RU2494962C1 - Method of manufacturing products from carbon-carbide-silicon material - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used in obtaining construction materials, operating in conditions of high thermal load and oxidative medium, for chemical, petrochemical, chemical-metallurgical industry and aeronautic engineering. Slip coating, based on composition of silicifying agent and temporary binding agent, is formed on work-piece from porous carbon-graphite material. Slip coating is made combined with internal layer basing on composition from silicon nitride powder and non-coke-forming polymer binding agent and external one, basing on composition from mixture of silicon carbide and quartz powders, taken in ratio 1:(2-3), and liquid glass, or siloxane binding agent, or colloidal solution of silica in water. After that, silicifying is performed by heating work-piece in vacuum to temperature 1800 °C, exposure for 1-2 hours at 1800-1850 °C and cooling. Silicifying is carried out in silicon vapours with pressure in reactor not higher than 35 mm Hg, for which purpose crucibles with silicon are additionally installed into charge. Heating in the interval 1400-1700 °C is carried out with rate not less than 300-350 degrees/hour.

EFFECT: simplified method of manufacturing large-sized products from carbon-carbide-silicon material, ensuring high purity of their surface and high strength.

3 cl, 1 tbl

Description

The invention relates to the field of structural materials operating under high thermal loading and an oxidizing environment and can be used in the chemical, petrochemical and chemical-metallurgical industries, as well as in aircraft to create products and structural elements exposed to aggressive environments.

A known method of manufacturing products from UKKM, including the manufacture of a workpiece from a porous carbon-graphite material and its silicification by the liquid-phase method by immersing the workpiece in a silicon melt [US Pat. US No. 4397901, CL C2311 / 08, 1983]

The disadvantage of this method is its complexity due to the complex hardware design when using it for the manufacture of bulky products.

The closest to the proposed technical essence and the achieved effect is a method of manufacturing products from UKKM, including the manufacture of a workpiece from porous carbon-graphite material, the formation of a slip coating on it based on a composition of a siliconizing agent and a temporary binder, followed by siliconizing the workpiece by heating it in vacuum to a temperature 1800 ° C, holding for 1-2 hours at 1800-1850 ° C and cooling. Moreover, silicon powder is used as a siliconizing agent in a slip coating [US Pat. RF №3084425, class C04B 35/52, 1997].

By simplifying the hardware design, manufacturing technology is simplified.

Nevertheless, it still remains quite complicated in relation to the manufacture of large-sized products because of the need to heat them from 1300 to 1650 ° C at a speed of at least 600 deg / h for quickly transferring the silicon melt to a low-viscosity state. Otherwise (at a low heating rate), a viscous silicon melt flows into the surface pores of the workpiece material and is carbonized, which leads to a loss of its ability to flow during subsequent heating and, as a result, to surface silicification with the formation of growths on the product. A partial carbidization due to the interaction with carbon-containing and reactor gases also leads to an increase in the viscosity of the silicon melt.

The objective of the invention is to simplify the method of manufacturing large-sized products from UKKM with high purity of their surface and high strength UKKM.

This problem is solved due to the fact that in the method of manufacturing products from UKKM, including the manufacture of a workpiece from porous carbon-graphite material, the formation of a slip coating on it based on a composition of a siliconizing agent and a temporary binder, followed by siliconizing the workpiece by heating it in vacuum for 1- 2 hours at 1800-1850 ° C and cooling, in accordance with the claimed method, a slip coating is performed combined with the inner layer based on a composition of silicon nitride powder and is non-coke-forming polymeric binder and external based on a composition of a mixture of powders of silicon carbide and quartz, taken in the ratio 1: (2: 3), and liquid glass or a siloxane binder, or a colloidal solution of silica in water; while siliconization is carried out in silicon vapor at a pressure in the reactor of not more than 35 mm Hg, for which crucibles with silicon are installed in the charge.

In one of the more preferred embodiments of the method, silicification is carried out in saturated silicon vapors.

In another preferred embodiment of the method, heating in the range of 1400-1700 ° C is carried out at a rate of at least 300-350 degrees / hour.

The use of a composition of silicon nitride powder and a non-coking polymer binder in combination with siliconization in silicon vapors at a pressure in the reactor of not more than 35 mm Hg in the slip composition for the inner layer of slip coating. provides the ability to obtain a silicon melt from silicon nitride. Under these conditions, the decomposition of silicon nitride occurs at a temperature of 1600-1650 ° C with ~ 40% yield of the melt and ~ 60% of the silicon vapor from stoichiometric.

When the pressure in the reactor is more than 35 mm Hg the concentration of silicon vapors decreases due to a decrease in its evaporation rate, and also the temperature at which decomposition of silicon nitride occurs.

In the temperature range 1600-1650 ° C, the silicon melt has an acceptable (not too high and not too low) chemical activity, which eliminates the carbidization of carbon fibers.

