Spray powder for highly porous coatings

Abstract

The present invention relates to a powder comprising solid ceramic particles, characterized in that the particles have an internal porosity of at least 30%, preferably at least 45%, preferably at most 65%, and that the powder has a d10 grain size of at least 100 μm.

Description

[Technical Field]

[0001] The present invention relates to powders for thermal spraying. [Background technology]

[0002] Atmospheric plasma spraying (APS) of powder is a well-known and established method for producing coatings.

[0003] Powders as thermal spray materials make it possible to produce coatings for different applications. The coatings produced in this way are dense or porous depending on the application. The porosity of the coatings ranges from less than 5% by volume for dense coatings to 30% by volume for highly porous coatings. In this state-of-the-art coating, spray powders with particle sizes in the range of 5 to 200 μm are used.

[0004] Specialized applications such as ceramic abradable coatings for aircraft turbines or industrial gas turbines require coatings with porosity greater than 30% by volume.

[0005] Abradable coatings are made from materials that easily shear when rubbed against a stronger component. The former wears away, while the latter does not experience significant damage. Abradable coatings are used in the compressor and turbine sections of jet engines and stationary gas turbines, where they offer a technique for minimizing clearances between the casing and rotating parts, such as the turbine blades. This tighter seal improves overall efficiency. Abradable coatings not only enable a tighter seal, but also offer the ability to adjust clearances in-situ to withstand physical events and thermal mismatch scenarios during operation.

[0006] To produce ceramic coatings with porosity greater than 30% by volume, the state of the art is to use a mixture of spray powder and pore formers, both of which are simultaneously deposited by a thermal spray process. The pore formers are volatile materials that, after decomposition, leave macropores in the applied coating. Pore formers can be, for example, polymers, graphite, nitrides, and other volatile materials. The use of such mixtures requires an additional burn-off step to ensure that all pore formers are gone and that the final coating is ready for use as an abradable coating.

[0007] A drawback of known methods for producing state-of-the-art powders and highly porous coatings is their relatively low deposition efficiency (DE) of less than 30% due to the additional step of burning off the pore former and the compromise of spray parameters when simultaneously depositing raw materials with very high melting points (ceramic) and very low melting points (pore former). The low DE requires the use of much more spray powder to achieve the same coating thickness as for standard porous coatings. This is a waste of raw material and requires the development of recycling methods for the lost powder, along with additional equipment and energy effort.

[0008] The general principles of APS coatings for abrasive applications are disclosed, for example, in US Pat. No. 5,434,210A.

[0009] Many attempts have been made in the past to produce highly porous coatings without the addition of pore formers.

[0010] For example, U.S. Patent No. 8,486,520 B2 describes the production of highly porous coatings by spraying particles with high internal porosity in the particle size range of 53 to 160 μm produced by agglomerated ceramic nanoparticles. While the coatings have nanostructured porous microzone areas of 10 to 80%, the achieved porosity is in the range of 15 to 20% by volume, far below the optimal range of 30% or more by volume required for abradable coatings.

[0011] A further example is disclosed in EP 2444514A1, which describes the production of a low-density porous metal coating for use as a seal on a gas turbine engine component. The coating is formed by depositing a low-density porous abradable metal sealing layer on the component. The sealing layer is formed by co-depositing metal sponge particles and metal precursor particles, which decompose during a post-deposition heat treatment, leaving a metal wall with micropores. In this case, a metal precursor that must be post-heat treated is used to create the porous coating. The coating exhibits porosity of 40 to 80% by volume, but the principle is similar to the standard pore former method, which also requires post-heat treatment to achieve the desired porosity.

[0012] A further example is disclosed in European Patent No. 2514850B1, which describes the production of a thermal barrier coating. The coating is formed by thermal spraying of spray particles having a particle size distribution in which the 10% cumulative particle size is greater than or equal to 30 μm and less than or equal to 100 μm. The porosity of the coating is 16% by volume, which is far below the optimal range of 30% by volume or more required for abradable coatings.

[0013] WO 2022 / 197827 claims, as an example, a porosity of 30% or more, and, as another example, both the use of fugitives and the absence of fugitives. No experimental data is given.

[0014] All of the described state-of-the-art spray powders require either the addition of pore formers and / or an additional post-heat treatment to produce coatings with a porosity of more than 30% by volume. The disadvantages of this method of producing highly porous coatings are: · Low deposition efficiency of the spray powder / pore former mixture used. The need for additional raw materials to produce a coating with a defined thickness. · The need for post-heat treatment to produce a ready-to-use coating with high porosity. · Increased time and cost to produce highly porous coatings with porosity greater than 30% by volume.

[0015] Another state-of-the-art process is described in the Journal of Thermal Spray Technology, January 2016, in a paper titled "Microstructure and Properties of Porous Abradable Alumina Coatings Flame-Sprayed with Semi-Molten Particles." Here, porosity is created by particles that are fused, crushed, and then plasma-spheronized. Due to the manufacturing process, the particles are dense internally. Therefore, the process, not the particles, is the primary reason for the porosity created. During flame spray deposition, a solid core creates a skeleton with small interparticle pores. Melt from the partially molten shell more or less fills the skeleton. As more material melts and fills, porosity decreases. The advantage of this process is that it allows the use of readily available dense particles. The disadvantage is that it only works with very low spray distances (less than 60 mm) and small particles with a narrow particle size distribution range. In the paper, particles with diameters of 40 to 50 μm were used. Small variations result in unfused particles, which further lead to poor bonding and very low deposition efficiency. The use of particles in such a small range results in very low yields and high manufacturing costs.

