RU2761813C1 - Additive method for obtaining dimension products from conductive ceramics by spark plasma sintering - Google Patents

Abstract

FIELD: material processing.

SUBSTANCE: method relates to the technique of hot pressing with simultaneous sintering of powders and to the thermomechanical class of welding using thermal energy and pressure. An additive method for producing dimensional products from conductive materials by spark plasma sintering involves prebaking of the connected parts, placing the powder of the base material between the connected parts, while only the first connected part is presintered, and the second and subsequent parts are obtained by filling the powder of the original ceramics into a mold in which one of the punches is already sintered, followed by joint electric spark plasma sintering. Sintering is carried out in a vacuum or in a protective argon atmosphere at high temperatures released in the powder. Finished products (carbides, borides: SiC, TiC, TaC, ZrB₂, HfB₂, metals: W, Mo, Ta, Nb and their alloys, including high-entropy) are obtained by combining several successively baked parts and powder layers into one whole.

EFFECT: expansion of the range of solutions in large size equipment production.

5 cl, 6 dwg

Description

The proposed invention (hereinafter referred to as the Invention) relates to the field of production of materials with an elevated operating temperature, as well as to the field of hot pressing technology, in which simultaneously pressing and sintering of powders occurs, which makes it possible to obtain virtually non-porous products not only from metals, but also products from carbides , borides and other superhard materials; and to a thermomechanical class of welding carried out using heat energy and pressure.

The invention involves the production of finished products by combining several sequentially baked parts into one whole and can be used in the manufacture of dimensional and complex profile products. The greatest effect from the application of the invention is expected in the production of high-temperature products from oxygen-free types of ceramics, such as carbides, nitrides and borides, as well as metals W, Mo, Ta, Nb and their alloys, including high-entropy ones.

Traditional sintering methods for these types of ceramics and metals are very difficult. Therefore, they use a new, innovative method of spark plasma sintering (SPS) - first developed by the Japanese company Sumitomo Coal Mining Co., Ltd. and widespread in the USA, Germany and other developed countries. The process is based on a modified hot pressing method, in which an electric current from a special DC pulse generator is passed directly through the mold and the blank to be pressed. The duration of a single pulse is 3 ms. A sequence of 12 pulses is used, and then the current is turned off for a time equal to the duration of the two pulses.

Sintering is carried out in a vacuum or in a protective argon atmosphere at high temperatures while simultaneously applying pressure to the sample according to a uniaxial scheme, which allows very fast heating and an extremely short operating cycle. At the same time, grain growth does not occur and it opens up opportunities for creating materials with previously inaccessible compositions and nanoscale grains, which gives them unusual properties. An electric current passes directly through the sintered powder sample, which has electrical conductivity, discharges occur between the powder particles and a spark plasma arises, which contributes to their cleaning and heating, and with subsequent uniaxial compression, a workpiece of high density and strength is formed. The contacts are heated in a short period of time, so that the sintering of the sample as a whole occurs at temperatures (200 ÷ 400) ° C lower than the sintering temperatures of the same sample by conventional methods.

It is known that when performing the pressing operation, the height of the initial powder filling is several times higher than the height of the pressed product. In the case of spark plasma sintering, the filling height is approximately twice the height of the sample compacted after sintering. It is this circumstance, in view of the small volume of the working chamber of the SPS installation, that is a limitation for obtaining large-sized items of great height.

Large, complex shaped articles are typically made from simple shaped ceramic components and then assembled together to form the desired final article. Any connection is not only a critical place, in which it is required to maintain the strength equal to the base material, but also, in extreme conditions of high temperatures, close values of the coefficients of thermal expansion of the components to be joined. Thus, an ideal joint should have close values of all strength and physical and mechanical properties.

A number of methods of joining ceramic parts are known, for example, such as brazing, diffusion welding, hot pressing. A common disadvantage of all these methods is that the connection is inferior to the basic ceramics in terms of its thermomechanical characteristics.

In view of the above, there is a need for a method of joining ceramic parts using spark plasma sintering, which allows joining ceramic parts without using materials that differ in chemical composition from the composition of the base ceramic. The retention of the chemical composition in the volume of the entire product provides it with high thermomechanical characteristics that meet the requirements of new technology.

In practice, several methods are known for making such connections.

