US20130101820A1

US20130101820A1 - Thermal spray coated member and thermal spraying method therefor

Info

Publication number: US20130101820A1
Application number: US13/697,829
Authority: US
Inventors: Nobuo Yonekura
Original assignee: Individual
Current assignee: Nippon Steel Hardfacing Corp
Priority date: 2010-05-24
Filing date: 2010-05-24
Publication date: 2013-04-25
Also published as: JPWO2011148515A1; KR20130113941A; MX2012013660A; WO2011148515A1; BR112012029727A2; CN102906298B; CN102906298A

Abstract

It is an object to extend the service life of a thermal-sprayed coating thermal-sprayed onto a molten metal resistant member and to prevent adhesion of a molten metal.

In the molten metal resistant member, a contact portion coming into contact with a molten metal including Zn and/or Al has been covered with the thermal-sprayed coating. The thermal-sprayed coating is formed by using thermal-spraying oxide-based ceramic particles having an average particle diameter as a median diameter of 10 μm or smaller. The thermal-spraying particles are thermal-sprayed at a high flying particle velocity of 1,000 m/sec or higher with only the surfaces of the flying thermal-spraying particles being in a semi-molten state and the inside of the particles being in a solid state. The resistance to corrosion by a molten metal, insulating properties, the resistance to washing with acids, and the ability to prevent adhesion of the molten metal are thereby improved.

Description

TECHNICAL FIELD

The present invention relates to a thermal spray coated member produced by thermally spraying thermal-spraying particles thereonto and to a method for producing the thermal spray coated member.

BACKGROUND ART

In one known method for forming a coating on the surface of a steel plate, the steel plate is immersed in a bath containing a molten metal such as zinc, aluminum, or a zinc-aluminum alloy. Conveying rollers for conveying the steel plate are provided in the bath. However, the molten metal may permeate and erode the conveying rollers. One known measure to prevent the permeation and erosion is to coat the surfaces of the conveying rollers with a protective coating.
One known method for forming such a protective coating is a high velocity gas spraying method. Patent Literatures 1 and 2 disclose methods in which a WC—Co-based or WC—B—Co-based cermet material is thermal-sprayed using the high velocity gas spraying method. Patent Literature 3 discloses a method in which a thermal-sprayed layer formed by plasma-spraying a ceramic such as chromia onto the coating is subjected to pore sealing. Patent Literature 4 discloses a method in which chromium carbide is formed on the surface of a thermal-sprayed layer and in pores therein to seal the pores. Patent Literature 5 discloses a method in which a composite ceramic composed of SiO₂—Cr₂O₃—Al₂O₃is thermal-sprayed onto the surface of a substrate using a plasma gun or a gas spray gun to seal pores with chromium oxide. Patent Literature 6 discloses a method in which, in order to prevent corrosion in molten zinc caused by local cells formed from WC in a thermal-sprayed cermet coating and Co therein serving as a binder, the components of the binder are controlled so that the difference in immersion potential is 80 mV or lower.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No. Sho. 48-11237
Patent Literature 2: Japanese Patent No. 2553937
Patent Literature 3: Japanese Patent Application Laid-Open No. Hei. 05-209259
Patent Literature 4: Japanese Patent Application Laid-Open No. Hei. 08-109458
Patent Literature 5: Japanese Patent Application Laid-Open No. 2002-4016
Patent Literature 6: Japanese Patent Application Laid-Open No. 2009-19271

SUMMARY OF INVENTION

Technical Problem

However, with thermal spraying methods and pore sealing treatment described in Patent Literatures 3 to 5, the bonding strength between the ceramic particles is weak, and the porosity of the thermal-sprayed ceramic layers is high, so that their structures are brittle. In addition, since the porosity is high, the insulating properties of the thermal-sprayed ceramic layers are low, so that a corrosion potential cannot be prevented.
Even with the method described in Patent Literature 6, the corrosion potential cannot be reduced to zero, so that the progress of dissolution loss cannot be prevented.
Accordingly, it is an object of the present invention to provide a thermal spray coated member including a thermal-sprayed oxide-based ceramic coating that includes thermal-spraying particles bonded with high bonding strength, is dense, resists deterioration, and has high insulating properties.

Solution to Problem

To achieve the above object, the thermal spray coated member according to the present invention includes a thermal-sprayed oxide-based ceramic coating formed using thermal-spraying oxide-based ceramic particles having an average particle diameter as a median diameter of 10 μm or smaller. The thermal-sprayed oxide-based ceramic coating is dense, resists deterioration, and has high insulating properties. The above average particle diameter is a median diameter measured by a laser diffraction scattering measurement method.

