EP3746405A1

EP3746405A1 - Powder for a thermal barrier

Info

Publication number: EP3746405A1
Application number: EP19701562.1A
Authority: EP
Inventors: Alain Allimant; Howard Wallar
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2018-01-31
Filing date: 2019-01-31
Publication date: 2020-12-09
Also published as: WO2019149857A1; FR3077288A1; US20210061716A1; CN111670163A

Abstract

The invention relates to a particle powder, a number higher than 95% of the particles having a circularity greater than or equal to 0.85, the powder containing more than 98% of a stabilised oxide which is selected from stabilised zirconium oxides, stabilised hafnium oxides and the mixtures thereof, the stabilised oxide being stabilised by a stabiliser which is selected from the oxides of Y, Ca, Ce, Se, Mg, In, La, Gd, Nd, Sm, Dy, Er, Yb, Eu, Pr and Ta, which are referred to as "stabilising oxides", and the mixtures of these stabilising oxides, as a percentage by mass on the basis of the oxides, and having: - a mean particle size D₅₀ less than 15 μm, a 90th percentile of the particle sizes D₉₀ less than 30 μm, and a size dispersion index (D₉₀ - D₁₀)/D₁₀ less than 2; - a relative density greater than 90%.

Description

POWDER FOR THERMAL BARRIER

Technical area

Thermal barrier coatings, or thermal barrier coatings, or TBC, are thermally insulating coatings. Although generally porous, TBC can be dense and in this case vertically cracked (DVC), in English "dense and vertically cracked".

The invention relates to a feed powder intended to be deposited by plasma spraying to form a TBC, a method for producing such a feed powder, and a body coated with a TBC obtained by plasma spraying said feed powder.

State of the art

The TBC were described by HL BERSTEIN in "High temperature coatings for industrial gas turbine users", Proceedings of the 28 ^th symposium "Turbomachinery". Conventionally, a TBC consists of zirconia partially stabilized by about 8% by weight of yttrine or magnesia applied by electron beam vapor deposition, or EBPVD, or deposited by thermal spraying. , and in particular by plasma projection under air.

A TBC conventionally has a thickness of between 3 and 15 mm.

It is conventionally arranged on a bonding layer made of NiCrAlY, itself deposited on a metal substrate. The tie layer improves the adhesion of the TBC.

The TBC advantageously isolates the metal substrate from the hot gases of the environment, in particular by providing thermal insulation.

TBCs are thus commonly used to protect the components of gas turbines from oxidation and corrosion at high temperatures.

Under the effect of thermal cycling and corrosion, the TBC may however be subject to spalling.

Deposition by EBPVD leads to a columnar microstructure oriented substantially perpendicular to the surface of the substrate, that is to say "vertically". This microstructure is resistant to chipping. However, EBPVD deposition is much more expensive than thermal spray deposition. Moreover, a thermal projection TBC has a lower thermal conductivity than a TBC obtained by EBPVD. It is therefore a more effective thermal barrier. Conventionally, however, it does not allow to obtain vertical cracking.

US 2004/0033884 or US 6 893 994 discloses thermal barrier coatings. However, they are not vertically cracked.

WO2007 / 139694, WO2008 / 054536 or US 2014/0334939 disclose vertically cracked coatings. According to the teaching of these documents, coatings based on zirconia strongly stabilized with Yttrine are not very resistant to thermal shocks.

There is thus a continuing need for a vertically cracked TBC coating that can be deposited by plasma thermal spraying, with high efficiency, and having an improved compromise between chipping resistance and constant thickness thermal insulation capability.

An object of the invention is to respond, at least partially, to this need.

Summary of the invention

According to the invention, this object is achieved by means of a powder (hereinafter "feed powder") of melted particles (hereinafter "feed particles"), preferably obtained by plasma melting,

said powder containing, in percentage by weight on the basis of the oxides, more than 98% of a stabilized oxide chosen from stabilized zirconium oxides, stabilized hafnium oxides and their mixtures, the stabilized oxide being stabilized by a stabilizer chosen from among the oxides of Y, Ca, Ce, Sc, Mg, In, La, Gd, Nd, Sm, Dy, Er, Yb, Eu, Pr and Ta, referred to as "stabilizing oxides", and the mixtures of these oxides stabilizing,

said powder having:

a median particle size D50 of less than 15 mHi, a 90 percentile of particle sizes D90 of less than 30 mHi, and a size dispersion index relative to the percentile of particle sizes D10, (D90-Dio) / Dio; less than 2;

a relative density greater than 90%, preferably greater than 95%, the cumulative specific volume of the pores having a radius of less than 1 mhi being preferably less than 10% of the apparent volume of the powder.

