JPH0428668B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing a silicon carbide material that has improved physical properties by forming a dense crystalline phase on its surface. More specifically, the present invention is a carbonized silicon carbide that precipitates a dense crystalline phase of silicon carbide by subjecting the surface to a specific treatment, thereby imparting excellent oxidation resistance, heat resistance, thermal shock resistance, and mechanical strength. The present invention relates to a method for producing a silicon material. Conventional technology Conventionally, silicon carbide sintered bodies have been used as heat-resistant structural materials for applications such as gas turbines and automobile engines because they have excellent heat resistance, corrosion resistance, and mechanical properties such as hardness, bending strength, and elastic modulus. It is a material that has been used extensively and is expected to develop in the future. However, this silicon carbide sintered body has a common weakness with non-oxides: it is easily oxidized in high-temperature atmosphere, and it is particularly eroded in the coexistence of water vapor, forming a glassy substance with pores on its surface layer, resulting in a decrease in physical properties. It has the serious drawback of causing Therefore, in order to improve this drawback, various methods have been used so far, such as an atmospheric pressure sintering method in which specific metals and carbon are added to silicon carbide powder and reacted at high temperatures, and carbon is added to silicon carbide powder. ,
Proposed methods include a reactive sintering method in which silicon is supplied from the outside, and a hot press method in which a catalyst is added to silicon carbide powder and heated at high temperature and pressure. However, such a method requires special equipment and consumes a lot of energy, and the obtained sintered body is not necessarily dense and often has pores. For example, even in a silicon carbide sintered body obtained by hot pressing at a temperature of 2000°C, when observing a cut surface cut out from a block, many pores of about 1 to 2 μm are present. When the sintered body has such pores, it is easily oxidized in high-temperature atmosphere, leading to various undesirable situations. For example, oxygen in the atmosphere diffuses into the interior through the pores of the sintered body, first forming an amorphous thin film of silicon dioxide at a temperature of about 700°C, and then crystallizing as cristobalite at an even higher temperature of about 1200°C. , Al ₂ O ₃ , B ₂ O ₃ ,
Silicates and glass containing numerous fine pores and cracks containing sintering accelerators such as Fe ₂ O ₃ are formed on the surface layer. will begin to occur. In this way, silicon carbide materials tend to be dense and not
If the material has pores, its high-temperature oxidation resistance is poor, and its use as a high-temperature heat-resistant structural material is inevitably limited. Therefore, research is underway to form a protective film on the surface of the substrate as a means of surface modification of silicon carbide materials. Until now, no method has been found that can uniformly and economically form an oxidation-resistant protective film on the surface of a silicon carbide substrate having a large area and a complicated shape. Problems to be Solved by the Invention Under these circumstances, the present invention provides an extremely simple method of forming a coating layer having a thermal expansion coefficient similar to that of the base material on the surface of a silicon carbide base material having an arbitrary shape. This was done with the aim of economically providing a silicon carbide material that can be formed uniformly and with good adhesion by a simple operation, and has excellent oxidation resistance and other physical properties. Means for Solving the Problems As a result of extensive research in order to develop a silicon carbide material with excellent oxidation resistance, the present inventors discovered that an alkali metal compound and carbonaceous material were added to the surface of a silicon carbide base material. By providing the mixture layer and firing it at a predetermined temperature, a homogeneous silicon carbide crystal phase is formed on the surface layer of the base material, and a coating layer made of glass containing an alkali metal is formed on top of the homogeneous silicon carbide crystal phase. Then,
It has been discovered that by removing this coating layer, an oxidation-resistant silicon carbide material having a dense crystalline phase in the surface layer can be obtained, thereby achieving the above object, and based on this knowledge, the present invention has been completed. It came to this. That is, the present invention provides a layer of a mixture of an alkali metal compound and carbonaceous material on the surface of a silicon carbide base material, and then sintering the mixture layer in the presence of oxygen at a temperature of 800 to 1300°C. The present invention provides a method for producing an oxidation-resistant silicon carbide material having a dense crystalline phase in the surface layer portion, which comprises removing a coating layer formed on the surface and made of glass containing an alkali metal. The present invention will be explained in detail below. As the silicon carbide base material used in the method of the present invention, conventionally used methods such as pressureless sintering method, reaction sintering method, hot press sintering method, etc.