Advantageous Effects of Invention

The present invention can provide a thermal spray coated member including a thermal-sprayed oxide-based ceramic coating that includes thermal-spraying particles bonded with high bonding strength, is dense, resists deterioration, and has high insulating properties.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an optical microscope photograph of a cross section of a conventional high-velocity-gas-sprayed WC—B—Co-based cermet coating that has been immersed in a molten metal including 55% by weight of aluminum and 45% by weight of zinc at 873K for 16 days.

FIG. 2 is an optical microscope photograph of a coating in Patent Literature 3 formed by thermal-spraying an oxide-based ceramic (chromia) onto a high-velocity-gas-sprayed WC—B—Co-based cermet coating and then subjecting the resultant coating to pore sealing.

FIG. 3 is an optical microscope photograph of a cross section of a test piece immersed in a molten metal including 55% by weight of aluminum and 45% by weight of zinc at 873K for 16 days, the test piece including: a thermal-sprayed cermet coating thermal-sprayed using a high velocity gas spraying apparatus; and a coating with unsealed pores that has been formed thereon by thermal-spraying 6 μm fine thermal-spraying gray alumina particles to 50 μm using a high velocity gas spraying apparatus in a manner according to the present invention.

FIG. 4 is a cross-sectional view of a high velocity gas spraying apparatus.

FIG. 5 is a cross-sectional view of a high velocity gas spraying apparatus in Japanese Patent Application Laid-Open No. 2009-179846.

FIG. 6 is a schematic diagram of a blast abrasion tester.

