JPS60215528A - Chromium oxide for spraying and its production - Google Patents

Abstract

PURPOSE:To provide chromium oxide for spraying, composed of the single crystal particles of chromium oxide having particular shape, and having excellent fluidity and high spraying yield. CONSTITUTION:The objective chromium oxide for spraying is composed of the single crystal particles of chromium oxide having spherical or polyhedral shape, an average particle diameter of 10-50mu and an angle of repose of <=40 deg.C. The above chromium oxide can be produced by calcining powdery chromium oxide in an oxidizing atmosphere (e.g. air) at a temperature between 1,500 deg.C and the melting point of chromium oxide (2,265 deg.C), disintegrating the product into individual particles (e.g. by suspending the calcined product in a proper amount of water, and disintegrating with a Henschel mixer, etc.), and collecting the particles having desired diameter by classification.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to chromium oxide for thermal spraying and a method for producing the same, and its purpose is to provide a material suitable for thermal spraying to be used for thermal spray coating for surface modification of metals, ceramics, etc. The purpose of the present invention is to provide chromium oxide for thermal spraying.

Powdered chromium oxide, which has traditionally been used in pigments, abrasives, refractory materials, etc., can be produced industrially by thermal decomposition of chromic acid anhydride, reduction roasting of dichromates, thermal dehydration of chromium hydroxide, etc. are manufactured, but all of these have a particle size of 1
11. It is in a fine powder state consisting of very fine primary particles of around m size.

Recently, wear-resistant, #
Thermal spray coating is carried out by so-called plasma jet, which instantaneously melts and sprays various high-performance thermal spray materials for the purpose of imparting properties such as corrosion resistance and heat resistance. For example, it is used to impart properties such as wear resistance and corrosion resistance.

It is said that it is important for thermal spray chromium oxide used for such purposes to satisfy the following requirements.

(1) A chromium oxide coating with high chemical purity and 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) During thermal spraying, the proportion of chromium oxide powder added to the plasma gas that reaches the surface of the target substrate without scattering or loss and effectively forms a coating, that is, the yield of thermal spraying is high. .

In general, in order to satisfy these requirements, especially (2) and (3) above, the particle shape of chromium oxide must be as close to spherical as possible, the particle surface must be smooth, the particle size distribution must be uniform, and each individual particle must be as close to spherical as possible. It is thought that it is necessary to have a mass (size) of at least a certain degree. Especially, the particle size is about 571
This is because fine particles smaller than m have a small inertia and are easily scattered and lost during injection.

However, conventional chromium oxide for thermal spraying is produced by heating ordinary chromium oxide powder in an electric furnace at a high temperature above the melting point (approximately 24
Because it is manufactured by heating it to 00°C to melt it, cooling it to solidify it, crushing it, and then sieving it to recover the desired particle size, there are no furnace materials or Disadvantages include contamination due to electrode erosion and the contamination of some metallic chromium generated based on the reduction reaction with carbon, as well as increased contamination and decreased yield due to strong crushing and classification. Not only that, but microscopic observation shows that the grains are all angular, irregularly shaped, fragmented polycrystals, with irregular and rough fractured surfaces. However, the fluidity as a powder is not necessarily sufficient, and it also has drawbacks such as being easily scattered during thermal spraying and easily reducing the yield of thermal spraying.

On the other hand, methods for obtaining rather large single crystals of chromium oxide have been known in the past; for example, there is a literature that states that when fine chromium oxide powder is heated at 1,300 to 1,500°C in an air atmosphere, it grows into rather coarse particles of chromium oxide. (Ceramic Industry Association Journal 8? [5] 1979, pp. 253-258). Another method is to heat and sinter a mixture of chromium oxide and alkali chromate (Japanese Unexamined Patent Publication No. 53-262flie). However, in the former method, it takes an extremely long time to grow fine chromium oxide particles into coarse particles by heating up to 15,000C, and the resulting coarse particles are formed by sintering several single crystals together. The fluidity is not necessarily sufficient if it is used as it is because it may form lump-like aggregates, and the latter method uses an extremely large amount of alkali chromate, twice the amount of raw chromium oxide. Since the purpose is to recover flaky single crystals of chromium oxide that have been evaporated and condensed based on this, the obtained chromium oxide not only lacks fluidity as a powder but also has a low yield as chromium oxide. There is a problem that the container is extremely small and corrosion of the container is inevitable.

