RU2625845C1 - Method for silicon carbide ceramic material production - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: implementation of the method for silicon carbide ceramic material production, comprises batch preparation from a mixture of silicon carbide micropowders and sintering activator oxides, moulding of the material preform, preform drying and subsequent sintering, the micropowders mixture is obtained by grinding of a silicon carbide particles and activator oxides mixture with particles larger than 5 microns on a rotary abrading mill where grinding inserts and working chamber lining are made from a ceramic composite material comprising vol. %: diamond - 20-75, silicon carbide - 20-75, silica - 3-40.

EFFECT: increased grinding efficiencyand increased chemical purity of the obtained ceramic material.

2 cl

Description

The invention relates to the technology of ceramic materials, and in particular to methods for producing silicon carbide ceramic materials operating at high temperatures in aggressive and abrasive environments, as well as in conditions of shock-dynamic impact.

Ceramic materials based on carbides have a useful combination of mechanical and physicochemical properties and are widely used in engineering. A special place among carbide materials is occupied by silicon carbide ceramics, which have a low specific gravity, good mechanical properties, high hardness and wear resistance, low temperature coefficient of linear expansion (TEC), and stability in air at high temperatures. Along with this, silicon carbide ceramics also have significant advantages in specific mechanical characteristics.

Silicon carbide, by virtue of its chemical structure, determined by covalent bonds between silicon and carbon atoms, refers to difficult to sinter materials. Therefore, the preparation of silicon carbide ceramics by sintering requires the use of high purity silicon carbide micropowders. To reduce the sintering temperature, various activator additives are used. Such activators may be various oxides.

As the closest analogue of the proposed technical solution, the authors chose a method of manufacturing a ceramic material described in RF patent No. 2402507 (CL 04B 35/565, priority June 24, 2008). The method includes the preparation of micropowders (the preferred particle size is less than 1 μm) of silicon carbide, oxides of magnesium, aluminum, yttrium, a homogeneous charge, forming the workpiece, drying and sintering at a temperature above 1840 ° C.

The disadvantage of this method is the high complexity of the stage of obtaining a homogeneous mixture of micropowders, as well as significant contamination of the resulting mixtures of micropowders using known mixing methods.

The technical task of the invention is to increase the efficiency of obtaining silicon carbide ceramic material by reducing the complexity of preparing the mixture and increasing the chemical purity of the resulting ceramic material.

The problem is solved in the present invention by the fact that when implementing the method of manufacturing silicon carbide ceramic material, including the preparation of a mixture from a mixture of silicon carbide micropowders and sinter activating oxides, molding a material preform, drying the preform and its subsequent sintering, a micropowder mixture is obtained by grinding a mixture of silicon carbide particles and activator oxides larger than 5 μm in size on a rotary abrasive mill, in which the grinding inserts and the lining of the working chamber are made of core Amicheskogo composite material containing diamond - 20-75 vol.%, silicon carbide - 20-75 vol.%, silicon - 3-40 vol.%.

The use of a lining and grinding inserts of composite material with a diamond content of less than 20 vol.% And with a silicon content of more than 40 vol.% Is impractical because materials have low wear resistance, not providing an effective resource of the mill. The manufacture of lining and grinding inserts from a material with a diamond content of more than 75 vol.% And a silicon content of less than 3 vol.% Is impractical because obtaining such materials is technologically very difficult.

The use of silicon carbide particles and activator oxides of size less than 5 μm for grinding does not allow preparing a homogeneous preliminary mixture by mixing the initial powders for its feeding by the mill feeder, which also leads to heterogeneity of the composition of the obtained micropowder mixture. Preferred is the size of the source of crushed particles of 20-100 microns.

The invention consists in the following.

The manufacture of ceramic silicon carbide material based on silicon carbide and sinter activating oxides involves several successive stages, namely: preparation of a mixture of powders of silicon carbide and sinter activator oxides, preparation of a mixture from a mixture of powders, preparation of billets by molding a mixture, high-temperature sintering.

The first stage is the preparation of a mixture of silicon carbide micropowders and sintering activator oxides, such as alumina, yttrium oxide, magnesium oxide, and others. The preparation of a homogeneous mixture is hindered by the fact that mixing micron-sized powders is difficult due to their aggregation. Many hours of mixing in mixers leads, as a rule, to contamination of the mixture of micropowders with the material of the lining of the mixers due to the high abrasion of silicon carbide and oxides, which reduces the quality of the obtained silicon carbide ceramic material.

