JP2773439B2 - Method for producing silicon carbide-containing silicon nitride composite - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a silicon carbide-containing silicon nitride composite.

[0002]

2. Description of the Related Art In order to obtain a heat-resistant structural material having excellent properties of both silicon nitride and silicon carbide,
Development of a composite of silicon nitride and silicon carbide has been attempted.
Generally, a mixed powder of Si ₃ N ₄ powder and SiC powder is hot pressed. Japanese Patent Application Laid-Open No. 1-298069 discloses that silicon carbide obtained by hydrolyzing silicon alkoxide is 40 wt%.
A method of pressure sintering the following Si ₃ N ₄ —SiC mixed powder at a temperature of 1550 to 1730 ° C. is disclosed.

[0003]

SUMMARY OF THE INVENTION Generally, Si ₃ N ₄ —SiC
(Ultra fine powder) Since the composite material is difficult to sinter, special materials are used or it is forcibly sintered by hot pressing. Japanese Unexamined Patent Publication (Kokai) No. 1-298069 also uses a raw material obtained by hydrolyzing an alkoxide, and also employs a hot press in Examples. It is inconvenient to use special raw materials, and when hot pressing is used, there are restrictions on the product shape. Therefore, the present invention uses such a special starting material,
An object is to provide a method for densely sintering an i ₃ N ₄ —SiC composite material.

[0004]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a sintering aid containing Si ₃ N ₄ in a weight ratio of 4:
Y ₂ O ₃ and MgAl ₂ O ₄ in the range of 1 to 1: 1 are converted to Y ₂ O ₃ + MgAl ₂ O ₄
And 4～8Wt% of the total weight the Si ₃ N ₄ as 100wt% +, Y ₂ O ₃
+ MgAl ₂ O ₄ + Si ₃ N ₄ with total weight of 100 wt%,
And 20 wt% of the mixed powder, and the molded body is
Pre-sintering by heating to a temperature in the range of 00 ° C., and then increasing the atmospheric pressure to 1000 atm or more in the same temperature range, and performing a method for producing a silicon carbide-containing silicon nitride composite, which is characterized by performing main sintering. provide.

The first feature of the method of the present invention is that a sintering aid and
Then Y_TwoO_ThreeAnd MgAl_TwoO_FourThe point is to use. This allows Si_Three
N_Four-Sinterability of the SiC composite is improved. MgAl_TwoO_FourIs MgO
Al_TwoO _ThreeMgAl instead of mixed powder_TwoO_FourMust be powder
No. Y_TwoO_ThreeAnd MgAl_TwoO_FourIs in the range of 4: 1 to 1: 1 by weight.
And the total amount is Si_ThreeN_Four+ Y_TwoO_Three+ MgAl_TwoO_FourTotal weight of 100
4 to 8 wt% with respect to wt%. If the amount of sintering aid is small
Not densely sintered, and too much causes a decrease in strength at 1000 ° C
Because it becomes. Y_TwoO_ThreeAnd MgAl_TwoO_FourWeight ratio of 4: 1
Y_TwoO_ThreeDoes not densely sinter when the value of_TwoO_Three
When the amount decreases, sintering becomes denser, but the amount of spinel increases.
Too high, the strength at 1000 ℃ decreases.

It is desirable that the starting materials of Si ₃ N ₄ , Y ₂ O ₃ , and MgAl ₂ O ₄ have as fine and high purity as possible. Although not limited, Si ₃ N ₄ has a particle size of 0.6 μm or less, more preferably 0.1 to 0.4 μm, and a purity of 100 p in total metal impurities.
pm or less, Y ₂ O ₃ and MgAl ₂ O ₄ have a particle size of 0.6 μm or less, more preferably 0.1 to 0.4 μm, and a purity of 99.9% or more.

The SiC powder to be added is Si ₃ N ₄ + Y ₂ O ₃
+ Relative to the total weight 100 wt% of MgAl ₂ O _4, is in the range of 10-20 wt%. By adding an appropriate amount of SiC to Si ₃ N ₄ , high-temperature strength (for example, 1000 ° C. strength) and fracture toughness can be improved. Together with it. Y ₂ O ₃ -MgAl ₂ O ₄ is Si ₃ N ₄
Although the sinterability is improved, there is a disadvantage that the high-temperature strength is reduced. However, the addition of SiC improves high temperature strength. If the added amount of SiC is small, this effect is not sufficient. S
iC is preferably 0.1 μm or less, more preferably 0.05 μm
It is desirable to have a purity of 99.9% or more with the following particle size.

Mixing and molding of Si ₃ N ₄ , SiC and a sintering aid may be carried out by a conventional method, and typically, compression molding is performed under a pressure of about 300 MPa.

The second feature of the present invention is that the sintering method is not hot pressing but gas pressure sintering.
After presintering in the range of 1700 to 1800 ° C., the gas pressure is increased to 1000 atm or more at the same temperature, and the main sintering is performed. This sintering without hot pressing is performed with Y ₂ O ₃ as a sintering aid.
Enabled by using MgAl ₂ O ₄ .