By "stabilized oxide" is meant the oxide, namely zirconium oxide and / or hafnium oxide on the one hand, and the stabilizer on the other hand.

A feed powder according to the invention is therefore a remarkable powder, in particular because of the very small particle size dispersion, with respect to Dio, by the small quantity of particles having a size greater than 30 mHi and by a specific gravity. relative high.

This last characteristic implies a very small amount of hollow particles, or even substantially zero. The particle size distribution ensures a very homogeneous fusion during the projection.

As will be seen in more detail in the following description, a feed powder according to the invention allows, by simple thermal spraying, and in particular by plasma spraying, to obtain a TBC coating vertically cracked both highly insulating thermally and very resistant to thermal cycling.

A feed powder according to the invention may also include one or more of the following optional features:

More than 95%, preferably more than 99%, preferably more than 99.5% by number of said particles have a circularity greater than or equal to 0.85, greater than or equal to 0.87, preferably greater than or equal to 0 , 90;

The powder contains more than 99.9%, more than 99.950%, more than 99.990%, preferably more than 99.999% of said stabilized oxide; The quantity of the other oxides is so small that it can not have a significant effect on the results obtained with a feed powder according to the invention;

Oxides account for more than 98%, more than 99%, more than 99.5%, more than 99.9%, more than 99.95%, more than 99.985% or more than 99.99% of the mass of the powder;

The percentage by number of particles having a size of less than or equal to 5 mhi is greater than 5%, preferably greater than 10%;

The percentage in number of particles having a size greater than or equal to 0.5 μm is greater than 10%; The median particle size (D ₅₀ ) of the powder is greater than 0.5 mhi, preferably greater than 1 mhi, or even greater than 2 mhi, and / or less than 13 mhi, preferably less than 12 pm, preferably less than 10 mhi or less than 8 mhi; The percentile (D 10) of the particle sizes is greater than 0.1 μm, preferably greater than 0.5 μm, preferably greater than 1 μm, or greater than 2 μm;

The percentile 90 (D90) of the particle sizes is less than 25 μm, preferably less than 20 μm, preferably less than 15 μm;

The 99.5 percentile (D99, s) of the particle sizes is less than 40 μm, preferably less than 30 μm;

The size dispersion index (D 9 _O -D ₁₀ ) / D ₁₀ is preferably less than 1.5; This advantageously results in a higher coating density;

Preferably, the powder has a monomodal particle size dispersion type, i.e., a single main peak;

The cumulative specific volume of the pores with a radius of less than 1 μm is less than 8%, preferably less than 6%, preferably less than 5%, preferably less than 4%, preferably less than 3.5% of the apparent volume. powder;

The specific surface area of the feed powder is preferably less than 0.4 m ² / g, preferably less than 0.3 m ² / g.

The invention also relates to a method for manufacturing a feed powder according to the invention comprising the following successive steps:

a) granulation of a particulate filler so as to obtain a powder of granules having a median size of 50 of between 20 and 60 microns, the particulate filler comprising, in percentage by weight on the basis of the oxides, more than 98% of a stabilized oxide selected from stabilized zirconium oxides, stabilized hafnium oxides and mixtures thereof, the stabilized oxide being stabilized by a stabilizer selected from oxides of Y, Ca, Ce, Sc, Mg, In, La, Gd, Nd, Sm, Dy, Er, Yb, Eu, Pr, and Ta, so-called "stabilizing oxides", and the mixtures of these stabilizing oxides,

b) injecting said granule powder, via a carrier gas, through at least one injection port to a plasma jet generated by a plasma gun, under conditions causing a burst before melting of more than 50% , preferably more than 60%, preferably more than 70%, preferably more than 80%, preferably more than 90% by number of the injected granules, as a percentage of number, then a melting of the granules and pieces of granules so as to obtain droplets,

c) cooling said droplets, so as to obtain a feed powder according to the invention;

d) optionally, granulometric selection, preferably by sieving or by pneumatic classification, of said feed powder.

The violent injection of the powder advantageously makes it possible simultaneously to reduce the median size of the feed powder and to reduce the proportion of hollow particles. It thus makes it possible to obtain a very high relative density.

Preferably, the plasma gun has a power greater than 40 kW, preferably greater than 50 kW and / or less than 65 kW, preferably less than 60 kW.

