RU2610744C1 - Silicon nitride based charge and method for production of articles thereof - Google Patents

Abstract

FIELD: chemisty.

SUBSTANCE: invention refers to the field of production of articles of silicon nitride based high-temperature construction materials which may be used in engine production, mechanical engineering and other high-technology industrial branches, in particular in manufacture of complex geometry parts requiring mechanical treatment, e.g., ceramic ball bearings. Silicon nitride based charge contains 10-15% wt% sintering additives and technological connective with sintering additives containing fraction of nanodispersive powders at the level 40-60%. Nanodispersive powders with specific surface of 30-50 m²/g are produced by methods of heterophase precipitation or co-precipitation. A method for production of articles using above silicon nitride based charge includes grinding of silicon nitride powders and sintering additive powders in planetary-type mill, introduction of technological connective (PVAL), pressing of blank and sintering under nitrogen pressure. The pressed blank is subject preliminarily to mechanical treatment using hard-alloy tools, then technological connective is removed in the air at temperatures of 500-1100°C, then sintering is carried out in vacuum with heating up to a temperature of 1600°C and after that in nitrogen medium under pressure of 3-4 MPa and temperature of 1800-1850°C. The final mechanical treatment is performed using suspensions based on super-hard abrasive materials.

EFFECT: production of ceramics with porosity max 0,1%, strength min 800 MPa and simplified technology for manufacture of complex geometry articles.

4 cl, 2 ex, 4 dwg

Description

The invention relates to the field of production of products from high-temperature structural materials based on silicon nitride, which can be used in engine building, mechanical engineering and other high-tech industries, in particular in the manufacture of complex parts that require machining, for example, ceramic balls of bearings.

Known mixture for the manufacture of ceramic materials (RU 2443659 С04B 35/593, publ. 02.27.2012) based on silicon nitride and sintering additives (powdered magnesium oxide GOST 4526-75; powdered yttrium oxide grade It0-I according to TU 48-4 -524-90; aluminum oxide (alumina) grade G-00 according to GOST 6912-87). The insufficient chemical activity of industrial powders of sintering additives and the low values of their contact surface after grinding - mixing in a centrifugal mill made of ceramic material (specific surface no more than 10-15 m ² / g) leads to the formation of porosity in the finished material and a decrease in its strength properties.

The manufacture of complex parts, including spherical, of silicon nitride (Si ₃ N _{4) is} associated with significant difficulties. A known method of manufacturing products of ceramic material based on silicon nitride, including joint grinding-mixing in a ball mill of powders of silicon nitride and sintering additives, the introduction of a technological binder of organic plasticizer, forming blanks, sintering and hot pressing at a temperature of 1650-1800 ° C (EP 1967503, С04B 35/593, publ. 09/10/2008). The disadvantages of the method are the duration of the operation of mixing-grinding powders in a ball mill, sintering and hot pressing in series, in two stages, which increases energy consumption. The use of industrial micron-sized powders without significant grinding causes the formation of pores in the sintered ceramic material, which causes accelerated destruction of the mating parts in the friction units.

A known method of producing products based on silicon nitride (RU 2458023 (C1), С04B 35/591, publ. 08/10/2012), which includes the preparation of a mixture based on powders of silicon nitride and the addition of sintering components, the introduction of a technological binder into the mixture, molding the product and its sintering under conditions of self-propagating high-temperature synthesis (SHS) in the backfill at a temperature of 1900-1950 ° C under a nitrogen pressure of 5 to 35 MPa. The disadvantages of the method are the heterogeneity of the material obtained by reaction sintering under SHS conditions, its high porosity (7%) and low strength (400-600 MPa).

The closest in technical essence is a method of manufacturing products from a ceramic material based on silicon nitride (RU 2443659, С04В 35/593, publ. 02/27/2012), which consists in grinding and mixing powders of silicon nitride and sintering additives of at least magnesium oxide and oxide yttrium, in a centrifugal mill made of ceramic material, introducing into the mixture a technological binder in the form of an organic plasticizer, forming blanks for hot pressing and hot pressing at a temperature of 1600-1700 ° C, press pressure Ia 25-35 MPa. The disadvantages of this method is the inability to obtain products of complex shape by hot pressing, pollution of the surface layers of products with carbon from the mold, which reduces the quality of the products. The disadvantages of the method are the low degree of grinding of industrial powders of additives, the absence of a technological operation to remove a temporary technological binder (20% solution of polyisobutylene P-20C in gasoline), which leads to the formation of pores in the finished material, which cause the destruction of mating parts in friction units at high speed rotation. Hot pressing allows to obtain blanks of products with a large allowance and a simple shape (square or cylindrical section). The high hardness of the ceramic material based on silicon nitride (20-26 GPa) causes significant difficulties to achieve the required dimensional accuracy and surface roughness of the parts of rotation. To obtain complex, for example spherical parts, it is necessary to carry out multi-stage machining, including a multi-transition process of rough grinding, medium grinding with diamond tools of various grain sizes, fine grinding and polishing with suspensions based on superhard abrasive materials.

