METHOD FOR FORMING THERMAL SPRAYED COATING

Abstract

Provided is a method for forming a thermal sprayed coating, including a step of spraying a non-oxide ceramic material on a substrate to form a coating by a high velocity oxy-fuel spraying process. An average particle size of the non-oxide ceramic material is 0.1 to 5.0 .mu.m, and a particle size distribution of the non-oxide ceramic material contains one or more peaks in each of: a particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and a particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m. The non-oxide ceramic material is dispersed in a solvent to prepare slurry 11, and the slurry 11 is fed from the exterior to flame 10 jetted from a thermal spray gun 2 to form a dense thermal sprayed coating structure.

Description

TECHNICAL FIELD

[0001] The present invention relates to methods for forming a thermal sprayed coating, in which a dense thermal sprayed coating is formed on a substrate with a non-oxide ceramic material by a high velocity oxy-fuel spraying process.

BACKGROUND ART

[0002] In order to improve functionality of a surface of a structure, it is widely practiced to form various thermal sprayed coatings on a surface of constituent members. A thermal spraying method is a surface treatment technology in which thermal spray materials such as metals, ceramics and cermets are fed into flame generated by combustion gas, plasma arc, and the like, and these materials are softened or melted and sprayed at high velocity on a surface of an object to be thermally sprayed, to coat the surface with a thermal sprayed coating.

[0003] Various materials can be used for thermal spraying, but on the other hand, because a thermal spraying process is performed at high temperature, evaporation and oxidation of the thermal spray material may occur during the process. Therefore, a good quality coating cannot be obtained unless thermal spraying conditions are sufficiently selected according to the material used. In particular, non-oxide ceramics such as aluminum nitrides are generally considered to be more difficult to select thermal spraying conditions than other materials, and have been variously studied conventionally.

[0004] Patent Literature 1 describes a method for producing a coating, in which an aluminum nitride coating is formed on a substrate by using an explosive thermal spray apparatus having a combustion cylinder, a gas supply means for supplying fuel gas or the like, an ignition means for igniting fuel mixed gas, and a powder feed means. In this literature, aluminum nitride powder having an average particle size of 1 .mu.m to 5 .mu.m is processed into granulated powder having the average particle size of 20 .mu.m to 60 .mu.m, and the granulated powder is used as powder to be fed.

[0005] Patent Literature 2 describes a method for forming a coating, in which an aluminum nitride thermal sprayed coating is formed on a substrate by atmospheric plasma spraying by adjusting temperature and spraying speed of aluminum nitride powder.

[0006] Patent Literature 3 describes a method for forming a coating, in which powder particles of nitride are continuously deposited on a substrate of parts for a semiconductor manufacturing apparatus without melting the powder particles.

[0007] Patent Literature 4 describes a method for forming a coating on a surface of a substrate, in which raw material powder mainly containing particles of a metal nitride having sublimation and no melted phase is dispersed in an organic solvent to prepare slurry, and the slurry is flame sprayed under predetermined thermal spraying conditions.

CITATION LIST

Patent Literature

[0008] [Patent Literature 1] Japanese Laid-Open Patent Publication No. 2017-071835

[0009] [Patent Literature 2] Japanese Laid-Open Patent Publication No. 2009-235558

[0010] [Patent Literature 3] International Publication WO 2010/027073

[0011] [Patent Literature 4] Japanese Laid-Open Patent Publication No. 2014-198898

SUMMARY OF INVENTION

Technical Problem

[0012] A problem common to the above Literatures 1 to 4 is that when the size of the thermal spray material is too large, the particles are un-melted and it is difficult to form a coating, and even when the coating can be formed, it is difficult to obtain a dense coating or a coating having sufficient adhesion to a substrate. In contrast, when the size of the thermal spray material is too small, oxidation of the particles proceeds excessively and it is difficult to obtain a coating having the required composition.

