JP3207045B2 - Method for producing silicon nitride based sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to automotive parts such as pistons, cylinders, valves, cam rollers, rocker arms, piston rings, and piston pins, as well as turbine rotors, turbine blades, nozzles, combustors, scrolls, nozzle supports, and the like. The present invention relates to a method for producing a high-strength silicon nitride sintered body which is suitably used for parts for gas turbine engines such as a seal ring, a spring ring, a diffuser, and a duct, and particularly requires dimensional accuracy.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been known to have heat resistance,
Due to its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbo rotors. This silicon nitride sintered body is generally made of Y ₂ O ₃ , Al ₂ O ₃
Alternatively, by adding a sintering aid such as MgO, high density and high strength characteristics are obtained. As such silicon nitride sintered bodies are required to be further improved in strength and oxidation resistance at high temperatures when their use conditions are further increased. In response to such demands, various improvements have been attempted, for example, by studying sintering aids and improving firing conditions.

Among them, from the viewpoint that oxides such as Al ₂ O ₃ and MgO, which have been conventionally used as sintering aids, deteriorate the high-temperature characteristics, the period of silicon nitride, such as Y ₂ O ₃ Simple ternary system (Si ₃ N ₄ —SiO ₂ —RE ₂ O ₃ ) composed of Group 3a element (RE) and silicon oxide
In the sintered body having the composition of
It has been proposed to increase the melting point and stabilize the grain boundaries by precipitating a crystal phase such as a YAM phase and an apatite phase composed of i-RE-ON. For example, JP-A-63
No. 100067 proposes a sintered body containing two or more rare earth elements of Y, Er, Tm, Yb, and Lu, in which a crystalline phase of an apatite structure is formed at a grain boundary. Have been.

[0004]

Problems to be Solved by the Invention Sinters containing various rare earth elements which have been conventionally proposed have a certain degree of high temperature strength and oxidation resistance. However, when such a conventional silicon nitride-based sintered body is actually used for a gas turbine or an automobile part, it may be exposed for a long time in a state where a high load is applied in a high temperature state. There is a problem that the sintered body is gradually deformed, that is, so-called creep occurs. The occurrence of such creep deformation has been a major factor impeding practical application even if the strength is high. For example, JP-A-63-1000
As described in No. 67, when a composite oxide is formed with two or more kinds of rare earth elements, the eutectic point is lowered, so that the creep resistance at high temperatures is also low.

Further, as described above, in a system using only a silicon nitride powder as a silicon nitride source and an oxide of a Group 3a element of the periodic table such as Y ₂ O ₃ as a sintering aid and silicon oxide, As phase sintering progresses, firing shrinkage occurs, so when manufacturing a complicated shape product requiring high dimensional accuracy after firing, it is difficult to control the size to be set to a large shrinkage, or a polishing process Is complicated. Therefore, a method of increasing the density of a molded body by adding silicon powder as a starting material and nitriding silicon in a nitrogen atmosphere before firing has been conventionally proposed. Not considered.

Accordingly, the present invention has excellent resistance to oxidation from a low temperature to a high temperature, and has sufficient strength characteristics for use in automobile parts and gas turbine engines from a room temperature to a high temperature, particularly from room temperature to 1500 ° C. It is an object of the present invention to provide a method for producing a silicon nitride sintered body which is excellent in bending strength and creep resistance up to a high temperature and which is applied to parts requiring high dimensional accuracy.

[0007]

Means for Solving the Problems The present inventors have studied the factors that influence the creep resistance and found that the type of rare earth element used and its composition are important for the creep resistance. As a result of repeated examinations based on this, it was found that Lu was used as a rare earth element, that other rare earth elements were not contained as much as possible, and that the ratio with impurity oxygen present in the sintered body was controlled to a specific range. It has been found that creep resistance can be improved while maintaining high-temperature strength, and the present invention has been accomplished.