Preferably, the plasma gun has a power of between 40 to 65 kW and the ratio of the mass quantity of granules injected by injection orifice, preferably by each injection orifice, on the surface of said injection orifice is greater than 15, preferably greater than 17, preferably greater than 20, preferably greater than 23 g / min per mm ² of area of said injection port and / or less than 30 g / min per mm ² of surface of said orifice injection.

The injection orifice, preferably each injection orifice is preferably constituted by a channel whose length is greater than once, preferably twice or even 3 times the equivalent diameter of said injection port.

Preferably, the flow rate of the injected granule powder is less than 2.4 g / min, preferably less than 2 g / min per KW of power of the plasma gun.

There is no intermediate sintering step, preferably no consolidation between steps a) and b). This lack of intermediate consolidation step advantageously improves the purity of the feed powder. It also facilitates the bursting of the granules in step b).

A method of manufacturing a powder according to the invention may also include one or more of the following optional features:

In step a), the granulation is preferably an atomization method or spray drying ("spray drying" in English) or pelletization (transformation into pellets); In step a), the mineral composition of the granule powder comprises more than 98.5%, preferably more than 99%, preferably more than 99.5%, preferably more than 99.9%, more than 99.95%, more than 99.99%, preferably more than 99.999% of said stabilized oxide, in weight percent based on the oxides;

The median circularity C 50 of the granule powder is preferably greater than 0.85, preferably greater than 0.90, preferably greater than 0.95, and even more preferably greater than 0.96;

The circularity percentile of the granule powder, C ₅ , is preferably greater than or equal to 0.85, preferably greater than or equal to 0.90;

The median form ratio A50 of the granule powder is preferably greater than 0.75, preferably greater than 0.8;

The specific surface area of the granule powder is preferably less than 15 m ² / g, preferably less than 10 m ² / g, preferably less than 8 m ² / g, preferably less than 7 m ² / g;

The cumulative pore volume having a radius less than 1 mHi, measured by mercury porosimetry, of the granule powder is preferably less than 0.5 cm ³ / g, preferably less than 0.4 cm ³ / g or preferably less than 0.3 cm ³ / g;

The bulk density of the granule powder is preferably greater than 0.5 g / cm ³ , preferably greater than 0.7 g / cm ³ , preferably greater than 0.90 g / cm ³ , preferably greater than 0 , 95 g / cm ³ , preferably less than 1.5 g / cm ³ , preferably less than 1.3 g / cm ³ , preferably less than 1.1 g / cm ³ ;

The percentile (10 10) particle size of the granule powder is preferably greater than 10 μm, preferably greater than 15 μm, preferably greater than 20 μm;

The 90th percentile (90) particle size of the granule powder is preferably less than 90 μm, preferably less than 80 μm, preferably less than 70 μm, preferably less than 65 μm;

The granule powder preferably has a median size of S n between 20 and 60 microns;

The granule powder preferably has a percentile of 10 between 20 and 25 μm and an D'9 _O between 60 and 65 μm; The 99.5 percentile (99.5) particle size of the granule powder is preferably less than 100 μm, preferably less than 80 μm, preferably less than 75 μm;

The size dispersion index with respect to D ₅₀ , (90-D'10) / D ₅₀ , of the granule powder is preferably less than 2, preferably less than 1.5, preferably less than 1.2, more preferably less than 1.1;

In step b), the diameter of each injection orifice is less than 2 mm, preferably less than 1.8 mm, preferably less than 1.7 mm, preferably less than 1.6 mm;

In step b), the injection conditions are equivalent to those of a plasma gun having a power of 40 to 65 kW and generating a plasma jet in which the mass quantity of granules injected by an injection orifice , preferably by each injection orifice, in g / min and per mm ² of the surface of said injection orifice, is greater than 10 g / min per mm ² , preferably greater than 15 g / min per mm ² ; "equivalent" means "adapted so that the burst rate of the granules (number of granules burst on number of granules injected) is the same";

An injection port, preferably each injection port, defines an injection channel, preferably cylindrical, preferably of circular section, having a length at least once, preferably at least twice, or even three times greater the equivalent diameter of said injection port, the equivalent diameter being the diameter of a disc of the same surface as the injection port;

In step b), the flow rate of granule powder is less than 3 g / min, preferably less than 2 g / min, per kW of power of the plasma gun;

The flow rate of the carrier gas (by injection orifice (that is to say by "powder line")) is greater than 5.5 l / min, preferably greater than 5.8 l / min, preferably greater than 6.0 l / min, preferably greater than 6.5 l / min, preferably greater than 6.8 l / min, preferably greater than 7.0 l / min;

The granule powder is injected into the plasma jet at a feed rate greater than 20 g / min, preferably greater than 25 g / min, and / or less than 60 g / min, preferably less than 50 g / min. min, preferably less than 40 g / min, per injection port;

The total granular feed rate (cumulative for all injection ports) is greater than 70 g / min, preferably greater than 80 g / min, and / or preferably lower at 180 g / min, preferably less than 140 g / min, preferably less than 120 g / min, preferably less than 100 g / min;

Preferably, in step c), the cooling of the melt droplets is such that, up to 500 ° C, the average cooling rate is between 50 000 and 200 000 ° C / s, preferably between 80 000 and 150 000 ° C / s.

