JPH0568405B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a porous single particle chromium carbide having a coarse particle size and a method for producing the same. Specifically, the present invention relates to a thermal spray coating for surface modification of metals, ceramics, etc. The present invention relates to providing chromium carbide which is suitable as a thermal spraying material used in or as a ceramic material. <Prior Art> Conventionally, it has been known to produce chromium carbide by mixing a chromium source and carbon powder and subjecting the mixture to reduction firing. For example, Special Publication No. 49-3639, Special Publication No. 50-
Representative publications include Publication No. 17040 and Japanese Patent Publication No. 54-8361. However, in these methods, impurities and contamination are unavoidable during production, and the particles obtained are fine powders, sintered bodies, etc., and each method has its advantages and disadvantages. On the other hand, it is also known to use chromium carbide as a thermal spraying material, but in both cases the sintered product is pulverized to adjust the particle size. (Tokuko Showa 54-
(Publication No. 8361) <Problems to be solved by the invention> Recently, various high-performance thermal spraying materials have been instantly applied for the purpose of imparting wear resistance, corrosion resistance, heat resistance, etc. to the surfaces of various metals and ceramics. Thermally sprayed coatings are commonly performed by melting, spraying, or, for example, plasma jet, and chromium carbide is also used as one of the thermal spraying materials to impart wear resistance and high-temperature oxidation resistance. . It is said that it is important for thermal spray chromium carbide used for such purposes to satisfy the following requirements. (1) A chromium carbide film with the desired performance can be obtained on the surface of the base material. (2) It has good fluidity as a powder and can be smoothly supplied at a constant speed without clogging the thin tube from the powder supply device to the plasma gun nozzle during use. (3) Of the chromium carbide powder added to the plasma gas during thermal spraying, the proportion of the chromium carbide powder that reaches the surface of the target substrate without scattering or loss and effectively forms a film, that is, the yield of thermal spraying is high. Generally, in order to satisfy these requirements (2) and (3) above, it is considered necessary that the particle shape of chromium carbide be as close to a spherical shape as possible, and that the particle size distribution be narrow. In particular, fine particles with a diameter of about 5 μm or less have small inertia and are easily scattered and lost during injection. However, conventional chromium carbide for thermal spraying is produced by mixing metal chromium or chromium oxide powder with carbon powder, press-molding the mixture, and carbonizing it in a carbon coating or in a carburizing gas. Since it is manufactured by pulverizing the fired product in the state and then sieving it to recover the desired particle size, the pulverization process is essential, and therefore the desired particle size after classification Not only does it have the disadvantage that the yield is extremely low, but the particles obtained are all angular, amorphous, flaky polycrystals, with irregular and rough fractured surfaces. Fluidity as a granule is not necessarily sufficient,
Further, during thermal spraying, there were disadvantages such as easy scattering and a low yield of thermal spraying. Furthermore, the particle size of chromium carbide particles produced by conventional methods varies considerably depending on the manufacturing method, but in all cases, a considerable amount of free carbon remains in the produced chromium carbide, and free carbon must be separated by some method. Removal is an important step in refining chromium carbide and has been one of its drawbacks. (Japanese Patent Publication No. 51-24480) In view of the above-mentioned problems, the present inventor conducted intensive research and found that by using chromium oxide single crystals with coarse grain size as a raw material, carbonization with high purity and unique physical properties can be achieved. The present invention was completed by discovering that chromium single particles can be obtained. <Means for solving the problems> and <Operation> That is, the present invention is characterized in that it is a porous single particle with an average particle size in the range of 5 to 50 μm and an angle of repose of 40 degrees or less. Chromium carbide and its manufacturing method. The first feature of the chromium carbide according to the present invention is that it is a porous single particle. In the accompanying drawings, FIGS. 1, 2, and 4 are scanning electron micrographs showing the particle structure of chromium carbide, and FIG. 3 is a scanning electron micrograph showing the particle structure of chromium oxide. 