Method of producing coatings by heat spraying.

Abstract

Provided is a method for forming a thermal spray coating in which a coating is formed by impacting a non-oxide ceramic material on a substrate by a high-speed flame spraying. A slurry 11 is prepared by dispersing, in a solvent, a non-oxide ceramic material having an average particle diameter of 0.1 to 5.0 μm and having, in a particle size distribution of a powder material, one or more peaks in each of the following: the range of at least 0.1 μm and less than 1.0 μm and a range of at least 1.0 μm and less than 10.0 μm. The thermal spray coating with fine and dense coating structure is formed by externally feeding the slurry 11 to a flame 10 that is ejected from a thermal spray gun 2.

Description

Method for forming thermal spray coating

[0001] The present invention relates to a method for forming a thermal spray coating, in which a dense thermal spray coating made of a non-oxide ceramic material is formed on a substrate by a high-velocity flame spraying method.

[0002] To improve the functionality of structural surfaces, various thermal spray coatings are commonly formed on the surfaces of structural components. Thermal spraying is a surface treatment technology in which a thermal spray material such as metal, ceramic, or cermet is supplied into a flame generated by combustion gas or a plasma arc, softened or molten, and then sprayed at high speed onto the surface of the object to be sprayed, thereby forming a thermal spray coating on the surface.

[0003] Although a wide variety of materials can be used in thermal spraying, the high-temperature heating and melting process can cause evaporation and oxidation of the spray material during the process, and a high-quality coating cannot be obtained unless the thermal spraying conditions are carefully selected to suit the material being used. In particular, it is generally considered more difficult to select the thermal spraying conditions for non-oxide ceramics such as aluminum nitride than for other materials, and various studies have been conducted in the past.

[0004] Patent Document 1 describes a coating production method for forming an aluminum nitride coating on a substrate using a detonation thermal spraying device having a combustion tube, a gas supply means for supplying fuel gas or the like, an ignition means for igniting the fuel gas mixture, and a powder supply means. In this document, aluminum nitride powder with an average particle size of 1 μm to 5 μm is used as the supplied powder, which is granulated to have a particle size of 20 μm to 60 μm.

[0005] Patent Document 2 describes a film formation method in which an aluminum nitride thermal spray coating is formed on a substrate by atmospheric plasma spraying while adjusting the temperature and flight speed of aluminum nitride powder.

[0006] Patent Document 3 describes a method for forming a coating on a semiconductor manufacturing equipment part by continuously depositing nitride powder particles on a substrate without melting them.

[0007] Patent Document 4 describes a method in which a raw material powder containing, as a main component, particles of a metal nitride that has sublimability and no molten phase is dispersed in an organic solvent to prepare a slurry, and the slurry is flame sprayed under predetermined spraying conditions to form a coating on the surface of a substrate.

[0008] JP 2017-71835 A JP 2009-235558 A International Publication No. 2010 / 027073 JP 2014-198898 A

[0009] A common problem in the above-mentioned documents 1 to 4 is that if the size of the spray material is too large, the particles will not melt, making it difficult to form a film, and even if a film can be formed, it is difficult to obtain a dense film or a film that has sufficient adhesion to the substrate. Also, if the size of the spray material is too small, oxidation of the particles will proceed excessively, making it difficult to obtain a coating with the required composition.

[0010] In the method described in Patent Document 1, in which aluminum nitride powder with an adjusted average particle size is used to form a film using an explosive thermal spraying device, the average particle size of the material used is large, so the material does not melt sufficiently to form a film, or even if a film can be formed, it does not become a dense film.

[0011] In the deposition method of Patent Document 2, which deposits aluminum nitride by atmospheric plasma spraying, the flame temperature generated by the plasma heat source is so high that aluminum nitride sublimes. Also, the addition of rare earth metal ceramics is essential to improve density.

[0012] Patent Document 3 describes that 90% or more of the nitride powder particles in the formed thermal spray coating are deposited unmelted, and that this was achieved by modifying the thermal spray nozzle of the ultra-high speed flame thermal spraying equipment, but does not describe what specific modifications were made.