The invention also relates to a method for producing a vertically cracked TBC coating, said method comprising a step of thermal spraying, preferably by plasma, of a feed powder according to the invention, in particular manufactured according to a method according to the invention. invention, on a substrate.

Preferably, the substrate is metal. The substrate may be a blade of a propeller or a blade of a gas turbine.

The invention also relates to a body comprising a substrate and a vertically cracked TBC coating and at least partially covering said substrate, said TBC coating being preferably separated from the substrate by a bonding layer, preferably NiCrAlY, and being manufactured with a method according to the invention. This body is particularly well suited for use in an environment at a temperature above 1200 ° C.

The coating preferably has a thermal conductivity of less than 3 W / m.K.

Preferably, said coating comprises more than 98% of said stabilized oxide and preferably has a porosity, measured in a photograph of a polished section of said coating, as described below, less than or equal to 1.5%. Preferably, the porosity of said coating is less than 1%.

Preferably, said coating comprises more than 98.5%, preferably more than 99%, preferably more than 99.5%, preferably more than 99.9%, more than 99.95%, more than 99.97%. %, more than 99.98%, more than 99.99%, preferably more than 99.999% of said stabilized oxide, in weight percent based on the oxides.

Such a coating can be manufactured with a thermal spraying method according to the invention.

The invention further relates to the use of such a vertically cracked TBC coating to protect a workpiece in an environment having a temperature in excess of 1000 ° C, 1200 ° C, 1200 ° C or 1300 ° C. Definitions

"Impurities" are the inevitable constituents, involuntarily and necessarily introduced with the raw materials or resulting from reactions between the constituents. Impurities are not necessary constituents but only tolerated constituents. The level of purity is preferably measured by GDMS (Glow Discharge Mass Spectroscopy) which is more accurate than the AES-ICP (Inductively Coupled Plasma Atomic Emission Spectrometer).

The "circularity" of the particles of a powder is conventionally determined as follows: The powder is dispersed on a flat pane. The images of the individual particles are obtained by scanning the dispersed powder under an optical microscope, while keeping the particles in focus, the powder being illuminated by the underside of the glass. These images can be analyzed using a device of the type Morphologi ^® G3 marketed by the company Malvenu

As shown in FIG. 4, to evaluate the "circularity" C of a particle P ', the perimeter P _D of the disk D having an area equal to the area A _p of the particle P' is determined on an image of this particle. The perimeter P _p of this particle is also determined. Circularity is equal to the ratio of

² * _L / ^

PD / P _p . So C = -. The longer the particle is elongated, the more

pp

circularity is weak. The SYSMEX FPIA 3000 user manual also describes this procedure (see "detailed specification sheets" on www.malvem.co.uk).

To determine a circularity percentile (described below), the powder is poured onto a flat pane and observed as explained above. The number of particles counted should be greater than 250 so that the measured percentile is substantially the same, regardless of how the powder is poured onto the glass. The shape ratio A of a particle is defined as the ratio of the width of the particle (its largest dimension perpendicular to the direction of its length) and its length (its largest dimension).

To determine a shape ratio percentile, the powder is poured onto a flat pane and observed as explained previously, to measure the lengths and widths of the particles. The number of particles counted should be greater than 250 so that the measured percentile is substantially the same, regardless of how the powder is poured onto the glass.

Percentiles or "percentiles" 10 (Mio), 50 (M ₅₀ ), 90 (M90) and 99.5 (M99, s), and more generally "n" M _n of a property M of particles of a powder of particles are the values of this property for the percentages, by number, of 10%, 50%, 90%, 99.5% and n%, respectively, on the cumulative distribution curve relating to this property of the particles of the powder , the values relating to this property being ranked in ascending order. In particular, the percentiles D _n (or D ' _n for the powder of granules), A _n , and C _n are relative to the size, the aspect ratio and the circularity, respectively.

