CA1273317A

CA1273317A - Method of plasma coating carbon bodies with silicon powder

Info

Publication number: CA1273317A
Application number: CA000497989A
Authority: CA
Inventors: Heinrich Kuhn; Olaf Stitz; Karl Wimmer
Original assignee: Sigri GmbH
Current assignee: Sigri GmbH
Priority date: 1984-12-19
Filing date: 1985-12-18
Publication date: 1990-08-28
Anticipated expiration: 2007-08-28
Also published as: CN1006556B; EP0186800A2; EP0186800A3; JPS61190891A; US4716572A; DE3446286A1; EP0186800B1; JPH0341955B2; CN85109647A; DE3571967D1

Abstract

ABSTRACT OF THE DISCLOSURE

A process for coating graphite and carbon bodies with a protective layer containing predominantly silicon, by plasma spraying a silicon powder with a grain size under 0.05 mm and with an argon/hydrogen mixture as the plasma gas at reduced atmospheric pressure of at most 200 h Ps is disclosed. The layer thickness is 0.1 to 0.5 mm and has a density which is at least 95% of the theoretical density. The coated bodies, for instance, graphite electrodes have a lower burnoff rate than unprotected ones in an oxidizing or corrosive atmosphere.

Description

- æ~ 7 25~84/l4 METHOD FOR COATING CARBON AND GRAPHITE ~ODIE~
-The inven~ion relates to a method for applying a protec-tive silicon-containing layer to the surface of carbon or graphite bodies by plasma spraying.
Several me-thods have been described for the protection of carbon and graphite bodies, primarily against oxidation and erosion, in which the surface of the body is coated with oxidation and erosion-resistant substances. Typical coating subs-tances are ceramics, compounds of refractory me~als, and also metals i-f the application temperatures of the coated body are not too high, or if the protection is required only for a given tempera-ture range or if a coa-ting is needed which conducts electric current. The technically important coatings from the group of metallic coating substances are silicon and alloys consisting substantially of silicon, such as ferrosilicon. These coating substances exhibit a relatively high resistance against aggressive agents and can be converted into silicon carbide entirely or partially, either in the generation of the protective layer itself, or by a suitable thermal post-treatment. Silicon-coated carbon and graphite has, for instance, been proposed for crucibles and other metallurgical vessels, electrodes heat exchangers, nuclear reactors, nozzles and heat shields.
The durability of the protective layer applied to the surface of carbon and graphite bodies is determined primarily by the adhesion of the layer. This is often insufficien-t, and results in failure especially where repeated fast temperature changes are encoun-tered. A separation of the layer and/or the formation of ~q~