[0013] In Patent Document 4, powder of metal nitride particles having a particle size of about 0.5 to 3 μm is used, and unless the thermal spraying conditions are set with extremely high precision, as described above, oxidation of the particles proceeds excessively, making it difficult to obtain a coating with the required composition.

[0014] In view of the problems of the prior art, the present invention aims to provide a method for forming a thermal spray coating that can produce a dense coating with high adhesion, even when non-oxide ceramics are used as the material.

[0015] The inventors investigated a method for forming a thermal spray coating by colliding a non-oxide ceramic material onto a substrate, and by adopting a high-velocity flame spraying method using a material with a predetermined average particle size and particle size distribution, they succeeded in forming a dense coating with high adhesion, thereby solving the problem.

[0016] That is, the method for forming a thermal spray coating of the present invention is a method for forming a thermal spray coating by colliding a non-oxide ceramic material onto a substrate using a high-velocity flame spraying method, wherein the average particle size of the non-oxide ceramic material is 0.1 to 5.0 μm, and the particle size distribution of the non-oxide ceramic material has one or more peaks in the range of 0.1 μm or more and less than 1.0 μm, and in the range of 1.0 μm or more and less than 10.0 μm.

[0017] The present invention employs a high-velocity flame spraying method, which prevents excessive oxidation of the non-oxide ceramic material during the spraying process and allows for the production of a thermal spray coating primarily composed of non-oxide ceramics. Here, "primarily composed of non-oxide ceramics" means that non-oxide ceramics account for the largest proportion of the constituent components of the thermal spray coating, by mass. Furthermore, the present invention also employs a non-oxide ceramic material with a smaller average particle size than typical thermal spray materials, including both relatively large and relatively small particle groups. Specifically, the average particle size of the non-oxide ceramic material is 0.1 to 5.0 μm, and the particle size distribution of the non-oxide ceramic material has one or more peaks in the range of 0.1 μm to less than 1.0 μm and the range of 1.0 μm to less than 10.0 μm. Even when a high-velocity flame spraying method is employed, slight oxidation progresses from the outer periphery of the particles when thermal spraying is performed in an oxygen-containing environment (e.g., in air). In this case, particles in the range of 0.1 μm or more and less than 1.0 μm are mostly oxidized during the thermal spraying process, while particles in the range of 1.0 μm or more and less than 10.0 μm are only partially oxidized and are not oxidized as a whole. When these materials are formed into a coating, the particles in the range of 0.1 μm or more and less than 1.0 μm act as a binder that binds together particles in the range of 1.0 μm or more and less than 10.0 μm. In other words, when using a non-oxide ceramic material with a small average particle size, by including a certain amount of relatively large particles and a certain amount of relatively small particles, the relatively small particles function as a binder that binds together the relatively large particles, resulting in a dense coating with high adhesion.

[0018] The volume ratio of the non-oxide ceramic material having a particle size of 1.0 μm or more and less than 10.0 μm to the material having a particle size of 0.1 μm or more and less than 1.0 μm is preferably 60% or more, and more preferably 90% or less, in order to obtain a denser coating with stronger adhesion.

[0019] The non-oxide ceramic material is preferably supplied to the flame as a suspension dispersed in a solvent. By forming a film by such a suspension high-velocity flame spraying method, aggregation of the spray material during transportation can be suppressed, and a dense coating can be more reliably formed.

[0020] The suspension is preferably supplied to a flame that is sprayed from the tip of a thermal spray nozzle. High-velocity flame spraying using an internal supply system is prone to spitting, in which the spray material accumulates inside the nozzle and the deposits are ejected as clumps. By contrast, using an external supply system, in which the suspension is supplied from the tip of a thermal spray nozzle to a flame that is sprayed, can prevent spitting.

[0021] The non-oxide ceramic material may be one containing one or more ceramics selected from the group consisting of carbide ceramics, nitride ceramics, and boride ceramics. These non-oxide ceramics are harder than oxide ceramics, but are generally difficult to form by thermal spraying. The method for forming a thermal spray coating of the present invention can form a dense coating with high adhesion even from these materials, resulting in a hard, high-quality coating.

