JPH0143021B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a thermal spray powder material for forming a thermal spray coating on the surface of a metal material, and particularly to a molybdenum-based metal powder material for efficiently obtaining a dense and uniform thermal spray coating. Spray dissimilar metals, ceramics, or composite materials of these materials onto the surface of metal materials to form a film.
Methods of improving material properties are commonly used. Various thermal spraying materials are used depending on the properties required for the coating, including heat resistance,
Molybdenum-based metals such as molybdenum and ferromolybdenum are used in parts that require corrosion resistance and wear resistance. Molybdenum-based metal thermal spray materials are used after being processed into wires and powders. Conventionally, powdered molybdenum-based thermal spray materials have been used by adjusting the particle size within a certain range by pulverizing and classifying the base material. However, molybdenum-based metals are brittle and are over-pulverized during the crushing process, resulting in a large amount of fine powder with a particle size below the target, resulting in extremely low product yields of less than 70%. In the case of expensive materials such as molybdenum, the generation of unnecessary particle size increases costs and must be avoided as much as possible. Furthermore, even if the particles are sharply classified into a certain particle size range, the size, shape, and apparent density of each particle will vary, resulting in inconsistent coating properties after thermal spraying.
It also has the disadvantage that the thermal spraying yield is not stable. In the thermal spraying process, the thermal spray material powder is ejected through a small nozzle, so unless a powder with stable properties is used, smooth operation will be difficult. Furthermore, if the sizes of individual particles differ, the amount of heat received will also differ, and the dissolution of the particles will not be uniform, resulting in the disadvantage that the resulting film will not be uniform. Furthermore, since molybdenum oxidizes in the air at room temperature, oxides are produced in the usual pulverization method, and the thermal spray coating made using such a thermal spray material has the disadvantage that it is not a uniform coating. The object of the present invention is to eliminate these drawbacks and to provide a molybdenum-based metal powder suitable for thermal spraying at a low cost in order to obtain a homogeneous thermal spray coating. The thermal spray material of the present invention is produced by crushing metallic molybdenum or low carbon ferromolybdenum into fine particles of 40 microns or less, preferably 20 microns or less, and adding a completely decomposable organic binder to the resulting fine particles.
Granulated to 150 microns and further sintered at a temperature of 700 to 1050°C in a non-oxidizing atmosphere to reduce the total oxygen content to 0.5%.
It can be obtained by doing the following. The molybdenum-based thermal spray powder obtained in this way has very little oxide and the particle shape and size are uniform, resulting in high thermal spraying efficiency, and the resulting thermal spray coating is dense and homogeneous, with strong adhesion. Become something. The thermal spray materials of the present invention are used for combustion gas spraying and plasma spraying. As the molybdenum-based metal, commercially available metal molybdenum or low carbon ferromolybdenum containing 60% or more molybdenum and 0.10% or less carbon can be used. Since high carbon ferromolybdenum contains carbide, it is unfavorable in terms of the lubricating properties and wear resistance of the film. The base material nodules are first pulverized to 40 microns or less. In order to improve granulation properties, it is desirable to pulverize to 20 microns or less. Then, the obtained fine particles are
Granulate to microns, preferably 20 to 105 microns. If it is less than 10 microns, it will scatter and will not contribute to film formation, worsening thermal spraying efficiency. Coarse particles of 150 microns or more melt slowly and are present in the film as unmelted particles, deteriorating the film properties. For granulation, we use fully thermally decomposable organic binders such as polyvinyl alcohol (PVA), polyethylene glycol, ethyl cellulose, carboxyl methyl cellulose (CMC), and corn starch, which completely decompose and volatilize at the sintering temperature, and can be rolled, flowed, and spray-dried. This can be done using the following methods. If the organic binder remains, it will form molybdenum carbide, so use one that completely dissipates in the temperature range of the firing process. For example, when using PVA in rolling granulation, 0.5
It is better to use a % aqueous solution. A solution with a high concentration of 1% or more has too high a viscosity and is difficult to granulate. In the case of spray granulation, PVA may be added so that the slurry viscosity becomes 120 to 150 centipoise. This makes it possible to obtain a powder having the desired particle size without causing a change in composition. In addition, if particles other than the target particle size are returned to the pulverization process, the target particle size can be achieved with good yield. Next, the secondary particles obtained by granulation are sintered in a non-oxidizing atmosphere for the purposes of decomposing and removing the binder, removing oxides generated in the pulverization and classification process, and ensuring particle strength through sintering. As the non-oxidizing atmosphere, a reducing atmosphere, preferably a hydrogen atmosphere, an inert atmosphere, or a vacuum atmosphere can be used. In the case of an inert atmosphere or a vacuum atmosphere, improvement in purity is achieved by sublimation of molybdenum oxide rather than reduction and removal of oxides. Processing temperature is 700℃~1050℃, preferably 850℃~
The temperature is 1000℃ and the processing time is 30 minutes or more. 700
At temperatures below .degree. C., both reduction and sublimation are insufficient, and the strength of the particles obtained is also insufficient, resulting in decomposition into fine powder during thermal spraying, resulting in a decrease in thermal spraying efficiency.
Further, since there is a concern that undecomposed organic binder may remain, temperatures below 700°C are not preferable. At temperatures above 1050°C, the particles will fuse together, making it impossible to obtain powder with the desired particle size. The metal powder obtained by the above method has particles close to spherical in shape and has extremely good fluidity. Also, the oxygen content will be 0.5% or less. Furthermore, when this powder is used for plasma spraying, the sprayed coating obtained is dense and has excellent adhesion, with a spraying efficiency of over 80%. Furthermore, the above method has a product yield of 90% or more and is extremely economical. Next, the present invention will be explained by showing examples. Example 1 Molybdenum with a purity of 99% or more was pulverized in a vertical mill to 20 microns or less, with an average particle diameter (D ₅₀ ) of 7 to 8 microns. To 100 parts of the pulverized material, 5 parts of polyvinyl alcohol was added as a binder, and 100 parts of water was mixed and stirred to form a slurry, which was then formed into micropellets using a spray dryer. The particles obtained at this time were approximately spherical in shape, and their size was approximately 20 to 105 microns, precisely 5% of which was less than 20 microns and 3% of which was greater than 105 microns, and the yield was 96% based on the amount of feed. Ta. Although this particle size distribution can be used as is as a thermal spray material, the obtained secondary particles were classified into a range of 30 to 105 microns just to be sure. Next, the secondary particles were heated at 950℃ in a hydrogen stream for 2 hours.
Baked for an hour. As a result of experiments, it was found that 700°C is sufficient for reducing the oxide and volatilizing the binder, but 950°C for 2 hours is optimal in order to give the secondary particles strength. Care must be taken as secondary particles begin to sinter when the temperature exceeds 1100°C. The fired secondary particles were further passed through a 105 micron sieve to obtain a powder material for thermal spraying. On the other hand, for comparison, a powder obtained by pulverizing metal molybdenum and classifying it into particles of 30 to 105 microns was used as a conventional product. The properties of these powders are shown in Table-1.

