RU2443659C1 - Manufacturing method of items from ceramic material by hot pressing based on silicon nitride - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: manufacturing method of items from material on the basis of silicon nitride consists in preparation of the charge by mixing silicon nitride with additives at least of magnesium oxide and yttrium oxide, together with mechanical activation, addition to the charge of organic plasticising agent and formation of workpieces for hot pressing. Hot pressing is performed at temperature of 1600-1700°C, pressing pressure of 25-35 MPa, and exposure during 10-20 minutes. Mechanical activation is performed in planetary centrifugal mill at centrifugal acceleration 10-20 g during 10-20 minutes and ratio of the mass of balls to mass of mixture of 6:1 to 12:1; at that, balls and drum of planetary mill are made from ceramic material.

EFFECT: obtaining ceramic material with homogeneous fine-grain structure at shortening the time of the process cycle and reducing the hot pressing temperature.

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Description

The invention relates to powder metallurgy and can be used for the manufacture of ceramic structural parts, for example, in the manufacture of engine parts and rotors with high rotation speed, in the production and modernization of centrifuges, gas turbine engines, molds for casting heat-resistant glasses, bearing cages, etc. .

Known patent RU 2010783, С04В 35/58, publ. 04/15/94., B.I. No. 7 "The mixture for the manufacture of ceramic material" containing boron nitride 10-60 wt.% And yttrium oxide introduced into the mixture as part of an ultrafine plasma-chemical powder composition (silicon nitride - yttrium oxide) 40-90 wt.%, While yttrium oxide 12-18 wt.%, Silicon nitride 82-88 wt.%. Mixing and grinding of the starting components is carried out in a ball mill for 100 hours. The mixture is wiped through a 063 sieve, the bulk density is measured, the required amount is weighed and preliminary pressing is carried out in a metal mold. The resulting briquette is hot pressed in a graphite mold at 1700 ° C for 2 hours. The finished ceramic material has a low hardness of 60-65 HRc and insufficient mechanical strength of 55-60 kgf / mm ² , which limits its use in highly loaded friction units.

There is also a known method of manufacturing a high-strength material based on silicon nitride with additives up to 10% MgO and up to 10% Y ₂ O ₃ , used for the manufacture of cutting tools (US patent No. 4652276, B24D 3/02, publ. 24.03.87). The mixture is prepared by mixing the components in a rotary ball mill for 12 hours with WC-Co hard alloy balls. The resulting mixture was placed in graphite molds and subjected to hot pressing at a temperature of 1750 ° C, a compression pressure of 31.6 MPa and holding for 60 minutes. The density of the finished parts was 3.23 g / cm ² .

The disadvantages of the method are the length of the process of preparing the mixture, the high temperature of hot pressing and prolonged exposure at this temperature.

The closest in technical essence is a method of manufacturing products from a ceramic material based on silicon nitride, including the preparation of a mixture consisting of silicon nitride and additives (magnesium oxide, aluminum oxide, etc.), by joint grinding in a ball mill, the introduction of an organic plasticizer, molding , sintering and hot pressing at a temperature of 1650-1800 ° C (patent EP 1967503, СВВ 35/593, publ. 09/10/2008).

The disadvantages of the method are its duration due to the duration of the preparation of the mixture and the sintering and hot pressing in series, in two stages, which also increases energy consumption. In addition, the use of industrial powders without significant grinding does not allow obtaining a homogeneous microstructure of ceramics, structural inhomogeneities on the friction surface cause accelerated destruction of the counterbody.

The objective of the invention is to improve the quality of products due to the formation of a homogeneous fine-grained microstructure while reducing the time of the technological cycle and lowering the temperature of hot pressing.

When using the present invention, the following technical result is achieved:

- the density of the products 3.10-3.22 g / cm ³

- the coefficient of friction paired with steel U9A (HRc = 64) is ≤0.01

- wear after 8500 hours of continuous operation 1 · 10 ^-2 microns / km

- microhardness of the material 20-26 GPa

- compressive strength σ = _SJ 1000-1080 MPa

- ultimate strength in bending σ _bending = 700-770 MPa

- increasing labor productivity by reducing the time of the technological cycle.

