OXIDATION PROHIBITIVE COATINGS FOR CARBONACEOUS ARTICLES

Abstract: A carbonaceous article, e.g., a carbon or graphite crucible or mold, is provided with an oxidation prohibitive coating comprising a rigid, glassy, ceramic, refractory mate­ rial containing mullite and silicon carbide or boron carbide or both. The carbonaceous article has a coefficient of thermal expansion which is greater than about 1.5 × 10⁻⁶ inch/inch/­ °C.

Description: Related Application Copending EP application No. 84 850 235.7 (EP-A-0134770) entitled "Carbonaceous Articles Having Oxidation Prohibitive Coatings Thereon", filed by R. V. Sara on even date herewith, disclose and claims subject matter which is related to the present application. Technical Field The present invention relates to carbonaceous articles and more particularly to carbonaceous articles having oxidation prohibitive coatings thereon and to improved methods for applying such coatings onto the surfaces of carbonaceous articles. In a more specific aspect, the present invention is directed to improvements in oxidation protection of carbon or graphite articles having relatively high coefficients of thermal expansion (CTE), i.e., above about 1.5 x 10⁻⁶ m/m/°C. Such articles include particularly graphite crucibles or molds for casting molten refractory materials, furnace lining brick and the like. Background Art Carbonaceous articles are widely used today in both the chemical and metallurgical industries because they exhibit exceptional properties at high temperatures. For example, graphite crucibles or molds exhibit good mechanical strength and excellent thermal stability when subjected to high casting temperatures. However, a major deficiency of carbonaceous articles is that they rapidly oxidize and erode at high temperatures. i.e. above about 500°C. This deficiency can be particularly troublesome in those instances where the articles must be periodically replaced as they are oxidized and consumed. The final cost of the product is therefore significantly increased. It has already been proposed to employ oxidation prohibitive coatings on the surfaces of carbonaceous articles, such as carbon or graphite electrodes. These protective coatings have been composed of carbides, silicides, oxides and metals, for example. They have been applied to the articles using various known methods such as by electric arc, flame spraying, vacuum heating and the like. Chemical Abstracts, Vol. 99, No. 6, 1983, page 249, no. 42519a, discloses oxidation prevention of graphite-containing refractory materials by coating with a mixture of water glass solution, feldspar and B₄C, Co oxide, MnO₂, and/or Fe oxide. The coating gives a reduced oxidation loss at 800°C. Chemical Abstracts, Vol. 77, 1972, page 183, no. 104592a, discloses a protective coating with insulating properties on metal and graphite molds. The coating contains inorganic fillers, water glass, phenol-sugar-resol and water and is sprayed on a hot mold. Suitable filler are corundum, chromemagnesite, powdered SiO₂, mullite, sillimanite or graphite. Oxidation prevention is not mentioned. U.S. Patent No. 3,140,193 to Kane, for example, discloses a graphite electrode having a protective coating consisting of an inner layer made of porous silicon carbide and an outer layer composed of silicon metal. The inner layer is applied to the graphite by vacuum heating in a furnace at elevated temperatures. A problem with these coatings has been that they bond to the carbon or graphite substrate with difficulty and they are also expensive. Thus, there is a need in this industry for improved oxidation prohibitive coatings for use on carbonaceous articles which avoid the difficulties of the prior art and provide very effective protection against oxidation even under severe or abusive conditions, and which are easy to apply, inexpensive and readily adaptable to full-scale production. Summary of the Invention In accordance with the present invention there is provided an article comprising a carbonaceous substrate having a coefficient of thermal expansion greater than about 1.5 x 10⁻⁶ m/m/°C and an oxidation prohibitive coating overlying and bonded to said substrate upon being heated to an elevated temperature above 1100°C, said coating being formed from a particulate mixture essentially consisting of a) mullite, in an amount of at least 20 % by weight, b) silicon carbide and/or boron carbide, c) silica and d) a silicon containing binder material selected from sodium silicate, potassium silicate, colloidal silica and ethyl silicate, and that the