RU2695423C1 - Article from graphite with modified near-surface layer and method of modification of article surface, having base from graphite - Google Patents

Abstract

FIELD: technological processes.SUBSTANCE: group of inventions relates to thermal protection and antioxidation coatings and can be used to increase chemical inertia and operating temperature of articles used in aerospace industry, fuel and energy complex, in chemical industry, etc. Product is characterized by presence of base from graphite, in which near-surface modified layer consists of graphite of base and silicon carbide. Near-surface layer is obtained by substituting part of the graphite of the base with silicon carbide to form a branched dendritic crystal structure of silicon carbide inside the graphite. Modification of the article surface is carried out in a vacuum furnace by providing the thermochemical heterogeneous reaction of the base graphite with the silicon melt in contact with the base surface and carbon monoxide fed into the vacuum furnace. Said reaction is accompanied by substitution of part of graphite of base with silicon carbide to form branched dendritic crystal structure of silicon carbide inside graphite.EFFECT: near-surface layer formed according to the proposed method is retained in the base due to branched dendritic intrusion of silicon carbide into the graphite of the base, providing higher strength characteristics, chemical and thermal resistance of the articles.8 cl, 9 dwg

Description

The group of inventions relates to the field of thermoprotective and antioxidant coatings and can be used to increase the chemical inertness and operating temperature of materials used in the aerospace industry, fuel and energy complex, in the chemical industry, etc. More specifically, the invention allows to modify the surface of a graphite product by forming in its surface layer of silicon carbide layer embedded in graphite. The invention allows to combine the advantages of both materials: graphite and silicon carbide.

In the context of this application, the term "product" is understood in its broadest sense. It can be: three-dimensional indivisible design (detail); flat plate; profiled or bent sheet, including as part of a curly shell or casing; tubular product; thread for the formation of carbon-carbon composite material (CCM), etc.

In the context of this application, the term "surface layer" is understood as a layer located directly below the outer surface of the product. The surface layer, in contrast to the surface (covering) layer, is buried inside the base material of the product and practically does not go beyond the original (before modification) dimensions of the base. In this case, the surface layer can be formed on one or several surfaces of the product, including completely covering the base material. The near-surface layer can be continuous or fragmentary, that is, located only under a part of the outer surface, and is made using masking of the surface that is not to be processed.

Methods of protecting and / or hardening product surfaces by applying various protective coatings, including silicon carbide (SiC) coatings, are widely known.

Known methods for producing silicon carbide coatings on various materials can be divided into two groups. The methods of the first group use physicochemical processes in which silicon and carbon for the formation of silicon carbide are fed into the synthesis zone in the form of chemical compounds or steam: hydrides, halides, etc. [see, for example, USSR Author's Certificate No. 145106]. That is, they use "external" sources of matter.

The formation of coatings of SiC on various products (substrates) is widely used in the manufacture of microelectronics products. These are mainly chemical precipitation methods. Thus, for example, US Pat. No. 3,520,740 proposes the use of a mixture of gases: silane (SiH ₄ ), propane (C ₃ H ₈ ) and hydrogen H ₂ , in which silicon carbide is formed, deposited on the surface of the substrate. The disadvantage of this and other similar methods is low productivity, high energy consumption, as well as the cost of manufacturing precursors.

Another group of methods uses "internal" sources, i.e. the source of the silicon carbide forming material is the material itself (base) on which silicon carbide is grown. For example, in the patent RU 2363067, a silicon substrate is used as the silicon source, onto which a layer of silicon carbide is deposited by annealing in the atmosphere of CO or CO ₂ gas or a mixture thereof. As a result, a thin SiC film grows on the surface of the substrate. Also known is a method of producing a silicon carbide film on silicon [Haq K.E., Khan I.N. Surface Characteristics and Electrical Conduction of β-SiC Films Formed by Chemical Conversion. Journal of Vacuum Science and Technology, (1970) 7 (4), 490-493. doi: 10.1116 / 1.1315373] in which the silicon substrate is heated at temperatures from 950 ° C in an atmosphere of C ₂ H ₂ , resulting in the formation of a SiC film on the surface. The disadvantages of such methods are that, firstly, they only allow the growth of silicon carbide films on the surface of silicon products, and secondly, only films of small thickness (up to several hundred nanometers).

All of these sources describe the product in the form of a plate (substrate) with a surface layer of silicon carbide deposited.

