Short fiber coating process

Abstract

Method for coating short fibers. The present invention relates to a method for coating short fibers, comprising: - the desizing of sieved silicon carbide short fibers, - the deposition, on the desizing short fibers, of an adhesion promoter made of a metal oxide ceramic material having pendant groups on its surface, the adhesion promoter being deposited by atomic layer deposition technique, and - the deposition, on the desizing short fibers coated with the adhesion promoter, of boron nitride by atomic layer deposition technique, comprising (a) the grafting of a first Lewis acid precursor comprising boron to the surface of the adhesion promoter by addition to the pendant groups at the boron atom, and (b) the reaction of the first precursor thus grafted with a second precursor comprising nitrogen to obtain boron nitride. Fig. 2.

Description

Title of the invention: Short fiber coating process technical field

[0001] The present disclosure relates to a method for coating short silicon carbide fibers using the atomic layer deposition technique, enabling uniform growth of boron nitride on the fiber surface. The present disclosure also relates to an associated reinforcing fibrous element and a ceramic matrix composite part whose reinforcement is formed from such reinforcing fibrous elements. Previous technique

[0002] Ceramic matrix composite materials (CMC materials) withstand temperatures ranging from 600°C to 1400°C. Due to their superior resistance to high temperatures, CMCs require less cooling. Since this cooling is traditionally obtained from the compressor, which impacts the turbomachine's efficiency, CMC materials therefore improve engine efficiency, thereby reducing fuel consumption.

[0003] CMC materials possess good thermostructural properties, that is, high mechanical properties that make them suitable for structural components, and the ability to retain these properties at high temperatures. CMC components may include a textile reinforcement made of long, continuous fibers woven along the stress directions of the final composite component. The weaving step is costly, and the use of a woven fiber reinforcement may present limitations in the case of components with complex or small geometries. Indeed, the minimum weave pitch may prove too coarse for producing a small geometric detail.

[0004] To solve this problem, it was considered using a reinforcement made not of continuous long fibers but of fibrous elements dispersed within the matrix. However, it is well known that the performance of a CMC material requires specific management of the interfacial bonds between the fibers and the matrix in order to achieve the damage-prone nature of the final composite. This modulation of the interfaces is conventionally achieved by interposing an interphase between the fiber and the matrix.

[0005] When seeking to coat dispersed fibrous elements, it is desirable to avoid the formation of agglomerates and to obtain a homogeneous and unified coating of these elements in order to obtain the desired protective character.

[0006] Solutions have been proposed in the prior art to address this problem. In this regard, WO 2022 / 003269 can be cited, which proposes a short fiber coating by chemical vapor deposition in a fluidized bed by adding spacer particles distinct from the reinforcement to be coated.

[0007] It remains desirable to provide new techniques enabling the obtaining of a homogeneous and unitary coating of short fibers, and enabling in particular the avoidance of the addition of third particles to the filler. Description of the invention

[0008] The present description relates to a method for coating short fibers, comprising: - the desizing of coated short silicon carbide fibers having an average length less than or equal to 5 mm, - the deposition, on the desizing short fibers, of an adhesion promoter made of a metal oxide ceramic material having -OH groups on its surface, the adhesion promoter being deposited by atomic layer deposition technique while maintaining a temperature less than or equal to 400°C during deposition, and - the deposition, on the desizing short fibers coated with the adhesion promoter, of boron nitride by atomic layer deposition technique, comprising (a) the grafting of a first Lewis acid precursor comprising boron to the surface of the adhesion promoter by addition to the -OH groups at the boron atom,and (b) the reaction of the first precursor thus grafted with a second precursor comprising nitrogen to obtain boron nitride.

[0009] In the following, the "Atomic Layer Deposition" (ALD) technique will be referred to as the "ALD technique".

[0010] The formation of the adhesion promoter on short fibers after desizing promotes two-dimensional nucleation and uniform growth of boron nitride (BN) on the fiber surface. This results in uniformly and homogeneously coated short fibers, providing improved performance. The ALD technique offers excellent conformity, meaning it can uniformly coat complex, three-dimensional structures with high aspect ratios. ALD coatings have a uniform thickness and excellent composition control. However, the inventors have observed that if the adhesion promoter is omitted, BN exhibits island growth, leading to inhomogeneous deposition. A temperature of 400°C or lower is imposed during promoter formation to avoid any risk of removing surface -OH groups.

[0011] The first Lewis acid precursor has an electron deficiency at the boron which is intended to be filled by electrons from the oxygen of the groups -OH of surface. During grafting, a covalent bond is formed between the oxygen atom of the -OH groups and the boron of the first precursor.

[0012] In one embodiment, the thickness of the adhesion promoter is between 0.5 nm and 10 nm, for example between 2 nm and 6 nm.

