JP2695070B2 - Method for producing silicon nitride-silicon carbide composite sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural ceramic material used for automobile parts, wear resistant tools and the like, and more particularly to a method for producing a silicon nitride-silicon carbide composite sintered body having an excellent function in this field.

[0002]

2. Description of the Related Art Silicon nitride has strength, fracture toughness, corrosion resistance,
It is a material with a good balance of abrasion resistance, thermal shock resistance, oxidation resistance, etc., and has recently attracted attention as engineering ceramics for structural members at room temperature and at high temperatures. However, in order to use silicon nitride ceramics in fields where high reliability is required for materials such as automobile parts, fracture toughness is further improved to overcome its brittleness and strength is also improved. It is essential to plan. Conventionally, ceramics, which are polycrystals, have been made stronger by refining individual crystal grains.
This method reduces the fracture toughness of the material and makes it even more brittle. On the other hand, as a technique for improving fracture toughness, for example, as disclosed in Japanese Patent Publication No. 62-265173, there is a method of compounding and dispersing silicon carbide whiskers in a silicon nitride matrix.

According to this method, it is considered that the crack toughness which develops at the time of fracture is deflected by the whiskers, or the whiskers are pulled out or crosslinked to improve the fracture toughness. However, whisker composite improves fracture toughness, but conversely, the size of the added whiskers is on the order of 1 to 10 μm, and it is practically difficult to completely remove the agglomerates from the whiskers. As a grain, it becomes the starting point of fracture, which reduces the material strength. Such a decrease in strength is also observed in the long fiber composite material. Also, micron-order particles are mechanically mixed,
In the fired particle-dispersed composite material, the composite effect of the dispersed particles is not remarkably exhibited in terms of both strength and toughness.

[0004]

As described above, conventionally, when the strength is improved by refining the structure of the silicon nitride system,
Fracture toughness decreases, and conversely, when whisker is added or silicon nitride is grown to form large columnar crystals to improve the fracture toughness, the strength decreases, so it is extremely difficult to improve strength and toughness at the same time. It was difficult. Therefore, in silicon nitride ceramics, it has been a major issue to achieve both strength and improvement in toughness.

[0005]

In view of the above problems, the present invention relates to a method for controlling a fine structure for obtaining a sintered body having high strength and high toughness. That is, after adding a sintering aid to a mixed powder of amorphous silicon nitride and carbon to obtain a green compact, it is fired at 1350 to 1700 ° C. to make carbon a solid solution in the amorphous silicon nitride, Furthermore, after being crystallized into α-type silicon nitride and simultaneously undergoing a phase transition to a β-type crystal phase, 1700 to 2300 ° C
The silicon nitride-silicon carbide composite calcination, characterized in that the carbon solid-dissolved in the step of reacting with silicon of silicon nitride precipitates α or β type silicon carbide crystals in and / or at grain boundaries of silicon nitride. It is a method for producing a bound body. Japanese Patent Application Laid-Open No. 2-160669 describes a method for producing a composite sintered body using silicon nitride, silicon carbide composite powder or mixed powder as a raw material.
On the other hand, the present invention uses a mixed powder obtained by adding carbon powder to silicon nitride powder as a raw material, and uses silicon nitride and carbon as a solid phase in the final densification step in which β-type silicon nitride grains grow in liquid phase sintering. The major feature is that the reaction is carried out to precipitate silicon carbide particles.

In the above, for mixing the starting materials, there is mechanical mixing such as ball mill, attrition mill, ultrasonic mixing, etc., which is used for mixing conventional ceramic powders. The firing may be performed by hot pressing or atmospheric pressure sintering in a nitrogen atmosphere.

In the obtained sintered body, the average grain size of the crystal grains of silicon nitride as the base material is 3 μm or less, preferably 1
μm or less and aspect ratio of 20 or less, preferably 10
Silicon carbide particles having a matrix structure consisting of the following columnar crystals and / or equiaxed crystals having a diameter of 1 μm or less, and having a size of several nm to several hundred nm in the crystal grains of silicon nitride and / or in the grain boundaries. Have a dispersed structure.

[0008]

When a sintering aid is added to a mixture of amorphous silicon nitride and carbon and the mixture is fired, silicon carbide is produced during firing, and the obtained sintered body is a silicon nitride-silicon carbide composite sintered body. It has been discovered that it has higher strength and higher toughness than a single sintered body of silicon nitride or a sintered body obtained by mechanical mixing of silicon nitride and silicon carbide crystal powder. The greatest feature of the present invention resides in that solid-state reaction of amorphous silicon nitride and carbon occurs. That is, carbon is dissolved in the grains of amorphous silicon nitride in the temperature range of 1300 to 1700 ° C, and after the crystallization of silicon nitride to the α phase → the transition to the β phase occurs, the solid solution occurs. By reacting the formed carbon with silicon of silicon nitride to precipitate silicon carbide in the grains of silicon nitride and / or in the grain boundaries of the silicon nitride, which is a conventional method for producing a composite material. Alternatively, as compared with the case where whiskers are mechanically mixed and fired, silicon carbide, which is a dispersed phase, can be more uniformly dispersed, and the particle size thereof can be made finer to form ultrafine particles of nanometer order.

