JPH03290376A - Production of composite ceramic article - Google Patents

Abstract

PURPOSE:To obtain a preliminarily sintered material having high relative density and formable with a direct hot hydrostatic press by preparing a slurry containing ceramic powder, silicon carbide whisker and silicon nitride whisker and having a viscosity lower than a specific level and forming the slurry while orienting the whiskers, CONSTITUTION:A slurry having a viscosity of <=3.0 poise is produced by dispersing ceramic powder and silicon carbide whisker or silicon nitride whisker in a solvent. The slurry is poured into a gypsum mold and molded while orienting the whiskers parallel to the wall surface of the gypsum mold to obtain a green molded article. Since the whiskers are free from mutual entanglement, the slurry is completely filled in the mold and the relative density of the green article reaches >=60%. The control of the orientation of the whisker can be carried out by adjusting the viscosity of the slurry within the above range. The green molded article is sintered under normal pressure to obtain a preliminarily sintered material having a relative density of >=95%. Since the preliminarily sintered material has high relative density, it can be sintered with a hot hydrostatic press without using a glass capsule.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method of manufacturing a composite ceramic product having high toughness and high density.

[Prior Art] Ceramics are widely used in heater tubes, crucibles, cylinder liners, and the like because they have high strength even at high temperatures and excellent corrosion resistance. However, ordinary ceramics have the problem of being brittle and easily damaged by thermal shock.

According to the findings of the present inventors, composite ceramics in which whiskers are dispersed have excellent toughness and are resistant to thermal shock. However, composite ceramics in which whiskers are dispersed without special efforts have a relative density of less than 95% after pressureless sintering because the whiskers prevent densification. has insufficient wear resistance.

A method is now known to manufacture a pre-sintered body using pressureless sintering and re-sintering this pre-sintered body using hot isostatic pressing to produce a strong, dense ceramic product with a high Weibull coefficient. It is being

In this method, if the pressureless sintered ceramic serving as the pre-sintered body has a relative density of less than 95%, the pre-sintered body is encapsulated in a glass capsule and re-sintered using hot isostatic pressing. If the relative density of the pressureless sintered ceramic that becomes the pre-sintered body is 95% or more, the pre-sintered body can be directly re-sintered using hot isostatic pressing without encapsulating it in a glass capsule. .

As already mentioned, conventional pressureless sintered bodies with dispersed whiskers have a relative density of less than 95%. Therefore, in order to resinter a conventional pressureless sintered body in which whiskers are dispersed by hot isostatic pressing, it is necessary to encapsulate it in a glass capsule.

However, the work of encapsulating the preliminary sintered body in a glass capsule is extremely troublesome.

[Problem to be solved by the invention] Although it has not been possible to produce ceramics in the past, if it is possible to produce ceramics by pressureless sintering with dispersed whiskers and a relative density of 95% or more, it will be possible to produce ceramics directly without encapsulating them in a glass capsule. Since it can be resintered using hot isostatic pressing, the troublesome work of encapsulating it in a glass capsule can be omitted.

The present invention produces ceramics by pressureless sintering in which whiskers are dispersed and has a relative density of 95% or more, and the ceramics produced by pressureless sintering are directly used as a pre-sintered body without being enclosed in a glass capsule. The objective is to provide a method for producing ceramic products that are strong, dense, and have a high Weibull coefficient by resintering using hot isostatic pressing.

[Means for Solving the Problems] In the first step of the present invention, ceramic powder and silicon carbide whiskers and/or silicon nitride whiskers are dispersed in a solvent to form a slurry with a viscosity of 3.0 poise or less. Manufacture.

Various silicon nitride powders, alumina powders, silicon carbide powders, magnesia powders, and ittria powders are commercially available as ceramic powders, and in the present invention, these powders can be used alone or in combination. Further, compound powders such as cordierite and spinel can also be prepared and used. Although the particle size of the ceramic powder is not particularly limited, a fine powder of 1 μl or less is preferable.

