JP7742750B2 - Thermal spray powder and method for producing thermal spray coating - Google Patents

Description

The present invention relates to a thermal spray powder. The present invention also relates to a method for producing a thermal spray coating using the thermal spray powder.

Technology for imparting new functions by coating the surface of a substrate with various materials has long been used in a variety of fields. One example of this surface coating technology is the thermal spraying method, in which molten ceramic particles are sprayed onto the surface of a substrate to form a thermal spray coating made of the ceramic.

For example, in the field of semiconductor device manufacturing, microfabrication is sometimes performed on the surface of semiconductor substrates by dry etching using plasma of halogen-based gases such as fluorine, chlorine, and bromine. After dry etching, the inside of the chamber from which the semiconductor substrate is removed is cleaned using oxygen gas plasma. Inside this chamber, components exposed to highly reactive oxygen gas plasma or halogen gas plasma can be corroded. For this reason, in semiconductor device manufacturing equipment, ceramic spray coatings are applied to components exposed to oxygen gas or halogen gas plasma to prevent corrosion by the plasma.

In the thermal spraying methods disclosed in Patent Documents 1 to 3, the powder material for thermal spraying (thermal spray powder) is supplied to the thermal spraying device in a dry state. To ensure a stable supply of thermal spray powder to the thermal spraying device, efforts are underway to develop thermal spray powders with improved fluidity.

Patent No. 6811188 Patent No. 4630799 Patent No. 6262716

On the other hand, from the perspective of improving the durability of thermal spray coatings, it is desirable to form thermal spray coatings with smoother surfaces and higher density. One way to improve the smoothness and density of thermal spray coatings is to reduce the size of the ceramic particles that make up the thermal spray powder. However, simply reducing the size of the ceramic particles may result in a decrease in the flowability of the thermal spray powder. Therefore, there is still room for improvement in the design of ceramic particles.

In light of this situation, the present invention aims to provide a technology that maintains the appropriate fluidity for thermal spraying and can supply finer ceramic particles to the substrate during thermal spraying.

The inventors noticed that when granulated and sintered particles are thermally sprayed, the primary particles that make up the particles melt due to the thermal energy generated during the spraying process and penetrate into the gaps, thereby reducing the particle size. The inventors also confirmed that in thermal spray coatings formed using granulated and sintered particles, some of the sprayed granulated and sintered particles reach the substrate in an unmelted state and may remain in the thermal spray coating. After extensive research, the inventors found that intentionally increasing the gaps between the granulated and sintered particles allows the sprayed granulated and sintered particles to be completely melted. Furthermore, the inventors discovered that a thermal spray coating can be produced by supplying particles to a substrate in a smaller particle size than conventional particles, leading to the completion of the present invention.

The thermal spray powder disclosed herein is composed of ceramic particles. The thermal spray powder is prepared by subjecting the thermal spray powder to the following conditions:
Plasma working gas:
Argon (Ar) gas: 50 psi; and
Helium (He) gas: 50 psi
Plasma output: 36 kW
Supply rate of thermal spray powder: 20 g/min
Spraying distance: 400 mm
The thermal spray powder is characterized in that, when comparing the average particle size (D 50 ) of the thermal spray powder after atmospheric pressure plasma spraying in water with the average particle size before the thermal spraying, the average particle size (D ₅₀ ) is at least 25% smaller as determined by a laser diffraction scattering method.

The thermal spray powder having such a configuration is configured so that the average particle diameter ( _D50 ) becomes at least 25% smaller than before thermal spraying due to the thermal energy during thermal spraying. This allows for a finer thermal spray powder to be supplied to the substrate during thermal spraying. In addition, the particle size of the thermal spray powder can be set so as to achieve flowability suitable for thermal spraying.

In a preferred embodiment, the angle of repose of the thermal spray powder disclosed herein is 40 degrees or less. The angle of repose of the thermal spray powder having such a configuration is set to achieve fluidity suitable for thermal spraying.

Another preferred embodiment of the thermal spray powder disclosed herein is composed of oxide ceramic particles. By using a thermal spray powder of this composition, a thermal spray coating made of oxide ceramic can be produced.

In another preferred embodiment of the thermal spray powder disclosed herein, the ceramic particles are granulated and sintered particles of primary particles made of the ceramic. Preferably, the average ratio of the area of open pores to the total area of the surface, based on SEM observation of the surface of the ceramic particles, is 20% or more. In a thermal spray powder of this configuration, the ratio of the area of open pores is within the above range, thereby increasing the degree of thermal contraction of the thermal spray powder. This allows for the supply of finer thermal spray powder to the substrate during thermal spraying.

In another preferred embodiment of the thermal spray powder disclosed herein, the ceramic particles are granulated and sintered particles of primary particles made of the ceramic, and when the surface of the ceramic particles is observed with an SEM, the ratio of the maximum diameter Dmax to the minimum diameter Dmin of the open pores present on the surface (Dmax/Dmin) is 1 to 1.8. A thermal spray powder in which the shape of the open pores observed on the surface of the ceramic particles by SEM observation is specified as described above has an increased degree of thermal contraction. This allows for the supply of finer thermal spray powder to the substrate during thermal spraying.

In another preferred embodiment of the thermal spray powder disclosed herein, the primary particles have an average particle size ( _D50 ) of 0.5 μm or more and 5 μm or less, as measured by a laser diffraction scattering method. This configuration makes it possible to prevent the thermal spray powder containing unmelted portions from being mixed into the thermal spray coating and to prevent defects from occurring in the thermal spray coating, thereby enabling the production of a denser thermal spray coating.

In another preferred embodiment of the thermal spray powder disclosed herein, the bulk specific gravity is 1.0 or less. This configuration further enhances the effect of refining the thermal spray powder through thermal spraying.

The technology disclosed herein also provides a method for producing a thermal spray coating. That is, a method is provided in which any of the thermal spray powders disclosed herein is thermally sprayed onto the surface of an object to be sprayed (substrate) to produce a thermal spray coating. In this method for producing a thermal spray coating, finer thermal spray powder is supplied to the substrate during thermal spraying, making it possible to preferably produce a dense, flat thermal spray coating, for example, with a porosity of 4% or less and a surface roughness (arithmetic mean roughness) Ra of 3.5 μm or less.

