Method for coating short fibers

Abstract

The present invention relates to a method for coating short fibers, comprising: - de-sizing sized silicon carbide short fibers, - depositing, on the de-sized short fibers, an adhesion promoter made of metal oxide ceramic material having pendent groups at its surface, the adhesion promoter being deposited by atomic layer deposition, and - depositing, on the de-sized short fibers covered with the adhesion promoter, boron nitride by atomic layer deposition, comprising (a) grafting a first Lewis acid precursor comprising boron to the surface of the adhesion promoter by addition onto the pendent groups at the boron atom, and (b) reacting the first grafted precursor with a second precursor comprising nitrogen in order to obtain the boron nitride.

Description

Description Title of the invention: Short fiber coating process Technical Field

[0001] This paper describes a method for coating short silicon carbide fibers using atomic layer deposition, enabling uniform growth of boron nitride on the fiber surface. It also describes an associated reinforcing fibrous element and a ceramic matrix composite part reinforced by such reinforcing fibrous elements. Previous technique

[0002] Ceramic matrix composite materials (CMCs) can withstand temperatures ranging from 600°C to 1400°C. Due to their superior high-temperature resistance, CMCs require less cooling. Since this cooling is traditionally drawn from the compressor, impacting turbomachine efficiency, CMCs improve engine efficiency, thereby reducing fuel consumption.

[0003] CMC materials possess good thermostructural properties, meaning high mechanical properties that make them suitable for structural components, and the ability to retain these properties at high temperatures. CMC components can include a textile reinforcement made of long, continuous fibers woven along the stress directions of the final composite part. The weaving process is costly, and the use of woven fiber reinforcement can be limited for parts with complex or small geometries. Indeed, the minimum weave pitch may be too coarse for producing small geometric details.

[0004] To address this issue, the use of reinforcement composed not of continuous long fibers but of fibrous elements dispersed within the matrix was considered. 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 to achieve the desired damage-prone nature of the final composite. This modulation of the interfaces is typically 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, adding spacer particles distinct from the reinforcement to be coated.

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

[0008] This presentation concerns a short fiber coating process, comprising: - the desizing of short sieved silicon carbide fibers having an average length less than or equal to 5 mm, - the deposition, on the desensitized 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 of 400°C or less during deposition, and - the deposition, on the short desensitized 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 level of 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 "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 conformability, meaning it can uniformly coat complex, three-dimensional structures with high aspect ratios. ALD coatings have a uniform thickness and excellent composition control. Nevertheless, the inventors have It was 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 atom, which is intended to be filled by electrons from the oxygen atoms of the surface -OH groups. During grafting, a covalent bond is formed between the oxygen atom of the -OH groups and the boron atom of the first precursor.

[0012] In one example 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 thermomechanical stresses related to differential expansion. This characteristic can be disregarded, particularly when the adhesion promoter has a coefficient of expansion close to that of silicon carbide, as indicated below.

[0014] In an example implementation, the following condition is met: |Ci - C2I < 2.10' 6 °C 1 , 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' 1 .

[0016] This characteristic helps to limit thermomechanical stresses during operation. However, the use of an adhesion promoter with 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 in themselves.

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

[0020] The implementation of an alumina promoter is advantageous for further improving performance, as 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 containing a metal can, for example, be trimethylaluminum, dimethylaluminum isopropoxide (DMAI), aluminum chloride (AlCh), or aluminum alkoxides. In particular, the precursor containing a metal can be trimethylaluminum.

[0022] In one example embodiment, the first precursor is chosen from: boron trichloride BCh, boron tribromide BB, 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 BCh, boron tribromide BBr3, triethylborane B(C2H5)3, or borazine B3N3H6. More specifically, the first precursor can be boron tribromide BB.

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

[0025] In one example embodiment, boron nitride deposition is carried out in a reaction vessel 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 performed for a duration 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] Performing several successive deposition cycles increases the thickness of the deposited boron nitride, and the number of cycles to be performed depends on the desired thickness.

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

[0031] In the specific case of an alumina promoter and a first BB 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: [Chem. 1] Os-OH+BBrs AhOs-O-BBri+HBr

[0032] After reacting the first precursor grafted with the second precursor, the compound Al2O3-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.

