RU2716323C9 - Method of protecting carbon-containing composite materials of large-size articles from oxidation - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to methods of protecting carbon-containing materials from oxidation and to protection against oxidation of large-size articles. According to the method, workpiece from porous carbon-containing composite material is subjected to preliminary silicification by liquid-phase method at heating to 1,650–1,750 °C at pressure in reactor 600–760 mm Hg in argon using silicon nitride as precursor of liquid silicon. Remaining non-carbidized silicon is distilled in vacuum at 1,800–1,850 °C. Silicisation and distillation of free silicon from the pores of the material are carried out in a single process. Then high-porous carbon is formed in pores of billet material using one of known methods, preferably, by growing nanocarbon in form of particles, fibers or tubes. Thereafter, on billet surface formed a slurry coating based on diborides of refractory metals and chemically active to silicon component(s): nanodispersed carbon, molybdenum carbides (MoC and Mo₂C) or Mo₅Si₃, or a mixture of carbon with molybdenum with particle size of up to 5 mcm. Final siliconisation is carried out using vapor-liquid-phase method. Coating based on oxides of refractory metals is formed over obtained coating.

EFFECT: high reliability of protecting carbon-containing materials of large-size articles from oxidation at temperatures of 2,000 °C and higher while simultaneously increasing reproducibility of the obtained results.

1 cl, 4 ex, 2 tbl

Description

The invention relates to methods for protecting carbon-containing materials from oxidation and for protection against oxidation of large-sized items,

There are many methods of protecting carbon-containing materials of large-sized products from oxidation by siliconizing them, for example, [1) patent SU No. 1834839, class. CO1B 31/02, 1993; 2) patent RU No. 2468991; 3) patent RU No. 2084425, 1997 and others].

These methods do not provide the possibility of long-term operation of materials in an oxidizing environment and high-temperature exposure.

Closest to the claimed is a method of protecting carbon-containing materials of large-sized products from oxidation, including the formation of a slip coating on the workpiece from a porous carbon-containing composite material based on refractory metal diborides and a component (s) reactive to silicon and siliconizing the workpiece materials and slip coating by the vapor-phase method [ RF patent No. 2082694, 1997].

The method provides more reliable protection of the carbon-containing composite material of large-sized items from oxidation due to the formation of oxidation-resistant coatings. Nevertheless, protection against oxidation at temperatures ≥2000 ° C would be even more reliable if there were no free silicon in the materials of the product and the coating and if the coating were even more oxidation resistant. In addition, the disadvantage of this method is its poor reproducibility due to unstable results of siliconization by the vapor-phase method. This is also the reason for the decrease in the reliability of protection of the product material from oxidation.

The objective of the invention is to improve the reliability of the protection of carbon-containing materials of large-sized items from oxidation at temperatures of 2000 ° C or more, while increasing the reproducibility of the results obtained.

The problem is solved due to the fact that in the method of protecting carbon-containing composite materials of large-sized products from oxidation, including the formation of a slip coating on a workpiece from a porous carbon-containing composite material based on refractory metal diborides and a composite (s) chemically active to silicon, siliconizing the workpiece and slip materials coatings by the vapor-liquid-phase method, in accordance with the claimed technical solution, an oxide coating based on refractory oxides is formed over the coating obtained after siliconizing; Before the formation of a slip coating based on diborides of refractory metals, preliminary siliconization of the workpiece from a porous carbon-containing composite material is carried out by the liquid-phase method when heated to 1650-1750 ° C at a pressure in the reactor of 600-760 mm Hg. Art. in argon using silicon nitride as a precursor of liquid silicon, free silicon is distilled out of the material in vacuum at a temperature of 1800-1850 ° C, carrying out preliminary siliconization and distillation of free silicon in a single technological process, high-porous carbon is formed in the pores of the workpiece material and only after that having formed a slip coating, the final siliconizing is carried out; at the same time, when forming a slip coating, either nanodispersed carbon, or finely dispersed molybdenum carbides MoC and Mo ₂ C, or lower molybdenum silicide Mo ₅ Si ₃ , or a mixture of carbon with molybdenum with a particle size of no more than 5 microns, is used as an active component (s) their possible content in the slip composition, sufficient only for the complete chemical bonding of silicon, and the final siliconizing by the vapor-liquid phase method is performed with the initial introduction of silicon into the pores of the material by capillary condensation of its vapors in the temperature range 1300-1600 ° C on the workpiece at the temperature of silicon vapor, exceeding the temperature of the workpiece, respectively, by 100-10 degrees, followed by heating the workpiece to 1700-1850 ° C and cooling at a workpiece temperature exceeding the temperature of silicon vapor.