[0016] Furthermore, another publication (Curry et al.) described the performance characterization of highly porous thermal barrier coatings produced by high-power plasma spray processing. The results of the study showed that low-density powders with specific gravities less than 1.6 g / cc achieved high-porosity transfer to the final coating at very high feed rates of up to 280 g / min. While deposition efficiencies similar to conventional coating processes for thermal barrier coatings were achieved, coatings with less than 30% porosity were only obtained (Curry N, Leitner M, Koerner K. High-Porosity Thermal Barrier Coatings from High-Power Plasma Spray Equipment—Processing, Performance and Economics. Coatings. 2020; 10(10):957).

[0017] WO 2022 / 0214553 A1 discloses a suspension comprising solid ceramic particles and a solvent, wherein the suspension has a fines fraction of 0.5 or less. The density of the ceramic particles is between 3.0 and 7.0 g / cm. 3 and the particles have a volume-based d in the range of 2 μm to 10 μm. 50 It is characterized by having a value.

[0018] WO 20222 / 14556 A1 discloses a suspension for suspension thermal spraying, comprising solid ceramic particles having an average particle size of 2 μm or less and a liquid phase comprising an organic solvent. The organic solvent is characterized by a flash point of at least 60°C and at most 400°C. The suspension has a viscosity of less than 20 mPa·s, and the concentration of the solid ceramic particles in the suspension ranges from 5% to 95% by weight.

[0019] Further state of the art is known from Guo et al., 2017, Materials and Design, 139, 343-350, US Patent Application Publication No. 2017 / 166485 A1, WO 2018 / 169753 A1, Roy et al., 2005, Powder Technology, 157, 1-3, 20-16 and US Patent Application Publication No. 2013 / 202912 A1. [Prior art documents] [Patent documents]

[0020] [Patent Document 1] U.S. Patent No. 5,434,210A [Patent Document 2] U.S. Patent No. 8,486,520B2 [Patent Document 3] European Patent No. 2444514A1 [Patent Document 4] European Patent No. 2514850B1 [Patent Document 5] International Publication No. 2022 / 197827 [Patent Document 6] International Publication No. 2022 / 0214553A1 [Patent Document 7] International Publication No. 20222 / 14556A1 [Patent Document 8] U.S. Patent Application Publication No. 2017 / 166485Al [Patent Document 9] International Publication No. 2018 / 169753Al [Patent Document 10] U.S. Patent Application Publication No. 2013 / 202912Al [Non-patent literature]

[0021] [Non-Patent Document 1] “Microstructure and Properties of porous Abradable Alumina Coatings flame-sprayed with semi-molten particles” in the Journal of Thermal Spray Technology January 2016 [Non-patent document 2] Curry N, Leitner M, Koerner K. High-Porosity Thermal Barrier Coatings from High-Power Plasma Spray Equipment-Processing, Performance and Economics. Coatings. 2020; 10 (10): 957 [Non-patent document 3] Guo et al., 2017, Materials and Design, 139, 343-350 [Non-patent document 4] Roy et al., 2005, Powder Technology, 157, 1-3, 20-16 Summary of the Invention [Problem to be solved by the invention]

[0022] The object of the present invention is to overcome the drawbacks of state of the art spray powders and methods for producing highly porous coatings with porosity levels above 30% by volume. [Means for solving the problem]

[0023] The object is to provide a powder comprising solid ceramic particles, the particles having an internal porosity of at least 30%, preferably at least 45%, preferably at most 65%, and the d of the powder 10 The problem is solved by a powder comprising solid ceramic particles, characterized in that the particle size is at least 100 μm.

[0024] The present invention is particularly useful for thermal spraying, preferably atmospheric plasma spraying.

[0025] A coated substrate can be obtained by coating a substrate with a powder according to the invention, the resulting coating having a porosity of at least 30% by volume. [Brief explanation of the drawings]