Obtaining a mixture of micropowders of silicon carbide and sinter activating oxides in the proposed technical solution is carried out by grinding a mixture of larger particles of the mixture components in a rotary grinding mill. The preparation of a homogeneous mixture from relatively large powders, for example, 50 microns in size, is technologically simple in “drunk barrel” mixers or vibrating mixers lined with polymeric materials.

The design of the rotor-abrasive mill provides the grinding of powders of materials by combining the simultaneous effect of crushing and shear forces on the crushed particles. The impact on the particles of the material of shear forces can significantly increase the grinding efficiency of solid materials. This is due to the mechanical properties of solid materials: high compressive strength combined with relatively low tensile and shear strength. The mill used has a vertical closed housing with nozzles for input and output of products. The input pipe of the product (the initial ground powders) is located in the upper part of the housing, for example, on its upper cover. The outlet pipe of output (crushed material) is located in the lower part of the body - on the lower cover. In the middle of the case is a cylindrical working chamber. The working chamber has a cooling water jacket, necessary to maintain the temperature in the working chamber and to remove excess heat generated during the grinding of materials. The inner surface of the working chamber has a lining made of a ceramic composite material diamond - silicon carbide - silicon.

A rotor having a shaft is installed in the mill body. The rotor shaft is installed along the axis of the working chamber, passes through the top cover of the housing and is connected to a drive that provides rotation of the shaft during operation of the mill. In the area of the working chamber on the shaft, perpendicular to it, supporting disks are installed, the diameter of which is smaller than the inner diameter of the lining. Several support discs can be installed on the shaft, but not less than two. The distance between the discs may be the same or different depending on the design of the mill. In the outer zone of the supporting discs, perpendicular to it, the axis of rotation of the grinding elements is installed. These axes can be made, for example, as cylindrical shafts mounted between two adjacent support disks and fixed in them. The number of axles installed between the two discs depends on the design of the mill, but they cannot be less than two. In this case, it is advisable to symmetrically set the axes in a circle, because this provides the best balance of rotor rotation during mill operation.

Grinding elements are installed on the axes of rotation. On one axis of rotation, in height, several grinding elements can be installed. The grinding elements have a housing made of metal, in which a grinding insert of a ceramic composite material diamond - silicon carbide - silicon is fixed. The grinding elements are mounted on the axes of rotation so that the center of gravity of the element does not coincide with the axis of rotation, and when the rotor rotates, the grinding element is rotated on the axis of rotation and the grinding insert is pressed against the surface of the lining. The grinding insert made of a ceramic composite diamond - silicon carbide - silicon and fixed in the housing of the grinding element, provides the necessary wear resistance of the element.

Obtaining a mixture of micropowders is as follows. Turn on a rotary abrasive mill. In this case, the mill drive provides the rotation of the rotor. Grinding elements in which the center of gravity is displaced relative to the axis of their rotation, due to the centrifugal force acting on them, rotate on the axis of rotation, thereby moving the grinding insert to the surface of the lining. Grinding inserts are pressed to the surface of the lining and slide on it with the angular velocity of rotation of the rotor.

A pre-prepared mixture of starting powders of silicon carbide and sinter activating oxides is fed through an inlet pipe located above the working chamber. Sinking down due to their own weight into the working chamber, the powder particles are discarded by the rotor on the surface of the lining and are subjected to the abrasive effect of grinding inserts moving along the surface of the lining, thereby causing the destruction of particles, grinding of the material with simultaneous mixing of the crushed components. The increase in peripheral speed, the rotation of the rotor increases both the linear speed of the grinding insert on the surface of the lining, and the pressing force of the grinding insert to the surface of the lining. Thereby, the degree of grinding of the starting materials is controlled. If several grinding elements are installed along the height of the mill, on the axis of rotation, in the zone of the working chamber, then the crushed material after exposure to the upper grinding elements under its own weight falls into the zone of influence of the next highest elements on it. Thereby, an increase in the degree of grinding of powdered materials and their mixing is achieved. Then the resulting mixture under its own weight is discharged from the mill through the outlet pipe located below the working chamber.

