POWDER FOR A THERMAL BARRIER

Abstract

A powder of fused particles. The powder includes, in percentage by weight based on 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 the oxides of Y, Ca, Ce, Sc, Mg, In, La, Gd, Nd, Sm, Dy, Er, Yb, Eu, Pr, and Ta, called stabilizing oxides, and the mixtures of these stabilizing oxides. The powder has: a median particle size D.sub.50 under 15 m, a 90th percentile of the particle sizes, D.sub.90, under 30 m, and a size dispersion index (D.sub.90D.sub.10)/D.sub.10 below 2, and a relative density above 90%. The percentiles D.sub.n of the powder are the particle sizes corresponding to the percentages, by number, of n %, on the cumulative distribution curve of the powder particle size and the particle sizes are classified by increasing order.

Claims

1. A powder of fused particles, said powder containing, in percentage by weight based on 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 the oxides of Y, Ca, Ce, Sc, Mg, In, La, Gd, Nd, Sm, Dy, Er, Yb, Eu, Pr, and Ta, called stabilizing oxides, and the mixtures of these stabilizing oxides, said powder having: a median particle size D.sub.50 under 15 m, a 90th percentile of the particle sizes, D.sub.90, under 30 m, and a size dispersion index (D.sub.90D.sub.10)/D.sub.10 below 2; a relative density above 90%, the percentiles D.sub.n of the powder being the particle sizes corresponding to the percentages, by number, of n %, on the cumulative distribution curve of the powder particle size, the particle sizes being classified by increasing order.

2. The powder as claimed in claim 1, having: a percentage by number of particles having a size less than or equal to 5 m that is greater than 5%, and/or a median size of the particles D.sub.50 below 10 m, and/or a 90th percentile of the particle sizes D.sub.90 below 25 m, and/or a 99.5 percentile of the particle sizes D.sub.99.5 below 40 m, and/or a size dispersion index (D.sub.90D.sub.10)/D.sub.10 below 1.5.

3. The powder as claimed in claim 1, in which the median size of the particles D.sub.50 is below 8 m.

4. A method of making a powder as claimed in claim 1, said method comprising the following steps: a) granulation of a particulate charge so as to obtain a granular powder having a median size D.sub.50 between 20 and 60 microns, the particulate charge comprising, in percentage by weight based on 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 the oxides of Y, Ca, Ce, Sc, Mg, In, La, Gd, Nd, Sm, Dy, Er, Yb, Eu, Pr, and Ta, called stabilizing oxides, and the mixtures of these stabilizing oxides; b) injection of said granular powder, by means of a carrier gas, through at least one injection orifice into a plasma jet generated by a plasma gun, in conditions causing break-up of more than 50% by number of the granules injected, in percentage by number, so as to obtain molten droplets; c) cooling said molten droplets, so as to obtain a feed powder as claimed in claim 1; d) optionally, granulometric selection of said feed powder.

5. The method as claimed in claim 4, in which the injection conditions are determined such as to cause break-up of more than 70% of the granules injected, in percentage by number.

6. The method as claimed in claim 5, in which the injection conditions are determined such as to cause break-up of more than 90% of the granules injected, in percentage by number.

7. The method of making a powder as claimed in claim 4, in which, in step b), the injection conditions are adjusted to cause a degree of break-up of the granules identical to a plasma gun having a power from 40 to 65 kW and generating a plasma jet in which the amount by weight of granules injected by each injection orifice, in g/min and per mm.sup.2 of the surface area of said injection orifice is above 10 g/min per mm.sup.2.

8. The method as claimed in claim 7, in which the amount by weight of granules injected by each injection orifice, in g/min and per mm.sup.2 of the surface area of said injection orifice is above 15 g/min per mm.sup.2.

9. The method of making a powder as claimed in claim 4, in which said injection orifice defines an injection channel having a length at least once greater than the equivalent diameter of said injection orifice.

10. The method as claimed in the claim 9, in which said length is at least twice greater than said equivalent diameter.

11. The method of making a powder as claimed in claim 4, in which, in step b), the flow rate of granular powder is below 3 g/min per kW of power of the plasma gun.

12. The method as claimed in claim 4, in which granulation comprises atomization.

13. A method of making a dense, vertically cracked thermal barrier coating, said method comprising a step of plasma spraying, on a substrate, of a powder as claimed in claim 1.

14. The method as claimed in claim 1, in which the substrate is a propeller blade or a turbine vane.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0107] Other features and advantages of the invention will become clearer on reading the following description and on examining the appended drawings, in which:

[0108] FIG. 1 shows schematically step a) of a method according to the invention;

[0109] FIG. 2 shows schematically a plasma torch for making a feed powder according to the invention;

[0110] FIG. 3 shows schematically a method for making a feed powder according to the invention;

[0111] FIG. 4 illustrates the method that is used for evaluating the circularity of a particle.

DETAILED DESCRIPTION

[0112] Method of Making a Feed Powder

[0113] FIG. 1 illustrates an embodiment of step a) of a method of making a feed powder according to the invention.

[0114] Any known method of granulation may be used. In particular, a person skilled in the art knows how to prepare a slip suitable for granulation.

[0115] In one embodiment, a binder mixture is prepared by adding PVA (polyvinyl alcohol) 2 to deionized water 4. This binder mixture 6 is then filtered through a 5-m filter 8. A particulate charge, consisting of the powdered stabilized oxide 10 (for example of purity 99.99%), with a median size of 1 m, is mixed into the filtered binder mixture to form a slip 12. The slip may comprise, by weight, for example 55% of stabilized oxide and 0.55% of PVA, made up to 100% with water. This slip is injected into an atomizer 14 to obtain a granular powder 16. A person skilled in the art knows how to adjust the atomizer to obtain the desired granulometric distribution.

[0116] Preferably, the granules are agglomerates of particles of an oxide material having a median size preferably less than 3 m, preferably less than 2 m, preferably less than 1.5 m.

[0117] The granular powder may be sieved (5-mm sieve 18, for example) in order to remove any residues that have fallen from the walls of the atomizer.

[0118] The resultant powder 20 is a spray-dried only (SDO) granular powder.

[0119] FIGS. 2 and 3 illustrate an embodiment of the fusion step b) of a method of making a feed powder according to the invention.

[0120] An SDO granular powder 20, for example as made according to the method illustrated in FIG. 1, is injected by an injector 21 into a plasma jet 22 produced by a plasma gun 24, for example a ProPlasma HP plasma torch. The conventional devices for injection and plasma spraying may be used, for mixing the SDO granular powder with a carrier gas and injecting the resultant mixture into the center of the hot plasma.

[0121] However, the granular powder injected must not be consolidated (SDO) and injection into the plasma jet must be done violently, to promote rupture of the granules. The violent nature of the shocks determines the intensity of break-up of the granules, and therefore the median size of the powder produced.

[0122] A person skilled in the art knows how to adapt the injection parameters for violent injection of the granules, in such a way that the feed powder obtained at the end of steps c) or d) has a granulometric distribution according to the invention.

[0123] In particular, a person skilled in the art knows that: [0124] approximation of the angle of injection between the injection axis of the granules Y and the axis X of the plasma jet to 90, [0125] an increase in the flow rate of powder per mm.sup.2 of surface area of the injection orifice, [0126] a decrease in the flow rate of powder, in g/min, per kW of gun power, and [0127] an increase in the flow rate of the plasmagene gas are factors that promote rupture of the granules.

[0128] In particular, WO2014/083544 does not disclose injection parameters allowing rupture of more than 50% by number of the granules, as described in the examples hereunder.

[0129] It is preferable to inject the particles quickly so as to disperse them in a very viscous plasma jet flowing at a very high velocity.

[0130] When the injected granules come into contact with the plasma jet, they are subjected to violent shocks, which may break them into pieces. For penetration into the plasma jet, the granules to be dispersed, which are not consolidated, and in particular are not sintered, are injected at a high enough velocity so that they have high kinetic energy, but limited to ensure good efficacy of break-up. Absence of consolidation of the granules reduces their mechanical strength, and therefore their resistance to these shocks.

[0131] A person skilled in the art knows that the velocity of the granules is determined by the carrier gas flow rate and the diameter of the injection orifice.

[0132] The velocity of the plasma jet is also high. Preferably, the flow rate of plasmagene gas is greater than the median value recommended by the torch manufacturer for the selected anode diameter. Preferably, the flow rate of plasmagene gas is greater than 50 l/min, preferably greater than 55 l/min, preferably greater than 60 l/min.

[0133] A person skilled in the art knows that the velocity of the plasma jet can be increased by using an anode of small diameter and/or by increasing the flow rate of the primary gas.

[0134] Preferably, the flow rate of the primary gas is greater than 40 l/min, preferably greater than 45 l/min.

[0135] Preferably, the ratio of the flow rate of secondary gas, preferably dihydrogen (H.sub.2), to the flow rate of plasmagene gas (made up of the primary and secondary gases) is between 20% and 25%.

[0136] Of course, the energy of the plasma jet, influenced notably by the flow rate of the secondary gas, must be high enough to cause the granules to melt.

[0137] The granular powder is injected with a carrier gas, preferably without any liquid.

[0138] In the plasma jet 22, the granules are melted into droplets 25. Preferably, the plasma gun is set so that fusion is substantially complete.

[0139] Fusion advantageously makes it possible to reduce the level of impurities.

[0140] On leaving the hot zone of the plasma jet, the droplets are cooled rapidly by the surrounding cold air, but also by forced circulation 26 of a cooling gas, preferably air. The air advantageously limits the reducing effect of the hydrogen.

[0141] Preferably, the plasma torch comprises at least one nozzle arranged so as to inject a cooling fluid, preferably air, so as to cool the droplets resulting from heating of the granular 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 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 axis X of the plasma jet intersect.

[0142] Preferably, the angle of injection between the injection axis Y and the axis X of the plasma jet is greater than 85, preferably about 90.

[0143] Preferably, forced cooling is generated by a set of nozzles 28 arranged around the axis X of the plasma jet 22, so as to create a roughly conical or annular flow of cooling gas.

[0144] The plasma gun 24 is oriented vertically toward the ground. Preferably, the angle between the vertical and the axis X of the plasma jet is less than 30, less than 20, less than 10, preferably less than 5, preferably approximately zero. Advantageously, the flow of cooling gas is therefore perfectly centered relative to the axis X of the plasma jet.

[0145] 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.

[0146] Advantageously, forced cooling limits the generation of satellites, resulting from contact between very large hot particles and small particles in suspension in the densification chamber 32. Moreover, a cooling operation of this kind makes it possible to reduce the overall size of the processing equipment, in particular the size of the collecting chamber.

[0147] 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.

[0148] The densification chamber may be connected to a cyclone 34, the exhaust gases from which are directed to a dust collector 36, so as to separate very fine particles 40. Depending on the configuration, some feed particles according to the invention may also be collected in the cyclone. Preferably, these feed particles may be separated, in particular with an air classifier.

[0149] Optionally, the feed particles collected 38 may be filtered, so that the median size D.sub.50 is less than 15 microns.

[0150] Table 1 below gives the preferred parameters for making a feed powder according to the invention.

[0151] The characteristics in one column are preferably, but not necessarily, combined. The characteristics of both columns may also be combined.

TABLE-US-00001 TABLE 1 Step b) Preferred characteristics Even more preferred characteristics Gun High-performance gun with ProPlasma HP gun low wear (for processing the powder without contaminating it) Anode Diameter >7 mm HP8 anode (8 mm diameter) Cathode Doped-tungsten cathode ProPlasma cathode Gas injector Injection partially radial ProPlasma HP setup (swirling gas injection) Current 500-700 A 650 A Power >40 kW >50 kW, preferably about 54 kW Nature of the primary gas Ar or N.sub.2 Ar Flow rate of the primary gas >40 l/min, 50 l/min preferably >45 l/min Nature of the secondary gas H.sub.2 H.sub.2 Flow rate of the secondary gas >20 vol % of the plasmagene 25 vol % of the plasmagene gas mixture gas mixture Injection of the granular powder Total flow rate of injected powder <180 g/min <100 g/min (g/min)(3 injection orifices) (preferably <60 g/min per injector) Flow rate in g/min per kW of power <5 <2 Diameter of the injection orifices <2 mm <1.5 mm (mm) preferably <1.8 mm Flow rate in g/min per mm.sup.2 of >10 >15 and <20 surface area of injection orifice Nature of the carrier gas Ar or N.sub.2 Ar Flow rate of the carrier gas per >6.0 l/min, 7.0 l/min injection orifice preferably >6.5 l/min Angle of injection relative to the >85 90 axis X of the plasma jet (angle in FIG. 2) Distance between an injection >10 mm >12 mm orifice and the axis X of the plasma jet Cooling of the droplets Cooling parameters Conical or annular air curtain, oriented downstream of the plasma jet Angle between the direction of Downstream of the plasma Downstream of the plasma jet, injection of the cooling fluid, from a jet, 10 30 and <60 nozzle, and the axis X of the plasma jet Total flow rate of the forced cooling 10-70 Nm.sup.3/h 35-50 Nm.sup.3/h fluid Flow rate of the exhaust gas 100-700 Nm.sup.3/h 250-500 Nm.sup.3/h

[0152] The ProPlasmaHP plasma torch is sold by Saint-Gobain Coating Solutions. This torch corresponds to torch T1 described in WO2010/103497.

Examples

[0153] The following examples are supplied for purposes of illustration and do not limit the scope of the invention.

[0154] The feed powders 1 and 2 according to the invention and comparative 1 were made with a plasma torch similar to the plasma torch shown in FIG. 2 of WO2014/083544, starting from a source of zirconia powder yttriated at 8 wt %, called zirconia powder hereinafter, having a median size D.sub.50 of 1.5 micron, measured with a Microtrac laser particle analyzer.

[0155] In step a), a binder mixture is prepared by adding PVA (polyvinyl alcohol) binder 2 (see FIG. 1) to deionized water 4. This binder mixture is then filtered through a 5-m filter 8. The powdered zirconia 10 is mixed into the filtered binder mixture to form a slip 12. The slip is prepared so as to comprise, in percentage by weight, 55% of zirconia powder and 0.55% of PVA, the balance to 100% being deionized water. The slip is mixed intensively using a high shear rate mixer.

[0156] The granules are then obtained by atomization of 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 0.381/min.

[0157] The speed of the rotating atomizing wheel, driven by a Niro FS1 motor, is set so as to obtain the targeted sizes of the granules 16.

[0158] The air flow rate is adjusted to maintain the inlet temperature at 295 C. and the outlet temperature close to 125 C. so that the residual moisture content of the granules is between 0.5% and 1%.

[0159] The granular powder is then sieved with a sieve 18 in order to extract the residues therefrom and obtain SDO granular powder 20.

[0160] In step b), the granules from step a) are injected into a plasma jet 22 (see FIG. 2) produced with a plasma gun 24. The injection and fusion parameters are given in Table 2 below.

[0161] In step c), for cooling the droplets, seven Silvent 2021L nozzles 28, sold by Silvent, were fixed on a Silvent 463 annular nozzle holder, sold by Silvent. The nozzles 28 are spaced regularly along the annular nozzle holder, so as to generate an approximately conical air stream.

TABLE-US-00002 TABLE 2 Treatment of the powder Spray dried + plasma spraying Granules (particles obtained after spray drying) Type of granules Spray-dried powder of yttriated zirconium oxide Granules D.sub.10 (m) 25.8 Granules D.sub.50 (m) 42.1 Granules D.sub.90 (m) 66.1 Average bulk density 1.2 Step b): injection Total feed flow rate of granules 90 g/min 120 g/min Flow rate in g/min per kW of gun power 1.7 2.5 Number of injection orifices (powder lines) 2 3 Angle of injection relative to 90 (normal to the jet) 80 downstream the X axis of the plasma jet (FIG. 2) Distance of each injector 12 mm 12 mm (radially from gun axis) Diameter of the injection orifice 1.5 mm 2.0 mm of each injector Flow rate of the argon carrier 7.0 l/min 4.0 l/min gas per injection orifice Flow rate in g/min per mm.sup.2 25.5 12.7 of surface area of injection orifice Step b): fusion Plasma gun ProPlasma HP Diameter of the anode of the plasma gun 8 mm Voltage (V) 83 74 Power (kW) 54 48 Plasmagene gas mixture Ar + H2 Flow rate of the plasmagene gas 67 l/min 48 l/min Proportion of H.sub.2 in the plasmagene gas 25% Nature of the primary gas Ar Calculated flow rate of the primary gas 50 l/min 36 l/min Current intensity of the plasma arc 650 A Step c): cooling Annular cooling nozzles 7 nozzles Silvent 2021 L fixed Silvent 463 Total flow rate of cooling air (Nm.sup.3/h) 20 20 Air flow rate in the cyclone (Nm.sup.3/h) 650 650 Step d): granulometric selection Upper threshold of granulometric selection 20 microns 10 microns No selection (by sieving) (air classif.) Lower threshold of granulometric selection 5 microns 2.5 microns No selection (air classif.) (air classif.) Feed particles collected (feed powder) Reference Invention 1 Invention 2 Comparative 1 D.sub.10 (m) 7.5 3.2 19.2 D.sub.50 (m) 15.1 6.5 37.7 D.sub.90 (m) 18.3 9.2 62.2 (D.sub.90 D.sub.10)/ 1.4 1.9 2.2 D.sub.10 Fraction by number: 10 m (%) 23 100 2 Fraction by number: 5 m (%) 0 34 1 Relative density calculated in % 91 92 81 after mercury porosimetry at a pressure of 200 MPa

[0162] The cumulative specific volume of the pores having a radius less than 1 m, in the granules, was 340.10.sup.3 cm.sup.3/g.

[0163] The tests show that a feed powder according to the invention has a relative density greater than 90%.

[0164] The invention thus supplies a feed powder having a size distribution and a relative density that give the coating a very high density. Furthermore, this feed powder may be effectively sprayed by plasma, with good productivity.

[0165] 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 relative to a powder not fused by plasma of the same size, which allows injection without complex feeding means.

[0166] Of course, the invention is not limited to the embodiments described and presented.