[0022] In the present invention, a non-oxide ceramic spray material is used that has an average particle size of 0.1 to 5.0 μm and a particle size distribution that has one or more peaks in a predetermined range of particle sizes smaller than 1.0 μm and in a predetermined range of particle sizes larger than 1.0 μm. By spraying this material with high velocity flame spraying, the particles in the predetermined small particle size range act as a binder that binds together particles in the predetermined large particle size range, thereby obtaining a dense coating with high adhesive strength.

[0023] FIG. 1 is a schematic diagram of the main parts of a thermal spraying device for carrying out a high velocity flame thermal spraying method used in a method for forming a thermal sprayed coating. FIG. 2 is a graph showing the particle size distribution of titanium carbide powder having a single-peak and a double-peak particle size distribution. FIG. 3 is a photograph showing the results of film formation properties. FIG. 4 is a graph showing the particle size distribution of aluminum nitride powder having a double-peak particle size distribution. FIG. 5 is a table showing the relationship between the surface roughness of a substrate and adhesion strength. FIG. 6 is a table showing an image of cross-sectional structure observation and the coating components. FIG. 7 is an image of cross-sectional structure observation showing the bonding state between particles in the coating.

[0024] An embodiment of the present invention will be described. A high-velocity oxygen-fuel (HVOF) thermal spraying method is used as a method for forming a thermal spray coating in this embodiment. A thermal spray coating is formed by colliding a thermal spray powder onto a substrate using the high-velocity oxygen-fuel (HVOF) thermal spraying method. The high-velocity oxygen-fuel (HVOF) thermal spraying method uses the combustion energy of combustion gas as a heat source. A supersonic flame is generated by increasing the pressure in a combustion chamber, and the thermal spray powder is supplied to the center of the supersonic flame jet flow, accelerated, brought to a molten or semi-molten state, and continuously sprayed at high speed.

[0025] The molten spray particles collide with the substrate at supersonic speed, allowing for the formation of a dense, highly adhesive thermal spray coating. The continuous formation of the thermal spray coating results in a homogeneous thermal spray coating. The combustion gas used as the heat source is a combustible gas, such as hydrogen or a carbon-hydrogen-based gas such as acetylene, ethylene, or propane, or an oxygen-containing combustion-supporting gas. Liquid fuels, such as kerosene, may be used instead of the combustible gas.

[0026] Specifically, a mixed gas such as oxygen / propane, oxygen / propylene, oxygen / natural gas, oxygen / ethylene, or oxygen / hydrogen is used as the combustion gas, a supersonic flame is generated with a flame velocity of 900 to 2500 m / sec and a flame temperature of 1800 to 3800°C, the spraying distance is maintained at 100 to 350 mm, and the substrate temperature during spraying is controlled to 200°C or less, thereby enabling thermal spraying.

[0027] The substrate is not limited and may be a metal, ceramic, polymer material, or the like. Specific examples of metallic materials include a metal selected from Fe, Cr, Ni, Al, Ti, and Mg, or an alloy containing one or more elements selected from Fe, Cr, Ni, Al, Ti, and Mg. Such metallic materials are formed by extrusion, cutting, plastic processing, or forging. The substrate may be a metallic material on which a coating is formed by welding, plating, or thermal spraying. An undercoat may be provided between the substrate and the thermal spray coating.

[0028] The thermal spray material used is a non-oxide ceramic material, which includes one or more ceramics selected from the group consisting of carbide ceramics, nitride ceramics, and boride ceramics.

[0029] Specific examples include carbide ceramics, nitride ceramics, boride ceramics, and mixtures thereof, each containing one or more elements selected from the group consisting of Ni, Cr, Co, Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr, Fe, Hf, and La.

[0030] Carbide ceramics include TiC, WC, TaC, B ４ C, SiC, HfC, ZrC, VC, Cr ３ C ２ Examples of nitride ceramics include TiN, CrN, CrN, TaN, AlN, BN, and Si. ３ N ４ , HfN, NbN, YN, ZrN, Mg ３ N ２ , Ca ３ N ２ Examples of boride ceramics include TiB ２ , ZrB ２ , HfB ２ , V.B. ２ , TaB ２ , NbB ２ , W ２ B ５ , CrB ２ , LaB ６ Examples include:

[0031] 1 is a schematic diagram of the essential parts of a thermal spraying apparatus 1 for carrying out the high velocity oxygen flame thermal spraying method used in the method for forming a thermal spray coating according to this embodiment. This thermal spraying apparatus 1 is configured as an apparatus for suspension HVOF thermal spraying, in which the spray material is supplied from the outside in the form of a slurry (suspension). The thermal spraying apparatus 1 is of an external supply type, in which a slurry in which the spray powder is dispersed in a solvent is supplied from the outside, and is equipped with a thermal spraying gun 2 and a slurry supply nozzle 3.