FIG. 2 is a schematic diagram illustrating an example of producing a thermal spray coating using a thermal spray powder according to one embodiment. FIG. 1 is a schematic diagram illustrating an example of the production of a thermal spray coating using a conventional thermal spray powder. 1 is a SEM observation image of the surface of the thermal spray powder of Sample 1. 1 is a cross-sectional SEM image of the thermal spray powder of Sample 1. 1 is a SEM observation image of the surface of the thermal spray powder of Sample 2. 1 is a cross-sectional SEM image of the thermal spray powder of Sample 2. 1 is a SEM observation image of the surface of the thermal spray powder of Sample 3. 1 is a cross-sectional SEM image of the thermal spray powder of Sample 3. 1 is an SEM observation image of flying particles of Sample 1. 10 is an SEM observation image of flying particles of Sample 2. 10 is an SEM observation image of flying particles of Sample 3. 1 is a cross-sectional SEM image of the thermal spray coating of Sample 1. 1 is a cross-sectional SEM image of the thermal spray coating of Sample 2. 1 is a cross-sectional SEM image of the thermal spray coating of Sample 3. 1 is a graph showing the surface roughness Ra (μm) of the thermal spray coatings of Samples 1 to 3.

Preferred embodiments of the present invention are described below. Matters necessary for implementing the present invention other than those specifically mentioned in this specification can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and the common general technical knowledge in the relevant field. In the specification, the expression "X to Y" indicating a numerical range means "X or more and Y or less" unless otherwise specified, and it is understood that such a numerical range also includes a range "greater than X and less than Y."

<Definition>
As used herein, the term "thermal spray powder" refers to a powdered material used for thermal spraying. The thermal spray powder disclosed herein is a thermal spray powder material substantially composed of ceramic particles and may contain unavoidable impurities other than the intended ceramic particles (e.g., trace amounts of minute non-ceramic particulate matter).
As used herein, the term "primary particle" refers to the smallest unit that can be identified as a granular material from its appearance among the morphological components that make up the thermal spray powder. When the ceramic particles that make up the thermal spray powder disclosed herein include secondary particles (e.g., granulated particles), the particles that make up the secondary particles can be referred to as primary particles.
Here, "secondary particles" refers to particulate matter (in the form of particles) in which primary particles are bonded three-dimensionally and behave as a single unit. Granulated particles and granulated and sintered particles obtained by sintering after granulation are examples of "secondary particles" as used herein. Note that "bonding" as used herein refers to the direct or indirect bonding of two or more primary particles, and includes, for example, bonding between primary particles due to a chemical reaction, bonding between primary particles that attract each other due to simple adsorption, bonding that utilizes the anchor effect of allowing an adhesive or the like to penetrate into the unevenness of the primary particle surface, bonding between primary particles that utilizes the effect of electrostatic attraction, and bonding in which the surfaces of primary particles are melted and integrated.
In addition, in this specification, the term "raw material particles" refers to particles that constitute the powder in the raw material stage used to produce the thermal spray powder disclosed herein.

<Method for measuring the average particle size of thermal spray powder>
In this specification, the "average particle size ( _D50 )" of the primary particles constituting the thermal spray powder and secondary particles refers to the average particle size at 50% of the integrated value in the volume-based particle size distribution measured using a particle size distribution measuring device based on a laser scattering/diffraction method (50% volume average particle size). In this specification, the "average particle size ( _D50 )" is also sometimes referred to as " _Dv50 ."

<Method for measuring angle of repose>
In this specification, the "angle of repose" refers to the base angle calculated from the diameter and height of a cone-shaped deposit formed by dropping a thermal spray powder from a funnel of a certain height onto a horizontal substrate. The angle of repose can be measured in accordance with the provisions of JIS R9301-2-2:1999 "Methods for measuring physical properties of alumina powder - 2: Angle of repose."

<Method for measuring bulk specific gravity>
In this specification, the term "bulk specific gravity" in relation to a thermal spray powder refers to the density (specific gravity) calculated from the mass of the thermal spray powder when the thermal spray powder flows naturally through an orifice with a diameter of 2.5 mm and fills a container of a predetermined volume in a natural filling state. The bulk specific gravity can be measured in accordance with the provisions of JIS Z2504:2012 "Metal powders -- Measurement method of apparent density."

<Method for measuring granule strength (compressive strength)>
In this specification, the "granule strength" of thermal spray powder is measured using an electromagnetic force loading compression tester. Specifically, a measurement sample is fixed between a pressure indenter and a pressure plate, and a load is applied at a constant increasing rate using electromagnetic force. Compression is performed using a constant load rate compression method, and the amount of deformation of the measurement sample during compression is measured. The measured deformation characteristics of the sample are processed using a dedicated program to calculate the strength value.

<Configuration of thermal spray powder>
The thermal spray powder disclosed herein is a thermal spray powder used to produce a thermal spray coating by a thermal spraying method. As described above, this thermal spray powder is composed of ceramic particles. The proportion of ceramic particles in the thermal spray powder is preferably 95 mass % or more, more preferably 99 mass % or more, and even more preferably 99.9 mass % or more, and can be, for example, 99.99 mass % or more. The type of ceramic may be appropriately selected depending on the composition of the thermal spray coating to be produced. Examples of ceramics include oxide ceramics made of oxides, and non-oxide ceramics such as carbides, borides, nitrides, and apatite.

The oxide ceramic can be any of a variety of metal oxides, without any particular limitations. Metallic or non-metallic elements that make up such oxide ceramics include, for example, semimetallic elements such as boron (B), silicon (Si), germanium (Ge), antimony (Sb), and bismuth (Bi); typical elements such as magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn), aluminum (Al), gallium (Ga), indium (In), tin (Sn), and lead (Pb); scandium (Sc), yttrium (Y), titanium (Ti), and zirconium (Z). Examples of suitable elements include one or more transition metal elements such as Cr, hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), silver (Ag), and gold (Au); and lanthanoid elements such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Er), and lutetium (Lu). Among these, one or more elements selected from Mg, Y, Ti, Zr, Cr, Mn, Fe, Zn, Al, and Er are preferred.

Examples of oxide ceramics include alumina, zirconia, yttria, chromia, titania, cobaltite, magnesia, silica, calcia, ceria, ferrite, spinel, zircon, nickel oxide, silver oxide, copper oxide, zinc oxide, gallium oxide, strontium oxide, scandium oxide, samarium oxide, bismuth oxide, lanthanum oxide, lutetium oxide, hafnium oxide, vanadium oxide, niobium oxide, tungsten oxide, manganese oxide, tantalum oxide, terbium oxide, europium oxide, neodymium oxide, tin oxide, antimony oxide, antimony-containing tin oxide, indium oxide, tin-containing indium oxide, zirconium oxide aluminate, zirconium oxide silicate, hafnium oxide aluminate, hafnium oxide silicate, titanium oxide silicate, lanthanum oxide silicate, lanthanum oxide aluminate, yttrium oxide silicate, titanium oxide silicate, and tantalum oxide silicate.

