Part made of cmc material with pre-cracked environmental barrier for thermomechanical accommodation

Abstract

The present invention relates to a coated part (1) intended to be mounted in a turbomachine, comprising: - a substrate (3) made of composite material with an at least partially ceramic matrix, and - an environmental barrier (12) on the substrate and comprising: (i) a fastening layer (5) comprising silicon present on a surface of the substrate, and (ii) an environmental barrier layer (7) covering the fastening layer, the environmental barrier layer comprising an internal region (73) which is a barrier to oxidizing and corrosive species and non-cracked and situated towards the fastening layer, and an external region (71), opposite the fastening layer and covering the internal region, having a network of cracks (75) for thermomechanical adaptation having a distance (d) between adjacent cracks of between 10 µm and 50 µm and of which the cracks extend to a depth (p75) of between 50% and 95% of a thickness (e7) of the environmental barrier layer.

Description

[0001] Description Title of the invention: CMC material part with pre-cracked environmental barrier for thermomechanical accommodation

[0002] Technical Field

[0003] A particular field of application of the invention is the protection of at least partially ceramic matrix composite materials ("CMC materials") forming hot parts of gas turbines, such as combustion chamber walls, or turbine rings, turbine nozzles or turbine blades, for aeronautical engines or industrial turbines.

[0004] Prior art

[0005] Improving efficiency and reducing pollutant emissions means that ever higher temperatures are required in gas turbines.

[0006] It has therefore been proposed to replace metallic materials with CMC materials, particularly for combustion chamber walls or turbine rings. Indeed, CMC materials are known to have both good mechanical properties allowing their use for structural elements and the ability to maintain these properties at high temperatures. Due to their better resistance to high temperatures, CMC materials require less cooling. This cooling traditionally comes from a draw in the compressor which impacts the efficiency of the turbomachine, CMC materials therefore make it possible to improve engine efficiency which reduces fuel consumption.Furthermore, their use contributes to optimizing the performance of turbomachines, in particular by reducing the overall mass of the turbomachine, which further contributes to a reduction in fuel consumption and therefore to a significant reduction in polluting emissions.

[0007] CMC materials may include a fibrous reinforcement of refractory fibers, typically carbon or ceramic, which is densified by a ceramic matrix, for example silicon carbide. Under the operating conditions of aeronautical turbines, i.e. at high temperature in an oxidizing and humid atmosphere, CMC materials are sensitive to corrosion. Corrosion of CMC results from the oxidation of silicon carbide into silica which, in the presence of water vapor, volatilizes in the form of silicon hydroxides Si(OH)4. Corrosion phenomena lead to a recession of the CMC and affect its service life. In order to limit this degradation in operation, it has been considered to form environmental barriers on the surface of CMC materials ("Environmental Barrier Coating"; "EBC").Environmental barriers may include a silicon bonding layer and a rare earth silicate layer positioned on the bonding layer. The bonding layer serves, on the one hand, to improve the adhesion of the rare earth silicate layer and, on the other hand, to form a protective silica layer, whose low oxygen permeability helps protect the CMC against oxidation. The rare earth silicate layer, for its part, limits the diffusion of water vapor towards the silica layer formed by oxidation of the silicon and consequently limits its recession.

[0008] It remains nevertheless desirable to further improve the protection of CMC materials in oxidizing and corrosive environments at high temperatures, particularly those greater than or equal to 800°C.

[0009] Statement of the invention

[0010] The invention relates to a coated part intended to be mounted in a turbomachine, comprising:

[0011] - a substrate made of composite material with an at least partially ceramic matrix, and

[0012] - an environmental barrier on the substrate and comprising: (i) a bonding layer comprising silicon present on a surface of the substrate, and (ii) an environmental barrier layer covering the bonding layer, said environmental barrier layer comprising an internal barrier region to oxidizing and corrosive species which is not cracked and is located on the side of the bonding layer, and an external region, opposite the bonding layer and covering the internal region, having a network of thermomechanical adaptation cracks having a distance between adjacent cracks of between 10 pm and 50 pm and the cracks of which extend over a depth of between 50% and 95% of a thickness of said environmental barrier layer.

