JPH11171669A - Production of boron carbide film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for the production of a baron carbide film having extremely dense texture and free from peeling trouble from the substrate and the crack generation on the boron carbide film formed on the substrate. SOLUTION: A boron carbide coating film free from boron oxide is formed on the surface of a substrate in a non-oxidizing atmosphere by using a plasma spray coating technique. The formed boron carbide film is heat-treated in a non-oxidizing atmosphere at 1100-2400 deg.C to effect the rebonding of boron and carbon generated by the decomposition of boron carbide to reproduce boron carbide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a boron carbide film, and more particularly, it is effectively used as a wear-resistant material, a sliding material, a heat-resistant tile such as a rocket, and a control material or a shielding material for a nuclear reactor. And a method for producing a boron carbide film.

[0002]

2. Description of the Related Art Boron carbide has a high melting point and thus is excellent in heat resistance, and is chemically stable and therefore has excellent oxidation resistance. Further, it has excellent mechanical strength such as abrasion resistance, strength, and hardness, and also has neutron absorption. For this reason, materials coated with boron carbide have heretofore been used as wear-resistant materials and sliding materials such as sandblast nozzles and linings of crushers, heat-resistant tiles of rockets, control materials and shielding materials of nuclear reactors, and the like.

[0003] The coating of boron carbide is generally performed by a CVD method, a plasma spraying method or the like. However, when a boron carbide film is formed on a substrate by using the above-described plasma spraying method or the like, there has been a problem that cracks occur in the film or the film is peeled off.

In view of such a problem, Japanese Patent Application Laid-Open No. 5-33907
In Japanese Patent No. 9, after converting the surface of a carbon material as a base material into boron carbide, after arranging boron carbide powder on the formed conversion layer, or after forming a layer of boron carbide by a CVD method or the like, A method for forming a dense film by hot pressing is disclosed.

In Japanese Patent Application Laid-Open No. Hei 7-33567,
The particle size is 0.01 to 1 μm on the surface of the carbon material as the base material.
Agglomerate the fine particles of nitride or carbide of
A secondary pseudo particle of 50 μm is manufactured, and the secondary pseudo particle is coated on the surface of the substrate by a plasma spraying method, and a metal, an alloy, and an oxide or non-oxide ceramic are further formed thereon. A method is disclosed in which at least one layer is spray-coated and laminated.

[0006]

However, even with the above method, cracks in the boron carbide film coated on the substrate and peeling from the substrate cannot be sufficiently prevented. In, the tendency was remarkable. For this reason, characteristics such as heat resistance and wear resistance inherent to boron carbide cannot be sufficiently enjoyed.

[0007] The present invention provides a method for producing a boron carbide film in which cracks do not occur in the boron carbide film formed on the base material and peeling from the base material does not occur.

[0008]

The present inventors have found that when a boron carbide (B ₄ C) film is formed on the surface of a substrate by a plasma spraying method or the like, the cause of cracks in the boron carbide film, and The cause of the peeling of the boron carbide film from the substrate was examined in detail. As a result, it was found that boron oxide (B ₂ O ₃ ) mixed in the film was involved in the generation of these cracks.

That is, for example, when boron carbide is formed on the surface of a substrate by a plasma spraying method, the surface of a molten droplet is oxidized by oxygen in a spraying atmosphere, and boron oxide is mixed into the film. Since this boron oxide has a hygroscopic property, when it is left in the air, it reacts with water present in the air and expands. As a result, the boron carbide film collapses to cause cracks or peel off from the substrate.

[0010] The present invention has been made based on the findings of these detailed studies. That is, the present invention provides a method for forming boron carbide on a surface of a substrate by plasma spraying in a non-oxidizing atmosphere, and then forming the boron carbide in a non-oxidizing atmosphere.
A method for producing a boron carbide film, wherein the heat treatment is performed at a temperature of 00 to 2400 ° C.

