RU2415109C1 - Nanostructured ceramic matrix composite material and method of producing said material - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to machine engineering ceramics, particularly ceramic matrix composite material based on silicon carbide and reinforced with carbon fibre. The composite material contains a matrix of reaction-sintered silicon carbide, reinforced with bundles of carbon filaments, separated from the matrix by a barrier layer. The filaments in the bundles are joined to each other by a dense sintered nanosize inter-filament phase which contains carbon, silicon, boron, nitrogen, as well as aluminium and yttrium oxide compounds. The barrier layer and the inter-filament phase do not contain free carbon. The method of producing the composite material involves pre-treatment of the bundles of carbon filaments with a suspension which contains polymer binder and nanoparticles of silicon nitride, aluminium and yttrium oxide compounds and at least one component selected from: boron, silicon, boron or silicon compound. After drying, the fibre mass is saturated with coke-forming binder which contains silicon carbide particles, the workpiece is moulded, the binder is cured, followed by a carbonisation step, synthesis and sintering of the inter-filament phase, and then a step for saturating the workpiece with solutions or melts of coke-forming polymers and silicification.

EFFECT: obtaining material with high characteristic strength and impact viscosity.

8 cl, 1 tbl, 2 dwg

Description

The invention relates to the field of engineering ceramics, in particular to a ceramic-matrix composite material based on silicon carbide reinforced with carbon fibers (hereinafter, composite) with enhanced strength and toughness characteristics and a method for its preparation. The proposed composite can be used for the manufacture of structural parts operating under high mechanical and dynamic loads.

For mass production of products from C / SIC composites, it is most expedient to use reaction sintering technology in which a matrix of reactive silicon carbide is formed by impregnating porous carbon-carbon preforms with a molten silicon [Krenkel W. Carbon Fiber Reinforced CMC for High-Performance Structures // International Journal of Applied Ceramic Technology. - 2004. - V.1, No. 2. - P.188-200]. The main disadvantage of this technology is the interaction of liquid silicon with carbon fibers with the formation of silicon carbide, which degrades the strength of the fibers themselves and the properties of the fiber-matrix interface and thereby leads to a significant decrease in the strength characteristics and the brittle nature of the destruction of the material. In order to avoid such negative phenomena, carbon fibers are protected from contact with the silicon melt by creating barrier coatings (layers) at the fiber - matrix interface.

As a rule, the reinforcing fibers in the composite are represented by bundles of elementary fibers (filaments), which requires the protection of two interfaces. The barrier layer at the interface “carbon filament bundle - silicon carbide matrix” protects the entire filament bundle from the penetration of the silicon melt into the bundle. Individual filaments can be protected by filament coatings or by an interfilament matrix (phase) that binds individual filaments within the bundle. The role of the inter-filament phase is not only to protect the filaments from the effects of the silicon melt and from oxidation, but also to create the necessary level of adhesion at the “filament - inter-filament phase” interface, in which the filaments are pulled out of the inter-filament phase when the material breaks, which causes a plastic (non-fragile) ) the nature of the destruction and high toughness of the composite.

Most of the known methods for protecting reinforcing fibers are based on the application of barrier coatings, for example, of carbon, silicon carbide, boron nitride to the initial fibers by chemical vapor deposition (CGD). The common and main disadvantage of the methods for the manufacture of composites, including the coating of reinforcing carbon fibers by the CGO method (for example, implemented in the inventions US 6110527 (IPC С04В 35/573, С04В 35/628, publ. 08.29.2000) RU 2137732 (С04В 35/80, publ. . September 20, 1999), is the inaccessibility and high cost of raw materials, the long deposition cycle, the need for sophisticated technological equipment, which leads to significant complication and cost of the material manufacturing process and reduces the main advantages of reaction sintering technology. A drawback of these methods, as well as manufactured composites, is that thin coatings applied from the gas phase to individual filaments may have poor protective properties due to the presence of microcracks and pores. Increasing the thickness of coatings requires long impregnation cycles, which not only complicate process, but also lead to the formation of closed pores inside the bundles of filaments and between bundles, preventing uniform impregnation of the workpiece with a molten silicon [Y. Xu, L. Cheng, Carbon. - 1999, 37, 1179-1187]. In addition, the stages of preliminary deposition of the CGO coating are usually incompatible with the subsequent operations of forming blanks by winding and weaving due to the increased stiffness of the fiber bundle and the destruction of the coating during bending of the filament during molding, which imposes limitations on the technological capabilities of manufacturing products.