[0013] The inventors have found that the environmental barriers of the prior art may exhibit uncontrolled cracking during operation, which is explained by the fact that the barrier is initially, upon manufacture, in compression. This compressive stress relaxes by creep upon subjection to the high temperatures encountered in operation, which then results in cracking upon cooling due to a return to a tensile field. This cracking may reach the bonding layer, which leads to uncontrolled growth of the protective silica layer, which may result in spalling of the environmental barrier by increasing localized stresses at the interface of the bonding layer with the environmental barrier layer.Remarkably, the inventors found that the use of an environmental barrier, pre-cracked in a controlled manner before its first use with a controlled distance between adjacent cracks and a controlled cracking depth, makes it possible to favorably accommodate thermomechanical stresses in operation. The thermomechanical adaptation crack network makes it possible to avoid uncontrolled cracking of the barrier by dissipation of energy via the pre-existing crack network, while retaining an internal uncracked region providing the desired seal against oxidizing and corrosive species in order to control the growth of the protective silica layer and prevent spalling.

[0014] In an exemplary embodiment, the distance between adjacent cracks of the thermomechanical adaptation crack network is between 15 pm and 30 pm.

[0015] Such a feature helps to further improve the accommodation of thermomechanical constraints in operation.

[0016] In an exemplary embodiment, the cracks of the thermomechanical adaptation crack network extend to a depth of between 75% and 90% of the thickness of said environmental barrier layer. Such a feature helps to further improve the accommodation of thermomechanical stresses in operation, while providing optimal sealing against oxidizing and corrosive species, ensured by the internal region.

[0017] In one exemplary embodiment, the environmental barrier layer comprises a silicate of at least one rare earth, in particular the environmental barrier layer may comprise a yttrium disilicate, an ytterbium disilicate, or a mixture of these two compounds. According to one variant, the environmental barrier layer comprises mullite.

[0018] In an exemplary embodiment, the part further comprises an additional coating located on the external region of the environmental barrier layer, said additional coating being a thermal barrier with a thickness greater than or equal to 100 μm, or a protective layer against calcium and magnesium aluminosilicates with a thickness greater than or equal to 50 μm.

[0019] Such a characteristic advantageously makes it possible to further functionalize the protective coating and thus improve the protection provided.

[0020] In an exemplary embodiment, the inner region and the outer region of the environmental barrier layer are made of the same material.

[0021] Such a characteristic advantageously makes it possible to optimize the thermomechanical compatibility between the two regions and therefore to optimize the control of the pre-cracking of the environmental barrier.

[0022] In particular, the inner region and the outer region of the environmental barrier layer may be made of yttrium disilicate, ytterbium disilicate, or a mixture of these two compounds.

[0023] Brief description of the drawings

[0024] [Fig. 1] Figure 1 shows, schematically and partially, an example of a part coated according to the invention.

[0025] [Fig. 2] Figure 2 represents, schematically and partially, a precursor deposit of the environmental barrier layer of the part of Figure 1. [Fig. 3] Figure 3 represents, schematically and partially, the effect of a cracking heat treatment on the precursor deposit of Figure 2.

[0026] [Fig. 4] Figure 4 is a snapshot of an example of an environmental barrier layer usable within the framework of the invention obtained after cracking treatment of a precursor deposit.