Conventionally, when a boron carbide film is formed by a plasma spraying method, the boron carbide powder melts at the time of thermal spraying and partially decomposes into boron (B) and carbon (C) as shown in the following formula (1). I do. B ₄ C → 4B + C (1)

Since boron at the time of thermal spraying is at a high temperature, it reacts immediately with oxygen in the atmosphere to form boron oxide as shown by the following equation (2). 4B + 3O ₂ → 2B ₂ O ₃ (2)

On the other hand, there are carbon atoms that remain as they are, and carbon atoms that react with oxygen to form carbon monoxide (CO) and scatter to the outside during thermal spraying, as shown by the following equation (3). 2C + O ₂ → 2CO (3) Accordingly, as shown in FIG. 1, a (boron oxide + carbon) layer b composed of boron oxide and residual carbon is formed on the surface of the dissolved boron carbide droplet a.

As shown in FIG. 3, when such a droplet is hit on the substrate d and cooled, the boron carbide sprayed film c has a laminated structure because it contains a large amount of boron oxide.

On the other hand, in the method for producing a boron carbide film of the present invention, the boron carbide film is formed by plasma spraying in a non-oxidizing atmosphere, so that the decomposed boron reacts with oxygen as described above. Does not produce boron oxide. Accordingly, in this case, as shown in FIG. 2, a (boron + carbon) layer f composed of decomposed and generated boron and carbon is formed on the surface of the boron carbide droplet e.

Further, according to the present invention, the boron carbide film formed by plasma spraying is formed in a non-oxidizing atmosphere.
Heat treatment is performed at a temperature of 1100 to 2400 ° C. As a result, as shown in the following equation (4), the decomposed boron and carbon are recombined to form boron carbide. 4B + C → B ₄ C (4)

On the other hand, when the boron carbide film formed by the conventional method for manufacturing a boron carbide film is subjected to a heat treatment as in the present invention, as shown in the following formula (5), the carbon in which boron oxide remains is removed. Reacts to produce boron carbide and carbon monoxide. This carbon monoxide is a gas and forms pores in the boron carbide film and remains, or these pores are connected to form open pores and scatter outside. 2B ₂ O ₃ + 7C → B ₄ C + 6CO (5)

After the boron carbide film is formed according to the method of the present invention, the boron carbide film is subjected to a heat treatment in a non-oxidizing atmosphere to produce only boron carbide without generating carbon monoxide. can do. Therefore, according to the method of the present invention, an extremely dense boron carbide film can be produced. That is, by using the method for producing a boron carbide film of the present invention, an extremely dense boron carbide film containing no boron oxide in the film can be produced. Therefore, it is possible to prevent the boron carbide film from expanding, to prevent the occurrence of cracks in the boron carbide film, which has been conventionally regarded as a problem, and to provide a boron carbide film having excellent heat resistance and oxidation resistance. Obtainable.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for producing a boron carbide film according to the present invention will be described in detail based on embodiments of the present invention.

The substrate that can be used in the present invention must have thermal shock resistance in order to prevent damage due to plasma spraying. In addition, at the time of temperature decrease after performing the heat treatment of the present invention, it is necessary to have a thermal expansion coefficient equal to or higher than that of boron carbide in order to prevent the occurrence of cracks in boron carbide. .

The lower limit of the thermal expansion coefficient of the substrate is from room temperature to 100
At 0 ° C., it is preferably 4.5 × 10 ⁻⁶ / deg, more preferably 5.5 × 10 ⁻⁶ / deg. The thermal expansion coefficient of the substrate is 4.5 × 10 ⁻⁶ / deg
If the diameter is smaller than the temperature, the tensile stress from the base material acts on the boron carbide film at the time of the temperature decrease after the heat treatment described later is performed, and the boron carbide film is easily cracked and easily peeled from the base material. Is not preferred.

The upper limit of the coefficient of thermal expansion of the substrate is from room temperature to
At 1000 ° C., it is preferably 7 × 10 ⁻⁶ / deg, and more preferably 6 × 10 ⁻⁶ / deg. If the coefficient of thermal expansion of the base material is larger than 7 × 10 ⁻⁶ / deg, the compressive stress from the base material acts on the boron carbide film at the time of the temperature decrease after the heat treatment, thereby causing the peeling from the base material Is likely to occur, which is not preferable. Examples of the base material satisfying such conditions include carbon, recrystallized silicon carbide (SiC), silicon (Si) -silicon carbide (SiC) composite, mullite, and aluminum nitride. From the viewpoint of properties, it is preferable to use carbon, recrystallized silicon carbide, and a silicon-silicon carbide composite.

