SK157795A3 - Abrasion-proof protective layer against oxidation of silicon(iv) carbide bodies - Google Patents

Abstract

Abrasion-resistant substrate mouldings of porous SiC are protected against oxidn., by O2, by a surface coating of matrix glass, SiC and MoSi2. The coating consists of a mixt. of a polysiloxane (I) and finely dispersed MoSi2, obtd. by heat treatment at up to 1700 degrees C, and contains some unreacted MoSi2. Pref. the coating is 5-500, esp. 30-300 mu m thick. Also claimed is a method of producing the coating.

Description

Silicon carbide (SiC) components, due to their strength, resistance to oxidation and creep at high temperatures, are used for heating, load-bearing, protective and structural purposes in high temperature equipment. Examples are heating elements, susceptors, crucibles, protective tubes, radiation shields, gas guidance and impact plates, support bases and pulleys for various types of furnaces in industry and laboratories. The creep limits of such components, when comprised of porous, sintered, reaction-bonded or recrystallized SiC-types, lie in the range of 1500 degrees Celsius. At higher temperatures, the stability decreases as it is first under the action of atmospheric oxygen according to the formula SiC + 20 _and - _SiO2 + 2C0 forming the silicon dioxide layer, protecting all the gas available to the surface to oxidation, breaking the continuous conveyance of the gaseous components of sub-oxide of silicon (SiO), and thus, it cannot fully fulfill its protective function. The use of silicon carbide parts in an even higher temperature zone in an oxidizing atmosphere requires therefore more stable protective layers which either cover all gas-accessible pore systems or which all the outer surface including

-2-pole inputs close reliably. It has also been repeatedly tried to make protective layers of this kind available. Often an important component of such layers is molybdenum silicide ₍ MoSi ₂ ).

According to German Laid File No. 11 42 042, the resistance to oxidation of SiC heating conductors increases when their surface is first covered with a layer of molybdenum silicide and is then annealed at temperatures between 1300 and 1500 degrees Celsius to a glassy, oxidation-protective layer containing molybdenum, silicon, oxygen, and partly carbon. In the later published DE 14 96 660 patent, the effectiveness of the protective layers thus produced is again questioned. These layers should always have pores which allow the access of oxidizing gases. To overcome these difficulties, it is proposed in a later publication to make a dense, oxidation-protective layer of glass containing a metal silicide consisting of silica, alumina, alkaline earth oxide and alkali metal oxide. The bound silicide, for example MoSi ₂ , is intended to act as a getter in this layer and bind the diffusing oxygen in the reaction to exclusively not only volatile products, especially SiO ₂ . Another protective layer comprising molybdenum silicide and a low-melting matrix such as glass or glass ceramics, as components which degrade at high temperature mechanical stress, for essentially carbon-based substrates, is disclosed in DE 40 34 001 A1.

The main problem in the use of protective layers with glass matrices containing molybdenum silicide lies in the stability against temperature changes of the materials involved in the respective technical solution. At a constant change of high temperatures, which mean temperatures from 1700 to 1750 degrees Celsius, to room temperature, on the boundary surfaces between the substrate and the adhesion protective layer,

- the requirements which, due to the different characteristics of the substances, in particular the different coefficients of thermal expansion, cannot always be met. The peeling of the layers and cracks are then a consequence in the cooled state. Such deterioration occurs in particular when the boundary surfaces mediating the adhesion are subjected to additional mechanical stresses due to the mechanical loading of the components, for example by the influence of the furnace stacked for heat treatment. Upon renewed heating to working temperature, these damages are not adequately remedied, if at all possible, and the substrate body is damaged.

Another requirement for high temperature-resistant, oxidation-resistant coatings is their erosion resistance, which is insufficient if the layers at high temperatures contain relatively strongly softened glasses containing, for example, alkali metals and / or alkaline earth metals and have no abrasion resistant substances.

SUMMARY OF THE INVENTION It is therefore an object of the invention to provide firmly adhering, up to 1700 degrees Celsius, stable and additionally erosion-resistant oxidation protective layers for porous silicon carbide parts, and to provide a method for producing such layers.

