder having a particle size distribution of an average particle diameter of 2 μm or less and a metal impurity amount of 100 ppm or less is preferable. The upper limit of the oxygen content of the silicon nitride powder used as the starting material is 1.5% by weight (however, when firing without performing the oxygen volatilization treatment described below, the upper limit is 1.0% by weight). . If the oxygen content exceeds these upper limits, the added amount of the sintering aid increases, and the oxidation resistance is impaired.

Lutetium oxide Lu ₂ O ₃ is added to the sintering aid, and the amount of the lutetium added is such that the composition becomes Lu ₄ Si ₂ O ₇ N ₂ , specifically, 1 to 12% by weight. Excellent oxidation resistance is realized in this range. If the addition amount of Lu ₂ O ₃ is less than 1% by weight, the SiO ₂ component in the liquid phase at the time of firing increases, and Lu ₄ Si ₂ O ₇ N ₂
A substantially single crystal phase cannot be obtained. If it exceeds 12% by weight, the grain boundary phase becomes a single phase of the Lu ₄ Si ₂ O ₇ N ₂ crystal phase, but the grain boundary phase increases and the oxidation resistance decreases. Lu ₂ O ₃ added amount is 2.2
Assuming 5 to 9% by weight, Lu ₄ Si ₂ O ₇ N in the silicon nitride sintered body
The content of ₂ becomes 2.5 to 10% by weight.

In the method for producing a silicon nitride sintered body according to the invention of the present application, the sintering is performed in a nitrogen atmosphere at 1 to 100 atm.
Performed at ~ 2200 ° C. The firing method is not particularly limited. For example, hot pressing can be adopted as the simplest method. Hot pressing, for example, puts the raw material powder in a graphite mold, and in a nitrogen atmosphere of 1 to 100 atm.
Multiplied by the pressure of 100~500kg / cm ^2, 30~120 at 1700~2200 ℃
It can be performed by firing for about a minute.

If the atmosphere is lower than 1 atm, silicon nitride is decomposed and the silicon nitride sintered body is not densified. 100
When the pressure exceeds the atmospheric pressure, a high-pressure gas is confined in the silicon nitride-based sintered body, remains as bubbles, and does not become dense at 95% or more. On the other hand, if the firing temperature is lower than 1700 ° C., a liquid phase is not sufficiently generated, and thus the powder is not densified. If the temperature exceeds 2200 ° C., the grain growth becomes severe, and as a result, the strength at room temperature decreases.

In the method for producing a silicon nitride-based sintered body of the invention of the present application, in order to minimize the amount of the sintering aid added and to secure oxidation resistance, the oxygen in the silicon nitride powder is preceded by firing. An oxygen volatilization treatment for further reducing the content can be performed. In the oxygen volatilization process, specifically, the raw material mixed powder
It can be carried out by heating to 1600 ° C. or less in a nitrogen atmosphere of 1 atm or less. By this oxygen volatilization treatment, the oxygen content is reduced to the oxygen content of the above-described composition of the silicon nitride sintered body. The heating time can be about 30 to 120 minutes. By the oxygen volatilization treatment, the impurity oxygen contained in the silicon nitride powder is removed through a reaction such as Si ₃ N ₄ + 3SiO ₂ = 6SiO + 2N ₂ . If the heating temperature exceeds 1600 ° C., densification starts, so that oxygen cannot be removed efficiently. On the other hand, when the nitrogen atmosphere exceeds 1 atm, the decomposition reaction such as the above becomes difficult to proceed, and the efficiency of removing oxygen decreases.

The decomposition reaction as described above proceeds efficiently under reduced pressure.

In order to remove oxygen as an impurity from the silicon nitride powder as the raw material, it is effective to add and mix the silicon powder with the raw material in addition to the oxygen volatilization treatment described above. The added silicon (Si) reacts with Si + SiO ₂ = 2SiO or the like and contributes to the removal of oxygen. The amount of silicon powder to be added depends on the amount of oxygen contained in the silicon nitride powder, but can be approximately 1 to 10% by weight. In general,
If it is less than 1% by weight, the effect of removing oxygen is poor, and 10% by weight
%, Si may remain in the silicon nitride sintered body, which affects the properties.