To solve this problem and achieve a technical result in a method of manufacturing hot-pressed articles of ceramic material based on silicon nitride, which consists in preparing the mixture by mixing silicon nitride with additives of metal oxides, at least magnesium oxide and yttrium oxide, the introduction of an organic plasticizer, molding blanks, sintering during hot pressing,

according to the invention, the mixing of the starting components is carried out together with mechanical activation under centrifugal acceleration of 10-20g for 10-20 minutes, and hot pressing is carried out in one stage at a temperature of 1600-1700 ° C, pressing pressure of 25-35 MPa, holding for 10-20 min Mechanical activation is carried out in a planetary centrifugal mill with a ratio of the mass of the balls to the mass of the mixture from 6: 1 to 12: 1, and the balls and the drum of the planetary mill are made of ceramic material.

The advantage of the proposed method is that when applying mechanical activation by co-grinding and mixing the starting components, it is possible to obtain an active ultrafine mixture. In this case, the size of the powder particles decreases, the particles acquire an internal ultrafine structure, a large internal energy is accumulated in the crystal lattice of the material in the form of dislocation formations, vacancies, packing defects, etc. The components of powder mixtures are evenly distributed in microvolumes at the level of powder particles. This technique allowed to reduce the temperature of hot pressing (compared with the prototype) by 50-150 ° C and reduce the exposure time by 3-6 times due to the accumulation during the mechanical activation of internal energy in the processed powder. Conducting hot pressing in one stage can reduce the duration of the process and reduce energy consumption. In addition, the use of mechanical activation made it possible to obtain ceramics with a homogeneous fine-grained structure, which had a positive effect on the operational characteristics of finished ceramics.

The preliminary compacting operation (forming blanks for hot pressing) allows to reduce shrinkage during hot pressing, as a result of which the dimensions of the molds have decreased, the quality of manufacturing of parts has increased, and the service life of parts of graphite molds has increased.

Depending on the composition of the charge and the technological properties of the powders used, the modes of mechanical activation in a planetary centrifugal mill are selected: centrifugal acceleration (from 10g to 20g), processing time (from 10 to 20 min) and ball load (the ratio of the mass of balls to the mass of the powder mixture is from 6: 1 to 12: 1).

Figure 1 shows the microstructure of a material based on silicon nitride with the addition of magnesium oxide, yttrium oxide and alumina, × 1000.

Figure 2 shows the microstructure of a material based on silicon nitride with the addition of magnesium oxide and yttrium oxide, × 1000.

Figure 3 presents the test scheme of the thrust bearings of a material based on silicon nitride,

where 1 is a thrust bearing made of a silicon nitride-based material made by the claimed method;

2 - clip of aluminum alloy;

3 - vacuum oil VM-12;

4 - counterbody (needle) made of steel U9A HRc = 64.

The tests were carried out in a vacuum.

Figure 4 shows a General view of the finished thrust bearings.

The proposed method of manufacturing products from a ceramic material based on silicon nitride is implemented as follows.

Mixing of the starting components and simultaneous grinding of particles is carried out by mechanical activation in a planetary centrifugal mill with centrifugal acceleration of 10-20 g for a time from 10 minutes to 20 minutes, depending on the composition of the charge. The drum and balls are made of ceramic material, the ratio of the mass of balls to the mass of the mixture is from 6: 1 to 12: 1. Then a preliminary compaction of the powder mixture is carried out by introducing an organic plasticizer and subsequent compaction, for example, by pressing or rolling. The resulting blanks are placed in graphite molds and are hot pressed in nitrogen or in vacuum with a residual pressure of 1-10 Pa at a temperature of 1600-1700 ° C, a pressing pressure of 25 MPa to 35 MPa with isothermal exposure for 10-20 minutes .

The inventive method were made glides of materials based on silicon nitride. To prepare the mixture, powdered silicon nitride was used in accordance with TU 6-09-03-312-77; powdered magnesium oxide GOST 4526-75; powdered yttrium oxide of the It0-I brand according to TU 48-4-524-90 with a purity of 99.995%; aluminum oxide (alumina) grade G-00 according to GOST 6912-87.