However, silicon carbide is applied to the base not only for the purpose of obtaining microelectronics products. So known is the Method of applying a gas-tight coating of silicon carbide, described in patent RU 2601049, aimed at increasing the thermo-oxidative strength of coated parts from various materials, including graphite. The method includes the following stages: placing the product from high-temperature material in the thermal zone of the furnace; supplying a gaseous silicon-containing component and gaseous hydrocarbons to the surface of the product. In this case, to obtain a silicon-containing component using a molten silicon placed in a crucible in the thermal zone of the furnace. After holding the product in the oven in the specified mixture of reagents for a predetermined time and subsequent cooling of the product, the product is coated with a layer of silicon carbide. The method allows to obtain nano- and microcrystalline gas-tight coatings with different structures, adjustable polytype composition and particle size of silicon carbide from 50 nm to hundreds of microns.

The specified method according to patent RU 2601049 adopted as a prototype of the proposed method.

As a prototype of the claimed product adopted adopted in the same patent RU 2601049 a plate of graphite, which is coated with a protective surface layer of silicon carbide.

As noted above in the prototype, a SiC layer is formed on the surface of the graphite base of the product, carried out from the reagents in the furnace. The presence of any coating (surface layer) increases the geometric dimensions of the product itself, which is not always acceptable, especially for products of complex configuration with a given dimensional accuracy. In addition, on curly products, obtaining a coating uniform in thickness is very problematic. The coating is based on adhesive forces. Under shear loads, peeling of the coating is possible, due to the presence of a clear boundary between silicon carbide and the surface of the base of the product. Peeling of the coating is also possible under thermal cyclic loads due to differences in thermal expansion coefficients. This is especially negatively manifested in the presence of protrusions and depressions on the surface of the product, causing the appearance of zones of thermal stresses and the formation of thick layers of coatings.

However, the creation of a protective layer is possible not only by coating the base. Another direction of changing the properties of the base material is chemical-thermal treatment, during which the surface of the base material is enriched with certain elements, for example, processes such as cementation, silicification, carbonization. The present invention relates to this direction, however, information sources that disclose methods for modifying the surface of graphite with silicon carbide are not identified.

The basis of the invention is the task of expanding the arsenal of funds and creating a new product with a base of graphite and the corresponding method for modifying the surface layer of this product. The technical result achieved is an increase in the thickness of the protective layer and strength characteristics of the product due to the transformation of the structure of the surface layer of the base of the product and without changing the geometric dimensions of the product itself.

The problem in the first object of the invention - the product - is solved in that it is characterized by the presence of a base of graphite and a surface modified layer consisting of graphite base and silicon carbide. The near-surface layer was obtained by replacing part of the graphite base with silicon carbide, with the formation of a branched dendritic crystalline structure of silicon carbide inside graphite.

A surface layer of silicon carbide can be further formed on the surface layer of the article. However, unlike the prototype, this additional coating is applied not on a graphite base, but on a modified layer, including silicon carbide, the concentration of which is maximum at the surface.

The problem in the second object of the invention - the method - is solved by the fact that the surface modification of the product is carried out in a vacuum furnace by forming a near-surface modified layer, which is carried out by providing a thermochemical heterogeneous reaction of the graphite base with molten silicon in contact with the surface of the base, and carbon monoxide, which served in a vacuum oven. Moreover, the above reaction is accompanied by the replacement of part of the base graphite with silicon carbide with the formation of a branched dendritic crystal structure of silicon carbide inside graphite.

The contact of the silicon melt with the surface of the base can be obtained in various ways, for example, it is formed:

- when molten silicon, previously applied in powder form to the surface of the base,

- when the silicon film is molten, pre-formed on the surface of the base,

- when molten silicon wafers, previously placed on the surface of the base,

- by dipping the base into a silicon melt and drawing it from the melt formed in the crucible, which is placed in a vacuum furnace and pre-filled with silicon.

It is possible to additionally carry out chemical etching or vacuum annealing.

In order to better demonstrate the distinctive features of the invention, as examples without any limiting nature, the preferred embodiments are described below with respect to an article based on a graphite plate. Examples of implementation are illustrated by the figures of the drawings, which show:

FIG. 1 is an implementation of a method in which a silicon film is preliminarily deposited on a surface of a substrate;

Figure 2 - schematically shows the implementation of the method in which silicon powder is preliminarily applied to the surface of the substrate;

FIG. 3 - implementation of a method in which dipping the base into a silicon melt and pulling it out of the melt;

FIG. 4. - implementation of the method in which a silicon wafer (single-crystal or amorphous) is placed on the surface of the base (above).

