PROCESS FOR PRODUCING MOLDING COMPOSED OF SILICON CARBIDE AND CARBON

Abstract: A process for producing a molding composed of silicon carbide and carbon and utilized as heat-resistant material, abrasion-resistant material, chemical resistant material, etc. is disclosed. This process intends to form a deep, uniform-thickness silicon carbide layer by mere procedure of penetrating a siliceous material into a carbonaceous molding to effect a reaction, and comprises molding into a desired shape a carbonaceous material having a lattice constant, c, of up to 6.9 ANGSTROM and a density of up to 1.7 g/cm<3>, penetrating a siliceous material thereinto to effect a reaction. Thus a molding of silicon carbide and carbon having about the same shape as that of the carbonaceous molding can be obtained.

Description: This invention relates to a method of forming a blocked solid composed of silicon carbide and carbon which can be used as heat resistant, wear resistant, or chemical resistant materials in various fields of industry. BACKGROUND ART Various methods of obtaining a composite composed of silicon carbide and carbon have been proposed. In order to improve the resistance to oxidation and wear, and other properties of carbon material, there have been proposed methods of forming a film of silicon carbide on the surface of a carbon substrate, such as the method of forming a silicon carbide layer on the surface of a substrate by decomposition and reaction of a gaseous silicon compound (e.g. SiO gas) , and the method of forming a silicon carbide film on the surface of a substrate by reaction of a gaseous silicon compound (e.g. SiCl₄) and a gaseous carbon compound (e.g. CCl₄ gas). However, the maximum thickness of the silicon carbide layer or film formed by such methods is about 2 mm. On the other hand, in order to form silicon carbide in a blocked solid of carbon there have been proposed a method which uses the above-mentioned gas phase reaction, and a method which uses a penetration reaction of molten silicon into a blocked carbon solid. By these methods it is possible to form silicon carbide in the blocked carbon solid to the depth of only several millimeters. The present inventors once developed a method by which, in order to form silicon carbide deep in a thick carbon block, the carbon block is first oxidized at 400 ∼ 600°C to make it porous and light in weight, and then a silicon containing material such as molten metallic silicon is caused to penetrate into and react with the porous block. By this method it is possible to attain a high rate of conversion of carbon to silicon carbide and form a blocked solid in which silicon carbide is uniformly formed to the depth of above 1 cm. Further studies and experiments, however, have disclosed that there are carbon materials in which silicon carbide is not uniformly formed, but the reaction is limited to the surface thereof. The DE-A-28 34 315 discloses a process for producing anti-friction products, in which porous carbon material is impregnated with molten silicon at temperature-time units which are to be adhered to specifically. A graphitized porous blank may be used as carbon material for the impregnation, the graphitization being carried out at temperatures of 2000 to 3000°C. However, problems may arise in that the strength of the composite material formed by this method may not be sufficiently high. This invention has been made in view of the above-mentioned problems, and its object is to provide a method of forming a new blocked solid composed of silicon carbide and carbon, wherein by selecting the crystallinity and density of a carbon base and simply causing a silicon containing material to penetrate into and react wtih the carbon base, it is possible to form a uniformly thick silicon carbide layer on the surface of the base with ease and without fail. DISCLOSURE OF THE INVENTION To attain the object, in the method according to the present invention, a carbon material which has been graphitized at temperatures of from 1550°C to below 2000°C to obtain a lattice constant c of 0.69 nm or below is formed into a block of a desired shape having a density of 1.3 to 1.7 g/cm³, and a molten silicon containing material is caused to penetrate into and react with the carbon block thereby to obtain a blocked solid composed of silicon carbide and carbon having substantially the same shape as the carbon block. The basic concept of the invention for solving the problem is that with due consideration given to the chemical reactivity and density of the carbon material constituting a carbon block, by selecting