mixture forms a rigid, glassy, ceramic, refractory coating having a coefficient of thermal expansion of about 5.5 x 10⁻⁶ m/m/°C. The ceramic coating adheres or bonds exceptionally well to these relatively high CTE (coefficient of thermal expansion) carbonaceous materials even after the article has been subjected to severe or abusive conditions such as thermal shock. The ceramic coating is most advantageously employed in conjunction with carbonaceous articles such as carbon or graphite crucibles or mods used in the casting of molten refractory materials, furnace lining brick and the like. The present invention also comprehends an improved method of protecting a carbonaceous article having a coefficient of thermal expansion which is greater than about 1.5 x 10⁻⁶ m/m/°C against oxidation and erosion at an elevated temperature above about 1100°C which comprises: (a) applying to the surfaces of said carbonaceous article a particulate mixture consisting essentially of between about 20 and 70% by weight mullite, between about 5 and 14% by weight silica, between about 5 and 70% by weight of silicon carbide and/or boron carbide, and between about 5 and 14% by weight of a silicon-containing binder material selected from the group consisting of sodium silicate, potassium silicate, ethyl silicate and colloidal silica, all percentages being based upon the total weight of the dry ingredients; and (b)heating said particulate mixture to a temperature at which a rigid, glassy, ceramic, refractory coating is formed. Preferably, the particulate mixture is applied to the carbonaceous article in the form of an aqueous slurry and then dried under conditions to produce a hard coherent layer adhering to the surfaces of the article. The coated article is then heated to an elevated temperature sufficient to cure the coating composition. Mullite (3Al₂O₃·2SiO₂) is a well known ceramic material. It is widely used for many applications in the ceramic industry. Mullite is commonly produced by heating clays, feldspar, hyanites, etc., to a temperature in excess of 1300°C. Methods for producing mullite are disclosed in U.S. Patent Nos. 3,857,923 and 3,922,333, for example. Description of the Preferred Embodiment The present invention is based upon the discovery that a rigid, glassy, ceramic, refractory coating containing silicon carbide or boron carbide or both and having a coefficient of thermal expansion (CTE) which approaches that of the carbonaceous article, i.e., above about 1.5 x 10⁻⁶ m/m/°C, can be bonded to the surfaces of the carbonaceous article by applying thereto a particulate mixture containing mullite, silica, silicon carbide or boron carbide or both and a silicon-containing binder selected from sodium silicate, potassium silicate, colloidal silica and ethyl silicate, and then heating the mixture to an elevated temperature above about 1100°C. Since the coefficient of thermal expansion of the coating approaches that of the carbonaceous substrate, high internal stresses do not build up inside the coating due to differences in thermal expansion between the coating and the substrate. Consequently, there is less of a tendency for the coating to crack or even spall under severe or abusive conditions. Moreover, the ceramic coating is plastic and flowable at temperatures above about 1100°C and is therefore able to seal off any tiny microcracks that may develop in the coating during its useful life. In the practice of the present invention, a coating composition is prepared by admixing together particles of mullite, silica, silicon carbide or boron carbide or both and a silicon-containing binder material selected from sodium silicate, potassium silicate, colloidal silica and ethyl silicate. The coating composition comprises between about 20 and 70% by weight mullite, between about 5 and 14% by weight silica, between about 5 and 70% by weight silicon carbide or boron carbide or both and between about 5 and 14% by weight of the binder material, all percentages being based upon the total weight of the dry ingredients. A sufficient amount of water is added to the mixture to form a slurry of a consistency which will allow the coating composition to be uniformly applied to the surfaces of the carbonaceous article by brushing, spraying or the like. The particle size of the various mixture ingredients used in the coating composition is not narrowly critical. Generally, the mullite and silica particles should be kept within a range of between about 5 and 350 µm, whereas the silicon carbide or boron carbide particles should be maintained