[0013] This characteristic facilitates the grafting of the boron-based precursor while maintaining a controlled thickness which, during operation, helps to limit the thermomechanical stresses related to differential expansion. This characteristic can be disregarded, in particular, when the adhesion promoter has a coefficient of expansion close to that of silicon carbide, as indicated below.

[0014] In one embodiment, the following condition is verified: ICi - C2I < 2.106 T1, where Ci denotes the coefficient of thermal expansion of silicon carbide and C2 denotes the coefficient of thermal expansion of the adhesion promoter.

[0015] Unless otherwise stated, the coefficients of thermal expansion (denoted "CTE") are measured at 750°C and are expressed in °C'.

[0016] Such a characteristic helps to limit thermomechanical stresses during operation. However, the use of an adhesion promoter having a CTE significantly different from SiC is not excluded from the invention. In this latter case, a thinner adhesion promoter may be preferred.

[0017] In one embodiment, the adhesion promoter can be deposited from a precursor comprising a metal and an oxygenated precursor selected from: water (H2O), hydrogen peroxide (H2O2), air or ozone (O3).

[0018] The operating conditions to be implemented to deposit the metal oxide promoter by atomic layer deposition technique are known per se.

[0019] In particular, the adhesion promoter can be made of alumina.

[0020] The implementation of an alumina promoter is advantageous for further improving performance, because the alumina deposited by ALD technique exhibits high conformity due to self-limiting reactions, stable surface chemistry, high surface coverage, low surface diffusion, chemical inertness and a wide process window.

[0021] In the case of alumina, the precursor comprising a metal may, for example, be trimethylaluminum, dimethylaluminum isopropoxide (DMAI), aluminum chloride (AlCl3), or aluminum alkoxides. In particular, the precursor comprising a metal may be trimethylaluminum.

[0022] In one embodiment, the first precursor is chosen from: boron trichloride BC13, boron tribromide BBr3, boron trifluoride BF3, triethylborane B(C2H5)3, or borazine B3N3H6, and the second precursor is chosen from: ammonia NH3, dinitrogen N2 or hydrazine N2H4.

[0023] In particular, the first precursor can be chosen from: boron trichloride BC13, boron tribromide BBr3, triethylborane B(C2H5)3, or borazine B3N3H6. More particularly, the first precursor can be boron tribromide BBr3.

[0024] The second precursor can, more particularly, be ammonia NH3. In particular, a first precursor boron tribromide BBr3 and a second precursor ammonia NH3 can be used.

[0025] In one embodiment, the boron nitride deposition is carried out in a reaction chamber maintained at a pressure between 133 Pa and 1333 Pa, and one or more successive deposition cycles are performed, each comprising: - the grafting of the first precursor by exposing the adhesion promoter to the first precursor for a period of at least 1 second, for example at least 5 seconds, for example at least 10 seconds, for example at least 15 seconds; this period may be between 1 second and 100 seconds, for example between 5 seconds and 100 seconds, for example between 10 seconds and 100 seconds, for example between 15 seconds and 100 seconds or between 1 second and 60 seconds, for example between 5 seconds and 60 seconds, for example between 10 seconds and 60 seconds, for example between 15 seconds and 60 seconds, - purging the reaction chamber after this grafting and before the introduction of the second precursor, and - the reaction of the first precursor thus grafted with the second precursor by exposing the first precursor to the second precursor for a period of at least 1 second, for example at least 5 seconds, for example at least 10 seconds, for example at least 15 seconds, this period can be between 1 second and 100 seconds, for example between 5 seconds and 100 seconds, for example between 10 seconds and 100 seconds, for example between 15 seconds and 100 seconds or between 1 second and 60 seconds, for example between 5 seconds and 60 seconds, for example between 10 seconds and 60 seconds, for example between 15 seconds and 60 seconds.

[0026] The purging of the reaction vessel can, for example, be carried out by purging with an inert gas, such as argon. This purging can be carried out for a period of at least 2 seconds, for example, at least 10 seconds.

[0027] The first precursor can be introduced into the reaction chamber during a pulse lasting at least 0.1 seconds. The second precursor can be introduced into the reaction chamber during a pulse lasting at least 1 second.

[0028] In general, the durations just described can vary considerably depending on the surface to be covered and will be adapted according to the intended application.

[0029] Carrying out several successive deposition cycles makes it possible to increase the thickness of the deposited boron nitride and the number of cycles to be carried out depends on the desired thickness.

[0030] The temperature imposed in the reaction chamber during the deposition of boron nitride depends on the precursor used. Generally, it can be between 100°C and 1000°C. For example, in the specific case of a first precursor, boron tribromide BBr3, this temperature can be between 500°C and 900°C. In the specific case of a first precursor, triethylborane B(C2H5)3, this temperature can be between 300°C and 900°C.