~733~7 cracks in the layer occurs, which largely cancels the ~rotection against oxidizing fluids. ~umerous processes have been proposfed for preparing protective layers on carbon and graphite which meet the requirements and consists substantially of silicon. Accordiny to U.S. Patent 3,275,471, carbon and graphite bodies are immersed in a slurry of fine silicon powder which also contains silicon carbide, and the thus coated bodies are heated to produce a pro-tective layer which consists of a silicon matrix having silicon carbide particles dispersed in the matrix. Protective layers prepared according to this method are comparatively porous, and accordingly are permeable to fluids. I'he behaviour of layers which are generated by deposition of silicon from the vapor phase or by flame spraying, are similar. It has been proposed to elim-inate the porosity of the silicon layer applied by brushing or flame spraying on graphite bodies, by local fusing, i.e. melting the silicon ~British Patent 866,818). With this treatment, the adhesion of the layer is improved because part of the melted material penetrates into the pores of the graphite body, and sili-con carbide is formed in a transition zone. It is also known in coating smaller graphite bodies to move the body along a capillary filled with melted liquid silicon, where the melt issuing from the capillary forms a thin film on the surface of the body. In another method, carbon and graphite bodies are provided with a protective silicon layer in contact with reactive gases such as chlorosilanes at a higher temperature (West German OLS 2,739,258).
The protective layers prepared by this method are not free of shortcomings; in particular, the adhesion and gas tightness meet the technical requirements only in part.
Finally, it is known to provide the surface of carbo~
and graphite bodies with a protective layer which consists sub-stantially of silicon by plasma spraying (West German OLS 1,671,065; Wes-t German Patent 1,271,007). The porosity of the protective layers prepared by plasma spraying is less than the porosity of other layers except for CVD (chemical vapor deposi-tion) layers without the diffusion o-f oxidizing fluids being impeded sufficiently by the layer. For this coating method, the application of several layers, the melting of the protective layer or its sealing with vitreous substances is accordingly provided as means for the necessary reduction of the permeability.
The present inven-tion seeks to simplify and improve the methods for preparing protective layers on carbon and graphite bodies and generate a substantially gas-impervious protective layer consisting substantially of silicon, and which adheres firmly to the surface of the body.
Thus, there is provided in accordance with the invention a method for applying a silicon-containing protective layer to the surface of a carbon or graphite body which comprises, plasma spraying a silicon powder having a grain size under 0.05 mm with an inert gas/hydrogen mixture as a plasma gas at an atmospheric pressure of at most 200 h Pa to generate a protective layer 0.1 to 0.5 mm thick on the surface of the body, with the generated pro-tective layer having a density which is at least 95~ of the theo-retlcal density of silicon.
Other features which are considered as characteristic ~ ~7 for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for coating carbon and graphite bodies, it is nevertheless not intended to be limited to the details shown, since various mo~ifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The invention however, together with additional objects and advantages thereof will be best understood from the following description.
The problem of overcoming the difficulty in generating a substantially gas-impervious protective layer consisting substan-tially of silicon on ~arbon and graphite bodies, is solved by plasma spraying, using a silicon powder with a grain size below 0.05 mm and an inert gas/hydrogen mixture as the plasma gas at an atmospheric pressure of at most 200 h Pa on the surface of the carbon or graphite body to generate a layer 0.1 to 0.5 mm thick of substantially silicon, the density of which is at least 95% of the theoretical density.
~0 The basis of the invention is the surprising discovery that a layer prepared in accordance with the invention is imper-vious to fluids, adheres well to carbon or graphite bodies and protects the bodies effectively against the attack of corrosive substances. The protection is achieved in a single operation, and consequently multiple coatings, or at least partial melting of the layer~ or the application of special sealing compounds, can be omitted. Another advantage is the small variation in the quality ~33~) of the layer and, correspondingly, a small variation in its ser-vice life. The effects would seem to be based primarily on the fact that the silicon particles are not oxidized, and reach a higher speed, than with plasma spraying at normal pressure. Both factors favor the development of a more ductile protective layer which is anchored on the surface profile of the carbon or yraph-ite. The grain size of the silicon powder used for the coating should not be more than 0.05 mm, because with grain sizes above this limit, the permeability of the layer increases and its adhes-ion decreases. Silicon powder fractions with grain sizes of 0.02 to 0.04 mm are particularly desirable with respect to the uniform-ity of the layer development. ~he term "silicon powder" is under-stood to include pure silicon as well as silicon alloys, such as ferrosilicon, preponderantly consisting of silicon, each capable of producing, under the conditions in accordance with the invent-ion, and impermeable and strongly adhering layer. Particularly desirable as plasma gas is a mixture of a noble gas and hydrogen;
especially argon and hydrogen. The volume ratio is preferably 60 to 70~ noble gas such as argon and 40 to 30~ hydrogen. With this composition of plasma gas, oxide layers on the silicon particles are substantially reduced and ductility required for the quality of the layer is obtained. The atmospheric pressure in the coating chamber should not exceed 200 h Pa, since the protective layer becomes permeable for fluids at higher pressure and also its adhesion is unsatisfactory. With decreasing atmospheric pressure in the coating chamber, the apparatus , . ~ .

required to obtain decreased atmospheric pressure i5 necessarily more complex, without a substantial change in the quality of t,he protective layer. An ade~uate balance oE apparatus reyuirements relative to the quality of the layer appears to be obtained with pressures of 50 to 100 h Pa.
In preparing an impermeable protèction layer on t'ne surface of carbon and graphite bodies, silicon must be applied to the surface in a mean layer thickness of at least 0.1 mm. The relatively large layer thickness is necessary because of the sur-face profile, which is largely determined by the porosity of the body, and of the geometric arrangement between the surface or the body and the plasma nozzle. With increasing layer thickness, the mechanical stresses formed in the layer as a result of temperature changes or gradients increase, since the coefficients of expansion of the protective layer and the carbon or graphite bodies are different. As a consequence, cracks are formed in the protective layer, through which oxygen and other harmEul fluids can penetrate the layer. The maximum layer thickness should therefore be at most 0.5 mm. Especially advantageous are protective layers with a mean thickness between 0.25 and 0.35 mm. The density of the layer should be at least 95% of the theoretical density. With a density above 95% of theoretical density, pores contained in the protec-tive layer do not extend through the entire thickness of the layer and the residual porosity has no adverse effect on the operation of the coating. Under some circumstances it may be advantageous to change the composition while it is being generated, for instance, to have the base of the layer consist of pure silicon ~2~33,3iL7 and its surface of a silicon alloy. Under certain conditions of application, such alloys can be more resistant than pure silicon which, on the other hand, adheres well to the carbon surface.
Concentration gradien-ts of silicon in the layer are generated by changes of the powder composition during the coating.
The method according to the invention is suitable for coating carbon and graphite bodies of any shape, for instance, arc lamp carbons, graphite crucibles for the manufacture of semicon-ductors, block heat exchangers of graphite, etc. The method is ln particularly well suited for coating the sections of graphite electrodes for arc furnaces which are screwed together to form a continuous column. Because of the fast heating-up and cooling-down of these electrodes, -the coatings are subjected to particu-larly large thermal stresses and must withstand an aggressive and corrosive atmosphere. The protective silicon layers applied on graphite electrodes by the method according to the invention are stable, do not separate from the electrode surface and shield the electrode against fluids. For coating carbon or graphite bodies, ~ for instance, an electrode, the bodies optionally after roughing o~
the surface to be coated, cleaning and preliminary degassing, are placed and stored in a coating chamber. The bodies and the plasma nozzle can be moved and rotated relative to each other. The cham-ber is then evacuated to about 1 Pa and flooded with argon, while the atmospheric pressure rises to at most 200 h Pa and preferably to 50 to 100 h Pa. At the same time the plasma arc is drawn. The arc voltage is about 68 V, and the plasma gas consis-ts of an argon/hydrogen mixture with 60 to 70 volume percent argon and the ~q~3~L~