[0026] [Figure 1] Figure 1a shows an example scheme of standard-sized particles (50 μm diameter). Figure 1b shows a cross-sectional SEM image of a coating produced from standard-sized particles. Figure 1c shows a cross-sectional SEM image of a coating produced from standard-sized particles with fully molten particles and pores around the particles (white arrows). Figure 1d shows an example scheme of coarse powder-sized particles (130 μm diameter) of the present invention. Figure 1e shows a cross-sectional SEM image of a highly porous coating produced from coarse powder-sized particles of the present invention. Figure 1f shows a cross-sectional SEM image of a highly porous coating produced from an atomized powder of the present invention with partially molten particles (also referred to as "semi-molten splats") having an outer dense, molten shell with a porous core. The image shows the exterior (white arrow) and internal pores of the semi-molten splats. Figure 1g shows a cross-sectional SEM image of a highly porous coating produced from an atomized powder of the present invention, focusing on the pores distributed inside the semi-molten splats (white arrows). [Figure 2a] 1 shows an SEM image at 1000x magnification of a portion of a cross section of an inventive coarse powder-sized particle of ytterbium zirconate according to Examples 37 to 40. [Figure 2b] 10 shows further SEM images at 1000x magnification of a portion of a cross section of an inventive coarse powder-sized particle of yttrium zirconate according to Examples 48 to 51. [Figure 3]Cross-sectional SEM image of a coating made from a state-of-the-art powder with a sieving cut of 38 μm. PSD: d10-43 μm; d50-90 μm; d90-190 μm. The average porosity was 12 vol% (magnification: 100×, accelerating voltage: 15.0 kV, WD: 12.2 mm). [Figure 4] Cross-sectional SEM image of a coating made from a state-of-the-art powder with a sieve cut of 63 μm. PSD: d10-43 μm; d50-90 μm; d90-190 μm. The average porosity was 19 vol% (magnification: 100×, accelerating voltage: 15.0 kV, WD: 13.2 mm). [Figure 5] Figure 1 shows a cross-sectional SEM image of a coating made from an atomized powder of the present invention with a 100 μm sieve cut. PSD: d10-107 μm; d50-162 μm; d90-251 μm. The average porosity was 35% by volume (magnification: 100x, accelerating voltage: 15.0 kV, WD: 11.4 mm). [Figure 6] 1 is a schematic representation of a manufacturing process for producing a powder according to the present invention. [Figure 7] 1 shows a cross-sectional SEM image of a coating produced from an atomized powder of the present invention having partially molten particles with an outer dense molten shell (magnification: 500x, accelerating voltage: 15.0 kV, WD: 11.3 mm). [Figure 8] An overview of the powder PSD d50, the spray distance SD in mm, and the resulting porosity values ​​of all the examples disclosed below with respect to the sieves used is given. The numbers in square brackets represent the sieve size, and the numbers in parentheses represent the different test series. Examples according to the invention are marked by a square. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention provides a powder for atmospheric plasma spraying (APS), which has an internal porosity of at least 30% by volume and 10 It comprises solid ceramic particles having a particle size greater than 100 μm.

[0028] The present invention makes it possible to transfer the internal porosity of particles to coatings produced by state-of-the-art thermal spray processes. The much coarser particle size and significantly higher internal porosity of the powders of the present invention compared to prior art powders are the primary factors for producing this highly porous coating. The time that the large particles spend in the plasma flame is long enough to melt the particle's shell, yet the internal porosity remains. This remaining internal porosity is the primary cause of coating porosity, and therefore must exceed 30% by volume to obtain coatings with porosity of 30% or more by volume.

[0029] The particles of the present invention are agglomerates of fine so-called primary particles.

[0030] The top two micrographs in Figure 1 show particles (standard and in accordance with the invention), respectively, which are agglomerates of fine primary particles.

[0031] In the following, when the term "particle" is used without further characterization, it refers to an agglomerate of primary particles.

[0032] As used herein, the term "particle size" refers to the particle size distribution determined by 10 refers to the value. 50 The value is known as the median diameter or median of the particle size distribution determined from a volumetric representation or from a number representation. x The values ​​refer to a volumetric expression, i.e., particle diameter at "x" volume % in the cumulative distribution (e.g., d of at least 100 μm). 10 means that 10% by volume of the particles have a diameter smaller than 100 μm).

[0033] In the present invention, particle size distribution (PSD) was determined using laser diffraction particle analysis, preferably using a "CILAS" particle analyzer. The particle sizes of the samples described in this patent were measured with a CILAS 1190 particle analyzer in wet mode, and the solvent was deionized water. The measurements were performed without ultrasound to avoid particle destruction during the measurements. The PSD measurement range of the CILAS 1190 was 0.04 μm to 2500 μm, and the analysis mode was Fraunhofer.

[0034] In such particle analyzers, a laser beam is scattered by the particles of the sample, the angle of light scattering being inversely proportional to the particle size. The particle size distribution is typically determined at intervals ("boxes").

[0035] According to the invention, it is particularly advantageous to minimize the volume fraction of particles having a particle size of 100 μm or less in order to obtain a highly porous coating.

[0036] Other instruments exhibiting a sensitivity and measurement range comparable to that of the CILAS 1190 may be utilized to measure particle size distributions in accordance with the present invention. Those skilled in the art will be aware of suitable instruments that are commercially available.

[0037] In the present invention, the porosity of the coating was measured using an image analysis method, in which 20 images (using SEM) at 500x magnification were taken along with the cross-section of the coating to fully depict the microstructure.

[0038] The images were then processed using freely available image analysis software (ImageJ 1.52a, National Institutes of Health, USA). The images were converted to binary format and an automatic binarization technique (intermodes) was applied to avoid operator-induced errors. The software was able to detect the amount of pores and calculate the pore area, which was then compared to the total area of ​​the SEM, and the value was output as a volume percentage (vol%). The values ​​reported in this document are the average values ​​of the measured SEM images.

[0039] The internal porosity of the particles according to the invention was measured in the same way, but at a magnification of 1000 (SEM). Figures 2a and 2b show two sample images of the cross section of a particle according to the invention measured at that magnification.

[0040] The internal porosity or particles of the powders of the present invention may range from 30% to 65% by volume, preferably from 45% to 60% by volume, and most preferably from 45% to 55% by volume.

[0041] As is apparent from the above and from the figures, the internal porosity of the particles refers to the porosity inside the particles, rather than the bulk porosity of the majority of the particles.