Examples of non-oxide ceramics include carbide ceramics such as tungsten carbide, chromium carbide, vanadium carbide, niobium carbide, molybdenum carbide, tantalum carbide, titanium carbide, zirconium carbide, hafnium carbide, silicon carbide, and boron carbide; boride ceramics such as molybdenum boride, chromium boride, hafnium boride, zirconium boride, tantalum boride, and titanium boride; nitride ceramics such as titanium nitride, silicon nitride, and aluminum nitride; composites such as forsterite, steatite, cordierite, mullite, barium titanate, lead titanate, lead zirconate titanate, Mn-Zn ferrite, Ni-Zn ferrite, and sialon; and phosphate compounds such as hydroxyapatite and calcium phosphate.

The above ceramics may be doped or substituted with any element. Furthermore, these ceramics may be contained alone or in combination of two or more. For example, when two or more ceramics are contained, some or all of them may form a composite. Examples of such composite ceramics include yttria-stabilized zirconia, partially stabilized zirconia, gadolinium-doped ceria, lanthanum-doped lead zirconate titanate, the above-mentioned sialon, and the above-mentioned composite oxides.

The thermal spray powder disclosed herein is prepared by subjecting the thermal spray powder to the following conditions:
Plasma working gas:
Argon (Ar) gas: 50 psi; and
Helium (He) gas: 50 psi
Plasma output: 36 kW
Supply rate of thermal spray powder: 20 g/min
Spraying distance: 400 mm
The thermal spray powder is characterized in that the average particle size ( _D50 ) of the thermal spray powder is reduced by at least 25% when compared before and after atmospheric plasma spraying in water at 200 K. The degree of thermal shrinkage of the thermal spray powder due to thermal spraying can be evaluated, for example, by the percentage reduction in the average particle size ( _D50 ) of the thermal spray powder (hereinafter also referred to as " _D50 reduction percentage") caused by atmospheric plasma spraying under the above conditions. Note that the value of plasma power may contain a certain amount of error. The value of plasma power is typically an arithmetic mean value.

When the average particle diameter ( _D50 ) of the thermal spray powder before the atmospheric plasma spraying is defined as _D1 and the average particle diameter ( _D50 ) of the thermal spray powder recovered from the water after the thermal spraying is defined as _D2 , the _D50 reduction rate (%) of the thermal spray powder is calculated using the following formula (1):
D ₅₀ reduction rate (%) = (D ₁ - D ₂ )/D ₁ × 100 (1)
The _D50 reduction rate (%) of the thermal spray powder is 25% or more as described above, but is preferably 30% or more, and more preferably 35% or more. The greater the _D50 reduction rate (%), the denser and smoother the thermal spray coating can be produced.

Alternatively, the degree of thermal shrinkage of the thermal spray powder may be evaluated by comparing the _D10 or _D90 of the thermal spray powder before and after atmospheric plasma spraying (APS). For example, the _D10 reduction rate (%) or _D90 reduction rate (%) may be used as an index for such evaluation. D10 refers to the particle size at ₁₀ % of the cumulative total from the smallest particle size in a volume-based particle size distribution measured using a particle size distribution measuring device based on a laser scattering/diffraction method. _D90 refers to the particle size at 90% of the cumulative total from the smallest particle size in a volume-based particle size distribution measured using a particle size distribution measuring device based on a laser scattering/diffraction method.

The _D10 reduction rate (%) of the thermal spray powder is calculated using the following formula (2):
(D ₁₀ before APS −D ₁₀ after APS)/D ₁₀ before APS × 100 (2)
The _D10 reduction rate (%) of the thermal spray powder is preferably 15% or more, more preferably 20% or more, and even more preferably 25% or more. The greater the _D10 reduction rate (%), the denser and smoother the thermal spray coating can be produced.

The _D90 reduction rate (%) of the thermal spray powder is calculated using the following formula (3):
(D ₉₀ before APS −D ₉₀ after APS)/D ₉₀ before APS × 100 (3)
The _D90 reduction rate (%) of the thermal spray powder is preferably 35% or more, more preferably 40% or more, and even more preferably 45% or more. The greater the _D90 reduction rate (%), the more fine the thermal spray powder can be made as a whole, and ultimately the more dense and smooth the thermal spray coating can be produced.

The angle of repose of the thermal spray powder disclosed herein can be 40 degrees or less. The angle of repose is one of the indicators that has traditionally been widely used to indicate the fluidity of powder. The angle of repose of a thermal spray powder can be an indicator that reflects the fluidity of the thermal spray powder within a supply device, during transport from the supply device to the thermal spray device, etc. Therefore, specifying a smaller angle of repose makes it possible to achieve a thermal spray powder with higher fluidity, and ultimately to produce a more uniform thermal spray coating with greater productivity. The angle of repose of the thermal spray powder is preferably 39 degrees or less, more preferably 38 degrees or less, and even more preferably 37 degrees or less. There is no particular lower limit for the angle of repose, but if the angle of repose is too small, the thermal spray powder may be more likely to scatter or it may become difficult to control the supply rate of the thermal spray powder. From this perspective, the angle of repose of the thermal spray powder is preferably 20 degrees or more. Note that the angle of repose referred to here is the angle of repose before the APS.

The thermal spray powder disclosed herein can have a bulk density of 3.0 g/cm ³ or less. For this thermal spray powder, bulk density can be one index for indicating the likelihood of thermal shrinkage during thermal spraying. Therefore, the smaller the bulk density specified, the more easily a thermal spray powder can be realized that is more susceptible to thermal shrinkage, and ultimately, a denser thermal spray coating can be produced. The bulk density of the thermal spray powder is, for example, 2.0 g/cm ³ or less, preferably 1.5 g/cm ³ or less, more preferably 1.2 g/cm ³ or less, and even more preferably 1.0 g/cm ³ or less. Note that if the bulk density is too low, the productivity of the thermal spray coating may decrease. From this perspective, the bulk density of the thermal spray powder is preferably 0.5 g/cm ³ or more. Note that the bulk density referred to here refers to the bulk density before APS.