Known methods of protecting carbon fibers by applying a layer of coke-forming binder, forming a carbon matrix of the workpiece in the carbonization process. At the carbonization stage, the polymer layer on the fibers (usually on fiber bundles) is converted to carbon, and at the stage of siliconization, the carbon is converted to silicon carbide. Krenkel and Cern [W. Krenkel, F. Gern, ICCM-9 Proc., July 12-16, 1993, Madrid, Spain] investigated this method of protecting carbon fibers, i.e. in fact, using a carbon matrix that was used for reaction sintering. By controlling the adhesion between the fiber and the carbon source, they obtained a carbon matrix containing dense protective sections and a controlled crack structure, which made it possible to quickly impregnate silicon, but limited the degree of reaction sintering. As a result of this, the composite obtained had an inhomogeneous structure and did not possess sufficiently good mechanical properties. In most of these methods, for example, DE 4438456 (IPC С04В 35/573; C04B 35 / 80D; С04В 37 / 00В, publ. 02.05.1996) the formed barrier coatings on the beams contain silicon carbide and carbon. The main disadvantage of these methods is the poor protection of the filaments due to the lack of filament coating or inter-filament phase.

The best protection of filaments and the realization of high strength properties are achieved in methods that provide both the creation of an external barrier coating on the beams and the impregnation of the beams with the formation of an interfilament phase.

A known method of manufacturing a composite RU 2337083 (IPC С04В 35/83, С04В 35/532, С04В 35/577, publ. 10/27/2008), in which at the first stage of impregnation with a weakly concentrated binder solution, the preliminary formation of the interfilament phase occurs, at the second stage of impregnation with concentrated a solution of a binder with fillers forms a polymer coating on fiber bundles simultaneously with a polymer matrix filled with ceramic particles. After passing through the subsequent stages of carbonization and silicification, the inter-filament phase is porous carbonized carbon, and the coating on bundles of filaments is silicon carbide. The disadvantage of this method is the relatively weak protection of the filaments from the penetration of the silicon melt into the beam, since the interfilament phase is carbonized carbon, which inevitably contains microcracks and pores. In addition, impregnation with a concentrated binder solution with preliminary formation of both the matrix and the coating on the beams in one stage leads to the production of a sufficiently thick layer of carbon on the beams after carbonization, which cannot be completely precipitated and remains in the microstructure as free carbon, worsening the properties of the composite . On the other hand, the introduction of solid particles of ceramic fillers during the mentioned impregnation is associated with the risk of breaking the uniformity and continuity of the preliminary polymer film on the beams, reducing the protective properties of the barrier layer. These disadvantages of the method degrade the performance of the finished composite.

The known method EP 1818320 (IPC С04В 35/80; СВВ 41/50, publ. 08/15/2007), relating to the fibrous composite material and the method of its manufacture. This composite contains a Si-SiC matrix and a reinforcing structure of filaments, each of which includes at least one bundle of carbon fibers and a carbon component that is not a carbon fiber. A method of manufacturing a fibrous composite material includes: introducing a powder component (from resin, coke etc.) inside the bundles; creating a coating of thermoplastic polymer around bundles of carbon fibers; high temperature (up to 2500 ° С) annealing of the formed or annealed product. The disadvantages of this method are the need to use the aforementioned high-temperature annealing to graphite the carbon of the interfilament phase, as well as the need to use hot pressing during molding to impregnate layers of filaments with a thermoplastic polymer. These operations complicate the process. In addition, impregnation with a thermoplastic polymer cannot ensure its uniform and uniform distribution over the surface of the filaments and good adhesion of the future barrier coating to the surface of the beams. The disadvantage of the composite according to this invention is the presence of regions of free carbon forming the interfilament phase of the composite, which reduces its oxidative stability and degrades mechanical properties.

Closest to the present invention is the patent RU 2184715, (IPC C04B 35/83, 35/573, 41/88, publ. 02.24.1998), which protects the reaction-sintered composite with improved properties and its production method suitable for large-scale production. The composite is represented by a carbide-silicon matrix reinforced with short bundles of carbon fibers (filaments), which are externally surrounded by a barrier coating containing at least a surface layer of carbon. Moreover, inside the bundles, the fibers (filaments) are interconnected by a continuous inter-filament phase with a possible content of ceramic particles.