within a range of between about 20 and 200 µm. A commercial grade silicon carbide powder of a size which is less than 74 µm is preferably employed. The silicon-containing binder material is preferably sodium silicate as indicated, although other materials may be used such as potassium silicate, ethyl silicate and colloidal silica. The sodium silicate can be employed either as a dry powder or a liquid, e.g. waterglass. The slurry is applied to the carbonaceous article using conventional methods such as by brushing or by spraying. Care should be taken to cover all of the surfaces to be protected with a relatively thick, moist uniform layer. The applied layer is then dried for a sufficient time to remove water and produce a hard coherent layer which adheres to the surfaces of the article. The layer may be dried suitably at room or ambient temperature for about one or two hours. The layer may also be dried under accelerated conditions at moderately elevated temperatures (e.g., 80°C) in a drying oven, for example. If necessary, a second application of the slurry may be applied in the same manner to the dried, first applied layer in order to build up the thickness of the coating. Preferably, the thickness of the dry coating is in a range of between about 0.020 and 0.100 inch (0.5-2.5mm). After drying has been complete, the coated article is then heated to an elevated temperature above about 1100°C in order to cure the composition and form a rigid, glassy, ceramic coating. Optionally, the coating may be heated directly to curing temperature while simultaneously drying the applied layer. The coating adheres or bonds exceptionally well to the carbon or graphite substrate and is able to withstand severe thermal shock. Typically, ceramic coatings for carbonaceous articles according to the present invention have a coefficient of thermal expansion of about 5.5 x 10⁻⁶ m/m/°C. By comparison, high CTE graphite materials used to make casting molds and the like have coefficients of thermal expansion of about 4 x 10⁻⁶ m/m/°C. The present invention also provides a carbonaceous article having an uncured or "green" oxidation prohibitive coating on its surfaces. This uncured or green coating can be applied to the carbonaceous article in the same manner as described hereinabove using a coating composition with the same ingredients and proportions. However, in this case, the manufacturer would not bake or cure the coating at elevated temperatures before storage or shipment to the customer. The ceramic coating of the present invention is particularly well suited to such storage or shipment since the uncured, dried coating adheres very well to the carbonaceous substrate and is hard and very durable. As a precautionary measure, however, the uncured coating should not be exposed directly to the elements for prolonged periods since the coating before curing is water soluble. The present invention also comprehends a novel and improved coating composition for use in protecting the surfaces of a carbonaceous article, e.g., carbon or graphite crucible or mold, against oxidation. The coating composition comprises between about 20 and 70% by weight mullite, between about 5 and 14% by weight silica, between about 5 and 70% by weight silicon carbide or boron carbide or both and between about 5 and 14% by weight of the silica containing binder, all percentages being based upon the total weight of the dry ingredients. The coating composition may be advantageously applied to the surfaces of the carbonaceous article in the form of a wet paste by brushing, trowling or a similar technique. Alternatively, the coating composition may be applied to the surfaces of the carbonaceous article by spraying or dipping, for example. In these cases, additional water is added to the coating composition as required to adjust its consistency to the desired level. In the preferred practice of the present invention, the coating composition comprises between about 48 and 60% by weight mullite, between about 9 and 12% by weight silica, between about 16 and 35% by weight silicon carbide and between about 8 and 11% by weight of the sodium silicate binder, all percentages being based upon the total weight of the dry ingredients. Other ingredients may also be used in the coating composition to adjust or modify its physical properties. For example, the thermal expansion of the coating can be lowered by adding low expanding constituents like fused SiO₂, graphite, Si₃N₄, etc. Glazing can be enhanced with borates, and low temperature oxidation protection improved by