1 and 2 show chromium carbide according to an embodiment of the present invention, and FIG. 4 shows ordinary chromium carbide for comparison. As is clear from the observation of the drawings, the chromium carbide of the present invention has a large number of pores on the particle surface, making the entire particle porous, and furthermore, there is no bonding of each particle due to sintering, and the particles are substantially porous. It is an independent single particle. The apparent density of such porous particles is generally in the range of 2.4 to 2.8 g/ml. Figure 3 shows the raw material chromium oxide single crystal, and it can be seen that the particle size and external shape of the chromium carbide are maintained almost as they are. On the other hand, in the comparative chromium carbide shown in FIG. 4, there are not many porous particles, and the particles are not bonded together by sintering to form a single particle body. In this way, single-particle chromium carbide refers to an independent particle state in which the raw material chromium oxide single-crystal particles are converted into carbide particles while substantially retaining the external shape and particle size. Further, "substantially" means that most of the particles are in the above state, and that small particles are sintered and a secondary particle state is tolerable to some extent. Next, the second feature of the chromium carbide in the present invention is that the average particle size is in the range of 5 to 50 μm, preferably in the range of 10 to 50 μm as a thermal spray material. The width of the particle size distribution is in the range of 1 to 100 μm, but the width of the distribution is narrow and centered around the average particle size. Furthermore, as mentioned in the previous section, the size and external shape of such particles depend on that of the raw material chromium oxide. Therefore, the chromium carbide of the present invention contains less impurities than the conventional chromium carbide, and it is easy to separate carbon and the like, so it can be easily obtained as a highly pure product. In addition, chromium carbide includes Cr ₃ C ₂ ,
Examples include Cr ₇ C ₃ and Cr ₂₃ C ₆ , but in the present invention, Cr ₃ C ₂ and Cr ₇ C ₃ are used in most cases. In addition, in the present invention, when discussing particle size,
This refers to the value determined by the Coulter-Kauter particle size distribution measurement method, which is a well-known method in the field of particle size measurement. Furthermore, a third feature of the chromium carbide in the present invention is that the particles have very good fluidity (free flow properties) due to the above-mentioned particle characteristics. The quality of the fluidity of particles can be evaluated by the angle of repose, and the smaller the value, the better the fluidity, and it is better to set it to 40 degrees or less. Because the chromium carbide according to the present invention has the above-mentioned particle size and is a rounded single particle, it has very good fluidity compared to pulverized sintered compacts with many sharp fractured surfaces, and has an angle of repose of 40 Never go overboard. Conventionally, chromium carbide has been typically used as a thermal spray material, but since the product of the present invention is a chromium carbide single particle having the above characteristics, it can be used as is.
It can be used as a suitable material because it has desirable properties required for thermal spray materials. In addition, chromium carbide can be used as a new ceramic material because it has corrosion resistance, heat resistance, and wear resistance. It can be pulverized by a desired pulverizing means and used as a ceramic material if necessary. The method for producing chromium carbide in the present invention is not particularly different from the conventional method for operating limbs, but its distinctive feature is that coarse single-crystal chromium oxide particles with an average particle size of 5 to 50 μm are used as raw materials. That is, it is produced by mixing single-crystal chromium oxide particles with an average particle size of 5 to 50 μm with a carbon-containing substance, and then heat-treating the mixture at a temperature of 1250° C. or higher in a reducing atmosphere. Conventionally, commercially available chromium oxide (Cr ₂ O ₃ ) is a fine powder with an average particle size in the range of 0.5 to 3 μm, but when such a fine powder raw material is used,
As shown in Figure 4, the chromium carbide obtained cannot be obtained as a single crystal due to its remarkable sinterability, and when used as a thermal spray material, it is essential to use pulverization methods. Pulverized products are not preferred as raw materials because they not only have poor quality characteristics such as fluidity, but also reduce the yield of the desired particle size. Therefore, a coarse chromium oxide single crystal can be obtained by firing the above-mentioned general chromium oxide fine powder again at a temperature of 1400° C. or higher in a normal atmosphere. The method for producing such coarse chromium oxide has already been developed by the present inventors, for example,
No. 169351 (see Japanese Patent Application Laid-open No. 60-65727) or Japanese Patent Application No. 59-70013 (see Japanese Patent Application Laid-Open No. 60-215528)
It is detailed in the. That is, general chromium oxide fine powder is heated and calcined in an oxidizing atmosphere at a temperature of 1400°C or higher below the melting point of chromium oxide, with or without the addition of auxiliaries such as alkali metal salts or alkylated earth metal salts. , then
