RU2084426C1 - Method of preparing blend for moldings containing silicon carbide - Google Patents

Abstract

FIELD: constructional materials. SUBSTANCE: method consists in mixing carbon-containing ingredient, filler, and temporary binder, the first of which being thermally expanded graphite powder which has been preliminarily modified by way of compressing into briquets with density 0.25-1.15 g/cu.cm and subsequently ground into powder. This powder is applied in amount exceeding 100/(2ρ₁/(ρ₂+1) wt % where ρ₁ is filler density and ρ₂ density of thermally expanded graphite. EFFECT: diversified carbon carbide-based materials prepared with improved structure and mechanical properties.

Description

The invention relates to the field of producing ceramic products containing silicon carbide by silicon firing. Such products find application in high temperature technology as refractories and heating elements, in mechanical engineering in friction units, in instrumental practice as bonded abrasives, blade tools for high-speed processing of graphite, etc. [1]
Known methods for producing a mixture of various compositions for molding products, which are subsequently subjected to siliconizing firing, i.e. impregnation with molten silicon and additional heat treatment, consisting in mixing the necessary components [2, 3, 4] In all cases, the composition of the charge includes: a carbon-containing component — petroleum coke powder [2] graphite [3] pyrocarbon [4] filler, which can used so-called "primary" silicon carbide [2] a mixture of powders of silicon carbides and boron [3] powders of any other refractory carbides or borides inert with respect to molten silicon and a temporary binder or plasticizer. The required amounts of component powders are mixed, and preforms are pressed from the resulting mixture. During the heating of the preforms in an inert medium or vacuum together with the loading of silicon metal, the temporary binder burns out or carbonizes at temperatures lower than the melting point of silicon, after which the carbon-containing component and the residual carbon of the binder interact with the silicon melt impregnating the preform and form the so-called. "secondary" silicon carbide, interconnecting filler particles. The properties of heat-treated products depend mainly on the content of the carbide (with fillers) phase in the resulting material and on its structure. Silicon metal, filling the pores of the product, reduces the strength and chemical resistance of the material [2], therefore, its content is usually tried to minimize.

The closest in technical essence to the claimed invention is a method for producing products from silicon carbide by mixing the components of a mixture containing 20-60 wt. coking binder, 20-70 wt. activated carbon and 10-60 wt. alpha-silicon carbide, molding and silicification of preforms [5] The disadvantage of this method is the appearance in the structure of the siliconized material of inclusions (in the form of interlayers) of metallic silicon caused by the use of a significant amount of a binder, and the incompleteness of conversion of the carbon of the binder and activated carbon into silicon carbide. If this material is annealed at a temperature of 1900-2000 ^o C (silicon stripping mode [2]), then pores are detected at the site of inclusions. The bending strength of the material obtained from this mixture is relatively low and does not exceed 27.5 kg / mm ² .

 The technical result achieved by the present invention is to improve the mechanical characteristics and chemical resistance of the reaction-sintered materials containing silicon carbide by increasing the proportion of the carbide phase in the composition of the material and optimizing its structure.

The essence of the proposed method consists in the fact that when mixing powders of a carbon-containing component, filler and a temporary binder solution, a highly plastic thermally expanded graphite (TEG) powder [6] is used as the carbon-containing component [6], which is pre-modified by pressing the initial low-density TEG into briquettes with density ρ _trg = 0 , 25-1.15 g / cm ³ , followed by grinding the resulting briquettes again into powder. In the manufacture of the charge, the indicated powder is introduced in an amount of x, which is selected depending on the density of the filler substance ρ _n (weighted average, if the filler is a mixture of several powders) and must be large X ^crit , determined in accordance with the formula
x ^rhbn = 100 / (2ρ _n / ρ _trg +1), mass%.

 As a temporary binder, an alcohol solution of phenol-formaldehyde resin or a solution of polyacrylonitrile in dimethyl sulfoxide is used at a rate of 2-5 wt. dry matter binder in the mixture.

Comparative analysis with the prototype allows us to conclude that the claimed composition of the charge differs from the known application of a different carbon material, namely thermally expanded graphite, modified by pressing / grinding and setting its minimum amount in the charge depending on the density of the filler substance. If the filler, as in the prototype, is silicon carbide (ρ = 3.2 g / cm ³ ), then the mass fraction of TEG, depending on its density, should be at least 4-14 wt. Modification of the TEG powder is carried out by briquetting the initial low-density powder, followed by grinding and sieving into fractions (this procedure can be carried out repeatedly), and the choice of the density of briquettes in the claimed range from 0.25 to 1.15 g / cm ³ depends on the particle size and some physico-chemical characteristics of the filler powder. The use of TEG (without filler and without pressing / grinding) as a charge for molding preforms and subsequent silicification has been published by the authors of this application earlier [7] However, the use of TGN for molding the initial low-density powder, although it allows not to use binders, leads to a significant, reaching 50 % non-density of the molded blanks, resulting in heterogeneity, delamination, low mechanical strength of siliconized products. Thus, the known use of TWG in the composition of the charge does not provide siliconized products with those properties that are achieved in the claimed solution.