[0027] Description of the embodiments

[0028] Figure 1 shows an example of part 1 comprising a 3 CMC material provided with an environmental barrier 12. The 3 CMC material may comprise a fibrous reinforcement which may be made of carbon fibers (C) or ceramic fibers, for example silicon carbide fibers (SiC) or formed essentially of SiC, including Si-C-O or Si-CON fibers, that is to say also containing oxygen and possibly nitrogen. Such fibers are produced by the company NGS under the reference “Nicalon” or “Hi-Nicalon” or “Hi-Nicalon Type-S”, or by the company Ube Industries under the reference “Tyranno-ZMI”. The ceramic fibers may be coated with a thin interphase layer of pyrolytic carbon (PyC), boron nitride (BN) or boron-doped carbon (BC, with 5% at. to 20% at. of B, the remainder being C). The fibrous reinforcement is densified by a matrix that is at least partially ceramic, for example predominantly ceramic in volume.The ceramic matrix may comprise silicon carbide or a Si-BC ternary system, for example. The matrix may be at least partly formed by CVI in a manner known per se. Alternatively, the matrix may be at least partly formed by liquid means (impregnation with a precursor resin of the matrix and transformation by crosslinking and pyrolysis, the process being able to be repeated) or by infiltration of silicon in the molten state (Melt-Infiltration process). In the latter case, a powder is introduced into the possibly partially densified fibrous reinforcement, this powder being able to be a carbon powder, a ceramic powder for example silicon carbide, or a mixture of such powders, and a metallic composition based on silicon in the molten state is then infiltrated to form a SiC-Si type matrix.The fibrous reinforcement may or may not be woven; it does not go beyond the scope of the invention when the fibrous reinforcement is in the form of short fibers dispersed in the material 3. Alternatively, a particulate reinforcement may be used in the form of grains dispersed in the material 3.

[0029] The environmental barrier 12 may be formed on the entire external surface S of the CMC material 3 or on only a portion of this surface S, for example when only a portion of the surface S must be protected. The environmental barrier 12 comprises a bonding layer 5 and an environmental barrier layer 7 covering the bonding layer 5. In the example illustrated, the bonding layer 5 is present in contact with the surface S of the composite material 3. Furthermore, in this example, the layer 7 is in contact with the bonding layer 5. The bonding layer 5 may, in a manner known per se, form in operation a protective silica layer against oxidation (so-called “TGO” layer for “Therally Grown Oxide”). The bonding layer 5 may be made of silicon.

[0030] Layer 7 provides protection against oxidation and corrosion at high temperature by limiting in particular the diffusion of water vapor and oxygen towards the bonding layer 5 and the CMC material 3. Layer 7 may comprise a rare earth silicate, for example a rare earth monosilicate and / or a rare earth disilicate. Layer 7 may comprise at least one rare earth element chosen from yttrium Y, scandium Sc and the lanthanides. In particular, the rare earth element may be chosen from yttrium Y and ytterbium Yb. It will be noted that it is not outside the scope of the invention if layer 7 comprises several rare earth elements, for example yttrium and ytterbium.

[0031] Figure 1 shows an environmental barrier layer 7 which comprises an internal region 73 which is not cracked and is located on the side of the bonding layer 5. The internal region 73 provides a seal against oxidizing and corrosive species. The region 73 may be located in contact with the bonding layer 5. The layer 7 further comprises an external region 71, opposite the bonding layer 5, and having a network of cracks 75 for thermomechanical adaptation. The region 73 and the region 71 may be made of the same material, for example yttrium disilicate, ytterbium disilicate, or a mixture of these two compounds. Details relating to the formation of the environmental barrier 12 will be described below. The outer region 71 covers the inner region 73. The cracks 75 extend over the entire thickness of the region 71 to the region 73 which is not cracked. The network of cracks 75 can extend over the entire surface area of ​​the region 71.The crack network 75 has a controlled distance d between adjacent cracks of between 10 pm and 50 pm, preferably between 15 pm and 30 pm. In particular, the average distance between adjacent cracks 75 is between 10 pm and 50 pm, preferably between 15 pm and 30 pm. Generally, the distance d between adjacent cracks may be regular over the layer 7 and correspond to a cracking pitch. Alternatively, this distance d may be variable between different pairs of adjacent cracks while always being between 10 pm and 50 pm, preferably between 15 pm and 30 pm, for all the cracks 75 present. According to one example, all the distances d between adjacent cracks may be between m / 2 and 2m, where m denotes the average distance between adjacent cracks 75.The cracks 75 may each extend over a depth p75 of between 50% and 95%, preferably between 75% and 90%, of the thickness e7 of the environmental barrier layer 7. The cracks 75 do not reach the bonding layer 5 and make it possible to preserve the internal sealing region 73 in order to control the growth of the protective silica layer in operation, and to avoid flaking. The thickness e7 of the layer 7 may be greater than or equal to 50 μm, for example greater than or equal to 200 μm, for example greater than or equal to 250 μm. This thickness e7 may be between 50 pm and 1000 pm, for example between 200 pm and 1000 pm or between 250 pm and 1000 pm, for example between 200 pm and 700 pm or between 250 pm and 700 pm. Generally speaking, the thickness e73 of the internal sealing region 73 may be between 1 pm and 10 pm, for example between 2 pm and 5 pm.If desired, an additional thermal barrier or CMAS protection coating may be deposited on region 71 in a manner known per se, or region 71 may define an external surface of the coated part 1.