these factors it is possible to form a blocked solid of silicon carbide and carbon having a silicon carbide layer of a uniform, large thickness. From the above technical point of view, the present inventors have conducted penetration reactions of a silicon containing material on the surfaces of carbon blocks having different lattice constants c (crystallinities) and densities, compared the results and confirmed that in the blocks having a lattice constant c of 0.6900 nm (6.900 Å) or below which results from being graphitized at temperatures of from 1550 to below 2000°C, and a density of 1.3 g/cm³ to 1.7 g/cm³, a uniform formation of silicon carbide is effected. Generally, there are various forms of carbon having different crystallinities from an almost amorphous form such as carbon black to an almost perfectly crystallized form such as natural graphite. Except the natural form, the crystallinity depends on the temperature at which carbon is treated for graphitization in manufacturing the carbon material. The higher the temperature of the graphitization treatment is, the higher the degree of graphitization is. At the same time, the lattice constant c of the material decreases to 0.6708 nm (6.708 Å), the constant c of natural graphite, as will be described later in the embodiment 1. Therefore, the degree of crystallization (graphitization) can be estimated by measuring the lattice constant c of the carbon material. It is known that generally the same materials have a higher activity in their amorphous state than in their crystal state. From the above, it is believed that the reactivity of carbon material can be estimated by measuring its lattice constant c by X-ray diffractiometry. It is believed that the density of a carbon material has a great influence on the void rate, in particular, on the penetration reaction of molten silicon with carbon as in the present invention. In fact, as shown in embodiments 2 and 3 to be described later, a sufficient penetration reaction does not occur on the carbon material being graphitized at temperatures of 1550°C to below 2000°C below if its density exceeds 1.7 g/cm³. On the basis of the above observation and consideration, this invention has been made to provide a manufacturing method wherein a carbon material having 0.6900 nm (6.900 Å) or below which has been graphitized at temperatures of from 1550°C to below 2000°C is formed into a block having a density of 1.3 g/cm³ to 1.7 g/cm³, into which a silicon containing material is caused to penetrate and react therewith thereby to form silicon carbide uniformly and more deeply in the block than with the prior art method, while substantially maintaining the original shape of the block. The reason why the carbon material having a suitable crystallinity and a density of 1.3 g/cm³ to 1.7 g/cm³ can be converted into silicon carbide uniformly and deeply in the block from the surface thereof is believed to be due to the following mechanism. First, the reaction of the invention for forming silicon carbide is a reaction in which molten silicon penetrates into voids in the carbon material while reacting with the material to form silicon carbide. Since the conversion of carbon to silicon carbide accompanies expansion of the volume by about 2.4 times, the voids are filled with silicon carbide. Therefore, it is necessary that the carbon material should have voids into which molten silicon, etc. can penetrate. On the other hand, if the carbon material has too many voids, a sufficient penetration reaction may take place, but a problem is posed that the mechanical strength of the composite made of silicon carbide and carbon becomes weak. Thus, with the method of invention there is an optimum density range of the carbon material. This is the reason why the density of the carbon material is between 1.3 g/cm³ and 1.7 g/cm³. However, as shown in the embodiment 3 to be described later, even with a carbon material having a density of 1.5 g/cm³, if it has a low crystallinity caused by graphitization treatment at low temperature, the penetration reaction is restricted to a shallow depth in the surface of the carbon material. This is believed to be due to the speeds at which the following two phenomena constituting the reaction of conversion of carbon to silicon carbide proceed: (I) the speed at which molten silicon, etc. penetrate into the voids of carbon by capillary action; and (II) the speed at which molten silicon, etc. react with carbon in the surface of the voids to form silicon carbide, which expands about 2.4 times in volume thereby to fill the voids in the