The granular strength (compressive strength) of the thermal spray powder disclosed herein can be 0.7 kgf/ ^mm2 or more. The granular strength is preferably 0.9 kgf/ ^mm2 or more, and more preferably 1.0 kgf/ ^mm2 or more. Setting the granular strength (compressive strength) within this range can prevent the thermal spray powder from scattering or collapsing when supplied to a thermal spraying device or during thermal spraying, thereby enabling the production of a homogeneous thermal spray coating with higher productivity. On the other hand, if the granular strength is too high, it may be difficult to sufficiently melt the thermal spray powder. From this perspective, the granular strength of the thermal spray powder is appropriately 500 kgf/ ^mm2 or less, preferably 400 kgf/ ^mm2 or less, and more preferably 200 kgf/ ^mm2 or less. Note that the granular strength referred to here refers to the granular strength before APS.

The thermal spray powder disclosed herein may have an average particle size (D ₅₀ ) of 10 μm or more. The average particle size (D ₅₀ ) is preferably 14 μm or more, more preferably 18 μm or more, and even more preferably 22 μm or more. By setting the average particle size (D ₅₀ ) within this range, the flowability of the thermal spray powder can be made suitable for supplying the powder, which in turn enables the production of a uniform thermal spray coating with higher productivity. On the other hand, the average particle size (D ₅₀ ) may be, for example, 55 μm or less, preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 35 μm or less. Setting the average particle size (D ₅₀ ) within this range allows the thermal spray powder to be sufficiently melted during thermal spraying. In one example, it is particularly preferable for the average particle size (D ₅₀ ) to be 20 μm to 30 μm. It is possible to produce a dense thermal spray coating with a smooth surface while achieving the desired flowability for the thermal spray powder. Note that the average particle size (D ₅₀ ) referred to here refers to the average particle size (D ₅₀ ) before APS.

The particle size distribution of the thermal spray powder disclosed herein can be appropriately set depending on the type and conditions of the thermal spraying equipment used to produce the thermal spray coating. For example, the _D90 of the thermal spray powder is preferably set to 60 μm or less (e.g., 50 μm or less). The _D10 of the thermal spray powder is preferably set to 20 μm or less. Note that the _D90 and _D10 referred to here refer to the _D90 and _D10 before APS.

When the ceramic particles constituting the thermal spray powder disclosed herein are secondary particles, the average particle diameter D50 of the ceramic primary particles constituting the secondary particles can be 10 μm or less. The average particle diameter _D50 is preferably 9 μm or less, more preferably 7 μm or less, and even more preferably 5 μm or less. By setting the average particle diameter _D50 within this range, heat from _a heat source can easily reach the center of the primary particles during thermal spraying of the thermal spray powder, thereby preventing the thermal spray powder containing unmelted portions due to insufficient heating from being mixed into the thermal spray coating, and ultimately enabling the production of a denser thermal spray coating. On the other hand, the average particle diameter _D50 is preferably 0.1 μm or more, preferably 0.3 μm or more, and more preferably 0.5 μm or more. Setting the average particle diameter _D50 within this range prevents the occurrence of defects in the thermal spray coating that are thought to be caused by overheating, enabling the production of a denser thermal spray coating.

From the perspective of thermal shrinkage as described above, the thermal spray powder disclosed herein is preferably a granulated and sintered powder. This thermal spray powder is, for example, a thermal spray powder composed of granulated and sintered particles formed by mixing and granulating raw ceramic particle particles (primary particles), followed by sintering, resulting in secondary particles formed by three-dimensionally bonding the primary particles with gaps between them. In other words, the ceramic particles that make up the thermal spray powder can be granulated and sintered particles of primary ceramic particles. Below, we will explain one example of a granulation and sintering method that suitably produces such granulated and sintered particles, but it is not intended to be limited to this method.

<Method of manufacturing thermal spray powder>
The granulation sintering method is a technique in which raw material particles (primary particles) are granulated into the form of secondary particles and then sintered to firmly bond (sinter) the raw material particles together. In this granulation sintering method, granulation can be carried out using a granulation method such as dry granulation or wet granulation. Examples of granulation methods include tumbling granulation, fluidized bed granulation, stirring frame granulation, crushing granulation, melt granulation, spray granulation, and microemulsion granulation. Among these, spray granulation is a preferred granulation method.

Using the spray granulation method, thermal spray powder can be produced, for example, by the following procedure. First, raw material particles with the desired composition are prepared, and their surfaces are stabilized with a protective agent or other agent, if necessary. The stabilized raw material particles are then dispersed in an appropriate solvent, along with spacer particles made of an organic material or other material, and optional binders and various additives (e.g., dispersants), to prepare a spray liquid. Dispersion of the raw material particles in the solvent can be achieved using, for example, a mixer or disperser, such as a homogenizer or blade stirrer. The spray liquid is then atomized using an ultrasonic atomizer or other device to form droplets. These droplets can be entrained in an airflow and passed through a spray dryer to form granulated particles. The resulting granulated particles are then introduced into a designated firing furnace and fired, sintering the raw material particles. In this way, a thermal spray powder composed of granulated and sintered particles in the form of secondary particles, in which primary particles are bonded with gaps, can be obtained. The primary particles may have approximately the same dimensions and shape as the raw material particles, or may be formed by growing and bonding the raw material particles through firing.

To impart a desired thermal shrinkage to the granulated and sintered particles (thermal spray powder) produced in this manner, it is preferable to use spacer particles in the preparation of the spray liquid. In this way, when the droplets are dried in the above-mentioned manufacturing process, the raw material particles, spacer particles, and binder are in a uniform mixture. The raw material particles and spacer particles are bound by the binder to form mixed particles. When these mixed particles are fired, the spacer particles and binder disappear (burn out), and the raw material particles are sintered. In this way, secondary particles are formed in which primary particles are bonded together with gaps. Note that during sintering, some of the raw material particles may become liquid, depending on their composition and size, and contribute to bonding with other particles. As a result, the average particle size of the primary particles may be larger than that of the raw material particles in the starting material. Furthermore, between drying and firing, due to the loss of components other than the raw material particles and the densification of the raw material particles during firing, the average particle size of the resulting secondary particles becomes significantly smaller than the size of the droplets. The average particle size of these secondary particles and primary particles, as well as the size and proportion of the gaps formed between the primary particles, can be designed appropriately according to the desired shape of the secondary particles.