DESCRIPTION OF EMBODIMENTS

Thermal spray coated members of the present invention include various members that require to have a thermal-sprayed oxide-based ceramic coating that includes thermal-spraying particles bonded with high bonding strength, is dense, resists deterioration, and has high insulating properties. Examples of such members include members that come into contact with a molten metal, conveying rollers that convey high-temperature glass with a temperature of 473K or higher, and hearth rolls disposed in a heat treatment furnace for steel plates.
Examples of the members that come into contact with a molten metal include: conveying rollers disposed inside a storage container storing a molten metal for plating steel plates to convey the steel plates; conveying rollers disposed outside the storage container to convey steel plates with the molten metal adhering thereto; molds into which a molten metal is poured; dippers used for the molds; and feed pumps for feeding a molten metal.
Embodiments of the present invention will next be described in detail, and a member that comes into contact with a molten metal is used as an example.
As described in Patent Literatures 4 and 6, examples of the properties required for a thermal-sprayed coating thermal-sprayed onto a molten metal resistant member such as a conveying roller used for molten zinc plating include: (1) erosion by molten zinc is less likely to occur; (2) the thermal-sprayed coating resists abrasion even when it comes into contact with a strip (steel plate); (3) adhering molten zinc can be easily removed, and maintenance is easy; (4) the thermal-sprayed coating used as a plating member has long life and is low cost; (5) the thermal-sprayed coating can easily resist thermal shock when it is immersed in high-temperature molten zinc; and (6) the corrosion potentials of the components of a thermal-sprayed cermet coating in molten zinc are low.
However, in the thermal-sprayed coatings in Patent Literatures 1 and 2 produced by thermal-spraying a WC—Co-based or WC—B—Co-based cermet material by the high velocity gas spraying method, the Co binder is eroded by molten zinc or aluminum, and the molten metal permeates and erodes the thermal-sprayed coatings through through-pores and pores. This causes a corrosion potential to occur, and the thermal-sprayed coating flakes off and wears. The molten metal permeating the thermal-sprayed coating is embedded in the thermal-sprayed coatings and cannot be easily removed, causing a reduction in plating quality of the steel plate.
The coating described in any of Patent Literatures 3 to 5 produced by thermal-spraying an oxide-based ceramic onto a thermal-sprayed cermet coating easily cracks and flakes off by thermal shock and has low coating strength and wear resistance. The thermal-spraying ceramic particles are bonded to each other by subjecting the coating to pore sealing. However, since the bonding structure is brittle, cracks and exfoliation easily occur when external mechanical force or thermal stress is applied to the coating, and the insulating properties thereof are low. The thermal-sprayed ceramic coating has a pumice-like structure containing pores and through pores in an amount of about 15% to about 25%. Therefore, a molten metal easily permeates the pores and through pores in the sprayed ceramic coating, and the molten metal and the oxide thereof (dross) are embedded in the thermal-sprayed coating and cannot be easily removed, causing a reduction in plating quality.
In recent years, in the context of reducing cost, improving production efficiency, etc., there is a need for a further improvement in the service life of conveying rollers. In addition, since requirements for the quality of plated steel plates are becoming more strict, there is a strong need for a molten metal and oxides thereof (dross) adhering to the conveying rollers to be easily removed, in order to take measures against plating quality defects caused by the molten metal and oxides thereof.
Since the conveying rollers are reused after zinc and aluminum adhering thereto are dissolved by washing with an acid such as sulfuric acid or hydrochloric acid, the conveying rollers are required to have a long life against corrosion by acids.
The present inventor has analyzed a conventional thermal-sprayed coating to analyze the cause of the exfoliation and wear of the thermal-sprayed coating. FIG. 1 is an optical microscope photograph of a cross section of a conventional high velocity gas sprayed WC—B—Co-based cermet coating that has been immersed in a molten metal including 55 percent by weight of aluminum and 45 percent by weight of zinc at 873K for 16 days. Zinc and aluminum being molten metal components permeated the thermal-sprayed coating, and the thermal-sprayed coating was cracked and nearly flaked off. The molten metal eroded and permeated Co used as a binder of the thermal-sprayed coating and also caused permeation and erosion through through-pores and pores.
FIG. 2 is an optical microscope photograph of a cross section of a coating in Patent Literature 3 formed by thermal-spraying an oxide-based ceramic (chromia) onto a high-velocity-gas-sprayed WC—B—Co-based cermet coating. Generally, a ceramic has a high melting point and is therefore thermal-sprayed using a plasma gun. Since the temperature of the plasma from the plasma gun is about 30,000K, the thermal-spraying particles are completely melted and impinge on a substrate as molten droplets. However, since the velocity of the thermal-spraying particles is as low as about 250 m/sec, the thermal-sprayed coating contains pores in an amount of 15% to 25%. The bonding strength between the thermal-spraying particles is, however, low, and the thermal-spraying particles are bonded through a pore sealing material. However, not all the pores can be filled. Although the oxide ceramic itself does not react with the molten metal, the molten metal is embedded in the pores, and this prevents the molten metal from being removed. The pore sealing material is cracked and flakes off when thermal stress or an external mechanical force caused by contact with a steel plate is applied to the coating, and the molten metal is further embedded in the cracks and exfoliation. This makes the molten metal more difficult to remove.
The plasma sprayed oxide-based ceramic particles are quenched from a molten droplet state. Therefore, a large number of micro-cracks are present, and this weakens mechanical strength and causes a reduction in insulating properties.
The present inventor has found that a long-service life molten metal resistant member that allows zinc to be easily removed, can prevent occurrence of a corrosion potential, can prevent permeation of acids so as to resist repeated washing with the acids, and has wear resistance and resistance to corrosion by a molten metal can be produced if a thermal-sprayed oxide-based ceramic coating that includes particles bonded with high bonding strength, is dense, resists deterioration, and has high insulating properties can be obtained.