[0032] The thermal spray gun 2 has a combustion vessel section 5 that forms a combustion chamber 4, a thermal spray nozzle 6 that is continuous with the combustion vessel section 5, and an ignition device 7. A gas containing high-pressure oxygen and fuel is supplied to the combustion chamber 4, and the gas is ignited by the ignition device 7. The flame generated in the combustion chamber 4 is once constricted by the thermal spray nozzle 6 and then expanded to become a supersonic flame, which is then sprayed at high speed from the tip of the thermal spray nozzle 6. A slurry 11 is supplied from the slurry supply nozzle 3 to the sprayed flame 10. The thermal spray powder in the slurry 11 becomes molten or semi-molten, is accelerated by the flame 10, and collides with the substrate 100 at high speed, forming a thermal spray coating on the substrate 10.

[0033] The slurry 11 is prepared by dispersing the thermal spray powder in an organic solvent containing a dispersion medium made of water or alcohol and an organic dispersant. The slurry 11 contains 5 to 40% by mass of the thermal spray powder particles. The slurry 11 is supplied to a flame 10 that is sprayed from the tip of the thermal spray nozzle 6.

[0034] In an internal supply system where the slurry is supplied from inside the spray nozzle, the spray material may accumulate inside the nozzle tube and be ejected as a clump, resulting in spitting. In contrast, in this embodiment, as shown in Figure 1, an external supply system is used where the slurry 11 is supplied from the outside to the frame 10, which prevents spitting.

[0035] The non-oxide ceramic material used as the thermal spray powder has an average particle size of 0.1 to 5.0 μm, and the particle size distribution of the non-oxide ceramic material has one or more peaks in the range of 0.1 μm or more to less than 1.0 μm and in the range of 1.0 μm or more to less than 10.0 μm. That is, there is one or more mountain-shaped peaks in the particle size distribution in the range of 0.1 μm or more to less than 1.0 μm, and there is one or more mountain-shaped peaks in the particle size distribution in the range of 1.0 μm or more to less than 10.0 μm. The average particle size of the particles is defined as the particle size (median diameter) at which the cumulative value reaches 50% when the particle size distribution is measured by a laser diffraction / scattering method (Microtrac method).

[0036] Two or more peaks may be present in the range of 0.1 μm or more to less than 1.0 μm and the range of 1.0 μm or more to less than 10.0 μm. A typical example is a non-oxide ceramic material having one peak in the range of 0.1 μm or more to less than 1.0 μm and one peak in the range of 1.0 μm or more to less than 10.0 μm. Another example is a non-oxide ceramic material having multiple peaks in the range of 0.1 μm or more to less than 1.0 μm and multiple peaks in the range of 1.0 μm or more to less than 10.0 μm.

[0037] It is necessary that a considerable number of particles of the non-oxide ceramic material have particle sizes in the range of 0.1 μm or more and less than 1.0 μm, and also a considerable number have particle sizes in the range of 1.0 μm or more and less than 10.0 μm. Furthermore, the volume ratio of the particles of the non-oxide ceramic material having particle sizes in the range of 1.0 μm or more and less than 10.0 μm to the particles having particle sizes in the range of 0.1 μm or more and less than 1.0 μm is preferably 60% or more, and more preferably 90% or less.

[0038] Particles with a particle size ranging from 0.1 μm to less than 1.0 μm are very small, and therefore oxidize upon contact with the air during thermal spraying, with many of them becoming oxides. By setting the average particle size of the thermal spray powder made of a non-oxide ceramic material to 0.1 to 5.0 μm and setting the particle size distribution to have one or more peaks in a predetermined range of particle sizes smaller than 1.0 μm and in a predetermined range of particle sizes larger than 1.0 μm, the particles in the predetermined small particle size range, most of which become oxides, are given a binder function that binds together particles in the predetermined large particle size range. The small particle size particles fill in the gaps between the larger particle sizes, joining them together. This makes it possible to obtain a very dense coating.

