JPS6287462A - Silicon nitride sintered body and manufacture - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a silicon nitride sintered body and a method for producing the same, and particularly relates to a silicon nitride sintered body that has high strength at high temperatures.

[Conventional technology]

Since silicon nitride sintered bodies have excellent high-temperature strength, they are being developed as the most promising ceramics for structural materials such as automobile engine parts that require heat resistance.

Since silicon nitride has poor sintering properties, there are two methods for sintering silicon nitride: a reactive sintering method in which a metal silicon compact is fired in a nitrogen atmosphere, and a method in which a sintering aid is added to silicon nitride powder. There is a method of firing in an inert atmosphere. However, from the viewpoint of the strength of the sintered crystal, a method using a sintering aid is mainly used especially for structural ceramics. As a sintering aid, alumina (8j!z(h),
There are many oxides such as yzos and magnesia (MgO).

[Problem that the invention seeks to solve]

Silicon nitride sintered using a sintering aid has relatively high strength, but there is a problem in that its strength decreases at high temperatures of 1000° C. or higher, especially 1200° C. or higher.

[Means for solving problems]

Means for solving the above problems according to the present invention are β-
The main component is Si3N, and the general formula YX/2 is used as the second phase.
Si6□□7□87! y-Xoy N5-y (in the formula, 0
The present invention is a silicon nitride sintered body containing one or more of yttrium sialon crystals, (Ca, re, Mg)Sin crystals, and MgYSiO crystals represented by <Xxy<8).

In other words, in the silicon nitride sintering method in which a sintering aid is added and sintered, the sintering aid melts at a lower temperature than silicon nitride, and the presence of the liquid phase promotes the sintering of silicon nitride. Conventionally, in the silicon nitride sintered body obtained, the phase based on these sintering aids exists as a glass phase at the grain boundaries of silicon nitride. The presence of this glass phase is the cause of the decrease in high temperature strength of the silicon nitride sintered body. Therefore,
In the present invention, the high-temperature strength of the silicon nitride sintered body is increased by converting the glass phase based on these sintering aids into a crystalline state.

In order to make the grain boundary phase of the silicon nitride sintered body crystalline, the present invention uses Y2O3, A # 20° and MgO (All 2oz and MgA j! in place of all or part of MgO) as sintering aids. z04) is selected, and heat treatment is performed after firing. For more details, see Si3Ng
Y20*, ^1203, and MgO are added to the powder as sintering aids in an amount of 2 to 5% by weight (preferably, each sintering aid is in an equal weight%), molded, and then subjected to pressure or non-pressure treatment. After baking in an inert atmosphere at 1600 to 1900°C, preferably 1700 to 1750°C for 1 to 10 hours, preferably 2 to 6 hours to obtain a Si3N, sintered body, N2
1 to 10 under inert atmosphere such as gas, inert gas, vacuum, etc.
Atmosphere, preferably 1 to 2 atm pressure, 1100 to 140
At a temperature of 0°C, preferably 1300-1350°C, 30
The heat treatment is carried out for 5 minutes to 5 hours, preferably 2 to 4 hours.

In the method of the present invention, the grain boundary phase is not crystallized during the Si3N4 sintering step, but is crystallized by the subsequent heat treatment.

Yttrium sialon crystal (Yx/zSi6-y+xzz A l1V-X 0I
INII-F), the ratio of X and y is 0.8 <x
/Y<1.0 is preferable.

This is because if x/y is less than 0.8, vitrification is likely to occur at high temperatures. Y38e 5itOJ, YzSis^7!05
N3 is a stable crystal that satisfies the condition 0.8 < x/y xt, and is therefore preferable.

(Ca, Fe, Mg) SiO + crystalline Ca
, Fe is thought to be mixed with impurities in the furnace, and these crystals are mainly found on the surface of the sintered body. MgYSi(h
The crystal is a phase generated by the addition of MgO, and together with yttrium sialon, it is a main component of the grain boundary crystal.

〔Example〕

Silicon nitride raw material powder with an average particle size of 0.84 μm and a purity of 98% or more, 4 wt% vzOs and 4 wt% MgA 7! O
a was blended and a test piece was molded using a mold press. The obtained test piece was fired at 1750° C. for about 4 hours in a nitrogen atmosphere pressurized to 9 atmospheres to obtain a silicon nitride sintered body.

When the surface and interior of the fired body were analyzed by X-ray diffraction,
It was observed that only β-SiJ4 was fixed as the crystalline phase, and the grain boundary phase was vitrified.

The relative density of the fired body is 92%, and the 4-point bending strength at 1200°C is 60kg/1Ill11! Met.

Next, this fired body was heat treated in a nitrogen atmosphere at normal pressure under the following conditions.

(11 Heating up to 800°C at a heating rate of 15°C/min (2180°C)
Hold at 0°C for 30 minutes (3) Raise the temperature to 1150°C at a heating rate of 15°C/min (4) Continue to raise the temperature to 1300°C at a heating rate of 5°C/min (51130°C)
Hold at 0°C for 4 hours (6) Cooling in the furnace The reason why the temperature is held at 800°C and 1150°C for 30 minutes during the temperature rise is to remove distortion since the temperature rise rate is as fast as 15°C/min. Furthermore, the temperature increase rate from 1150° C. to 1300° C. was made small because crystallization begins, so it is necessary to raise the temperature slowly. And 1300℃×4h
Crystallization is completed at r.

Analysis of the surface and interior of the fired body after heat treatment by X-ray diffraction revealed that it was β-Si3Na and Y38j! Si, O, N
, (Ca, Fe, Mg) Sin,, YSiO
, crystals were identified.

(Ca, Fe, Mg) SiO+ was detected only in the surface layer.

The relative density of the fired body was 95% or more, and the four-point bending strength at 1200°C was 68 kg/mm''.

〔Effect of the invention〕

According to the present invention, a silicon nitride sintered body having high strength at high temperatures can be obtained, and the reliability of ceramics can be improved.

Claims (1)

[Claims] 1. β-Si_3N_4 is the main component, and the second phase is expressed by the general formula Y_x_/_2Si_6_-_y_+_x_/_2
Al_y_-_xO_yN_8_-_y (where 0<x
<y≦8) Yttrium SiAlON crystal,
(Ca, Fe, Mg)SiO_3 crystal and MgYSi
A silicon nitride sintered body characterized by containing one or more types of O_4 crystals. 2. Y_2O_3 as a sintering aid to Si_3N_4 powder
, Al_2O_3 and MgO are added, molded and fired to obtain a Si_3N_4 sintered body, and then heat-treated to obtain β
-Si_3N_4 is the main component, and the second phase is the general formula Y_x_/_zSi_6_-_y_+_x_/_zAl
_y_-_xO_yN_8_-_y (where 0<x<y
≦8) Yttrium SiAlON crystal, (C
a, Fe, Mg) SiO_3 crystal and MgYSiO_
A method for producing a silicon nitride sintered body, characterized by obtaining a Si_3N_4 sintered body containing one or more of four crystals.