[0010] The sintering temperature ranges from 1700 to 1800 ° C. If the temperature is too low, sintering will not be sufficient, whereas if the temperature is too high, grain growth will occur and the room temperature and 1000 ° C. strength will decrease. The firing atmosphere is a nitrogen atmosphere, but may be a mixed atmosphere of nitrogen and an inert atmosphere. Before the main sintering, in order to make gas pressure sintering effective, preliminary sintering is performed near normal pressure to make the sintered body closed pores. As a temporary measure of closed pore formation, the theoretical density
About 90%. After the preliminary sintering, the atmospheric pressure is increased to 1000 atm or more, usually 1000 to 2000 atm, and the main sintering is performed at the same temperature. In the present invention, 98-99% of the theoretical density
It is possible to achieve the above densification. If it is less than 1000 atm, it does not sinter densely. And since this invention is gas pressure sintering, there is no restriction in a product shape like a hot press.

According to the method of the present invention, the four-point bending strength from room temperature to 1000 ° C. is in the range of 900 to 1100 MPa, particularly 1000
High-temperature high-strength Si ₃ whose average strength at ℃ is less than 900MPa
N ₄ -SiC composite is obtained.

[0012]

[Action] Y_TwoO_Three, MgAl_TwoO_FourSi as a sintering aid
_ThreeN_Four−High pressure of SiC hard-to-sinter materials without hot pressing
Densification by force gas pressure sintering and Y_TwoO
_Three, MgAl_TwoO_FourHas the disadvantage that the high temperature strength is reduced.
In particular, the addition of SiC_ThreeN_FourSintering
Si enters the grain boundary triple point_ThreeN_FourOxidation resistance of sintered body
The strength at 1000 ° C does not decrease compared to the strength at room temperature.
It becomes.

[0013]

[Example] Si_ThreeN_FourPowder (average particle size 0.2μm, metal impurities
(Total amount 30ppm, alpha conversion rate almost 100%)
Silicon chloride (Production method: Plasma CVD method, specific surface area 47m^Two/
g, average particle size 0.03μm, purity 99.9%, β phase almost 100
%), Y as a sintering aid_TwoO_ThreePowder (average particle size 0.3μm, pure
99.9%), MgAl_TwoO_FourPowder (average particle size 0.3μm, purity 9
9.9%) with the composition shown in Table 1 (Si_ThreeN_Four
200kgf / cm^TwoPressure
Press molding, packing the molded product in thin rubber, vacuum sealing and CIP
3000kgf / cm with (hydrostatic molding)^TwoPressure.
Then, the molded body was N 2 under the conditions shown in Table 1._TwoIn the atmosphere
Sintered in a furnace. Heating rate is 5 ℃ / min, maximum temperature
0kgf / cm for up to 4 hours after arrival^Two(1 atm) N _Twoatmosphere
Below, then 15kgf / cm per minute to the maximum pressure shown in Table 1.^Two
Pressurized at the pressure rising speed, and the total holding time at the highest temperature is 8 hours
Heating and pressurizing until This heating / boosting schedule
FIG. Room temperature 4-point bending strength of these sintered bodies
(JIS R 1601), high temperature four-point bending strength (JIS R 1604, air
Medium) and fracture toughness (JIS R 1607, SEPB method)
The results shown in Table 1 were obtained. The relative density of the sintered body is n-butano
The value obtained by dividing the bulk density obtained by the
is there. The balance is porosity, but the surface is
This was supported by the observation of the polished surface. N number of bending test is
15. The n number of the fracture toughness value is 5, and the value is the average value.
The standard deviation of flexural strength was 80MPa or less at room temperature for all samples.
Below, it was 50 MPa or less at 1000 ° C. Sample No. 2 of Example
As a result of observing the structure of the sintered body of
The silicon carbide powder is homogeneously dispersed by the
Silicon nitride exists at the grain boundary of silicon nitride in the sintered body or at the triple point of the grain boundary
I could observe it. Some SiC is Si_ThreeN_Fourgrain
Also existed within.

[0014]

[Table 1]

[0015]

As described above, the high-temperature strength of the Si ₃ N ₄ —SiC composite is improved, and the degree of freedom of the product shape is increased.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schedule for raising and lowering the temperature of sintering in an example.

Claims (1)

(57) [Claims] 1. A weight ratio of Si ₃ N ₄ to sintering aid of 4:
Y ₂ O ₃ and MgAl ₂ O ₄ in the range of 1 to 1: 1 are converted to Y ₂ O ₃ + MgAl ₂ O ₄
And 4～8Wt% of the total weight the Si ₃ N ₄ as 100wt% +, Y ₂ O ₃
+ MgAl ₂ O ₄ + Si ₃ N ₄ 10%
And a mixed powder containing 1700 to 20 wt%.
A method for producing a silicon carbide-containing silicon nitride composite, comprising: pre-sintering by heating to a temperature in the range of 1800 ° C .;