A thermal-sprayed coating used for a molten metal resistant member undergoes strong thermal shock when the thermal-sprayed coating comes into contact with a molten metal. The toughness of a thermal-sprayed ceramic material is lower than that of thermal-sprayed metal and cermet materials. Therefore, in a conventional method, the amount of pores in the thermal-sprayed ceramic material is increased to form a pumice-like microstructure, and thermal stress is relaxed by thermal deformation of the microstructure. Since the temperature of the plasma from a plasma gun is about 30,000K, the thermal-spraying particles are completely melted and impinge on a substrate as molten droplets. However, since the velocity of the thermal-spraying particles is as low as about 250 m/sec, the thermal-sprayed coating inevitably contains pores in an amount of 15% to 25%. When the ceramic is once melted and thermal-sprayed, it undergoes a phase change and is transformed, and the ceramic after the phase change is similar to but different from the original ceramic in bulk form.
The present inventor has found a high thermal shock resistant thermal spray coated member including a thermal-sprayed coating formed by thermally spraying thermal-spraying oxide-based ceramic particles having an average particle diameter as a median diameter of 10 μm or smaller. More specifically, the thermal spray coated member comes into contact with a molten metal, and each of the thermal-spraying particles in the thermal-sprayed coating includes a surface layer portion that has been once thermally melted and then solidified and an inner layer portion that has not been thermally melted during thermal spraying.
FIG. 4 is a cross-sectional view of a high velocity gas spraying apparatus. The thermal-sprayed coating is thermal-sprayed from the high velocity gas spraying apparatus shown in FIG. 4. Kerosene used as fuel is fed inside a combustion chamber 2 and combusted with high-pressure oxygen, and the pressure inside the combustion chamber becomes as high as about 0.7 MPa. The combustion gas is converted to high-temperature supersonic gas with a velocity of about 3,270K and 3,000 m/sec through a Laval nozzle 3 disposed at the outlet of the combustion chamber. Thermally spraying particles sprayed from a thermal-spraying particle spraying nozzle 4 are supplied to the high-temperature supersonic gas. The supplied thermal-spraying particles are heated, accelerated, and then sprayed onto an member to be spray-coated.
The configuration of the high velocity gas spraying apparatus shown in FIG. 4 will next be described in detail. The high velocity gas spraying apparatus includes three main components, i.e., a combustion chamber tail plug 1 disposed on its rear end, the combustion chamber 2 disposed in front of the plug 1 in the direction of spraying, and a spray nozzle 5 connected to the combustion chamber 2.
A fuel supply port 7 for supplying fuel such as kerosene at high velocity in the forward direction of spraying and an oxygen supply port 8 for supplying oxygen gas at high velocity in the forward direction of spraying are disposed in the combustion chamber tail plug 1.
The combustion chamber 2 to which the combustion chamber tail plug 1 is attached is formed into a cylindrical shape, and the Laval nozzle 3 having a shape in which its diameter tapers and then gradually increases is formed at the connection portion between the combustion chamber 2 and the spray nozzle 5.
The spray nozzle 5 connected to the combustion chamber 2 through the Laval nozzle 3 is a copper tube having an inner diameter of about 11 mm and a length of about 10 cm to about 20 cm and is cooled by water from the outside. A thermal spraying material supply section 4 for supplying the thermal spraying material is disposed in the spray nozzle 5 and located on a side close to the Laval nozzle 3. A material prepared by adding a material suitable for the required characteristics such as wear resistance to a Ni-, Ni—Cr-, or Co-based alloy is used as the thermal spraying material.
When thermal spraying is performed, first, the fuel and oxygen supplied from the kerosene supply port 7 and from the oxygen supply port 8 disposed in the combustion chamber tail plug 1 are combusted in the combustion chamber 2. During combustion, the combustion gas in the combustion chamber 2 has a pressure of about 0.7 MPa and a combustion temperature of about 3,000° C. The combustion gas is fed to the Laval nozzle 3, accelerated to a sonic to supersonic speed when the gas passes through the Laval nozzle 3, and then supplied to the spray nozzle 5. The thermal spraying material is sprayed from the thermal spraying material supply section 4 into the accelerated combustion gas at the connection portion between the Laval nozzle 3 and the spray nozzle 5. The thermal spraying material is accelerated and heated by the combustion gas. The flow of the combustion gas and the thermal spraying material is smoothened when they pass through the spray nozzle 5, and the resultant flow with improved convergence properties is sprayed from the tip of the spray nozzle 5. The thermal spraying material is thereby sprayed at very high velocity and can be thermal-sprayed onto an member to be spray-coated.
White alumina having an average particle diameter as a median diameter of 40 μm, which is an oxide-based ceramic commonly used in the high velocity gas spraying apparatus in FIG. 4, was sprayed into the spraying apparatus to perform a thermal spray test. However, no film was deposited.
Therefore, an Accuraspray thermal spray measuring apparatus, manufactured by Sulzer Metco, was used to perform measurement. The temperature of the surfaces of the flying thermal-spraying particles was found to be 2,053K, and the velocity of the thermal-spraying particles was found to be 815 m/sec. However, the melting point of the white alumina is 2,302K, and therefore a film was not deposited because the temperature and velocity were insufficient.
Assuming that the thermal-spraying particles are perfect spheres. Then their surface area is proportional to the square of the diameter, and the volume is proportional to the cube of the diameter. Therefore, the specific surface area increases inversely proportional to the diameter. For example, when the diameter of the thermal-spraying particles is reduced from 40 μm to 4 μm, the specific surface area increases by a factor of 10. The weight becomes 1/1,000. The thermal-spraying particles sprayed into the combustion gas are heated from their surfaces and sprayed through the flow of the combustion gas. Therefore, when finer thermal-spraying particles are used, it is expected that the temperature and velocity of the thermal-spraying particles increase.