[0013] In the method described in Patent Literature 1, in which the granulated aluminum nitride powder having the adjusted average particle size is used to form the coating by using the explosive thermal spray apparatus, the average particle size of the powder used is large. Therefore, the powder cannot be sufficiently melted, so that a coating cannot be formed. Alternatively, even when a coating can be formed, the coating is not dense.

[0014] In the method for forming a coating of Patent Literature 2, in which aluminum nitride is used to form the coating by the atmospheric plasma spraying, temperature of flame due to a plasma heat source is very high, so that the aluminum nitride will be sublimated. Furthermore, it is necessary to add rare earth metal ceramics in order to improve denseness of the coating.

[0015] Patent Literature 3 describes that 90% or more of the nitride powder particles in the thermal sprayed coating formed are un-melted and deposited, and that this was realized by modifying a thermal spray nozzle of an ultra-high velocity oxy-fuel spraying equipment. However, there is no specific description of what kind of modification was made.

[0016] In the method of Patent Literature 4, powder containing the metal nitride particles having a particle size of about 0.5 to 3 .mu.m is used. Therefore, unless the thermal spraying conditions are set with extremely high accuracy, oxidation of the particles proceeds excessively and it is difficult to obtain a coating having the required composition, as described above.

[0017] In view of the problems of the prior art, the present invention has an object of providing a method for forming a thermal sprayed coating, capable of obtaining a dense and highly adhesive coating even when non-oxide ceramics are used as the thermal spray material.

SOLUTION TO PROBLEM

[0018] The present inventors have investigated a method for forming a thermal sprayed coating, in which a non-oxide ceramic material is sprayed on a substrate to form a coating, and resultantly succeeded to form a dense and highly adhesive coating by adopting a high velocity oxy-fuel spraying process with a material having predetermined average particle size and particle size distribution, leading to solution of the problems.

[0019] The method of the present invention is a method for forming a thermal sprayed coating, including a step of spraying a non-oxide ceramic material on a substrate to form a coating by a high velocity oxy-fuel spraying process, wherein an average particle size of the non-oxide ceramic material is 0.1 to 5.0 .mu.m, and a particle size distribution of the non-oxide ceramic material contains one or more peaks in each of: a particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and a particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m.

[0020] Because the high velocity oxy-fuel spraying process is adopted in the present invention, it is possible to prevent the non-oxide ceramic material from being excessively oxidized in a process of thermal spraying and to obtain a thermal sprayed coating mainly composed of non-oxide ceramics. Here, "mainly composed of non-oxide ceramics" means that non-oxide ceramics are the most abundant in terms of mass unit among the constituent components of the thermal sprayed coating. Furthermore in the present invention, the non-oxide ceramic material has an average particle size which is smaller than that of general thermal spray materials, and contains a relatively large size particle group and a relatively small size particle group among them. Specifically, the average particle size of the non-oxide ceramic material is 0.1 to 5.0 .mu.m, and the particle size distribution of the non-oxide ceramic material contains one or more peaks in each of: the particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and the particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m. Even when the high velocity oxy-fuel spraying process is adopted, some oxidation proceeds from the outer peripheral side of particles in case thermal spraying is performed under environment containing oxygen (for example, in the atmosphere). At this time, most of each of particles having a particle size in a range of 0.1 .mu.m or more and less than 1.0 .mu.m are oxidized in the process of thermal spraying, whereas not all part of each of particles having a particle size in a range of 1.0 .mu.m or more and less than 10.0 .mu.m is oxidized because only a part of that is oxidized. When a coating is formed with a material containing these particles, the particles having a particle size in a range of 0.1 .mu.m or more and less than 1.0 .mu.m serve as a binder which binds the particles having a particle size in a range of 1.0 .mu.m or more and less than 10.0 .mu.m to each other. That is, when a non-oxide ceramic material having a small average particle size is used, in case a certain amount of particles having relatively large size and a certain amount of particles having relatively small size are contained, the particles having relatively small size function as a binder for binding the particles having relatively large size to each other. As a result, a dense and highly adhesive coating can be obtained.