[0032] Details relating to the manufacture of the coating on the CMC material 3 will now be described, in particular concerning the formation of the primer layer 5 and the environmental barrier layer 7. The primer layer 5 may be formed by chemical vapor deposition from a precursor comprising silicon comprising, for example, a silane, a monochlorosilane, a dichlorosilane, and / or a trichlorosilane. Two examples of formation of the primer layer 5 by chemical vapor deposition are described below.

[0033] According to a first example, the temperature imposed during the deposition of the bonding layer 5 may be between 900°C and 1150°C, for example between 1100°C and 1150°C, and the pressure imposed during this deposition may be between 15.3 kPa and 20 kPa, for example between 16.7 kPa and 18 kPa. During the deposition, the precursor comprising silicon may be introduced into the reaction chamber in which the CMC material 3 is present with a flow rate of between 0.05 grams / minute and 0.3 grams / minute, for example between 0.1 grams / minute and 0.2 grams / minute. According to this first example, the bonding layer 5 obtained has a crystalline microstructure. In particular, the bonding layer 5 may be made of silicon, this bonding layer 5 comprising, for example, columnar grains of crystalline silicon. Alternatively, the bonding layer 5 may be made of silicon alloy, for example a eutectic silicon alloy or silicide.According to a second example, the bonding layer 5 comprises an amorphous silicon phase having crystalline silicon grains distributed therein, these grains possibly having an average size of between 0.03 pm and 3 pm. The amorphous silicon phase may be formed from pure silicon or silicon with boron, oxygen and / or nitrogen dispersed therein. According to this second example, the bonding layer 5 may be formed at a deposition temperature which prevents crystallization of the deposited silicon, followed by a heat treatment of the bonding layer at a treatment temperature higher than the temperature imposed during deposition in order to form the crystalline silicon grains distributed in the amorphous silicon phase. The temperature imposed during deposition may be between 300°C and 700°C or between 700°C and 1000°C, and the pressure imposed during deposition may be between 1.2 kPa and 1013 hPa.The operating conditions are chosen according to the precursor used. The treatment temperature may be between 1000°C and 1400°C, for example between 1200°C and 1350°C. During deposition, the precursor comprising silicon may be introduced into the reaction chamber in which the CMC material 3 is present with a flow rate of between 0.1 grams / minute and 2 grams / minute. Those skilled in the art will recognize that the bonding layer 5 may be formed by other techniques.