White alumina particles having an average particle diameter as a median diameter of 4 μm were sprayed, and measurement was performed using the Accuraspray thermal spray measuring apparatus, manufactured by Sulzer Metco. The surface temperature of the flying thermal-spraying particles was found to be 2,700K, and the velocity of the thermal-spraying particles was found to be 2,750 m/sec. This surface temperature is higher than the melting point of the white alumina being 2,302K. The white alumina particles were actually thermal-sprayed, and a 6 μm film was deposited in one pass. The coating was subjected to X-ray diffraction. It was found that an α phase which is the phase of the white alumina in bulk form remained unchanged and that a γ phase generated by fusion was present only in very limited regions. Therefore, the physical properties of the coating were very close to those of the white alumina in bulk form.
White alumina particles having an average particle diameter as a median diameter of 10 μm were sprayed, and measurement was performed using the Accuraspray thermal spray measuring apparatus manufactured by Sulzer Metco. The surface temperature of the flying thermal-spraying particles was found to be 2,400K, and the velocity of the thermal-spraying particles was found to be 1,000 m/sec. This surface temperature is higher than the melting point of the white alumina being 2,302K. The white alumina particles were actually sprayed, and a 1 μm film was deposited in one pass.
It was found that the formation of a film of an oxide-based ceramic by thermal spraying, which had been considered impossible, could be achieved by using the high velocity gas spraying apparatus. More specifically, this can be achieved by using thermal-spraying oxide-based ceramic particles having an average particle diameter as a median diameter of 10 μm or smaller. In addition, the thermal-spraying particles are thermal-sprayed at a high flying particle velocity of 1,000 m/sec or higher such that only the surfaces of the flying thermal-spraying particles are semi-melted while the inside of the particles is in a solid state.
With the conventional high velocity gas spraying apparatus, thermal spraying could be performed for a short time. However, the inner surface of the nozzle 5 wore, and molten thermal-spraying particles adhered to the inner surface of the nozzle 5 and then flaked off, causing spray defects called spitting.
FIG. 5 is an example of a high velocity gas spraying apparatus disclosed in Japanese Patent Application Laid-Open No. 2009-179846 by the present inventor. The inner diameter of an inner tube 2 which is disposed at one end of the high velocity gas spraying apparatus 1 is the same as the inner diameter of a nozzle of the high velocity gas spraying apparatus. Air, nitrogen gas, or the like injected from a gas injection port 4 is injected through a cylindrical gap between the inner tube 2 and an outer tube 3 and forms a cylindrical gas tunnel to prevent oxidation of the thermal-spraying particles. When an oxide-based ceramic is sprayed, the gas tunnel may not be used. The fine powdery thermal-spraying particles are sprayed at one end from a thermal spraying powder spraying port 5 into the inside and center of high-temperature and high-velocity combustion gas 6 and then converted to high temperature and high velocity particles.
An oxide-based ceramic was thermal-sprayed using the high velocity gas spraying apparatus in FIG. 5. No abrasion of the nozzle and no spitting occurred, and the oxide-based ceramic could be thermal-sprayed stably.
The thermal-spraying oxide-based ceramic particles that are in the form of fine powder having an average particle diameter as a median diameter of 10 μm or smaller and have been sprayed into the high velocity gas spraying apparatus are heated such that their surface temperature is close to the temperature of the combustion gas in the high velocity gas spraying apparatus and are accelerated to a velocity close to the velocity of the combustion gas. However, the oxide-based ceramic has low thermal conductivity, and the distance between the spraying apparatus and the member to be spray-coated is short (200 mm in this test example). The heating time is therefore short, and the flying thermal-spraying oxide-based ceramic particles impinge on the member to be spray-coated at a supersonic velocity of 1,000 m/sec of higher, with only the surfaces of the particles being in a semi-melted state and the inside being in a solid state. Therefore, in contrast to a thermal-sprayed oxide-based ceramic coating formed using a conventional plasma gun, a thermal-sprayed coating that is dense, includes thermal-spraying particles bonded with high bonding strength, strongly resists deterioration, and has insulating properties can be obtained. Since high compressive residual stress remains present in the thermal-sprayed coating, high bonding strength between the particles is obtained, and resistance to cracks and exfoliation caused by external mechanical force and thermal shock is obtained. More specifically, each of the thermal-spraying particles impinging on the member to be spray-coated includes a surface layer portion that has been once thermally melted and then solidified and an inner layer portion that has not been thermally melted and is in a state before the thermal-spraying particles are fed to the high velocity gas spraying apparatus. The surface layer portions of adjacent thermal-spraying particles are firmly bonded with no gaps, and a thermal-sprayed coating having the above-described properties can thereby be obtained. In the conventional technology, pore sealing treatment is necessary. Thermal spraying takes one day for, for example, a roller to be used in a zinc bath, and pore sealing and heat treatment takes 4 days. In the present invention, pore sealing is not necessary. This is highly effective for the cost and the time for completion.
FIG. 3 is an optical microscope photograph of a cross section of a test piece immersed in a molten metal including 55% by weight of aluminum and 45% by weight of zinc at 873K for 16 days, the test piece including: a thermal-sprayed Mo—Co, Cr—B-based cermet coating thermal-sprayed using the high velocity gas spraying apparatus; and a coating with unsealed pores that has been formed thereon by thermal-spraying 6 μm fine thermal-spraying gray alumina particles to 50 μm using the high velocity gas spraying apparatus. The thermal-sprayed fine powdery gray alumina coating layer was dense, and no adhering molten metal was found. The fine powdery gray alumina layer on the surface protected the thermal-sprayed cermet coating therebelow, and the thermal-sprayed cermet coating was completely sound. The test piece was immersed in sulfuric acid for 12 hours to perform an acid resistance test. The fine powdery gray alumina coating in the surface layer and the thermal-sprayed cermet coating therebelow were completely sound, and the test piece exhibited good properties as a molten metal resistant member. Therefore, it was found that even a thermal-sprayed oxide-based ceramic coating that has molten metal resistance and undergoes thermal shock can be practically used when the thermal-sprayed oxide-based ceramic coating is formed not to have a pumice-like porous structure, is dense, includes thermal-spraying particles bonded with high bonding strength, resists deterioration, and has high insulating properties.
Embodiments of the present invention based on the above findings will next be described in detail.

