DE102010036815A1 - Coating bodies, useful as a diffusion barrier against diffusing carbon or carbon compounds from the support body, comprises applying a first layer of silicide and applying a second layer of a low-melting glass and sintering - Google Patents

Abstract

Coating of bodies made of graphite and/or carbon fiber reinforced carbon using silicide, preferably molybdenum silicide and a glass comprises applying a first layer of silicide including molybdenum silicide, hafnium silicide, zirconium silicide on the body, sintering at above 1500[deg] C for more than 30 minutes to produce an intense chemical bond with the body to obtain non-porous dense first coating layer and applying a second layer of a low-melting glass to the first coating layer chemically associated with the body and sintering at above 1300[deg] C to obtain a second coating layer.

Description

The present invention relates to a method for coating carrier bodies, in particular plates, of carbon-fiber-reinforced carbon or graphite with a glass surface and such a coated carrier plate with a glass surface.

Carrier bodies, in particular plates, are used as carriers for sintered parts originating from powder metallurgical production methods for motor vehicle transmissions, gear wheels, turbine blades, tools, engine components or the like. In these manufacturing processes, metal powders are mechanically compacted into greenware in molds at pressures of between 1 ton and 10 tons per square centimeter, and then sintered at high temperatures below the melting point of the lowest melting component in sintering furnaces. The sintering is carried out in vacuum or under special unpressurized or pressurized atmospheres, at temperatures of, for example, 1200 ° C to 1300 ° C, often with subsequent shock cooling using liquid nitrogen. Especially this thermal shock treatment loads the carrier body especially.

In principle, the carrier bodies must not react with the metals or alloys nor with the atmospheric constituents such as hydrogen, hydrocarbons, nitrogen, argon or forming gas, nor wear out excessively due to the frequent feed changes and the associated high mechanical load, nor become warping or softening. These bodies must be temperature-resistant and also resistant to thermal shock. The materials carbon fiber reinforced carbon and graphite are therefore fundamentally disadvantageous, since they react easily with hydrogen to form methane or higher hydrocarbons and thus form pores, which leads to softening. These materials are relatively soft, are easily damaged and are rather disadvantageous in terms of abrasion resistance, mechanical stability, chemical inertness, temperature and thermal shock resistance and adhesion properties. In direct contact with the green bodies to be sintered, they lead to their carburizing by diffusion of the carbon contained in the material of the carrier body in the metal. Such a contact must also be avoided because it leads to a wetting, ie to an intensive adhesion of the metal parts to be sintered to the carrier body, on which it rests.

In order to meet these requirements in the different areas of application, carrier bodies made of the materials mentioned must be provided with appropriate coatings.

In particular for very clean sintering furnaces without dust, fibers or carbon particles, a diffusion-proof, hard and mechanically loadable glass coating of the carrier bodies made of graphite or carbon-fiber-reinforced carbon is desirable. The direct application of glass suspensions does not lead to the goal due to the lack of wetting. There is no connection between the glass layer and the carrier, a durable, load-bearing coating can not be achieved. From the prior art, therefore, other methods are known to produce a glassy surface.

For example, the DE 40 34 001 A1 a protective layer of carbon-containing carbon or carbon-containing SiC containing carbonaceous components which is only directed against high-temperature oxidation, a mechanical load-bearing capacity due to overlying green compacts or a thermal load capacity are not described. The initially sponge-like layer disclosed therein is produced by applying a nonaqueous suspension of a MoSi ₂ powder, drying it and briefly sintering it at 1700 ° C. This produces a porous layer which adheres relatively well to the component and onto which a nonaqueous suspension of a mullite-cordierite powder (Al ₈ [(O, OH, F)] (Si, Al) O ₄ ] ₄ - (Mg, Fe ²⁺ ) ₂ (Al ₂ Si) ^[4] [Al ₂ Si ₄ O ₁₈ ]) and melted at 1600 ° C. This fills the pores of the MoSi ₂ layer and creates a shiny surface. The protective layer thus produced consists of a network of MoSi ₂ particles firmly embedded in the low-melting vitreous phase. For carbon fiber reinforced carbon bodies, a diffusion barrier of TiC or SiC is proposed between the body and the coating.