[0042] The internal porosity of the particles according to the invention is expressed in pores / mm 2 It can also be expressed as its pore density in units of .

[0043] Pore ​​density can be measured by image analysis methods, based on eg 5 sample images at eg 1000x magnification, again using the same software, to determine the number of pores per area inside the particle.

[0044] The particles of the present invention have 20,000 to 200,000 pores / mm 2 , preferably 30,000 to 180,000 pores / mm 2 The pore density is

[0045] Both the above values ​​of internal porosity as well as pore density indicate a very uniform distribution of numerous pores inside the particle.

[0046] Deposition efficiency (DE) values ​​were determined in accordance with DIN EN ISO 17836 2018-03 on annular specimens with a diameter of 300 mm and a height of 100 mm. Deposition efficiency is therefore the ratio, expressed as a percentage, calculated by dividing the weight of the applied coating by the weight of the raw material powder used to apply the coating.

[0047] As used herein, the term "ceramic" refers to an inorganic, non-metallic solid material, including metals or non-metals in the form of oxides, nitrides, or carbides. Ceramic materials are brittle, hard, and strong in compression, but weak in shear and tension. They resist the chemical attack that occurs in other materials exposed to acidic or caustic environments. Ceramics can generally withstand high temperatures, ranging from 1000°C to 1600°C.

[0048] In another embodiment, said solid ceramic particles comprise or consist of an oxide of at least one of a transition metal, or a rare earth metal, or a metal from Group 13 or 14 of the periodic table, or a mixture thereof.

[0049] As used herein, the term "transition metal" refers to a transition metal element or a mixture thereof, i.e., two or more transition metal elements. According to IUPAC, a transition metal is an element whose atom has an incomplete d subshell or is capable of giving rise to a cation due to an incomplete d subshell. However, for purposes of this specification and claims, the term "transition metal" shall include only the elements of Groups 4-11 of the periodic table, plus Zn.

[0050] As used herein, the term "rare earth metal" refers to the chemically similar metallic elements of Group 17, which include the lanthanides, yttrium, and scandium. The lanthanides are defined as the set of elements having atomic numbers 57 to 71, all of which, except for promethium, occur naturally (Extractive Metallurgy of Rare Earths, C.K. Gupta, N. Krishnamurthy, CRC).

[0051] When the solid ceramic particles comprise or consist of a mixture of transition metal oxides and rare earth metal oxides, the oxides are preferably ytterbium zirconate (YbZrO, "YbZr") or gadolinium zirconate (GdZrO, "GdZr") or yttria-stabilized zirconia (YSZ).

[0052] The solid ceramic particles may also comprise or consist of oxides of rare earth metals, such as yttrium oxide (yttria, Y2O3).

[0053] When the solid ceramic particles comprise or consist of an oxide of a metal of group 13 of the periodic table, the oxide is preferably aluminum oxide (alumina, Al2O3).

[0054] When the solid ceramic particles comprise or consist of an oxide of a metal of group 14 of the periodic table, the oxide is preferably a silicate mineral, such as a rare earth silicate, including silicon dioxide (silica, SiO2).

[0055] Preferably, the rare earth silicate may be yttrium monosilicate (YMS), yttrium disilicate (YDS), ytterbium monosilicate (YbMS), ytterbium disilicate (YbDS), yttrium-ytterbium monosilicate (YYbMS), yttrium-ytterbium disilicate (YYbDS), or mixtures and / or complexes thereof.

[0056] As used herein, the term "mixture" refers to a material made of at least two components that are not chemically bonded to each other, so that each component material retains its chemical properties and composition. A mixture is a physical combination of components that are intermixed together in the form of solutions, suspensions, and colloids.

[0057] As used herein, "complex" means a combination of at least two components that are chemically bonded to each other.

[0058] Preferably, the powder of the invention consists essentially of said ceramic particles, which in particular means that the powder is essentially free of pore formers or the like.

[0059] According to the present invention, the internal porosity of the particles is important: as mentioned above, the internal porosity of the particles must be at least 30% or more.

[0060] During the plasma spray process, the shell of the particle melts, creating a dense area at the edge of the particle that is preserved in the coating. For example, Figure 1f shows a partially melted particle (right) of a spray powder of the present invention that has a dense outer molten shell with a porous core, and a particle-free area around the melted particle.

[0061] However, it has surprisingly been found that the interactions between the large gaps between the large particles, due to their large size together with the internal porosity of the particles, result in obtaining coatings with porosity of 30% by volume or more.

[0062] It can be seen that the coating produced from particles of standard size (see Figures 1a and b) is much denser than the coating derived from particles of the spray powder of the present invention (see Figures 1d and e).

[0063] Furthermore, a cross-sectional SEM image of a standard-sized particle shows a completely molten particle (Fig. 1c). In contrast, an SEM image of a coating made from the spray powder of the present invention (Fig. 1f) shows a partially molten particle with an outer dense molten shell with a porous core (=fine porosity) and particle-free areas around the molten particle (=coarse porosity).

[0064] Furthermore, Figures 1f and 1g show that the coatings produced from the powders of the present invention are characterized by a uniform distribution between semi-molten splats and pores within the semi-molten splats (white arrows, Figure 1g). As used herein, the term "splat" refers to resolidified, molten, or partially melted ceramic particles. The semi-molten splats form a stable skeleton due to the formation of sinter bridges between the primary particles.