When spacer particles disappear from the mixed particles due to sintering of the granulated particles, pores are created in the mixed particles where the spacer particles were previously present. Therefore, using spacer particles increases the porosity of the granulated and sintered particles. Increasing the porosity of the granulated and sintered particles increases the degree of thermal contraction of the granulated and sintered particles (i.e., the thermal spray powder) during thermal spraying. The porosity of the granulated and sintered particles can be adjusted appropriately by adjusting the amount, particle size, shape, etc. of the spacer particles. For example, the mixing ratio of raw material particles to spacer particles (volume of raw material particles:volume of spacer particles) is preferably 80:20 to 10:90. The higher the mixing ratio (volume ratio) of spacer particles, the higher the porosity of the granulated and sintered particles, resulting in a denser and smoother thermal spray coating. From this perspective, when the total volume of the raw material particles and the volume of the spacer particles is taken as 100% by volume, the volume fraction of the spacer particles is more preferably 30% by volume or more, and even more preferably 40% by volume or more. On the other hand, in order to improve the physical stability and handleability of the thermal spray powder and produce a uniform thermal spray coating with greater productivity, the volume fraction of the spacer particles is appropriately 80% by mass or less, and preferably 70% by volume or less.

The particle size of the spacer particles is, for example, 0.1 μm to 10 μm, and preferably 1 μm to 5 μm. The spacer particles can be any pore-forming material (e.g., resin particles such as acrylic resin particles, epoxy resin particles, polyimide resin particles, polyolefin particles, etc.) used to produce this type of granulated and sintered particles, without any particular restrictions. The shape of the spacer particles is also not particularly limited, but may be, for example, spherical or nearly spherical, plate-like, fibrous, etc.

In addition, in the above manufacturing process, the concentration of raw material particles in the prepared spray liquid is preferably 10% to 50% by mass. The amount of binder added is preferably adjusted to a ratio of 0.05% to 10% by mass (e.g., 1% to 5% by mass) relative to the mass of the raw material particles. Examples of binders that can be added include carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinylpyrrolidone (PVP).

Furthermore, in the above manufacturing process, firing can be carried out in air, vacuum, or an inert gas atmosphere. To remove the spacer particles and binder, firing is preferably carried out in an atmosphere in which oxygen is present. The firing temperature is preferably set to, for example, 600°C or higher and 1600°C or lower. After firing, the produced secondary particles may be crushed and/or classified, if necessary.

SEM observation of the surface of a thermal spray powder composed of granulated and sintered particles produced as described above reveals that the granulated and sintered particles have a ceramic matrix portion (white portion in Figure 3) and open pores (black portion in Figure 3). These open pores are independent of each other and are formed when the spacer particles are burned away during firing from the mixed particles of raw material particles and spacer particles formed in the above-mentioned manufacturing process. As described in the Examples below, the matrix portion and the independent open pore portion of the granulated and sintered particles can be separated using commercially available image analysis software.

The independent open pores may be substantially circular. "Substantially circular" means that the ratio (Dmax/Dmin) of the maximum diameter Dmax to the minimum diameter Dmin of the independent open pores is 1 to 1.8. The ratio (Dmax/Dmin) can be determined by observing the surface of the thermal spray powder with an SEM and analyzing the obtained surface SEM image. For example, five or more (e.g., ten or more) ceramic particles are randomly selected from the surface SEM image, and the ratio (Dmax/Dmin) is calculated for the independent open pores that occupy an area inside an imaginary circle, drawn from the center of each particle and having a diameter equal to half the particle diameter of the particle. Furthermore, it is desirable to exclude minute regions of less than 1 ^μm2 during binarization during the calculation. This allows for a more realistic arithmetic mean value to be obtained. The arithmetic mean value calculated here is used to evaluate the shape of the independent open pores.

The independent open pores may each have approximately the same diameter. "Approximately the same diameter" means, for example, that the Dmax and Dmin used in measuring the ratio (Dmax/Dmin) are both within 20% (preferably within 10%) of the average diameter of the independent open pores. The diameter of the independent open pores may be -20% to +20% of the size of the spacer particles used in the manufacturing process.

An average of 20 or more independent open pores may be present per granulated and sintered particle. The greater the number of independent open pores present on the surface of the granulated and sintered particles, the greater the degree of thermal shrinkage of the thermal spray powder, resulting in a denser and smoother thermal spray coating. The average number of independent open pores per granulated and sintered particle is preferably 30 or more, more preferably 40 or more, and even more preferably 50 or more. Considering the flowability and granular strength of the thermal spray powder, the average number of independent open pores per granulated and sintered particle should be 200 or less (e.g., 150 or less, or 100 or less). The number of independent open pores may be counted for five or more (e.g., 10 or more) ceramic particles randomly sampled from a surface SEM image, and the arithmetic average value calculated.

Alternatively, in a thermal spray powder produced by the above-described production method, the porosity of the surfaces of the granulated and sintered particles may be an average of 20% or more. This porosity refers to the average value of the ratio of the total area of open pores (independent open pores) to the area (projected area) of any number of particles (e.g., 1 to 5 particles per image) that are the subject of analysis in each of a plurality of SEM images of the surfaces of granulated and sintered particles (e.g., 5 to 10 images). Note that, although only approximately half of the particle is observed in a surface SEM image, the area of this half surface and the pore area can be conveniently doubled to determine the area and pore area of the entire particle surface.
Porosity is an index for evaluating the porosity of granulated and sintered particles, and is preferably 25% or more, and more preferably 30% or more. The higher the porosity, the greater the degree of thermal shrinkage of the thermal spray powder, and ultimately the more dense and smooth the thermal spray coating can be produced. On the other hand, taking into consideration the physical stability and handleability of the thermal spray powder, the porosity is, for example, 70% or less, preferably 60% or less, and more preferably 50% or less. Specific methods for measuring porosity are as described in the Examples below.

The mechanism by which the thermal spray powder disclosed herein exhibits a higher degree of thermal shrinkage than conventional methods is described below with reference to Figures 1 and 2. However, the mechanism by which the effects of the thermal spray powder disclosed herein are achieved is not intended to be limited to the following. As shown in Figure 1, when ceramic particles 11 constituting the thermal spray powder disclosed herein are sprayed, the ceramic particles 11 are melted into flying particles 12 by the thermal energy generated during spraying. When the flying particles 12 reach the substrate S, a thermal spray coating L1 is formed. When the ceramic particles 11 melt, the molten ceramic penetrates the gaps P1 (open pores and closed pores). In other words, the volume of the gaps decreases. As a result, the flying particles 12 shrink during spraying compared to before spraying. In this way, the flying particles 12 that have shrunk due to spraying form the thermal spray coating L1, which increases the density of the thermal spray coating L1 and further enhances the smoothness of the surface of the thermal spray coating.