The task of the invention is to simplify the manufacturing technology of complex products that require machining, such as ceramic balls of bearings, reducing the stages of machining.

The technical result of the invention is the production of ceramics with porosity of not more than 0.1%, strength not less than 800 MPa.

The essence of the claimed invention lies in the fact that the mixture is based on silicon nitride, sintering additives and technological bonds, and sintering additives in an amount of 5-20 wt.%. contain a fraction of nanodispersed powders in an amount of 40-60%; powders obtained by heterophase deposition or coprecipitation with a specific surface area of 30-50 m ² / g are used as nanodispersed powders.

A method of manufacturing products from a mixture based on silicon nitride, which consists in grinding and mixing powders of silicon nitride, powders of sintering additives, maintaining a binder, pressing the workpiece and sintering under nitrogen pressure, the grinding and mixing being carried out in a planetary mill, using PVA as the technological binder then the technological binder is removed in air at temperatures of 500-1100 ° C, sintering is carried out in vacuum to a temperature of 1600 ° C, then in a nitrogen medium at a pressure of 4 MPa to a temperature of 1850 ° C.

Preliminary machining of the pressed billet is carried out with a carbide tool.

The final machining is carried out with suspensions based on superhard abrasive materials.

The inventive method of manufacturing products from a ceramic material based on silicon nitride consists in grinding and mixing in a planetary mill a mixture of silicon nitride powder with micron and nanosized powders of sintering additives, followed by the introduction of a technological binder into the mixture, pressing the workpiece, pre-turning it (options), removing technological bonding, sintering, fine grinding and polishing of the surface (options).

The amount of sintering additives is 10-15 wt.%. As sintering additives, at least one additive selected from the range including: alumina, yttrium, magnesium oxide, taken separately or in mixture, is used. The ratio of fractions of micron and nanosized powders of sintering additives is from 40 to 60%. As nanodispersed powders are used powders obtained by heterophase deposition or coprecipitation with a specific surface area of 30-50 m ² / g Polyvinyl alcohol (PVA) is used as a technological bond. Pre-turning (options) of the pressed billet is carried out with a carbide tool. Technological ligament removal is carried out in air at temperatures that do not allow oxidation of silicon nitride (500-1100 ° C). Sintering is carried out in a vacuum compression furnace, first in vacuum to a temperature of 1500-1600 ° C, then in a nitrogen atmosphere at a pressure of 3-4 MPa to a temperature of 1800-1850 ° C. Fine grinding and polishing (options) of the external surface of parts is carried out with suspensions based on superhard abrasive materials.

The advantage of the proposed method is that the use of a fraction of sintering additives from nanosized powders in an amount of 40-60% dispersion of 30-50 m ² / g can significantly increase the contact surface and the chemical activity of the powder system of silicon nitride - sintering additives, which contributes to a more intensive course of heterogeneous heat and mass transfer processes at all stages of manufacture.

At the stage of pressing, the particles acquire stronger mechanical adhesion with PVA macromolecules. The strong coagulation structure of the material, in which thin bonding layers of PVA are located between the powder particles, allows turning of pressed blanks with the help of carbide tools.

The use of a fraction of sintering additives from nanodispersed powders with a dispersion of less than 30 m ² / g in an amount of less than 40% does not allow turning of pressed blanks using carbide tools because of their low mechanical strength. The use of a fraction of sintering additives from nanosized powders with a dispersion of more than 50 m ² / g in an amount of more than 60% does not allow obtaining high-quality compacts due to the formation of delamination cracks.

After turning the product, a carbon-containing organic binder is burned out at temperatures that do not allow oxidation of silicon nitride (500-1100 ° C).

Sintering in vacuum ensures the removal of gases from the pores even in the early stages of sintering, as a result of which the gas does not prevent pore overgrowth. Nanodispersed powders contribute to the acceleration of physicochemical processes of the formation of the liquid phase, the development of adhesion forces between individual particles and compaction of products during sintering. The exposure time at maximum temperature is determined by the overall dimensions of the products.

The result of the use of a fraction of nanosized powders of sintering additives is the production of a complex product from a ceramic material based on silicon nitride with a porosity of not more than 0.1% and strength up to 800 MPa.