In FIG. 1 - FIG. 4 is indicated: 1 - a vacuum furnace (casing walls), 2 - an inlet pipe for supplying CO, 3 - an outlet pipe for the removal of gaseous reaction products, 4 - a product from graphite (initial state), 5 - thermal zone of the furnace, 6 - silicon film, previously deposited on a graphite base, 7 — silicon powder, 8 — crucible with molten silicon, 9 — movable product holder, 10 — silicon plate.

In FIG. 5. SEM image of the surface of the product manufactured according to Example 1, obtained by scanning electron microscope,

FIG. 6. SEM image of the cross section of the product with a fragmented modified near-surface layer,

FIG. 7. - SEM image of SiC microcrystals on a cross section of a modified near-surface layer,

FIG. 8. Micro-Raman spectrum of the initial surface of the product (graphite), and the surface of the product, after the implementation of the method.

FIG. 9. Micro-Raman spectra of individual SiC crystallites found in the volume of the modified near-surface layer.

To implement the method illustrated in FIG. 1, the base (a flat plate made of graphite grade MPG-7 with a size of 30 * 30 * 5 mm) of a product with a silicon film 1 mm thick, previously applied to the surface, is placed in the furnace 1. The internal volume of the furnace is heated in vacuum 10 ^-2 -10 ^{- 3} Pa to a temperature of about 950 ° C. Then, CO gas is supplied through the inlet pipe 2 to the internal volume of the furnace (CO may be mixed with an inert gas — argon or xenon, the reagent remains CO) at a pressure of 5–20 Pa and the furnace chamber is heated to the silicon melting temperature (1410 ° C) . Stand for 5 minutes, then the temperature is lowered to a temperature below the melting point of silicon (up to 1350 ° C) while maintaining the flow of CO and pressure in the furnace. At this temperature, the product is kept for 20 minutes, after which the gas is pumped out, the furnace is cooled, and the product is removed with a modified surface layer with a thickness of the order of 950 μm or more, which depends on the thickness of the deposited silicon layer.

In contrast to the prototype, in which, in the presence of hydrocarbons, a surface coating of SiC is formed on a graphite-based product, in the present method, the surface layer is modified in the presence of CO gas in a vacuum furnace. This is due to another process taking place in the furnace.

The formation of the near-surface modified layer is carried out due to the thermochemical heterogeneous reaction of the reactants with the graphite base as follows.

Before reaching the melting temperature of silicon (T <1410 ° C), carbon monoxide interacts with silicon previously deposited on graphite, coating silicon with a thin (up to 100 nm thick) layer of silicon carbide according to the reaction:

This SiC layer protects the silicon being melted from evaporation and further conversion of the entire volume of Si to silicon carbide. We note the last circumstance is very important. If all of the silicon turned into SiC, it would not have melted at the melting temperature of silicon T = 1410 ° C and could not penetrate deep into the graphite base. Then, with a further increase in temperature to the melting point of silicon (T = 1410 ° C), the following occurs. The authors found that the CO gas can easily and deeply penetrate through the initially formed SiC crust saturated with silicon vacancies. In this case, gas diffusion does not occur, but a process resembling a gas flow through channels. At the time of silicon melting, the penetration of CO gas is greatly enhanced. Unlike CO gas, the reaction product — SiO gas — leaves the reaction zone much more slowly than CO. This is due to the fact that the SiO molecule is larger, and also due to the fact that when passing CO during heating to the melting temperature, the initial channels in Si are “overgrown” with the SiC reaction product to a greater depth, changing its diameter. As a result, the SiO gas cannot completely come out and the rate of chemical reaction (1) slows down and equilibrium is established. The melt under the SiC “boils”, which can lead to cavitation at the boundary of the SiC layer and at the boundary with the graphite base. As a result, the melt is well convective mixed. When this occurs, the outflow of SiO in the direction of the graphite base by the reaction:

At the boundary of the melt with graphite, reaction (2) occurs, as a result of which the following processes occur.

1. Part of the reaction product (1) is spent on reaction (2), which leads to a shift in the equilibrium of reaction (1) towards the formation of SiC.