In addition, the formation of porous carbon in the pores of the material is carried out by growing nanocarbon in the form of particles, fibers or tubes.

The formation of an oxide coating based on refractory oxides on top of the coating obtained after siliconizing allows to reduce the thermal load on the product material, as well as to protect the underlying coating from the direct effect of a high-temperature oxidative flow.

Prior to the formation of a slip coating based on diborides of refractory metals, preliminary siliconizing of a workpiece from a porous carbon-containing composite material by a liquid-phase method when heated to 1650-1750 ° C at a pressure of 600-760 mm Hg. Art. in argon using silicon nitride as a precursor of liquid silicon, followed by distillation of free silicon, makes it possible to obtain fairly stable results in terms of the degree and uniformity of siliconizing at this stage and thereby create the prerequisites for obtaining stable results at the final siliconizing stage.

In addition, conditions are created for the formation of secondary carbon in the pores (from which free silicon has sweated out; and these are, first of all, large pores) for its transformation into silicon carbide during the final siliconization.

In addition, during the distillation of free silicon from the pores, it is also removed from the surface of the workpiece, which creates favorable conditions for the formation of a slip coating based on diborides. At temperatures below 1650 ° C and a pressure of 600-760 mm Hg. Art. silicon nitride has not yet decomposed to form liquid silicon. At a pressure below 600 mm Hg. Art. the probability of carbidization of silicon nitride particles in the slip coating increases under the influence of carbon-containing reactor gases, the diffusion rate of which increases with decreasing pressure in the reactor. Carrying out heating at a pressure of more than 760 mm Hg. Art., as well as at a temperature of more than 1750 ° C and a pressure of 600-760 mm Hg. Art. impractical because leads to an unreasonable increase in the thermal load on the water-cooled current leads of the siliconizing installation. At temperatures below 1800 ° C, free silicon does not sweat out of the pores. Heating above 1850 ° C is not advisable, because the thermal load on the current leads increases.

Carrying out preliminary siliconizing of the carbon-containing composite material and distillation of free silicon in a single technological process makes it possible to most completely remove it from the surface of the workpiece, because the time of its contact with carbon-containing reactor gases is reduced, due to which it does not have time to carbonize and carbidize. It also allows for a simplified method.

The formation (after distillation of free silicon) in the pores of the preform material of highly porous carbon makes it possible to reduce the pore size and thereby reduce the content of free silicon in the material by reducing the amount of silicon entering each individual pore at the stage of final siliconizing.

Formation (in a preferred embodiment of the method) of porous carbon by growing nanocarbon in the form of particles, fibers or tubes allows it to be most completely converted into silicon carbide.

The use in the formation of a slip coating based on diborides of refractory metals and a component (s) active to silicon as the latter, either nanodispersed carbon, or finely dispersed molybdenum carbides MoC and Mo ₂ C or lower molybdenum silicide Mo ₅ Si ₃ , or a mixture of carbon with molybdenum with a size particles no more than 5 microns allows you to get a slip coating with pore sizes not exceeding 2.5 microns. The content of components active to silicon in the composition in the minimum possible amount, sufficient only for complete chemical bonding of silicon, creates conditions for the conversion of almost all silicon material entering the pores into the corresponding refractory compound at the stage of final siliconization. At the same time, as a result of final siliconizing, a barrier coating material is obtained with a high content of refractory metal diboride in it, which makes it possible to impart high oxidation resistance to the coating material while maintaining the barrier function (excluding chemical interaction between the workpiece material and the oxide coating).

During the final siliconizing of the workpiece, the initial introduction of silicon into the pores of the material by capillary condensation of its vapors in the range of 1300-1600 ° C on the workpiece at a silicon vapor temperature exceeding the temperature of the workpiece by 100-10 degrees, respectively, allows it to be introduced into arbitrarily small pores. Thus, in conjunction with the presence of such pores in the material and their formation by active components to it, taken in the above amount, conditions are created for the conversion of almost all silicon into refractory compounds.

At temperatures below 1300 ° C, there is a high probability of the formation of a solid condensate of silicon vapor on the surface of the slip coating, which results in the impossibility of introducing silicon into the pores of the workpiece material and the slip coating.