In the temperature range 1600-1650 ° C, the silicon melt has a low viscosity. Therefore, he could impregnate a preform of porous carbon-graphite material to its full thickness, and its excess would drain from the preform. However, at a low heating rate, and hence prolonged contact with carbon-containing reactor gases, silicon nitride partially carbides with the formation of refractory compounds, namely silicon carbide and carbonitride. Their presence in the silicon melt leads to an increase in its viscosity and, as a consequence, to surface silicification with the formation of growths on the surface of the preform.

The siliconization process in silicon vapors allows chemically bonding at least a portion of the carbon-containing gases. However, in the range of 1000–1300 ° C, the concentration of silicon vapors is still low, which means that the efficiency of their binding of carbon-containing gases is low.

Performing a slip coating combined with an outer layer based on a composition of a mixture of powders of silicon carbide and quartz, taken in the ratio 1: (2: 3), and liquid glass or siloxane or colloidal solution of silica in water, in conjunction with the process of silicification in pairs silicon can significantly reduce the amount and time of contact of carbon-containing gases with particles of silicon nitride.

In the range of 1000-1300 ° C, this is achieved due to the fact that the inner layer is protected from access to it by carbon-containing gases with an outer dense layer of slip coating.

Above 1300 ° C, as the concentration of silicon vapors increases, they chemically interact with silicon dioxide (SiO ₂ ), which is part of liquid glass (or siloxane or colloidal silica in water) and a mixture of powders (SiC and SiO ₂ ), with the formation silicon monoxide (SiO)

$$Si{O}_2+S{i}_{P\mathrm{but}R}\to 2Si{O}_{P\mathrm{but}R}\text{(one)}$$

Silicon monoxide (SiO) diffuses into the reactor volume, moving towards the flow of carbon-containing gases. Silicon monoxide has a higher vapor pressure (concentration) than silicon, and therefore is able to chemically bind carbon-containing gases, in particular carbon monoxide (CO), to a greater extent by the reactions:

$$SiO+CO\to Si+C{O}_2\text{(2)}$$

$$SiO+3CO\to SiC+C{O}_2\text{(3)}$$

In the range 1300-1500 ° C, a decrease in the number and time of contact of silicon nitride particles with carbon-containing gases is achieved due to the fact that part of the thickness of the outer layer is still dense and protects the inner layer from access to it by carbon-containing gases.

Above 1500 ° C, a reaction begins to occur between SiC and SiO ₂ with the formation of volatile SiO:

$$SiC+2Si{O}_2\to 3SiO+CO\text{(four)}$$

As a result of its course, and also due to the fact that SiC and SiO ₂ are taken in the ratio 1: (2: 3), the outer layer of the slip coating is completely decomposed. This eliminates the possibility of the penetration of refractory SiC particles into the silicon melt at the time of decomposition of silicon nitride into a melt and silicon vapor, i.e. by the time the temperature range of 1500-1650 ° C is reached, the Si ₃ N ₄ particles are protected from carbidization by SiO and Si vapors, namely: SiO vapor moves towards carbon-containing gases and thereby reduces their diffusion rate to Si ₃ N ₄ , and partially silicon vapor chemically bind carbon-containing gases by the reactions:

$$S{i}_{P\mathrm{but}R}+2CO\underset{\text{t}>{\text{1500}}^{\text{o}}\text{C}}{\to }SiC+C{O}_2\text{(5)}$$

Siliconization in saturated silicon vapors provides, on the one hand, the possibility of binding carbon-containing gases in a greater amount than in unsaturated vapors, and, on the other hand, allows more to counteract the process of premature (before reaching the temperature range 1600-1650 ° C) decomposition of silicon nitride.

Carrying out heating in the temperature range of 1400-1700 ° C with a speed of at least 300-350 deg / h allows you to reduce the contact time of silicon nitride particles with carbon-containing and gases and thereby to even more protect them from carbidization, as well as to ensure rapid runoff and evaporation excess silicon melt from the surface of the siliconized workpiece.

Continued heating from 1700 to 1800 ° C provides the possibility of completing the process of swelling and evaporation of the excess silicon melt from the surface of the workpiece.

Holding for 1-2 hours at a temperature of 1800-1850 ° C ensures the completion of carbidization of the silicon workpiece that has entered the pores.

Cooling ensures the completion of the siliconization process.

In the new set of essential features, the object of the invention has a new property: the ability to give a silicon melt a low viscosity at the time it is impregnated with a porous carbon-graphite material at a temperature not exceeding 1650 ° C, when the silicon melt has an acceptable (not low and not too high) chemical activity, without the need for heating from 1400 to 1700 ° C at a high speed, namely: at least 600 deg / h (in the proposed method, it is possible to heat with less than 600 deg / h, speed, namely: 250-300 deg / h).