In the present invention, the whiskers used are silicon carbide short fibers or silicon nitride short fibers having a diameter of 0.1 to 1 μm and a length of 5 to 100 μm. For example, the product name Toka Whisker (manufactured by Tokai Carbon Co., Ltd.) is a commercially available short fiber of silicon carbide, which can be used directly as the whisker of the present invention. In addition, commercially available silicon carbide long fibers and silicon nitride long fibers have a capacity of 5 to 1100 tL.
It can be cut to length and used as the whisker of the present invention.

In the first step of the present invention, a ceramic raw material is dispersed in a solvent to produce a slurry having a viscosity of 3.0 poise or less.

In the present invention, ceramic powder, 5 to 20% by weight of whiskers based on the ceramic powder, and 3 to 15% by weight of a cordierite sintering aid based on the ceramic powder are mixed in water containing a dispersant. as a solvent to form a slurry with a viscosity of 3.0 poise or less9 In the present invention, the viscosity of the slurry refers to the viscosity measured with a rotational viscometer calibrated with a standard solution for viscosity meter calibration according to JIS Z8809. The viscosity of the slurry is set to 3.0 poise or less by adjusting, for example, the particle size of the ceramic powder, the amount of dispersant added, and the pH.

In the second step of the present invention, this slurry is injected into a plaster mold and inked to form a green molded body.

The relative density of this green molded body is 60% or more. In order to make the relative density 60% or more, in the present invention, the orientation of whiskers in the green molded body is controlled. If a green molded body is manufactured using a slurry containing whiskers without special efforts, the whiskers will be oriented randomly in three dimensions, but in this orientation, the whiskers will intertwine with each other and the filling of the slurry will be insufficient. Therefore, the relative density of the green molded body is 60% or less. As will be explained in more detail below, in the present invention the whiskers are oriented parallel to the walls of the plaster form.

Further, it is parallel to the wall surface of the plaster mold and randomly oriented in two dimensions. When the whiskers are oriented in this manner, the whiskers do not become entangled with each other, so that the slurry is sufficiently filled and a green molded body having a relative density of 60% or more is obtained. According to the findings of the present inventors, in a slurry with a viscosity of 3.0 poise or higher, the two-dimensional orientation of the whiskers in the green molded body becomes loose, and as the viscosity increases, the whiskers become three-dimensionally random and their relative density increases. is less than 60%.

Control of the orientation of the whiskers in the present invention can be achieved by appropriately adjusting the viscosity of the slurry to 3.0 poise or less, although it varies depending on the type, particle size, and shape of the ceramic powder and whiskers constituting the slurry.

In the third step of the present invention, the green molded body formed in the second step is pressureless sintered. This pressureless sintering is performed under normal pressureless sintering conditions, such as sintering temperature, sintering time, and atmospheric gas. As already mentioned, the present invention uses a green molded body in which the direction of whiskers is controlled. A green molded body in which the whiskers are parallel to the wall surface of the plaster mold and randomly mixed in two dimensions over the entire wall thickness from the outer surface to the inner surface of the green molded body has a relative density of This results in a pre-sintered body of 95% or more.

In the fourth step of the present invention, the preliminary sintered body produced in the third step is directly sintered again by hot isostatic pressing without encapsulating it in a glass capsule. Since the pre-sintered body with a relative density of 95% or more has no air permeability, it can be heated, for example, by hot isostatic pressing at a pressure of 1000 to 2000 kgf/Cm and a temperature of 1500 to 1800°C, without putting it into a capsule. When sintered, it becomes a high-density ceramic product close to the theoretical density.

[Function] Fig. 1 is an explanatory diagram of the action of the slurry of the present invention, (A) is a diagram showing the shape of a green molded body or a composite ceramic product, (Bl), (82) Fig. 11 is an illustration of the action of the slurry of the present invention. It is an explanatory view of direction. In the green molded body of the present invention, the whiskers 2 are arranged approximately parallel to the surface of the green molded body (the wall surface of the plaster mold) as shown in FIG. 1 (131), and also as shown in Figure 1 (B2). They are arranged in two-dimensional random directions and approximately evenly throughout the entire thickness of the green molded body. This orientation of the whiskers 2 in the green molded body 1 is maintained as it is even in the pre-sintered body after pressureless sintering and the composite ceramic product after hot isostatic pressing.