There is a method based on the use of reactive precursors as connecting materials, which, when heated, interact with each other, forming a finely dispersed, well-sintered material, chemically identical to the material of the ceramics being joined. Thus, the authors connected parts made of silicon carbide in (RU 2604530, IPC С04В 37/00, 2016) and from ZrB ₂ in works (CN 110041090, IPC С04В 37/00, 2019).

The disadvantages of this technical solution are: high requirements for non-flatness of the joined sides, the complexity of uniform placement of the precursor powder at the junction, the lack of control over the degree of conversion of the precursor into the final product, the difficulty of achieving the same density of the newly formed separating layer with that for pre-fired parts, which can lead to to the occurrence of stresses and defects at the junction.

In the method (CN 101516804, IPC S04B 37/00, 2009), it is proposed to connect the pre-fired parts directly to each other by the method of spark plasma sintering by applying high pressure at the stage of “softening” the ceramics, followed by an increase in temperature to complete the solid phase joining process.

The disadvantages of this technical solution are: the highest requirements for the non-flatness of the sides (less than 100 microns), the use of increased pressure at high temperatures, which complicates the technological equipment for carrying out spark plasma sintering, the excess of the final temperature of the connection, the sintering temperature of the joined parts made of powders, which promotes the growth crystals and deterioration of the properties of previously fired products.

An example of the implementation of the following joining method is (EN 200700052, IPC S04B 37/00, 2008), in which pre-fired parts are joined using a composite film consisting of ceramic particles of the original powder and an organic binder. The base material is used as the ceramic powder. The final bonding result is achieved by solid phase sintering with the application of pressure.

The disadvantages of the technical solution are: high requirements for the flatness of the sides, especially in cases where the thickness of the connecting film is several microns, the complexity and duration of the operation for preparing the green film, the use of additional firing in order to remove the organic component of the ceramic film.

The closest to the proposed technical solution (prototype) is a method of joining parts (patent EN No. 200700052, IPC С04В 37/00, 2008), in which pre-fired parts are joined using a composite film consisting of ceramic particles of the original powder held together by a plasticized polymer binder ...

The disadvantage of the prototype is, first of all, the presence of this film and other organic components, which include polymer binders, plasticizers, dispersants, solvents, etc., requiring their removal to obtain a strong bond of ceramic particles. In addition, the prototype has the following limitations:

1) by the amount of pressure (less than 5 MPa), so as not to destroy the film with ceramic particles in their raw state;

2) on the specific surface of ceramic particles, which should be in the range from 2 to 10 m2 / g;

3) for the average size of ceramic particles from 0.1 to 1.0 microns;

4) the maximum deviation from flatness of the sintered parts to be joined, which should be less than 500 microns;

5) for the preferred thickness of the raw joint material from 5 μm to 1 mm;

6) for the temperature of the joint, which should be less than the sintering temperature by about 100 ° C.

The proposed by us additive method for producing dimensional products from conductive ceramics by the method of spark plasma sintering is free from these disadvantages and limitations.

The development of the proposed additive method is aimed at solving a technical problem: to provide a method for the formation of a connection between the first sintered product, from a conductive material and a powder layer of the same material (without foreign additives), which would be sintered with this connection, forming a chemically and mechanically integral whole with the first sintered product. In addition, it is desirable that no discernible interfacial layer remains after the formation of such a bond and that the resulting bilayer material exhibits comparable stability in thermodynamic cycles.

The technical result of the proposed solution is:

- building up the first pre-fired electrically conductive part, used as one of the electrodes, with a layer of ceramic particles of the initial powder, clamped by the second graphite electrode and sintered together by passing a pulsed electric current. In this case, thermal energy is released mainly in the powder layer and is defined as:

where I (τ) is the electric current that varies with time, R (τ) is the electrical resistance of the compact, that changes with time, and M is the mass of the compact.

- obtaining large-sized products of complex geometric shape from conductive ceramics with maximum use of the volume (dimensions) of the working chamber of the SPS installation.

Carbides, borides SiC, TiC, TaC, Zr ₂ B, HfB ₂ , as well as metals W, Mo, Ta, Nb and their alloys, including high-entropy ones, can be used as conductive ceramics.

In the method, the first part is obtained from the initial powder by the SPS method, it is placed in a mold and its height is increased by filling the powder with a powder similar in mass, which was used to form the first part. The powder is placed between the end face of the first sintered part and the graphite electrode, and the spark plasma sintering operation is repeated. The newly obtained part, combined from two parts, is placed in a mold, a powder of the same mass is poured, and the operation of spark plasma sintering is carried out again, etc.