[0065] Figure 7 shows the macropores between the larger particles, along with the micropores within the particles. In contrast, other publications have described the presence of more abundantly distributed spherical pores within the coating, lacking the larger interstices between particles with fine internal porosity (Curry N, Leitner M, Korner K. High-Porosity Thermal Barrier Coatings from High-Power Plasma Spray Equipment - Processing, Performance and Economics. Coatings. 2020;10(10):957).

[0066] The process for producing the powder according to the invention comprises the following steps: a) providing a powder comprising primary solid ceramic particles having a particle size in the range of 0.5 to 10 μm, more preferably 1 to 5 μm; b) spray drying said powder by adding at least one organic binder to said powder; c) sintering the spray-dried powder at a temperature of 1300°C to 1600°C for 20 to 30 hours; d) the sintered powder has a d of at least 160 μm 50 and having a particle size distribution having e) sieving said powder through at least one sieving step in which a 100 μm sieve is used; Includes.

[0067] Thus, the powder according to the invention is also characterized in that it can be obtained by the process described above and further below.

[0068] The size and internal porosity of the spay powder are primarily influenced by the spray drying process provided in the process disclosed above.

[0069] To obtain the desired internal porosity, the primary particle diameter (d 50It is important to use powders having a particle size of preferably 0.5 to 10 μm, more preferably 1 to 5 μm, and particularly preferably 1.5 to 2.5 μm.

[0070] If the particle size is too fine, the internal porosity is reduced due to the sintering effect that occurs on the fine, high surface area particles.

[0071] If the primary particles are too coarse, the contact area between the particles will be reduced, leading to insufficient particle adhesion after spray drying, which can lead to the collapse of the spray-dried particles and an insufficient proportion of fine particles being lost after sieving.

[0072] Also, if the primary particle size is too coarse, the powder will contain only a few coarse particles, resulting in low porosity between them. Furthermore, coarse, dense primary particles will produce a harder coating. However, low hardness is important for applications as an abradable coating. If the coating is too hard, the tip of the blade may be damaged when cutting into the abradable coating.

[0073] Spray drying (step (b)) is important for forming agglomerates by causing the fine primary particles to stick together.

[0074] During spray drying, an organic binder can be used. When particles come into contact and coalesce during drying, a liquid bridge is formed, resulting in a solid bridge after the water evaporates. However, when low-water-soluble or water-insoluble particles must be agglomerated, a binder can be used to form a solid bridge between the particles of the powder. Before adding the binder, the powder can be mixed with a solvent, preferably deionized water. The binder is added to the suspension as an aqueous solution. The type of binder has a significant impact on agglomeration properties such as friability, density, porosity, bulk flow, and size distribution.

[0075] The binder may be selected from commonly known materials such as the group consisting of polyvinyl alcohol, polyethylene glycol, polyacrylate, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, hydroxyethyl cellulose, and any mixture thereof.

[0076] The binder may be burned off during sintering.

[0077] The sintered powder is essentially free of binders or the like.

[0078] Particle size can be affected by interactions between powders and heated air during the spray drying process. Process parameters such as the temperature difference between the dryer inlet and outlet, the amount of drying air, the humidity of the drying air, the solids content of the suspension, the nozzle diameter, and the air-to-suspension ratio in the case of a two-fluid bottom nozzle are also important for achieving the desired particle size.

[0079] The internal porosity can be influenced by the type of binder, the solids content of the suspension, the viscosity of the suspension and the sintering temperature.

[0080] Two-fluid bottom nozzles can atomize suspensions using gas (air or nitrogen). They consist of two concentric tubes, with the suspension supplied by the inner tube and the atomizing gas supplied by the outer tube. The major advantages of bi-material nozzles are their low pressure operation (approximately 1.5-10 bar) and the narrow droplet size distribution, depending on the nozzle design. The average droplet size can depend primarily on the ratio of the atomizing gas and suspension flow rates. As the pressure and, therefore, the atomizing gas flow rate increase, the droplet size becomes smaller. The same is true when the flow rate of the spray liquid decreases. Furthermore, the viscosity and surface tension of the suspension can play an important role in determining the droplet size generated.

[0081] The sintering temperature may depend on the material used and may have to be adjusted. The lower the melting temperature of the material, the lower the sintering temperature must be selected. If the sintering temperature is selected too high, the pores of the particles will partially close during sintering, resulting in a decrease in internal porosity. Adjustment of these parameters is within the knowledge of one skilled in the art.

[0082] The spray dried powder may be sintered at a temperature of 1200-1600°C for 20 to 30 hours.

[0083] As an example, zirconia must be sintered at higher temperatures in the range of 1400 to 1500°C, while silicates must be sintered at lower temperatures of 1200 to 1300°C.

[0084] Furthermore, the sintering parameters need to be optimized to create strong bonds between the primary particles to avoid fracture during plasma spraying, while avoiding particles that are too hard and dense. If the sintering temperature is too high, the porosity will be reduced due to the sintering effect, and the sprayed particles will not have enough internal porosity to produce a highly porous abradable coating.

[0085] For ceramic particles selected from the group consisting of YSZ, YbZr, and GdZr, a sintering temperature of 1400° C. to 1500° C. has been found to be useful.

[0086] Higher dumping heights of materials require longer sintering times and vice versa.