2. As a result of reaction (2), one of the carbon atoms is removed from the graphite base, forming vacancies that can merge into nanopores and microchannels, while liquid silicon saturated with SiO gas “breaks out” and penetrates into the graphite from the SiC film layer, filling nanopores, microchannels, and other voids in graphite. CO gas easily penetrates deep into the base, since the removed atoms and silicon, according to reaction (1) and carbon, according to reaction (2) form voids in graphite, greatly facilitating the process of penetration of gases and silicon into it. The synthesis lasts 20 minutes During this time, all silicon is converted to silicon carbide. Since micropores formed in carbon at the intermediate stage, which were filled with silicon carbide, the final structure consists of lacunae with a branched dendrite-like crystal structure filled with SiC inside a graphite base with an increase in the SiC concentration on the surface of the product to almost 100% (see Fig. 5 and Fig. 8).

The authors found that molten silicon without the addition of CO cannot completely penetrate deep into graphite. For complete penetration of Si into graphite, reactions (1) and (2) must necessarily occur simultaneously. They simultaneously ensure the occurrence of two processes, namely, the initial creation of a surface crust of SiC, which protects against evaporation of Si, and plays the role of a “lid” that holds SiO in the melt until its concentration is created in it for reaction (2), which prepares the base graphite for rapid penetration of silicon carbide melt into it. In addition, after the mixture of SiC and Si is already in graphite, reactions (1) and (2) complete the conversion of the residues of graphite and silicon into SiC.

Note that nothing of the kind happens in the technology described in the well-known publication (Haq K.E., Khan I.N. Surface Characteristics and Electrical Conduction of β-SiC Films Formed by Chemical Conversion. Journal of Vacuum Science and Technology, ( 1970) 7 (4), 490-493.doi: 10.1116 / 1.1315373), nor in the prototype (RU 2601049), in which the reaction of formation of SiC occurs when silicon is combined with hydrocarbons. In all these technologies, silicon combines with carbon, even if it is taken from silicon itself (Haq K.E., Khan I.N. Surface Characteristics and Electrical Conduction of β-SiC Films Formed by Chemical Conversion. Journal of Vacuum Science and Technology , (1970) 7 (4), 490-493. Doi: 10.1116 / 1.1315373)). In this case, the silicon atom is attached to carbon, which increases the molecular volume and does not allow the formation of SiC inside graphite, but only on its surface. On the contrary, in our case, both the silicon atom by reaction (1) and the carbon atom by reaction (2) are removed from the system, which reduces the atomic volume and leads to the formation of vacancies and voids for filling them with silicon carbide. In contrast to the method according to patent RU 2363067, in the claimed method, silicon is melted, which leads to reaction (2), which cannot occur if SiO gas is released on the SiC surface and does not create the required pressure inside the molten silicon.

Thus, in the inventive method in the furnace, a SiC film is created, silicon is melted under a SiC film on the base surface, a thermochemical heterogeneous reaction (1) with carbon monoxide and a thermochemical heterogeneous reaction of graphite with molten silicon and silicon monoxide and the formation of a near-surface modified layer.

The above modes of the method (temperature, time intervals, pressure) are obtained experimentally and can vary within certain limits. It depends on many factors (brand of graphite, specified thickness of the surface layer, thickness of the pre-applied film, etc.)

The product with a modified near-surface layer was studied by optical microscopy, scanning electron microscopy, Raman spectroscopy. The structure of the modified near-surface layer is presented in the images of scanning electron microscopy (Fig. 5. Fig. 6). The characteristic size of the crystals on the surface is of the order of several microns, which is an order of magnitude smaller than in the prototype method. From FIG. Figure 6 shows that the thickness of the SiC layer was about 950 μm. In FIG. 7 is a SEM image of individual crystallites in a cross section of a sample. Their size is also on the order of several microns. FIG. 8 shows the micro-Raman spectra of the surface of the article before (dashed line) and after modification (solid line). The micro-Raman spectrum of the initial surface corresponds to graphite, while the modified surface consists mainly of silicon carbide - the SiC line of the cubic polytype (796 obr. Cm) dominates in the spectrum. It should be noted that the spectrum shows averaging over a region of size 50 * 50 μm ² . The micro-Raman spectrum of individual crystallites, taken from the region of 300 * 300 nm ² (see Fig. 9), shows clear lines of pure silicon carbide on the outer surface of the product.

