JPH0139993B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for forming a ceramic coating on the surface of a base metal or non-metallic material by plasma spraying, and its purpose is to form a ceramic coating on the surface of some ceramic materials, which was previously considered impossible. It is an object of the present invention to provide a method for forming a ceramic coating that enables coating formation and has excellent heat resistance. According to literature, this type of thermal spraying equipment or thermal spraying method can be roughly divided into gas type using combustion energy and electric type using electric energy, depending on the type of heat source that heats the spray material, which is then sent to the thermal spraying process. It is said that thermal spraying can be roughly divided into powder spraying method and solid spraying method (ROKIDE method) depending on the physical state of the spray material. The latter classification is mainly used to subdivide the former gas type, and the solid material used in the gas type means a rod-shaped material or a wire-shaped material. On the other hand, electric type plasma spraying machines, especially those used industrially, fall into the category of powder spraying methods. According to the above-mentioned classification method, this method is an invention within the scope of an external powder feeding plasma spray method based on a powder atomization method. In general, plasma spraying is widely known as a coating formation method for imparting wear resistance, heat resistance, corrosion resistance, etc. to the surface of metal materials. This plasma spraying method usually uses the apparatus shown in the drawings.
That is, a negative electrode 2 made of a tungsten electrode provided on a plasma gun 1 and a positive electrode 3 made of a water-cooled copper electrode.
are connected to a control device 4, and a plasma arc is generated between the two poles 2 and 3 by a DC power supply 5.
Then, gases from the gas cylinders 6 and 7 of primary gas and secondary gas are supplied between the negative electrode 2 and the positive electrode 3 of the plasma gun 1 via the control device 4,
Due to the pressure of the plasma gas, the plasma arc becomes a plasma jet 8 and is ejected from the plasma gun 1. By supplying thermal spray powder from a powder supply device 9 into the plasma jet 8, thermal spraying is performed on the surface of the surface-polished base material 10 to form a coating. In addition,
In the figure, 11 is a water supply source for cooling the positive electrode 3,
It is supplied as appropriate via the control device 4. Plasma jet generated by such equipment8
The temperature is usually 1 to 3 x 10 ⁴ 〓, which is a temperature that can melt all substances, but from the perspective of thermal spray powder, coating formation (powder adhesion) by thermal spraying is performed smoothly. The conditions are considered as follows (quote: "Ultrahigh Temperature Research" Vol. 3, No. 4, 1966, P21-P33). Thermal spray powder must melt completely and uniformly. Give sufficient kinetic energy to the droplet. The molten droplets collide with the surface of the base material to ensure close contact or welding. The droplets solidify while remaining in close contact with the surface of the base material, and the droplets that adhere one after another closely bond with each other to form a film. There shall be no qualitative change in the sprayed material or the base material to be sprayed. These conditions must be satisfied at the same time during thermal spraying, and must be maintained continuously and stably; furthermore, these conditions are not independent and interfere with each other. In particular, and are related to the properties of plasma as a heat source, and are matters that are evaluated based on the requirements of the base material, that is, the needs. Here, an appropriate heating time is required to completely melt each powder particle, and the heating time is controlled by the melting point, specific heat, thermal conductivity, shape of the particles, etc. of the thermal spray material. In addition, the powder is heated and accelerated by the plasma flow and transported to the base material, and the acceleration that the particles receive varies depending on the shape and specific gravity of the particles, their relative velocity with the plasma flow, and the density of the plasma. . When thermal spraying ceramics, a plasma jet using N ₂ gas and H ₂ gas is usually used in order to form a film that satisfies the above-mentioned factors. This plasma jet using N ₂ - _{H 2} gas is due to the fact that ceramic materials generally have high specific heat and low thermal conductivity, and is selected mainly for the following reasons. That is, a low velocity jet flame can be created and the particles can be heated for a long time. Since it is a diatomic gas, it can take a large amount of heat. By using such a plasma jet using N ₂ - H ₂ gas,
Vol. 8, No. 8, 1973. P33-P38) can be thermally sprayed on ceramics.