[0021] In the non-oxide ceramic material, a volume ratio of a material having a particle size in a range of 1.0 .mu.m or more and less than 10.0 .mu.m to a material having a particle size in a range of 0.1 .mu.m or more and less than 1.0 .mu.m is preferably 60% or more, and more preferably 90% or less. In this case, a denser and more highly adhesive coating can be obtained.

[0022] It is preferred a suspension in which the non-oxide ceramic material is dispersed in a solvent is fed to flame. By adopting such a suspension high velocity oxy-fuel spraying process for forming a coating, aggregation between thermal spray materials during transportation is suppressed, so that a dense coating can be formed more reliably.

[0023] It is preferred the suspension is fed to flame jetted from a tip of a thermal spray nozzle. In the case of a high velocity oxy-fuel spraying process with internal feed mode, spitting easily occurs, in which: the thermal spray material is deposited in the thermal spray nozzle; and the deposits are agglomerated and discharged. In contrast, the spitting can be prevented by adopting a thermal spraying process with external feed mode in which the suspension is fed to the flame jetted from the tip of the thermal spray nozzle.

[0024] The non-oxide ceramic material may consist of a material containing one or more ceramics selected from the group consisting of carbide ceramics, nitride ceramics, and boride ceramics. Although these non-oxide ceramics are harder materials than oxide ceramics, they are generally difficult to be formed by thermal spraying. According to the method for forming a thermal sprayed coating of the present invention, a dense and highly adhesive coating can be formed even with these materials, thus, a hard and high quality coating can be obtained.

ADVANTAGEOUS EFFECTS OF INVENTION

[0025] As in the present invention, by using a material, as the thermal spray material composed of non-oxide ceramics, having an average particle size of 0.1 to 5.0 .mu.m and having a particle size distribution which contains one or more peaks in each of a predetermined range with a smaller particle size and a predetermined range with a larger particle size than 1.0 .mu.m as a boundary, and by subjecting the material to high velocity oxy-fuel spraying, particles in the predetermined range with a smaller particle size serve as a binder for binding particles in the predetermined range with a larger particle size to each other. As a result, a dense and highly adhesive coating can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

[0026] FIG. 1 is a schematic view of main part of a thermal spray apparatus for performing the high velocity oxy-fuel spraying process used in the method for forming a thermal sprayed coating.

[0027] FIG. 2 includes graphs each showing a unimodal particle size distribution of titanium carbide powder and a bimodal particle size distribution of titanium carbide powder.

[0028] FIG. 3 is a photographic view showing the results of coating formability.

[0029] FIG. 4 includes graphs both showing a bimodal particle size distribution of aluminum nitride powder.

[0030] FIG. 5 is a table showing the relationship between surface roughness of a substrate and adhesive force.

[0031] FIG. 6 includes an image of observation of cross-sectional structure and a table showing coating components.

[0032] FIG. 7 includes images of observation of cross-sectional structure showing bonding state between particles in a coating.

DESCRIPTION OF EMBODIMENTS

[0033] Embodiments of the present invention will be described. The high velocity oxy-fuel (HVOF) spraying process is used in the method for forming a thermal sprayed coating of the present embodiments. Thermal spray powder is sprayed on the substrate by the high velocity oxy-fuel spraying process to form the thermal sprayed coating. The high velocity oxy-fuel spraying process is a thermal spraying method which uses combustion energy of combustion gas as a heat source, in which supersonic flame is generated by increasing pressure in a combustion chamber, the thermal spray powder is fed to the center of supersonic flame jet stream and accelerated, and the thermal spray powder is melted or half-melted and continuously sprayed at high velocity.

[0034] Because the melted thermal spray particles are sprayed on the substrate at supersonic velocity, a dense and high adhesive thermal sprayed coating can be formed. In particular, the thermal sprayed coating is continuously formed, so that the formed thermal sprayed coating becomes to be homogeneous. As the combustion gas used as the heat source, there are exemplified flammable gases such as: hydrogen; and acetylene, ethylene and propane each mainly containing carbon and hydrogen, or combustion-supporting gases each containing oxygen. Liquid fuel such as kerosene may be used instead of the flammable gas.