For example, 10% by number of the particles of the powder have a size less than D10 and 90% of the particles in number have a size greater than or equal to D10. The percentiles for size can be determined using a particle size distribution using a laser granulometer.

Likewise, 5% by number of particles of the powder have a circularity lower than the C5 percentile. In other words, 95% by number of particles of this powder have a circularity greater than or equal to C5.

The percentile 50 is classically called the "median" percentile. For example, C50 is conventionally called "median circularity". Similarly, the percentile D50 is conventionally called "median size". The A50 percentile also conventionally refers to the "median form ratio".

By "particle size" is meant the size of a particle conventionally given by a particle size distribution characterization performed. with a laser granulometer. The laser granulometer used can be a Partica LA-950 from the company HORIBA.

The percentage or fraction by number of particles having a size smaller than or equal to a determined maximum size can be determined using a laser granulometer.

The cumulative specific volume of the pores of radius less than 1 mHi, expressed in cm ³ / g of powder, is conventionally measured by mercury porosimetry according to the ISO 15901-1 standard. It can be measured with a MICROMERITICS porosimeter.

The apparent volume of powder, expressed in cm ³ / g, is the inverse of the apparent density of the powder.

The "bulk density" ("bulk density") of a particle powder is conventionally defined as the ratio of the mass of the powder divided by the sum of the apparent volumes of said particles. In practice, it can be measured with a MICROMERITICS porosimeter at a pressure of 200 MPa.

The "relative density" of a powder is equal to its apparent density divided by its actual density. The actual density can be measured by helium pycnometry.

The "porosity" of a coating can be evaluated by image analysis of a polished cross section of the barrier. The coated substrate is sectioned using a laboratory cutting machine, for example using a Struers Discotom apparatus with an alumina cutting disc. The coating sample is then mounted in a resin, for example by using a Struvers Durocit type cold mounting resin. The mounted sample is then polished using polishing media of increasing fineness. Abrasive paper or, preferably, polishing discs can be used with a suitable polishing slurry. A typical polishing procedure begins with dressing the sample (for example with a Struers Piano 220 Abrasive Disc), then changing the polishing sheets associated with the abrasive suspensions. The abrasive grain size is decreased at each fine polishing step, the size of the diamond abrasives starting with example at 9 microns, then at 3 microns, to finish at 1 micron (Struers DiaPro series). For each abrasive grain size, the polishing is stopped as soon as the porosity observed under optical microscope remains constant. The samples are thoroughly cleaned between the steps, for example with water. A final polishing step, after the 1 μm diamond polishing step, is performed using colloidal silica (OP-U Struers, 0.04pm) associated with a soft felt type sheet. After cleaning, the polished sample is ready for observation under the light microscope or SEM (Scanning Electron Microscope). Because of its superior resolution and outstanding contrast, SEM is preferred for producing images for analysis. The porosity can be determined from the images using image analysis software (eg ImageJ, NIH), adjusting the thresholding. The porosity is given as a percentage of the surface of the cross section of the coating.

"Specific surface area" is conventionally measured by the BET method (Brunauer Emmet Teller), as described in the Journal of the American Chemical Society 60 (1938), pages 309-316.

The "granulation" operation is a process of agglomeration of particles using a binder, for example a binder polymer, to form agglomerated particles, which may optionally be granules. The granulation comprises, in particular, atomization or spray drying and / or the use of a granulator or a pelletizer, but is not limited to these processes. . Conventionally, the binder comprises substantially no oxides.

A "granule" is an agglomerated particle having a circularity of 0.8 or more.

A consolidation step is an operation to replace, in the granules, the links due to organic binders by diffusion links. It is generally carried out by a heat treatment, but without complete melting of the granules.

The "deposition efficiency" of a plasma spraying process is defined as the ratio, in percentage by mass, of the amount of material deposited on the substrate divided by the amount of feed powder injected into the plasma jet.

"Projection productivity" is defined as the amount of material deposited per unit of time.

The flow rates in l / min are "standard", that is to say measured at a temperature of 20 ° C, under a pressure of 1 bar.

"Include" or "understand" must be understood in a non-limiting manner, unless otherwise indicated.

Unless otherwise indicated, all percentages of composition are percentages by weight based on the mass of the oxides.

The properties of the powder can be evaluated by the characterization methods used in the examples.