Embodiment 1

(1) A thermal spray coated member having high thermal shock resistance, the thermal spray coated member comprising a thermal-sprayed coating formed by thermally spraying thermal-spraying oxide-based ceramic particles having an average particle diameter as a median diameter of 10 μm or smaller, wherein each of the thermal-spraying particles in the thermal-sprayed coating includes a surface layer portion that has been thermally melted and then solidified and an inner layer portion that has not been thermally melted during thermal spraying.
The oxide-based ceramic is an oxide, is therefore stable, and has good features such as wear resistance, corrosion resistance at high temperature, resistance to corrosion by acids, heat insulating properties, and electrical insulating properties. Therefore, such an oxide-based ceramic can be used for a long time for a molten metal resistant member that comes into contact with a molten metal including Zn and/or Al and is not used in a reducing atmosphere. In the present invention, the molten metal including Zn and/or Al is not a limitation, and high-temperature glass with a temperature of 475K or higher can also be used.

Embodiment 2

(2) In the configuration in (1), the thickness of the oxide-based ceramic coating is preferably 50 μm or smaller.
Conventional thermal-spraying particles have an average particle diameter as a median diameter of about 40 μm. Therefore, to prevent through-pores that reach a substrate, the thickness must be at least 5 times the minimum diameter of the particles, i.e., 200 μm. However, the coating of the present invention that includes particles with an average particle diameter as a median diameter or 10 μm or smaller, is dense, and has high insulating properties can exert its performance even when the sprayed thickness is 50 μm or smaller, and a significant reduction in cost can be achieved.

Embodiment 3

(3) In the configuration in (1) or (2), the thermal spray coated member preferably further comprises a thermal-sprayed undercoating, the thermal-sprayed undercoating being a cermet- or metal-based thermal-sprayed coating that has been thermal-sprayed as a primer coating for the thermal-sprayed oxide-based ceramic coating, and the thickness of the thermal-sprayed undercoating is preferably set to 200 μm or smaller.
TABLE 1 shows the thermal expansion coefficients of various materials. Generally, in a molten zinc bath, when the difference in thermal expansion coefficient is less than about 60%, the difference causes no cracks. However, when the difference in thermal expansion coefficient is larger than 60%, cracks are more likely to occur. Generally, the oxide-based ceramic used for the upper layer has a small thermal expansion coefficient, and stainless steel used for a substrate has a large thermal expansion coefficient. Therefore, by providing, between the substrate and the upper layer, a thermal-sprayed undercoating having an intermediate thermal expansion coefficient between those of the substrate and the upper layer, the occurrence of cracks due to the difference in thermal expansion coefficient can be prevented.
When gray alumina is thermal-sprayed and SUS316L being austenite stainless steel is used for the substrate, it is preferable that a metal-based material such as STELITE#6 and WC—B—Co-based cermet be thermal-sprayed to form an undercoating.
In the configuration in (1), gray alumina or white alumina may be used for the thermal-spraying particles. In this case, the thermal-spraying particles in the thermal-sprayed coating are such that the crystal structure of the surface layer portion has been changed from the α phase to the γ phase by heat fusion and the crystal structure of the inner layer portion has not been changed and is the α phase.