The boundaries of the proposed range of possible particle density of the TEG powder achieved by briquetting / grinding and the minimum content of TEG in the charge are established experimentally and are determined by the following circumstances:
at a density less than 0.25 g / cm ³ , the bonding of the filler particles to each other after firing is determined not by "secondary" silicon carbide, but by metallic silicon, the amount of which in the product is unacceptably high;
when the density of the TWG is greater than 1.15, the volume of "secondary" silicon carbide formed during the siliconization exceeds the volume of the starting material. This leads to unacceptable deformation of products;
if the mass fraction of TWG in the charge is less than the value of X ^crit determined by the formula x ^crit = 100 / (2ρ _n / ρ _trg +1), mass%, from the condition that the powder is filled with TWG at least 35% of the volume of the molded billet, the result will also be excessively high the silicon content in the material of the obtained product, the deterioration of mechanical properties and chemical resistance. The implementation of this condition is associated with another positive quality of the claimed composition, namely, a reduction in the pressing force and wear of expensive steel molds. The upper limit for the value of x is impractical to limit, since in the absence of filler (this is also possible, see example 10) x 100 wt. At the same time, in real practice, when working with compositions containing fillers inert with respect to molten silicon, x is selected within 1-2 x ^crit , in particular because silicon carbide powder is cheaper than TEG and to increase the share of “secondary” due to TEG silicon carbide in a heat-treated material is disadvantageous.

During the experimental verification of the proposed method, eleven mixtures of the ingredients described in the examples were prepared. In all cases, the initial powder of thermally expanded graphite with a bulk density of 5-25 g / cm ^{3 was} pressed into briquettes with a density of 0.25-1.15 g / cm ³ , after which the briquettes were ground and the resulting powder was dispersed into fractions, in some cases, to achieve The process of pressing briquettes and grinding them was repeated up to three times. All used powders (TEG, fillers) were preliminarily annealed in vacuum no worse than ^10-3 Pa at a temperature of 1500 ^o C for 1 hour, were mixed dry in a drum mill for 40 minutes, after adding a binder solution they were additionally mixed for 20 minutes, dried at room temperature temperature. The blanks were pressed in closed steel molds at pressures of 50-150 kg / cm ² . Silicon annealing was carried out in a Tammanov type furnace with a tubular graphite heater in an argon atmosphere at a temperature of 1950 ± 50 ^° C for 20 minutes.

Example 1 (analog). A mixture is prepared consisting of: silicon carbide 80 wt. petroleum coke 20 wt. An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 350 kg / cm ² . The material after siliconizing annealing contains: silicon carbide - 82 wt. silicon 15 wt. carbon 3 wt. has a density of 2.79 g / cm ³ , porosity of 5% and a bending strength of 18.5 kg / mm ² . The billet after annealing retained its original shape.

Example 2 (prototype). A mixture was prepared consisting of: silicon carbide - 20 wt. activated carbon 40 wt. phenol-formaldehyde resin 40 wt. The specific pressure during the pressing of the preform was 320 kg / cm ² . The material after siliconizing annealing contains: silicon carbide - 83 wt. silicon 15 wt. carbon 2 wt. has a density of 2.85 g / cm ³ , porosity of 7% and a bending strength of 24.5 kg / mm ² . The billet after annealing retained its original shape.

Example 3. A mixture is prepared consisting of: silicon carbide 88 wt. thermally expanded graphite with ρ _trg = 0.65 g / cm ³ 12 wt. which is greater than X ^crit 100 / (2 • 3.2 / 0.65 + 1) 9.2 wt.

An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 115 kg / cm ² . The material undeformed after siliconizing annealing of the product contains: silicon carbide -94 wt. silicon 6 wt. carbon less than 0.3 wt. has a density of 2.99 g / cm ³ , porosity of -4% and a flexural strength of 38.5 kg / mm ² .

Example 4. A mixture was prepared consisting of: silicon carbide 84 wt. thermally expanded graphite with ρ _trg = 1.20 g / cm ³ 16 wt. An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure for pressing the billet was 145 kg / cm ² . After siliconizing annealing, the workpiece is strongly deformed and is not suitable for any use.

Example 5. Prepared a mixture consisting of: silicon carbide 96 wt. thermally expanded graphite with ρ _trg = 0.15 g / cm ³ 4 wt. An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 85 kg / cm ² . The blank after siliconizing annealing is not deformed, the chemical composition of its material: silicon carbide 76 wt. silicon 24 wt. carbon less than 0.3 wt. density 2B59 g / cm ³ , porosity 11%, bending strength 10.5 kg / mm ² .