Brief description of the figures

Other characteristics and advantages of the invention will emerge more clearly on reading the description which follows and on examining the appended drawings, in which:

FIG. 1 schematically represents step a) of a method according to the invention;

FIG. 2 diagrammatically represents a plasma torch for the manufacture of a feed powder according to the invention;

FIG. 3 schematically represents a method for manufacturing a feed powder according to the invention;

Figure 4 illustrates the method that is used to evaluate the circularity of a particle.

detailed description

Method of manufacturing a feed powder

FIG. 1 illustrates an embodiment of step a) of a method for manufacturing a feed powder according to the invention.

Any known method of granulation can be used. In particular, those skilled in the art know how to prepare a slip suitable for granulation. In one embodiment, a binder mixture is prepared by adding PVA (polyvinyl alcohol) 2 in deionized water 4. This binder mixture 6 is then filtered through a filter 5 of 5 mhi. A particulate filler consisting of powder stabilized oxide (e.g., 99.99% purity), with a median size of 1 mhi, is mixed in the filtered binder mixture to form a slurry 12. The slurry may comprise mass, for example, 55% of stabilized oxide and 0.55% of PVA, the complement to 100% being water. This slip is injected into an atomizer 14 to obtain a powder of granules 16. Those skilled in the art knows how to adapt the atomizer to obtain the desired particle size distribution.

Preferably, the granules are agglomerates of particles of an oxide material having a median size preferably of less than 3 μm, preferably less than 2 μm, preferably less than 1.5 μm.

The granule powder can be sieved (5 mm sieve 18 for example) in order to eliminate the possible presence of residues fallen from the walls of the atomizer.

The resulting powder is a "spray-dried only" (SDO) granule powder.

Figures 2 and 3 illustrate an embodiment of step b) of melting a method of manufacturing a feed powder according to the invention.

An SDO granule powder 20, for example, as manufactured according to the process illustrated in FIG. 1, is injected by an injector 21 into a plasma jet 22 produced by a plasma gun 24, for example a plasma torch. ProPlasma HP. Conventional plasma injection and projection devices can be used to mix the SDO granule powder with a carrier gas and to inject the resulting mixture into the hot plasma core.

However, the injected granule powder must not be consolidated (SDO) and the injection into the plasma jet must be made brutally, to promote the breaking of granules. The shock force determines the intensity of the bursting of the granules, and therefore the median size of the powder manufactured.

The person skilled in the art knows how to adapt the injection parameters for a sudden injection of the granules, so that the feed powder obtained at the end of steps c) or d) has a particle size distribution according to the invention. In particular, those skilled in the art know that:

a comparison of the injection angle θ between the injection axis of the Y granules and the X axis of the plasma jet by 90 °,

an increase in the powder flow rate per mm ² of surface area of the injection orifice,

a reduction in the powder flow, in g / min, per kW of power of the gun, and

an increase in the flow of plasma gas

are factors that promote breakage of the granules.

In particular, WO2014 / 083544 does not disclose injection parameters allowing more than 50% by number of the granules to break, as described in the examples below.

It is preferable to inject the particles rapidly so as to disperse them in a very viscous plasma jet which flows at a very high speed.

When the injected granules come into contact with the plasma jet, they are subjected to violent shocks, which can break them into pieces. To penetrate the plasma jet, the unconsolidated, and in particular unsintered, granules to be dispersed are injected at a velocity sufficiently high to benefit from a high kinetic energy, but limited to ensure a good bursting efficiency. The lack of consolidation granules reduces their mechanical strength, and therefore their resistance to these shocks.

It is known to those skilled in the art that the speed of the granules is determined by the flow rate of the carrier gas and the diameter of the injection port.

The speed of the plasma jet is also high. Preferably, the plasma gas flow rate is greater than the median recommended by the torch manufacturer for the chosen anode diameter. Preferably, the plasma gas flow rate is greater than 50 l / min, preferably greater than 55 l / min, preferably greater than 60 l / min.

It is known to those skilled in the art that the plasma jet velocity can be increased by using a small diameter anode and / or by increasing the flow rate of the primary gas.

Preferably, the flow rate of the primary gas is greater than 40 l / min, preferably greater than 45 l / min.

Preferably, the ratio between the secondary gas flow rate, preferably the dihydrogen (H ₂ ) and the plasma gas flow (consisting of primary and secondary gases) is between 20% and 25%. Of course, the energy of the plasma jet, influenced in particular by the flow rate of the secondary gas, must be high enough to melt the granules.

The granule powder is injected with a carrier gas, preferably without any liquid.

In the plasma jet 22, the granules are melted into droplets 25. Preferably, the plasma gun is set so that the melting is substantially complete.

Fusion advantageously reduces the level of impurities.