The disadvantage of the obtained composite is the low density of the interfilament phase, the presence of pores and free carbon in it due to its unsintered state. In addition, the barrier layer on the filament bundles contains free carbon. All this leads to reduced mechanical characteristics of the composite.

In this method, bundles of filaments are impregnated (i.e., fill the space between filaments) and conditioned (i.e., coated externally) with a binder (including coke-forming) with the possible addition of particles of ceramic fillers, such as carbides, nitrides, borides. Then the preform is molded, the binder is vulcanized, carbonized and silicified.

A significant disadvantage of this method is that it does not provide sintering of the interfilament phase, since when impregnating bundles of filaments, functional additives conducive to sintering are not introduced. After low-temperature (800 ° C) carbonization of the binder, synthesis and sintering of the inter-filament phase are not carried out, which makes the presence of free carbon and pores inside the bundles of filaments inevitable. Pores reduce the protective properties of the interfilament phase, opening the access of the silicon melt to the filaments during silicification.

Another disadvantage of this method is that the beams are coated with a coke-forming binder prior to the vulcanization and carbonization of the binder. This leads to obtaining after carbonization a sufficiently thick layer of carbon in the beams, which cannot be completely precipitated, which leads to the formation of a barrier layer containing not only silicon carbide, but also free carbon. The use of hard-to-reach and expensive silicon and organosilicon polymers, according to additional variants of the method, significantly complicates the manufacturing process, making it less suitable for mass production.

The objective of the invention is to create a ceramic composite material with a matrix based on reactive silicon carbide reinforced with carbon fibers, which has high density, bending strength, impact strength and demonstrates the non-fragile nature of the fracture, made in an inexpensive way, suitable for mass production of products from available raw materials.

The present invention relates to a nanostructured ceramic matrix material comprising a matrix of reactive silicon carbide reinforced with carbon filament beams separated from the matrix by a barrier layer and containing an interfilament phase comprising elements such as carbon, silicon, boron, nitrogen, in which the barrier layer consists exclusively of silicon carbide. The interfilament phase has a dense sintered nanoscale microstructure and additionally contains oxide compounds of aluminum and yttrium in an amount of 5 to 10 wt.%, And the carbon entering it is firmly bound.

The protection of carbon filaments from the influence of a silicon melt is achieved by creating two protective structures: a barrier layer on bundles of filaments of silicon carbide and a dense sintered nanoscale interfilament phase made of ceramic, which additionally provides the necessary level of adhesion at the “filament - interfilament phase” interface.

The absence of free carbon in the composition of the barrier layer and the interfilament phase further contributes to the improvement of the mechanical characteristics of the composite, as well as its resistance to oxidation, wear resistance, and integral hardness.

When the content of aluminum and yttrium oxide compounds in an amount of less than 5 wt.%, The structure of the inter-filament phase is green and porous, characterized by reduced protective functions; and with a content of more than 10 wt.%, the strength of the inter-filament phase decreases and the adhesion level at the “filament-inter-filament phase” interface changes significantly.

The nanoscale microstructure of the interfilament phase determines its high density and high protective properties and, therefore, a high level of mechanical characteristics of the composite as a whole.

In preferred embodiments, the inter-filament phase contains grains no larger than 100 nm in size and the pore volume fraction of the inter-filament phase does not exceed 2%.

A method for producing a nanostructured ceramic composite material is proposed, which includes the steps of impregnating the pulp with a coke-forming binder containing silicon carbide particles, molding the billet, vulcanizing the binder, carbonizing and siliconizing, in which, before the stage of impregnating the pulp, pre-treatment of carbon filament beams under the influence of ultrasonic vibrations with a suspension containing binder and nanosized particles of silicon nitride and functional ext approx comprising oxides of yttrium and aluminum and at least one component from the group of boron, silicon, boron or silicon compound, and after the carbonization step is carried out sequentially a step of synthesizing and sintering phase and mezhfilamentnoy step impregnation of the preform coke-forming solutions or melts of polymers.

The proposed method for producing a composite provides the most complete protection of carbon filaments from the effects of a silicon melt during silicification due to the fact that the suspension at the stage of preliminary processing of carbon filament bundles contains functional additives, including a mixture of yttrium and aluminum oxides. Due to this, after an additional stage of synthesis and sintering, the formation of a dense sintered interfilament phase from ceramics containing no free carbon is achieved.