The new property allows us to simplify the method of manufacturing large-sized products from UKKM and at the same time ensure high surface cleanliness and high strength UKKM.

The method is as follows.

One of the known methods is made of a blank of porous carbon-graphite material. Then a slip coating is formed on it, which is combined, namely: with an inner layer based on a composition of silicon nitride powder and a non-coking polymer binder, and an outer layer based on a composition of a mixture of silicon carbide and quartz powders taken in the ratio 1: (2 : 3), and liquid glass, or a siloxane binder, or a colloidal solution of silica in water.

After that, the workpiece is silicified by heating it in vacuum to 1800 ° C, holding for 1-2 hours at 1800-1850 ° C and cooling.

In this case, silicification is carried out at a pressure in the reactor of not more than 35 mm Hg. in silicon vapors, for which crucibles with silicon are placed inside the retort along with the workpiece.

In one preferred embodiment of the method, silicon crucibles and a siliconized preform are placed in a retort having a quasiclosed volume, i.e. volume, the effect of leakage from which silicon vapors on the formation of their unsaturated state can be neglected, which ensures the formation of saturated silicon vapors.

In another preferred embodiment of the method, part of the crucibles is filled with powdered silicon.

When the billet is heated to the melting temperature of silicon, carbon-containing gases are absorbed by silicon powder, as well as silicon vapors. Starting from 1100 ° C, the absorption of carbon-containing gases is also carried out by silicon monoxide (SiO) formed by reaction (1). The consequence of this is a decrease in the probability of carbidization of Si ₃ N ₄ .

To an even greater extent, the function of protecting Si ₃ N ₄ from carbidization is performed by a dense outer layer of slip coating up to a temperature of 1500 ° C, which impedes the access of carbon-containing gases to the inner layer of slip coating.

By the time the temperature reaches 1600-1650 ° C, the entire (without residue) outer layer of the slip coating decomposes due to reaction (4) and escapes in the form of SiO vapor and gaseous CO, which eliminates the possibility of SiC particles entering the silicon melt resulting from the decomposition of Si ₃ N ₄ .

In the range of 1500-1650 ° C, the function of protecting Si ₃ N ₄ particles from carbidization is performed by silicon vapors that bind carbon-containing gases in SiC by reaction (5).

Due to the protective effect of the dense outer layer of the slip coating, as well as the absorption capacity of SiO and Si vapors, heating in the range of 1400-1700 ° C can be carried out at a speed of 250-300 deg / h.

The implementation of heating from 1400 to 1700 ° C at a speed of at least 300-350 deg / h, which provides yet another of the preferred embodiments of the method, allows to further reduce the likelihood of carbidization of particles of Si ₃ N ₄ .

In the range 1600-1650 ° C, when the outer layer of the slip coating disappears without a trace, silicon nitride, which is part of the inner layer of the slip coating, decomposes with the formation of 40% molten silicon and 60% silicon vapor.

Due to this, as well as the protective effect of silicon vapors, the silicon melt formed from Si ₃ N ₄ does not contain refractory particles of silicon carbide and carbonitride and therefore has a low viscosity corresponding to temperatures of 1600-1650 ° C.

Under the influence of capillary forces, a part of the silicon melt flows into the pores of the material, as a result of which the workpiece material is saturated with the melt throughout its thickness, and its excess flows off and evaporates from the surface of the workpiece upon subsequent heating.

The filling of the pores of the preform with silicon occurs in addition due to the diffusion of silicon vapors in them and / or capillary condensation of silicon vapors.

Subsequent heating is carried out to a temperature of 1800 ° C, after which they produce 1-2 hours exposure at 1800-1850 ° C. This completes the carbidization of the silicon material that has entered the pores.

After this, the cages are cooled and the workpiece is removed from the reactor.

The following are examples of specific implementation of the method.

Example 1

A carbon-plastic preform in the form of a plate 120 × 360 × 5 mm in size was made of UT-900 brand fabric and phenol-formaldehyde binder grade BZ, it was carbonized at a final temperature of 850 ° C followed by high-temperature treatment (WTO) at 1800 ° C. Then, the carbon-fiber-reinforced carbon fiber composite from the past WTO was saturated with pyrocarbon by a vacuum isothermal method.