On leaving the hot zone of the plasma jet, the droplets are rapidly cooled by the surrounding cold air, but also by a forced circulation 26 of a cooling gas, preferably air. Air advantageously limits the reducing effect of hydrogen.

Preferably, the plasma torch comprises at least one nozzle arranged to inject a cooling fluid, preferably air, so as to cool the droplets resulting from the heating of the granule powder injected into the plasma jet. The cooling fluid is preferably injected downstream of the plasma jet (as shown in FIG. 2) and the angle g between the path of said droplets and the path of the cooling fluid is preferably less than or equal to 80 °. , preferably less than or equal to 60 ° and / or greater than or equal to 10 °, preferably greater than or equal to 20 °, preferably greater than or equal to 30 °. Preferably, the injection axis Y of any nozzle and the X axis of the plasma jet are secant.

Preferably, the injection angle θ between the injection axis Y and the X axis of the plasma jet is greater than 85 °, preferably about 90 °.

Preferably, the forced cooling is generated by a set of nozzles 28 disposed about the X-axis of the plasma jet 22, so as to create a substantially conical or annular flow of cooling gas.

The plasma gun 24 is oriented vertically towards the ground. Preferably, the angle α between the vertical and the X axis of the plasma jet is less than 30 °, less than 20 °, less than 10 °, preferably less than 5 °, preferably substantially zero. Advantageously, the flow of cooling gas is perfectly centered with respect to the axis X of the plasma jet. Preferably, the minimum distance d between the external surface of the anode and the cooling zone (where the droplets come into contact with the injected cooling fluid) is between 50 mm and 400 mm, preferably between 100 mm and 300 mm. mm.

Advantageously, the forced cooling limits the generation of satellites, resulting from the contact between very large hot particles and small particles in suspension in the densification chamber 32. Moreover, such a cooling operation makes it possible to reduce the overall size of the treatment equipment, especially the size of the collection chamber.

The cooling of the droplets 25 makes it possible to obtain feed particles 30, which can be extracted in the lower part of the densification chamber 32.

The densification chamber may be connected to a cyclone 34, whose exhaust gas is directed to a dust collector 36, so as to separate very fine particles 40. According to the configuration, certain feed particles in accordance with FIG. The invention may also be collected in the cyclone. Preferably, these feed particles can be separated, in particular with an air separator.

Optionally, the collected feed particles 38 can be filtered, so that the median size D50 is less than 15 microns.

The following Table 1 provides the preferred parameters for making a feed powder according to the invention.

The characteristics of a column are preferably, but not necessarily, combined. The characteristics of the two columns can also be combined.

Table 1 The plasma torch "ProPlasmaHP" is sold by Saint-Gobain Coating Solutions. This torch corresponds to the torch T1 described in WO2010 / 103497.

Examples

The following examples are provided for purposes of illustration and do not limit the scope of the invention.

The feed powders 1 and 2 according to the invention and comparative 1 were manufactured with a plasma torch similar to the plasma torch shown in FIG. 2 of WO2014 / 083544, from a source of yttriated zirconia powder. at 8% by mass, hereafter called "zirconia powder", having a median size D50 of 1.5 microns, measured with a Microtrac laser particle analyzer.

In step a), a binder mixture is prepared by adding PVA (polyvinyl alcohol) binder 2 (see Figure 1) in deionized water 4. This binder mixture is then filtered through a filter 8 of 5 mhi. The powdered zirconia 10 is mixed in the filtered binder mixture to form a slurry 12. The slurry is prepared so as to comprise, as a percentage by weight, 55% of zirconia powder and 0.55% of PVA, the balance being 100%. % being deionized water. The slurry is mixed intensively using a high shear mixer.

The granules are then obtained by atomizing the slip, using an atomizer 14. In particular, the slip is atomized in the chamber of a GEA Niro SD 6.3 R atomizer, the slip being introduced at a flow rate of about 0.degree. , 38 l / min.

The speed of the rotary atomizing wheel, driven by a Niro FS1 engine, is adjusted to obtain the sizes of the targeted granules 16.

The air flow rate is adjusted to maintain the inlet temperature at 295 ° C and the outlet temperature near 125 ° C so that the residual moisture of the granules is between 0.5% and 1%.

The granule powder is then screened with a sieve 18 in order to extract the residues and obtain a powder of SDO 20 granules.

In step b), the granules of step a) are injected into a plasma jet 22 (see FIG. 2) produced with a plasma gun 24. The injection and melting parameters are given in table 2 following.