In the method, the active connection of the sintered part and the original powder occurs due to the fact that the powder has a higher electrical resistance compared to the electrical resistance of the already sintered part. Therefore, it is heated at the contact boundaries of the particles to higher temperatures, and the consolidation of the activated solid and powder parts occurs as a result of uniaxial compression.

The surface of the fired part to be joined in the proposed method is activated either by a chemical method, or by sandblasting or electroerosive processing and cleaned with alcohol.

Such an additive method makes it possible to manufacture large-sized products of complex geometric shapes with the maximum possible use of the volume of the working chamber of the SPS installation with the achievement of high strength and physicochemical properties inherent in the base material of the parts to be joined.

The essence of the technical solution is illustrated by the following description and the attached figures.

FIG. 1 shows a SPS sintered zirconium diboride powder part 1 (eroded).

FIG. 2 shows zirconium diboride-based powder baked by the additive SPS method to part 1 (after EDM).

FIG. 3 shows zirconium diboride-based powder baked by the additive SPS method to part 2 (without EDM).

FIG. 4 shows the microstructure of an SPS sintered zirconium diboride powder.

FIG. 5 shows an SPS sintered part with a hemispherical surface of zirconium diboride powder.

FIG. 6 shows an article made of zirconium diboride powder, obtained from two parts (solid and porous) by the additive SPS method (no electrical discharge machining).

Achievement of the technical result, provided by an additive method for producing electrically conductive ceramic products by the method of spark plasma sintering, including preliminary firing of the parts to be joined, placing the powder of the base material between the parts to be connected, is based on the fact that upon receipt of the product, only the first part to be connected is subjected to preliminary sintering, and the second and subsequent parts are obtained by pouring the original ceramic powder into a mold, in which one of the electrodes is already a previously sintered part, followed by joint electrospark plasma sintering. The sintering time is chosen in such a way that the entire porous layer can be heated.

For multiple use of the graphite mold, it is necessary that the values of the thermal expansion coefficients of the conductive sintered material and the graphite are close to each other.

Although sparks are observed on the electrical contacts during the passage of powerful current pulses, the contact surface of the fired part with the powder is additionally activated by electro-erosion or by sandblasting and cleaned with alcohol.

Conductive parts with a convex curved surface must mate with the concave curved surface of the graphite electrode so that the thickness of the sintered powder layer between them remains the same. On the contrary, if the part has a concave surface, then the mating electrode surface must have a convex curved surface so that the thickness of the powder layer to be sintered between them remains the same.

The first part to be joined is pre-fired to a density (2 ÷ 6)% less than the final density of the finished product, depending on the required number of parts to be joined and the number of corresponding sintering.

The possibility of reproducing the present invention and implementing the method with the achievement of the specified technical result is illustrated by the following examples of specific execution of articles made of conductive ceramics by the method of spark plasma sintering.

Example 1 - joining two pieces

Detail 1

A weighed portion of a composite powder based on ZrB2 in an amount of 360 g is placed in a graphite mold with a diameter of 60 mm of a Labox 1575 spark plasma sintering unit, air is pumped out to 6 Pa, preliminary compaction is performed to a pressure of 5 MPa, heating is performed at a rate of 20 ° C / min to a temperature of 1600 ° C on the surface of the graphite mold while increasing the pressure to 30 MPa, maintain such conditions for 7 minutes, after which the installation is smoothly cooled at the same rate of 20 ° C / min to room temperature. The fired 1st part (Fig. 1) has dimensions of 30 × 60 mm and a relative density of more than 90% and an electrical conductivity close to the electrical conductivity of graphite used for making molds and electrodes.

Detail 2 (Finished product)

In a graphite mold with an inner diameter of 60 mm, a fired part 1 is placed, pretreated at the place of the future connection by the electroerosive method and purified with ethyl alcohol, 360 g of a composite powder based on ZrB2 are poured on top, placed in the Labox 1575 installation, air is evacuated to 6 Pa, preliminary compaction is performed to a pressure of 5 MPa and heating at a rate of 20 ° C / min to a temperature of 1600 ° C while increasing the pressure to 30 MPa, hold it for 9 minutes, after which the installation is cooled at the same rate of 20 ° C / min to room temperature. The finished product, consisting of two parts (Fig. 2), has dimensions of 60 × 60 mm and a relative density of more than 90%.