In view of the above-mentioned problems, the inventors of the present invention have conducted intensive studies on chromium oxide for thermal spraying, which is most suitable as a material for thermal spraying, and its manufacturing method. The inventors completed the present invention by discovering that a powder made of particles has good fluidity and is ideal, and a method for specifically producing such a powder.

That is, the first invention of the present invention has an average particle size of 10 to 5.
The second invention is a thermal spraying chromium oxide powder consisting of spherical or polyhedral single crystal particles of chromium oxide having an angle of repose of 40 degrees or less. This is a method for producing chromium oxide for thermal spraying, which is characterized by heating and firing in an oxidizing atmosphere at a temperature of 1500°C to below the melting point of chromium oxide (2285°C).

The present invention will be explained in detail below.

First, to explain the first invention, the chromium oxide for thermal spraying of the present invention is a single-crystal chromium oxide powder with high purity, and has a density close to the theoretical value without containing bubbles.

Its shape is spherical or polyhedral, and according to observation using an electron microscope, it is spherical or polyhedral, consisting of single crystal grains surrounded by well-developed smooth crystal faces, and the polyhedron is preferably octahedral or larger. This includes things that are more than 14-sided and that are as close to a sphere as possible.

The average particle size of the chromium oxide for thermal spraying of the present invention is 10 to 50,
gm, preferably 20-3511.11, 10g
If it is less than 50gm, the mass of the powder will be small, so the inertia will be small, and it will be easily scattered and lost before it reaches the base material after being thrown into the thermal plasma. This is not preferable since it may reach the base material and form a rough coated surface.

In addition, the chromium oxide for thermal spraying of the present invention has an angle of repose of 40 degrees or less, preferably 34 degrees or less, as a characteristic of powder particles regarding fluidity. This is undesirable because it blocks the thin tube leading to the gun nozzle, making it difficult to supply in a steady state.

The chromium oxide for thermal spraying of the present invention having the shape, particle size, and fluidity characteristics described below is a chromium compound having completely new physical properties for thermal spraying of metals, ceramics, etc.
It has the following advantages over chromium oxide produced by conventional melting methods, making it ideal as a material for thermal spraying.

(1) Since it has high chemical purity and does not contain any metallic chromium, a chromium oxide coating with desired performance can be applied to the surface of a substrate with good reproducibility.

(2) Since it is composed of single crystal particles with a spherical or thick polyhedral shape and a smooth surface, it has good fluidity as a powder, and together with the fact that it is within a specified particle size range, it is easy to use from a powder supply device. Smooth supply is possible up to the plasma gun nozzle, and the yield of thermal spraying is high.

Next, a method for producing chromium oxide for thermal spraying, which is the second invention, will be explained.

The chromium oxide for thermal spraying of the first invention described above is produced by heating and firing the raw material chromium oxide powder in an oxidizing atmosphere at a temperature of 1500°C to below the melting point of chromium oxide (2285°C) to form a single crystal. It can be easily manufactured by growing it.

The chromium oxide used as a starting material in the production method of the present invention is generally industrially available powdered chromium oxide, but its precursors, chromic acid anhydride, chromium hydroxide, , ammonium chromate, or
It is also possible to reuse ammonium dichromate or a portion of chromium oxide recovered from the classification step described below in the production method of the present invention.

On the other hand, heating and firing in the production method of the present invention is preferably carried out in an oxidizing atmosphere; it is unsuitable in a reducing atmosphere because the crystal growth of chromium oxide is extremely slow. As a means for realizing the oxidizing atmosphere, simple firing in air, introduction of oxygen gas or air, addition of an appropriate amount of an oxidizing agent such as chromic anhydride to the raw material chromium oxide powder, etc. are all effective.

The heating and firing temperature in the manufacturing method of the present invention is 1500°C
to below the melting point of chromium oxide (2285°C), preferably 1500°C to 1900°C, and the reason why the firing temperature is specified in the above range is based on the following fact discovered by the present inventors.