[0034] Generally speaking, environmental barrier layer 7 can be formed by implementing the following steps:

[0035] (a) deposition of a precursor layer by chemical vapor deposition of organometallic compound(s) (“Metal Organic Chemical Vapor Deposition”; “MOCVD”), optionally by chemical vapor deposition of organometallic compound(s) by direct liquid injection (“DLI-MOCVD”), then

[0036] (b) cracking treatment of the precursor layer deposited in step (a) by subjecting it to a temperature of between 1250°C and 1350°C for a period of between 5 hours and 50 hours, for example between 10 hours and 50 hours. According to one example, the deposit of the precursor layer carried out during step (a) may comprise a silicate of at least one rare earth and is formed at least from an alkoxysilane silicon oxide precursor and a rare earth beta-diketonate rare earth oxide precursor in the presence of a gaseous source providing oxygen. In particular, the silicon oxide precursor may be di-t-butoxydiacetoxysilane and the rare earth oxide precursor RE(thd)3, where RE denotes a rare earth element and the group (thd) denotes 2,2,6,6-tetramethyl-3,5-heptanedionate, the rare earth oxide precursor being for example Y(thd)3 (CAS No. 15632-39-0).The gaseous source supplying oxygen may comprise at least one of the following gases: O2, N2O, H2O, CO2, O3. The temperature imposed during step (a) may be between 1000°C and 1250°C. The cracking treatment of step (b) may be carried out in an oxidizing atmosphere, for example in air. Figures 2 and 3 schematically represent the deposit obtained after steps (a) and (b) respectively. After step (a), the deposit 700 has a growth cone microstructure with a first portion 701 located on the side of the bonding layer 5 formed by first contiguous growth cones having a first average size, and a second portion 702 formed by second growth cones having a second average size, greater than the first average size, and having a lower compactness than the first growth cones. According to one example, a 700 deposition of an alpha phase of yttrium disilicate can be obtained during step (a).In step (b), the structure illustrated in Figure 3 is obtained with preferential cracking at the junctions of the growth cones 702 while ensuring the sealing of the coating at the interface with the bonding layer 5 by the presence of the uncracked internal region 73 obtained from the cones 701. The heat treatment of step (b) can cause sintering of the coating, which leads to the coalescence of the growth cones 701 of smaller size but greater number in the area close to the interface with the bonding layer 5, thus leading to the formation of a sealed region 73. The heat treatment of step (b) can also cause a volume contraction, for example due to a transition between the alpha phase and the beta phase of the yttrium disilicate in the case where it is implemented, which results in preferential cracking at the junctions of the growth cones 702.

[0037] The part 1 thus manufactured may be a part for aeronautical or aerospace application. The part 1 may be a hot end part of a gas turbine of an aeronautical or aerospace engine or of an industrial turbine. The part 1 may be a turbomachine part. The part 1 may constitute at least a part of a distributor, at least a part of a nozzle or a thermal protection coating, a wall of a combustion chamber, a turbine ring sector or a turbomachine blade.

[0038] Once obtained, part 1 is mounted and assembled with the other elements of the turbomachine for use at high temperature, greater than or equal to 800°C, in an oxidizing and corrosive atmosphere. In particular, it can be used at a temperature between 800°C and 1500°C, or even between 800°C and 1300°C. Part 1 can, in particular, be used in humid air.

[0039] Example

[0040] A precursor deposit was made in a hot-wall direct liquid injection (DLI-MOCVD) organometallic compound(s) chemical vapor deposition reactor, using a solution of di-t-butoxydiacetoxy silane (DADBS) and tris(2,2,6,6-tetramethyl-3,5-heptanedionato) yttrium (Y(thd)3), diluted in toluene. The precursors are injected and vaporized using a thermostatically controlled direct liquid injection system marketed under the reference Vapbox 300 by the company KEMSTREAM, using car engine type injectors to form an aerosol. After preparation, the precursor deposit was heat-treated for 5 hours at 1350°C in air. During this heat treatment, the deposit passed from the alpha phase in the precursor deposit to the beta phase, causing regular microcracking of the upper part of the coating. Details of the preparation conditions are provided below:

[0041] - deposition temperature: 1030°C,

[0042] - deposition pressure: 5 mbar,

[0043] - concentration of DADBS: 0.1 mol / L,

[0044] - concentration of Y(thd)3: 0.15 mol / L,

[0045] - precursor solution flow rate: 0.1 g / minute,

[0046] - O2 flow rate: 54 standard cubic centimeters per minute (“standard cubic centimeters per minute”; “seem”),

[0047] - N2 flow rate: 200 standard cubic centimeters per minute,

[0048] - injector frequency: 2 Hz,

[0049] - evaporation temperature of precursors: 210°C.