At temperatures above 1600 ° C, the likelihood of the formation of liquid silicon vapor condensate on the surface of the slip coating increases, which results in the corrosion of the slip coating and the impossibility of high-quality impregnation of the pores of the workpiece material and slip coating (It is known that liquid silicon does not flow into pores less than 3 microns in size) ...

At a workpiece temperature of 1300 ° C and a silicon vapor temperature below 1400 ° C (i.e., when the temperature difference between silicon vapor and the workpiece is less than 100 degrees), the condensation rate of silicon vapor is low, which leads to an unjustified lengthening of the process.

At a workpiece temperature of 1600 ° C and a silicon vapor temperature of more than 1610 ° C (i.e., when the temperature difference between silicon vapor and the workpiece is more than 10 degrees), the condensation rate of silicon vapor increases excessively, which leads to washout of the slip coating.

Due to the absence of free silicon in the barrier coating, the possibility of degradation of the properties of the oxide coating as a result of their chemical interaction with silicon is excluded. Thus, its (oxidizing coating) high oxidation resistance and heat-insulating properties in relation to the product material are preserved. The latter circumstance makes it possible to reduce the temperature on the carbon-silicon-carbide composite material and thereby ensure operation at a gas flow temperature of ≥2000 ° C. It should also be noted that it is much easier to introduce silicon into small pores using the vapor-liquid-phase siliconizing method than into larger pores, which works to increase the reproducibility of the results.

In the new set of essential features, the subject of the invention has a new property: the ability to more efficiently protect CCCM of large-sized items from oxidation at temperatures of 2000 ° C or more, as well as the ability to increase the likelihood of obtaining materials for the item and a barrier coating with a reproducible component composition.

Thanks to the new property, the task is solved, namely: the reliability of the protection of carbon-containing materials of large-sized items from oxidation at temperatures of 2000 ° C and more is increased, while the reproducibility of the results obtained is increased.

The method is carried out as follows.

A workpiece made of a porous carbon-containing composite material is subjected to preliminary siliconization by the liquid-phase method when heated to 1650-1750 ° C at a pressure in the reactor of 600-760 mm Hg. in argon using silicon nitride as a precursor of liquid silicon. Remaining non-carbidized (mainly in large pores) silicon is distilled off in vacuum at a temperature of 1800-1850 ° C. The operations of siliconizing and stripping free silicon from the pores of the material are carried out in a single technological process. Then, highly porous carbon is formed in the pores of the workpiece material by one of the known methods.

In a preferred embodiment of the method, the formation of porous carbon in the pores of the material is carried out by growing nanocarbon in the form of particles, fibers or tubes. After that, a slip coating is formed on the surface of the workpiece based on diborides of refractory metals and a component (s) reactive to silicon. In this case, either nanodispersed carbon, or molybdenum carbides (MoC and Mo ₂ C) or lower molybdenum silicide Mo ₅ Si ₃ , or a mixture of carbon with molybdenum with a particle size of up to 5 μm with their minimum possible content in the slip composition sufficient only for complete chemical bonding of silicon.

Then the final siliconizing is carried out by the vapor-liquid phase method. In this case, the final introduction of silicon into the pores of the workpiece materials and slip coating is carried out by capillary condensation of its vapors in the temperature range of 1300-1600 ° C on the workpiece at a silicon vapor temperature exceeding the workpiece temperature by 100-10 degrees, respectively. Thereafter, a coating based on refractory metal oxides is formed on top of the resulting coating. The method is illustrated by the following examples of its specific implementation.

In all examples, the product to be manufactured was a 120 x 360 x 6-8 mm plate.

For each of the examples, 2 plates were made.

Example 1

On the surface of a workpiece made of a porous carbon-containing composite material (namely: CM based on a frame made of UT-900 carbon fabric and a coke-pyrocarbon matrix having a density of 1.45 g / cm ³ and an open porosity of 9.7%), a slip coating was formed based on the composition from silicon nitride (precursor of liquid silicon) and a temporary binder, namely: 8% aqueous solution of polyvinyl alcohol (PVA) in the amount of 50% by weight of the workpiece. Then the billet with the formed slip coating was heated to a temperature of 1750 ° C at a pressure in the reactor of 720 mm Hg. Art. in argon. When the temperature reaches 1750 ° C and the pressure in the reactor is 720 mm Hg. silicon nitride decomposes to form liquid silicon in an amount of 100% of stoichiometric (i.e., in an amount of 34: 140 ~ 0.6 by weight of silicon nitride in a slip coating). In this case, bulk impregnation of the workpiece material with liquid silicon occurs in parallel with its carbidization, and the excess of liquid silicon flows down.