That is, in general, particle growth associated with heating of chromium oxide in an oxidizing atmosphere proceeds by a unique mechanism called "evaporation-condensation", but according to the results of the study by the present inventors, the growth of crystal particles is It starts at temperatures above 1300°C and becomes sharp. However, simply heating a powder consisting of fine particles of chromium oxide at 1,500°C requires heating for more than 100 hours to grow it to an average particle size of 1,107p or more, but heating at a higher temperature or adding a small amount of alkali metal salts It has become clear that single crystal grains with an average grain size of 1 gm or more can be grown in a practically short heating time by heating in their presence. Therefore, if the heating and firing temperature is below 1500°C, which is outside the above range, the crystal growth of the chromium oxide powder will not be sufficient, and if it exceeds the melting point of chromium oxide, it will become a melt, and a severe crushing process after cooling and solidification is essential. This is undesirable because it results in a product that is irregularly shaped, fragmented polycrystalline, and contains pores, impurities, metallic chromium, and the like.

The firing time can be arbitrarily selected depending on the heating and firing temperature, and there is no need to limit it in particular.The longer the firing time, the greater the crystal growth, but on the other hand, more energy is required, so take this into consideration. It can be arbitrarily set to suit the size of the desired single crystal.

The heating and firing method can be carried out by placing the raw material chromium oxide powder or raw material mixture in a suitable fireproof container and firing it in an oxidizing atmosphere at a temperature of 1500°C to below the melting point of chromium oxide for the desired time. In many cases, the baked product obtained is a powder consisting of single crystal grains with uniform grain size, so the grain size is adjusted and used as is, or if necessary, further classified to a desired grain size and used as a product. Various methods such as sieving, dry or wet cyclone, and water sieving can be used as a means for classification.

In addition, if multiple single crystal particles are sintered during the crystal growth process due to heating and firing, resulting in a "bumpy" shape, a mechanical crushing process is performed to separate the single crystal particles in order to separate them from each other. It is desirable to have sufficient fluidity.

The crushing treatment may be either dry crushing or wet crushing, but it is not necessary to crush the single crystal particles of chromium oxide itself, and it is sufficient to separate or loosen the single crystals based on impact force or shear force. It is best to suspend the mixture in an appropriate amount of water and use a grinder such as a Henschel mixer.

In addition, in the production method of the present invention, if necessary, an appropriate amount of an alkali metal salt, alkaline earth metal salt, etc. as a crystal growth promoter may be mixed with the raw material chromium oxide powder and heated.

The alkali metal salts and alkaline earth metal salts to be added include carbonates, hydroxides, chromates, dichromates, chlorides, and sulfates thereof. Particularly preferred are one or more salts selected from the above-mentioned sodium, potassium, lithium or barium salts.

The amount of such metal salt to be used with respect to the raw material chromium oxide is Cr
It may be at most 20% by weight based on 203. The reason for this is that crystal growth of chromium oxide is expected as the amount of addition increases, but at around 20% by weight, it reaches saturation and crystal growth cannot be expected for that amount of addition, and the yield of chromium oxide decreases significantly. This is also because corrosion of the firing container and furnace materials will increase.

In this case, the fired product obtained by heating and firing is cooled and then subjected to crushing treatment to peel off the sintered and attached particles from each other, and washed with water or hot water to remove soluble groups. .

The purpose of this water washing is to remove chromium oxide partially oxidized and converted into chromate or dichromate and metal salts added as crystal growth agents.

After washing with water, the product is dried and classified as desired. In addition, the classification can be carried out by using a water sieve as well as washing with water.

Thus, according to the production method of the present invention, chromium oxide single crystals having an average particle size of 10 to 50 gm, a spherical to polyhedral shape, and an angle of repose of 40 degrees or less can be industrially advantageously produced. It can be produced and its purity is sufficient to suit various uses.

That is, according to the production method of the present invention, the product directly falls into the particle size of commercially available chromium oxide for thermal spraying, has a spherical to polyhedral shape, and has good fluidity. Further, according to this method, rough particle size adjustment can be performed only by adjusting the firing temperature and firing time, which is a further advantage.