[Table] As shown in Table 1, the product yield of the material of the present invention is extremely high, and since the upper and lower parts of the sieve can be reused by returning them to slurry, the product yield substantially reaches 95% or more. The particles have a nearly spherical shape and are a powder with good fluidity.
Furthermore, the apparent density is low because fine primary particles are granulated into secondary particles. Moreover, the oxygen content can also be made extremely low. Next, these materials were plasma sprayed and the spraying properties and properties of the sprayed coating formed on the surface of the SS base material were investigated. The plasma spraying conditions were 35V and 800A.
The results are shown in Table-2.

[Table] (1) Thermal spraying efficiency is the adhesion rate to the base material.
(2) Film hardness was measured at 200g x 15 seconds using a micro-Vickers hardness meter.
Table 2 shows that when the powder material according to the present invention is used, thermal spraying efficiency is high and a dense film can be obtained. The reason why such a film is obtained is that it is a granulated product made by aggregating fine primary particles, and the particle shape is almost spherical and the size is uniform. This makes the powder flow uniform,
When focusing on individual single particles, the heat-receiving area is large, the dissolution rate is uniform, and the oxygen content is low, so a homogeneous film can be obtained. Example 2 A powder material of low carbon ferromolybdenum consisting of Mo: 63.2%, C: 0.01%, Fe: remainder was obtained in the same manner as in Example 1. For comparison, a powder obtained by pulverizing and classifying into 30 to 105 microns as a conventional product was used, and plasma spraying was performed under the same conditions as in Example 1.
The film properties were compared. Powder properties are shown in Table 3. In addition, the film characteristics are shown.
4.

【table】

[Table] As described above, according to the present invention, a thermal sprayed coating with high product yield and excellent coating properties can be efficiently obtained.

Claims (1)

[Scope of Claims] 1. A powder material for thermal spraying, characterized by comprising a sintered body of fine particles of metallic molybdenum or ferromolybdenum. 2 40% of metal molybdenum or ferromolybdenum
Finely pulverized to less than a micron size, one type of pyrolyzable organic binder selected from polyvinyl alcohol (PVA), polyethylene glycol, ethyl cellulose, carboxymethyl cellulose (CMC), and corn starch is added to this fine powder.
A method for producing a powder material for thermal spraying, which comprises granulating to ~150 microns and sintering the resulting granulation in a non-oxidizing atmosphere at a temperature range of 700°C to 1050°C.