Comparing the thermal spray powder disclosed herein with a conventional example of a thermal spray powder, the conventional ceramic particles 21 shown in Figure 2 also have gaps P2. However, the volume of the gaps in ceramic particles 21 is smaller than the volume of the gaps in ceramic particles 11. Therefore, the degree of thermal contraction of ceramic particles 21 when they become flying particles 22 is smaller than the degree of thermal contraction of ceramic particles 11. Furthermore, compared to ceramic particles 11, the interior of ceramic particles 21 is more solid, and even if the particle surface is melted by thermal spraying, the interior of the particle may reach the substrate S in an unmelted state (unmelted particles 23). For these reasons, it is believed that the density and surface smoothness of the thermal spray coating L1 produced by thermal spraying ceramic particles 11 are greater than those of the thermal spray coating L2 produced by thermal spraying ceramic particles 21.

The thermal spray powder disclosed herein can be sprayed by various thermal spraying methods to produce thermal spray coatings on various substrates. The thermal spray powder disclosed herein is particularly suitable for producing thermal spray coatings by plasma spraying methods such as atmospheric plasma spraying (APS), low-pressure plasma spraying (LPS), and high-pressure plasma spraying. This thermal spray powder can also be suitably used for high-velocity flame thermal spraying, such as high-velocity oxygen flame (HVOF) thermal spraying, warm spray thermal spraying, and high-velocity air flame (HVAF) thermal spraying. The thermal spray powder may be supplied to the thermal spraying device in powder form, or in the form of a slurry dispersed in an appropriate dispersant.

There are no particular restrictions on the type of substrate used to produce the thermal spray coating. Examples of substrates include aluminum, aluminum alloys, iron, steel, copper, copper alloys, nickel, nickel alloys, gold, silver, bismuth, manganese, zinc, and zinc alloys. Among these, preferred examples include widely used metallic materials such as steel, including various SUS materials (which may be so-called stainless steels) used as corrosion-resistant structural steel; aluminum alloys, including 1000 series to 7000 series aluminum alloys useful as lightweight structural materials; and Ni-, Co-, or Fe-based corrosion-resistant alloys, including Hastelloy, Inconel, Stellite, and Invar.

The following describes examples of the present invention, but it is not intended that the present invention be limited to the examples shown below.

<Production Example>
[Sample production]
(Sample 1)
As the raw material powder, yttrium oxide powder (primary particles) having an average particle diameter _D50 of 0.9 μm and yttrium oxide powder (primary particles) having an average particle diameter _D50 of 3.0 μm were mixed to prepare a mixed powder. As the spacer particles, acrylic resin spacer particles (hereinafter also referred to as "acrylic particles") having a particle size (particle diameter) of approximately 3 μm (MX-300, manufactured by Soken Chemical & Engineering Co., Ltd.) were prepared. The mixed powder was blended at 70 vol% and 30 vol% of acrylic particles, and the blend was dispersed in an aqueous solution containing 2 wt% of a binder to prepare a spray liquid. In the spray liquid, the content of yttrium oxide particles was 90.7 wt%, and the content of acrylic particles was 9.3 wt%. This spray liquid was sprayed into an airflow using a spray granulator and dried to produce granulated particles. The resulting granulated particles were subjected to a firing treatment at 1600°C to sinter the primary particles, and then crushed and classified to produce a thermal spray powder composed of granulated and sintered particles (secondary particles). This thermal spray powder was designated Sample 1 of this Production Example. The average particle diameter _D50 of Sample 1 was 27 μm.

(Sample 2)
The blend ratio was 40% by volume of the mixed powder and 60% by volume of acrylic particles. In the spray liquid, the content of yttrium oxide particles was 73.5% by weight, and the content of acrylic particles was 26.5% by weight. Except for this, the thermal spray powder of Sample 2 was produced using the same materials and process as Sample 1. The average particle diameter _D50 of Sample 2 was 27 μm.

(Sample 3)
The raw material powder used was yttrium oxide powder (primary particles) with an average particle size _D50 of 2.4 μm. Acrylic particles were not used. The thermal spray powder of Sample 3 was manufactured using the same materials and process as Sample 1. The average particle size _D50 of Sample 3 was 27 μm.

(Sample 4)
A fused and pulverized powder of yttrium oxide was prepared as Sample 4. First, raw material powders were blended so as to obtain the desired yttrium oxide (Y ₂ O ₃ ). The raw material powder was heated to melt it, and then cooled to prepare a solidified product (ingot). This solidified product was pulverized by mechanical means and classified as necessary to obtain the fused and pulverized powder of Sample 4. The average particle diameter D ₅₀ of Sample 4 was 29 μm.

[Measurement of particle size distribution]
The volumetric particle size distribution of the thermal spray powders of Samples 1 to 4 was measured using a laser diffraction/scattering particle size analyzer (Malvern Panalytical, Mastersizer 3000). The values of D _v10 , D _v50 , and D _v90 are shown in the corresponding columns in Table 1.

[Angle of repose]
The angle of repose of the thermal spray powders of Samples 1 to 4 was measured in accordance with JIS R9301-2-2:1999. The angle of repose was a value obtained by subjecting each thermal spray powder to an A.B.D. powder property measuring instrument (Model ABD-72, manufactured by Tsutsui Rikakikai Co., Ltd.). The corresponding column in Table 1 shows the measured angle of repose value.

[Measurement of bulk density]
The bulk density (g/cm ³ ) of the thermal spray powders of Samples 1 to 4 was measured in accordance with JIS Z2504:2012. The bulk density (g/cm ³ ) was obtained by subjecting each thermal spray powder to a JIS bulk density measuring instrument for metal powders (manufactured by Tsutsui Scientific Instruments Co., Ltd.). The measured bulk density (g/cm ³ ) values are shown in the corresponding columns in Table 1.

[SEM Observation]
Planar and cross-sectional images of the thermal spray powders of Samples 1 to 4 were obtained using a benchtop SEM (Phenom ProX, manufactured by Phenom-World). The observation magnification was 5000x. For reference, Figures 3 to 8 show surface SEM observation images and cross-sectional SEM observation images of Samples 1 to 3. The scale bars in Figures 3 to 8 indicate 10 μm.