That is, in the method for producing a silicon nitride sintered body of the present invention, 1 to 10 mol% of a Group 3a element oxide of the periodic table is used.
Silicon oxide is composed of 1 to 20 mol% and silicon or silicon and silicon nitride in a proportion of 70 mol% or more in terms of nitride, and the Group 3a element oxide of the periodic table is mainly composed of Lu;
Other than the group 3a element oxides of the periodic table other than
A molded body having a composition of not more than 7 mol% of the group III element oxide and having a molar ratio of the amount of the silicon oxide to the group IIIa element oxide of the periodic table of less than 2 is 800 to 1500 ° C.
After nitriding the silicon by heat treatment in a nitrogen-containing atmosphere described above, the silicon is further densified by firing in a non-oxidizing atmosphere containing nitrogen.

Hereinafter, the present invention will be described in detail. According to the method for producing a silicon nitride-based sintered body of the present invention, silicon as a starting material,
It contains silicon and silicon nitride as main components, and contains, as additive components, an oxide of a Group 3a element of the periodic table and silicon oxide. The term “silicon oxide” as used herein means an oxygen content remaining in the final sintered body, such as impurity oxygen inevitably contained in silicon nitride, silicon powder, etc., in addition to that added as silicon oxide powder, in terms of SiO ₂ . Things are also included.

As a specific composition of the starting material, 1 to 10 mol%, particularly 1 to 7 mol%, and 1 to 20 mol%, particularly 1 to 12 mol%, of silicon oxide of Group 3a of the periodic table are contained. ,
70 mol% of silicon or silicon and silicon nitride in terms of silicon nitride
As described above, it is particularly important that the content is contained in a ratio of 85 to 98 mol%. This is because if the amount of Group 3a element oxide in the periodic table is less than 1 mol%, densification is insufficient, and if it exceeds 10 mol%, high-temperature strength and high-temperature creep resistance are deteriorated. If the amount is less than 1 mol%, a compound having low resistance to high-temperature oxidation such as melilite, which is a compound of silicon nitride and an oxide of an element of Group 3a of the periodic table, is likely to be formed at the grain boundary, which is not preferable. This is because the volume fraction of the grain boundary phase increases and the high-temperature characteristics deteriorate. On the other hand, if the amount of silicon nitride is less than 70 mol%, high temperature strength is not exhibited.

[0011] According to the present invention, the periodic table group 3a element oxide is the best that there is no periodic table group 3a element oxide other than Lu ₂ O ₃ consists only Lu ₂ O ₃ but Actually, there may be some contamination. Therefore, in the present invention, the oxide of the Group 3a element of the periodic table other than Lu ₂ O ₃ is at most 7 mol%, particularly at most 5 mol%, more preferably at most 3 mol%, based on the total amount of the group 3a element oxide of the periodic table. %. This is because, by selecting Lu ₂ O ₃ as the Group 3a element oxide of the periodic table, the high-temperature characteristics are significantly improved as compared with other Group 3a element oxides of the periodic table, and Lu ₂ O is also improved. _This is because the presence of a Group 3a element other than _{3 in} the Periodic Table lowers the eutectic point as compared to a single element, and as a result, the creep resistance of the sintered body is deteriorated. In the present invention, the third part of the periodic table other than Lu is used.
Group a element oxides include Y, Yb, Er, Dy, H
oxides such as o, Tb, Tm and Sc.

Further, according to the present invention, silicon oxide (S ₂ O) is used for the oxide of a group 3a element of the periodic table (RE ₂ O ₃ ).
It is important that the ratio of (iO ₂ ) (SiO ₂ / RE ₂ O ₃ ) be less than 2. This is a major factor in determining the composition of the grain boundary, and if this ratio is 2 or more, the creep resistance is deteriorated.