SUMMARY OF THE INVENTION

This object is solved by the characterizing parts of claims 1 and 5. The claims dependent on claims 1 and 5 set forth herein are preferred embodiments of the invention.

An essential feature of the solution according to the invention is the use of polysiloxanes as precursors for the production of matrix glass containing inter alia silicon carbide. When silicon carbide components are formed, they also grow on a substrate also consisting of silicon carbide

Thus, in connection with the other binding effects of the glass melt on the porous surface, they give a particularly good adhesion. The substrate may have any shape, such as plate, block, tubular, crucible, rod or meandering.

Polymers of organosilicon compounds, such as siloxanes, have already been used successfully for the production of ceramic fibers, powders and wear layers. However, against the enormous density jump of more than 100% occurring in the polymer / ceramic pyrolithic conversion and the associated shrinkage of up to 80%, no components or layers could be maintained at high density. It was only by incorporating the reaction filler powders into the polymer matrix that the shrinkage occurring during the transformation into the ceramic state was so minimized that more dimensioned components could be produced (EP 0 412 428 B1). The resulting composite materials are more characterized by low shrinkage, but are openly porous. The present invention now consists in mixing the polysiloxane resin in liquid form with molybdenum silicide in fine powder as the reaction filler until a slurry is formed, thereby coating the porous body of the silicon carbide substrate, and converting the coating from a composite material by subsequent heat treatment. gas-tight and thus against oxidation of the protective layer. This occurs by first removing the layer by evaporating a substance such as water or solvent by slightly increasing the temperature and / or applying vacuum. Then, the polysiloxane component is thermally crosslinked in the temperature range up to a maximum of 400 degrees Celsius. Subsequently, the component is pyrolyzed in a non-oxidizing atmosphere, such as argon or nitrogen, at temperatures of 1000 to 1500 degrees Celsius, whereby hydrogen, lower hydrocarbons and small amounts of CO escape as pyrolysis products. This pyrolysis essentially results in matrix glass

-5Si-OC into which MoSi ₂ is introduced and which may contain carbide components to some extent and, if nitrogen was used as the surrounding atmosphere in the reaction pyrolysis step, also nitride components. Since the nitride constituents are not as stable at high temperatures as the other constituents mentioned, the reaction pyrolysis should not be carried out, if carried out in nitrogen, to prevent the formation of a significant amount of nitride constituents below 1250 degrees Celsius. The molybdenum silicide and carbide components, in connection with the Si-OC matrix glass, provide high temperature stability, corrosion resistance as well as oxidation of the protective layer. If, in addition to molybdenum silicide, other ceramic silicon compounds or aluminum-containing components are present in the deposited layer, these may also participate in the reactions which take place in the process steps carried out at high temperatures. The glass layer thus formed now contains more pores and cracks that are less than 10 microns. This residual porosity is closed by subsequent heat treatment in air at a temperature of at least 800 degrees Celsius, in particular at temperatures of 1500 to 1700 degrees Celsius. At the high temperatures prevailing in the reaction pyrolysis and the glassing step, part of the MoSi ₂ can also be converted by reaction with the carbon present in the matrix glass to Mo ₂ C and SiC. The remainder of the filler component is available as unreacted MoSi ₂₃ or as Mo _b Si ₃ in the composite material thus formed. These substances, in particular molybdenum silicide, silicon carbide, as well as the Si-OC component derived from siloxane, produce an excellent combination of the properties of this layer: oxidation stability, layer stability and abrasion resistance.

The last heat treatment under oxidizing conditions need not be carried out directly following the sceramization process. It is also possible at the first designated

- to heat the body coated with the vitreous layer to operating temperature in an oxidizing atmosphere.

The invention also relates to a protective layer produced according to the method described.

