JP2892246B2 - Silicon nitride sintered body and method for producing the same - Google Patents

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 having excellent strength characteristics from room temperature to a high temperature and particularly used for parts for automobiles and parts for gas turbine engines, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been known to have heat resistance,
Because of its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbo rotators. 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 a low melting point oxide such as Al ₂ O ₃ which has been conventionally used as a sintering agent deteriorates high-temperature characteristics, the periodicity of Y ₂ O ₃ or the like with respect to silicon nitride is considered. in the sintered body having a composition of table group 3a element (RE) and a simple ternary system consisting of a silicon oxide _{(Si 3 N 4 -SiO 2 -RE} 2 O 3), the grain boundary of the sintered body Si
It has been proposed to increase the melting point and stabilize the grain boundary by precipitating a crystal phase such as a YAM phase and an apatite phase composed of -RE-ON.

[0004]

However, when the grain boundary phase is crystallized, the high-temperature characteristics are improved to some extent as compared with the case where the grain boundaries are amorphous, but the oxidation resistance at high temperatures is poor. Since it is not possible to produce a stable crystalline phase with sufficient mechanical properties, the practical use of such a sintered body is hindered, and further improvements in strength and oxidation resistance are required. Have been.

Therefore, the present invention is excellent in oxidation resistance especially from low temperature to high temperature, and has sufficient strength characteristics for use in automobile parts and gas turbine engines from room temperature to high temperature, especially from room temperature to 1500. It is an object of the present invention to provide a silicon nitride-based sintered body having excellent bending strength up to a high temperature of ° C.

[0006]

Means for Solving the Problems In order to enhance the strength characteristics and oxidation resistance of the sintered body, the present inventors have studied the composition of the sintered body and the subphases existing at the grain boundaries of the silicon nitride phase. As a result of repeated investigations based on the viewpoint that it is important to control the grain boundary phase, at least Lu is used as a Group 3a element of the periodic table that is added to silicon nitride to form a grain boundary phase, and a specific grain boundary phase is used. It has been found that the above object is achieved by forming a crystal phase.

That is, the silicon nitride sintered body of the present invention has a silicon nitride crystal phase as a main phase, and contains Lu, or at least one element of Group 3a of the periodic table other than Lu and Lu, Si, O And a sintered body composed of a sub-phase containing N, wherein the group 3a element of the periodic table has a total amount of 0.1 to 1 in terms of oxide.
0% by mole, and at least RE ₂ Si ₂ O ₇ (RE is a Group 3a element of the periodic table) crystal precipitated in the grain boundary phase of the sintered body. Then, as a manufacturing method, silicon nitride is used as a main component, and L
0.1 to 10 mol% of u ₂ O ₃ , or one or more of Lu ₂ O ₃ and one or more oxides of Group 3a other than Lu ₂ O ₃
And a composition in which the molar ratio represented by SiO ₂ / RE ₂ O ₃ is 2 or more with respect to the total amount of group 3a element oxides (RE ₂ O ₃ ) of the periodic table of silicon oxide (SiO ₂ ). After firing the compact in a non-oxidizing atmosphere, at least RE ₂ Si ₂ O ₇ is added to the grain boundary phase by a cooling process or heat treatment after firing.
(RE is an element belonging to Group 3a of the periodic table).

Hereinafter, the present invention will be described in detail. The silicon nitride-based sintered body of the present invention contains silicon nitride as a main component and, as an additive, Lu (lutetium) or one or more elements of Group 3a of the periodic table other than Lu and Lu and excess oxygen. It is a thing. Here, the excess oxygen means that when the group 3a element of the periodic table other than Si in the sintered body forms an oxide stoichiometrically from the total amount of oxygen in the sintered body, it binds to the element. a remaining amount of oxygen except for oxygen are taken into account its oxygen mostly contained in the silicon nitride raw material or is intended to be mixed as an additive such as SiO _2, as existing all in the present invention as SiO ₂ .

The sintered body of the present invention is structurally mainly composed of a silicon nitride crystal phase, and most of the sintered body is β-Si
Consisting of ₃ N _4. In addition, at least Lu, Si, N, O, and the like are present at the grain boundaries of the main phase. Lu, or one or more elements of Group 3a of the periodic table other than Lu and Lu, and excess oxygen (as silicon oxide) According to the present invention, RE is present in this grain boundary phase.
It is important to precipitate ₂ Si ₂ O ₇ . Where R
E is an element belonging to Group 3a of the Periodic Table present in the system,
In the present invention, Lu or Lu and other Group 3a elements of the periodic table are used.