[0039] Furthermore, if the volume ratio of non-oxide ceramic materials with particle sizes in the range of 1.0 μm or more and less than 10.0 μm to materials with particle sizes in the range of 0.1 μm or more and less than 1.0 μm is 60% or more, preferably 90% or less, the interparticle bonding strength is significantly increased, making it possible to form a denser coating with stronger adhesiveness. These volume ratios can be calculated by comparing the areas of the particle size distributions measured by laser diffraction / scattering method (Microtrac method).

[0040] Generally, if there is a considerable amount of powder with a particle size of about 0.1 to 1.0 μm, the fluidity of the thermal spray powder decreases, and there is a risk that stable supply will not be possible. In contrast, in this embodiment, the coating is formed using a suspension high-velocity flame spraying method in which the spray material is supplied in the form of a slurry, so the thermal spray powder can be transported in a state where aggregation is suppressed, and a stable supply of the thermal spray powder is possible. Generally, when non-oxide ceramics are sprayed, if a large amount of particles with a particle size of nearly 10.0 μm are included, there is a risk that the coating will become excessively porous and the quality of the coating will deteriorate. However, in this embodiment, the small particle size particles act as a binder, making it possible to form a high-quality, dense thermal spray coating.

[0041] The thickness of the thermal spray coating obtained by the above-mentioned method for forming a thermal spray coating is preferably in the range of 50 to 2000 μm, and this thickness is set appropriately depending on the intended use. Generally, if the thickness is 50 μm or more, the uniformity of the coating can be maintained and the coating function can be fully exerted, and if the thickness is 2000 μm or less, a decrease in mechanical strength due to the influence of residual stress inside the coating can be prevented.

[0042] The porosity of a ceramic thermal spray coating should be approximately 0.1 to 5%, but the porosity of the thermal spray coating obtained by the method for forming a thermal spray coating of this embodiment can also be less than 0.1%, although this depends on the particle size distribution of the thermal spray powder. If the porosity is high, it may lead to a decrease in mechanical strength or, for example, when used in a gas atmosphere, may make it easier for gas to penetrate into the coating. The coating formation conditions may be set appropriately depending on the substrate, raw material powder, film thickness, manufacturing environment, etc.

[0043] The following describes examples in which a coating was actually formed based on the present invention.

[0044] The relationship between material powder size and film-forming properties was investigated using two types of titanium carbide powder with different particle size distributions. Two types of titanium carbide powder (Material A and Material B) were used, each adjusted to the particle size distribution shown in Figure 2. One type of titanium carbide (Material A) has only one peak in the range of 1 to 10 μm, while the other type of titanium carbide (Material B) has one peak in the range of 0.1 to 1.0 μm and one peak in the range of 1.0 to 10.0 μm.

[0045] The average particle size of material A is 3.7 μm, and the average particle size of material B is 2.4 μm. The volume ratio of material A having a particle size in the range of 1.0 μm or more and less than 10.0 μm to material A having a particle size in the range of 0.1 μm or more and less than 1.0 μm is 100%. The volume ratio of material B having a particle size in the range of 1.0 μm or more and less than 10.0 μm to material B having a particle size in the range of 0.1 μm or more and less than 1.0 μm is 74%.

[0046] Each titanium carbide powder was suspended in water to form a slurry, and this material was used to form a coating on a stainless steel substrate by suspension HVOF thermal spraying. Figure 3 is a photograph showing the results of coating formation. SD in the table is the spraying distance (mm). It was found that even when powders with similar average particle sizes were used, almost no coating was formed with Material A, which has a unimodal particle size distribution, whereas a coating could be formed with Material B, which has a bimodal particle size distribution.

