RU2641358C2 - Method of obtaining technological trainings of ceramic articles from silicon nitride - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: method involves preparing the charge by mixing in a disk mill silicon nitride with additions of yttrium oxide and aluminium oxide at a ratio of 3:5 oxides. The total amount of yttrium oxide and aluminium oxide is 15 wt % of the total amount of the charge. The obtained batch is subjected to cold isostatic pressing at a pressure of 200 MPa in silicone elastic moulds with an exposure time of 90 s. Sintering of the preform is carried out in a nitrogen atmosphere at a heating rate of 525°C/h and further exposure for 1 hour at a sintering temperature of 1650°C.EFFECT: method allows you to obtain blanks of complex shape and large size, it is possible to achieve the planned indicators.3 dwg, 1 ex

Description

The invention relates to the field of powder metallurgy, namely the production of products from materials based on silicon nitride. In the future, such products can be widely used in the aviation and space industries, as well as rocket science and other branches of modern technology and industry.

To date, there are a number of technical solutions that allow to obtain ceramic materials and products based on silicon nitride powders. A known method of manufacturing ceramic products based on refractory nitrides by the method of self-propagating high temperature synthesis (SHS) in order to obtain products with low porosity, uniform chemical composition and high strength (Patent RU 1720258, C04B 35/58, published 05/10/1995). This method involves the preparation of an exothermic powder mixture containing one component from the group: Group III-VIII metal of the periodic system of elements, boron, silicon, carbon, as well as one inorganic compound from the group: boride, silicide, transition metal nitride of groups IVB-VIB; carbide, nitride, oxide, silicon; nitride, alumina; boron nitride, oxide of an element of group II - IV. A product blank is formed from the resulting mixture, which is then subjected to heat treatment in a combustion mode in a nitrogen-containing gas medium with temperature control from 2000 to 3000 ° C and pressure from 0.1 to 1000 MPa. The porosity of the obtained products is 1 - 10%, strength 25-40 kg / mm ² This type of material is used to obtain heat-resistant dielectric products. The disadvantages of this method include the need for heat treatment at very high temperatures, the presence of porosity of the obtained workpieces, as well as low bending strength.

A known method of producing products from refractory compounds using the products of the SHS process (Patent RU 2028997, C04B 41/87, published 02.20.1995). This method includes the following steps: pressing the silicon nitride powder, impregnating the pressing with sintering activating oxides, and sintering in a nitrogen medium. It should be noted that the preform is placed over a layer of an aluminum-containing exothermic mixture, the mixture is ignited, and the impregnation is carried out by condensation of aluminum oxide from the gaseous products formed during combustion in the pores of the compact. According to the described technology, a product is obtained from silicon nitride with a density after sintering of 2.47-2.49 g / cm ³ and a shrinkage during sintering of 22.5-22.9%. The disadvantages of the method include the impossibility of obtaining ceramic workpieces with high physical and mechanical properties, as well as the lack of information about the dielectric characteristics of the material obtained by this technology, the heterogeneity of the impregnation, the low density value, as well as the high shrinkage rate of the obtained product.

A known method of manufacturing sintered products based on silicon nitride (Patent RU 2458023, C04B 35/591, published 10.08.2012), including the preparation of the initial mixture containing powders of silicon nitride and technological additive, molding the product, placing it in the reactor, filling the product with an exothermic layer a mixture containing silicon nitride in an amount of 45.0-65.0 wt.% and silicon in an amount of 35.0-55.0 wt.%, the initiation of combustion and heat treatment in the combustion mode under nitrogen pressure from 5 to 35 MPa. As a technological additive in this method, Al ₂ O ₃ , Y ₂ O ₃ , mullite, yttrium-aluminum garnet, calcium aluminate, yttrium aluminosilicate in an amount of from 5 to 15 wt.% Are used. Silicon nitride powders obtained by plasma chemical, pyrochemical, or CB synthesis with a specific surface area of at least 5 m ² / g are used in the charge. No more than 30 wt.% Silicon powder can be added to the initial charge. The exothermic mixture for filling may additionally contain a metal oxide of group II-III, mainly MgO, Y ₂ O ₃ , Al ₂ O ₃ , in an amount of not more than 15 wt.%. Using the described technology, products with increased mechanical bending strength (150-600 MPa) and density (85-97% of theoretical) are obtained. The proposed technology allows to obtain products based on silicon nitride, which can be used by aviation. The disadvantage is the need to use high temperatures of about 1900 ° C, as well as the use of an exothermic mixture for backfilling sintered products.