[0031] In the particular case of an alumina promoter and a first BBr3 precursor, the grafting of the first precursor onto the surface of the promoter corresponds to the following chemical reaction, resulting in the creation of a covalent bond between the oxygen of the surface -OH group and the boron of the first precursor:

[0032] [Chem.l] AbOj-OH+BBrj AhOî-O-BBrz+HBr

[0033] After reaction of the first precursor grafted with the second precursor, the compound A12O3-O-BN is obtained. This compound has a covalent bond between the oxygen of the surface -OH group and the boron of the boron nitride.

[0034] In one embodiment, the deposition of the adhesion promoter and boron nitride is carried out in a reactor rotated on itself.

[0035] Such a characteristic advantageously avoids any risk of obtaining partial coverage of the fibers due to certain areas of fibers not exposed to reactive gases; it is of particular interest in the case where a very large quantity of short fibers is coated.

[0036] In one embodiment, the desizing of short silicon carbide fibers can be carried out by heat treatment. This heat treatment for removing the sizing can, for example, be performed at a temperature of 700°C or higher under a flow of N2 for a duration of 1 hour or higher, for example, between 1 and 2 hours. The desizing, the deposition of the adhesion promoter, and the deposition of the boron nitride can, in one example, be carried out in the same chamber.

[0037] The present description also relates to a fibrous reinforcing element, comprising: - a short silicon carbide fiber with a length less than or equal to 5 mm, - an adhesion promoter coating the short fiber in metal oxide ceramic material, and - boron nitride coating the adhesion promoter and the short fiber with covalent bonding of boron atoms of boron nitride to surface oxygen atoms of the adhesion promoter.

[0038] This fibrous reinforcing element can be obtained by implementing the process described above.

[0039] In particular, the adhesion promoter can be alumina.

[0040] In general, the covalent bond between the oxygen of the promoter and the boron of the BN can be characterized by infrared spectroscopy or nuclear magnetic resonance (NMR).

[0041] The present description also relates to a part made of ceramic matrix composite material, comprising: - a ceramic matrix, and - a fibrous reinforcement, dispersed in the ceramic matrix, comprising a plurality of fibrous reinforcing elements as described above.

[0042] In one embodiment, the part further comprises a layer of silicon carbide intercalated between the boron nitride of the reinforcing fibrous elements and the ceramic matrix.

[0043] The ceramic matrix of the part can be formed by any technique known per se, and in particular by infiltration of silicon or a silicon alloy in the molten state ("Melt-Infiltration)

[0044] In one embodiment, the part is a turbomachine component, for example, an aeronautical or industrial turbomachine component. The resulting part may be a turbine component. The resulting part may be a turbomachine blade, for example, a turbine blade. The resulting part may be a turbine distributor or a portion of a turbine distributor. Alternatively, the resulting part may be a turbine ring sector.

[0045] The aforementioned features and advantages, as well as others, will become apparent from the following detailed description which refers to the attached drawings. Brief description of the drawings

[0046] The attached drawings are schematic and are intended primarily to illustrate the principles of the exposition.

[0047] [Fig.1] Fig.1 corresponds to photographs obtained by scanning electron microscopy of a boron nitride deposit on short silicon carbide fibers carried out in the context of a process outside the invention.

[0048] [Fig.2] The [Fig.2] is a photograph obtained by scanning electron microscopy of a boron nitride deposit on short silicon carbide fibers carried out as part of a process according to the invention.

[0049] [Fig.3] The [Fig.3] corresponds to analysis results obtained by transmission electron microscopy of the A12O3 / BN deposit made in the process according to the invention associated with the [Fig.2].

[0050] [Fig.4] The [Fig.4] corresponds to an analysis result obtained by atomic emission spectroscopy (AES) for the A12O3 / BN coated fibers obtained in the process according to the invention associated with the [Fig.2]. Description of the implementation methods

[0051] Example 1 (outside the scope of the invention)

[0052] Short silicon carbide fibers were desensitized by implementing the following conditions: temperature increase at a rate of 200°C / hour up to 800°C (under N2), then application of a 6-hour plateau at 800°C and free cooling to ambient temperature.

[0053] Boron nitride was then deposited onto these SiC fibers desensitized from the precursors boron tribromide (BBr3) and ammonia (NH3) at 750°C. The conditions implemented are detailed in Table 1 below.

[0054] [Tables 1] Stage Duration Percentages^ of 8¾ 0.2 seOTs of 25% Exposure 20 «terns VVs Purge 25 Pà&atho B !dT> 25% 2^ Purge 105%

[0055] The deposition of BN onto SiC fibers using the atomic layer deposition technique was limited by surface nucleation. The state and surface chemistry of the native discontinuous fibers inhibit BN growth, resulting in island-like BN growth (see [Fig. 1]).

[0056] Example 2 (invention)

[0057] Short silicon carbide fibers were desensitized by implementing the same conditions as in Example 1.