Therefore, in the composite ceramic product manufactured by the method of the present invention, the whiskers 2 are arranged substantially parallel to the surface of the product and in two-dimensional random directions b).

When a ceramic product is heated by [f internal force 1] as shown in Fig. 1 (A), the inner surface thermally expands. The tensile thermal shock stress P1 in the parallel e-h direction and e
A tensile thermal shock stress P2 in the -f direction is generated.

Silicon carbide whiskers and silicon nitride whiskers have extremely high strength, but in the present invention, the whiskers are arranged parallel to the outer surface of the ceramic product and oriented randomly in two dimensions. Therefore, in the present invention, the whiskers can withstand the tensile thermal shock stresses P1 and P2 shown in FIG. 1 (B2) parallel to the outer surface, and also prevent the propagation of minute racks to prevent the occurrence of cracks. Therefore, the ceramic product of the present invention has excellent toughness and is resistant to thermal shock.

FIGS. 1 (CI) and (C2) are explanatory diagrams of comparative examples using slurry having a viscosity of 3.0 voices or more. In conventional green molded bodies and composite ceramic products in which whiskers are composited, the whiskers 2 are three-dimensionally arranged in random directions. When a ceramic product is used, for example, when it is heated from the inner surface as shown in FIG. 1(A), the inner surface thermally expands, but the tensile impact stress P□ in the thickness direction (a-b direction) of the ceramic product is small. In conventional composite ceramic products, the whiskers 2 are arranged randomly in three dimensions and are also oriented in the wall thickness direction where tensile thermal shock stress is small, so the number of whiskers oriented parallel to the outer periphery of the ceramic product is small. . For this reason, conventional composite ceramic products cannot withstand tensile thermal shock stresses P1 and P2 parallel to the outer periphery of the ceramic product during use, and are susceptible to thermal shock.

In the slurry of the present invention having a viscosity of 3.0 poise or less as shown in FIGS. 1 (Bl) and (B2), the whiskers 2 are sequentially arranged in layers when the slurry is formed. Therefore, the whiskers 2 do not become entangled with each other during inking.

As a result, the matrix material 1 is also densely deposited. Therefore, according to the present invention, a dense green molded body having a relative density of 60% or more can be obtained. Furthermore, when a green molded body in which the whiskers are parallel to the wall surface of the plaster mold and randomly oriented in two dimensions over the entire wall thickness from the outer surface to the inner surface of the green molded body is sintered under pressure, the relative density is 95% or more. A pre-sintered body is obtained.

This pre-sintered body with a relative density of 95% or more is dense and has no open pores on its surface, so there is no need to encapsulate it in a glass capsule. The result is a high-density composite ceramic product that is close to that of the original.

This is shown in Figure 1 (CI) and (C2). In conventional green molded bodies using slurry with a viscosity of 3.0 poise or more, the whiskers 2 become entangled with each other during slurry molding.
The matrix material 1 is not fully filled. Therefore, it is difficult to produce green molded bodies with a relative density of 60% or more using conventional methods. Furthermore, even if the green molded body is sintered under normal pressure, the relative density of the preliminary sintered body is 95% or less. Since the pre-sintered body with a relative density of 95% or less has open pores and communicating holes on one surface, it is essential to encapsulate it in a glass capsule, and then sinter it using an S hot isostatic press. However, high-density composite ceramic products cannot be obtained.