After carbonization of the polymer binder, the stage of synthesis and sintering of the inter-filament phase is carried out, in which the nanosized particles of functional additives, such as silicon, boron, a compound of boron or silicon with carbonized carbon inside the beams, are formed with the formation of nanoscale grains of secondary products between the grains of the starting silicon nitride and subsequent sintering . Nanosized oxide additives provide effective liquid-phase sintering and the formation of a continuous dense interfilament phase inside the beam.

Carrying out the impregnation of the preform with solutions or melts of coke-forming polymers after the synthesis and sintering stages promotes the preliminary formation of a barrier layer of small thickness, which allows for the complete siliconization of this layer in the subsequent stage and to avoid the presence of residual free carbon in it. Preliminary formation of the barrier layer occurs in the gap between the fiber bundles and the carbonized matrix, which is formed due to the shrinkage of the latter in the previous stages. In addition, the said impregnation allows you to fill microcracks and pores in the matrix of the workpiece with the achievement of a more dense microstructure.

In preferred embodiments, the particle size of the solid phase of the suspension, consisting of particles of silicon nitride and functional additives, does not exceed 100 nm, which contributes to the formation of a denser and finer microstructure.

In preferred embodiments, the content of the binder in the suspension does not exceed 15 wt.%, And the solids content does not exceed 35 wt.%.

An excess content of the binder and / or solid phase can lead to an increased viscosity of the suspension, in which it either does not penetrate into the bundles, or is distributed unevenly between the filaments.

Preferably, the step of synthesizing and sintering the inter-filament phase is carried out at 1350-1500 ° C. in a nitrogen atmosphere or in vacuum.

At the stage of synthesis and sintering, a temperature below 1350 ° C is insufficient for the synthesis of secondary products and sintering, and a rise in temperature above 1500 ° C causes excessive grain growth, worsening the properties of the interfilament phase.

An additional advantage of the method is that the strength properties of the siliconized composite are controlled by varying certain technological parameters, such as: chemical composition and amount of functional additives, temperature and atmosphere at the stage of synthesis and sintering of the interfilament phase. The proposed method is provided by the use of simple and suitable for mass production operations and available domestic raw materials.

Example 1

The carbon fiber of the LO brand is placed in a steel tray and poured with a pre-prepared suspension in an organic solvent of the following composition per 1 liter of solvent, for example ethyl alcohol:

polymer binder LBS-1 - 50 g,

Si ₃ N ₄ powder with a particle size of not more than 200 nm - 285 g,

a mixture of Al ₂ O ₃ and Y ₂ O ₃ with a particle size of not more than 100 nm - 18 g.

Si powder with a particle size of not more than 200 nm - 39 g,

Impregnation is carried out under the influence of ultrasonic vibrations. After impregnation and drying, a solution of coke-forming polymer binder grade LBS-1 containing 50 wt.% SiC powder with an average particle size of 8 μm is applied to the pulp and re-dried. The finished prepreg is cut to the desired shape and size and placed in the mold. The molding is carried out under a pressure of 10 MPa at a temperature of 180-200 ° C. Molded preforms are carbonized by heating from 200 to 1380 ° C in vacuum, and after holding for 1 hour, the temperature is raised to 1500 ° C and held for 1 hour at this temperature in a nitrogen atmosphere. Next, the heat-treated workpieces are impregnated with a melt of oil pitch in vacuum at a temperature of 150 ° C. The impregnated preforms are installed in cassettes and subjected to silicification in vacuum at a temperature of 1450 ° C with an exposure of 1 hour. The properties of the obtained material are given in the table.

Figures 1 and 2 respectively show a photograph of the microstructure of a flat thin section of a composite in the field of view of an optical microscope (× 1750) and a fractogram of the fracture surface in the field of view of a scanning electron microscope, which show carbon filaments inside the beam that are interconnected by a dense continuous nanoscale interfilament phase .