Received a blank of carbon-carbon composite material (CCCM) with a density of 1.48 g / cm ³ and an open porosity of 9.7%. Then, a combined slip coating was formed on the workpiece, namely: its inner layer was formed on the basis of a composition of silicon nitride powder and a non-coking binder (which was used as a 4% aqueous solution of polyvinyl alcohol (PVA)), and the outer layer was formed on the basis of the composition from a mixture of powders of silicon carbide and quartz, taken in a ratio of 1: 3 and liquid glass. The weight of the inner layer of the slip coating was 60% of the weight of the plate from the CCM, and the weight of the outer layer was 38%. Then the plate with the slip coating formed on it was installed in a tool for siliconizing; in addition to the plate, 2 crucibles with silicon were installed in the cage. Then, the workpiece was heated to 1800 ° C at a reactor pressure of 27 mm Hg. in silicon vapors. In a specific case, heating to 1000 ° C was performed at a speed of 100 deg / h, from 1000 to 1400 ° C - 180 deg / h, from 1400 to 1700 ° C - 280 deg / h and from 1700 to 1800 ° C - 90 deg / hour.

Starting from 1100 ° C, carbon-containing gases were chemically bonded with silicon monoxide formed by reaction (1). Starting from 1500 ° C, the chemical bonding of carbon-containing gases continued with silicon monoxide, which is already formed by reaction (4).

In addition, throughout the process of silicification, carbon-containing gases were also bound by silicon vapors.

By the time the temperature reaches 1600-1650 ° C, the entire (without residue) outer layer of the slip coating was decomposed due to reaction (4) and volatilization in the form of SiO vapor and gaseous CO, which excluded the possibility of SiC particles entering the silicon melt resulting from the decomposition of nitride silicon.

In the range 1600-1650 ° C, when the outer layer of the slip coating disappeared without a trace, silicon nitride, which is part of the inner layer of the slip coating, decomposed. The silicon melt formed in this case, which has a low viscosity in the indicated temperature range, impregnated the pores of the CCCM, and its excess drained and evaporated from the surface of the preform upon subsequent heating from 1650 to 1800 ° C.

Then produced exposure at 1800-1850 ° C for 2 hours and cooling.

The result was a plate made of UKKM with a density of 1.74 g / cm ³ , open porosity of 4.8% and a silicon content of 14.9%. There were no growths on the surface of the plate. The study FMH UKKM on samples cut from the plate obtained following parameters: σ _p ^o - 144 MPa, σ _mfd ^about - 182 MPa, σ _SJ ^about - 112 MPa.

Example 2

The part was made analogously to example 1 with the following significant difference: 2 crucibles were installed in the cage, one of which was filled with pieces of silicon, and the other with silicon powder.

The result was a part from UKKM with an even higher surface finish.

Example 3

The part was made analogously to example 2 with the following significant difference: heating from 1400 to 1700 ° C was carried out at a speed of 350 deg / h.

The result was a part from UKKM with an even higher surface finish than in examples 1 and 2.

Example 4

The detail was made analogously to example 2 with the following significant difference: silicification was performed in saturated silicon vapors, for which the preform was placed in a quasiclosed volume of the retort. The result was a part from UKKM with an even higher surface finish than in examples 1-3.

Other examples of the manufacture of products from UKKM of the proposed methods are given in table 1, where examples 1-10 correspond to the claimed limits, and examples 11-13 to transcendental values.

Here are examples of the manufacture of products from UKKM using the prototype method (examples 14, 15).

As can be seen from the table, the manufacture of products from UKKM, including large ones (example 9), in accordance with the proposed method and the claimed limits allows us to obtain UKKM with high values of silicon content, with a high level of FMX in the absence of growths on their surface.

When going beyond the claimed limits of the proposed method, the surface of the product in one or another quantity has outgrowths and also the silicon content is not high enough for the specified type of material (examples 11-13).

In the manufacture of products from UKKM in accordance with the prototype method (example 15), the product is obtained without growths, but only if heating from 1300 to 1700 ° C is performed at a high speed (≥600 deg / h), which leads to complication way.

Otherwise, namely: at a relatively low heating rate (for example, at 350 deg / h), a product with growths on the surface is obtained, and the CCM has a low silicon content (see example 14).

Claims (3)

1. A method of manufacturing products from carbon-carbide-silicon material (UKKM), including the manufacture of a workpiece from porous carbon-graphite material, forming a slip coating on it based on a composition of a siliconizing agent and a temporary binder, followed by silicification by heating the workpiece in vacuum to a temperature of 1800 ° C, holding for 1-2 hours at 1800-1850 ° C and cooling, characterized in that the slip coating is combined with the inner layer based on a composition of silicon nitride powder and non-coke -forming polymeric binder and the outer - based on the composition of a mixture of silicon carbide and quartz powder, in a ratio of 1: (2-3), and waterglass or siloxane binder solution or colloidal silica in water; while siliconization is carried out in silicon vapor at a pressure in the reactor of not more than 35 mm Hg, for which crucibles with silicon are additionally installed in the cage. 2. The method according to claim 1, characterized in that at least part of the crucibles is filled with silicon powder. 3. The method according to claim 1, characterized in that the heating in the range of 1400-1700 ° C is carried out at a speed of at least 300-350 deg / h.