According to the present invention, the inclusion of Lu in the grain boundary phase increases the melting point of RE ₂ Si ₂ O ₇ and increases the melting point of RE ₂
The eutectic temperature of the binary system of Si ₂ O ₇ and Si ₃ N ₄ is increased, and the high-temperature strength can be increased. In the sintering process, this crystal phase exists as a liquid phase due to the reaction with the silicon nitride particles and enhances the sinterability. However, if the crystal phase remains as a glass phase in the grain boundary phase as it is after cooling, the high temperature strength is reduced. It degrades oxidation resistance. Therefore, by precipitating the RE ₂ Si ₂ O ₇ crystal phase by a predetermined cooling process or heat treatment, it is possible to increase the high-temperature strength and at the same time the oxidation resistance.

Further, in order to precipitate the crystal phase, the amount of excess oxygen in the sintered body (SiO ₂ ) in terms of silicon oxide (SiO ₂ ), which is the total amount of elements of Group 3a in the periodic table including Lu, and the amount in terms of oxide (RE
Molar ratio _{2 O 3) (SiO 2 /} RE 2 O 3) 2
It is necessary to be above. This is because when the above ratio is less than ₂ , RE ₁₀ Si ₂ other than RE ₂ Si ₂ O ₇ is added to the grain boundary phase.
An apatite phase such as O ₂₃ N ₄ or RE ₁₀ (SiO ₄ ) ₆ N ₂ starts to exist, and when the ratio is further reduced, the apatite phase alone or a YAM phase represented by RE ₄ Si ₂ O ₇ N ₂ Is deposited, thereby deteriorating the oxidation resistance, particularly at around 900 ° C.

The content of Lu or one or more elements of Group 3a of the periodic table other than Lu and Lu is 0.1 to 10 mol%, especially 0.3 to 5 mol% in terms of oxide. This is because, if it exceeds 10 mol%, the volume fraction of the grain boundary phase in the sintered body increases, and the high-temperature strength of the sintered body deteriorates. In a composite system of Lu and another element of Group 3a of the periodic table, it is necessary that Lu does not fall below 0.1 mol%, particularly 0.3 mol% in terms of oxide.

According to the present invention, Lu or L
one or more REs of group 3a elements of the periodic table other than u and Lu
₂ Si ₂ O ₇ (RE: element of Group 3a of the periodic table) is deposited. When the (SiO ₂ / RE ₂ O ₃ ) molar ratio described above is 2.0 or more and about 2.5 or less, RE is formed as a crystal phase. _Two
Precipitated only Si ₂ O ₇ crystal phase, but when the molar ratio is greater than 2.5, RE ₂ Si ₂ O ₇ in addition to crystalline Si ₂ N
₂ O starts to precipitate, but there is no problem in terms of characteristics.

The Group 3a element of the periodic table other than Lu used in the present invention includes Y and lanthanoid elements, and Yb and Er are particularly preferable.

Next, the method for producing the silicon nitride sintered body of the present invention will be described. First, silicon nitride is used as a main component as a raw material powder, and Lu ₂ O ₃ or Lu ₂ O ₃ and Lu ₂ are used as additional components. One or more powders of an oxide of Group 3a of the periodic table other than O ₃ , and optionally a silicon oxide powder are added. Also, instead of the Group 3a oxide powder and the silicon oxide powder in the periodic table, a composite compound powder of these can be used, or a composite compound powder of these and silicon nitride can be used. The silicon nitride powder used is itself α-S
Any of i ₃ N ₄ and β-Si ₃ N ₄ may be used, and their particle diameter is suitably 0.4 to 1.2 μm.

According to the present invention, using these powders,
0.1 to 10 mol% of Lu ₂ O ₃ , or at least one oxide of Group 3a oxide of the periodic table other than Lu ₂ O ₃ and Lu ₂ O ₃ ;
In particular, by mixing with 0.3 to 5 mol% and silicon oxide, the amount of excess oxygen in terms of silicon oxide and the total amount of group 3a element oxides (RE ₂ O ₃ ) of the periodic table containing Lu ₂ O ₃ are determined. SiO ₂ /
It is prepared and mixed so that the molar ratio represented by RE ₂ O ₃ becomes 2 or more. The amount of excess oxygen in terms of silicon oxide at this time is the total amount of the amount of impurity oxygen contained in the silicon nitride powder in terms of SiO ₂ and the amount of silicon oxide powder to be added or the amount of silicon oxide in terms of silicon oxide.