addition of Al and Ti. These metallic additions also increase the electrical conductivity of the coating. Metallic, inorganic or carbonaceous fibers can also be added to the coating composition to improve strength. The following examples will serve to further illustrate the practice of the present invention. EXAMPLE 1 A slurry was prepared using a commercial refractory cement containing mullite (3Al₂O₃·2SiO₂) and silica (SiO₂) produced under the tradename "Super 3000", a product of C.E. Refractories, Combustion Engineering, Inc., Valley Forge, PA. The cement had the following composition: approximately 57.0% by weight mullite, 11.4% by weight silica, and 31.6% waterglass. This composition as published by the manufacturer was calculated on the basis of a chemical analysis of the cement by ASTM C-753, which analysis comprises 52.2% by weight Al₂O₃, 44% by weight SiO₂, 0.2% Fe₂O₃ and 3.6% by weight alkali (as Na₂O). The waterglass is a 33.4% by weight solution of sodium silicate in the approximate form Na₂O 3.8SiO₂. Waterglass can also be expressed as 10.6 weight % Na₂O 3.8SiO₂ and 21 weight % water based upon the total weight of the cement. Silicon carbide powder was added to the cement in an amount of about 20% by weight based on the total weight of the cement to form the slurry. The final composition of the slurry was: 47.4% by weight mullite, 9.4% by weight silica, 26.1% by weight waterglass, and 16.7% by weight silicon carbide. The waterglass could be expressed as 8.7% by weight Na₂O·3.8SiO₂ and 17.4% by weight water, based upon the total weight of the slurry. On a water-free basis, the dry ingredients had the following composition: 57.7% by weight mullite, 11.4% by weight silica, 20.3% by weight silicon carbide and 10.6% by weight sodium silicate solids as Na₂O·3.8SiO₂. The silicon carbide used in the slurry was a commercial SiC powder, i.e., grade 280 RA Silicon Carbide, Carborundum Company, Niagara Falls, New York. The silicon carbide powders were of a size predominantly less than 74 µm. A block of isotropic graphite measuring 3.1 x 4.3 x 6.8 inches (7.9 x 10.9 x 17.3 cm) was coated by brushing the slurry onto all but one side of the block. The applied slurry was then allowed to dry in air at room temperature until all the water had been removed. A second application of the slurry was then brushed onto the dried first layer and was dried in air at room temperature layer in the same manner as described. The coating bonded extremely well to the graphite block and had an overall thickness of about 0.080 inch (2 mm). The coated block was then placed inside a Globar furnace with its uncoated side resting on a ceramic insulating brick. The coated block was then heated to 1350°C. Air was purged through the furnace chamber at 7.5 SCFM during the oxidation test. After six hours, the coated brick was quenched by pulling it from the furnace and allowing it to stand at ambient temperature. It was found that oxidation of the brick occurred on the uncoated side but no oxidation occurred on the five coated sides of the brick. Furthermore, quenching from 1350°C did not rupture the coating-graphite bond. This excellent result was attributed to the similar thermal contracting coefficients of both the coating and graphite brick. EXAMPLE 2 A brick measuring 3 x 4 x 8 inches (7.6 x 10.2 x 20.3 cm) was shaped from a cool-base carbon block and coated with two applications using the slurry prepared in Example 1. The slurry contained the same ingredients and proportions of mullite, silica, silicon carbide and waterglass and was prepared in exactly the same manner. The coal-base material is commonly used to line aluminum reduction cells. The two applications were applied to the brick in the same manner allowing the first application to dry completely in air in order to remove water before applying of the second application. The coated brick was then placed inside an oxidizing furnace along with uncoated bricks made from the same graphite material and heated for seven days at a temperature of about 550°C. The coated brick was about 20% oxidized whereas the uncoated brick was completely oxidized. In other examples, a number of standard graphite electrode samples having relatively low coefficients of thermal expansion (CTE) were coated with a slurry prepared as in Example 1 and were then subjected to oxidation tests. Although some of the coated electrode samples did successfully withstand thermal shock, in most cases, the coating did not bond well to the graphite, it cracked and spalled during cool-down to room temperature.