[0013] In the starting material, as an additive component, Lu ₂ O ₃ , or a compound comprising silicon oxide and one or more of Lu ₂ O ₃ and at least one oxide of a Group 3a element of the periodic table other than Lu ₂ O ₃ , Or silicon nitride and Lu ₂ O ₃ , or L
It is also possible to use a compound powder consisting of silicon oxide and one or more oxides of group 3a elements of the periodic table other than u ₂ O ₃ and Lu ₂ O ₃ .

Further, the silicon powder used has an average particle diameter of 10 μm or less, particularly 5 μm, in order to facilitate nitriding.
m or less are desirable. The silicon nitride powder is blended to improve the sintering characteristics. However, since the firing deformation increases at the same time, the addition amount of the silicon nitride powder is desirably 75 mol% or less. In this case, the powder used may be either α-type or β-type, and the particle size is suitably from 0.4 to 1.2 μm. It should be noted that the silicon nitride powder can be used by any method such as a direct nitriding method and an imide decomposition method.

Lu ₂ O ₃ used as an oxide of a Group 3a element in the periodic table preferably has a purity of 93% or more, particularly 95% or more, and an average particle size of about 0.2 to 10 μm.

The silicon oxide powder has a high purity of 99% or more and an average particle diameter of about 0.2 to 5 μm.
For example, fumed silica is preferably used.

Further, according to the present invention, in addition to the above-mentioned composition, TiN, TiC, TaC, TaN,
The periodic table 4a, 5a, or 6a group metal compound such as VC, NbC, WC, WSi ₂ , Mo ₂ C, or WO ₃ or SiC may be present in the sintered body of the present invention as dispersed particles or whiskers. Since the effect of deteriorating the characteristics is small, it is of course possible to improve the characteristics by adding an appropriate amount thereof based on a known technique. However, Al
_The presence of metal oxides such as ₂ O ₃ , MgO, and CaO inhibits crystallization of the grain boundary phase and deteriorates high-temperature strength and high-temperature oxidation resistance, so that the total amount of these oxides is 1% by weight or less. In particular, it is desirable to control the amount to 0.5% by weight or less, more preferably 0.1% by weight or less.

Next, the powder mixed as described above is sufficiently mixed by a rotary mill, a barrel mill, a vibration mill or the like, and the mixed powder is formed by a known molding method, for example, press molding, casting molding, extrusion molding, injection molding. , Sludge molding,
It is formed into a desired shape by cold isostatic pressing or the like.

Next, the compact is heat-treated at a temperature of 800 to 1500 ° C. in an atmosphere containing nitrogen to nitride silicon contained in the compact to produce silicon nitride. There is no dimensional change during the conversion to silicon nitride, and the density increases because the weight increases. It is desirable to control the ratio of the theoretical density of the compact after the nitriding treatment to 60% or more. In order to nitride all of the silicon contained in this nitriding treatment, it is desirable to gradually nitride while increasing the temperature in multiple steps within the above-mentioned temperature range. There is. The nitriding can be promoted by treating the atmosphere in a nitrogen pressurized atmosphere of 1 to 50 atm.

Next, the high-density molded body obtained as described above is fired by a known firing method, for example, a hot press method, a normal pressure firing method, a nitrogen gas pressure firing method, and a heat treatment after firing. Intermediate hydrostatic pressure treatment (HIP treatment) and HIP treatment after glass sealing are performed to obtain a dense sintered body having a theoretical density ratio of 95% or more. If the temperature at this time is too high, the silicon nitride crystal grains grow and the strength is reduced. Therefore, the temperature is desirably 1600 to 2000 ° C, particularly preferably 1650 to 1950 ° C.

Since it is desirable that the grain boundary phase of the finally obtained sintered body is crystallized as much as possible, the grain boundary phase is cooled by the above-mentioned firing step, or temporarily held in the cooling step, or by heat treatment after the end of the firing step. It is desirable to precipitate a YAM phase or an apatite phase on the substrate. Specifically, the cooling from the firing temperature is gradually cooled to 200 ° C./hr or less, or the temperature is lowered from the firing temperature or 1000 to 180 after the firing is completed.
Heat treatment may be performed at 0 ° C. in a nitrogen atmosphere.