In some cases, densification becomes difficult due to the oxygen volatilization treatment. In such a case, for example, sintering can be performed by hot isostatic pressing to ensure densification.

In addition to the above, in the method for producing a silicon nitride sintered body of the present invention, silicon powder can be used as a starting material. Silicon powder, compared to silicon nitride powder,
This is suitable for generating a liquid phase of Lu ₄ Si ₂ O ₇ N ₂ composition while having a low oxygen content and a reduced amount of a sintering aid. However, when silicon powder is used as a starting material, it is necessary to perform a nitriding treatment before firing. In particular,
After adding and mixing 1 to 12% by weight of lutetium oxide powder to silicon powder, the mixture is heated to 1500 ° C. or lower in a nitrogen atmosphere.
This nitriding treatment is a treatment for converting silicon to silicon nitride, and is essentially different from the oxygen volatilization treatment for adding silicon powder when the above-mentioned silicon nitride powder is used as a starting material. If the heating temperature exceeds 1500 ° C. in the nitriding treatment, silicon is undesirably melted. Further, the nitriding treatment can be performed until, for example, 90% or more of silicon is changed to silicon nitride. Whether it has been changed to silicon nitride can be confirmed by X-ray diffraction. After the nitriding treatment, as in the case where silicon nitride powder is used as a starting material, firing is performed at 1700 to 2200 ° C. in a nitrogen atmosphere of 1 to 100 atm. Also in this case, firing can be performed by hot pressing.

The silicon nitride sintered body of the invention of the present application is mostly crystallized by ordinary sintering. However, in order to further promote crystallization, 1 to 24 hours at 1300 to 1700 ° C. after sintering. It is effective to keep time.

Next, examples of the silicon nitride sintered body of the invention of the present application and a method for producing the same will be shown together with comparative examples.

[0040]

【Example】

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

[Table 4]

(Example 1) Average particle size 0.5 μm, oxygen content
1.0% by weight, 92% α-type silicon nitride powder (powder P1)
Lutetium oxide was added at 8.18% by weight and mixed and pulverized for 2 hours using a wet ball mill to which ethanol was added. Next, after drying using a rotary evaporator in the air, a molded body of 80 mm × 45 mm × 10 mm was formed by die molding at 20 MPa.

The compact was placed in a graphite mold and fired using a gas pressure hot press furnace. First, heat from room temperature to 1300 ° C at a rate of 500 ° C / hour in a vacuum of 10 ^-2 Pa,
After holding for 10 hours, nitrogen gas at 10 atm was introduced into the furnace,
While applying a pressure of MPa, the temperature was raised to 1800 ° C. at 500 ° C./hour and maintained at 1800 ° C. for 1 hour. The firing conditions are described in Table 1 as pattern S1.

The obtained sintered body is pulverized, put in a mixed solution of hydrofluoric acid and nitric acid, and kept at 180 ° C. for 10 hours in a Teflon (registered trademark) container to perform a heat dissolution treatment. ,
Si and L in solution using a high frequency induction emission spectrometer (ICP)
The concentration of u was quantified. Next, the pulverized material of the sintered body is sealed in a tin capsule together with the carbon powder, and dissolved by heating in a carbon crucible.The generated nitrogen and carbon monoxide are quantified, and the amount of oxygen and the amount of nitrogen in the sintered body are determined. Quantified.

As shown in Table 1, the quantitative value was Lu: 7.33.
75% by weight, Si: 55.011% by weight, O: 1.1586% by weight, N: 36.49
It was 3% by weight. From this result, the composition of the sintered body was 96.2
0 mol% Si ₃ N ₄ -0.703 mol% SiO ₂ -3.097 mol% Lu ₂ O ₃ , and the triangle ABC and the square DE shown in FIGS. 1 and 2 respectively.
The composition was within FG.

From the results of X-ray diffraction, it was confirmed that the phases formed in the sintered body were β-Si ₃ N ₄ and Lu ₄ Si ₂ O ₇ N ₂ .

