JPH04270174A - Composite sintered ceramic and its production - Google Patents

Abstract

PURPOSE:To improve the toughness of a molded article composed mainly of SiC by baking the article under specific condition. CONSTITUTION:The objective composite sintered ceramic material is composed mainly of Si3N4 at the surface layer and SiC and Si3N4 at the core part and has a compositional gradient continuously increasing the SiC/(SiC+Si3N4) ratio from the surface layer toward the core part. It can be produced by mixing (A) SiC powder having an average particle diameter of 0.1-2mum with (B) <=10wt.% of carbon powder or a substance capable of forming carbon by baking, e.g. a phenolic resin, coal tar pitch or a boron-containing compound and (C) a binder, forming the mixture, calcining at 200-800 deg.C and baking the calcined article in an atmosphere capable of forming Si3N4 and carbon by the reaction of SiC and nitrogen.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a ceramic composite sintered body mainly composed of silicon carbide and silicon nitride, and in particular to a ceramic composite sintered body whose surface layer has been modified to improve the toughness of the silicon carbide sintered body. Regarding its manufacturing method.

0002

[Prior Art] Non-oxide ceramics, represented by silicon carbide and silicon nitride, have attracted attention as materials with superior hardness, strength, toughness, and chemical stability compared to other ceramics and metals, and are suitable for any structure. It has already been put into practical use as a wear-resistant material and has recently been developed as a structural material for heat engines such as gas turbines.

[0003] Silicon carbide is produced by adding a boron-containing compound such as B4C or carbon as a sintering agent, and
It is obtained by firing in an inert atmosphere at ℃ and has excellent properties such as strength, hardness, and abrasion resistance, and almost no strength deterioration at high temperatures, so its application as a high-temperature structural material is progressing. It is being

On the other hand, since silicon nitride itself is difficult to sinter, Al2O3 and oxides of Group 3a elements of the periodic table are added as sintering aids to sinter the silicon nitride at 1600 to 2000°C.
It is densified by firing in a nitrogen atmosphere, and although it has the disadvantage that its strength deteriorates at high temperatures compared to silicon carbide, it has excellent toughness and thermal shock resistance. There is.

Therefore, from the viewpoint of compounding silicon nitride and silicon carbide, it has been conventionally attempted to bond a silicon carbide sintered body and a silicon nitride sintered body, or to mix and sinter silicon carbide and silicon nitride. Or, a method in which metal Si is made to exist in a silicon carbide sintered body using a reaction sintering method, and this is nitrided in a nitrogen atmosphere to form a mixed layer of silicon nitride and silicon carbide at least in the surface layer. etc. are published in 1986.
-264364 etc.

[0006]

[Problems to be Solved by the Invention] However, the method of joining sintered bodies requires a suitable bonding agent to join the sintered bodies, and furthermore, the strength of the joint itself is low due to sintering. Since the strength of the compact is weak compared to the strength of the compact itself, the original characteristics of each sintered compact cannot be exhibited by joining, and even with a simple mixing method, it is difficult to uniformly disperse the compact, and the sinterability is poor. Improvement is necessary to reduce the

[0007] Furthermore, even in the method using reactive sintering, it is difficult to completely carry out the nitriding reaction of metal Si, and the metal Si remains inside the sintered body, resulting in deterioration of the strength. Moreover, the surface layer of the obtained sintered body also consists of a mixed phase of silicon carbide and silicon nitride, which has the disadvantage that the original characteristics of silicon nitride cannot be exhibited.

[0008]

[Means for Solving the Problems] As a result of repeated studies on the above problems, the inventors of the present invention have created a molded body mainly composed of silicon carbide, and have created a molded body containing silicon carbide as a main component. At the same time as this firing, the conversion reaction of silicon carbide to silicon nitride proceeds, and the conversion reaction is controlled to produce a final sintered body whose surface layer is mainly made of silicon nitride and whose interior is made of carbide. It is made of silicon or silicon carbide and silicon nitride, and by forming a composition gradient in which the ratio of silicon carbide to silicon nitride increases continuously, it has the characteristics of silicon carbide while fully exhibiting the characteristics of silicon nitride. They have discovered that by doing so, a sintered body with excellent toughness, high thermal shock resistance, and high-temperature strength can be obtained.

The present invention will be described in more detail below. According to the present invention, silicon carbide powder is first prepared as a raw material powder. As the silicon carbide powder, either α-SiC or β-SiC, or a mixture thereof can be used. The average particle size of the powder is suitably 0.1 to 2 μm. Further, to the above silicon carbide powder, carbon powder such as carbon black or graphite, phenol resin or coal tar pitch that can generate carbon by firing, or a boron-containing compound such as B4C is added in a proportion of 10% by weight or less. be able to.

Next, a binder or the like is added to the above powder, and after molding into a desired shape by a well-known molding method such as press molding, extrusion molding, casting molding, cold isostatic pressing, etc., the powder is heated at 200 to 800°C if desired. After calcination, it is fired.

According to the present invention, this firing is performed according to the following equation 1.

[Math. 1] 3SiC+2N2 → Si3 N
4 +3C, the material is fired in an atmosphere where silicon nitride and carbon can be produced by the reaction between silicon carbide and nitrogen. Specifically, firing is performed at a temperature of 1000°C or higher, especially 1500°C or higher, with nitrogen gas as an essential component in the atmosphere, and the nitrogen gas pressure is 500 atmospheres or higher. The reaction can proceed.

[0012] If the firing conditions at this time deviate from the above-mentioned temperature and pressure, the nitriding reaction will not proceed or will be difficult to proceed.

[0013] According to this firing, high densification is achieved in both the internal and surface layers, and the above reaction occurs particularly actively in the surface layer of the sintered body, so that silicon nitride is deposited in the surface layer of the sintered body from the inside. A specific sintered body is formed in which the number of particles increases.