The known method (patent RU 2443659, C04B 35/593, published 2012.27.02), which consists in preparing the mixture by mixing silicon nitride with additives of at least magnesium oxide and yttrium oxide, together with mechanical activation, adding organic plasticizer to the mixture and molding blanks for hot pressing. Hot pressing is carried out at a temperature of 1600-1700 ° C, a pressing pressure of 25-35 MPa, exposure for 10-20 minutes. Mechanical activation is carried out in a planetary centrifugal mill with a centrifugal acceleration of 10-20 g for 10-20 minutes and the ratio of the mass of the balls to the mass of the mixture from 6: 1 to 12: 1, while the balls and the drum of the planetary mill are made of ceramic material. This invention allows to obtain ceramics with a homogeneous fine-grained structure while reducing the time of the technological cycle and lowering the temperature of hot pressing. The disadvantage of the proposed method is the complexity and duration of the process.

A known method of producing ceramic material from silicon nitride (patent BY No. 11890 dated 04/30/2009), including molding an initial preform from a powder of silicon nitride, compressing it to the required pressure, heating to sintering temperature, sintering under pressure, turning off heating, cooling the sample and reducing pressure to atmospheric. The described technology allows to obtain a material with a relative density of 90-100% and crack resistance of 3.1-5.4 MPa · m ^1/2 . The disadvantages of this method are the need to use high temperatures of 1750-2200ºС, restrictions on the shape and size of the final products in the form of tablets of small diameter 11 mm and a height of 5 mm. In addition, the dielectric properties of the finished material have not been established.

A known method of obtaining a shell of an antenna cone from a reactively-bound silicon nitride according to the patent of the Russian Federation 2453520 (C04B 35/591, C04B 41/84, published 06/20/2012), comprising preparing a thermoplastic slip from silicon particles with a nitriding accelerator (Fe ₂ O ₃ , Ni or NiO) in an amount of 0.1-3 wt.% with an average particle size of the powder mixture of 1.5-3 microns, molding the billet by hot injection molding into a thermostatic injection mold with two independent heating and cooling loops of the punch and die, nitriding the billet and satellite samples in an electric vacuum furnace in the temperature range 1000-1500 ° C and at a nitrogen pressure of 1.25 atm for 70 hours in a bed of silicon nitride on the outer surface. The material obtained in this way is used to produce high-quality large-sized shells of silicon nitride antenna fairings with a set of properties required for radiolucent materials and products: high strength, close to zero porosity, dielectric characteristics stable over a wide temperature range, including low dielectric losses. The tangent of the loss angle of the resulting material is 2.0 · 10 ^-3 .

The disadvantage is a rather complex and lengthy process and a long process of nitriding.

A known method of manufacturing products of silicon nitride according to patent RU 2540674 (C04B 35/587, C04B 35/626, published 02/10/2015). The invention relates to the production of heat-resistant products from ceramic materials, which may have an electrical purpose. The technical result of this invention is the improvement of physical and mechanical properties of products and the possibility of manufacturing products of complex shape. A composition of silicon nitride powder with additives of metal oxides from the group: Al ₂ O ₃ , Y ₂ O ₃ , MgO, taken in amounts of Al ₂ O ₃ 2-6 wt.%, Y ₂ O ₃ not more than 9 wt.%, MgO not more than 6 wt.% by weight of the charge (the total amount of additives is not more than 17% by weight of the charge) is subjected to mechanical activation in ethanol, the mixture is dried, cold uniaxial pressing is carried out in a closed mold at a pressure of 50-100 MPa. Then, the obtained preforms are annealed in air in a bed of silicon nitride at a temperature of 600 ° С and the blanks are sintered in a bed of silicon nitride at 1700-1750 ° С and a heating rate of 250 deg / h. The resulting products, as follows from the above examples, have high strength (for example, for a composition containing 87 wt.% Si ₃ N ₄ +7 wt.% Y ₂ O ₃ +6 wt.% Al ₂ O ₃ , the bending strength is 2014 MPA), with a relative density of 97%, low electrical conductivity - not more than 10-12 Ohm ^-1 cm ^-1 . The disadvantage of the described method is the probability of anisotropy of the properties of the obtained material in connection with the use of uniaxial pressing technology and the need for additional annealing, as well as the absence of a description of dielectric characteristics such as the dielectric loss tangent and permittivity for the material obtained by this technology.