[0087] According to a specific embodiment, the sintering temperature can be achieved by a continuous heating rate of 50-200°C per hour. The heating rate is an important parameter during sintering. If the heating rate is too high, the organic portion of the particles will be released too quickly, resulting in particle destruction. This effect will reduce the yield of particles with a particle size greater than 100 μm.

[0088] There may be no hold time between heating periods until the desired temperature is reached. The sintered powder is then 10 The particle size distribution may be from 100 μm to 110 μm.

[0089] The sintered powder has a d of at least 160 μm. 50 The particle size distribution may have a d of at least 160 μm. 50 A particle size distribution having the following formula is preferred to obtain a coarse thermal spray powder after sieving.

[0090] The powder may be sieved by at least one sieving step, in which a 100 μm sieve is used to remove particles in the range below 100 μm. The sieving step e) may be carried out both before and after the sintering step c).

[0091] An additional sieving step can avoid fine particles less than 100 μm in the powder, which can lead to dense fusion zones between larger particles.

[0092] The PSD of the powder is preferably measured after sintering, at which point if fine particles are still present a sieving step is required, or a further sieving step if one was already performed before sintering.

[0093] The powders of the present invention can be used to produce coatings with a porosity of more than 30% by volume by thermal spraying, preferably by atmospheric plasma spraying, without the need for post-heat treatment or the addition of pore formers to produce highly porous coatings. Those skilled in the art will be aware of suitable spraying parameters to obtain such coatings.

[0094] According to the present invention, the powder can be used to produce a coating on any suitable substrate, such as a metal, ceramic, polymer, ceramic matrix composite, metal alloy, or mixture or composite thereof. The term "ceramic matrix composite" refers to a composite material comprising ceramic fibers embedded in a ceramic matrix. For example, the substrate can be made of steel, preferably stainless steel, aluminum, or a superalloy.

[0095] In another embodiment, the coated substrate can be obtained by coating said substrate with the powder described herein.

[0096] In certain embodiments, the coating on the substrate has a porosity of 30% to 60% by volume.

[0097] Preferably, the coating on the substrate is characterized by a uniform distribution of pores, which refers to the distribution between the splats (as defined above) and the pores both inside and outside the splats (see Figures 1f and 1g).

[0098] Coatings produced in accordance with the present invention exhibit low thermal conductivity, low hardness, high porosity and good wear properties.

[0099] The present invention will be described in more detail by the following examples and comparative examples, without being limited thereto. [Example]

[0100] Every example describes a different tested powder, coating tests done with different parameters and porosity results. The process for producing the powder of the present invention is represented diagrammatically in FIG.

[0101] Powder raw materials 1 and 2 were mixed, melted 3, crushed 4, annealed 5, and pulverized 6. The powder was then added to deionized water with stirring, and a binder composition was added and the mixture was stirred for 100 seconds. -1The target mixture viscosity range of 200 to 300 mPa·s was achieved at shear rates of 100°C. Spray drying was performed using a GEA Niro 6.3 spray dryer with a two-fluid bottom nozzle, inlet temperatures of 240 to 300°C, outlet temperatures of 135°C, and flow rates of 8 to 16 mPa·s. 3 Spray drying was performed using an atomizing air flow of 1 / hour. After spray drying, the powders were sintered using a Nabertherm HT160 / 17 LBS oven for 8 hours at 1450°C (Comparative Examples 1 to 28, Examples 29 to 40, and Comparative Examples 41 to 43) and 30 hours at 1500°C (Comparative Examples 44 to 51). In some examples, the sintered powders were sieved9 to obtain the final product10.

[0102] The powders were then subjected to a plasma spraying process under various conditions, particularly with regard to the spraying distance and injector diameter, which were adjusted depending on the powder, as would be common knowledge to a person skilled in the art.

[0103] Comparative Examples 1 to 16 = Unsieved ytterbium zirconate powder. Approximately 5 kg of ytterbium zirconate spray powder, produced as shown in Figure 6 and described above, was used to produce an abradable coating by plasma spraying. The measured internal porosity of the powder was approximately 45%. The particle size was adjusted by spray drying as described above.

[0104] Plasma spraying was performed with a 100HE spray gun (Progressive Surface, USA). All important parameters and values ​​are summarized in Table 1. The constant plasma spraying parameters were as follows: the volumetric flow rates of the plasma gases were adjusted to 60 lsm argon, 40 lsm nitrogen, and 50 lsm hydrogen. The carrier flow rate was set to 21 lsm argon. The powder flow rate was 150 g / min. The power was 95 kW.

[0105] The porosity values ​​in the resulting coatings ranged from 15% to 24% by volume, depending on the spraying parameters. As can be seen in Figure 8, there is no clear trend in porosity depending on particle size. It could also be observed that spraying distances greater than 110 mm resulted in lower porosity.

[0106] [Table 1]

[0107] Comparative Examples 17 to 20 = ytterbium zirconate powder sieved through a 38 μm sieve. Comparative Examples 21 to 28 = ytterbium zirconate powder sieved through a 63 μm sieve. Approximately 26 kg of ytterbium zirconate spray powder, produced as shown in Figure 6 and described above, was used to produce an abradable coating by plasma spraying. The measured internal porosity of the powder was approximately 45%. The powder was partially sieved through a 38 μm vibrating sieve until the amount of spray powder greater than 38 μm was approximately 12 kg (Comparative Examples 17 to 20). The remaining powder was sieved through a 63 μm vibrating sieve (Comparative Examples 21 to 28). The total amount after sieving was: 11.9kg>38μm=spray powder 1.74kg<38μm 9.1kg>63μm=Atomized powder 3.3kg<63μm It was.