TABLE 1

		THERMAL
		EXPANSION
	MATERIAL	COEFFICIENT	NOTE

UPPER	GRAY	7.4 × 10⁻⁶/K	OXIDE-BASED
LAYER	ALUMINA		CERAMIC
	WHITE	8.0 × 10⁻⁶/K	OXIDE-BASED
	ALUMINA		CERAMIC
LOWER	WC-B—Co	9.3 × 10⁻⁶/K	CERMET-BASED
LAYER	BASED
	Mo—Co, Cr—B	9.2 × 10⁻⁶/K	CERMET-BASED
	BASED
	STELITE#
6	15.0 × 10⁻⁶/K	METAL BASED
SUB-	SUS316L	19.3 × 10⁻⁶/K	AUSTENITE-BASED SUS
STRATE	SUS410	11.7 × 10⁻⁶/K	MARTENSITE-BASED
			SUS
	SUS430	11.9 × 10⁻⁶/K	FERRITE-BASED SUS

Various oxide-based ceramics are listed in TABLE 3. Of these, gray alumina is produced by melting and reducing natural bauxite directly in an arc type electric furnace and is therefore inexpensive. In addition, the gray alumina has a low melting point, can be easily thermal-sprayed, and has high shock resistance, wear resistance, molten metal resistance, acid resistance, and molten metal releasability. When the gray alumina was immersed in a molten metal including 55% by weight of aluminum/45% by weight of zinc at 873K for 16 days, the gray alumina was not damaged, as shown in the optical microscope photograph in FIG. 3.
A test piece 1 was produced by thermal-spraying conventional gray alumina having an average particle diameter as a median diameter of about 25 μm using a plasma gun, and a test piece 2 according to the present invention was produced by thermal-spraying gray alumina having an average particle diameter as a median diameter of about 6 μm using the high velocity gas spraying apparatus in FIG. 5. The bonding strengths between the thermal-spraying particles in the thermal-sprayed coatings in the test pieces 1 and 2 were compared using a blast abrasion tester shown in FIG. 6. More specifically, 1 kg of a #70 alumina blasting material was sprayed onto each thermal-sprayed coating from a distance of 65 mm at an angle of 60°. A reduction in weight of the each sprayed coating was measured, and the results for the test pieces were compared. By causing the blasting material to impinge on the thermal-sprayed coating at an angle of 60°, thermal-spraying particles in the thermal-sprayed coating are separated from each other, and the weight of the thermal-sprayed coating decreases. By comparing the amounts of the reduction in weight, the bonding strengths between the particles can be compared. As shown in TABLE 2, the bonding strength between the particles in the test piece 2 according to the present invention was found to be higher by a factor of 5.6 than that in the test piece 1 including the conventional thermal-sprayed coating. The thermal-sprayed coating in the test piece 1 was brittle. This may be because, since the thermal-sprayed coating in the test piece 1 contains pores in an amount of 21%, the particles are not physically bonded. In addition, since the thermal-spraying particles have been completely melted in plasma at about 30,000K, the phase change may also contribute to the brittleness.

TABLE 2

	TEST PIECE 1
	CONVENTIONAL	TEST PIECE	2
	THERMAL-SPRAYED	PRESENT
ITEM	COATING	INVENTION

THERMAL SPRAYING	PLASMA GUN	HIGH VELOCITY
APPARATUS	Sulzer Metco	GAS SPRAYING
	10 MB Gun	APPARATUS
		FIG. 5
THERMAL-SPRAYED	25 μm	6 μm
PARTICLE DIAMETER
THICKNESS OF	200 μm	200 μm
THERMAL-SPRAYED
FILM
POROSITY	21%	0.1%
WEIGHT REDUCTION	5.6	1 (REFERENCE)
RATIO
INTERPARTICLE	1 (REFERENCE)	5.6
BONDING
STRENGTH RATIO

TABLE 3

	MAIN	LINEAR	MELT-
	COM-	EXPANSION	ING
	PONENT	COEFFICIENT	POINT
TYPE	(%)	10-6/K	(K)

ALUMINA	GRAY	Al₂O₃: Bal	7.4	2128
	ALUMINA	TiO₂: 2.5
		SiO₂: 1.0
	WHITE	Al₂O₃: +98	8.0	2272
	ALUMINA
ALUMINA-	15% TITANIA	Al₂O₃: Bal	5.3	2113
TITANIA		TiO₂: 15
	40% TITANIA	Al₂O₃: Bal	7.5	2113
		TiO₂: 40