The use of a lining and grinding inserts made of a ceramic composite material diamond - silicon carbide - silicon in the mill design allows its efficient use to obtain a mixture of silicon carbide micropowders with sinter activating oxides. This is due to the fact that the ceramic composite used in the construction of the mill has a high hardness (up to 55 GPa), which is higher than the hardness of the milled and mixed materials, for example silicon carbide and alumina, having a lower hardness than the composite (23 and 20 GPa, respectively ) The high hardness of the composite and the linings and grinding inserts made from it determines the very low wear of these elements of the mill, which not only ensures the efficient use of the grinding device without replacing the corresponding elements, but also allows us to obtain pure mixtures of ground powders that are not contaminated with “grinding” the details of the grinding devices. The absence of impurities in the mixture of micropowders, primarily iron, is a necessary requirement for the preparation of ceramic silicon carbide materials, since such impurities can interact with silicon carbide at the stage of high-temperature sintering with the formation of silicides and carbides, which significantly degrade the quality of ceramics.

The resulting mixture of silicon carbide micropowders with activating oxides is used to prepare a batch for molding, for example, by moistening the powders or adding a temporary binder to the powders, ensuring the formability of the batch and preserving the shape of the molded preform. Such a binder may be an aqueous solution of polyvinylpyrrolidone.

The mixture is molded into a workpiece of the required size, for example, in metal molds at a pressure of 100 MPa. The workpiece is dried in air at a temperature of 120 ° C.

Sintering blanks to obtain silicon carbide ceramic material is carried out in a protective gas environment - argon, carbon monoxide at a temperature of 1800-1950 ° C. During sintering, the activator oxides are melted, the surface of silicon carbide microparticles is melt wetted, and the silicon carbide particles are subsequently sintered by the liquid-phase sintering mechanism. The processes that occur develop more evenly throughout the entire volume of the preform if the preform is made from a homogeneous mixture of micropowders (preferably less than 1 micron in size) of all components. This is exactly what the proposed technical solution is aimed at.

The proposed technical solution is illustrated by example.

Example. The initial powders of silicon carbide with a particle size of 50-80 microns, alumina (particle size 20-40 microns), yttrium oxide (particle size 30-40 microns) are mixed in a drum mixer in the ratio of silicon carbide - 85 wt.%, Alumina - 9 wt.%, Yttrium oxide - 6 wt.%. Mixing is carried out for 30 minutes The resulting mixture is fed using a feeder into a rotary abrasive mill with a working chamber diameter of 105 mm, and a motor power for rotor rotation of 2 kW. The surface of the working chamber is made of a ceramic composite material having the composition: diamond - 42 vol.%, Silicon carbide - 50 vol.%, Silicon - 8 vol.%. The mill has 36 grinding elements with inserts from a composite of the same composition. The specific consumption of the supplied powders is 0.3 kg / h. During the operation of the mill, powders of silicon carbide, aluminum oxide and yttrium oxide are simultaneously crushed and mixed. The average particle size of the resulting mixture of micropowders is 0.8 microns. The iron content in micropowders is less than 0.04%. The mixture is moistened with a 6 wt.% 10% solution of polyvinylpyrrolidone in water, mixed and rubbed through a sieve with a mesh size of 400 microns. The mixture is molded in metal forms into a workpiece with a diameter of 63 mm and a height of 8 mm at a molding pressure of 120 MPa. The molded preform is dried at 120 ° C. for 1 hour. The preform is placed in a high-temperature furnace and in an argon medium, the preform is heated to a temperature of 1880 ° C and held for 1 hour. Then the workpiece is cooled with the furnace. As a result of the implementation of the method receive ceramic silicon carbide material. The density of the material is 3.20 g / cm ³ , the three-point bending strength (beam 5 × 5 × 50 mm based on the 40 mm test) is 480 MPa, the elastic modulus (dynamic method) is 410 GPa.

Thus, the proposed method provides the production of silicon carbide ceramic material from uncontaminated impurities of micropowders and reduces the complexity of the process at the stage of obtaining a mixture of micropowders by combining the grinding and mixing processes in one technological stage.

Claims (2)

1. A method of manufacturing a silicon carbide ceramic material, comprising preparing a mixture from a mixture of silicon carbide micropowders and activator oxides, forming a material preform, drying the preform and its subsequent high-temperature sintering, characterized in that the mixture of micropowders is obtained by grinding a mixture of silicon carbide particles and activator oxides in size more than 5 microns in a rotary abrasive mill, in which the grinding inserts and the lining of the working chamber are made of ceramic composite material containing solid diamond - 20-75 vol.%, silicon carbide - 20-75 vol.%, silicon - 3-40 vol.%. 2. The method according to p. 1, characterized in that the particle size of silicon carbide and oxides of activators in the crushed mixture of 20-100 microns.