A sintered silicon carbide body having pores obtained by any method can be used. Further, there are no particular restrictions on the shape or size, and a base material having any shape or size can be used. In the method of the present invention, examples of the alkali metal compound used to form a layer on the surface of the silicon carbide substrate include halides, carbonates, nitrates, and sulfates of alkali metals such as lithium, potassium, and sodium. These are preferably mentioned, and one type of these may be used or two or more types may be used in combination. Further, these alkali metal compounds may be used in powder form or in the form of an aqueous solution. When used in powder form, the particle size is usually preferably 20 μm or less. In the present invention, it is necessary to use carbonaceous material together with the alkali metal compound to form the layer. Preferred examples of the carbonaceous material include single carbon particles and carbohydrates such as sucrose and starch. The ratio of the alkali metal compound to the carbonaceous material is usually selected in a weight ratio of 1:0.1 to 1:2. The mixture of the alkali metal compound and carbonaceous material has a concentration of 2 in terms of alkali metal per unit area (cm ² ).
It is preferably applied by means of painting, spraying, etc. so that a layer of ~30 mg is formed. When forming this layer, a binder such as methylcellulose or dextrin may be used if desired in order to strengthen the coated surface after drying. In the method of the present invention, a silicon carbide base material with a mixture layer of an alkali metal compound and carbonaceous material provided on the surface thereof is heated at a temperature of 800 to 1300°C, preferably 850 to 1150°C, in the presence of oxygen. Calcinate at a temperature in the range of °C. Firing is usually performed in the atmosphere, but if desired, it may be performed in a mixed gas of oxygen and an inert gas such as nitrogen or argon, or a mixed gas of air and the inert gas. Further, a firing time of about 5 to 30 hours is usually sufficient. The above-described firing treatment produces a uniform silicon carbide crystalline phase on the surface layer of the silicon carbide base material, and further forms an alkali metal-containing vitreous coating layer thereon. The mechanism of the formation of such a silicon carbide crystalline phase and the formation of a glassy coating layer is not necessarily clear, but it is thought that the following reaction causes the formation of the crystalline phase and formation of a glassy coating layer. . That is, an alkali metal compound is applied to the surface of a silicon carbide substrate and heated at 800 to 1300°C in the presence of oxygen.
By firing at a temperature in the range of of silicon monoxide is produced. This gaseous silicon monoxide is
In the liquid phase, as shown in reaction formula (),
Decomposes into gaseous silicon dioxide and gaseous silicon. Next, this gaseous silicon reacts with the carbon contained in the liquid phase produced by reaction formula () and carbon derived from carbonaceous materials, and as shown in reaction formula (), crystalline silicon is formed. Silicon carbide is produced, and a dense crystalline phase of silicon carbide is produced on the surface layer of the base material. 3SiC＋〓O ₂ →3SiO (g) + 3C (S) …() 3SiO (g) → 1.5SiO ₂ (g) + 0.5Si ₃ (g)
...() 0.5Si ₃ (g) + 1.5C(S) → 1.5SiC(S) ...() According to X-ray diffraction, silicon carbide produced by the above reaction formula () is considered to be produced stably at low temperatures. β-SiC. On the other hand, most of the gaseous silicon dioxide produced in the reaction formula () dissolves in the liquid phase and reacts with the alkali metal oxide to form an alkali metal-containing glass as shown in the reaction formula (). As a result, a coating layer made of alkali metal-containing glass is formed on the silicon carbide crystal phase. M ₂ O (L) + 1.5SiO ₂ (g) → 1/2M ₂ SiO ₃ (S) + 1/2M ₂ Si ₂ O ₅ (S) ... () [However, M is an alkali metal, (L) is a liquid,
(S) represents a solid.] The alkali metal-containing vitreous coating layer formed in this way has a low melting point and must be removed. Regarding the removal of this coating layer,
For example, the substrate is immersed in a phosphoric acid solution, and
It can be removed by holding it at a temperature of 200 to 300°C for about 5 minutes to 1 hour,
Alternatively, it can also be removed by immersing it in a 5-20% by weight hydrofluoric acid solution at room temperature for about 5 minutes to 1 hour. In this way, on the surface layer of the silicon carbide base material,
A silicon carbide material having a uniform and dense silicon carbide crystal phase and having excellent oxidation resistance can be obtained. When this crystalline phase is observed under a microscope, it has a thickness of 2 to 4 μm and a length of 5 μm.
It is tightly packed with fibrous crystals of ~10 μm and partially flat crystals, and the crystal phase is
Linear diffraction reveals a single phase consisting of stable β-silicon carbide. Effects of the Invention According to the method for producing an oxidation-resistant silicon carbide material of the present invention, a uniform and dense crystal phase of silicon carbide can be formed on the surface layer of a silicon carbide base material having an arbitrary shape by an extremely simple operation. The resulting silicon carbide material can be formed economically and has excellent heat resistance and thermal shock resistance, as well as particularly improved oxidation resistance and mechanical strength, making it suitable for heat-resistant structural materials, such as automobile engines. It is useful as an industrial material for industrial components and gas turbines, and as a high-temperature oxidation- and corrosion-resistant material in fields such as space, ocean, and environmental chemistry, and in fields such as coal liquefaction, gasification development, and geothermal development. Examples Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples in any way. Example 1 A solution of 2 g of lithium carbonate, 1.2 g of starch, and 10 mg of methylcellulose dissolved in 20 ml of distilled water was placed on the surface of an atmospheric pressure silicon carbide sintered body heated to about 100°C, so that the amount of lithium was 20 mg per 1 cm ² . So that