Thus, expensive commercial methods are known from the prior art for producing ceramic products based on silicon nitride powders, such as hot isostatic pressing of ISU and spark plasma sintering (SPS-sparkplasmasintering), which allows to obtain a product with high physicomechanical properties and high dielectric properties, which prevents the use of the obtained materials for radio engineering purposes, in addition, the use of these methods is associated with significant limitations in form and size a measure of the final products. Reaction bonding methods make it possible to obtain a material with acceptable dielectric properties, but relatively low mechanical indicators.

Closest to the claimed invention is a method that describes the preparation of a ceramic material based on silicon nitride with the addition of aluminum and magnesium oxides in a total amount of 10% of the total composition of the mixture, mixing in a disk mill, dry pressing and sintering of the mixture at temperatures of 1550-1650 ° C. The resulting material has a dielectric constant of 8.5, a dielectric loss of 0.003 at a maximum density in the range of 3.16 g / cm ³ and with a flexural strength of about 500 MPa. (online source http://www.sciencedirect.com/science/article/pii/0025541685902368).

The disadvantage is a sufficiently high dielectric constant and dielectric loss.

The challenge facing the invention is to create a method for producing a ceramic material based on silicon nitride free from the disadvantages of the prototype.

Technical result:

- obtaining blanks of complex shape and large size through the use of cold isostatic pressing;

- achievement of the dielectric constant of not more than 7.1 at a density of products not less than 2.97 g / cm ³ , the tangent of the dielectric loss angle of not more than 1.4 · 10 ^-3 , the ultimate strength in bending is not lower than 265 MPa, the ultimate strength is compression not lower than 2115 MPa, microhardness not lower than 1375 HV, fracture toughness not lower than 6.0 MPa · m ^1/2 due to optimization of the sintering regime.

The solution to this problem is achieved by the proposed method, including the preparation of the mixture by mixing in a disk mill silicon nitride with the addition of aluminum oxides, dry pressing and sintering of the workpiece at a temperature of 1650 ° C, which introduced the following new features:

- yttrium oxide is additionally introduced into the composition of the charge at a ratio of 3: 5 to alumina, while in the total amount, yttrium oxide and alumina comprise 15% of the total amount of charge,

- mixing is carried out for 20 minutes at a speed of 250 rpm,

- after mixing, the resulting mixture is compacted in silicone elastic molds by cold isostatic pressing at a pressure of 200 MPa, the exposure time under pressure is 90 seconds,

- sintering is carried out in a nitrogen atmosphere at a pressure of 1 atm, the heating rate of 525 ° C / hour and further exposure for 1 hour at a temperature of 1650 ° C.

The application of the cold isostatic pressing method for compacting the prepared mixture makes it possible to obtain a workpiece of complex shape with a uniform distribution of properties throughout the volume. Sintering the product in a nitrogen atmosphere at a temperature of 1650 ° C and holding for an hour allows to obtain a sufficiently high strength characteristics in combination with low dielectric properties.

The proposed combination of composition and modes of obtaining ceramic material based on silicon nitride with the claimed properties is not identified from the prior art, which confirms compliance with the condition of "novelty" and inventive step.

The invention is illustrated by graphic materials.

In FIG. Figure 1 shows a scanning electron microscope image of the structure of a ceramic material based on silicon nitride with an average structural size of 300 nm.

In FIG. 2, the table shows the results of mechanical and dielectric tests of samples of the obtained ceramic material based on silicon nitride.

In FIG. Figure 3 shows the dynamics of the thermal expansion coefficient of the obtained ceramic material based on silicon nitride on temperature.

The method is as follows.

As a raw material for the preparation of a ceramic material based on silicon nitride, silicon nitride Si ₃ N ₄ powder is used. The composition of the charge includes silicon nitride powder and oxide additives: aluminum oxide and yttrium oxide at a ratio of 3: 5 in a total amount of not more than 15%. The resulting mixture is compacted by cold isostatic pressing. Sintering is carried out in a nitrogen atmosphere at a pressure of 1 atm with a heating rate of 525 ° C / hour and further exposure for 1 hour at a temperature of 1650 ° C.