The mixed powder thus obtained is molded into a desired shape by a known molding method, for example, press molding, cast molding, extrusion molding, injection molding, cold isostatic pressing, etc. The body is fired in a known firing method, for example,
Densification can be achieved by hot pressing, normal pressure firing, and nitrogen gas pressure firing. Furthermore, densification can be improved by performing treatment by hot isostatic pressing (HIP) after the calcination. As another method, the above-mentioned molded body or sintered body can be glass-sealed and fired by the HIP method. Specific firing conditions are firing at a temperature of 1500 to 1950 ° C. under a nitrogen pressure at which silicon nitride does not decompose. If the sintering temperature exceeds 1950 ° C., the silicon nitride crystal causes grain growth and lowers the strength.

Next, it is gradually cooled in a cooling process after firing, or it is temporarily 1000 to 1600 in the cooling process.
Hold at ° C. Alternatively, after firing, 1000 to 1
When heat treatment is performed for 1 to 48 hours in a non-oxidizing atmosphere at 600 ° C., at least RE ₂ Si ₂ O ₇ (R
E is a group 3a element of the periodic law) and can precipitate crystals.

According to the silicon nitride sintered body of the present invention and the method for producing the same, the presence of a metal oxide having a low melting point such as Al ₂ O ₃ , MgO, CaO inhibits crystallization of the grain boundary phase and increases the temperature. These oxides are desirably controlled to a total amount of 0.5% by weight or less in order to deteriorate strength and high-temperature oxidation resistance.

On the other hand, metals of Group 4a, 5a and 6a of the Periodic Table, and their carbides, nitrides, silicides, SiC, etc., have characteristics even when present in the sintered body of the present invention as dispersed particles or whiskers. Since the influence of deterioration is small, it is of course possible to improve the properties as a composite material by adding an appropriate amount of these based on a known technique.

[0021]

The mechanical properties and oxidation resistance of a silicon nitride sintered body are determined by the grain boundary phase.

The oxidation resistance can be improved by crystallizing the grain boundary crystal phase into RE ₂ Si ₂ O ₇ (RE is a Group 3a element in the periodic table). Among the elements of Group 3a of the periodic table, the smaller the ionic radius, the smaller the diffusion of oxygen and the better the oxidation resistance. At the same time, the melting point of RE ₂ Si ₂ O _{7 and} the melting point of silicon nitride and RE ₂ Si ₂ O ₇ The melting point of a two-component system can be improved. Among the lanthanide compounds, Lu has the smallest ionic radius, and by using the Lu element, excellent oxidation resistance and mechanical properties can be imparted from room temperature to high temperatures, particularly from 1500 ° C.

[0023]

EXAMPLES As raw material powders, silicon nitride powder (BET specific surface area 8 m ² / g, α rate 98%, oxygen content 1.2 wt%, metal impurity content 0.03 wt%), Lu ₂ O ₃ powder, Lu ₂ O
Powders of Group 3a element oxides and silicon oxide powders other than ₃ were weighed and mixed such that the composition of the compact was as shown in Table 1. Further, Lu ₂ O ₃ powder or RE ₂ Si ₂ O ₇ powder synthesized from Lu ₂ O ₃ powder, other oxide powder of Group 3a of the periodic table and silicon oxide powder (Sample No. 1) was used.
9) Similarly, it was prepared to have the composition shown in Table 1. Then, a binder was added to the mixture, and a mold was formed at 1 t / cm ² .

The obtained molded body was heated to a predetermined temperature, and after removing the binder, was fired.

In the firing, the sample No. Each of the molded products Nos. 1 to 14 was placed in a sagger in silicon carbide in order to reduce the composition fluctuation, and calcined at 1850 ° C. for 4 hours in a nitrogen gas flow of 10 atm (GPS method). Thereafter, in order to sufficiently crystallize the grain boundary phase, the obtained sintered body is placed in a nitrogen stream at 1400 μm.
Heat treatment was performed at 24 ° C. for 24 hours.

The sample No. For the molded bodies of Nos. 15 to 22, sintered bodies were produced by a glass seal HIP method.
Specifically, first, before firing, the BN powder having poor wettability with glass is slurried and applied to the molded body in order to prevent the molded body from reacting with glass or the like as a sealing material during firing. Alternatively, the slurry was spray-coated.
Next, the molded body coated with BN or the like was sealed in a glass capsule, and fired at 1700 ° C. and 2000 atm for 1 hour by the HIP method. Thereafter, in order to sufficiently crystallize the grain boundary phase, the obtained sintered body was subjected to a heat treatment at 1400 ° C. for 24 hours in a nitrogen stream.