[Example] The present inventors produced silicon nitride fine powder (trade name:
A slurry with a viscosity of 3.0 poise or less was prepared using 5NP-85, manufactured by Nihon Heavy Chemical Industries, Ltd.) and 10% by weight of silicon carbide whiskers (trade name: Toka Whisker, manufactured by Tokai Carbon Co., Ltd.) of fine silicon nitride powder. Using this, a cylindrical green molded body having an outer diameter of 70 mm and a height of 100 mm as shown in FIG. 1(A) was formed. This green molded body was subjected to normal pressure sintering for 3 hours in a 1 atm nitrogen atmosphere furnace to form a preliminary sintered body. The relative density of this pre-sintered body was 95% or more, and it was re-sintered directly at 1600° C. and 1500 atm for 1 hour using a hot isostatic press without using a glass capsule. The performance of the green molded body, the preliminary sintered body after pressureless sintering, and the sintered product after hot isostatic pressing is shown in Examples Nos. 001 to 6 in Table 1.

As seen in Nos. 1 to 6 in Table 1, according to the method of the present invention, the relative density of the green molded product is 60% or more and is dense, and the whiskers are formed over the entire wall thickness of the cylindrical green molded product. The particles were randomly oriented in two dimensions and approximately parallel to the outer surface of the molded body. The pre-sintered body produced using this green compact was dense and had excellent toughness, but by further sintering it using hot isostatic pressing, the bending strength and fracture toughness values were further improved. A ceramic product with extremely excellent performance was obtained.

In Table 1, No. 7 is a comparative example, and the same silicon nitride fine powder and silicon carbide whisker slurry as No. 1 to No. 6 were used, but since the viscosity of the slurry was 3.0 poise or more, Low relative density. Direct hot isostatic pressing was performed without using a glass capsule, but the relative density of the sintered product after hot isostatic pressing was low, and the bending strength and fracture toughness values were also insufficient.

In Table 1, No. 8.9 is a comparative example, in which a preliminary sintered body was manufactured by press molding and pressureless sintering using silicon nitride fine powder and 10% by weight silicon carbide whiskers of silicon nitride fine powder. This is an example. Also in this case, the relative density of the preliminary sintered body is small. Although hot isostatic pressing was performed directly without using a glass capsule, the relative density of the sintered product after hot isostatic pressing was low;
The bending strength and fracture toughness values were also insufficient.

In Table 1, No. 0 is an example containing no silicon carbide whiskers, and a slurry was formed only with fine powder of silicon nitride.

A pre-sintered body without composite silicon carbide whiskers has a high relative density. Although hot isostatic pressing was performed directly without using a glass capsule, the relative density of the sintered product after hot isostatic pressing was sufficiently high. However, since this sintered product did not contain silicon carbide whiskers, its bending strength and fracture toughness were insufficient compared to Examples Nos. 1 to 6 of the present invention.

[Effects of the Invention] The whisker composite ceramic product manufactured by the method of the present invention has the whiskers arranged in a specific direction, so it is more resistant to thermal shock than the conventional whisker composite ceramic product.

Since the ceramic product of the present invention is sintered by hot isostatic pressing, it is a high toughness and high strength ceramic product with a high Weibull coefficient.

Since the present invention uses a pre-sintered body produced by the slurry molding method and the pressureless sintering method, ceramic products with complicated shapes can be produced by hot isostatic pressing.

In the present invention, the pre-sintered body is sintered directly by hot isostatic pressing without using a glass capsule or the like, so the conventional complicated manufacturing process of encapsulating the pre-sintered body under reduced pressure in a glass capsule is simplified. , manufacturing cost is also low.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating the orientation of whiskers in a green molded body or ceramic product containing whiskers.

Claims (1)

[Claims] A first step of dispersing ceramic powder and silicon carbide whiskers and/or silicon nitride whiskers in a solvent to produce a slurry with a viscosity of 3.0 poise or less, and using the slurry to reduce the relative density by a slurry molding method. a second step of forming a green molded body of 60% or more; a third step of producing a pre-sintered body with a relative density of 95% or more by sintering the green molded body under pressure; and a third step of producing a pre-sintered body with a relative density of 95% or more; and a fourth step of sintering by hot isostatic pressing without a glass capsule.