Example 1

For the manufacture of ceramic thrust bearing prepared the mixture (wt.%):

silicon nitride 90 magnesium oxide 5 yttrium oxide 4,5 aluminium oxide 0.5

The mixture was prepared in a planetary centrifugal mill in a drum made of ceramic based on aluminum oxide. For joint grinding, mixing, and mechanical activation, Sialon type ceramic balls with a diameter of 10 mm were used with a mass ratio of balls: mixture = 8: 1. Mechanical activation was carried out at a centrifugal acceleration of 17 g for 20 min. An organic plasticizer (polyisobutylene P-20C in the form of a 20% solution in gasoline) was added to the resulting mixture and preliminary compacting was carried out by rolling, followed by cutting blanks for hot pressing. The blanks of the thrust bearings were hot pressed at a temperature of 1700 ° С, a specific pressing pressure of 30 MPa, and holding for 15 min.

Example 2

Analogously to example 1, a mixture of the following composition was prepared:

silicon nitride 90 magnesium oxide 5 yttrium oxide 5

Mechanical activation was carried out at a centrifugal acceleration of 20 g for 20 min with a mass ratio of "balls: mixture" = 10: 1. The blanks of the thrust bearings were hot pressed at a temperature of 1660 ° C, a specific pressing pressure of 35 MPa, and holding for 20 min.

The manufactured glides (FIG. 4) had an average density of 3.10-3.17 g / cm ³ .

Microstructural studies of the manufactured thrust bearings were performed on thin sections prepared according to the generally accepted technique in the plane of the upper end surface of the thrust bearing; photographing and image analysis were carried out using an Axiovert 200Mat microscope.

Figure 1 shows the microstructure of the thrust bearing of the composition material (wt.%): Silicon nitride 90, magnesium oxide 5, yttrium oxide 4.5, aluminum oxide 0.5, with magnification × 500 times. The microhardness of the material, determined at a load of 200 g, amounted to 1790-1950 kg / mm ² .

Figure 2 presents the microstructure of the thrust bearing from the material composition (wt.%): Silicon nitride 90, magnesium oxide 5, yttrium oxide 5, with an increase of × 1000 times. The microhardness of the material, determined at a load of 100 g, amounted to 2420-2460 kg / mm ² .

The microstructure of both materials is dense, consists of fine grains with an average grain size of the phase based on silicon nitride less than 1 μm. Separate pores are detected in the materials: in the material in Fig. 1, the size and number of pores is larger than in the material in Fig. 2.

The thrust bearings were paired with a counterbody (needle) made of U9A tool steel, hardened to a hardness of HRc = 64 with an axial load of 0.7 kgf to 1.2 kgf, and a rotation speed of 100 thousand rpm (Fig. 3). Tests for 8500 hours of continuous operation showed wear of the thrust bearing material of the order of 1 · 10 ^-2 μm / km with a friction coefficient of k _tr. no more than 0.01.

Products obtained by the claimed method have the following characteristics:

- the density of the products 3.10-3.22 g / cm ³

- microhardness of the material 2000-2600 kg / mm ²

- compressive strength σ = _SJ 1000-1080 MPa

- ultimate strength in bending σ _bending = 700-770 MPa

It should be noted that the resulting products are characterized by a homogeneous microstructure obtained by mixing with simultaneous mechanical activation. Significantly reduced the time of the technological process of obtaining the product (3-6 times compared with the prototype with the same initial components of the charge). The hot pressing temperature was 1660-1700 ° C.

Claims (3)

1. A method of manufacturing hot-pressed articles of ceramic material based on silicon nitride, which consists in preparing the mixture by mixing silicon nitride with the addition of metal oxides, at least magnesium oxide and yttrium oxide, the introduction of an organic plasticizer, the molding of blanks and sintering during hot pressing ,
characterized in that the mixing of the starting components is carried out together with mechanical activation during centrifugal acceleration of 10-20 g for 10-20 minutes, and hot pressing is carried out in one stage at a temperature of 1600-1700 ° C, pressing pressure of 25-35 MPa, holding for 10-20 minutes 2. The method according to claim 1, characterized in that the mechanical activation is carried out in a planetary centrifugal mill with a ratio of the mass of balls to the mass of the mixture from 6: 1 to 12: 1. 3. The method according to claim 2, characterized in that the balls and drum of the planetary mill are made of ceramic material.

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