Although the mechanism of this sintering is not clear, the reaction from the surface of silicon carbide particles to silicon nitride progresses under a high temperature and high pressure nitrogen atmosphere, resulting in volumetric expansion, which leads to some degree of densification. As the process progresses, once a dense layer is formed on the surface of the sintered body, the entry of nitrogen gas into the interior of the sintered body is suppressed, resulting in a dense body with a porosity of 10% or less both on the surface and inside. However, it is thought that a structure is formed in which the composition changes almost continuously between the surface layer and the interior.

According to the present invention, the firing temperature, firing time, etc. in the firing are controlled so that all the silicon carbide components present in the surface layer of the sintered body are converted into silicon nitride, while the silicon carbide inside is controlled. The interior is mainly composed of silicon carbide or silicon carbide and silicon nitride so that it remains. Also, Figure 1
According to the nitrogen distribution diagram of It is possible to form a composition gradient such that the composition ratio expressed by silicon/(silicon carbide+silicon nitride) increases.

[0016] According to this structure, while the interior has properties similar to silicon carbide, the surface layer portion has properties similar to silicon nitride, that is, excellent toughness. Furthermore, in a normal silicon nitride sintered body, a metal oxide used as a sintering aid exists between silicon nitride crystal grains as a grain boundary phase, but in the surface layer of this sintered body, nitride Another major feature is that substantially no metal oxide exists between silicon crystal grains, which reduces strength deterioration due to the presence of grain boundary phases at high temperatures. Note that the composition ratio expressed by silicon carbide/(silicon carbide+silicon nitride) in the surface layer portion is desirably 0.1 or less. The interior is mainly composed of silicon carbide or silicon carbide and silicon nitride, but silicon carbide/(silicon carbide +
The composition ratio of silicon nitride (silicon nitride) is preferably 0.2 or more.

[0017]

[Function] According to the present invention, the surface layer is mainly composed of silicon nitride, and the interior is mainly composed of silicon carbide or silicon carbide and silicon nitride. Only the surface layer of the body can have the characteristics of silicon nitride. In addition, since the composition of silicon carbide and silicon nitride gradually changes from the surface layer to the inside,
The generation of stress due to the difference in thermal characteristics between the surface layer and the inside can be alleviated.

Furthermore, according to the manufacturing method of the present invention, since the nitriding reaction proceeds gradually from the surface layer, the composition of the surface layer and the inside of the sintered body can be controlled as desired, and the composition of silicon carbide and nitride can be controlled as desired. In the coexistence region with silicon, a structure in which silicon carbide and silicon nitride are uniformly dispersed in each other is formed.

[0019]

[Example] To β-SiC powder (average particle size 0.4 μm, oxygen content 0.2% by weight), an appropriate amount of a 20% solution of resol type phenolic resin was added as a molding binder, and acetone was added as a solvent. After adding an appropriate amount and kneading and drying, the mixture was passed through a sieve to obtain moldable granules. A disk-shaped molded body having an outer diameter of 20 mm and a thickness of 10 mm was prepared from the granules using a mold press.

Next, the molded body is placed in an inert atmosphere (N
2 in an air stream) to carbonize the phenol resin, the composition of the calcined body was analyzed. Then, this calcined body was fired under the conditions shown in Table 1: N2 pressure, firing temperature, and firing time.

The density of the obtained sintered body was measured by the Archimedes method, and the surface layer of the sintered body was analyzed by X-ray diffraction method, and the sintered body was crushed and powdered
The composition of the sintered body was analyzed using line diffraction. In addition, a sample was cut out from the sintered body, and the distribution of nitrogen atoms on the cut surface was investigated by EPMA analysis, which is shown in FIG. The results are shown in Table 2.

To evaluate the mechanical properties, the flat surface of the obtained sintered body was polished by 100 μm, and the toughness (K1c) was measured using the mirror surface by the indentation method (IF method). In addition, Niihara's formula was used for the calculation.

As a comparative example, a molded body was prepared in the same manner as above except that 0.4 parts by weight of B4C powder was added to 100 parts by weight of β-SiC powder (average particle size 0.4 μm), and then , 2050 in an argon atmosphere of 1 atm.
C. for 1 hour to produce a dense silicon carbide sintered body (Sample No. 1 in the table), and its properties were measured and evaluated in the same manner as above.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Effects of the Invention] As detailed above, according to the present invention,
By a simple method, silicon carbide and silicon nitride can be composited without using methods such as bonding, and the sintered body has the excellent characteristics of silicon carbide inside. The high toughness of silicon nitride can be imparted to the surface layer. This makes it possible to further expand the use of the material as a structural material or a high-temperature material.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing nitrogen distribution in a cross section of a ceramic composite sintered body of the present invention, as determined by EPMA analysis.

Claims (2)

[Claims] Claim 1: The surface layer is mainly made of silicon nitride,
A composition gradient in which the center part is mainly composed of silicon carbide or silicon carbide and silicon nitride, and the composition ratio expressed by silicon carbide/(silicon carbide + silicon nitride) increases almost continuously from the surface part to the center part. A ceramic composite sintered body characterized by having. 2. A powder compact containing silicon carbide powder as a main component is fired under conditions where silicon carbide can be nitrided to silicon nitride, so that the surface layer is mainly silicon nitride and the center is silicon carbide or silicon carbide. and silicon nitride, and from the surface layer to the center, silicon carbide/(silicon carbide +
A method for producing a ceramic composite sintered body, characterized in that the silicon carbide is converted to silicon nitride to such an extent that a composition gradient is formed such that the composition ratio expressed by silicon nitride increases.