Example 2

Carbon thread grade N-205 is pre-treated by pulling through a container with an impregnating suspension in an organic solvent of the following composition, per 1 liter, for example, ethyl alcohol:

polyvinyl alcohol - 50 g

powder Si ₃ N ₄ with a particle size of not more than 300 nm - 220 g

Al ₂ O ₃ + Y ₂ O ₃ powder with a particle size of not more than 100 nm - 28 g

B ₂ O ₃ with a particle size of not more than 200 nm - 100 g,

The thread exiting the impregnation tank is pulled through the drying chamber. When passing through a container with an impregnating suspension and a drying chamber, the thread is subjected to ultrasonic treatment. The processed thread is installed on a winding machine and winding the workpiece after passing the thread through a container with a binder of the following composition

coke-forming polymer binder LBS-1 - 50%

SiC powder with an average particle size of 10 μm - 50%,

Molded preforms are subjected to vulcanization in air at a temperature of 180-200 ° C and then heat treatment when heated in a stream of nitrogen from 200 to 1500 ° C with an exposure of 1 hour at a specified temperature. Heat-treated preforms are impregnated under vacuum with LBS-1 bakelite varnish with exposure at a residual pressure of not more than 10 ³ Pa for at least 1 hour for every 15 cm ^{3 of} preform. The impregnated preforms are dried in air to a temperature of 180-200 ° C and subjected to silicification at a temperature of 1450 ° C with an exposure of 1 hour.

The properties of the obtained composite are shown in the table.

Example 3

The method according to example 2, in which to impregnate the carbon filament H-205, a suspension of an organic solvent of the following composition is used, calculated per 1 liter of isopropyl alcohol:

- silicone polymer binder - 200 g;

- Si ₃ N ₄ powder with a particle size of not more than 100 nm - 300 g;

- powder Al ₂ O ₃ + Y ₂ O ₃ with a particle size of not more than 100 nm - 35 g;

The properties of the obtained composite are shown in the table.

Example 4

The method according to example 1, in which for the impregnation of carbon fiber brand LO using a suspension of an organic solvent of the following composition, based on 1 liter of ethyl alcohol:

- silicone polymer binder - 150 g;

- Si ₃ N ₄ powder with a particle size of not more than 200 nm - 200 g;

- powder Al ₂ O ₃ + Y ₂ O ₃ with a particle size of not more than 100 nm - 30 g;

- Si powder with a particle size of not more than 200 nm - 100 g,

and heat treatment is carried out when heated in a stream of nitrogen from 200 to 1500 ° C with an exposure of 1 hour at the specified temperature.

The properties of the obtained composite are shown in the table.

Example 5

The composite and the method according to example 4, in which the impregnation of the carbon fiber is carried out without ultrasonic treatment. The properties of the obtained composite are shown in the table.

Example 6

The composite and the method of example 1, in which particles of Si ₃ N ₄ and Si with a size of 0.5-1.0 μm are used to prepare the suspension.

The properties of the obtained composite are shown in the table.

Example 7

The composite and the method of example 2, in which the solids content in the suspension is 45 wt.%.

The properties of the obtained composite are shown in the table.

Claims (8)

1. Nanostructured ceramic matrix material, comprising a matrix of reactive silicon carbide reinforced with carbon filament beams separated from the matrix by a barrier layer and containing an interfilament phase, including elements such as carbon, silicon, boron, nitrogen, characterized in that the barrier layer contains exclusively silicon carbide, and the interfilament phase has a dense sintered nanoscale microstructure and additionally contains oxide compounds of aluminum and yttrium in an amount of 5 d 10 wt.%, Wherein carbon is included in its composition, is completely bound. 2. The material according to claim 1, characterized in that the inter-filament phase contains grains no larger than 100 nm in size. 3. The material according to claim 1, characterized in that the volume fraction of pores of the inter-filament phase does not exceed 2%. 4. A method for producing a nanostructured ceramic matrix material, comprising the steps of impregnating the pulp with a coke-forming binder containing silicon carbide particles, molding the billet, curing the binder, carbonizing and siliconizing, characterized in that, before the stage of impregnating the pulp, they additionally process carbon fiber bundles under the influence of ultrasonic oscillations in a suspension containing a binder and nanosized particles of silicon nitride and functional ext wok comprising oxides of yttrium and aluminum and at least one component from the group of boron, silicon, boron or silicon compound, and after the carbonization step is carried out sequentially a step of synthesizing and sintering phase and mezhfilamentnoy step impregnation of the preform coke-forming solutions or melts of polymers. 5. The method according to claim 4, characterized in that the particle size of the solid phase in the suspension does not exceed 100 nm. 6. The method according to claim 4, characterized in that the content of the binder in the suspension does not exceed 15 wt.%. 7. The method according to claim 4, characterized in that the solids content in the suspension does not exceed 35 wt.%. 8. The method according to claim 4, characterized in that the stage of synthesis and sintering of the inter-filament phase is carried out at 1350-1500 ° C in nitrogen atmosphere or in vacuum.

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