The obtained sintered body was cut, processed, and polished to the shape specified in JIS-R1601, to prepare a sample. The specific gravity of this sample was measured based on the Archimedes method, the theoretical density ratio was calculated, and a four-point bending test at room temperature and 1500 ° C. based on JIS-R1601 was performed. Also. Sample in air at 900 ° C or 1500 ° C
The sample was exposed to air for 100 hours, and the weight change per unit surface area was determined from the weight increase and the surface area of the sample. Further, the crystal phase of the grain boundary phase in the sintered body was identified by X-ray diffraction measurement. The results are shown in Table 2.

For the composition of the sintered body, a sample was pulverized, the oxygen content was determined by an infrared absorption method after conversion with final carbon dioxide, and the nitrogen content was determined by the thermal conductivity based on the periodic rule containing silicon and Lu. The Group 3a elements in Table 3 were measured by ICP emission spectroscopy, but substantially did not change from the composition of the compact.

[0029]

[Table 1]

[0030]

[Table 2]

According to the results of Tables 1 and 2, Lu ₂ O
Sample No. ₃ to which only oxides of other Group 3a elements of the periodic rule not containing ₃ were added. 1-3, 15 have low high-temperature strength,
Sample No. having an O ₂ / RE ₂ O ₃ ratio of 1.5. In No. 10, apatite was crystallized at the grain boundary, and the oxidation resistance was significantly deteriorated. Sample No. 1 containing Lu ₂ O ₃ , or Lu ₂ O ₃ and at least one element of Group 3a of the Periodic Table exceeding 10 mol%. In No. 14, the oxidation resistance was deteriorated.

In contrast to these comparative examples, all of the other samples according to the present invention showed that RE ₂ Si ₂ O ₇ or RE ₂ Si ₂ O ₇ crystals and Si ₂ N ₂ O ₇ crystals were precipitated at the grain boundaries. All of them showed excellent bending strength and oxidation resistance.

[0033]

As described in detail above, according to the present invention,
Strength degradation is small from room temperature to high temperature, especially 1500 ° C,
A silicon nitride sintered body having excellent oxidation resistance can be provided. As a result, the application of the silicon nitride sintered body to various structural materials including heat engines such as gas turbines can be expanded.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Kenichi Tajima 1-4-4 Yamashitacho, Kokubu-shi, Kagoshima Inside Kyocera Research Institute (72) Inventor Tomohiro Iwaida 1-4-4 Yamashitacho, Kokubu-shi, Kagoshima Kyocera Corporation Examiner in the Research Institute Daiji Daikohara (56) References JP-A-63-100067 (JP, A) JP-A-3-218969 (JP, A) JP-A-1-93469 (JP, A) (58) Investigation Field (Int.Cl. ⁶ , DB name) C04B 35/584

Claims (3)

(57) [Claims] 1. A sintering method comprising a silicon nitride crystal phase as a main phase and a grain boundary phase containing Si, O, and N and at least one of Lu and at least one element of Group 3a of the periodic table other than Lu and Lu. Wherein the group 3a element of the periodic table is contained in a proportion of 0.1 to 10 mol% in terms of oxide in total, and at least RE ₂ Si ₂ O ₇ ( RE is a silicon nitride-based sintered body characterized in that crystals are precipitated from the 3a group element of the periodic table). 2. An element (R) of Group 3a of the periodic table in said sintered body.
2. The nitriding method according to claim 1, wherein the molar ratio expressed by SiO ₂ / RE ₂ O ₃ of the amount of E) in terms of oxide (RE ₂ O ₃ ) and the amount of excess oxygen in terms of silicon oxide (SiO ₂ ) is 2 or more. Silicone sintered body. 3. An oxide containing silicon nitride as a main component and containing at least one of Lu ₂ O ₃ or an oxide of a Group 3a element of the periodic table other than Lu ₂ O ₃ and Lu ₂ O _{3 in an} amount of 0.1 to 10 mol%. , Silicon oxide (SiO ₂ ) and an oxide of an element of Group 3a of the periodic table (RE)
After sintering in a non-oxidizing atmosphere a molded body having a composition in which the molar ratio represented by SiO ₂ / RE ₂ O ₃ to the total amount of ₂ O ₃ ) is 2 or more, a cooling process after sintering or after sintering At least RE ₂ Si ₂ O ₇ (RE
(3a group element of the periodic table). A method for producing a silicon nitride-based sintered body, comprising: