Method for manufacturing an abradable layer

A process for manufacturing an abradable layer, includes compressing a powder composition including at least micrometric ceramic particles having a number-average form factor greater than or equal to 3, a mass content of said micrometric ceramic particles in the powder composition being greater than or equal to 85%, the form factor of a particle being defined as the ratio [largest dimension of the particle]/[largest cross-sectional dimension of the particle], and sintering the powder composition thus compressed to obtain the abradable layer, wherein a temperature imposed during sintering, the sintering time and the compression pressure applied are selected so as to obtain a volume porosity rate of the abradable layer greater than or equal to 20%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of PCT/FR2019/051510, filed Jun. 20, 2019, which in turn claims priority to French patent application number 1855682 filed Jun. 25, 2018. The content of these applications are incorporated herein by reference in their entireties.

The present invention relates to a process for manufacturing an abradable layer and a substrate coated with this abradable layer.

BACKGROUND OF THE INVENTION

Abradable layers are currently used in gas turbines to minimize the functional clearance, and thus leaks, between rotating and static parts. For high-pressure turbine applications, abradable seals are deposited on ring sectors attached to the casing. When turbine blades come into contact with the abradable material, the latter should wear out as a matter of priority, thus maintaining the aerodynamic performance of the engine.

However, it is also necessary to protect the substrate from high temperatures, which can reach 1600° C., and from erosion by the flow of gas at high temperature and pressure. For this purpose, a ceramic or refractory metal-based coating is typically formed by thermal spraying on the static parts, namely the ring sectors, to form a thermal barrier-type protective coating. However, the coatings thus obtained may not exhibit high abradability, which can lead to wear of the blade tips during operation, resulting in complex and costly repairs.

In order to increase the abradable nature of thermal barriers, various solutions have been considered in the state of the art. In this respect, mention may be made of the incorporation of blowing agents in order to increase the porosity of the barrier. However, these solutions may not be fully satisfactory because they can lead to a significant degradation of the erosion resistance of the coating and therefore the service life of the barrier and the underlying substrate.

There thus exists a need to provide a process for manufacturing an abradable layer having both good abradability and good erosion resistance.

OBJECT AND SUMMARY OF THE INVENTION

The invention is aimed at a process for manufacturing an abradable layer, comprising the following steps:

compressing a powder composition comprising at least micrometric ceramic particles having a number-average form factor greater than or equal to 3, the mass content of said micrometric ceramic particles in the powder composition being greater than or equal to 85%, and

sintering the powder composition thus compressed to obtain the abradable layer.

Unless otherwise stated, the number-average form factor corresponds to the number-average value of the following ratio R calculated for each particle of a given set of particles, with R denoting the ratio [largest dimension of the particle]/[largest cross-sectional dimension of the particle].

The use of the powder composition defined above and a pressure-sintering technique advantageously makes it possible to obtain a layer with both good abradability and good erosion resistance. Moreover, the inventors noted that abradable layers formed by pressure sintering have better erosion resistance than layers formed by plasma spraying with the same porosity rate or, in certain cases, a higher porosity rate.

In an example embodiment, the powder composition further comprises nanometric ceramic particles having a number-average form factor comprised between 0.7 and 1.3, preferably between 1.0 and 1.3, the mass content of said nanometric ceramic particles in the powder composition being less than or equal to 15%.

The presence of nanometric ceramic particles in the indicated contents advantageously further improves the erosion resistance of the abradable layer obtained without affecting the abradability.

In particular, the mass content of said nanometric ceramic particles in the powder composition can be between 1% and 10%.

However, the presence of nanometric ceramic particles is not compulsory insofar as, according to a variant, the powder composition consists essentially of said micrometric ceramic particles.

In an example embodiment, the mass content of said micrometric ceramic particles in the powder composition is greater than or equal to 90%.

In an example embodiment, said micrometric ceramic particles comprise at least acicular particles having a number-average form factor comprised between 3 and 5. As a variant or in combination, said micrometric ceramic particles comprise at least fibrous particles having a number-average form factor strictly greater than 5.

In an example embodiment the volume porosity rate of the abradable layer is greater than or equal to 20%.

“Volume porosity rate” is understood to mean the ratio between the volume of interstitial spaces separating the grains of the material considered and the overall volume of said material.

Such a porosity rate is advantageous in order to further improve the abradability of the layer formed.

In an example embodiment a compression pressure comprised between 12.5 MPa and 100 MPa is applied to the powder composition during sintering.

Such values for the compression pressure help to optimize the porosity rate of the abradable layer, and to optimize the abradability/erosion resistance compromise.

In an example embodiment the sintering time is comprised between 1 minute and 10 minutes.

Such values for the sintering time help to optimize the porosity rate of the abradable layer and to optimize the abradability/erosion resistance compromise.

In an example embodiment the required temperature during sintering is between 900° C. and 1150° C.

Such values for the temperature imposed during sintering help to optimize the porosity rate of the abradable layer and to optimize the abradability/erosion resistance compromise.

In an example embodiment, the powder composition is sintered using the spark plasma sintering (SPS) technique.

The invention is also aimed at a process for manufacturing a substrate coated with an abradable layer, the substrate being a turbomachine part and the process comprising:the deposition of the powder composition on a surface of the substrate, andthe formation of the abradable layer on the substrate from the powder composition thus deposited by using a process as described above.

This variant concerns the case in which the powder composition is deposited on the substrate first and then the abradable layer is formed directly on the substrate by pressure sintering of the deposited powder composition.

As a variant, the invention is also aimed at a process for manufacturing a substrate coated with an abradable layer, the substrate being a turbomachine part and the process comprising:the formation of the abradable layer by using a process as described above,the deposition of the abradable layer thus formed on a surface of the substrate, andthe bonding of the abradable layer thus deposited on the surface of the substrate.

This variant concerns the case where the abradable layer is formed first, then this abradable layer is deposited on the substrate and then bonded to the substrate.

In an example embodiment, one of the two following conditions is verified:the substrate is metallic, and said micrometric ceramic particles, as well as said nanometric ceramic particles if present, comprise at least zirconia, for example yttria-stabilized zirconia (YSZ) or yttria-partially-stabilized zirconia (YPSZ) or a mixture of zirconia and alumina, orthe substrate is made of a ceramic matrix composite (CMC) material, and said micrometric ceramic particles, as well as any nanometric ceramic particles if present, are made of rare-earth silicate.

Such combinations of substrate and abradable layer materials are advantageous because they minimize differential expansion during operation.

In an example embodiment the substrate is a turbine or compressor ring sector.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG.1represents, in a vertical cross-section along a vertical plane passing through its main axis A, a turbofan engine1. It comprises, from upstream to downstream according to the flow of air, a fan2, a low-pressure compressor3, a high-pressure compressor4, a combustion chamber5, a high-pressure turbine6, and a low-pressure turbine7.

The high-pressure turbine6comprises a plurality of blades6arotating with the rotor and rectifiers6bmounted on the stator. The stator of the turbine6comprises a plurality of stator rings arranged opposite the moving blades6aof the turbine6.FIG.2illustrates a stator ring, which is divided into several sectors each comprising a substrate10coated with an abradable layer12. The rotor blades6aof the rotor rub against the abradable layer12in case of radial incursion of the rotor.

An example embodiment of the abradable layer12will be described in connection withFIGS.3A and3B. TheseFIGS.3A and3Bschematically illustrate an example embodiment of the process according to the invention.

The substrate10to be coated is placed in the cavity of a mold20. The powder composition30is then deposited on a surface S of the substrate10. As shown inFIG.3B, the mold20is then closed. A bearing surface of its lid25is applied against the layer of powder composition30so as to compress the latter on the substrate10. The compression pressure applied to the powder composition30can be uniaxial. The thickness of the layer of powder composition30is thus reduced due to the compression between the substrate10and the lid25. The powder composition30undergoing the compression pressure is then sintered. The abradable layer12is obtained at the end of this sintering step. The particles forming the powder composition30may be made of a thermal barrier material, such as yttriated zirconia, yttrium-partially-stabilized zirconia, a mixture comprising zirconia and alumina, or a rare-earth silicate, for example a rare-earth monosilicate or disilicate. A spark plasma sintering (SPS) technique can be used to produce the abradable layer12.

In the example shown, the abradable layer12obtained has a substantially uniform density. As a variant, abradable layers of varying density could be formed as described in WO 2017/103420.

An example is described in connection with the figures in which the abradable layer12is formed directly on the substrate10from the powder composition30previously deposited on the substrate10.

In a variant not shown, the abradable layer12can first be formed on a substrate separate from the substrate by using the pressure sintering process that was described above. According to this variant, the abradable layer12thus formed is then separated from the substrate to be positioned on the surface S of the substrate10. The abradable layer12thus positioned is then bonded to the surface S of the substrate10in order to obtain the coated substrate. This bonding can be carried out by brazing, sintering or by means of attachments (for example bolting).

The abradable layer12formed is especially suited to equip high- or low-pressure turbine rings or compressor rings, for example in the aeronautical field, and most especially in aircraft turbojet engines.

Various details relative to the substrate10, the powder composition30and the operating parameters that may be imposed during the process will now be described.

The substrate10can be a part for a turbomachine. The substrate10can be made of a metallic material, for example superalloy. When the substrate10is made of a metallic material, the latter can for example be formed from one of the following materials: alloy AM1, alloy C263 or alloy M509.

As a variant, the substrate10can be made of CMC material. In this case, the substrate10can have a woven fibrous reinforcement, formed from carbon or silicon carbide fibers, densified by a ceramic matrix, for example comprising silicon carbide. A detailed example of the fabrication of CMC ring sectors is described in particular in US 2012/0027572.

The substrate10can be coated with a bond coat (not shown) which the abradable layer12is intended to coat. In the case of a metallic substrate10, an MCrAlY bond coat, for example a CoNiCrAlY bond coat, can be used. In the case of a CMC substrate, a mullite bond coat can be used, for example.

Concerning the powder composition30, it has been indicated above that the mass content of micrometric ceramic particles having a number-average form factor greater than or equal to 3 in this composition is greater than or equal to 85%. This mass content may be greater than or equal to 90%, preferably greater than or equal to 95%.

As indicated above, the micrometric ceramic particles may comprise acicular particles having a number-average form factor comprised between 3 and 5, fibrous particles having a number-average form factor greater than 5, or a mixture of such particles. The number-average form factor of the fibrous particles may, in particular, be comprised between 15 and 25.

According to a particular variant, the entirety of the micrometric ceramic particles can be constituted by the fibrous particles. According to another particular variant, the entirety of the micrometric ceramic particles can be constituted by the acicular particles. According to still another particular variant, the entirety of the micrometric ceramic particles can be constituted by a mixture of acicular and fibrous particles.

The acicular particles may have an average diameter in the non-agglomerated state (or average width) greater than or equal to 15 μm, for example comprised between 15 μm and 35 μm. The acicular particles may have an average length greater than or equal to 55 μm, for example comprised between 55 μm and 75 μm.

The average diameter and average length can be measured using a field-effect scanning electron microscope (SEM-FEG). The average diameter and average length are number averages.

The acicular particles usable in the context of the invention can be obtained by sol-gel process under the conditions described in the following article: C. Viazzi & al., 2006, Solid State Sciences 8 1023-1028,“Synthesis of Yttria Stabilized Zirconia by sol-gel route: Influence of experimental parameters and large scale production”.

By way of example, a succession of possible steps to synthesize acicular yttriated zirconia particles usable in the context of the invention is provided below:mix acetyl-acetone in 1-propanol and zirconium propoxide (Zr(OC3H7)4), then mix under stirring in 1-propanol,mix the composition thus obtained with a 0.5 mol/L solution of yttrium nitrate in 1-propanol, then mix under stirring in 1-propanol,mix the composition thus obtained with 10 mol/L water in 1-propanol for 15 minutes at 20° C. in order to obtain first a sol and then a gel,dry the gel conventionally at 70° C. for 24 hours in order to obtain a xerogel,heat treat the xerogel obtained at 1000° C. for 2 hours in order to obtain the acicular yttriated zirconia particles.

The fibrous particles may have an average diameter in the non-agglomerated state (or average width) greater than or equal to 6 μm, for example comprised between 6 μm and 8 μm. The fibrous particles may have an average length greater than or equal to 125 μm, for example comprised between 125 μm and 215 μm.

By way of example of fibrous particles usable in the context of the invention, mention may be made of the particles marketed under the name ZYBF-5 (CF010) by the firm Zircar.

As indicated above, the powder composition may comprise nanometric ceramic particles having a number-average form factor comprised between 0.7 and 1.3, preferably between 1.0 and 1.3, and present in a limited amount so as not to degrade the abradability of the layer obtained.

The mass content of the nanometric ceramic particles in the powder composition is preferably less than or equal to 10%, preferably less than or equal to 5%.

The mass content of the nanometric ceramic particles in the powder composition can, for example, be comprised between 1% and 15%, for example between 5% and 15%, for example between 5% and 10% or between 10% and 15%. The mass content of the nanometric ceramic particles in the powder composition can, for example, be comprised between 1% and 10%, for example between 1% and 5%.

The nanometric ceramic particles can have an average diameter in the non-agglomerated state of less than or equal to 70 nm, for example comprised between 30 nm and 70 nm.

The nanometric ceramic particles can have a number-average form factor comprised between 0.9 and 1.1, preferably between 1.0 and 1.1, for example substantially equal to 1. The nanometric ceramic particles can thus have a substantially spherical shape.

By way of example of usable nanometric ceramic particles, mention may be made of the particles marketed under the name Zirconia TZ 6Y by the firm Tosoh.

The powder composition30can consist essentially of micrometric ceramic particles, and any nanometric ceramic particles that may be present.

By way of illustration, photographs of particles usable in the invention are provided inFIGS.4A to4C.FIG.4Ais a photograph of nanometric ceramic particles having a substantially spherical shape and a form factor substantially equal to 1.FIG.4Bis a photograph of acicular micrometric ceramic particles having a number-average form factor comprised between 3 and 5.FIG.4Cis a photograph of fibrous micrometric ceramic particles having a number-average form factor strictly greater than 5.

Various details concerning the substrate10and the powder composition30have just been described. Details concerning the abradable layer12that can be obtained and the operating conditions under which it can be applied are now described.

The volume porosity rate of the abradable layer can be greater than or equal to 20%, for example 30%, for example greater than or equal to 35%. This volume porosity rate can be comprised between 20% and 50%, for example between 30% and 50%, for example between 35% and 50%.

The temperature imposed during sintering, the sintering time and/or the applied compression pressure can be modified in order to vary the volume porosity of the abradable layer12obtained. The temperature, the sintering time and/or the compression pressure can be increased in order to decrease the volume porosity of the abradable layer12.

A possible example of the change in compression pressure and temperature during the manufacture of the abradable layer12is shown inFIGS.5A and5B.

The assembly of the substrate10and the powder composition30is initially brought to a first temperature T1, for example greater than or equal to 600° C. While the assembly is brought to this first temperature T1, the compression pressure increases until it reaches, at a first time t1, a plateau at a value Pc which corresponds to the compression pressure that will be applied during the sintering of the powder composition30.

The compression pressure Pc imposed on the powder composition30during sintering may be less than or equal to 100 MPa, for example less than or equal to 50 MPa. This compression pressure Pc can be comprised between 12.5 MPa and 100 MPa, for example between 25 MPa and 100 MPa, for example between 25 MPa and 50 MPa or between 50 MPa and 100 MPa. The compression pressure Pc is maintained throughout the sintering of the powder composition30.

From the first time t1, the temperature imposed on the substrate10and on the powder composition30is increased to the sintering temperature Tf. The temperature reaches the sintering temperature Tfat a second time t2and is then maintained at this value. The sintering temperature Tfdepends on the nature of the powder composition30used. The sintering temperature Tfcan be comprised between 900° C. and 1150° C., for example between 1050° C. and 1150° C.

The sintering temperature Tfand the compression pressure Pc are maintained until the third time t3. The sintering time (t3−t2) can be greater than or equal to 1 minute, for example comprised between 1 minute and 10 minutes, for example between 1 minute and 6 minutes. Once sintering is finished, the compression pressure and the temperature are gradually reduced and the substrate10coated with the abradable layer12is then recovered.

In the example shown, a first rate of temperature increase between the first temperature T1and a second, higher temperature T2reached at the intermediate time tiis imposed, followed by a second rate of temperature increase, lower than the first rate of temperature increase, between the second temperature T2and the sintering temperature Tf. By way of illustration, the first rate of temperature increase may be greater than or equal to 100° C./minute and the second rate of temperature increase may be less than or equal to 50° C./minute. Still by way of illustration, the second temperature T2can be greater than or equal to 1000° C. However, it is not beyond the scope of the invention when the rate of temperature rise is constant between the first temperature T1and the sintering temperature Tf.

EXAMPLES

A powder composition consisting of 100% by mass of micrometric yttriated zirconia fibers marketed under the name ZYBF-5 (CF010) by the firm Zircar was shaped by spark plasma sintering under the following conditions:sintering temperature: 1100° C.,dwell time at sintering temperature: 6 minutes,compression pressure applied during sintering: 50 MPa.

The layer obtained had a volume porosity rate of about 50%.

Abradability tests were carried out under the following experimental conditions:3 blades are fixed on the test disc,blade rotation speed: 210 m/s,rate of incursion into the abradable layer: 50 μm/s,penetration distance: 500 μm.

The trace obtained in the layer and the condition of the blade tips are visible respectively onFIGS.6and7.

The area where the contact took place has very low metal transfers and therefore a very satisfactory abradable behavior due to the fact that the blade tips are little worn. Pre- and post-test measurements of blade thickness (measurement in the center of the blade) and mass made it possible to account for blade wear. The appearance of striations was observed at the top of the blades, which indicates low wear. This was confirmed by the results of the thickness and mass variation measurements which are reported in the table below.

By way of comparison, a powder composition comprising the following mixture was sintered by spark plasma sintering:80% by mass of micrometric yttriated zirconia fibers marketed under the name ZYBF-5 (CF010) by the firm Zircar, and20% by mass of nanometric ceramic particles marketed under the name Zirconia TZ 6Y by the firm Tosoh.

The conditions imposed during spark plasma sintering were as follows:sintering temperature: 1110° C.,dwell time at sintering temperature: 6 minutes,compression pressure applied during sintering: 50 MPa.

It was noted that such a material has a much less abradable behavior and wear of the blade tips (seeFIGS.8and9respectively). This behavior is explained by the excessive content of nanometric ceramic particles (20% by mass) which negatively affects the abradability.

The expression “comprised between . . . and . . . ” should be understood to include the bounds.