A specific example of the method

For the manufacture of workpieces, parts for electrical purposes (insulator cap) were prepared with a total weight of 50 g, which included 42.5 g of German silicon nitride powder (Stark, GradeM11) obtained by furnace synthesis, with the addition of 4.68 g of nanosized Al ₂ powder O ₃ (A16 SG) and 2.81 g of submicron powder Y ₂ O ₃ (Stark, GradeB) (ratio of Y ₂ O ₃ to Al ₂ O ₃ 3: 5). The total content of oxide additives does not exceed 15%. Powder mixing was performed in a RetschRS-200 disc mill for 20 minutes at a speed of 250 rpm. The resulting mixture was compacted in elastic silicone molds in an isostatic press of the company EPSICIP 400 - 200 * 1000 Y at room temperature and a pressure of 200 MPa for 90 sec. Sintering was carried out in a high-temperature furnace NaberthermVHT 8/22-GR in a nitrogen atmosphere at 1 atm and the following temperature conditions:

1. Heating up to 1650ºС in a nitrogen atmosphere, heating rate 525ºС / hour,

2. Exposure at 1650ºC in an atmosphere of nitrogen, duration 1 hour,

3. Cooling with the oven

The workpiece was cut into 8 samples, each of which was previously mechanically processed on a grinding machine, after which tests of physical and mechanical properties were carried out, as well as studies of the structure of the obtained material. The phase and structural composition of the silicon nitride-based ceramic material was monitored using a Quanta 600 FEG scanning electron microscope equipped with an EDAX energy dispersion analysis attachment and a JEOL JEM-210 transmission electron microscope. Volumetric and pycnometric density of sintered samples of the obtained ceramic composite material based on silicon nitride were determined by weighing and helium pycnometry. For this purpose, a Micromeritics AccuPyc 1340 helium pycnometer was used. To assess the mechanical characteristics, compression and bending tests were performed according to GOST 473.6-81 and GOST 473.8-81. Mechanical tests were carried out on an Instron universal electromechanical testing machine model 300LX-B1-C3-J1C. Microhardness was determined using an Affri DM8 micro-Vickers hardness tester according to GOST 9450-76. Crack resistance was measured by indentation. The linear expansion coefficient of samples of a ceramic material based on silicon nitride was measured using a Netzsch DIL 402 C / 4 / G dilatomer. Analyzing the data given in the table in figure 2, we can conclude that the obtained material is characterized by high isotropic properties in terms of volume, which is confirmed by a narrow range of values of the measured mechanical characteristics.

According to the test results, the following indicators were obtained (figures 1-3)

- fine-grained homogeneous structure,

- the density of articles made of ceramic composite material based on silicon nitride is 2.97 g / cm ³ ,

- microhardness not lower than 1375 HV,

- compressive strength σ _n not lower than 2115 MPa,

- ultimate strength in bending σ _and not lower than 265 MPa,

- dielectric loss - 1.4 · 10 ^-3 ,

- dielectric constant ε -7,05,

the coefficient of linear expansion is 3 · 10 ^-6 K ^-1 .

In addition, fracture toughness values of 6.0 MPa · m ^1/2 and water absorption coefficient <0.1 were measured.

Thus, the problem facing the invention is solved. The proposed method allows to obtain a material with a sufficiently high strength characteristics in combination with low dielectric properties: dielectric constant and dielectric loss is lower than that of the prototype.

Claims (1)

A method of obtaining technological blanks of ceramic products from silicon nitride, including the preparation of a mixture by mixing in a disk mill silicon nitride with the addition of aluminum oxides, dry pressing and sintering the billet in a nitrogen atmosphere at a temperature of 1650 ° C, characterized in that yttrium oxide is additionally introduced into the mixture at a ratio of 3: 5 to alumina, while the total amount of yttrium oxide and alumina is 15 wt.% of the total amount of charge, after mixing, compacting is carried out Acquiring charge in elastic silicone molds by cold isostatic pressing at a pressure of 200 MPa, holding time 90 seconds under pressure, and sintering produce a heating rate of 525 ° C / hour and further aging for 1 hour at the sintering temperature.

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