DE19743723A1 - Coating reinforcing material for ceramic composites - Google Patents

Abstract

A process for coating reinforcing components for ceramic composites comprises treating fibers, woven fabric or mats with a sol by means of a liquid coating method and then compacting the coating by thermal treatment.

Description

The invention relates to a method for manufacturing of coatings on reinforcement components for Composite ceramics, especially non-oxide Ver bundle ceramics. The different reinforcement compo fibers, mat made from them ten, fabric, short fibers or similar components, made of carbon, silicon carbide, silicon (oxy) carbide, Silicon nitride or silicon (carbo) nitride.

These can be used in the production of composite ceramics Fabrics can also be used as matrix elements. There at an embedding is to be used in each case the gain component in a pre-stage matrix, which is then pyrolytically converted into the ceramic form becomes. Ceramic materials produced in this way then have inherently a certain porosity.

Contrary to the use of metallic or on Plastic-based matrices must be observed that the integration of the fibers or the other Ver Strengthening components are not made in solid form may. The integration should have a so-called In interface. The Inter to be used face a significantly reduced strength than that  Fibers or the matrix. With the additional use The properties are related to the use of an interface Lich deflection, crack bridging and pull-out improved (pull-out effect) and a composite ceramics assembled in this way has a high Strength and fracture toughness. As materials graphite is preferred for such an interface and hexagonal boron nitride because they have a Have layer structure.

For the fabrics used for the reinforcement components problems occur when it is used exposed to higher temperatures. Here, for. B. Carbon fibers already at temperatures from approx. 500 ° C degraded oxidatively and accordingly the mechanical Strength of the composite formed in the ceramic irreversibly destroyed.

Since, as already mentioned, the ceramic is porous, bie the matrix only provides limited protection against it thermal influences and the application of an ent protective layer on the reinforcement used Component is essential if the Composite ceramics even at high temperatures and in be used in an oxygen-containing atmosphere what with ceramic materials with great certainty is the case. So can conveniently Beschich SiC are applied to the thermal Increase resilience.

The required layer thicknesses are on the Fibers or corresponding tissues made from them, in the area between 50 and 5000 nm.

Known procedures for applying such protection  layers are e.g. B. PVD and CVD processes, wherein Problems of a technological nature in particular occur that complicated geometries anyway must be coated evenly and in particular re when coating fiber mats with large difficult to access inner surface areas of the Technological effort increases and not all Coating processes lead to success without further ado ren. A possibly usable coating The CVI process (Chemical Vapor In filtration), for which very high investment and Operating costs are required and the Be stratification occurs at low speed and only very simple stoichiometries can be mastered can. So with such a CVI process for example layers of carbon and borni trid as a lubricant and with SiC as an inner oxide tion protection.

It is therefore an object of the invention to provide protective layers on reinforcement components for use in ver bundle ceramics on simple and relatively inexpensive Way to manufacture in the form that a firm adhesive, dense and even coating follows, even at temperatures above 1300 ° C is stable.

According to the invention, this object is achieved by the in claim 1 resolved characteristics. Favorable off design forms and developments of the invention result when using the in the subordinate Features mentioned claims.

In the process according to the invention, this is the case worked that precursors in the form of stable brine  and then in a liquid process, such as B. a sol-gel process, by dipping, spraying hen, brushes, squeegees or in the form of the coating of individual continuous fibers in the respective fiber system stem (bundles, mats and felts) in one continuous process, applied and subsequently thermal treatment is carried out on it, with which the applied sol in an oxidic Layer is transferred.

This will make it a suitable one Stoichiometry possible that good wettability and liability on the Ver strengthening component is achieved. The so made The layer is non-porous, coherent and adheres firmly after the thermal treatment is carried out has been.

In a simple way, the viscosity of the Sols are set and use that each time de order process then determines the necessary for this viscosity of the sol.

Suitable oxidic protective layers are α-aluminum oxide, Si-Al mixed oxides, which are preferred in the stoichiome the mullite or yttrium aluminum minium grenade because the compounds mentioned too sufficient at temperatures above 1500 ° C Secure protection.