Claims: An article comprising a carbonaceous substrate having a coefficient of thermal expansion greater than about 1.5 x 10⁻⁶ m/m/°C and an oxidation prohibitive coating overlying and bonded to said substrate upon being heated to an elevated temperature above 1100°C, said coating being formed from a particulate mixture, characterized in that the mixture essentially consists of a) mullite, in an amount of at least 20 % by weight, b) silicon carbide and/or boron carbide, c) silica and d) a silicon containing binder material selected from sodium silicate, potassium silicate, colloidal silica and ethyl silicate, and that the mixture forms a rigid, glassy, ceramic, refractory coating having a coefficient of thermal expansion of about 5.5 x 10⁻⁶ m/m/°C. An article according to Claim 1, wherein said coating consists essentially of between about 20 and 70% by weight mullite, between about 5 and 14% by weight silica, between about 5 and 70% by weight of the silicon carbide and/or boron carbide carbide and between about 5 and 14% by weight of the silicon-containing binder material, all percentages being based upon the total weight of the dry ingredients. An article according to Claim 2, wherein said coating consists essentially of about 57.7% by weight mullite, about 11.4% by weight silica, about 20.3% by weight silicon carbide and about 10.6% by weight sodium silicate. An article according to Claim 1, wherein said coating consists essentially of between about 48 and 60% by weight mullite, between about 9 and 12% by weight silica, between about 16 and 35% by weight silicon carbide and between about 8 and 11% by weight of sodium silicate, all percentages being based upon the total weight of the dry ingredients. An article according to any of the preceding claims, wherein said carbonaceous substrate has a coefficient of thermal expansion of about 4 x 10⁻⁶ m/m/°C. An article according to any one of the preceding Claims, characterized in that it is a graphite crucible. An article according to any one of the preceding Claims, characterized in that it is a furnace brick lining. A coating composition for forming an oxidation prohibitive coating upon a carbonaceous substrate upon being heated to an elevated temperature above 1100°C, characterized in consisting essentially of between about 20 and 70% by weight mullite, between about 5 and 14% by weight silica, between about 5 and 70% by weight silicon carbide or boron carbide or both and between about 5 and 14% by weight of a silicon-containing binder material selected from the group consisting of sodium silicate, potassium silicate, ethyl silicate and colloidal silica, all percentages being based upon the total weight of the dry ingredients. A coating composition according to Claim 8, consisting essentially of about 57.7% by weight mullite, about 11.4% by weight silica, about 20.3% by weight silicon carbide and about 10.6% by weight sodium silicate. A coating composition according to Claim 8, consisting essentially of between about 48 and 60% by weight mullite, between about 9 and 12% by weight silica, between about 16 and 35% by weight silicon carbide and between about 8 and 11% by weight sodium silicate, all percentages being based upon the total weight of the dry ingredients. A method of protecting a carbonaceous article having a coefficient of thermal expansion which is greater than about 1.5 x 10⁻⁶ m/m/°C against oxidation and erosion at an elevated temperature above about 1100°C, characterized by: (a) applying to the surfaces of said carbonaceous article a particulate mixture consisting essentially of between about 20 and 70% by weight mullite, between about 5 and 14% by weight silica, between about 5 and 70% by weight of silicon carbide and/or boron carbide, and between about 5 and 14% by weight of a silicon-containing binder material selected from the group consisting of sodium silicate, potassium silicate, ethyl silicate and colloidal silica, all percentages being based upon the total weight of the dry ingredients; and (b) heating said particulate mixture to a temperature at which a rigid, glassy, ceramic, refractory coating is formed. A method according to Claim 11, wherein said particulate mixture consists essentially of about 57.7% by weight mullite, about 11.4% by weight silica, about 20.3% by weight silicon carbide and about 10.6% by weight sodium silicate. A method according to Claim 11, wherein said particulate mixture is applied to the surfaces of said carbonaceous article in the form of an aqueous slurry. A method according to Claim 13, wherein said slurry is dried under conditions to produce a hard coherent layer adhering to the carbonaceous article. A method according to Claim 14, wherein said slurry is dried in air at ambient temperature. A method according to Claim 15, wherein said dried coherent layer is heated to a temperature of at least about 1100°C. A method according to Claims 11 and 13-16, wherein said particulate mixture consists essentially of between about 48 and 60% by weight mullite, between about 9 and 12% by weight silica, between about 16 and 35% by weight silicon carbide and between about 8 and 11% by weight sodium silicate, all percentages being based upon the total weight of the dry ingredients.

Inventors: SARA, RAYMOND VINCENT
Applicants: UNION CARBIDE CORPORATION
Application year: 1984
Main classification: C04B41
IPC codes: C04B41