Coarse chromium oxide can be obtained by performing appropriate loosening treatment. It goes without saying that if an auxiliary agent is added, it should be thoroughly removed by washing with water after firing. The chromium oxide obtained by this manufacturing method is, for example, a spherical or polyhedral coarse single crystal as shown in FIG. 3, and according to the manufacturing method of the present invention, these are sintered to form bonds between particles. The chromium carbide single particles can be converted into chromium carbide single particles in which the particle properties are substantially maintained without any problems. On the other hand, other carbon-containing raw materials include:
Graphite is generally used. It is preferable to use a carbon-containing substance with as high a chemical purity as possible because the produced chromium carbide can be obtained in a highly pure state. On the other hand, if the purity of the carbon material is poor, it will not only lead to a decrease in product purity, but also if the impurities include ash components with a low melting point, it will cause fusion between chromium carbide particles, resulting in poor quality after firing. Makes loosening difficult. The amount of carbon mixed varies depending on the type of target carbide and carbon material, but for Cr ₃ C ₂ and Cr ₇ C ₃ , which are commonly used as chromium carbide for thermal spraying, 3Cr ₂ O ₃ + 13C = 2Cr ₃ C ₂ +9CO (1) 7Cr ₂ O ₃ +27C=2Cr ₇ C ₃ +21CO (2) The amount of carbon in the starting material is a chemical that satisfies the reactions of equations (1) and (2) in the case of solid carbon materials such as graphite. Although the required amount is stoichiometrically, if necessary, a slight excess of carbon may be added in consideration of the presence of air during carbonization and firing. As for the mixing method, a homogeneous mixture can be easily obtained as it is using a conventional general mixing device, but when mixing, it is necessary to prevent dust during mixing and when taking it out from the device after mixing, and to take precautions for the next operation. To facilitate kneading, it is effective to add a small amount of water, a carbon material-containing aqueous solution, tar, pitch, etc. to the mixture. Incidentally, the added carbon content is, of course, a part of the carbon content. Next, the mixture is filled into a desired reaction container such as a crucible, but considering the working environment of the process so as not to generate dust, it is recommended to fill the mixture into a reaction container as a pressure-molded product. Cover the space between the molded body and the container and the top with graphite, or
A desired method is used to fill the reaction container, such as putting a small amount of the mixture into the reaction container so that it sticks, and then covering the top with graphite. The heating and firing method involves filling the raw material mixture, covering the top with graphite, etc., and placing it in a gas furnace such as a shuttle kiln or tunnel kiln or an electric furnace under a reducing atmosphere, and heating it at a temperature of approximately 1250°C or higher. Carbonization and firing are performed by processing. The carbonization reaction proceeds at a temperature of about 1250° C. or higher, preferably at a temperature of 1350 to 1400° C. The temperature may be adjusted depending on the amount of baking, etc., and baking may be carried out for a desired time. Baking time is 0.5 hours or more, preferably 1 hour
~3 hours is best. Thus, after calcination, the carbide according to the present invention:
Although it is obtained as a porous particle as shown in FIG. 1, separation from the carbon powder may occur as necessary depending on the firing method. In this case, separation is easy if carried out using the filling method as described above, and the separation can be carried out effectively due to the difference in particle size structure between chromium carbide and powdered carbon. Separation can be easily carried out by a desired operation such as separation using a sieve, water separation, or suction using a pneumatic transport device using a vacuum. The fired product obtained in this way is recovered as a molded body or an aggregate filled in a reaction vessel, but it is in a state that it collapses under slight pressure because there is virtually no bonding between particles due to sintering. The size and shape of the particles obtained by the disintegration maintain almost the same original shape as the raw material chromium oxide, as described above. Therefore, the disintegration into primary particles does not require a normal crushing operation, and single particles of chromium carbide with a predetermined particle size can be obtained by, for example, sizing with a vibrating sieve of the desired maximum particle size while adding a disintegration operation. . The loosened chromium carbide is suitable as a thermal spray material as it is. Furthermore, in order to substantially remove the carbon content or to classify the particles to a desired particle size, the purpose can be effectively achieved by elutriation or separation by methods such as wet gravity sedimentation classification. <Example> Example 1 Commercially available chromium oxide was heated in a gas furnace in an oxidizing atmosphere.