[0022]

According to the method of the present invention, among the oxides of Group 3a elements of the periodic table, the following components are added to silicon nitride:
Simultaneously satisfying the condition of selecting Lu and adding it with the other Group 3a element of the periodic table without using as much as possible, and controlling the ratio to impurity oxygen (SiO ₂ / RE ₂ O ₃ ) to less than 2. By doing so, it is possible to exhibit high creep resistance properties without deformation of the sintered body even when the sintered body is maintained at a high temperature, particularly 1500 ° C. under a high load state for a long time, while maintaining a high high-temperature strength of the sintered body.

The reason for this is that Lu has the smallest ionic radius among lanthanoid-based elements, and has a large viscosity at high temperatures as a glass or crystal phase with other grain boundary components such as Si ₃ N ₄ and SiO _2. High, and when held at high temperatures, one of the creep mechanisms, Si ₃
This is considered to be because slippage of the N ₄ particles hardly occurs. In addition, it is considered that the crystallization of the grain boundary can further increase the viscosity of the grain boundary, thereby improving the creep resistance.

Further, by introducing a step of nitriding silicon to produce silicon nitride before firing by using silicon powder or silicon powder and silicon nitride powder as main components, there is no dimensional change and weight increases. In addition, the density of the compact can be improved. Therefore, the amount of shrinkage in the firing stage is reduced, and a sintered body with a small amount of deformation and excellent dimensional accuracy can be obtained.

[0025]

EXAMPLE A silicon powder (having an average particle diameter of 3 μm) was used as a raw material powder.
m, oxygen amount 1.1% by weight, metal impurities such as Al, Mg, Ca, Fe, etc. 0.05% by weight) and silicon nitride powder (BE
T specific surface area 8 m ² / g, α rate 98%, oxygen content 1.2% by weight, metal impurities such as Al, Mg, Ca, Fe 0.0
3 wt%), purity 91% to 99% of Lu ₂ O ₃ powder (Yb ₂ O ₃ as an impurity, Er ₂ mainly includes O _3, etc.), a purity of 99% or more of Lu ₂ O ₃ Periodic Table 3 other than
It was prepared using a group a element oxide powder. Then, a binder was added to the mixture, and a blade was formed by casting. In Table 1, 2, SiO ₂ content in the raw material composition, SiO oxygen amount of silicon powder and silicon nitride powder ₂
It is the total amount of the converted amount and the added amount of the SiO ₂ powder.

The following (A) to (A)
Nitriding was performed using the nitridation pattern of (D). First, (A) ~
(C) is 2 hours at 1100 ° C and 2 hours at 1200 ° C.
Nitriding was performed by treating at -1300 ° C. for 2 hours and -1400 ° C. for 2 hours. In this case, a pattern in which the nitrogen gas pressure was set to 1 atm in a temperature range of 1100 ° C. or higher was set as (A), 1
The pattern set at 0 atm was defined as (B), and the pattern set at 30 atm was defined as (C). Furthermore, from 1100 ° C to 1
The temperature was gradually raised to 400 ° C over 10 hours, and 1400
A pattern in which the temperature was lowered after holding at 2 ° C. for 2 hours and the nitrogen pressure was set to 10 atm was defined as (D). At this time, it was confirmed from the weight increase rate that all the added silicon was nitrided in all the samples.

The nitride was fired by various firing methods shown in Tables 1 and 2. In the table, GPS is obtained by firing the molded body at 1750 ° C. at a nitrogen pressure of 1 atm for 10 hours, and further firing at 1920 ° C. at a nitrogen pressure of 10 atm for 5 hours. The hot press was fired at 1820 ° C. under a pressure of 36.3 MPa for 1 hour. HIP fills the compact in Vycor glass and melts the 175 Vycor glass.
It was fired for 1 hour at a temperature of 0 ° C. with an Ar gas applied at 2000 atm. In addition, GPS + HIP
Is fired at 1750 ° C. and a nitrogen pressure of 20
Hot isostatic firing at 00 atm for 1 hour.