[0035] Specifically, thermal spraying can be performed under the following conditions. That is, mixed gas such as oxygen/propane, oxygen/propylene, oxygen/natural gas, oxygen/ethylene, or oxygen/hydrogen is used as the combustion gas; supersonic flame having a flame velocity of 900 to 2500 m/sec and a flame temperature of 1800 to 3800.degree. C. is generated; a thermal spraying distance is maintained at 100 to 350 mm; and a substrate temperature during thermal spraying is controlled to 200.degree. C. or lower.

[0036] The substrate is not limited, and examples thereof include metal materials, ceramic materials, polymer materials, and the like. Specific examples of the metal materials include, for example: an elementary metal selected from Fe, Cr, Ni, Al, Ti, and Mg; and an alloy containing one or more elements selected from Fe, Cr, Ni, Al, Ti, and Mg. The substrate was produced with such metal materials by extrusion molding, cutting process, plastic process, and forging. It may be a substrate in which a coating is formed on a metal material by weld hardfacing, plating, or thermal spraying. An undercoating may be provided between the substrate and the thermal sprayed coating.

[0037] The non-oxide ceramic material is used as the thermal spray material. The non-oxide ceramic material includes a material containing one or more ceramics selected from the group consisting of carbide ceramics, nitride ceramics, and boride ceramics.

[0038] Specific examples thereof include carbide ceramics, nitride ceramics, boride ceramics, and mixtures thereof, each containing one or more elements selected from the group of Ni, Cr, Co, Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr, Fe, Hf, and La.

[0039] Examples of the carbide ceramics include TiC, WC, TaC, B.sub.4C, SiC, HfC, ZrC, VC, and Cr.sub.3C.sub.2. Examples of the nitride ceramics include TiN, CrN, Cr.sub.2N, TaN, AIN, BN, Si.sub.3N.sub.4, HfN, NbN, YN, ZrN, Mg.sub.3N.sub.2, and Ca.sub.3N.sub.2. Examples of the boride ceramics include TiB.sub.2, ZrB.sub.2, HfB.sub.2, VB.sub.2, TaB.sub.2, NbB.sub.2, W.sub.2B.sub.5, CrB.sub.2, and LaB.sub.6.

[0040] FIG. 1 is a schematic view of main part of a thermal spray apparatus 1 for performing the high velocity oxy-fuel spraying process used in the method for forming a thermal sprayed coating of the present invention. This thermal spray apparatus 1 is configured as an apparatus for suspension HVOF spraying, which feeds slurry (suspension) of a thermal spray material from the exterior. The thermal spray apparatus 1 is an apparatus with external feed mode, which feeds slurry prepared by dispersing thermal spray powder in a solvent from the exterior, and has a thermal spray gun 2 and a slurry feed nozzle 3.

[0041] The thermal spray gun 2 has a combustion container part 5 constituting a combustion chamber 4, a thermal spray nozzle 6 continuous with the combustion container part 5, and an ignition device 7. Gas containing high-pressure oxygen and fuel is supplied to the combustion chamber 4, and the gas is ignited by the ignition device 7. Flame generated in the combustion chamber 4 is once throttled by the thermal spray nozzle 6 and then, the throttled flame is expanded to give supersonic flame, and the supersonic flame is jetted at high velocity from a tip of the thermal spray nozzle 6. Slurry 11 is fed from the slurry feed nozzle 3 to the jetted flame 10. The thermal spray powder in the slurry 11 is melted or half-melted, and the slurry 11 containing the melted or half-melted thermal spray powder is accelerated by the flame 10 and sprayed on a substrate 100 at high velocity to form a thermal sprayed coating on the substrate 100.