Particle size range obtained after firing at 1800℃ for 10 minutes
Using 100 parts by weight of coarse chromium oxide (Fig. 3) with a particle diameter of 15 to 75 μm and an average particle size of 34 μm as a raw material, 34.25 parts by weight of commercially available graphite (C = 99%) and 6.7 parts by weight of water were added, and the mixture was mixed in a mixer. Make a homogeneous blend. This formulation is filled to approximately 2/3 volume in a refractory alumina cylindrical reaction vessel. On top of that, the same graphite used in the formulation was coated all over the container, and then an alumina lid was placed over the container. Next, this was placed in an electric furnace, heated to 1400°C, and fired at the same temperature for 3 hours. After cooling, the reaction vessel was discharged, and the remaining graphite coating in the reaction vessel could be easily separated from the fired product by tilting the vessel and removing it with a spoon. The container was turned upside down and the fired product, which had the same shape as the container, was taken out. This fired product was an aggregate with weak sintering properties that could easily penetrate into the interior when a spatula was inserted, and easily crumbled when pressed with the back of a spoon. This was placed in a 150 mesh sieve and sieved while being crushed lightly with a spoon to recover most of the material. This recovered material was 97% of the amount of raw chromium oxide converted to chromium carbide (Cr ₃ C ₂ ). As a result of X-ray diffraction, this recovered material showed only chromium carbide (Cr ₃ C ₂ ) and no unreacted chromium oxide, and a low peak was observed at the position of the strongest line of graphite, which shows a remarkable diffraction peak even in the presence of a small amount. was recognized. The obtained chromium carbide particles were porous single particles as shown in FIG. 1, but they had almost the same shape as the raw material chromium oxide, and the average particle size measured by Coulter Counter showed almost the same value. Therefore, the measured value of the angle of repose, which is used as an indicator of fluidity, also showed good values, and the fluidity was also good. The measured values are shown in Table 1 below.

[Table] Take a certain volume of sample into the hanging density measuring device,
It was calculated from its mass.
Example 2 Carbonization firing was carried out in the same manner as in Example 1, except that 5% more graphite was added than in Example 1 when blending the raw materials. As a result of X-ray diffraction, chromium carbide and graphite were identified in the obtained fired product. The baked product is passed through a 150 mesh sieve to loosen it.
After stirring and suspending in water, elutriation was performed several times to remove fine particles, and by over-drying, coarse porous chromium carbide single particles (Figure 2) were obtained. This chromium carbide was identified only as chromium carbide (Cr ₃ C ₂ ) by X-edge diffraction, and no graphite or other impurities were observed. Furthermore, the measurement results of particle size, angular velocity of repose, and apparent density are shown in Table 2, and a material with particle size distribution and fluidity suitable for a thermal spray material was obtained.

[Table] Comparative example 1 Commercially available fine chromium oxide (average particle size 1.8 μm)
Heat treatment was carried out in the same manner as in Example 1, except that 34.25 parts by weight of graphite was used in 100 parts by weight to form a mixture. The obtained chromium carbide was identified as Cr ₃ C ₂ by X-ray diffraction, and the chromium oxide was completely converted to carbide, but the sintered product was so sintered that it could not be powdered without mechanical crushing. In addition, when the powder obtained by pulverizing a part of the elutriated product of the fired product was observed using an electron microscope, it was confirmed that the powder was a remarkable agglomerate in which the particles were bonded and integrated, as shown in FIG. <Effects of the Invention> According to the present invention, as detailed above, chromium carbide with unique particle characteristics can be produced as a highly pure porous chromium carbide single particle, and this material can also be used as a new ceramic material. I can say that. Therefore, the chromium carbide according to the present invention can be suitably used as a material for thermal spraying without being mechanically pulverized, or by just classifying it if necessary, and it also has excellent heat resistance, corrosion resistance, wear resistance, etc. As a new ceramic material that takes advantage of the

[Brief explanation of drawings]

Figures 1 to 4 are scanning electron micrographs showing the particle structure (However, Figures 1 to 3 are magnified by ×400.
Figure 4 is a magnification of ×3000), Figures 1 and 2 are chromium carbide showing an embodiment of the present invention, Figure 3 is a raw material chromium oxide used in an embodiment of the present invention,
FIG. 4 shows the particle structure of chromium carbide shown as a comparative example.

Claims (1)

[Scope of Claims] 1. A chromium carbide characterized by being a porous single particle having an average particle size in the range of 5 to 50 μm and an angle of repose of 40 degrees or less. 2. The chromium carbide according to claim 1, wherein the single particle chromium carbide is Cr ₃ C ₂ or Cr ₇ C ₃ . 3 Single particle chromium carbide has an apparent density of 2.4 to 2.8
Chromium carbide according to claim 1 or 2, which is g/ml. 4. A method for producing chromium carbide, which comprises mixing single-crystal chromium oxide particles with an average particle size of 5 to 50 μm with a carbon-containing substance, and then heat-treating the mixture at a temperature of 1250° C. or higher in a reducing atmosphere. 5. The method for producing chromium carbide according to claim 4, wherein the single-crystal chromium oxide having an average particle size of 5 to 50 μm is produced by firing fine chromium oxide powder at a temperature of 1400° C. or higher to grow crystals. .