Example 6. Prepared a mixture consisting of: silicon carbide 92 wt. thermally expanded graphite with ρ _trg = 0.65 g / cm ³ 8 wt. which is less than X ^crit 9.2 wt. An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 185 kg / cm ² . The blank after siliconizing annealing is not deformed, the chemical composition of its material: silicon carbide 86 wt. silicon 14 wt. carbon less than 0.3 wt. density 1.82 g / cm ³ , porosity 7%, bending strength 10.5 kg / mm ² .

Example 7. Prepared a mixture consisting of: boron carbide 83 wt. thermally expanded graphite with ρ _trg = 0.60 g / cm ³ 17 wt. which is greater than X $\genfrac{}{}{0ex}{}{\text{rhbn}}{\text{tg}}$ = 100 / (2 • 2.5 / 0.60 + 1) = 10.7 wt.%. An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 135 kg / cm ² . The material undeformed after silico-annealing of the product contains: "boron silicocarbide" (B ₄ , C, Si, B) 58 wt. silicon carbide 35 wt. silicon 7 wt. carbon - less than 0.3 wt. has a density of 2.46 g / cm ³ , porosity of 8% and a flexural strength of 33 kg / mm ² .

Example 8. A mixture was prepared consisting of: graphite powder MPG-6 63 wt. thermally expanded graphite with ρ _trg = 0.75 g / cm ³ 37 wt. which is greater than X ^crit 100 / (2 • 1.6 / 0.75 + 1) 19 wt. An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 85 kg / cm ² . The material undeformed after siliconizing annealing of the product contains: silicon carbide 59 wt. silicon 23 wt. graphite 18 wt. has a strength of 2.22 g / cm ³ , porosity of 12% and a bending strength of 11 kg / mm ² .

Example 9. A mixture is prepared consisting of: titanium deboride 93 wt. thermally expanded graphite with ρ _trg = 0.45 g / cm ³ 7 wt. which is greater than X ^crit 100 / (2 • 4.4 / 0.45 + 1) 5 wt. An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 115 kg / cm ² . The material undeformed after siliconizing annealing of the product contains: titanium diboride 74 wt. silicon carbide 16 wt. silicon 10 wt. carbon less than 0.3 wt. has a density of 3.60 g / cm ³ , porosity of 5% and a flexural strength of 42 kg / mm ² .

Example 10. A mixture was prepared consisting of: thermally split graphite with ρ _trg = 0.90 g / cm ³ 62 wt. thermally expanded graphite with ρ _trg = 0.65 g / cm ³ 38 wt. An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 65 kg / cm ² . The material undeformed after siliconizing annealing of the product contains: silicon carbide 85 wt. silicon 15 wt. carbon less than 0.3 wt. has a density of 2.91 g / cm ³ , porosity of 4% and a bending strength of 27 kg / mm ² .

Example 11. A mixture is prepared consisting of: silicon carbide 44 wt. boron carbide 22 wt. graphite powder 22 wt. thermally expanded graphite with ρ _trg = 0.65 g / cm ³ 12 wt. which is greater than X ^crit 100 / (2 • 2.4 / 0.65 + 1) 11.8 wt. Here, the weighted average density of the mixture of powders of fillers is determined taking into account their mass fractions in the mixture:
P _n 1 / (0.5 / 3.2 + 0.25 / 2.5 + 0.25 / 1.6).

An alcohol solution of phenol-formaldehyde resin in the amount of 300 ml of a 25% solution per 1 kg of dry mixture was used as a binder. The specific pressure during the pressing of the preform was 115 kg / cm ² . The material undeformed after siliconizing annealing of the product contains: silicon carbide 69 wt. boron silicocarbide 14 wt. silicon 8 wt. graphite 9 wt. has a density of 2.67 g / cm ³ , porosity of -3% and a bending strength of 17 kg / mm ² .

From the examples considered it follows that the method of manufacturing a charge, which contains the required amount of thermally expanded graphite powder with particles of a certain density, allows to obtain a variety of materials based on silicon carbide with improved structure and mechanical properties. In addition, there is an additional positive effect associated with the fact that the pressing force does not exceed 150 kg / cm ³ and the wear of expensive steel molds is reduced.

Claims (1)

A method of producing a charge for molded products containing silicon carbide, comprising mixing an active carbon-containing ingredient, a filler and a temporary binder, characterized in that thermally expanded graphite powder is used as the carbon-containing ingredient, which is pre-modified by compression molding into briquettes with a density ρ _mp = 0.25 - 1.15 g / cm ³ followed by grinding into a powder and enter the specified powder in an amount of X, satisfying the ratio, wt. X> 100 / (2 ρ _n / ρ _trg + 1),
where ρ _n is the density of the filler substance (average density, if the filler is a mixture of several substances), g / cm ³ ;
ρ _trg is the density of thermally expanded graphite, g / cm ³ .