[0042] The slurry 11 is prepared by dispersing the thermal spray powder in water or an organic solvent containing a dispersing medium composed of an alcohol and an organic dispersant. The slurry 11 contains particles of the thermal spray powder in a mass ratio of 5 to 40%. The slurry 11 is fed to the flame 10 to be jetted from a tip of the thermal spray nozzle 6.

[0043] In the case of the internal feed mode in which the slurry is fed in the interior of the thermal spray nozzle, spitting may occur, in which: the thermal spray material is deposited in a nozzle tube; and the deposits are agglomerated and discharged. In contrast, as shown in FIG. 1 in the present embodiment, the spitting can be prevented by adopting the process with the external feed mode in which the slurry 11 is fed to the flame 10 from the exterior.

[0044] The average particle size of the non-oxide ceramic material which is the thermal spray powder is 0.1 to 5.0 .mu.m, and the particle size distribution of the non-oxide ceramic material contains one or more peaks in each of: a particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and a particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m. That is, one or more peaks exist within the particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m in the particle size distribution, and simultaneously one or more peaks exist within the particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m in the particle size distribution. The average particle size of particles is defined as a particle size (median size) at which an accumulation value is 50% when the particle size distribution is measured by a laser diffraction-scattering method (micro-track method).

[0045] Two, or three or more peaks may exist within each of: the particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and the particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m. A typical example is a non-oxide ceramic material of which particle size distribution contains: one peak in the particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and simultaneously one peak in the particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m. Other examples are non-oxide ceramic materials of which particle size distribution contains: a plurality of peaks in the particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and simultaneously a plurality of peaks in the particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m.

[0046] It is necessary that: a considerable number of the particles of the non-oxide ceramic material exist within the particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and simultaneously a considerable number of the particles of the non-oxide ceramic material exist within the particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m. Furthermore, the volume ratio of the material having a particle size in the range of 1.0 .mu.m or more and less than 10.0 .mu.m to the material having a particle size in the range of 0.1 .mu.m or more and less than 1.0 .mu.m is preferably 60% or more, and more preferably 90% or less.

[0047] Because the particles having a particle size in the range of 0.1 .mu.m or more and less than 1.0 .mu.m are very small, oxidation thereof proceeds when they come into contact with the atmosphere during thermal spraying, so that most of them become oxides. When the average particle size of the thermal spray powder consisting of the non-oxide ceramic material is 0.1 to 5.0 .mu.m and the particle size distribution thereof contains one or more peaks in each of the predetermined smaller particle size range and the predetermined larger particle size range than 1.0 .mu.m as the boundary, the particles in the predetermined smaller particle size range, most of which will become oxides, are imparted with binder function for binding the particles in the predetermined larger particle size range to each other. Gaps between the particles having the larger particle size are filled with the particles having the smaller particle size, so that the particles having the larger particle size are bound to each other. As a result, a very dense coating can be obtained.

[0048] In the non-oxide ceramic material, when the volume ratio of the material having a particle size in the range of 1.0 .mu.m or more and less than 10.0 .mu.m to the material having a particle size in the range of 0.1 .mu.m or more and less than 1.0 .mu.m is 60% or more, and preferably 90% or less, bonding force between the particles is remarkably increased. As a result, a denser and more highly adhesive coating can be formed. The volume ratio can be calculated by comparing the areas of the respective particle size distributions measured by a laser diffraction-scattering method (micro-track method).

[0049] Generally, when there are used a considerable number of particles having a particle size of about 0.1 to 1.0 .mu.m, fluidity of the thermal spray powder may lower and stable feed may not be possible. In contrast, in the present embodiment, because the coating is formed by the suspension high velocity oxy-fuel spraying process in which the slurry of the thermal spray material is fed, the thermal spray material can be transported, in which the thermal spray powder is suppressed from agglomeration. As a result, the thermal spray powder can be stably fed. Generally, when non-oxide ceramics containing a large amount of particles having a particle size close to 10.0 .mu.m are thermal sprayed, it is likely that the formed coating becomes to be excessive porous and quality of the coating deteriorates. In contrast, in the present embodiment, the particles having the smaller particle size serve as the binder, so that a high quality and dense thermal sprayed coating can be formed.