Pyrolysis of siloxanes to silica carbide containing products is known. See, for example, White et. al., Preparation from Silicon Carbide from Organosilicon Gels, Parts I and II, Advised Ceramic Materials, Vol. 2, No. 1 (1987). 45 to 59. There is also described the preparation of a suitable polysiloxane gel. However, useful polysiloxane forms are also available from known manufacturers of silicon-organic compounds. Both the condensation crosslinking and addition crosslinking siloxane resins are suitable as the starting material for the production of the layers according to the invention. The siloxanes used may have a large number of organic ligands, such as alkyl, aryl, especially phenyl or alkoxy groups. According to one preferred embodiment of the invention, siloxane resins are used whose yield on the ceramic residue after thermal decomposition is at least 70% by weight. This group includes silseskvioxany, e.g., (RSiO ^ _p) with ligands as the R radical vinyl, allyl or phenyl, which are either very reactive, has a high content of carbon, or have both of these features. Low-viscosity siloxane forms can be blended directly with the fillers in fine powder and then applied to the surface of the substrates. The multi-viscous gels are adjusted to the desired viscosity with a suitable solvent. Likewise, they are diluted with diluents if the viscosity of mixtures of polysiloxane and solids has increased too much. In order to improve the wetting of the substrate with the deposited suspension and to improve the adhesion of the layer to the substrate, the surface of the parts to be coated is coated in particular with a silane-based adhesion promoter.

The substrates contemplated for deposition comprise between 10 and 60, in particular between 20 and 50 percent by volume to fine

-Dust of molybdenum molten silicide, based on the siloxane component = 100% by volume.

To improve the abrasion resistance and to further adjust the thermal expansion of the ceramic layer, additional fine powder components may be added to the slurry. In particular, silicon-containing substances such as SiC, CrSi are used.

TaSi.

Nbsi or WSi which are additionally added to the suspension in amounts of up to 60% by volume, the suspension being determined to be equal to 100% by volume prior to the addition of these additives. In general, additives of this kind also improve the adhesion of the layer to the substrate. Particularly preferred for the oxidative inhibiting action of the protective layer is the addition of alumina or aluminosilicate glass forming additives such as aluminides such as AISi, AlZr, or aluminum silicide (Al ₄ S ₃ ) or mulite or hydroxid such as boehmite or hydrargillite which form a reaction combination with silicide fillers in an oxidizing atmosphere, a partially crystalline high temperature stable coating.

The suspension may be applied to the substrate according to any of the known methods, i. for example, by dipping, brushing or rolling, spraying, or, if deep penetration of the suspension into the pores of the body is intended, by vacuum processing the substrate body immersed in the suspension. Adjustment of the consistency of the suspension to the particular deposition method used may be accomplished by dilution with a suitable solvent such as toluene, acetone, or trichloromethane. These solvents must be removed after application of the layer and before the chemical cross-linking reaction is started by methods known to one skilled in the art, i.e. they must be removed in gaseous form.

The applied coatings according to the invention serve as abrasion-resistant protective layers for porous silicon carbide substrate bodies, resistant to high

-8 temperatures against oxidation, up to 1700 degrees Celsius. Particularly suitable porous substrates of this kind are sintered, reaction-bonded or recrystallized silicon carbide molds.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the invention is explained on the basis of exemplary embodiments.

Example 1

The mixture polymetylsilseskvioxanu (symbol NH 2100, Chemiewerk Nuenchritz) containing 40 percent by volume of the powder MoSi _{of the} mean grain size d _a = 3 microns (Type Grade C, f HC Starck, Goslar) was vigorous stirring with the addition of acetone as the solvent, converted into a homogeneous suspension , which had a viscosity at about 20 degrees Celsius about water. A 5 x 5 x 1 cm plate of reaction sintered silica carbide, a pore volume of 16%, a mean pore diameter of about 30 microns, was dipped several times at normal pressure into the slurry until about 100 microns thick slurry was applied. The adhering solvent was removed after each soaking under slight warming. The plate was then heated to 180 degrees Celsius in an air dryer for three hours and left for a further 4 hours at this temperature to complete crosslinking of the siloxane. The plate was then transferred to a shielding gas furnace, heated to 1200 degrees Celsius under argon for 10 hours, and left for an additional hour at the final temperature to complete the reaction pyrolysis. The resulting ceramic tie layer consisted of an amorphous Si-OC matrix component into which MoSi ₂ and SiC particles were embedded. This layer was firmly anchored in

-The surface of the substrate body and already substantially gas-tight. No subsequent weight change due to oxidation was detected at the next air-laying at 1600 degrees Celsius for 10 hours.