Example 2 - joining three pieces

Detail 1

A weighed portion of a composite powder based on ZrB2 in an amount of 360 g is placed in a graphite mold with a diameter of 60 mm of a Labox 1575 spark plasma sintering unit, air is pumped out to 6 Pa, preliminary compaction is performed to a pressure of 5 MPa, and heating is performed at a rate of 20 ° C / min to a temperature of 1600 ° With a simultaneous increase in pressure to 30 MPa, hold for 5 minutes, after which the installation is smoothly cooled to a temperature of 50 ° C. The fired 1st part (Fig. 1) has dimensions of 30 × 60 mm and a relative density of more than 90%.

Detail 2

A fired part 1 is placed in a graphite mold, pretreated in the shape of an electrode and activated by the electroerosive method and purified with alcohol at the place of the future connection, then 360 g of composite powder based on ZrB2 is poured on top and preliminary compaction is performed under a pressure of 5 MPa, air is pumped out to 6 Pa and heating is performed at a rate of 200 ° C / min to a temperature of 1600 ° C with a simultaneous increase in pressure to 30 MPa, hold for 7 minutes, after which the installation is smoothly cooled to a temperature of 50 ° C. The fired part 2 (Fig. 2), consisting of two parts, has dimensions of 60 × 60 mm and a relative density of 98%.

The resulting part 2 is processed from all sides by the electroerosive method for close contact with the surrounding graphite and activation of the surface, to which a weighed portion of a composite powder based on ZrB _{2 is} poured in an amount of 360 g. , heating is performed at a rate of 20 ° C / min to a temperature of 1600 ° C with a simultaneous increase in pressure to 30 MPa, holding is carried out for 9 minutes, after which the installation is smoothly cooled at the same rate of 20 ° C / min to room temperature. The finished product, consisting of three parts (Fig. 3), has a size of 90 × 60 mm and a relative density of more than 90%. The finished product at the joints has a homogeneous microstructure (Fig. 4), which does not differ from the microstructure of the base. When testing the strength for three-point bending, fracture occurred along the base material, and not at the joint, and the strength values (170 ÷ 200) MPa were obtained.

Thus, the proposed additive method of sequentially increasing the height of the workpiece with the initial powder and sintering the obtained layers by the SPS method makes it possible to obtain high-quality conductive ceramics with the maximum use of the height of the working chamber of the spark plasma sintering installation.

Example 3 - joining parts with a curved shape

The contact surface of the fired part with the powder can have a curved shape, for example, in the form of a hemisphere (Fig. 5).

Further additive sintering by the SPS method of such a part can be carried out both from flat and curved surfaces after their activation and cleaning. The addition of the powder layer to the flat surface of the part is similar to example 1. To add the powder layer to the convex curved surface, the powder was placed between it and a graphite electrode, the surface of which, facing the powder, had a concave curved surface. Thus, the thickness of the sintered layer of powder was kept the same between the curved surfaces of the part and the electrode. As a result of this additive SPS method, the article shown in FIG. 6.

The proposed additive method can be carried out on any known SPS equipment, which, along with the achieved positive technical and economic result, allows us to conclude that its widespread implementation is advisable.

Claims (5)

1. An additive method for producing electrically conductive ceramic products by spark plasma sintering, including obtaining the first part from a ceramic powder by spark plasma sintering, placing the resulting first part in a mold and increasing its height by filling in the powder used to form the first part, and conducting a second spark plasma sintering operations, wherein the powder is placed between the end face of the formed first part, which is used as one of the electrodes, and the second electrode made of graphite, obtaining a combined part that is a finished product, or obtaining a combined part, re-placing it in the mold and repeating the above process of building up the required number of times. 2. An additive method according to claim 1, characterized in that before the start of the joining operation, the contact surface of the fired part with the powder is activated by the electroerosive method and cleaned with alcohol. 3. The additive method according to claim 1, characterized in that before the start of the joining operation, the contact surface of the fired part with the powder is activated by sandblasting and cleaned with alcohol. 4. The additive method according to claim 1, characterized in that for a part with a convex curved surface, the mating electrode surface must have a concave curved surface so that the thickness of the powder layer between them remains the same, and vice versa, if the part has a concave surface, then the mating surface the electrode must have a convex curved surface so that the thickness of the powder layer between them remains the same. 5. The additive method according to claim 1, characterized in that the first part to be joined is subjected to preliminary firing to a density (2 ÷ 6)% less than the final density of the finished product, depending on the number of parts to be joined.

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