[0047] Next, the relationship between material powder size and film formability was investigated using two types of aluminum nitride powder (Materials C and D), each having a bimodal particle size distribution as shown in Figure 4. The average particle diameter of Material C was 1.8 μm, and the average particle diameter of Material D was 1.4 μm. In Material C, the volume ratio of material having a particle size in the range of 1.0 μm or more and less than 10.0 μm to material having a particle size in the range of 0.1 μm or more and less than 1.0 μm was 83%, and in Material D, the volume ratio of material having a particle size in the range of 1.0 μm or more and less than 10.0 μm to material having a particle size in the range of 0.1 μm or more and less than 1.0 μm was 70%.

[0048] Each aluminum nitride powder was suspended in alcohol to form a slurry, and a test was conducted in which this material was used to form a film on a stainless steel substrate by suspension HVOF thermal spraying. It was found that all materials could be used to form a film. Therefore, a new coating sample was prepared using material C, and coating evaluations were performed, including a tensile test to investigate the relationship between the substrate surface roughness and adhesion, cross-sectional structure observation, porosity measurement, coating component analysis, and electrical property investigation.

[0049] To investigate the relationship between the surface roughness of the substrate and adhesion, several stainless steel substrates were prepared for tensile testing, each adjusted to a desired surface roughness by blasting. Figure 5 is a table showing the relationship between the substrate's surface roughness and adhesion. Regardless of the substrate's surface roughness Ra, and regardless of whether or not blasting was performed as a pretreatment, all samples had sufficient adhesion. Furthermore, some of these coatings had very smooth surfaces with a surface roughness Ra of 1.0 μm or less.

[0050] Figure 6 shows an image of one of the cross-sectional structures and a table showing the coating composition. The abundance ratios (mass%) of each component in the coating were N: 23.52, O: 17.58, and Al: 58.89, indicating that nitrides and oxides were present in a balanced manner. The coating hardness was Hv 472, the thermal conductivity was 7.4 W / m K, the porosity was 0.1%, the breakdown voltage was 135 kV / mm, and the volume resistivity was 5.2 × 10 １３ This indicates that the thermal spray coating formed in this example has a dense coating structure and exhibits high electrical insulation properties.

[0051] The coating structure was observed under magnification using an FE-SEM. Figure 7 shows an image of the cross-sectional structure observed using an FE-SEM. An oxide layer was formed at the boundaries of the aluminum nitride particles, and this served as an adhesive layer. In other words, while the material is primarily nitride, the uniform and random presence of nitrides and oxides without any significant bias is an important factor in forming a dense thermal spray coating with high adhesive strength.

[0052] The methods for forming a thermal spray coating in the above embodiments and examples are illustrative and not restrictive. The methods for forming a thermal spray coating may include other steps depending on the object on which the thermal spray coating is to be formed and the application mode. The configurations and steps described in the above embodiments can be modified as long as the effects of the present invention are not impaired, and the forms of other configurations and steps that may be provided as needed are not limited.

[0053] REFERENCE SIGNS LIST 1 Thermal spraying device 2 Thermal spraying gun 3 Slurry supply nozzle 4 Combustion chamber 5 Combustion vessel part 6 Thermal spraying nozzle 7 Ignition device 10 Frame 11 Slurry 100 Substrate

Claims

1. The thermal spray coating method consists of a process of spraying a non-oxide ceramic material onto a workpiece to produce a coating by a high-speed fuel spraying process, where the average particle size of the non-oxide ceramic material is 0.1 to 5.0 microns and the particle size distribution of the non-oxide ceramic material consists of one or more peaks in each of: particle size ranges of 0.1 microns or more and less than 1.0 microns; and particle size ranges of 1.0 microns or more and less than 10.0 microns.

2. The thermal spray coating method according to claim 1, where the volume ratio of non-oxide ceramic material with particle sizes in the range of 0.1 microns or more and less than 1.0 microns to material with particle sizes in the range of 1.0 microns or more and less than 10.0 microns is 60% or more.

3. The thermal spray coating method according to claim 1 or 2, where a suspension of non-oxide ceramic material dispersed in a solvent is fed to a flame. 4.

5. Any of the thermal spray coating production methods under Patents 1-4 in which the non-oxide ceramic material consists of one or more ceramic materials selected from a group comprising ceramic carbide, ceramic nitride, and ceramic boride;