Example 2

Of 100 parts by volume of polymethylsilsesquioxane (type as in Example 1), 25 parts by volume of MoSi ₂ (type as in example 1) and 25 parts by volume of CrSi ₂ , mean particle size of 1.8 microns (according to FSSS, manufactured by HC Starch, Goslar) addition of 90% by weight of acetone, based on a mixture of polysiloxane, MoSi ₂ and CrSi ₂ = 100% by weight, by means of high-speed stirring, a suspension having a water-like viscosity. For the coating, a piece of reaction-sintered silica carbide tube having dimensions of 15 mm outside diameter, 11 mm inside diameter, 150 mm length having a pore volume of 16% and a mean pore diameter of 3 microns was considered. Prior to applying the slurry layer to the tube surface, the tube was still coated with a silane adhesion promoter layer (type VTMO, Chemiewerke Huels). This measure should further improve the binding of the polysiloxane functional groups to the surface of the SiC substrate. The adhesive layer was applied by dipping the tube and following drying for half an hour at 50 degrees Celsius in air. Then, the prepared suspension was applied by repeated soaking until a layer thickness of 150 microns was reached. After removal of the solvent, the polysiloxane fraction of the layer was slowly crosslinked to about 180 degrees Celsius after slow heating (od / min) to 180 degrees Celsius at temperatures from 180 to 220 degrees Celsius. The reaction pyrolysis was then carried out at 1500 degrees Celsius with a break of 4 hours under a nitrogen atmosphere. The ceramic protective layer thus formed consisted of a mixture of Si N, Mo C, MoSi, Cr C and SiC.

32

The above substances were placed in a master glass. The body thus produced showed no oxidation-related weight loss after air treatment at 1500 degrees Celsius in air.

Example 3

A mixture of 100 volume% polyfenylmetylsiloxanu (type H62C, f Wacker Chemie) and 30 vol% of the powder MoSi _a (type as in example 1), as well as 10% by volume of Al ^ Si ₃ (maximum particle size of 150 microns, f and Goodfellow, Bad Nauheim ) was adjusted to a water-like consistency with trichloromethane. The slurry was sprayed onto a 10 x 12 x 1 cm plate of reaction-sintered silica and the layer was dried, repeating the procedure until a layer thickness of about 50 microns was reached. The thermal crosslinking of the polymer was performed as described in Examples 1 and 2 at 200 degrees Celsius. This was also followed in principle by the reaction pyrolysis, which in this example was carried out under an argon atmosphere at 1250 degrees Celsius. The plate was then tempered from 1200 to 1600 degrees Celsius with a heating rate of 2 K / min. The surface-dense protective layer formed in this way was formed next to the aluminosilicate glass matrix component and crystalline mullite (Al ₂ O ₃ xSiO ₂ ). Up to temperatures of 1600 degrees Celsius was stable against oxidation, i.e. constant in weight.

Example 4

The SiC tube as described in Example 2 was immersed in a vacuum vessel which was filled with a thin suspension of polysiloxane and MoSi _α (water-like viscosity),

11syntetickým mullite (firm VAW Aluminum AG, Schwandorf), grain size 0 to 0.1 mm, a silicide oxide (type as in Example 3), and the vessel was evacuated to a pressure of less than 10 ² Pa and allowed 5 minutes at this pressure. Then the container was vented. As a result, the suspension was pushed up to approximately 2 mm from the fitting surface into the pores of the tube. After the tube was removed from the slurry, it was subjected to the crosslinking and pyrolysis conditions of the coating as described for this process in Example 3. Polyphenylmethylsiloxane (type H62C, from Wacker Chemie) was added to the slurry to which 25 vol% MoSi powder was added. ₂ (type as in Example 1) as well as 5 volume% mulite 5 volume% achieved by% Al Si based on siloxane component = 100%. The thin consistency in this case was with toluene. After the tube was deposited in air at 1500 degrees Celsius, anchoring against oxidation of the permanent layer on the tube surface was again found. The layers thus produced again exhibit improved stability against long-lasting oxidation at temperatures above 1600 degrees Celsius.