According to the present invention, silicon carbide particles having a large coefficient of thermal expansion (coefficient of thermal expansion: silicon nitride = 3.2 × 10 ⁶ / ° C, silicon carbide = 4.4 × 10 ⁶ / ° C) are contained in the crystal grains of silicon nitride. Dispersion on the order of nanometers causes residual stress due to the difference in shrinkage rate during cooling from the final sintering temperature to room temperature. This residual stress induces lattice defects such as dislocations in the silicon nitride crystal grains, and as a result, subgrain boundaries are formed around the silicon carbide particles and the silicon nitride particles are substantially divided. Even if the diameter becomes coarse, stress concentration does not occur at the crack tip at the time of fracture, and the strength is not reduced. Further, there is no glass phase or impurity phase at the respective interfaces of silicon carbide dispersed in the silicon nitride grains and the silicon carbide grains at the grain boundaries and the silicon nitride grains, and as a result, the nitriding which is the parent phase is performed. It becomes a material in which the inside and the grain boundaries of silicon are strengthened, are not easily deformed by the stress applied from the outside, and have large fracture energy when forming a fracture surface. As a result, the strength (σ) and the toughness (Kic) are simultaneously improved from the following formula for the brittle fracture of Griffith.

[0010]

(Equation 1)

[0011]

(Equation 2)

That is, Γ and E in the equation (1) are increased and Kic is increased due to the strengthening of silicon nitride grains and grain boundaries by the silicon carbide dispersed particles. Furthermore, even if there are coarse silicon nitride particles, the inside of the particles is divided and refined due to the formation of subgrain boundaries, and no defects occur. Since a does not increase in Eq. (2), σ increases in proportion to Kic. Also increases.

[0013]

EXAMPLES Examples and comparative examples will be described below.

[0014] Examples 1-6 amorphous silicon nitride powder and carbon powder (acetylene black) and a sintering aid _{(5wt% Y 2 O 3 ·} 2wt% Al 2 O 3)
Were mixed in a ball mill, press-molded, and then fired by a hot press method. Table 1 shows the amount of carbon added, the sintering conditions,
Content of silicon carbide in the obtained sintered body and 3 of the sintered body
Shows point bending strength and fracture toughness.

[0015]

[Table 1]

[0016] sintering aid in the silicon nitride crystal powder of Comparative Example 1 to 6 alpha phase _{(5wt% Y 2 O 3 ·} 2
wt% Al ₂ O ₃ ) was added and press molding was performed, followed by firing by a hot pressing method (Nos. 1 to 3). In addition, an example in which carbon was added to silicon nitride crystal powder was similarly prepared (No. 4 to
6). Table 2 shows the amount of carbon added, the sintering conditions, the amount of silicon carbide dispersed in the obtained sintered body, and the three-point bending strength and fracture toughness of the sintered body.

[0017]

[Table 2]

The silicon carbide crystal powder of Comparative Example 7 to 12 alpha-phase silicon nitride crystal powder β phase (average particle size 0.5 [mu] m) and a sintering aid _{(5wt% Y 2 O 3 ·} 2wt
% Al ₂ O ₃ ) in a ball mill and press-molded,
It was baked by the hot press method. Table 3 shows the amount of silicon carbide added, the sintering conditions, and the three-point bending strength and fracture toughness of the obtained sintered body.

[0019]

[Table 3]

[0020]

EFFECTS OF THE INVENTION According to the present invention, silicon nitride ceramics having excellent strength and fracture toughness can be obtained and can be used for various structural members such as automobile parts which require high strength and high toughness. Can be expected.

Claims (2)

(57) [Claims] 1. A sintering aid is added to a mixed powder of amorphous silicon nitride and carbon to obtain a green compact, which is then fired at 1350 to 1700 ° C. to solidify carbon into amorphous silicon nitride. After being melted and further crystallized into α-type silicon nitride and at the same time undergoing a phase transition to a β-type crystal phase, the carbon solid-solved at 1700 to 2300 ° C is reacted with silicon nitride silicon to form α or β type. 2. A method for producing a silicon nitride-silicon carbide composite sintered body, which comprises depositing the silicon carbide crystal of claim 1 in a grain of silicon nitride and / or at a grain boundary. 2. The method for producing a silicon nitride-silicon carbide composite sintered body according to claim 1, wherein the firing is performed by a hot pressing method or an atmospheric pressure sintering method in a nitrogen atmosphere.