It was applied evenly with a brush and dried. This sample was placed on a silicon carbide boat, placed almost in the center of a high alumina combustion tube, and then fired at 1100°C for 24 hours while flowing air at a rate of 500 ml/min. A covered silicon carbide mass was obtained. This item was observed through a metal microscope.
A glass phase with cracks and pores is observed, and
Linear diffraction confirmed that β-SiC and tridymite crystals were formed. Next, the silicon carbide mass was placed in 12 ml of phosphoric acid solution, heated to volatilize water, gradually raised to temperature, held at 28°C for 15 minutes, taken out, washed with water, and dried. As a result of observing the surface layer of this material using a scanning electron microscope (see figure), it was found that the thickness was 2~
Fibrous crystals of 3 μm and 5 to 10 μm in length were formed over the entire surface with some flat crystals. As a result of powder X-ray diffraction, this crystalline phase was confirmed to be a single phase of β-silicon carbide. Example 2 2 g of lithium fluoride, 800 mg of sugar, and methylcellulose were placed on the surface of a silicon carbide hot-pressed sintered body.
A solution prepared by dissolving 12 mg of lithium in 20 ml of distilled water was applied with a brush so that the amount of lithium was 20 mg per 1 cm ² , and then dried. This sample was placed in a furnace in the same manner as in Example 1, and while drying air was flowing at a rate of 500 ml/min, the temperature was rapidly raised from room temperature, and when it reached 950°C, the heating rate was slowed down to 1100°C. After raising the temperature to 16 hours, the temperature was maintained for 8 hours for firing. In this way, a silicon carbide mass uniformly covered with lithium glass was obtained. When this material was observed under a metallurgical microscope, a glass phase with cracks was observed on one surface, and X-ray diffraction confirmed that β-silicon carbide and tridymite were formed. Next, the silicon carbide mass was placed in 12 ml of phosphoric acid, heated at 280°C for 15 minutes, and then taken out and dried. As a result of observing the surface layer of this material using a scanning electron microscope, it was found that the thickness was 2-3 μm and the length was 5-5 μm.
Fibrous crystals of 10 μm were formed all over the surface, with some flat crystals. As a result of powder X-ray diffraction, this product was found to be a single phase of β-silicon carbide. Example 3 A sample was prepared under the same conditions as in Example 1, using sodium carbonate instead of lithium carbonate in Example 1, and using 400 mg of carbon for emission analysis as the carbonaceous material, placed in a furnace, and heated with air and argon. While flowing at a speed of 300ml/min and 100ml/min, respectively,
After rapidly raising the temperature from room temperature, it was held at 1000°C for 24 hours and fired to obtain a silicon carbide block covered with sodium silicate glass. According to observation using a metal microscope, tridymite crystals were formed throughout the product, and a molten bubbling phenomenon was observed. Since sodium salts react rapidly, good results were obtained by introducing argon, lowering the oxygen partial pressure, and lowering the reaction temperature. Next, the silicon carbide block was treated with 46% by weight hydrofluoric acid, washed with water, and dried. Crystals with a thickness of 2 to 3 μm and a length of 4 to 7 μm were formed all over the surface layer, with some flat crystals. As a result of powder X-ray diffraction, this product was found to be a single phase of β-silicon carbide. Example 4 A sample was prepared in the same manner as in Example 1 except that potassium carbonate was used instead of lithium carbonate in Example 1, and the sample was placed in a furnace and air and argon were pumped at 300% each.
While flowing at a rate of ml/min and 100 ml/min, the temperature was rapidly raised from room temperature and kept at 1100°C for 24 hours to perform calcination, thereby obtaining a silicon carbide mass covered with potassium-containing glass. Most of this material was a glass phase, but tridymite crystals were densely formed. Next, the silicon carbide lumps were added to a solution of 1 part by weight of 46% hydrofluoric acid and 2 parts by weight of distilled water.
After soaking for 15 minutes, it was placed in an ammonia aqueous solution to remove residual hydrofluoric acid, washed with water, and dried. Fibrous crystals with a thickness of 2 to 3 .mu.m and a length of 5 to 10 .mu.m were formed all over the surface layer of this material, together with flat crystals. As a result of powder X-ray diffraction, this product was found to be a single phase of β-silicon carbide. Example 5 In Example 1, a mixture of lithium fluoride, potassium chloride and sodium carbonate (weight ratio 2:1:1) was used as the alkali metal compound, and Example 1
A sample was prepared in the same manner as above, placed in a furnace, and rapidly heated from room temperature to 1000 ml while air and argon were flowing at a rate of 300 ml/min and 100 ml/min, respectively.
After raising the temperature from ℃ to 1100℃ over 16 hours,
It was held at this temperature for 8 hours and fired to obtain a silicon carbide mass covered with an alkali metal-containing glass phase. Next, the silicon carbide lumps were added to a solution of 1 part by weight of 46% hydrofluoric acid and 3 parts by weight of distilled water.
After soaking for 15 minutes, it was placed in an aqueous ammonia solution to remove residual hydrofluoric acid, washed with water, and dried. Fibrous crystals with a thickness of 2 to 3 μm and a length of 5 to 11 μm were formed all over the surface layer of this product, with some flat crystals. As a result of powder X-ray diffraction, this product was found to be a single phase of β-silicon carbide. Examples 6 to 19 Examples 6 to 19 were conducted using a silicon carbide sintered body, an alkali metal compound, reaction conditions, and glass removal method as shown in Table 1, and fibrous β was formed on the surface layer of the sintered body.
- Uniform formation of silicon carbide crystals. The results are shown in Table 1.