[0108] Plasma spraying was performed with a 100HE spray gun (Progressive Surface, USA). All important parameters and values ​​are summarized in Tables 2 and 3. The constant plasma spraying parameters for Comparative Examples 17 to 24 were as follows: the volumetric flow rates of the plasma gas were adjusted to 60 lsm argon, 40 lsm nitrogen, and 50 lsm hydrogen. For Comparative Examples 25 to 28, the volumetric flow rates of the plasma gas were 60 lsm argon, 37 lsm nitrogen, and 71 lsm hydrogen.

[0109] The carrier gas for Comparative Examples 17-20 was constant at 21 lsm argon, and for Comparative Examples 18-28 it was constant at 18 lsm argon.

[0110] In Comparative Examples 17 to 28, the powder flow rate was set at 150 g / min and the power output was 95 kW.

[0111] Porosity values ​​ranged from 11 to 25% by volume, depending on the spraying parameters. For example, Figure 3 shows a cross-sectional SEM image of a coating made from a state-of-the-art powder with a sieve cut of 38 μm and an average porosity of 12% by volume (Table 2, Comparative Example 19). Figure 4 shows an SEM image of a coating made from a state-of-the-art powder with a sieve cut of 63 μm and an average porosity of 19% by volume (Table 3, Comparative Example 25).

[0112] As can be seen in Figure 8, there is no clear trend in porosity depending on particle size.

[0113] [Table 2]

[0114] [Table 3]

[0115] Examples 29 to 36 Approximately 5 kg of the already sieved spray powder (>38 μm) of Comparative Examples 17 to 20 was further sieved through a 100 μm vibrating sieve. The amount of spray powder greater than 100 μm was 2.4 kg. The measured internal porosity of the powder was approximately 52%, and the particle size was at least 100 μm. Plasma spraying was carried out with a 100HE spray gun (Progressive Surface, USA). All important parameters and values ​​are summarized in Table 4. The constant plasma spraying parameters were as follows: the volumetric flow rates of the plasma gases were adjusted to 60 lsm argon, 37 lsm nitrogen, and 71 lsm hydrogen. The carrier flow rate was set to 18 lsm argon. The powder flow rate was 150 g / min. The power was 95 kW.

[0116] Porosity values ​​ranged from 27 to 38 vol.%, depending on the spray parameters. For the first time, we observed that nearly all tests resulted in coatings with porosity values ​​above 30 vol.%. Only two values ​​were slightly below 30 vol.%, apparently as a result of increased spray distance. Figure 5 shows a representative SEM image of a coating produced from a spray powder of the present invention with a sieve cut of 100 μm and an average porosity of 35 vol.% (Table 4, Example 1).

[0117] In particular, the spray distance, the injector diameter and the current have an influence on the formation of the coating. It can be seen that a very long spray distance and a small injector diameter - for example, an SD of 110 mm or more and a 0.073 inch injector - can in the worst case result in a porosity of less than 30% by volume, even if the powder according to the invention is used. The values ​​of the two examples of the powder according to the invention with a porosity of less than 30% by volume are close to 30% by volume and even higher, as obtained with the powder used in the comparative example.

[0118] [Table 4]

[0119] Examples 37 to 40 Approximately 33.9 kg of spray-dried powder, produced as shown in Figure 6 and described above, was sieved through a 100 μm vibrating sieve after sintering. The measured internal porosity of the powder was approximately 52%. The amount of sprayed powder >100 μm was 15.46 kg. Plasma spraying was performed with a 100HE spray gun (Progressive Surface, USA). All important parameters and values ​​are summarized in Table 5. The constant plasma spraying parameters were as follows: the volumetric flow rates of the plasma gases were adjusted to 60 lsm argon, 37 lsm nitrogen, and 71 lsm hydrogen. The carrier flow rate was set to 18 lsm argon. The powder flow rate was 150 g / min. The power was 95 kW.

[0120] Figure 2a shows an SEM image of a portion of the cross section of the particle at 1000x magnification.

[0121] The porosity values ​​were between 30 and 36% by volume, depending on the spraying parameters. Evaluation of the DE measurements of the spraying process showed 42% DE for sample no. 39 and 34% DE for sample no. 40.

[0122] [Table 5]

[0123] Comparative Example 41 Approximately 5 kg of gadolinium zirconate spray powder, produced as described in Figure 6, was used to fabricate an abradable coating by plasma spraying. The measured internal porosity of the powder was approximately 36%. The particle size was adjusted by spray drying as described above. Due to the higher PSD range, the material was sieved through a 425 μm sieve before sintering, and the coarser material was discarded. The remaining gadolinium zirconate powder was then sieved through a 100 μm sieve before sintering. After sintering, the powder contained 10.5 wt.% fine particles less than 100 μm.

[0124] Plasma spraying was performed with a 100HE spray gun (Progressive Surface, USA). All important parameters and values ​​are summarized in Table 6. The plasma spraying parameters were as follows: the volumetric flow rates of the plasma gases were adjusted to 85 slm argon, 47 slm nitrogen, and 71 slm hydrogen. The carrier flow rate was set to 18 slm argon. The powder flow rate was 150 g / min. The power was 95 kW.