In step c), to cool the droplets, 7 Silvent 2021L nozzles 28, sold by Silvent, were attached to a Silvent 463 annular nozzle holder, sold by Silvent. The nozzles 28 are spaced regularly along the annular nozzle holder, so as to generate a substantially conical air flow.

Table 2

5 The cumulative specific volume of the pores having a radius of less than 1 mHi, in the granules, was 340 × 10 ³ cm ³ / g.

The tests show that a feed powder according to the invention has a relative density greater than 90%.

The invention thus provides a feed powder having a size distribution and a relative density imparting a very high density to the coating. In addition, this feed powder can be efficiently projected by plasma and with good productivity.

The powder according to the invention makes it possible to produce coatings with a lower concentration of defects, in particular horizontal cracks. Moreover, such a powder has improved flowability compared to a plasma unmelted powder of the same size which allows an injection without complex feed means.

Of course, the invention is not limited to the embodiments described and shown.

Claims

1. Powder of melted particles,

said powder having:

a median particle size D50 of less than 15 mHi, a 90 percentile of particle sizes, D90, of less than 30 mHi, and a size dispersion index (D90-Dio) / D 10 of less than 2;

a relative density greater than 90%,

the percentiles D _n of the powder being the particle sizes corresponding to the percentages, by number, of n%, on the cumulative distribution curve of the particle size of the powder, the particle sizes being ranked in ascending order.

2. Powder according to the immediately preceding claim, having:

a percentage by number of particles having a size less than or equal to 5 mhi which is greater than 5%, and / or

a median particle size D50 of less than 10 mhi, and / or

a 90th percentile of particle sizes D90 less than 25 mhi, and / or

a 99.5 percentile of 99.5 μm particle sizes less than 40 μm, and / or a size dispersion index (D 9 _O -D ₁₀ ) / D ₁₀ less than 1.5.

A powder according to any one of the preceding claims, wherein the median size of the D50 particles is less than 8 mhi.

4. A method of manufacturing a powder according to any one of the preceding claims, said method comprising the following steps:

a) granulation of a particulate filler so as to obtain a powder of granules having a median size of 50 of between 20 and 60 microns, the particulate filler comprising, in percentage by weight on the basis of the oxides, more than 98% of a stabilized oxide selected from stabilized zirconium oxides, stabilized hafnium oxides and mixtures thereof, the stabilized oxide being stabilized by a stabilizer selected from oxides of Y, Ca, Ce, Sc, Mg, In, La, Gd, Nd, Sm, Dy, Er, Yb, Eu , Pr, and Ta, said "stabilizing oxides", and mixtures of these stabilizing oxides;

b) injecting said granule powder, via a carrier gas, through at least one injection port to a plasma jet generated by a plasma gun, under conditions causing a burst of more than 50% by number injected granules, in percentage by number, so as to obtain melt droplets;

c) cooling said melt droplets so as to obtain a feed powder according to any one of the preceding claims;

d) optionally, granulometric selection of said feed powder.

5. Method according to the immediately preceding claim, wherein the injection conditions are determined so as to cause bursting of more than 70% of the injected granules, in percentage by number.

6. Method according to the immediately preceding claim, wherein the injection conditions are determined so as to cause a burst of more than 90% of the injected granules, in percentage by number.

7. A method of manufacturing a powder according to any one of claims 4 to 6, wherein, in step b), the injection conditions are adapted to cause a burst rate granules identical to a gun with a plasma power having a power of 40 to 65 kW and generating a plasma jet in which the mass quantity of granules injected by each injection orifice, in g / min and per mm ² of the surface of said injection orifice is greater than 10 g / min per mm ² .

8. Method according to the immediately preceding claim, wherein the mass quantity of granules injected by each injection port, in g / min and per mm ² of the surface of said injection port is greater than 15 g / min per mm ^2. .

9. A method of manufacturing a powder according to any one of claims 4 to 8, wherein said injection port defines an injection channel having a length at least once greater than the equivalent diameter of said injection port.

The method of the immediately preceding claim, wherein said length is at least two times greater than said equivalent diameter.

11. A method of manufacturing a powder according to any one of claims 4 to

10, wherein in step b) the granule powder flow rate is less than 3 g / min per kW of plasma gun power.

12. Method according to one of claims 4 to 11, wherein the granulation comprises an atomization.

13. A method of manufacturing a dense and vertically cracked thermal barrier coating, said method comprising a step of plasma spraying, on a substrate, a powder according to any one of claims 1 to 3 or manufactured according to any one of any of claims 4 to 12.

14. The method of claim immediately preceding, wherein the substrate is a blade of a propeller or a blade of a turbine.