However, very particularly suitable are oxides which have a layer structure and can therefore also take on the function of the oxidation protection and also the function which the previously used interface had to fulfill, which leads to a simplification of the manufacture of composite ceramics. Such layer materials are, for example, silicates or aluminosilicates from the illite (e.g. muscovite KAl ₂ [(OH) ₂ ¦AlSi ₃ O ₁₀ ]), the montmorillonite ((Al, Mg) ₂ [(OH) ₂ ¦Si ₄ O ₁₀ ]) or the kaolinite group (Al ₄ [(OH) ₈ ¦Si ₄ O ₁₀ ) for use at temperatures up to around 1100 ° C.

For higher temperatures up to 1800 ° C, appropriate compounds of the Al ₂ O ₃ -CaO-MgO (SrO) system derived from hibonite (CaAl ₁₂ O ₁₉ ) can be used. For calcium or in combination with calcium, however, Mg and / or Sr can also be introduced into the crystal structure.

In contrast, in the method according to the invention to known wet coating processes of fibers or fabrics are not polymers, but brine and as target compounds oxides or oxide mixtures received. This can, in addition to the improved Protection against oxidation also the pull-out behavior of the Fibers with only a single composition be improved, as z. B. with alkaline earth alumina ten possible.

The coating of the reinforcement components with egg nem liquid process is much cheaper and easier to control than with the known ones CVD process is possible and can also automati be settled.

Use the viscosity and surface tension of the ten Sols may vary depending on the order process used be set and thereby a gore-free Order, with a coherent layer, too  with very small radii of curvature of the company used sern, can be achieved.

With the invention it is in contrast to the known solutions possible to produce layers that in addition to the interface effect also a sufficient one Protection against oxidation, which was previously only possible multiple coating was possible. This can not only the effort for the production of Ver bundle ceramics reduced, but also their quality be improved. The method can be used both at Fa fibers, fiber mats and ceramics den, whereby very firmly adhering, dense oxidic Protective layers with a thickness of 40 to 20,000 nm can be generated, with a layer thickness between between 100 and 1000 nm is particularly favorable. Depending on Composition of the protective layer produced can the composite ceramics produced in this way then at Tempe temperatures up to at least 1300 ° C but also at 1800 ° C can be used easily.

Various components can be used as precursors for the production of the brine, at least two of the components mentioned below being used. The sol can consist of the components

• A) M ^II R ¹ _x R ² _2-x , where M ^II can be Mg, Ca, Sr or Ba and is 0 ≦ x ≦ 2;
• B) M ^III R ¹ _y R ² _3-y , where M ^{III can be} Al or Y and is 0 ≦ y ≦ 3 and
• C) M ^IV R ¹ _z R ² _3-z , where M ^IV can be Ti, Zr, but preferably Si and 0 ≦ z ≦ 4. In the components, R ¹ and R ² can then be, for example, alkoxy and alkacyl radicals of up to 20 carbon atoms. Examples of these are methoxy, ethoxy, propoxy, (isopropoxy) ethoxy, acetyl or propionyl radicals.

If component C) contains silicon, R but also a short-chain alkyl radical with a maximum of 8 Be carbon atoms. examples for this are Methyl, ethyl or isopropyl.

These mixtures of alcoholates or acylates are by adding water to brine that is stable in storage leads, but the addition also in the form of air moisture can occur. In addition to the Sta bilization the mixtures complexing agents, such as acety contain laceton, ethylenediamine or maleic acid.

In the event that this is necessary to a ge suitable viscosity for coating by spray hen, diving, squeegees or the other known Ver stop driving, it can be cheap inert Ver add thickener. Shall create a shift who is in the range of a mullite phase an aluminum and a silicon for the brine compound used, the silicon content between 10 and 40 mol%, preferably between 20 and 30 Mol% is.

