JPH07102993B2 - Method for manufacturing silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention has excellent heat resistance, mechanical properties, and is chemically stable. Therefore, it is being applied to automobile engine parts, etc. It relates to a sintered body.

<Problems to be Solved by Conventional Techniques and Inventions> Reactive sintering methods, hot pressing methods, atmospheric pressure sintering methods, gas pressure sintering methods and the like are known as conventional methods for producing a silicon nitride sintered body. However, atmospheric pressure and gas pressure sintering methods are suitable for complex shaped parts that require high strength, such as engine parts.
In these methods, a mixed powder in which an oxide-based sintering aid such as MgO is added to silicon nitride raw material powder is molded by a method such as rubber press, slip cast, injection molding, etc. Sintering method) or a step of heating in an atmosphere of 1 atm or more (gas pressure sintering).

However, when molding defects such as voids and cracks occur in the powder molding process, these defects remain in the sintered body even after firing, reducing the strength of the product, resulting in a low average strength and variations in strength. Is also known to increase.

Hot isostatic pressing (hereinafter abbreviated as HIP) treatment is known as a method for reducing defects in the sintered body. This aims at crushing the vacancy defects by causing a high-pressure gas to act on the sintered body at a high temperature. However, this effect was small for the conventional silicon nitride sintered body. This is for the following reason.

Conventionally, as a starting material, a silicon nitride raw material powder whose main component is α type is used, and during the firing, the powder is densified and the α phase to β
Causes a transition to the phase (at this time the β phase becomes acicular),
It has been generally practiced to obtain a silicon nitride-based sintered body containing a large amount of acicular phases. This is because the sintered body containing many acicular phases was a kind of composite material and was considered to have excellent fracture toughness, strength and the like. However, HI
Even if the P treatment is performed, the acicular crystals surrounding the defect prevent deformation of the defect, and thus the defect is difficult to collapse.

<Means for Solving Problems> The inventors of the present invention have conducted research to eliminate this drawback. As a starting material of silicon nitride, the nitriding material containing 70% or more of β phase instead of α phase mainly Using silicon, adding a sintering aid that is usually used to this, mixing, molding, and then treating at a temperature of 1600 ° C. or higher in a nitrogen atmosphere of 500 atm or lower, and then
By hot isostatic pressing, even if the compact contains defects, it can be removed by HIP treatment, leading to the invention of a method for producing a silicon nitride sintered body with high strength and little variation. .

That is, by using silicon nitride powder containing 70% or more of β phase as the raw material powder, the amount of α-to-β transition is reduced, so that the amount of needle-like crystals is reduced and HIP works effectively. I aimed at

In the present invention, the silicon nitride raw material powder contains 70% or more of β phase. The balance may be alpha phase or amorphous. If the β phase is less than 50%, many needle-like crystals are present in the sintered body, so that the HIP defect removal effect is reduced. The present invention is effective even if 100% of β phase is used as a starting material, but when strength of the sintered body is required, a material containing α phase is preferable as long as the HIP effect cannot be prevented. Next, the sintering aid is an additive used in general pressureless sintering of silicon nitride, for example, MgO, Y ₂ O ₃ -Al ₂ O ₃ , etc. is the total weight of the silicon nitride powder and the additive. It can be used in a proportion of about 7 to 15% by weight.

Molding is mold molding, rubber press, slip cast,
A general ceramic molding method such as injection molding may be used.

Next, calcination is carried out at a temperature of 1600 ° C. or higher with nitrogen of 500 atm or lower. When the pressure exceeds 500 atm, a dense sintered body cannot be obtained because the high-pressure gas trapped in the closed pores hinders final sintering. The firing temperature varies depending on the type and amount of the sintering aid used, but below 1600 ° C, the liquid phase does not form, and therefore it does not become dense. The lower limit of the nitrogen pressure in the above step, the upper limit of the calcination temperature is not particularly as a rule of prospects, nitrogen atmosphere pressure is more than 1 atm, the firing temperature may be carried out the process at about about 2200 ° C. or less.