[Table] By the way, this does not mean that plasma spraying of ceramics under the conditions using N ₂ - H ₂ gas as described above is possible for all ceramics. In other words, SiO ₂ , MgO, CaO, BN, which are important components for refractory materials, especially refractories for steel,
Regarding Si ₃ N ₄ and SiC, no examples are found in Table 1 or other documents as ceramic materials for thermal spraying. In addition, as is clear from the example of cermets in Table 1, some carbides contain Co,
Since a self-fusing alloy consisting of Ni and a combination thereof is added and a film is formed by the adhesion of the molten particles of this additive, it has heat resistance only at temperatures far lower than the heat resistance temperature of ceramics. The present invention eliminates the conventional drawback that the N ₂ - H ₂ plasma jet has a radial frame tip, making it difficult to spray _{ceramics .} This method uses Ar- _N2 gas plasma instead of gas plasma, and the details will be explained below. In the present invention, Ar-N ₂ gas is used, but among the gas cylinders 6 and 7 in the drawing, cylinder 6 contains Ar gas as the primary gas, and cylinder 7 as the secondary gas.
Using _N2 gas, its mixed gas flow rate ratio (Ar/ _N2 )
The plasma jet 8 is generated by adjusting the ratio to be in the range of 100/0 to 70/30. in this way,
By increasing the primary gas Ar content to 70% or more, a high-temperature, high-velocity, sharp plasma jet can be produced.
Since the spraying focal point becomes small (i.e., the tip shape becomes conical), the surface of the base material 10 is partially heated to a red-hot softening or melting state [i.e., the load softening point of the base material (T ₁ ) (“JIS P2209
(1974)) and at a temperature below the melting point temperature], it is possible to prevent excessive heat from being applied to the base material 10. As for N ₂ , which is a secondary gas, if the mixing ratio is 30% or more, the shape of the frame will collapse, and thermal energy due to excessive dissociation of N ₂ gas will be given to the base material 10. Make sure to keep it below 30% to avoid melting more than necessary. In this case, due to the high speed jet frame,
Although the ceramic particles cannot be completely molten in the jet frame, it is possible to adhere the ceramic particles to the surface of the base material by heating or semi-molten the ceramic particles in the jet frame. In other words, ceramic particles are heated by a jet flame or in a semi-molten state, and the kinetic energy of the particles accelerated at high speed is converted after particle collision, and the thermal energy is converted from the irradiation energy by the high-temperature jet flame. Due to thermal energy, the ceramic particles become semi-molten or molten, and the surface of the base material 10 becomes red-hot softened or molten (i.e., at a temperature that is above the softening point under load (T ₁ ) and below the melting point of the base material). By being in a heated state), it can adhere to the surface of the base material 10. In particular, if the base material 10 is made of the same ceramic material, it will also have a sintering effect to adhere and solidify. The ceramic materials mentioned here include not only conventional ceramic thermal spray materials, but also SiO ₂ , MgO, CaO, BN, SiC,
It also contains Si ₃ N ₄ and Ti (C, N), and examples of using these ceramic materials as thermal spraying materials are shown in Table 2 below. Moreover, the base material here refers to materials other than polymeric materials, and refers to nonmetallic materials, particularly firebricks. The surface of the base material is thermally sprayed while being heated at a temperature above the load softening point (T ₁ ) and below the melting point of the base material in order to promote adhesion and solidification of the sprayed material.
In particular, for base materials such as silica bricks that have a large coefficient of thermal expansion, materials that have a transition point, and materials that have high thermal conductivity, it is highly effective to separately preheat to 500° C. or higher to alleviate thermal strain. Furthermore, for materials such as carbon-based materials that are susceptible to surface oxidation, it is a matter of course that the treatment be carried out in a vacuum or in an inert gas atmosphere. Furthermore, when heating a base material that dissociates and sublimates below its melting point, such as BN, it must be heated at a set temperature below its dissociation point and sublimation point. At this time, for example, some of the BN particles may change to B ₂ O ₃ and adhere, but the amount of change is only 10% or less, and there is almost no major change in the properties of the sprayed film. In the case of preheating using a jet frame, thermal effects on the base material can be sufficiently avoided by supplying ceramic powder while keeping the tip of the frame in contact with the surface of the base material. Next, Table 2 shows specific examples in which films were formed using SiO ₂ , MgO, and CaO as ceramic spray materials. As is clear from the results of this example, the adhesion strength is superior to that of the alumina sprayed film. Note that a thermal spraying device manufactured by Metco, Inc. in the United States was used as the device. The specifications of the equipment used are plasma generator: Metco-7MBH (80Kw type), maximum allowable output: 80Kw, maximum allowable current: 1000A, usage rate: 100% (at 80Kw), gas used: Ar or
N ₂ (primary gas), H ₂ or He (secondary gas), thermal spray material supply: External powder feeding method. In the example, the output current is 600~1000A and 35Kw ~
The plasma was operated in the range of 60Kw. As mentioned above, the conditions under which this device can output are the maximum output current.
The withstand voltage is 80V at 1000A. Therefore, if the amount of _N2 gas is increased, the operating current will be lowered to 600A.
Due to constraints on electrode cooling capacity, the mixed gas flow rate was adjusted to keep the operating power below 60Kw.

[Table] As explained above, the present invention uses a plasma jet generated at a mixing ratio of Ar and N ₂ gases (Ar/N ₂ ) in the range of 100/0 to 70/30. The surface of the base material is heated in the frame, and ceramic particles heated or semi-molten in the frame are sprayed onto the surface of the base material to form a ceramic coating on the surface of the base material.
The surface of the base material is partially heated with high temperature using _N2 gas to make it red-hot, softened or melted, and ceramic particles are blown at high speed and the particles collide, converting kinetic energy into thermal energy. It is possible to reliably adhere to the base material without completely melting it in the flame, there is no need to add low melting point substances for adhesion, and therefore it has the same heat resistance as the material to be thermally sprayed. A film with properties can be formed. Furthermore,
This is a very effective invention as it enables the formation of coatings on some ceramic materials, which was previously considered impossible.

[Brief explanation of drawings]

The drawing is a schematic illustration of a plasma spraying apparatus in which the method of the present invention is used. 1 is a plasma gun, 6 and 7 are gas cylinders, 8 is a plasma jet, and 10 is a base material.

Claims (1)

[Claims] 1. In the powder spray plasma spray method using electrical energy, a plasma spray gun equipped with a mechanism for supplying the material to be sprayed by an external powder feeding method is used to mix Ar- _N2 gas at its mixing ratio (Ar / _N2 )
The plasma jet has a value in the range of 100/0 to 70/30, and the frame shape is a sharp cone at the tip. While heating the surface of the metal base material at a temperature above the load softening point (T ₁ ) and below the melting point of the base material, the powder ceramic particles to be supplied to the gun are heated in the frame to a molten or semi-molten state. 1. A method for forming a ceramic film, which comprises spraying the powder onto a base material to form a ceramic film on the surface of the base material.