Further, a slice was cut out from the sintered body, and after being subjected to an argon ion mill treatment, observed using a transmission electron microscope (TEM). As shown in FIG. 3, a microstructure composed of β-type silicon nitride particles, and two-particle and multi-particle boundaries was observed. Lu ₄ Si ₂ O ₇ by electron diffraction
Crystallization of N ₂ was confirmed. Lu ₄ Si ₂ O ₇ N ₂ was crystallized in all 20 randomly selected multi-grain boundaries.

Then, the sintered body was subjected to surface grinding using an 800 mesh diamond wheel, processed into dimensions of 3 mm × 4 mm × 40 mm, and the bending strength was measured by room-temperature and high-temperature four-point bending according to JIS-R1601. . As shown in Table 2, the porosity of the sintered body was 1.2%, and the four-point bending strength at room temperature was 1050MP.
a, The high-temperature four-point bending strength at 1500 ° C. was 820 MPa. In addition, the processed test piece was heated to 1500 ° C. in an air atmosphere furnace and held for 100 hours. The weight increase after this oxidation test was 0.05 mg / cm ² , and the four-point bending strength at room temperature was 1020.
MPa. Oxidation resistance is evaluated by weight change when heated in air and room temperature strength. As the oxidation proceeds, an increase in weight and a decrease in strength are observed due to the formation of an oxide film.

From the above results, it is determined that the obtained sintered body is a silicon nitride wet sintered body having sufficiently high strength even at a high temperature and having excellent oxidation resistance. (Comparative Example 1) Average particle size 0.3 μm, oxygen content 1.8% by weight, α
12% by weight of ytterbium oxide was added to a silicon nitride powder (powder P2) having a mold content of 90%, mixed and pulverized in the same manner as in Example 1 to obtain a molded body.

The compact was placed in a graphite mold and fired using a gas pressure hot press furnace. First, after heating from room temperature to 800 ° C. at a rate of 500 ° C./hour in a vacuum of 10 ⁻² Pa, a nitrogen gas of 10 atm is introduced into the furnace, a pressure of 20 MPa is applied, and 1800 ° C. at 500 ° C./hour The temperature was raised to 1800 ° C. for 1 hour. The formed phases were β-Si ₃ N ₄ and Yb
It was confirmed to be ₄ Si ₂ O ₇ N ₂ . Further, from observation using a TEM, it was confirmed that all of the grain boundary phases were Yb ₄ Si ₂ O ₇ N ₂ .

The bending strength of the obtained sintered body was measured in the same manner as in Example 1. As a result, as shown in Table 4, the porosity was 1.5%, and the four-point bending strength at room temperature was 1220MPa.
a, The high-temperature four-point bending strength at 1500 ° C. was 470 MPa. A decrease in high-temperature strength is observed as compared with Example 1. This is determined from the comparison with Example 1 as being caused by the fact that the grain boundary phase is not Lu ₄ Si ₂ O ₇ N ₂ but Yb ₄ Si ₂ O ₇ N ₂ . (Examples 2 to 7) As shown in Table 1, as the silicon nitride powder, the same powder P as the silicon nitride powder used in Example 1 was used.
1, or average particle size 0.8 μm, oxygen content 0.8% by weight, α
Silicon nitride powder (powder P3) having a mold content of 95% was used, and lutetium oxide was added and mixed in the same manner as in Example 1 at the content shown in Table 1 to produce a molded body.

This compact was placed in a graphite mold and fired using a gas pressure hot press furnace. As a firing condition, a pattern S2 different from that in Example 1 was employed. That is, first 10 ^-2 Pa
In a vacuum, heat from room temperature to 1300 ° C at a rate of 500 ° C / hour, introduce nitrogen gas at 1300 ° C at 0.8 atm into the furnace, and then hold at 1500 ° C for 1 hour. And introduce a pressure of 20MPa,
The temperature was raised at 500 ° C. per hour to each of the temperatures shown in Table 1 and maintained at that temperature.

The obtained sintered body was prepared in the same manner as in Example 1.
The determination of Si and Lu, and the determination of oxygen and nitrogen were performed.