[0050] A thickness of the thermal sprayed coating obtained by the above-mentioned method for forming a thermal sprayed coating is preferably in a range of 50 to 2000 .mu.m, and the thickness is appropriately set according to the purpose of use. Generally, when the thickness is 50 .mu.m or more, uniformity of the coating is maintained and coating function can be sufficiently exhibited. When the thickness is 2000 .mu.m or less, decrease in mechanical strength due to influence of the residual stress inside the coating can be prevented.

[0051] A porosity of a ceramic thermal sprayed coating may be about 0.1 to 5%. However, the porosity of the thermal sprayed coating obtained by the method for forming a thermal sprayed coating of the present embodiment can be further less than 0.1% depending on the particle size distribution of the thermal spray powder. When the porosity becomes to be large, it is likely that the mechanical strength decreases and gas easily enters into the coating in the case of, for example, being used in a gas atmosphere. Forming conditions of the coating may be appropriately set according to the substrate, raw material powder, coating thickness, manufacturing environment, and the like.

EXAMPLES

[0052] Examples in which each of coatings is actually formed based on the present invention will be described below.

[0053] The relationship between the size of material powder and coating formability was investigated using two types of titanium carbide powders each having different particle size distributions. There were used two types of titanium carbide powders (Material A and Material B) each adjusted to the particle size distributions shown in FIG. 2. One titanium carbide (Material A) has the particle size distribution containing only one peak in the particle size range of 1 to 10 .mu.m. The other titanium carbide (Material B) has the particle size distribution containing one peak in the particle size range of 0.1 to 1.0 .mu.m, and one peak in the particle size range of 1.0 to 10.0 .mu.m.

[0054] The average particle size of Material A is 3.7 .mu.m, and the average particle size of Material B is 2.4 .mu.m. In Material A, the volume ratio of the material having a particle size in the range of 1.0 .mu.m or more and less than 10.0 .mu.m to the material having a particle size in the range of 0.1 .mu.m or more and less than 1.0 .mu.m is 100%. In Material B, the volume ratio of the material having a particle size in the range of 1.0 .mu.m or more and less than 10.0 .mu.m to the material having a particle size in the range of 0.1 .mu.m or more and less than 1.0 .mu.m is 74%.

[0055] A test was conducted, in which each titanium carbide powder was suspended in water to give slurry, and this slurry was subjected to the suspension HVOF spraying to form a coating on a stainless steel substrate. FIG. 3 is a photographic view showing the results of the coating formability listed on a table. The code "SD" in the table means a thermal spraying distance (mm). It was found that even when two types of powders having approximately the same average particle size were used, the coating was hardly formed with Material A having a unimodal particle size distribution, whereas the coating could be formed with Material B having a bimodal particle size distribution.

[0056] The relationship between the size of material powder and the coating formability was investigated using two types of aluminum nitride powders (Material C and Material D) each having the bimodal particle size distribution shown in FIG. 4. The average particle size of

[0057] Material C is 1.8 .mu.m, and the average particle size of Material D is 1.4 .mu.m. In Material C, the volume ratio of the material having a particle size in the range of 1.0 .mu.m or more and less than 10.0 .mu.m to the material having a particle size in the range of 0.1 .mu.m or more and less than 1.0 .mu.m is 83%. In Material D, the volume ratio of the material having a particle size in the range of 1.0 .mu.m or more and less than 10.0 .mu.m to the material having a particle size in the range of 0.1 .mu.m or more and less than 1.0 .mu.m is 70%.

[0058] A test was conducted, in which each aluminum nitride powder was suspended in an alcohol to give slurry, and this slurry was subjected to the suspension HVOF spraying to form a coating on a stainless steel substrate. As a result, the coating could be formed with any Materials. Hence, a coating sample was prepared again using Material C, and there were performed evaluations of the coating, such as a tensile test for investigating the relationship between surface roughness of the substrate and adhesive force, observation of cross-sectional structure, measurement of the porosity, analysis of the coating components, and survey of electrical properties.