For a protective layer made of "β-alumina", a Aluminum connection, in the manner already described and way, with an alcoholate or acylate of a Me talles of the second main group or mixtures of used this. However, this should be preferred Strontium in a range between 1 and 25 mol%, preferably between 5 and 20 mol%. Molar ratios of Al: Sr of 4: 1 are favorable and 12: 1.  

The subsequent thermal treatment should be favorable sometimes in several stages, at different temperatures repairs are carried out. With a thermal treatment at least temporarily in an inert atmosphere also has a favorable effect and thereby the burn-up of carbon fibers can be prevented.

Possible examples with which the invention Approach to be made clearer will be followed described as follows:

example 1

0.1 mole of Al (OBu ^S ) _{3 is added} , 0.1 mole of isopropoxyethanol is added dropwise and then, with simultaneous cooling, 0.5 mole of propionic acid is also slowly added. Subsequently, 0.0333 mol of methyl-trimethoxysilane are added dropwise and after the addition, the hydrolysis is carried out over a period of 1.5 h with simultaneous passage of humidified compressed air. The highly viscous, clear sol thus formed is then diluted with isopropanol in a ratio of 1: 3. The sol produced in this way has a solids content of 1.5% by mass and a dynamic viscosity of 16.8 mPas, with a dilution in a ratio of 1: 3 the solids content is 0.575% by mass and the dynamic viscosity drops to 3.8 mPas.

The brine thus produced is then over several months nate (above three months) stable. With the ther mixing treatment of the resulting gels, at Tempe In any case, mullite is found at temperatures up to 1000 ° C the.  

Example 2

In 10 g of propionic acid, 0.00508 mol Strontium metal dissolved while stirring. The strontium propionate solution thus prepared is then dropwise in 0.068 mol of an aluminum um-sec-butylate propionate, as already in Example 1 described, admitted. This mixture is then in the Diluted 1: 1 ratio with isopropanol. In the An after the hydrolysis there is a clear, colorless This highly viscous sol with a solids content of 3.73% mass fraction. This sol is then also over stable for more than three months.

Example 3

Carbon fiber mats (commercially available from Toray) are immersed in a sol of the composition 3 Al ₂ O ₃ .SiO ₂ with a solids content of 1.53% by mass without removing the size. After immersion, they are then pulled uniformly out of the sol at a constant speed of 10 cm / min and dried at room temperature for a period of up to 16 h and then at 110 ° C. for a period of 4 h. In order to largely oxidize the organic residues, the correspondingly coated mats are further subjected to a thermal treatment. A heating is carried out in steps of 100 ^° / h in air up to a total of 450 ° C and the mats are finished at a temperature of 450 ° C. A final thermal treatment is then carried out at 1000 ° C. before being added under a nitrogen atmosphere. The use of the inert gas is not necessary for the actual reaction, but it only serves to prevent the burning of the carbon fibers during the final compression of the mullite layer. After this thermal treatment, the layers formed are in a thickness of approximately 100 nm in crystalline form.

Example 4

In this example, the procedure is essentially as described in Example 3. In contrast, however, a sol with a mass fraction of 1.23% as a solid with the stoichiometry SrAl ₁₂ O _{19 is} used as a preliminary stage. The thermal treatment then takes place at temperatures of 1200 ° C.

Example 5

For the investigation of the pull-out behavior were instead of composite ceramics or fiber matted fiber bundles embedded in the model test rated.

A fiber bundle with a length of approx. 20 cm Carbon fibers (from Toray under the designation T 800 commercially available), were in the Form as described in Example 5 is coated with crystalline strontium aluminate. The correspondingly pretreated fibers are then in a silicon nitride / silicon carbonitride matrix been bedded. 50 g of polysilazane SLM 465012, 50 g silicon nitride with one particle through knives from 1 to 5 µm, 20 ml toluene and 1 g n-butyl ammonium chloride mixed over a period of 1 h and a slip using a ball mill manufactured. (Polymer and silicon nitride are more common  specifically at Wacker-Chemie GmbH, Munich, he available).