The quantitative value and the composition of the sintered body based on the quantitative value are as follows:
As shown in Table 1, each of the compositions was within the triangle ABC and the square DEFG shown in FIGS. 1 and 2, respectively.

From the results of X-ray diffraction, it was confirmed that all the generated phases were β-Si ₃ N ₄ and Lu ₄ Si ₂ O ₇ N ₂ . Further, as a result of TEM observation, a microstructure composed of β-type silicon nitride particles, and two-particle grain boundaries and multi-particle grain boundaries was observed.In the multi-particle grain boundaries, Lu ₄ Si ₂ O ₇ N was observed by electron diffraction. Crystallization of ₂ was confirmed. Lu ₄ Si ₂ O ₇ N ₂ was crystallized in all 20 randomly selected multi-grain boundaries.

The bending test and the oxidation test were also performed in Example 1.
The same was done. Porosity, room temperature 4-point bending strength, and 1500
The high-temperature four-point bending strength at ° C. was as shown in Table 2.

Also in Examples 2 to 7, the obtained sintered body is judged to be a silicon nitride wet sintered body having sufficiently high strength even at a high temperature and having excellent oxidation resistance. (Comparative Examples 2 and 3) As shown in Table 3, the powder P1 or P2 used in Example 1 and Comparative Example 1 was used as the silicon nitride powder, and contained lutetium oxide as shown in Table 3 The amount was added and mixed in the same manner as in Example 1 to further produce a molded body.

The compact was placed in a graphite mold and fired using a gas pressure hot press furnace. The firing conditions were the same as in Example 1,
The pattern S1 or the pattern S2 employed in Examples 2 to 7, respectively, was employed.

The obtained sintered body was treated in the same manner as in Example 1.
The determination of Si and Lu, and the determination of oxygen and nitrogen were performed.

The quantitative value and the composition of the sintered body based on the quantitative value are as follows:
As shown in Table 3. From the compositions around the triangle ABC and the square DEFG shown in FIGS. 1 and 2, respectively, Lu
It is shifted to ₂ O ₃ side. Since the composition of the grain boundary phase was Lu ₂ O ₃ excess, the result of X-ray diffraction confirmed that M phases other than Lu ₄ Si ₂ O ₇ N ₂ were generated.

A bending test and an oxidation test were performed in the same manner as in Example 1. The porosity, the room-temperature 4-point bending strength, and the high-temperature 4-point bending strength at 1500 ° C. were as shown in Table 4.

In the composition in which the M phase is generated, the viscosity of the liquid phase is high, which lowers the sinterability and does not sufficiently densify.
For this reason, the porosity is high, and the room temperature and high temperature strength are both low. Further, the oxidation resistance is also reduced due to the high porosity. (Example 8) As shown in Table 1, the same powder P1 as the silicon nitride powder used in Example 1 was used as the silicon nitride powder, and 4.84% by weight of the silicon powder and lutetium oxide were used.
2.435% by weight was added and mixed in the same manner as in Example 1 to produce a molded body.

The molded body was placed in a graphite mold, and the firing conditions employed in Examples 2 to 7 were determined using a gas pressure hot press furnace.
It was baked in pattern S2.

The obtained sintered body was prepared in the same manner as in Example 1.
The determination of Si and Lu, and the determination of oxygen and nitrogen were performed. As shown in Table 1, the quantitative values were as follows: Lu: 2.0774% by weight, Si: 5
8.63% by weight, O: 0.3286% by weight, N: 38.964% by weight. From this result, the composition of the sintered body, 99.00mol% Si ₃ N ₄ -
0.19 mol% SiO ₂ -0.81 mol% Lu ₂ O ₃ , which is recognized as being in the composition in the triangle ABC and the quadrangle DEFG shown in FIGS. 1 and 2, respectively.

From the results of X-ray diffraction, it was confirmed that the phases formed in the sintered body were β-Si ₃ N ₄ and Lu ₄ Si ₂ O ₇ N ₂ .