[0059] In order to investigate the relationship between the surface roughness of the substrate and the adhesive force, there were prepared a plurality of stainless steel substrates each adjusted to an arbitrary surface roughness by blasting in the tensile test. FIG. 5 is a table showing the relationship between the surface roughness of the substrate and the adhesive force. All samples had sufficient adhesive force regardless of: the value of the surface roughness Ra of the substrate; and existence of the blasting as a pretreatment. Some of them were coatings with a very smooth surface condition having the surface roughness Ra of 1.0 .mu.m or less.

[0060] FIG. 6 includes an image of the observation of the cross-sectional structure and a table showing the coating components. An abundance ratio (mass %) of each component in the coating was N: 23.52, O: 17.58, or Al: 58.89, and it was found that nitride and oxide existed in a well-balanced manner. A coating hardness was Hv472, a thermal conductivity was 7.4 W/mK, the porosity was 0.1%, a dielectric breakdown voltage was 135 kV/mm, and a volume resistivity was 5.2.times.10.sup.13 .OMEGA.cm. As a result, it was confirmed that the thermal sprayed coating formed in this example had a dense coating structure, and high electrical insulation was shown for the coating.

[0061] The coating structure was enlarged and observed using FE-SEM. Each of images of observation of the cross-sectional structure by FE-SEM is shown in FIG. 7. An oxide layer is formed at a boundary of the aluminum nitride particles, and this layer serves as an adhesive layer. That is, it can be understood that uniform and random presence of the nitride and the oxide without large bias in the coating while being mainly composed of the nitride is an important factor for forming a dense and highly adhesive thermal sprayed coating.

[0062] The methods for forming a thermal sprayed coating of the above-mentioned embodiments and examples are exemplary and not restrictive. Other steps may be included in the method for forming a thermal sprayed coating, depending on the objects on which a thermal sprayed coating is formed and construction modes. Each of configurations and steps described in the embodiments can be varied as long as effects of the present invention are not impaired, and other configurations and steps provided as necessary are not limited.

DESCRIPTION OF PREFERENCE CHARACTERS

[0063] 1 Thermal spray apparatus

[0064] 2 Thermal spray gun

[0065] 3 Slurry feed nozzle

[0066] 4 Combustion chamber

[0067] 5 Combustion container part

[0068] 6 Thermal spray nozzle

[0069] 7 Ignition device

[0070] 10 Flame

[0071] 11 Slurry

[0072] 100 Substrate

Claims

1. A method for forming a thermal sprayed coating, comprising a step of spraying a non-oxide ceramic material on a substrate to form a coating by a high velocity oxy-fuel spraying process, wherein an average particle size of the non-oxide ceramic material is 0.1 to 5.0 .mu.m, and a particle size distribution of the non-oxide ceramic material contains one or more peaks in each of: a particle size range of 0.1 .mu.m or more and less than 1.0 .mu.m; and a particle size range of 1.0 .mu.m or more and less than 10.0 .mu.m.

2. The method for forming a thermal sprayed coating according to claim 1, wherein in the non-oxide ceramic material, a volume ratio of a material having a particle size in a range of 1.0 .mu.m or more and less than 10.0 .mu.m to a material having a particle size in a range of 0.1 .mu.m or more and less than 10.0 .mu.m is 60% or more.

3. The method for forming a thermal sprayed coating according to claim 1, wherein a suspension in which the non-oxide ceramic material is dispersed in a solvent is fed to flame.

4. The method for forming a thermal sprayed coating according to claim 3, wherein the suspension is fed to flame jetted from a tip of a thermal spray nozzle.

5. The method for forming a thermal sprayed coating according to claim 1, wherein the non-oxide ceramic material comprises a material containing one or more ceramics selected from the group consisting of carbide ceramics, nitride ceramics, and boride ceramics.