From the DE 196 16 217 A1 For example, a protective coating for a carbon composite workpiece is known in which a first, inner protective layer contains refractory ceramic powder particles. This inner layer contains up to 86% by weight MoSi ₂ , up to 10% by weight B ₄ C, up to 12% by weight SiB ₄ and YB ₆ . For production, SiC, HfB ₂ and a preformed glass are first applied to the workpiece and heated. After cooling, a layer is applied which contains MoSi ₂ and a preformed glass and B ₄ C and ZrB ₂ . The overall coating thus produced contains a mixture of glass and MoSi ₂ in each layer in addition to other substances.

From the DE 692 05 013.2 T2 Again, an anti-oxidation coating for heat-resistant composite body of a carbon-silicon carbide ceramic (C-SiC ceramic) is known. Here, a layer applied on the first SiC layer of quartz glass, boron glass or borosilicate glass is to be provided, which in its transition to the plastic state as a self-healing layer should close holes and cracks in the underlying SiC coating. On a preform of carbon fiber fabric layers in the vapor phase, an intermediate coating of pyrocarbon and then applied in the vapor phase, a SiC matrix. This is followed by a first dip impregnation phase with a liquid solution of Na and K phosphates, followed by drying and heating. A second coating step is carried out by brushing an aqueous SiO ₂ / Al ₂ O ₃ suspension additionally containing P ₂ O ₅ , B ₂ O ₃ , alumina silicate, alkaline oxides and metallic oxides, followed by the treatment steps drying, heating to a maximum of 1000 ° C. as well as final aging.

From the GB-A-812 740 discloses a method for protecting graphite as a neutron brake in nuclear reactors using a mixture of a graphite powder and a binder of aluminum phosphate with kaolin, SiO ₂ , Al ₂ O ₃ and other refractory oxides.

From the DE 601 18 437 T2 Again, an oxidation-inhibiting coating for disc brakes made of graphite or carbon / carbon composite systems is known, which has a first coating of Si or SiC. A second coating comprises glass granule particles and P bound to O, O in turn bound to Si. Both layers should form a cementitious bond with each other.

Accordingly, it is generally known from the prior art to use MoSi ₂ in combination with glass formers within a coating as a more or less homogeneous mixture.

The present invention therefore has for its object to provide a coating for bodies of graphite or carbon fiber reinforced carbon or a mixture of both using a silicide, in particular MoSi ₂ and a glass, in which on the body a first layer of exclusively a silicide, such as MoSi ₂ , HfSi ₂ , ZrSi ₂ , and this is sintered above 1500 ° C for more than 30 min to produce an intense chemical bond with the body, so that a non-porous, solid and dense layer is obtained, and a second layer of low-melting glaze is applied to the first layer chemically bonded to the body and sintered above 1,300 ° C. With great advantage, the invention proposes to provide a thick, dense layer of a silicide, in particular MoSi ₂ , which is chemically bonded to the body material due to the sintering conditions. This chemical bond can be carried out by forming a Si-C bond from the silicide and the carbon of the carrier body. According to the invention, for example pure MoSi _{2 is} used, which also diffuses into the body during the treatment process. Especially the intimate chemical connection of the body with the first coating layer allows with great advantage the desired thermal shock resistance of the later finished coating. The second layer, a glass layer, improves the resistance of the inventive coated body against the aforementioned thermal and mechanical stresses and is particularly suitable as a diffusion barrier against diffusing carbon or carbon compounds from the carrier body in the resting, to be sintered workpiece extremely suitable.

In an embodiment of the invention, it is provided that a third layer of Al ₂ O _{3 is} applied to the second layer and sintered at more than 1500 ° C. for more than 30 min to produce an intensive chemical bond with the second layer and possibly the first layer , A third layer as described improves with great advantage the chemical tightness of the coating against diffusion of C-compounds from the carrier material into the workpiece and improves its mechanical strength, so that in later use of the invention coated carrier body many sintering cycles can be run through without damage.

Contains the third layer of the coating according to the invention additives in a proportion of 1 to 10 wt .-%, wherein the additives are selected from the group boron carbide, carbon, the physical and chemical properties of the coating can be adapted to the particular conditions of use with advantage.

With great advantage, it is provided that in the case of a three-layer coating, the second layer has a thickness of not more than 50 μm and therefore acts as an adhesion-promoting layer between the inner first and outer third layers of the coating. In this case, adhesion is also effected by Al ₂ O ₃ acting as a so-called intermediate oxide as a network former and converter of the basic structure of the intermediate glass layer. This results in a stable connection between Al ₂ O ₃ , glass and the inner silicide layer of the coating.