[Particle surface porosity]
The ratio of the area of open pores to the total area of the particle surface (porosity (%) of the particle surface) was measured for the thermal spray powders of Samples 1 to 4. The surface SEM images of each sample obtained in the above [SEM observation] were binarized by specifying only the particle portion of the surface SEM image using image analysis software (Image-Pro Plus, manufactured by Nippon Roper Co., Ltd.). Specifically, a contrast threshold was determined, and white areas were designated as the matrix (yttrium oxide) portion, and black areas were designated as open pores. The ratio of the area of open pores was then calculated when the area (projected area) subject to image analysis was taken as 100%. This measurement was performed on 10 randomly selected ceramic particles. The obtained arithmetic average value was taken as the particle surface porosity (%) for each thermal spray material. The measured particle surface porosity (%) values are shown in the corresponding columns in Table 1.

As shown in Figures 3 to 8 and Table 1, which show the results of this manufacturing example, open and closed pores were observed in the thermal spray powders (Samples 1 and 2) obtained by granulating and sintering using a spray liquid containing yttrium oxide particles and a specified amount of spacer particles (acrylic particles). These powders were found to have open and closed pores due to the loss of spacer particles during sintering. Furthermore, the porosity of the particle surfaces of Samples 1 and 2 was 20% or greater, and this porosity increased depending on the amount of spacer particles. Meanwhile, the particle size distributions of Samples 1 to 3, which were composed of granulated and sintered particles, were similar regardless of whether spacer particles were used. Furthermore, the angles of repose of Samples 1 to 4 were all 40 degrees or less, confirming that they had fluidity suitable for thermal spraying. Furthermore, it was confirmed that the bulk density of the thermal spray powder was reduced by using spacer particles to produce granulated and sintered particles.

<Creating flying particles>
The thermal spray powders of Samples 1 to 4 produced in the above Production Examples were subjected to atmospheric plasma spraying (APS) in water under the following thermal spraying conditions to produce flying particles of each sample.
[Thermal spraying conditions]
Thermal spraying machine: SG-100 (manufactured by Praxair)
Powder feeder: Model 1264 (manufactured by Praxair)
Plasma working gas:
Argon (Ar) gas (50 psi (0.34 MPa)); and
Helium (He) gas (50 psi (0.34 MPa))
Plasma output: 36 kW
Plasma generation voltage: 40V
Plasma generation current: 900A
Supply rate of thermal spray powder: 20 g/min
Spraying distance: 400 mm

To produce flying particles, first, Ar gas (primary gas) and He gas (secondary gas) were supplied to the thermal spray gun at atmospheric pressure as plasma working gases, and a voltage was applied between the cathode and anode to generate plasma. The plasma generation conditions during thermal spraying were as described above. The plasma generation current was set to 900 A, and the plasma generation voltage accordingly fluctuated between 38-42 V, resulting in a plasma output that could vary between 34-38 kW. Each sample of thermal spray powder was supplied to this plasma using the powder feeder, and then the powder was shot into cold water. The plasma irradiation angle relative to the water surface was 90 degrees. The "spraying distance" mentioned above for producing flying particles refers to the distance from the tip of the thermal spray gun to the water surface.

As described above, the water was collected after the spray powder was sprayed using atmospheric plasma spraying. The spray powder (ceramic particles) contained in the collected water was dried to form the flying particles for each sample. For reference, Figures 9 to 11 show SEM images (1000x magnification) of each sample. The scale bars in Figures 9 to 11 indicate 80 μm.

[Measurement of particle size reduction rate]
The volumetric particle size distribution of the flying particles of each example was measured in the same manner as in the measurement of the particle size distribution of the thermal spray powder in the above Production Examples. The percentage reduction in each particle size due to APS was calculated for each sample of thermal spray powder using _{the Dv10} , _Dv50 , and _Dv90 of the flying particles and the _Dv10 , _Dv50 , and Dv90 of the thermal spray powder before APS. Specifically, the percentage reductions in _Dv10 , _Dv50 , and _Dv90 before and after APS were calculated based on the above formulas (1) to (3), and were defined as the _Dv10% reduction percentage (%), _Dv50% reduction percentage (%), and _Dv90% reduction percentage (%). The corresponding columns in Table 2 show the values of _Dv10 , _Dv50 , and _Dv90 after APS, as well as the percentage reductions in each particle size.

As shown in Table 2, it was confirmed that the _Dv10 , _Dv50 , and _Dv90 of the ceramic particles decreased before and after APS in all samples (except for _Dv10 in Sample 4). When comparing thermal spray powders composed of granulated and sintered particles produced using spacer particles (Samples 1 and 2) with thermal spray powders not using spacer particles (Samples 3 and 4), it was confirmed that the particle size reduction rate (%) in Samples 1 and 2 was greater than that in Samples 3 and 4.

<Preparation of thermal spray coating>
Thermal spray coatings were produced by APS using the thermal spray powders of Samples 1 to 4 produced in the above Production Example. In this example, the substrate was an aluminum alloy (Al6061) plate (70 mm × 50 mm × 2.3 mm) whose surface was roughened by blasting with brown alumina abrasive (A#40). The equipment, plasma generation conditions, and thermal spray powder supply rate used in this example were the same as those in the "Production of Flying Particles" section above. In this example, the thermal spray powder of each sample was supplied into the plasma generated by the thermal spray gun, and the thermal spray gun was moved at a speed of 400 mm/sec, with the plasma irradiation angle relative to the substrate set to 90 degrees, to produce the thermal spray coatings of Samples 1 to 4. The thermal spray distance was set to 90 mm to 130 mm. The "spraying distance" used in the production of thermal spray coatings refers to the distance from the tip of the thermal spray gun to the substrate. For reference, Figures 12 to 14 show cross-sectional SEM images (observation magnification: 1000x) of the thermal spray coatings of Samples 1 to 3. The scale bars in Figures 12 to 14 indicate 80 µm.

[Film formation rate]
The coating rate (μm/pass) in the production of the above thermal spray coatings was calculated using the thermal spray powders of Samples 1 to 4. The coating rate (μm/pass) is the thickness (μm) of the thermal spray coating produced in one thermal spraying operation (1 pass) performed by a thermal spraying device (thermal spray gun) along the direction of travel of the thermal spraying device. Representative coating rates (μm/pass) for each sample are shown in the corresponding column in Table 3. The values listed in the "Spraying Distance" column in Table 3 are the spraying distances at which measurements of the coating rate, the density described below, and the surface roughness described below were obtained.