Then the workpiece was heated to a temperature of 1800 ° C in a vacuum, followed by holding at 1800-1850 ° C for 2 hours. During this period, silicon carbidization and distillation from the surface of the workpiece and from the large pores of the material are completed.

It has been experimentally established that the most stable results on the siliconization of large-sized items from CCCM are obtained by the above-described liquid-phase method in comparison with the vapor-liquid phase. Therefore, the use of the liquid-phase method for preliminary siliconization makes it possible to obtain reproducible results from process to process.

As a result of preliminary siliconizing and distillation of free silicon from large pores, a carbon-silicon carbide material (CCCM) with a density of 1.70 g / cm ³ and an open porosity of 4.4% was obtained, in which the content of total and free silicon was 14.7 and 0, respectively. 6 wt. %.

From one plate, 5 samples of ∅30 × δ 6 mm were cut out for subsequent operations (these samples were intended for testing for oxidation resistance on them). The rest of the wafer material was used to determine the apparent density and open porosity of the material, as well as to determine free silicon in it.

Then, highly porous carbon was formed in the pores of the workpiece and specimens из30 × δ 6 mm from CCCM by growing nanocarbon fibers in them by the gas-phase catalytic method. The result was an increase in the density of the material by 0.07 g / cm ³ .

After that, a slip coating was formed on the surface of the workpiece and samples based on powders of zirconium diboride and molybdenum carbide MoC with a particle size of 1-5 μm. The content of molybdenum carbide in the composition was 32 wt. %. The coating thickness was ~ 300 μm.

Then made the final siliconization of the materials of the samples and slip coating by the vapor-liquid phase method. For this, a blank with samples and crucibles with silicon were installed in a quasi-closed volume of the retort. In this case, the crucibles with silicon were consolidated in the lower part of the retort to provide them with additional heating with a bottom heater with an autonomous power source. The initial introduction of silicon into the pores of the workpiece materials with the samples and the slip coating was carried out by capillary condensation of its vapors in the temperature range on the workpiece and samples of 1300-1600 ° C. For this, crucibles with silicon during heating of the workpiece and samples from 1300 to 1600 ° C were heated to a temperature exceeding the temperature of the workpiece by 100-10 degrees, respectively. During this period, the pores of the workpiece material with samples and the slip coating were filled with silicon in parallel with its carbidization in the workpiece material (and samples) and transformation into molybdenum silicides and silicon carbide in the slip coating material.

Then the workpiece and specimens were heated to 1850 ° C, which was accompanied by the completion of the interaction of silicon with carbon and molybdenum carbide.

As a result of X-ray phase analysis of the coating material, the presence of the following phases was established: ZrB ₂ , SiC, MoSi ₂ , Mo ₅ Si ₃ . The presence of free silicon has not been established.

Thereafter, an oxide coating based on yttria-stabilized zirconia and aluminoxane polymer was formed over the formed barrier coating.

Example 2

The product and samples ∅30 × δ 6 mm were made analogously to example 1 with the significant difference that nanocarbon particles were used as an active component in a slip coating based on zirconium diboride in an amount sufficient for complete bonding of silicon. For this, the pore volume of the slip coating was calculated and multiplied by the silicon density. The amount of nanocarbon sufficient for silicon binding was 8.7 wt. %.

Example 3

The product and samples ∅30 × δ 6 mm were made analogously to example 1 with the significant difference that a mixture of carbon with molybdenum with a particle size of 1-5 μm was used as an active component in a slip coating based on zirconium diboride in the following weight ratio ZrB ₂ : Mo : C = 80: 15: 5, which ensures the complete consumption of silicon due to the possibility of interaction with molybdenum with the formation of higher and lower molybdenum silicides, which, unlike silicon carbide, do not have the property of opposing the diffusion of silicon to molybdenum.

As a result of X-ray phase analysis of the coating material, the presence of the following phases was established: ZrB ₂ , MoSi ₂ , Mo ₅ Si ₃ , SiC, Mo ₅ Si ₃ C. The presence of free silicon was not established.