remainder hydrogen. Silicon powder with a grain size less than 0.05 mm is blown into the plasma and is deposited on the carbon or graphite surface which is arranyed at a distance of about 250 to 300 mm from the plasma nozzle. ~ith a power of 60 kW and a plasma gas flow of about 50 l/min, the powder throughput is about 100 g/min. The coating performance can be adapted within wide limits to the technical requirements by the simultaneous use of several plasma nozzles.
The invention will be described with the aid of the following Examples 1 to 6. Sections for a graphite electrode with a diameter of 500 mm were roughened by lathe machining and sand blasting, whereby a mean roughness depth of about 0.05 to 0.09 mm was generated. The sections were placed in a coa-ting chamber and provided with a layer substantially consisting of silicon. The distance between the plasma nozzle and the graphite body was always 270 mm.
The coated sections were tested by comparison as parts of graphite electrodes in an arc furnace with a maximum transfor-mer power rating of 20 MVA. The electrodes are subjected to the 20 attack of oxidizing gases, slag, etc. and the loss, called lateral burnoff, is about 40 to 50% of the total electrode consumption.
In the protective layer of example 3, cracks developed when the electrode column was heated up, and the oxygen which diffused into, and reacted with, the carbon led to cavities between the protective layer and the graphite body. The pro-tec-tive layers of examples 5 and 6 were permeable due to their poros-ity and the adhesion of the layers was comparatively poor.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for applying a silicon-containing protective layer to the surface of a carbon body which comprises plasma spraying a silicon powder having a grain size under 0.05 mm with an inert gas/hydrogen mixture as a plasma gas at an atmospheric pressure of at most 200 h Pa to generate a protective layer 0.1 to 0.5 mm thick on the surface of the body, with the generated protective layer having a density which is at least 95% of the theoretical density, wherein the silicon powder is selected from the group consisting of pure silicon and silicon alloys preponderantly consisting of silicon, each capable of producing an impermeable and strongly adhering layer under the above conditions.

2. A method according to claim 1, wherein the grain size of the silicon powder is 0.02 to 0.04 mm.

3. A method according to claim 1, wherein the plasma gas is an argon/hydrogen mixture.

4. A method according to claim 2, wherein the plasma gas is a mixture of 60 to 70 volume percent argon and 40 to 30 volume percent hydrogen.

5. A method according to claim 1, wherein the atmospheric pressure is 50 to 100 h Pa.

6. A method according to claim 4, wherein the atmospheric pressure is 50 to 100 h Pa.

7. A method according to claim 1, wherein the protective layer thickness is 0.25 to 0.35 mm.

8. A method according to claim 6, wherein the protective layer thickness is 0.25 to 0.35 mm.

9. A method according to any one of claims 1 to 8, wherein during generation of the protective layer, the powder composition is changed to produce a concentration gradient of silicon in the layer over its thickness.

10. A method according to any of claims 1 to 8, wherein the surface of the body is roughened prior to generating the protective layer.

11. A method according to any one of claims 1 to 8, wherein the carbon body is a graphite electrode section of an arc furnace.

12. A graphite electrode section for an arc furnace, the surface of which section is coated with a silicon-containing protective layer obtained by plasma spraying a silicon powder having a grain size under 0.05 mm with an inert gas/hydrogen mixture as a plasma gas at an atmospheric pressure of at most 200 h Pa to generate a protective layer 0.1 to 0.5 mm thick on the surface of the body, with the generated protective layer having a density which is at least 95% of the theoretical density.