[0069] Furthermore, TEM observation showed that the β-type silicon nitride particles, the fine structure is observed comprising a second grain boundaries and multiparticulates grain boundary, the multiparticulate grain boundaries, Lu ₄ Si ₂ by electron beam diffraction O ₇
Crystallization of N ₂ was confirmed. Lu ₄ Si ₂ O ₇ N ₂ was crystallized in all 20 randomly selected multi-grain boundaries.

The bending test and the oxidation test were also performed in Example 1.
The same was done. As shown in Table 2, the porosity of the sintered body was 1.6%, the four-point bending strength at room temperature was 980 MPa, and the four-point bending strength at 1500 ° C. was 820 MPa. The weight increase after the oxidation test was 0.02 mg / cm ² , and the four-point bending strength at room temperature was 750 MPa.

Also in Example 8, the obtained sintered body was
It is judged to be a silicon nitride wet sintered body having sufficiently high strength even at high temperature and having excellent oxidation resistance. (Example 9) 5.58% by weight of lutetium oxide was added to silicon powder having an average particle diameter of 0.8 µm, mixed and pulverized in the same manner as in Example 1, and then a molded article was produced.

The molded body is heated from room temperature to 1200 ° C. by 10 ⁻² Pa
After heating in a vacuum at a rate of 10 ° C./hour to 1400 ° C., the temperature was maintained at 1400 ° C. for 24 hours to perform a nitriding treatment. Thereafter, the molded body was placed in a graphite mold and fired in the same manner as in Example 1 under the firing conditions for the pattern S1.

The obtained sintered body was treated in the same manner as in Example 1.
The determination of Si and Lu, and the determination of oxygen and nitrogen were performed. As shown in Table 1, the quantitative values were as follows: Lu: 3.02% by weight, Si: 57.
The content was 99% by weight, O: 0.457% by weight, and N: 38.538% by weight. From these results, the composition of the sintered body was 98.60 mol% Si ₃ N ₄ -0.19 mol
% SiO ₂ -1.21 mol% Lu ₂ O ₃ , which is recognized as being in the composition in the triangle ABC and the square DEFG shown in FIGS. 1 and 2, respectively.

From the results of X-ray diffraction, it was confirmed that the phases formed in the sintered body were β-Si ₃ N ₄ and Lu ₄ Si ₂ O ₇ N ₂ .

[0075] Furthermore, TEM observation showed that the β-type silicon nitride particles, the fine structure is observed comprising a second grain boundaries and multiparticulates grain boundary, the multiparticulate grain boundaries, Lu ₄ Si ₂ by electron beam diffraction O ₇
Crystallization of N ₂ was confirmed. Lu ₄ Si ₂ O ₇ N ₂ was crystallized in all 20 randomly selected multi-grain boundaries.

The bending test and the oxidation test were also performed in Example 1.
The same was done. As shown in Table 2, the porosity of the sintered body was 1.2%, the 4-point bending strength at room temperature was 970 MPa, and the 4-point bending strength at 1500 ° C. was 870 MPa. The weight increase after the oxidation test was 0.01 mg / cm ² , and the four-point bending strength at room temperature was 840 MPa.

Also in Example 9, the obtained sintered body was
It is judged to be a silicon nitride wet sintered body having sufficiently high strength even at high temperature and having excellent oxidation resistance.

Of course, the invention of this application is not limited by the above embodiments and examples. It goes without saying that various aspects are possible for details of the size, distribution, shape, purity, etc. of the particles of the raw material powder, firing conditions and the like.

[0079]

As described in detail above, the invention of the present application provides a silicon nitride sintered body having both high strength at high temperature and excellent oxidation resistance.

[Brief description of the drawings]

FIG. 1 is a phase diagram of a ternary system of Si ₃ N ₄ —SiO ₂ —Lu ₂ O ₃ .

FIG. 2 is a phase diagram of a ternary system of Si ₃ N ₄ —SiO ₂ —Lu ₂ O ₃ .

FIG. 3 is a schematic view of a transmission electron microscope image showing a microstructure of a silicon nitride based sintered body.

FIG. 4 is a phase diagram of a ternary system of Si ₃ N ₄ —SiO ₂ —RE ₂ O ₃ .