ALUMINA-ZIRCONIA	Al₂O₃: Bal	6.3	2143
	ZrO₂: 24

ZIRCONIA	8% YTTRIA	ZrO₂: Bal	9.7	2973
		Y₂O₃: 8
	25% YTTRIA	ZrO₂: Bal	8.7	2873
		MgO: 24

ZIRCON	ZrO₂: Bal	7.6	2048
	SiO₂: 33
ALUMINA-CHROMIA	Al₂O₃: 50	8.0	2403
	Cr₂O₃: Bal
CHROMIA	Cr₂O₃:	9.6	2573
MULLITE	3Al₂O₃•2SiO₂	5.6	2163

Claims

1. A thermal spray coated member having high thermal shock resistance, the thermal spray coated member comprising a thermal-sprayed coating produced by thermally spraying thermal-spraying particles of an oxide-based ceramic that have an average particle diameter of 10 μm or smaller, the average particle diameter being a median diameter, wherein each of the thermal-spraying particles in the thermal-sprayed coating includes a surface layer portion that has been once thermally melted and then solidified and an inner layer portion that has not been thermally melted during thermal spraying.

2. The thermal spray coated member according to claim 1, wherein a thickness of the thermal-sprayed oxide-based ceramic coating is 50 μm or smaller.

3. The thermal spray coated member according to claim 1, further comprising a thermal-sprayed undercoating, the thermal-sprayed undercoating being a cermet- or metal-based thermal-sprayed coating that has been thermal-sprayed as a primer coating for the thermal-sprayed oxide-based ceramic coating, and wherein a thickness of the sprayed undercoating is 200 μm or smaller.

4. The thermal spray coated member according to claim 1, wherein the oxide-based ceramic is gray alumina.

5. The thermal spray coated member according to claim 1, wherein the thermal spray coated member is a molten metal resistant member produced by coating a contact portion thereof with the thermal-sprayed coating, the contact portion coming into contact with a molten metal including Zn and/or Al or high-temperature glass with a temperature of 473K or higher.

6. The thermal spray coated member according to claim 1, wherein the thermal-spraying particles are grey alumina or white alumina particles, and the surface layer portion has a crystal structure of a γ phase changed from an α phase by heat fusion, and the inner layer portion has a crystal structure of the α phase.

7. The thermal spray coated member according to claim 2, further comprising a thermal-sprayed undercoating, the thermal-sprayed undercoating being a cermet- or metal-based thermal-sprayed coating that has been thermal-sprayed as a primer coating for the thermal-sprayed oxide-based ceramic coating, and wherein a thickness of the sprayed undercoating is 200 μm or smaller.

8. The thermal spray coated member according to claim 2, wherein the oxide-based ceramic is gray alumina.

9. The thermal spray coated member according to claim 3, wherein the oxide-based ceramic is gray alumina.

10. The thermal spray coated member according to claim 7, wherein the oxide-based ceramic is gray alumina.

11. The thermal spray coated member according to claim 2, wherein the thermal spray coated member is a molten metal resistant member produced by coating a contact portion thereof with the thermal-sprayed coating, the contact portion coming into contact with a molten metal including Zn and/or Al or high-temperature glass with a temperature of 473K or higher.

12. The thermal spray coated member according to claim 3, wherein the thermal spray coated member is a molten metal resistant member produced by coating a contact portion thereof with the thermal-sprayed coating, the contact portion coming into contact with a molten metal including Zn and/or Al or high-temperature glass with a temperature of 473K or higher.

13. The thermal spray coated member according to claim 7, wherein the thermal spray coated member is a molten metal resistant member produced by coating a contact portion thereof with the thermal-sprayed coating, the contact portion coming into contact with a molten metal including Zn and/or Al or high-temperature glass with a temperature of 473K or higher.

14. The thermal spray coated member according to claim 4, wherein the thermal spray coated member is a molten metal resistant member produced by coating a contact portion thereof with the thermal-sprayed coating, the contact portion coming into contact with a molten metal including Zn and/or Al or high-temperature glass with a temperature of 473K or higher.

15. The thermal spray coated member according to claim 8, wherein the thermal spray coated member is a molten metal resistant member produced by coating a contact portion thereof with the thermal-sprayed coating, the contact portion coming into contact with a molten metal including Zn and/or Al or high-temperature glass with a temperature of 473K or higher.

16. The thermal spray coated member according to claim 9, wherein the thermal spray coated member is a molten metal resistant member produced by coating a contact portion thereof with the thermal-sprayed coating, the contact portion coming into contact with a molten metal including Zn and/or Al or high-temperature glass with a temperature of 473K or higher.

17. The thermal spray coated member according to claim 10, wherein the thermal spray coated member is a molten metal resistant member produced by coating a contact portion thereof with the thermal-sprayed coating, the contact portion coming into contact with a molten metal including Zn and/or Al or high-temperature glass with a temperature of 473K or higher.