[0125] The porosity value in the resulting coating was 27% by volume. It was observed that already small amounts of fines below 100 μm reduced the porosity to below 30% by volume.

[0126] [Table 6]

[0127] Examples 42 and 43 Approximately 4 kg of the sintered gadolinium zirconate powder of Example 41 was further sieved to 100 μm. The fine material was discarded. The coarser material, which contained no fine particles below 100 μm, was used to produce an abradable coating by plasma spraying. The measured internal porosity of the powder was approximately 36%. The particle size was adjusted as described above.

[0128] Plasma spraying was performed with a 100HE spray gun (Progressive Surface, USA). All important parameters and values ​​are summarized in Table 7. The plasma spraying parameters were the same as in Example 41.

[0129] The porosity values ​​of the resulting coatings were 38 and 41% by volume. It should be noted that the only difference between Examples 41 and 42+43 was the additional sieving step to avoid fine particles below 100 μm in the spray powder.

[0130] This example also shows that it is possible to achieve a coating porosity that is higher than the internal porosity of the powder particles.

[0131] [Table 7]

[0132] Comparative Examples 44 to 47 Approximately 8.7 kg of 48YSZ (= 48 wt% Y2O3 and 52 wt% ZrO2) atomized powder, produced as described in Figure 6, was used to fabricate an abradable coating by plasma spraying. The measured internal porosity of the powder was approximately 45%. The particle size was adjusted by spray drying as described above. Due to the higher PSD range, the material was sieved through a 425 μm sieve before sintering to discard the coarser material. The remaining 48YSZ powder was sieved through a 100 μm sieve before sintering. After sieving, the powder contained 13 wt% fine particles less than 100 μm after sintering.

[0133] Plasma spraying was performed with a 100HE spray gun (Progressive Surface, USA). All important parameters and values ​​are summarized in Table 8. The plasma spraying parameters were as follows: the volumetric flow rates of the plasma gases were adjusted to 60 slm argon, 37 slm nitrogen, and 71 slm hydrogen. The carrier flow rate was set to 16 slm argon. The powder flow rate was 150 g / min. The power was 105 kW.

[0134] The porosity values ​​in the resulting coatings were between 23 and 30% by volume. It was observed that a small amount of fines less than 100 μm reduced the porosity to below 30% by volume.

[0135] [Table 8]

[0136] Examples 48 to 51 Approximately 8.5 kg of spray-dried and sintered powder, produced as described above and shown in Figure 6, was further sieved after sintering through a 100 μm vibrating sieve to cut fines and a 425 μm sieve to cut coarse particles. The amount of sprayed powder greater than 100 and less than 425 μm was 5.46 kg. Fine and coarse material was discarded. The 100 to 425 μm material was used to produce abradable coatings by plasma spraying and did not contain any fine particles less than 100 μm. The measured internal porosity of the powder was approximately 48%. The particle size was adjusted as described above.

[0137] FIG. 2b shows an SEM image of a portion of the cross section of the particle at 1000× magnification.

[0138] Plasma spraying was performed with a 100HE spray gun (Progressive Surface, USA). All important parameters and values ​​are summarized in Table 9.

[0139] The porosity values ​​of the resulting coatings were 35 and 41% by volume. It should be noted that the only difference between Examples 44 to 47 and 48 to 51 was the additional sieving step to avoid fine particles below 100 μm in the spray powder.

[0140] [Table 9]

Claims

1. A powder comprising solid ceramic particles, the particles having an internal porosity of at least 30%, preferably at least 45%, and preferably at most 65%, measured in accordance with the present specification, and a d of the powder measured in accordance with the present specification. １０ A powder comprising solid ceramic particles, characterized in that the particle size is at least 100 μm.

2. 10. The powder of claim 1, wherein the solid ceramic particles comprise or consist of an oxide of at least one of a transition metal, a rare earth metal, a metal from Group 13 or 14 of the periodic table, and any combination thereof.

3. The solid ceramic particles are ytterbium zirconate (Yb ２ Zr ２ O ７ ), gadolinium zirconate (Gd ２ Zr ２ O ７ ), yttrium stabilized zirconia (YSZ), yttrium oxide (Y ２ O ３ ), aluminum oxide (Al ２ O ３ ), silicate minerals including silicon dioxide, preferably rare earth silicates such as yttrium monosilicate (YMS), yttrium disilicate (YDS), ytterbium monosilicate (YbMS), ytterbium disilicate (YbDS), yttrium-ytterbium monosilicate (YYbMS), yttrium-ytterbium disilicate (YYbDS), or mixtures and / or complexes thereof.

4. 4. Use of the powder according to any one of claims 1 to 3 for thermal spraying, preferably for atmospheric plasma spraying.

5. 5. The use according to claim 4 for producing a coating having a porosity of at least 30% on a substrate, wherein the substrate is selected from the group consisting of metals, ceramics, polymers, ceramic matrix composites, metal alloys, and mixtures and / or composites thereof.

6. A coated substrate obtainable by coating said substrate with a powder according to any one of claims 1 to 3.

7. The coated substrate of claim 6 , wherein the coating has a porosity of 30% to 60% by volume.

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