The boron carbide coating of the present invention is formed in a non-oxidizing atmosphere using a plasma spraying apparatus as shown in FIG. Specifically, first, the dust filter 1
After the inside of the vacuum chamber 8 is evacuated with the vacuum pump 7 through the atmosphere control system 6, nitrogen (N ₂ ), helium
A non-oxidizing gas such as (He) and argon (Ar) is introduced into the vacuum chamber 8 for filling. However, the plasma spraying described below can also be performed without introducing these non-oxidizing gases and exhausting the inside of the vacuum chamber 8 with the vacuum pump 7.

Next, the thermal spray gun 12 is
In addition, a powdery boron carbide material as a raw material is supplied, and power and gas are supplied from a power supply 9 and a gas supply system 13 to the thermal spray gun 12 to generate plasma.

The boron carbide powder injected into the plasma is instantaneously melted and fired from the thermal spray gun 12 toward the substrate 4 installed on the substrate rotating device 3 to carbonize the surface of the substrate 4. Coat with boron. Regarding the portion of the substrate 4 to be covered,
By rotating the substrate rotating device 3 or operating the robot 5 attached to the thermal spray gun 12, the selection can be made as appropriate.

During the plasma spraying, the vacuum chamber 8
Vacuum pump 7 to maintain the degree of vacuum inside the
The non-oxidizing gas is supplied while the inside of the vacuum chamber 8 is evacuated. Further, in order to prevent the vacuum chamber 8 from being filled with boron carbide powder, a draft 2 is provided inside the vacuum chamber 8. The lower limit of the thickness of the boron carbide film thus formed is preferably 20 μm, and more preferably 50 μm. If the thickness of the boron carbide film is smaller than 20 μm, it is not preferable because characteristics such as heat resistance and oxidation resistance inherent to boron carbide cannot be imparted to the substrate.

On the other hand, the upper limit of the thickness of the boron carbide film is preferably 500 μm, more preferably 100 μm.
It is. Even if the thickness of the boron carbide film is larger than 500 μm, the inherent properties of boron carbide are saturated, and in addition to the result of simply causing a loss of the raw material, the mismatch of the thermal expansion coefficient with the base material causes It is not preferable because cracks easily occur in the boron carbide film.

In the present invention, since the boron carbide film is formed by plasma spraying in a non-oxidizing atmosphere, boron oxide does not enter the boron carbide film.

In the present invention, the boron carbide film formed by the above-described plasma spraying method is subjected to a heat treatment in a non-oxidizing atmosphere. The lower limit of the temperature of the heat treatment needs to be 1100 ° C., and preferably 1300 ° C. If the lower limit of the heat treatment temperature is lower than 1100 ° C., the decomposed boron and carbon do not react to form boron carbide, and have abrasion resistance.
The excellent properties inherent to boron carbide, such as hardness, decrease.
The upper limit of the temperature of the heat treatment needs to be 2400 ° C., preferably 2000 ° C., and more preferably 1600 ° C. If the heat treatment temperature is higher than 2400 ° C., the substrate is damaged, and as a result, the boron carbide film formed thereon is also undesirably damaged.

The heat treatment in the method for producing a boron carbide film of the present invention is specifically performed as follows. At first,
A substrate on which a boron carbide film is formed by plasma spraying is placed inside a device such as a high-temperature atmosphere furnace. Next, the inside of the apparatus is evacuated with a vacuum pump, and then filled with a non-oxidizing gas such as nitrogen, helium, and argon. Subsequently, after the temperature is raised to the predetermined temperature of the present invention at a temperature rising rate of 3 to 15 deg / min, the temperature is maintained at this temperature for 1 to 10 hours. Thereafter, the inside of the apparatus is cooled down to 200 ° C. or lower, and the substrate on which the boron carbide film is formed is taken out.