Example 5

Polyphenylsiloxane (type as in Example 3) corresponding to 100% by volume was mixed with 20% by volume MoSi ₂ (type as in Example 1) and 30% by volume of very fine SiC powder (FEPA grain size F 1200, manufactured by Lonzá-Werke Landshut) and by the addition of acetone it was worked up into a slurryable suspension. With this suspension, a 10 x 12 x 1 cm plate of recrystallized SiC was brushed until a layer thickness of 300 microns was reached. The conversion of the deposited layer into the resulting ceramic protective layer occurred as in Example 3. During the processing steps, very fine

-12 SiC particles into pores, cracks and surface roughness of the substrate body. Thus, up to 1500 degrees Celsius, a stable, well-anchored surface of the substrate body and in particular a smooth surface, which in addition showed excellent abrasion resistance, was formed against oxidation.

Claims (11)

An abrasion-resistant surface layer protecting a porous silica carbide substrate body from oxygen oxidation comprising matrix glass, silica carbide and molybdenum silicide, characterized in that the layer consists of a material formed from a mixture of polysiloxane and finely dispersed molybdenum silicide therein. processing up to 1700 degrees Celsius, and which still contains the thermal treatment of the unconverted molybdenum silicide. The coating according to claim 1, characterized in that the coating has a thickness of from 5 to 500 microns. The coating according to claim 1, characterized in that the coating has a thickness of from 30 to 300 microns. 4. A method for producing an abrasion resistant surface layer protecting a substrate of a carbide substrate against oxidation by oxygen, comprising at least a matt glass, silica carbide and molybdenum silicide, wherein the suspension is made of polysiloxane and molybdenum silicide particles; the part of the surface of the silicon carbide body to be protected, the body thus deposited is subjected to a first heat treatment up to a maximum of 400 degrees Celsius, the heat treated body relative to the oxidizing effects is subjected to pyrolysis treatment in an inert atmosphere of 1000 to 1600 degrees Celsius . After the pyrolysis treatment, the body is subjected to a heat treatment above 800 degrees Celsius in an oxidizing atmosphere. The process for producing a coating according to claim 4, characterized in that the suspension consists of a first fraction of polysiloxane = 100% by volume and at least a second fraction of 10 to 60% by volume of melybium silicide added to the polysiloxane. 6. The process of claim 4 wherein the suspension comprises a first portion of polysiloxane = 100 volume% and at least a second portion of 20 to 50 volume% molybdenum silicide added to the polysiloxane. The process for producing a surface layer according to claim 4, characterized in that the suspension contemplated for deposition additionally comprises 0-6% by volume of substances which, at least in one of the heat treatment stages, form alumina or an aluminosilicate component. The process for producing a coating according to claim 4, characterized in that the coating suspension consisting of polysiloxane and molybdenum silicide additionally comprises 0 to 60% by volume of the material from the group of silicon carbide and silicides of niobium, tantalum, chromium, tungsten and aluminum in fine. crumbled form. Method for producing a surface layer according to any one of claims 4, 5, 6, 7 and 8, characterized in that the siloxane resin used provides By thermally degrading the ceramic residue, at least 70 weight percent is obtained. Use of surface layers produced according to the preceding method claims as oxidation, abrasion resistant and high temperature stable layers for porous silicon carbide substrate bodies. Use of the surface layers produced according to the preceding method claims, according to claim 10 up to temperatures of 1700 degrees Celsius.