HIP is performed above 1600 ° C. If it is less than 1600 ° C, compositional deformation of the sintered body is small and the effect of defect removal is small. Also in this case, the upper limit of the temperature is not particularly limited, but it is preferable to perform the HIP at about 2200 ° C. or lower which is generally considered to be the upper limit of the sintering temperature of the silicon nitride sintered body. The kind and pressure of the gas are not particularly limited, but nitrogen gas is preferably used when the capsule-free processing is performed. The gas pressure of 500 atm or higher is particularly effective. Further, in the present invention, a method of sealing a sample with a capsule such as glass after firing at 500 atm or less, a method of performing firing and HIP in another device, a HIP device, performing firing and HIP by controlling temperature and pressure. The processing can be performed by a method or the like.

<Examples> Examples of the present invention will be given below.

Example 1 10% MgO was added to Si ₃ N ₄ powder consisting of the β phase 70% and the remaining α phase, mixed by a ball mill, dried and pulverized. 0.1 vol% of 70 μm plastic spheres as a defect source in this mixed powder
The mixture was added, molded with a die at 20 MPa, rubber-pressed at 200 MPa, heated to 500 ° C. to burn off plastics, and a molded body with defects of 5 × 6 × 50 mm was obtained. This was fired at 1700 ° C. for 1 hour in an atmosphere of nitrogen at 1 atm. A portion of this sample was placed at 1700 in a nitrogen atmosphere of 2000 atm.
Hot isostatic pressing was carried out at 0 ° C for 1 hour. Firing only and
Table 1 shows the average strength results of 15 pieces each of which was subjected to a room temperature bending test with a span of 30 mm by grinding a HIP-treated test piece into a shape of 3 × 4 × 40. As shown in Table 1, the strength was improved by the HIP treatment. Further, according to the fracture surface observation, it was found that the fracture was initiated from the spherical defect without HIP, but no spherical defect was found in the HIP-treated test piece.

Comparative Example 1 Table 2 shows the results of a bending test performed by the same process as in Example 1 except that Si ₃ N ₄ powder consisting of α phase 93% was used. As shown in Table 1, the strength improvement by HIP treatment was small, and spherical defects remained in the sintered body even after HIP.

Examples 2 to 4 Using the raw materials having the compositions shown in Table 1, defects were introduced in the same steps as in Example 1, and firing and HIP were performed under the conditions shown in Table 1. As shown in Table 1, the strength was improved and the spherical defects disappeared.

Comparative Examples 2 to 4 Using the raw materials having the compositions shown in Table 2, defects were introduced in the same steps as in Example 1, and firing and HIP were performed under the conditions of Table 2. As shown in Table 2, there was little improvement in strength, and spherical defects were found in the sintered body even after HIP.

Example 5 10% Y ₂ O ₃ , 5% Al ₂ O ₃ was added to a powder consisting of β phase 70% and the balance α phase.
Auxiliary agents were added so as to obtain the following, and the mixture was ball-milled, dried and pulverized to obtain mixed powder. An organic binder was added to the mixed powder and the kneaded material was injection-molded to obtain a molded body having a diameter of 20 mm and a length of 50 mm. When this was heated to 500 ° C. and the organic binder was removed and observed by X-ray, molding defects were found inside. This was treated with the schedule shown in FIG. 1 under N ₂ gas pressure, and the obtained sintered body was observed by X-ray. As a result, the defects disappeared.

<Effects of the Invention> As described above, according to the present invention, even if a defect occurs in the molding step, it can be removed during firing, and a dense and high-strength sintered body having no defect can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between firing temperature and pressure-time when producing Example 5 of the present invention.

Claims (1)

[Claims] 1. A silicon nitride powder containing 70% or more of β phase and a sintering aid are mixed and shaped, and then treated at a temperature of 1600 ° C. or more in a nitrogen atmosphere at 500 atm or less, and then at 1600 ° C. A method for producing a silicon nitride sintered body, comprising performing hot isostatic pressing at the above temperature.