The inventive method were made balls of materials based on silicon nitride. As micron starting components, powdered silicon nitride was used according to TU 6-09-03-312-77; powdered magnesium oxide according to GOST 4526-75; powdered yttrium oxide according to TU 48-4-524-90; aluminum oxide (alumina) grade G-00 according to GOST 6912-87. To prepare nanosized powders, we used aluminum nitrate powder according to GOST 3575-75, yttrium nitrate powder according to TU 6-09-4676-83, magnesium nitrate powder according to GOST 6203-77. The aluminum nitrate powder was dissolved in distilled water, the solution was precipitated in aqueous ammonia according to GOST 9-92, the aluminum hydroxide precipitate was filtered off and calcined at a temperature of 600 ° C to obtain a nanosized aluminum oxide powder with a specific surface area of 30-50 m ² / g. Yttrium oxide nanodispersed powder and magnesium oxide nanodispersed powder were prepared similarly.

Example 1

For the manufacture of the ball was prepared a mixture of components (wt.%):

A mixture of components was prepared in a planetary mill with grinding bodies made of yttrium oxide. After thorough grinding and mixing, a temporary technological binder (10% solution of polyvinyl alcohol) in the amount of 10% is added to the mixture, placed in a metal mold and pressed under a pressure of 120 MPa. After molding the workpiece in the form of a cylinder (Fig. 1), preliminary turning is carried out with a carbide tool to obtain a complex profile of the workpiece in the form of a ball (Fig. 2). The billet is heat-treated in air to remove the binder at a maximum temperature of 560 ° C and holding for 30 minutes. Sintering is carried out in a vacuum compression furnace, first in vacuum to a temperature of 1500-1600 ° C, then in a nitrogen atmosphere at a pressure of 3-4 MPa to a temperature of 1800-1850 ° C. Fine grinding and polishing (options) of the external surface of parts is carried out with suspensions based on superhard abrasive materials. The final grinding and polishing of the surface to obtain a given geometric accuracy and surface quality of the ball (Fig. 3) is carried out with suspensions based on superhard abrasive materials. Tests to determine the density and porosity of ceramic samples were determined according to GOST 473.4-81 by saturation of ceramic billets with water during boiling followed by hydrostatic weighing. The test results of the balls showed that the density is 3.27-3.28 g / cm ³ with a porosity of 0.05-0.08%. Tests to determine the tensile strength under static bending at a temperature of 20 ° C were carried out according to GOST 20019-74 on samples 5 × 5 × 50 mm in size. The tensile strength in bending was - 830 MPa.

Example 2 (prototype)

Analogously to example 1, a mixture of components was prepared (wt.%):

A mixture of components is prepared in a planetary mill with grinding bodies made of yttrium oxide. After thorough grinding and mixing, a temporary technological binder (10% solution of polyvinyl alcohol) in the amount of 10% is added to the mixture, placed in a metal mold and pressed plates with a pressure of 100 MPa. After molding the workpiece in the form of plates (Fig. 4). The billet is heat-treated in air to remove the binder at a maximum temperature of 600 ° C and holding for 20 minutes. Sintering is carried out in a vacuum compression furnace according to the regime, first in vacuum to a temperature of 1500 ° C, then in a nitrogen atmosphere at a pressure of 3 MPa to a temperature of 1800 ° C with holding at a maximum temperature for 1 h. The test results showed that the density is 3, 25-3.27 g / cm ³ with a porosity of 0.06-0.09%, tensile strength in bending was 800 MPa.

Thus, a mixture based on silicon nitride and a method of manufacturing products from it allows one to obtain complex products from a ceramic material based on silicon nitride with low porosity (not more than 0.1%) and high bending strength (800-830 MPa), to carry out preliminary turning of pressed moldings with carbide tools, to simplify and reduce the cost of the manufacturing technology of complex ceramic products, to simplify the machining of workpieces by eliminating coarse grinding operations, medium grinding Nia diamond tools, reduce labor intensity and reduce energy costs.

Claims (4)

1. A mixture based on silicon nitride, sintering additives and technological binder, characterized in that the sintering additives in an amount of 10-15 wt.% Contain a fraction of nanodispersed powders in an amount of 40-60 wt.%, Powders obtained by the method are used as nanodispersed powders heterophase deposition or coprecipitation with a specific surface of 30-50 m ² / g 2. A method of manufacturing products from a charge based on nitride according to claim 1, which consists in grinding and mixing powders of silicon nitride and powders of sintering additives, introducing a technological binder, pressing the workpiece and sintering under nitrogen pressure, characterized in that the grinding and mixing are carried out in a planetary a mill, PVA is used as a technological binder, the pressed billet is machined with a carbide tool, then the technological binder is removed in air at temperatures of 500-1100 ° С, sintering is carried out in a vacuum ume to a temperature of 1600 ° C, then in a nitrogen atmosphere at a pressure of 3-4 MPa to a temperature of 1800-1850 ° C. 3. The method according to p. 1, characterized in that the preliminary machining of the pressed billet is carried out with a carbide tool. 4. The method according to p. 1, characterized in that the final machining is carried out with suspensions based on superhard abrasive materials.

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