Example 4

The product and samples ∅30 × δ 6 mm were made analogously to example 1 with the significant difference that hafnium diboride (HfB ₂ ) was used as the metal diboride.

The remaining examples 5, 6 of a specific implementation of the method, including those discussed above 1-4, are shown in table 1. Here are also example 7 of manufacturing an article in accordance with the prototype method, as well as example 8 of manufacturing an article from CCCM with an oxide coating of the same composition, as in examples 1-6, characterized in that both the CCCM itself and the barrier coating contained free silicon in their composition.

Table 1 shows only those examples (meaning examples 1-6), the manufacture of products in accordance with which was carried out in full accordance with the claimed method.

The table does not show examples of the manufacture of products with a deviation from the declared limits concerning the technological parameters of the final siliconizing process, since formed on products and specimens ∅30 × δ 6 mm barrier coating was of poor quality. So, at the temperature of the workpiece (at the initial introduction of silicon into the pores of the material) below 1300 ° C, the coating simply fell off the workpiece. At a workpiece temperature above 1600 ° C, as well as at a temperature of 1600 ° C and a temperature difference between silicon vapor and a workpiece of more than 10 degrees, the coating had erosion in the form of furrows throughout its entire thickness.

Table 2 shows the results of testing specimens ∅30 × δ 6 mm for oxidation resistance.

Tests of samples with the developed protection against oxidation were carried out in an induction installation at a temperature of 2000 ° C for 30 minutes under conditions of natural air convection in an open atmosphere. The criterion for assessing the quality of protection of the material from oxidation was the decrease in the sample weight during the tests.

Table 1 shows that the manufacture of products with protection against oxidation, made in accordance with the inventive method, allows you to transfer CCCM to CCCM of a sufficiently high density with a silicon carbide content of 30-31.4 wt. % (based on the total silicon content of 21.0-22.0%) in the absence of free silicon in it, as well as to obtain a barrier (between the UKCM product and the oxide coating) coating that does not contain free silicon. The implementation of protection against oxidation in accordance with the prototype method does not provide such a possibility.

From table 2 it can be seen that the protection against oxidation, made in accordance with the claimed method, is more effective in comparison with the protection made according to the prototype method (compare examples 1-6 with example 7), which is due to both the absence in the material of the product and material barrier coating of free silicon, and the presence of an additional oxide coating.

Performing protection against oxidation with an oxide coating, but in the presence of free silicon in the CCCM and the material of the barrier coating, leads to a decrease in its effectiveness (compare examples 1-6 with example 8).

Claims (2)

1. A method of protecting carbon-containing composite materials of large-sized products from oxidation, including the formation of a slip coating on a workpiece made of a porous carbon-containing composite material based on refractory metal diborides and a component (s) reactive to silicon, siliconizing the workpiece and slip coating materials by a vapor-liquid phase method, which differs the fact that an oxide coating based on refractory oxides is formed on top of the coating obtained after siliconizing; moreover, before the formation of a slip coating based on diborides of refractory metals, preliminary siliconization of the workpiece from a porous carbon-containing composite material is carried out by the liquid-phase method when heated to 1650-1750 ° C at a pressure in the reactor of 600-760 mm Hg. in argon using silicon nitride as a precursor of liquid silicon, free silicon is distilled out of the material in vacuum at a temperature of 1800-1850 ° C, carrying out preliminary siliconization and distillation of free silicon in a single technological process, high-porous carbon is formed in the pores of the workpiece material and only after that having formed a slip coating, the final siliconizing is carried out; at the same time, when forming a slip coating, either nanodispersed carbon, or finely dispersed molybdenum carbides MoC and Mo ₂ C, or lower molybdenum silicide Mo ₅ Si ₃ , or a mixture of carbon with molybdenum with a particle size of no more than 5 microns, is used as an active component (s) their possible content in the slip composition, sufficient only for the complete chemical bonding of silicon, and the final siliconizing by the vapor-liquid phase method is performed with the initial introduction of silicon into the pores of the material by capillary condensation of its vapors in the temperature range 1300-1600 ° C on the workpiece at the temperature of silicon vapor, exceeding the temperature of the workpiece, respectively, by 100-10 degrees, followed by heating the workpiece to 1700-1850 ° C and cooling at a workpiece temperature exceeding the temperature of silicon vapor. 2. The method according to claim 1, characterized in that the formation of porous carbon in the pores of the material is carried out by growing nanocarbon in the form of particles, fibers or tubes.