As described above, in addition to performing boron carbide film formation by plasma spraying and heat treatment in a non-oxidizing atmosphere in separate batches, an apparatus provided with plasma spraying and heat treatment functions is used. , Can also be done inline.

FIG. 5 is an X-ray diffraction peak of a boron carbide film produced by a conventional boron carbide film production method.
FIG. 6 is an X-ray diffraction peak of the boron carbide film produced by the method for producing a boron carbide film of the present invention. In FIG.
Boron carbide such as 2θ = 35.0 and 37.8 degrees (B
₄ C), and 2θ = 26.4 and 44.
In addition to the peak from carbon (C) which is a base material such as 4 degrees,
2θ = 14.6 and 27.8 degrees boron oxide (B ₂ O ₃ )
From the peak.

On the other hand, in FIG. 6, 2θ = 35.0 and 3θ
Only peaks from boron carbide (B ₄ C), such as 7.8 degrees, and peaks from the base carbon (C), such as 2θ = 26.4 and 44.4 degrees, are seen, indicating that the method of the present invention. In the case where a boron carbide film was produced by using the method, it was found that boron oxide was not mixed in the boron carbide film.

FIG. 7 is an electron micrograph of a fracture surface of the boron carbide film formed by the conventional method for producing a boron carbide film. FIG. 8 is an electron micrograph of a fracture surface of the boron carbide film formed by the method for producing a boron carbide film of the present invention. As is clear from FIG. 7, the boron carbide film formed by the conventional method of manufacturing a boron carbide film has a droplet of boron carbide covered with participating boron, which is struck on a substrate, and has a scaly or small shape. It can be seen that they are stacked.

On the other hand, as is apparent from FIG. 8, the boron oxide film formed by the method for producing a boron carbide film of the present invention is made of very dense boron carbide with almost no generation of pores. Recognize.

[0036]

The present invention will be described in more detail with reference to examples. Examples 1 to 3 and Comparative Examples 1 and 2 each have a thermal expansion coefficient of 6 × 10 ⁻⁶ / deg,
A carbon substrate having a diameter of 50 mm and a thickness of 5 mm was used. Using a plasma spraying apparatus (manufactured by Plasma-Technic) having the configuration shown in FIG. 4, boron carbide having a thickness of 200 μm was formed on this substrate in an argon gas atmosphere. Thereafter, the substrate on which the boron carbide film was formed was placed in a high-temperature atmosphere furnace, evacuated, and replaced with argon gas. Then, Table 1
And heated at a rate of 3 deg / min.
The heat treatment was performed while holding for a time. After the heat treatment was completed, the substrate was cooled to 200 ° C. or lower in the apparatus, and the substrate was taken out. The presence or absence of boron oxide by X-ray diffraction, the occurrence of cracks by a humidification test, and the airtightness of boron carbide by a helium leak detector were examined. Table 1 shows the results.

COMPARATIVE EXAMPLE 3 The heat treatment after the formation of the boron carbide film was carried out by using a mixed oxidizing gas containing 5% by volume of oxygen and 95% by volume of argon.
Except that the test was performed at 0 ° C., the test was performed in the same manner as in the above Examples and Comparative Examples. The presence / absence of boron oxide, generation of cracks, and airtightness were examined in the same manner as in the above Examples and Comparative Examples. Table 1 shows the results.

Comparative Example 4 The procedure of Example 2 was repeated, except that the boron carbide film was formed by atmospheric plasma spraying. The presence / absence of boron oxide, generation of cracks, and airtightness were examined in the same manner as in the above Examples and Comparative Examples. Table 1 shows the results.

[0039]

[Table 1]

As is clear from Table 1, when the boron carbide film was formed according to the method for producing a boron carbide film of the present invention, no boron oxide was present in the boron carbide film, and a single boron carbide film was formed. It turns out that it is comprised. Further, the helium leak amount is extremely low, and it can be seen that the boron carbide film formed by the method of the present invention has very few pores and is dense.