carbon. As described above, the carbon which is less crystallized is more active than the carbon which is more crystallized, and rapidly reacts with molten silicon, etc. Therefore, the above-mentioned reaction (II) proceeds faster than the above-mentioned penetration (I), so that silicon carbide occupies the voids in the surface of the carbon, thereby preventing silicon from penetrating into the inside of the carbon. As a result, it is only adjacent the surface of the carbon that carbon can be converted to silicon carbide. On the contrary, in the carbon being graphitized at temperatures of from 1550°C to below 2000°C to suitably adjust crystallinity, the above-mentioned reaction (II) becomes moderate as shown in the embodiment 3, so that the above-mentioned penetration (I) progresses into the inside of the carbon, and then the above-mentioned reaction (II) progresses thereby to form silicon carbide deep in the carbon. As described above, in accordance with the method of the invention, by causing a molten silicon containing material to penetrate into a carbon block made of carbon having a density of 1.3 g/cm³ to 1.7 g/cm³ which has been graphitized at temperatures of from 1550°C to below 2000°C, it is possible to obtain a composite in which silicon carbide is formed uniformly to the depth of about 20 mm from the surface, as was very difficult in the prior art, and to manufacture a composite of silicon carbide and carbon having a required high density and strength with ease and certainty. BRIEF DESCRIPTION OF THE DRAWING The drawing shows a relation between the temperature of graphitization treatment and the lattice constant c of carbon. EXAMPLE 1 17 weight % of silicon is added to and mixed with each of the carbon materials graphitized at known temperatures of 880 °C, 1000°C, 1550°C, 1800°C, 2070°C, 2300°C, 2600°C and 2900 °C, respectively, and the mixtures are ground, and then measured by X-ray diffractometry (with Cu Kα, 35 kV-15 mA). The silicon is an internal standard for determining the lattice constant. The lattice constants c of the carbon materials determined by the d value (plane distance) of 002 diffraction line are plotted in the attached drawing against the temperature of graphitization. The materials treated at 880°C and 1000 °C, respectively, have very broad 002 diffraction lines, which shows that they have been scarcely crystallized. The lattice constant c determined by d₀₀₂ is about 0.70 nm (7.0 Å). On the other hand, the carbon materials treated at higher temperatures than 1550°C produce sharper diffraction lines as the temperature rises, which shows higher degrees of crystallization. At the same time, as shown in the drawing, the lattice constant c decreases approaching to 0.6708 nm (6.708 Å) of natural graphite. Thus, it has been found out that the degree of crystallization (graphitization) of carbon is mostly determined by the temperature of treatment for graphitization. COMPARATIVE EXAMPLE 1 As shown in Table 1, 0.05 parts by weight of molten silicon is caused to penetrate into and react with carbon blocks made of carbon material treated for graphitization at different temperatures, and the depth of penetration is measured. The reaction is conducted in the atmosphere of argon gas at 1800 °C for one hour. The reaction products obtained are observed with the naked eye. No residue of silicon is observed in the blocks of the carbons having a lattice constant c of 0.69 nm (6.9 Å) or below (treated for graphitization at temperatures above 1550 °C). On the other hand, some residue of solidified molten silicon is observed on the surface of the blocks of the carbons having lattice constants c above 0.69 nm (6.9 Å) (treated for graphitization at tempeatures of 880°C and 1000°C). The reaction products are then ground for measurement by X-ray diffractometry. The products obtained from the carbons having a lattice constant of 0.69 nm (6.9 Å) or below are composed of β-silicon carbide, carbon and a minute amount of silicon, with the reaction ratios calculated from the weights before and after the reaction being 88 ∼ 100 %. The sectioned surfaces of the reaction products are observed with an X-ray microanalyzer to investigate how deep from the surface of each block silicon has penetrated into the block and has been converted to silicon carbide. The results of the observation are given in Table 1. Carbon material Depth of penetration (mm) Temperature of treatment Density ρ c/Å 880 °C 1.66g/cm³ 7.00 Nearly zero 1000 °C 1.69g/cm³ 7.00 Nearly zero 1550 °C 1.78g/cm³ 6.88 1.0 1800 °C 1.76g/cm³ 6.86 1.5 2070 °C 1.79g/cm³ 6.86 1.0 2300 °C 1.79g/cm³ 6.76 1.2 2600 °C 1.79g/cm³ 6.74 0.8 2900 °C 1.8 g/cm³ 6.73 0.5 As shown in the above table, in the blocks of those carbons which have lattice constants c above 0.69 nm (6.9 Å), the reaction of forming silicon carbide does not sufficiently progress, and the penetration depth of silicon is nearly zero. On the other hand, in the blocks of those carbons which have a lattice constant c of 0.69 nm (6.9 Å) or below, the reaction is active, and silicon carbide is formed to the depth of 0.5∼1.5 mm from the surface. EXAMPLE 2 In comparative example 1, the depth of penetration of silicon fluctuates even in those blocks which have a lattice constant c of 0.69 nm (6.9 Å) or below and in which the reaction is good. The fluctuation is believed to have been caused by the use of carbon materials having different densities above 1.7 g/cm³. Therefore, by using carbon materials having the same density of 1.5 g/cm³ and treated at different temperatures silicon is caused to penetrate into and react with the carbon materials. The results are shown in Table 2. The penetration reaction is conducted in a heating device by heating the carbon blocks with metal silicon on the surface thereof in an atmosphere of argon gas at 1800 °C. The amount of silicon which is reacted with each of the carbon blocks is between 0.4 and 0.7 in weight ratio to carbon. As shown in the table, if the penetration reaction is conducted by using carbon materials having the same density, it is possible to increase the Si/C ratio from 0.05 to 0.4 ∼0.7. A good reaction is observed in all of the carbon materials being graphitized at temperatures of from 1550°C to below 2000°C and having a lattice constant c of 0.69 nm (6.9 Å) or below, with silicon having reacted with carbon uniformly to a depth of 1.4 mm or above from the surface. In particular, in the carbon materials which have been treated for graphitization at a temperature above 1800°C and have a lattice constant of 0.686 nm (6.86 Å) or below, the reaction is good, and observation of the reaction products with the naked eye and identification thereof by X-ray diffractometry reveal that silicon carbide is formed to a depth of about 20 mm from the surface. In the carbons which have lattice constants c exceeding 0.69 nm (6.9 Å), the depth of penetration of silicon is increased by making their density 1.5 g/cm³. However, the value is 1.2 mm or below, and a residue of silicon that has not reacted with carbon remains on the surface of the products. From the above it has been concluded that lowering of the density of carbon is not sufficient for uniform conversion of carbon to silicon carbide, and that the conversion is closely related to the crystallinity and density of carbon. The reason therefor is the difference in the reaction process caused by the crystallinity of the carbon as previously mentioned. Carbon material Degree of reaction Depth of penetration (mm) Temperature of treatment Density ρ 880 °C 1.5g/cm³ Bad 0.8 1000 °C 1.5g/cm³ Bad 1.2 1550 °C 1.5g/cm³ Good 1.4 1800 °C 1.5g/cm³ Good 2.4 or above POSSIBLE INDUSTRAIL APPLICATIONS As described above, the manufacturing method in accordance with the invention is applicable to manufacturing blocks composed of silicon carbide and carbon for use as heat resistant, wear resistant and chemical resistant materials. Such heat resistant and wear resistant materials are materials used in the manufacturing process of glass bottles such as a material for a pair of tongs for picking glass bottles from a finishing die, a plate for supporting glass bottles, or a material for forming a sliding guide surface. The chemical resistant materials can be used as a material for a die which contacts molten metal, a material for forming a trough for guiding molten metal into the die, etc.

Claims: Method of forming a block composed of silicon carbide and carbon by forming a carbon material into a block of a desired shape, causing a silicon containing material to penetrate into and react with the carbon block thereby to obtain a block composed of silicon carbide and carbon and having substantially the same shape as said carbon block, characterized by graphitizing the carbon material at temperatures of from 1550°C to below 2000°C to obtain carbon material having a lattice constant c of 0.69 nm (6.9Å) or below and by the carbon block having a density of 1.3 to 1.7 g/cm³.

Inventors: ARAI TAKEHITO, IZAWA HAJIME, YAMAMOTO TAIJI, ARAI, TAKEHITO, IZAWA, HAJIME, YAMAMOTO, TAIJI
Applicants: OSAKA CEMENT, OSAKA CEMENT CO., LTD.
Application year: 1988
Main classification: C04B41
IPC codes: C04B35, C04B41