The coating method of the invention is carried out under an inert gas atmosphere and / or using a temperature program, wherein the precursor substances of the coating layers have particle sizes of 0.1 to 200 .mu.m, preferably 0.5 to 25 .mu.m, and wherein the particle sizes and / or distributions of two precursor substances may differ.

In a process technology simple way, the invention proposes to apply the layers by spraying, dipping or dipping on the body to be coated.

With this method, the bodies to be coated can be present as powder, plate, rod, ball, cylinder or fiber or a mixture thereof, without this affecting the quality of the respective coating.

example 1

An approximately 1 cm thick plate of carbon fiber reinforced carbon is immersed in a MoSi ₂ -Sschlicht and then sintered at 1,780 ° C under inert gas for 4 h. After cooling, a low-melting glaze is applied and sintered at 1400 ° C.

1 shows a micrograph of a coating 1 according to Example 1. Good to see the body 2 made of carbon fiber reinforced carbon, on a rod-like MoSi ₂ layer 3 sintered by about 200 microns thickness. In the boundary layer 4 the result of diffusion as well as an intimate chemical combination of body material and MoSi ₂ layer can be recognized. Furthermore, it can be clearly seen that this first coating layer 3 dense and almost nonporous. On the MoSi ₂ layer 3 follows a glass layer to the outside 5 also about 200 microns thickness. Again, it is easy to see that the glass layer 5 does not serve as a matrix for a network of MoSi ₂ molecules but is formed as a separate, homogeneous coating layer, which rests on the first, inner coating layer.

Example 2

An approximately 1.5 cm thick sheet of graphite is treated as described above, with the glass layer 5 sprayed only with a thickness of 30 microns and sintered at 1300 ° C. After cooling again, a third layer 6 brushed from its Al ₂ O ₃ seals and sintered at 1,750 ° C for four hours.

2 shows as a microstructure a cross section through the coating thus produced 1 , At the bottom, a graphite layer is visible, which is the material of the uncoated body 2 forms. Above this is a first coating layer 3 from MoSi ₂ followed by a narrow glass layer 5 , The coating is terminated to the outside by the third outer coating layer formed by an Al ₂ O ₃ layer.

LIST OF REFERENCE NUMBERS

1

coating

2

body

3

MoSi ₂ layer (innermost coating layer)

4

interface

5

Glass layer (second coating layer)

6

Al ₂ O ₃ layer (third, outer coating layer)

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 4034001 A1 [0006]
• DE 19616217 A1 [0007]
• DE 69205013 T2 [0008]
• GB 812740A [0009]
• DE 60118437 T2 [0010]

Claims (7)

A method of coating bodies of graphite or carbon fiber reinforced carbon or a mixture of both using a silicide, in particular MoSi ₂ and a glass, characterized in that - on the body a first layer of exclusively a silicide, for example MoSi ₂ , HfSi ₂ , ZrSi ₂ , and this above 1500 ° C for more than 30 min to produce an intense chemical bond is sintered with the body so that a nonporous dense first coating layer is obtained, - on the first chemically bonded to the body coating layer a second layer of a low-melting glaze is applied and sintered above 1300 ° C, so that a second coating layer is obtained. Coating method according to claim 1, characterized in that applied to the second coating layer, a third layer of Al ₂ O ₃ and sintered at more than 1,500 ° C for more than 30 min to produce an intensive chemical bond with the second coating layer and optionally first coating layer so that a three-layer coating is produced. A coating method according to claim 2, characterized in that the third coating layer contains additives in a proportion of 1 to 10 wt .-%, wherein the additives are selected from the group boron carbide, carbon. Coating method according to claim 2, characterized in that in the case of a three-layer coating the second coating layer has a thickness of not more than 50 μm and therefore acts as an adhesion-promoting layer. Coating method according to one of the preceding claims, characterized in that it is carried out under an inert gas atmosphere and / or using a temperature program, wherein the precursor substances of the coating layers have particle sizes of 0.1 to 200 .mu.m, preferably 0.5 to 25 .mu.m and wherein can distinguish the particle sizes and / or distributions of two precursor substances. Coating method according to one of the preceding claims, characterized in that the individual layers are applied by spraying, dipping or dipping on the body. Coating method according to one of the preceding claims, characterized in that the bodies to be coated are present as powder, plate, rod, sphere, cylinder or fiber or a mixture thereof.

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