In order to suppress decomposition during firing, the furnace input during firing was kept constant at 300 g / liter. In the table, except for Sample No. 28, heat treatment was performed at 1400 ° C. for 24 hours for crystallization.

The dimensions of the obtained sintered body were measured,
Ratio to compact size (sintered compact size / compact compact size)
Was measured as a dimensional change (%). Then, the ratio of the theoretical density to the theoretical density of the obtained sintered body is calculated from the specific gravity according to the Archimedes method. A point bending flexural strength test was performed,
Tables 1 and 2 show the average values of the 0 test results. Further, the crystal of the grain boundary phase in the sintered body was identified by X-ray diffraction measurement. Further, the bending test piece was supported in the same manner as in the four-point bending test of JISR1601, and a load of 450 MPa was applied to the test piece.
It was kept at 00 ° C for a maximum of 100 hours, and the time until breakage was measured.

[0030]

[Table 1]

[0031]

[Table 2]

According to Tables 1 and 2, Samples Nos. 37 to 39 to which other Group 3a element oxides of the periodic table that do not contain Lu ₂ O ₃ alone were added, the high-temperature characteristics were deteriorated. SiO
Sample No. 21 having a ₂ / RE ₂ O ₃ ratio exceeding 2 had low creep resistance.

The oxide of an element belonging to Group 3a of the periodic table contains 10
Samples Nos. 15 to 17 exceeding mol% had poor high-temperature characteristics. Further, in Sample No. 1 in which the amount of Group 3a element oxide added to the periodic table was small, the densification was insufficient. In Sample No. 10 in which silicon powder was not added as a raw material, the dimensional change was 8%.
It was as large as 0%, and the amount of deformation at the blade tip was large. L
Samples Nos. 26 and 27 containing more than 7 mol% of Group 3a element oxides of the periodic table other than u showed significantly reduced creep resistance.

In contrast to these comparative examples, all of the other samples according to the present invention exhibited small dimensional changes, high high-temperature strength, and excellent creep resistance.

[0035]

As described in detail above, according to the present invention,
Reduces shrinkage and deformation during the sintering process, and produces a sintered body with high dimensional accuracy. At the same time, it has small strength deterioration from room temperature to high temperature, especially 1500 ° C, and has excellent creep resistance at high temperature. A silicon nitride sintered body can be obtained. As a result, it can be applied to various structural materials that require high dimensional accuracy used at room temperature and high temperature, including structural materials for heat engines such as ceramic gas turbine parts and ceramic turbo rotors made of silicon nitride sintered body. can do.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-6160 (JP, A) JP-A-63-100067 (JP, A) JP-A-6-271357 (JP, A) JP-A 8- 48565 (JP, A) JP-A-7-315938 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/584-35/596

Claims (1)

(57) [Claims] An oxide of a Group 3a element of the periodic table in an amount of 1 to 10 mol%, silicon oxide of 1 to 20 mol%, and silicon or silicon and silicon nitride in an amount of 70 mol% or more in terms of nitride. The Group 3a element oxide of the periodic table is mainly composed of Lu, and the Group 3a element oxide other than Lu is 7 mol% or less of the Group 3a element oxide of the entire Periodic Table; A molded product having a composition in which the molar ratio of the silicon oxide to the Group 3a element oxide of the periodic table is less than 2 is 800 to 1
A method for producing a silicon nitride-based sintered body, characterized in that the silicon is nitrided by heat treatment in a nitrogen-containing atmosphere at 500 ° C., and then fired and densified in a non-oxidizing atmosphere containing nitrogen.