[0058] An alumina adhesion promoter approximately 5 nm thick was then deposited by atomic layer deposition technique on SiC fibers using the sequence in Table 2 below at a temperature of 150°C and using trimethylaluminium (TMA) and water as precursors.

[0059] [Tables2] Step üœêe Argon Percentage WA Pulse. 1 sec 25% Exposure 38 sec Purge 48 seconds 183% Purge HjQ ■2 Exposure 38.SSOTdSS Purge % & [S

[0060] Boron nitride was then deposited by atomic layer deposition technique from boron tribromide (BBr3) and ammonia (NH3) precursors at 750°C. The conditions implemented are detailed in Table 3 below. Stage Duration Percentage of Argon Ptfeatkm of 8¾ 3.2 seconds 20% Exposure 23 seconds 0% Purge 1 Vu Pulse 5 seconds 23 .''S Exposure 23 seconds 0% Purge 3G

[0061] SEM and TEM observations and microanalysis of the surface of the A12O3 / BN coated fibers show, on the one hand, the presence of a continuous BN coating on the surface of the fibers and, on the other hand, an A12O3 / BN deposit on all the fibers (see Figures 2 to 4). Figure 3 shows a continuous deposit, adherent to the fiber, with a uniform thickness in the analyzed area. The alumina layer has a thickness of 2 to 5 nm, and the BN layer is 20 to 30 nm thick. According to the DEAS signal, the BN is poorly organized.

[0062] The MET check highlighted the following points: - The BN deposit is continuous, adheres to the fiber, and is uniform in thickness. - the deposited BN is isotropic and very poorly organized, - however, we note the presence of a layer of oriented BN (textured BN) with a thickness of 5 to 10 nm.

[0063] These observations are similar to those of a BN obtained by chemical vapor phase infiltration on a 3D woven fabric.

[0064] The expression "between ... and ..." should be understood as including the bounds.

Claims

Demands

1. A method for coating short fibers, comprising: - desizing coated short silicon carbide fibers having an average length less than or equal to 5 mm, - deposition, on the desizing short fibers, of an adhesion promoter of a metal oxide ceramic material having -OH groups on its surface, the adhesion promoter being deposited by atomic layer deposition technique while maintaining a temperature less than or equal to 400°C during deposition, and - deposition, on the desizing short fibers coated with the adhesion promoter, of boron nitride by atomic layer deposition technique, comprising (a) grafting a first Lewis acid precursor comprising boron to the surface of the adhesion promoter by addition to the -OH groups at the boron atom, and (b) reacting the first precursor thus grafted with a second precursor comprising nitrogen to obtain boron nitride.

2. A method according to claim 1, wherein the thickness of the adhesion promoter is between 0.5 nm and 10 nm.

3. A method according to claim 1 or 2, wherein the following condition is verified: ICi - C2I < 2.106 °C', where Ci denotes the coefficient of thermal expansion of silicon carbide and C2 denotes the coefficient of thermal expansion of the adhesion promoter.

4. A method according to any one of claims 1 to 3, wherein the adhesion promoter is deposited from a precursor comprising a metal and an oxygenated precursor selected from: water, hydrogen peroxide, air or ozone.

5. A method according to any one of claims 1, 2 or 4 related to claim 1 or 2, wherein the adhesion promoter is alumina.

6. A process according to any one of claims 1 to 5, wherein the first precursor is selected from: boron trichloride BC13, boron tribromide BBr3, boron trifluoride BF3, triethylborane B(C2H5)3, or borazine B3N3H6, and wherein the second precursor is selected from: ammonia NH3, dinitrogen N2 or hydrazine N2H4.

7. A method according to any one of claims 1 to 6, wherein the boron nitride deposition is carried out in a reaction chamber maintained at a pressure between 133 Pa and 1333 Pa, and wherein one or more successive deposition cycles are carried out, each comprising: - grafting the first precursor by exposing the adhesion promoter to the first precursor for a period of at least 1 second, - purging the reaction chamber after this grafting and before the introduction of the second precursor, and - reacting the first precursor thus grafted with the second precursor by exposing the first precursor to the second precursor for a period of at least 1 second.

8. Fibrous reinforcing element, comprising: - a short silicon carbide fiber having a length less than or equal to 5 mm, - an adhesion promoter coating the short fiber in metal oxide ceramic material, and - boron nitride coating the adhesion promoter and the short fiber with covalent bonding of the boron atoms of the boron nitride to the surface oxygen atoms of the adhesion promoter.

9. Part made of ceramic matrix composite material, comprising: - a ceramic matrix, and - a fibrous reinforcement, dispersed in the ceramic matrix, comprising a plurality of reinforcing fibrous elements according to claim 8.

10. Part according to claim 9, wherein the part is a turbomachine part.

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