[Denseness]
The density of the thermal spray coatings of Samples 1 to 4 formed as described above was evaluated by measuring the porosity (%) of the thermal spray coating. The porosity was determined by image analysis of cross-sectional SEM images taken approximately perpendicular to the substrate. First, the thermal spray coating was cut together with the substrate perpendicular to the substrate surface, and an arbitrary cross section in the thickness direction was extracted. The cross-sectional SEM image of the thermal spray coating was analyzed using image analysis software (Image-Pro Plus, manufactured by Nippon Roper Co., Ltd.) to perform binarization to separate the pores and solid phase, and the porosity (%), defined as the ratio of the pore area to the total cross-sectional area, was calculated. The representative porosity (%) of each sample is shown in the corresponding column in Table 3.

[Surface roughness measurement]
The surface roughness (arithmetic mean roughness) Ra of the thermal spray coatings of Samples 1 to 4 was measured in accordance with JIS B0601:2013. The surface roughness Ra was determined by measuring the surface roughness at five random points on each thermal spray coating using a surface roughness measuring instrument "SV-3000S CNC" (manufactured by Mitutoyo Corporation) and calculating the arithmetic mean value of these measurements. Representative surface roughness Ra (μm) for each sample is shown in the corresponding column of Table 3. For reference, FIG. 15 shows the surface roughness Ra (μm) of thermal spray coatings produced by spraying the thermal spray powders of Samples 1 to 3 at a spraying distance of 90 mm or 100 mm. The vertical axis "Ra/μm" in the graph of FIG. 15 represents the surface roughness Ra (μm). The horizontal axis "spraying distance/mm" in the graph of FIG. 15 represents the spraying distance (mm). In the graph of FIG. 15, in each section divided by spraying distance, the left bar is Sample 1, the center bar is Sample 2, and the right bar is Sample 3.

As shown in Table 3, the coating rate was similar when using the thermal spray powder of each sample, regardless of the thermal spray powder preparation method. Furthermore, when comparing thermal spray coatings prepared using thermal spray powders prepared using spacer particles (Samples 1 and 2) with thermal spray coatings prepared using thermal spray powders that did not use spacer particles (Samples 3 and 4), it was confirmed that the surface roughness Ra (μm) of the thermal spray coatings of Samples 1 and 2 was smaller than that of Samples 3 and 4. Furthermore, the porosity (%) of the thermal spray coating of Sample 2 was smaller than that of the thermal spray coatings of the other samples, indicating that the use of spacer particles can contribute to the production of thermal spray coatings with flatter and denser surfaces.

As is clear from the above results, it was found that the use of thermal spray powders (Samples 1 and 2) that contain ceramic particles and that, when compared before and after atmospheric plasma spraying in water under specified conditions, have an average particle size ( _D50 ) that is at least 25% smaller than that before spraying, enables the supply of finer particles to the substrate, thereby producing a thermal spray coating. The use of such thermal spray powders makes it possible to produce thermal spray coatings with smoother surfaces. Furthermore, such thermal spray powders are suitable for producing denser thermal spray coatings.

11, 21 ceramic particles 12, 22 flying particles 23 unmelted particles L1, L2 thermal spray coating S substrate

Claims (10)

A thermal spray powder comprising:
It is composed of ceramic particles,
The thermal spray powder was subjected to the following conditions:
Plasma working gas:
Argon (Ar) gas: 50 psi; and
Helium (He) gas: 50 psi
Plasma output: 36 kW
Supply rate of thermal spray powder: 20 g/min
Spraying distance: 400 mm
when comparing the average particle size (D ₅₀ ) of the thermal spray powder after atmospheric pressure plasma spraying in water with the value before the spraying, the average particle size (D 50 ) determined by a laser diffraction scattering method is reduced by at least 25% ;
the ceramic particles are granulated and sintered particles of primary particles made of the ceramic,
A thermal spray powder characterized in that the average ratio of the area of open pores to the total area of the surface, based on SEM observation of the surface of the ceramic particles, is 20% or more . A thermal spray powder comprising:
It is composed of ceramic particles,
The thermal spray powder was subjected to the following conditions:
Plasma working gas:
Argon (Ar) gas: 50 psi; and
Helium (He) gas: 50 psi
Plasma output: 36 kW
Supply rate of thermal spray powder: 20 g/min
Spraying distance: 400 mm
When comparing the average particle diameter (D) of the thermal spray powder based on a laser diffraction scattering method after atmospheric pressure plasma spraying in water with that before the thermal spraying, ₅₀ ) is reduced by at least 25%;
the ceramic particles are granulated and sintered particles of primary particles made of the ceramic,
the ratio (Dmax/Dmin) of the maximum diameter Dmax to the minimum diameter Dmin of open pores present on the surface is 1 to 1.8 when the surfaces of the ceramic particles are observed with an SEM. A thermal spray powder comprising:
It is composed of ceramic particles,
The thermal spray powder was subjected to the following conditions:
Plasma working gas:
Argon (Ar) gas: 50 psi; and
Helium (He) gas: 50 psi
Plasma output: 36 kW
Supply rate of thermal spray powder: 20 g/min
Spraying distance: 400 mm
When comparing the average particle diameter (D) of the thermal spray powder based on a laser diffraction scattering method after atmospheric pressure plasma spraying in water with that before the thermal spraying, ₅₀ ) is reduced by at least 25%;
A thermal spray powder having an angle of repose of 40 degrees or less. 3. The thermal spray powder according to claim 1, having an angle of repose of 40 degrees or less. the ceramic particles are granulated and sintered particles of primary particles made of the ceramic,
4. The thermal spray powder according to claim 2 , wherein the average ratio of the area of open pores to the total area of the surface of the ceramic particles, based on SEM observation of the surface, is 20% or more. the ceramic particles are granulated and sintered particles of primary particles made of the ceramic,
4. The thermal spray powder according to claim 1, wherein, when the surfaces of the ceramic particles are observed with an SEM, the ratio (Dmax/Dmin) of the maximum diameter Dmax to the minimum diameter Dmin of open pores present on the surfaces is 1 to 1.8. 7. The thermal spray powder according to claim 1, wherein the primary particles have an average particle size (D ₅₀ ) of 0.5 μm or more and 5 μm or less , as determined by a laser diffraction scattering method. The thermal spray powder according to any one of claims 1 to 7, which is composed of oxide ceramic particles. The thermal spray powder according to any one of claims 1 to 8 , having a bulk specific gravity of 1.0 or less. A method for producing a thermal spray coating by thermally spraying the thermal spray powder according to any one of claims 1 to 9 onto the surface of a substrate.