【table】

[Table] The silicon carbide materials obtained in each of the above examples were
in a Super Kanthal box electric furnace maintained at 20 °C.
After being inserted for a minute, the process of rapidly cooling in the air was repeated 10 times, but no peeling or shedding of the fibrous tissue was observed in any case, indicating that it had sufficient spall resistance. In addition, oxidation resistance was determined based on weight increase when heated at 1200°C for 500 hours. The results are shown in Table 2. Both have remarkable oxidation resistance, and
From observation using SEM images, almost no change in the fibrous structure of silicon carbide was observed.

【table】

【table】 [Brief explanation of drawings]

The figure is a scanning electron micrograph showing the structure of the surface layer of the oxidation-resistant silicon carbide material obtained by the method of the present invention.

Claims (1)

[Claims] 1. After providing a layer of a mixture of an alkali metal compound and carbonaceous material on the surface of a silicon carbide base material, this is fired at a temperature of 800 to 1300°C in the presence of oxygen, and then the base material is A method for producing an oxidation-resistant silicon carbide material having a dense crystalline phase in the surface layer portion, the method comprising removing an alkali metal-containing vitreous coating layer formed on the surface. 2. The manufacturing method according to claim 1, wherein the alkali metal compound is at least one selected from alkali metal halides, carbonates, nitrates, and sulfates. 3. A mixture layer of an alkali metal compound and carbonaceous material is provided on the surface of a silicon carbide base material so that the amount of the alkali metal is 2 to 30 mg per unit area (cm ² ). The manufacturing method described in Section 2. 4. The manufacturing method according to claim 1, 2, or 3, wherein the coating layer made of glass containing an alkali metal is removed using a phosphoric acid solution or a hydrofluoric acid solution.