Even if the product is required to have a more strict particle size in practical terms, if a water sieving step is provided in step 3 after the particle size adjustment step during firing, the product will have a uniform spherical or polyhedral shape, so water can be reduced. It is easy to classify using a sieve, and after recovering particles with the required particle size for thermal spraying materials, coarse and fine particles can be removed using materials with the required particle size, even if they are hard, heat resistant, and corrosion resistant. The use of chromium oxide, which is expensive to obtain by melting and pulverizing, can be supplied at a low cost to ceramic-related aggregates and abrasive materials, which have been hesitant due to the use of expensive chromium oxide, which is obtained by melting and grinding. The contribution is extremely large.

EXAMPLES The present invention will be explained in more detail below with reference to Examples and Comparative Examples.

Examples 1 to 4 and Comparative Examples 80 g of a mixture of commercially available chromium oxide powder (average particle size 1.5 gm) as a raw material and additives added as necessary were placed in an alumina crucible with an internal volume of 80 m iL, and the mixture was covered with a lid. The product was placed in an electric furnace, heated and fired in an air atmosphere under each firing temperature condition shown in Table 1, and then left to cool until the next day to obtain a fired product. The fired product is made into a slurry by adding water, subjected to a grinding process 4 using a household mixer, and then the particle size is adjusted using three water sieve tubes, followed by solid-liquid separation and drying to obtain chromium oxide for thermal spraying. Ta.

The yield, average particle size, and angle of repose of each of the obtained chromium oxides were measured, and the results shown in Table 1 were obtained. Furthermore, Table 1 lists commercially available product A as a reference example.

The particle size was measured using the Coulter counter method, and the angle of repose was measured using the injection method (however, the ring was
0mm).

Moreover, the shape shows the result of observation with an electron microscope.

FIG. 1 shows an electron micrograph showing the particle structure of commercially available product A as a reference example, and FIG. 2 shows an electron micrograph of the chromium oxide obtained in Example 1 above.

From Table 1 and Figure 2, it can be seen that the product of this example consists of polyhedral single-crystal particles of chromium liquefied with a thickness close to a spherical shape, and has a small angle of repose and good fluidity as a powder. it is obvious.

As a result of chemical analysis of the results of this example, each sample showed a Cr203 content of 99% or more. On the other hand, commercial product A contains C by containing metallic chromium.
r20:L content 101%, hydrochloric acid elution Cr203
was 2%, and the loss on ignition was -3.3%.

On the other hand, an electron micrograph of chromium oxide obtained in a comparative example is shown in FIG.

From Table 1 and FIG. 3, the resulting product of this comparative example had fine particles, was plate-shaped, and had no fluidity.

Example 5 7 Put 3 kg of commercially available chromium liquefied powder as a raw material into an aluminium crucible with an internal volume of 3 cm, cover with a lid, place in a gas furnace with a cloudy atmosphere, and heat it to 1800°C for 10 hours at the second temperature. Fired. After that, the temperature was lowered and cooled to obtain a fired product.

The fired product was subjected to wet straining using a Henschel mixer, and the viscosity was adjusted using three water sieve tubes, followed by solid-liquid separation and drying.
．． I got 4 kg. This chromium oxide had good fluidity, and as a result of observation with an electron microscope, it showed a shape similar to that shown in FIG.

This chromium oxide was sprayed onto a test piece with a diameter of 50 mm and a thickness of 9+ nm using a commercially available plasma spraying device to form a coating, and the cut surface of the cutter was observed using a scanning electron microscope photograph. As a result, this coating was found to have a higher
It was observed that there were few pores at the substrate-coating interface and the coating itself, and the coating had good adhesion to the substrate and was dense.

[Brief explanation of drawings]