[0050] The deposit obtained after the cracking heat treatment is illustrated in Figure 4.

[0051] The resulting coating successfully passed 1500 hours of corrosion testing without degradation. The interface with the bonding layer did not change during the corrosion test. This result confirms that the low thickness of the sealed portion of the environmental barrier is sufficient to ensure resistance to flaking. In addition, the pre-cracking network is quite repeatable.

[0052] More generally, the following ranges of conditions can be used for the deposition of the precursor layer:

[0053] - deposition temperature: 1000°C to 1250°C,

[0054] - deposition pressure: 5 mbar,

[0055] - concentration of DADBS: 0.01 mol / L to 0.1 mol / L,

[0056] - concentration of Y(thd)3: 0.015 mol / L to 0.15 mol / L,

[0057] - precursor solution flow rate: 0.1 g / minute to 2 g / minute, - O2 flow rate: 25 to 200 standard cubic centimeters per minute,

[0058] - N2 flow rate: 100 to 400 standard cubic centimeters per minute,

[0059] - injector frequency: 0.5 Hz to 3 Hz,

[0060] - precursor evaporation temperature: 170°C to 250°C, - potentially also applicable in a reactor in cold wall configuration.

[0061] The expression "between ... and ..." must be understood as including the limits.

Claims

Demands

1. Coated part (1) intended for mounting in a turbomachine, comprising: - a substrate (3) made of composite material with a matrix at least partially composed of ceramic, and - an environmental barrier (12) on the substrate and comprising: (i) an adhesion layer (5) comprising silicon present on a surface of the substrate, and (ii) an environmental barrier layer (7) covering the adhesion layer, said environmental barrier layer comprising an internal region (73) of barrier to oxidizing and corrosive species not cracked and located on the side of the adhesion layer, and an external region (71), opposite the adhesion layer and covering the internal region, having a network of thermomechanical adaptation cracks (75) having a distance (d) between adjacent cracks of between 10 pm and 50 pm and whose cracks extend over a depth (p75) of between 50% and 95% of a thickness (e7) of said environmental barrier layer.

2. Part (1) according to claim 1, wherein the distance (d) between adjacent cracks (75) of the thermomechanical adaptation crack network is between 15 pm and 30 pm.

3. Part (1) according to claim 1 or 2, wherein the cracks (75) of the thermomechanical adaptation crack network extend over a depth (p75) between 75% and 90% of the thickness (e7) of said environmental barrier layer.

4. Part (1) according to any one of claims 1 to 3, wherein the environmental barrier layer (7) comprises a silicate of at least one rare earth.

5. Part (1) according to claim 4, wherein the environmental barrier layer (7) comprises yttrium disilicate, ytterbium disilicate, or a mixture of these two compounds.

6. Part according to any one of claims 1 to 5, wherein the part further comprises an additional coating situated on the external region of the environmental barrier layer, said additional coating being a thermal barrier of thickness greater than or equal to 100 pm, or a protective layer against calcium and magnesium aluminosilicates of thickness greater than or equal to 50 pm.

7. A component according to any one of claims 1 to 6, wherein the internal region (73) and the external region (71) of the environmental barrier layer (7) are made of the same material.

8. Part according to claim 7, wherein the inner region (73) and the outer region (71) of the environmental barrier layer (7) are made of yttrium disilicate, ytterbium disilicate, or a mixture of these two compounds.

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