The carbon fibers were then coated by Immerse in the brine from the precursor mixtures. After immersion, the coated coal fa in a stick at a temperature of 80 ° C Dried atmosphere. Following that the carbon fiber bundles also under an inert gas condition in stages of 150 ° C / h Heated 350 ° C and then 1 h at this temperature held. As a result, the polymer crosslinks to insoluble ones and infusible products. For pyrolysis of the sili Organocium compound was at a speed from 30 ° C / h to 800 ° C and then again heated faster to 1200 ° C at 150 ° C / h. At the The final temperature was the coated carbon fiber held for 2 hours and then in Steps cooled from 150 ° C / h to room temperature.

The carbon fiber bundles thus coated were then broken and the break points optically in the open light and also under the scanning electron microscope is looking for. This showed a very pronounced her pull-out effect (pull-out effect) that is far off protruding fibers was recognizable.

In the experiments carried out according to the invention, it was found that mullite and SrAl ₁₂ O ₁₉ coated carbon fiber filaments are significantly more resistant to oxidation than uncoated carbon fiber filaments. In comparative investigations, it was found that the tear-off temperatures in air with a tensile load of 5 MPa result in clear differences and for uncoated carbon fiber filaments at temperatures between 635 and 645 ° C, in contrast to the simply coated carbon fiber filaments, higher tear-off temperatures could be measured could, the latter only cracked at temperatures of 795 ° C for mullite layers and at 925 ° C for a SrAl ₁₂ O ₁₉ layer.

Claims (14)

1. Process for coating reinforcement comm components for composite ceramics, where fibers, fabrics or mats with one Liquid process coated with a sol and then the layer is thermally compressed becomes. 2. The method according to claim 1, characterized in that the gain comm components made of carbon, silicon (oxy) carbides, Silicon nitrides or silicon (carbo) nitrides by dipping, spraying, brushing, knife coating or by coating the individual continuous fibers in the bundles, mats and felts are coated the. 3. The method according to claim 1 or 2, characterized in that the sol selects from mixtures of at least 2 components
M ^II R ¹ _x R ² _2-x , where M ^II can be Mg, Ca, Sr or Ba and is 0 ≦ x ≦ 2;
M ^III R ¹ _y R ² _3-y , where M ^{III can be} Al or y and is 0 ≦ y ≦ 3 and
M ^IV R ¹ _z R ² _4-z , where M ^IV can be Ti, Zr or Si and is 0 ≦ z ≦ 4 and
R ¹ and R ^{2 can be} an alkoxy, alkacyl radical with up to 20 carbon atoms or a short-chain alkyl radical with a maximum of 8 carbon atoms,
is produced with the addition of water. 4. The method according to claim 3, characterized in that the mixture Complexing agents such as acetylacetone, ethylenediamine or maleic acid can be added. 5. The method according to claim 3 or 4, characterized in that the sol to Ein position of viscosity and surface tension additionally water or inert thickeners be added. 6. The method according to any one of claims 3 to 5, characterized in that an aluminum and a silicon compound form the sol, so that a silicon content between 10 and 40 mol% is kept and a layer of a mullite phase or a mixture with Al ₂ O _{3 is} formed. 7. The method according to claim 6, characterized in that a silicon content is set between 20 and 30 mol%. 8. The method according to any one of claims 3 to 5, characterized in that the sol from a Aluminum compound and an alcoholate or Acylate of a metal of the second main group  or mixtures of these is produced. 9. The method according to claim 8, characterized in that an alcoholate or Strontium acylate with the aluminum compound is used and strontium with 1 to 25 mol% is included. 10. The method according to claim 9, characterized in that the mixture with a strontium content between 5 and 20 mol% will be produced. 11. The method according to claim 9, characterized in that a sol with Molver ratio Al: Sr of 4: 1 is produced. 12. The method according to claim 9, characterized in that a sol with Molver ratio Al: Sr of 12: 1 is produced. 13. The method according to any one of claims 1 to 12, characterized in that after the order of the Sols on the reinforcement components the thermi treatment in several temperature levels is carried out. 14. The method according to any one of claims 1 to 13, characterized in that the thermal loading action in an inert protective gas atmosphere, is carried out at least partially.