FIG. 5 is a phase diagram of a ternary system of Si ₃ N ₄ —SiO ₂ —RE ₂ O ₃ .

Continued on the front page (72) Inventor: Mamoru Mitomo 1-1-1, Tsunami, Tsukuba, Ibaraki Pref. F-term (reference) in the Agency for Science and Technology Agency, Inorganic Materials Research Laboratory

Claims (11)

[Claims] 1. A silicon nitride sintered body comprising silicon nitride particles and a grain boundary phase, wherein the grain boundary phase is substantially a single phase of a crystal phase of Lu ₄ Si ₂ O ₇ N _2. The composition of the silicon nitride sintered body is point A on the phase diagram of the ternary system of Si ₃ N ₄ —SiO ₂ —Lu ₂ O ₃ :
Si ₃ N ₄ , point B: 28 mol% SiO ₂ -72 mol% Lu ₂ O ₃ , and point C: 16 m
ol% SiO ₂ -84mol% Lu ₂ O ₃ Triangle ABC with three vertices
A silicon nitride-based sintered body characterized by having a composition around or inside. 2. The composition of a silicon nitride based sintered body according to claim 1.
Point D on the described triangle ABC: 99 mol% Si ₃ N ₄ -0.28 mol% Si
O ₂ -0.72 mol% Lu ₂ O ₃ , point E: 99 mol% Si ₃ N ₄ -0.16 mol% SiO _2-
0.84mol% Lu ₂ O ₃ , F point: 94mol% Si ₃ N ₄ -1.68mol% SiO ₂ -4.32
mol% Lu ₂ O ₃ , and point G: 94 mol% Si ₃ N ₄ -0.96 mol% SiO ₂ -5.04
mol% Lu ₂ O ₃ of the ambient or internal composition of the square DEFG is claim 1 silicon nitride sintered body according to the vertices of four points. 3. The method according to claim 1, wherein the composition contains 2.5 to 10% by weight of Lu ₄ Si ₂ O ₇ N _2.
Or the silicon nitride-based sintered body according to 2. 4. The silicon nitride sintered body according to claim 1, wherein the content of elements other than Lu, Si, O, and N is 1% by weight or less. 5. Among the grain boundary phases existing in the multi-grain grain boundary, 90 bodies are present.
Lu is more than product%_FourSi_TwoO₇N _Two5. The crystalline phase of claim 1, wherein
A silicon nitride-based sintered body according to any of the above. 6. A silicon nitride powder having an oxygen content of 1.0% by weight or less, a lutetium oxide powder of 1 to 12% by weight is added and mixed, and then the mixture is charged at 1700 to 2200 ° C. in a nitrogen atmosphere of 1 to 100 atm. 3. A method for producing a silicon nitride-based sintered body, characterized by firing until the composition described in 1 or 2 is reached. 7. Addition and mixing of 1 to 12% by weight of lutetium oxide powder to silicon nitride powder having an oxygen content of 1.5% by weight or less, and prior to firing, 1600 ° C. in a nitrogen atmosphere of 1 atm or less.
Heat below to evaporate oxygen until the oxygen content reaches the oxygen content of the composition of claim 1 or 2, then 1-100
A method for producing a silicon nitride-based sintered body, characterized by firing at 1700 to 2200 ° C. in a nitrogen atmosphere at atmospheric pressure. 8. The method for producing a silicon nitride-based sintered body according to claim 7, further comprising adding silicon powder. 9. The method for producing a silicon nitride-based sintered body according to claim 8, wherein the addition amount of the silicon powder is 1 to 10% by weight. 10. Addition and mixing of 1 to 12% by weight of lutetium oxide powder to silicon powder;
After heating to below ℃ to convert silicon to silicon nitride,
A method for producing a silicon nitride-based sintered body, characterized by firing at 1700 to 2200 ° C. in nitrogen at 1 to 100 atm until the composition according to claim 1 or 2 is obtained. 11. The method for producing a silicon nitride sintered body according to claim 6, wherein the firing is performed by hot pressing.