8 Figure 1 is an electron micrograph showing the particle structure of commercial product A as a reference example, Figure 2 is an electron micrograph showing the particle structure of the product of Example 1, and Figure 3 is the product of Comparative Example. These are electron micrographs showing the particle structure of
00 times. Applicant Nihon Kagaku Kogyo Co., Ltd. Agent Yutaka 1) Yoshio 9 Procedural amendment January 29, 1985 Manabu Shiga, Commissioner of the Patent Office 1, Indication of Case Patent Application No. 1983-070013 2, Invention Name: Chromium oxide for thermal spraying and its manufacturing method 3; Relationship with the amended case; Patent applicant: 9-15-1 Kameido, Koto-ku, Tokyo; Representative: Miki Tanahashi, 4, representative of Nihon Kagaku Kogyo Co., Ltd. Room 204, Sanshin Building, 1-4-1 Yurakucho, Chiyoda-ku, Tokyo Telephone: 501-2138 Toyota Naigai Patent Office (5941) Patent Attorney Yutaka 1) Yu Yoshi Detailed description of the invention
Column 6, Contents of amendment 6-1 The scope of claims is amended as shown in the attached sheet. 6-2 The detailed description of the invention is corrected as follows. 1) On page 6, line 4 of the specification, "to be fired" is corrected to "after firing, a crushing process is performed for peeling." 2) tr Page 7, line 7, "less than or equal to" is corrected to "less than or equal to." 3)ll Add the following item after "High yield" on page 8, line 6. (3) Good fluidity as a powder means that the angle of repose is 4.
It is easily identified by its low value of 0 degrees or less, but it is also characterized by its high apparent density value, since the particles tend to be in a close-packed state. 4) On page 8, line 12, ``Let it grow'' is corrected to ``After growing, the crushing process is carried out.'' 5) On page 10, lines 1 and 2, "100 hours" is corrected to "approximately 10 hours, although there may be some differences depending on the reaction conditions." 6)ll Page 10, line 8, "Not carried out," is replaced by "Not carried out." On the other hand, at high temperatures of 1800°C or higher, the amount of energy required is enormous and it is somewhat economically disadvantageous, and it also causes corrosion of the furnace and container materials. Impurity contamination tends to increase. ] to be corrected. 7) Page 11, line 4, “Adjust the particle size” was changed to “However, the fluidity of the powder is insufficient if it is left as is, so the particle size is adjusted by crushing it to remove it.” Corrected to ``. 8)ll Page 11, lines 9 to 13, "It is preferable to heat and bake..." is deleted. 9) On page 11, line 18, "fired product" is corrected to "fired product in a dry state, or". 10) On page 13, line 10, "oxidation" is corrected to "meltening with good fluidity." 11) On page 13, line 20, "during firing" is corrected to "by crushing the loosening pot during and after firing." 12)〃 On page 14, line 20, ``housing process'' is corrected to ``grinding process''. 13) On page 15, line 5, "average grain size and angle of repose" is corrected to "average grain size, angle of repose, and apparent density." 10 〃 On page 15, line 6, “Table 1” is corrected to “Table 1.” 15) Use page 15, line 10 r 30mm). '', then add ``, and the apparent density was calculated from the mass of a sample of a certain volume taken with an apparent density measuring device in accordance with JIS K 1487-1984.'' 16)〃 Page 16, Table 1, Table 1 is corrected as follows. 17) // On page 18, line 4, "after firing" is corrected to "after firing." 18)〃 On page 18, line 6, ``Hugoshi processing'' is corrected to ``Hugashi processing.'' 19) On page 18, line 7, "viscosity adjustment" is corrected to "particle size adjustment." 20) tt Page 18, line 10, “Fluidity” is changed to “Angle of repose is 29 degrees, apparent density is 2.8 g/
Corrected to "liquidity in ml". 21) tt Page 18, line 18, after "Approved." Add the following. ``The fired product before being crushed for crushing is in a lightly sintered state, and when it is crushed lightly with a spoon to form a powder, the angle of repose is 45 degrees and the apparent density is The fluidity was insufficient at 1.7 g/lan.'' Claims. l) Chromium oxide for thermal spraying, which is a powder consisting of spherical or polyhedral single crystal particles of chromium oxide with an average particle size of 10 to 50 pm, and an angle of repose of 40 degrees or less. 2) A method for producing chromium oxide for thermal spraying, which comprises heating and firing chromium oxide powder in an oxidizing atmosphere at a temperature of 1500° C. or below the melting point of chromium oxide, and then subjecting it to crushing treatment.

Claims (1)

[Scope of Claims] 1) A powder for thermal spraying, characterized in that it is a powder consisting of spherical or polyhedral single crystal particles of chromium oxide with an average particle size of 10 to 507 hm, and an angle of repose of 40 degrees or less. Chromium oxide. 2) A method for producing chromium oxide for thermal spraying, which comprises heating and firing chromium oxide powder in an oxidizing atmosphere at a temperature of 1500° C. or below the melting point of chromium oxide.