On the other hand, as is apparent from Comparative Examples 1 and 2 in Table 1, when the heat treatment is not performed within the temperature range of the present invention after forming the boron carbide film, boron and carbon remain inside the boron carbide film. You can see that it is doing.

In Comparative Example 3, boron oxide was present in the boron carbide film. Therefore, unlike the present invention, when heat treatment is performed in an oxidizing atmosphere, decomposed boron and carbon are recombined to form boron carbide, and decomposed boron reacts with oxygen to form boron oxide. It turns out that it forms. Furthermore, it can be seen that in Comparative Example 3, the helium leak amount was increased as compared with the Example. That is, as described above, it is understood that the carbon remaining after being decomposed reacts with oxygen to generate carbon monoxide, and this carbon monoxide contributes to the formation of pores.

In Comparative Example 4, although the boron carbide film was formed of a single boron carbide, the amount of helium leak was larger than that of the Example. This is because a boron carbide film was formed in an oxidizing atmosphere, so that boron oxide was contained in the boron carbide film, and in a subsequent heat treatment, carbon monoxide was generated by a reaction represented by the above formula (5). , To contribute to the formation of pores.

In Comparative Example 3, cracks were generated in the boron carbide film after the humidification test because boron oxide was contained in the boron carbide film. In other examples, the boron oxide film did not contain boron oxide. It can be seen that no cracks occurred in the boron carbide film.

[0045]

As described above, by using the method for producing a boron carbide film of the present invention, it is possible to obtain a boron carbide film containing no boron oxide, which has caused cracks and peeled off from the substrate. Can be manufactured.

Further, in the present invention, at the stage of forming the boron carbide film by using the plasma spraying method, the incorporation of boron oxide into this film is prevented. Therefore, even when the heat treatment is performed thereafter, the boron oxide does not react with the remaining carbon to generate carbon monoxide, and an extremely dense boron carbide film with few pores can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram showing a state of a plasma sprayed droplet formed when a conventional method is used.

FIG. 2 is a view showing a state of a plasma-sprayed droplet formed by using the method of the present invention.

FIG. 3 is a diagram showing a state of a boron carbide film when a boron carbide film is manufactured by a conventional method.

FIG. 4 is a schematic view of a plasma spraying apparatus for performing the method for producing a boron carbide film of the present invention.

FIG. 5 is an X-ray diffraction peak of a boron carbide film produced by a conventional boron carbide film production method.

FIG. 6 is an X-ray diffraction peak of a boron carbide film produced by the method for producing a boron carbide film of the present invention.

FIG. 7 is an electron micrograph of a fracture surface of a boron carbide film formed by a conventional method for producing a boron carbide film.

FIG. 8 is an electron micrograph of a fracture surface of the boron carbide film formed by the method for producing a boron carbide film of the present invention.

[Explanation of symbols]

a Droplet of boron carbide, b (boron oxide + carbon) layer, c
Sprayed film of boron carbide, d substrate, e droplet of boron carbide, f
(Boron + carbon) layer, 1 dust filter, 2 draft, 3 substrate rotating device, 4 substrate, 5 robot, 6
Atmosphere control system, 7 Vacuum pump, 8 Vacuum chamber, 9
Power supply, 10 raw material supply system, 11 plasma frame,
12 Thermal spray gun, 13 Gas supply system

Claims (4)

[Claims] 1. Boron carbide formed on a surface of a substrate by plasma spraying in a non-oxidizing atmosphere, and then heat-treated at a temperature of 1100 to 2400 ° C. in the non-oxidizing atmosphere. Manufacturing method of the film. 2. The thermal expansion coefficient of the substrate is 4.5 to 7 × 1.
Method for manufacturing a boron carbide coating according to claim 1, characterized in that the 0 ^-6 / deg. 3. The method according to claim 1, wherein the base material is carbon, recrystallized silicon carbide,
The method for producing a boron carbide film according to claim 2, comprising at least one selected from the group consisting of silicon and a silicon-silicon carbide composite. 4. The temperature of the heat treatment is from 1300 to 1600.
The method for producing a boron carbide film according to any one of claims 1 to 3, wherein the temperature is ℃.