Similarly to the above, it is possible to obtain a modified surface layer, previously, before loading into the furnace, by applying a layer of silicon powder with a thickness of 50 μm or more and a particle size of 1 μm or more on the graphite base of the product.

Similarly to the above described, it is possible to obtain a modified near-surface layer, previously, prior to loading into the furnace, by placing a silicon wafer with a thickness of 100 μm or more on a graphite-based product.

With preliminary deposition of a silicon film, it is possible to obtain a more uniform modified surface layer on the curved surface of the base and provide more precise control over the thickness of the coating than when applying a layer of powder or applying a plate. However, the method using a plate or a layer of powder, applicable only to a flat surface of the base, has its own advantage - there is no need to use specialized reactors for spraying thin films of silicon, which greatly simplifies the technology as a whole.

Also similar to that described above, it is possible to modify the surface layer using the technological method of wetting the graphite surface with a silicon melt by dipping the base into the melt and drawing it out. For this, the graphite base is fixed on a movable holder 9 (Fig. 3), under which a crucible with silicon is installed 8. When the furnace is heated above the melting temperature of silicon, silicon in the crucible forms a melt. The product is immersed in a silicon melt using a holder and then slowly drawn out in the presence of CO gas. After that, the furnace is cooled to a temperature below the melting temperature of silicon and kept at a temperature in the range of 800-1380 ° C. Then, the CO gas supply is stopped and the furnace is cooled to room temperature. After cooling the furnace, a product with a modified near-surface layer is removed over the entire surface of the product.

After the formation of the surface layer, if necessary, the surface of the product can be subjected to additional etching and heat treatment, for example, in air at 500-800 ° C to remove unwanted contaminants. As liquid etchants, acids, for example, nitric or perchloric acids, can be used. Boiling in these acids makes it possible to clean the surfaces of silicon carbide from technological impurities. It is also possible to conduct additional heat treatment of the product at temperatures up to 1400 ° C (vacuum annealing) to affect the structural features of the silicon carbide film.

Thus, according to the claimed method, a new graphite product was obtained with a surface layer formed by replacing part of the base graphite with silicon carbide, with the formation of a branched dendritic crystal structure of silicon carbide inside graphite. This inner (deep into the product) layer increases the strength properties of the product, its chemical and thermal resistance. The technology allows the formation of a surface layer up to 1 mm thick.

In contrast to known technologies in which the protective surface layer is held on the basis of adhesive surface interaction with the base material, the surface layer formed according to the claimed method is held at the base due to the branched dendritic incorporation of silicon carbide into the graphite base.

Claims (8)

1. The product, characterized in that it has a base of graphite, while the surface modified layer of the product consists of base graphite and silicon carbide and is obtained by replacing part of the base graphite with silicon carbide with the formation of a branched dendrite-like crystalline structure of silicon carbide inside graphite. 2. The product according to claim 1, characterized in that in addition to the surface modified layer there is a surface layer of silicon carbide. 3. A method of modifying the surface of an article having a graphite base, characterized in that the surface modification of the article is carried out in a vacuum furnace by forming a near-surface modified layer, which is carried out by providing a thermochemical heterogeneous reaction of the graphite base with the molten silicon in contact with the surface of the base and monoxide carbon supplied to the vacuum furnace, wherein the reaction is accompanied by the replacement of part of the base graphite with silicon carbide with the formation of a branched dendritic crystalline structure of silicon carbide inside graphite. 4. The method according to p. 3, characterized in that the contact of the silicon melt with the surface of the base is formed during the melt of silicon, previously applied in powder form to the surface of the base. 5. The method according to p. 3, characterized in that the contact of the silicon melt with the surface of the base is formed during the melt of a silicon film pre-formed on the surface of the base. 6. The method according to p. 3, characterized in that the contact of the silicon melt with the surface of the base is formed during the melt of the silicon plate previously placed on the surface of the base. 7. The method according to p. 3, characterized in that the contact of the silicon melt with the surface of the base is formed by dipping the base into the silicon melt and pulling it from the melt formed in the crucible, which is placed in a vacuum furnace and pre-filled with silicon. 8. The method according to any one of paragraphs. 3-7, characterized in that after surface modification, chemical etching or vacuum annealing is additionally carried out.

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