[0033] In one embodiment, the adhesion promoter and boron nitride are deposited in a reactor that is rotated on itself.

[0034] This characteristic advantageously avoids any risk of obtaining partial fiber coverage due to certain fiber areas not exposed to reactive gases; it is of particular interest when a very large quantity of short fibers are coated.

[0035] In one embodiment, the desizing of short silicon carbide fibers can be performed by heat treatment. This heat treatment for removing the sizing can, for example, be carried out at a temperature of 700°C or higher under a flow of N2 for a duration of at least one hour, for example, between one and two 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.

[0036] This presentation also concerns 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.

[0037] This fibrous reinforcing element can be obtained by implementing the process described above.

[0038] In particular, the adhesion promoter can be alumina.

[0039] 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).

[0040] This presentation also concerns 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.

[0041] In one embodiment, the part further includes a layer of silicon carbide intercalated between the boron nitride of the reinforcing fibrous elements and the ceramic matrix.

[0042] 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")

[0043] In one embodiment, the part is a turbomachine component, for example, an aeronautical or industrial turbomachine component. The resulting part could be a turbine component. The resulting part could be a turbomachine blade, for example, a turbine blade. The resulting part could be a distributor of a turbine or part of a turbine distributor. The resulting part may alternatively be a turbine ring sector.

[0044] The aforementioned features and advantages, as well as others, will become apparent upon reading the detailed description that follows, which refers to the attached drawings. Brief description of the drawings

[0045] The attached drawings are schematic and are primarily intended to illustrate the principles of the presentation.

[0046] [Fig. 1] Figure 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.

[0047] [Fig. 2] Figure 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.

[0048] [Fig. 3] Figure 3 corresponds to analysis results obtained by transmission electron microscopy of the Al2O3 / BN deposit made in the process according to the invention associated with Figure 2.

[0049] [Fig. 4] Figure 4 corresponds to an analysis result obtained by atomic emission spectroscopy (AES) for the AI2O3 / BN coated fibers obtained in the process according to the invention associated with Figure 2. Description of the implementation methods

[0050] Example 1 (excluding invention)

[0051] 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 room temperature.

[0052] Boron nitride was then deposited onto these SiC fibers, which had been desensitized from the precursors boron tribromide (BB) and ammonia (NH3) at 750°C. The conditions used are detailed in Table 1 below.

[0053] [Table 1]

[0054] The deposition of BN onto SiC fibers using atomic layer deposition techniques 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 Figure 1).

[0055] Example 2 (invention)

[0056] Short silicon carbide fibers were desensitized by implementing the same conditions as in Example 1.

[0057] 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.

[0058] [Table 2]

[0059] Boron nitride was then deposited by atomic layer deposition technique from boron tribromide (BB) and ammonia (NH3) precursors at 750°C. The conditions used are detailed in Table 3 below. [Table 3]

[0060] SEM and TEM observations and microanalysis of the surface of the Al₂O₃ / BN-coated fibers show, on the one hand, the presence of a continuous BN coating on the fiber surface and, on the other hand, an Al₂O₃ / BN deposit on all the fibers (see Figures 2 to 4). Figure 3 shows a continuous, adherent deposit with a uniform thickness in the analyzed area. The alumina layer is 2 to 5 nm thick, and the BN layer is 20 to 30 nm thick. Based on the DEAS signal, the BN is poorly organized.

[0061] The MET analysis 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.

[0062] These observations are similar to those of a BN obtained by chemical vapor phase infiltration on a 3D woven fabric.

[0063] The expression "between ... and ..." should be understood as including the boundaries.

Claims

Demands

1. A method for coating short fibers, comprising: - the desizing of short sieved silicon carbide fibers having an average length less than or equal to 5 mm, - the deposition, on the desensitized 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 of 400°C or less during deposition, and - the deposition, on the short desensitized 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 level of the boron atom, and (b) the reaction of 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 met: |Ci - C2I < 2.10 -6 °C 1 , 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 BCh, 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 vessel maintained at a pressure between 133 Pa and 1333 Pa, and wherein one or more successive deposition cycles are performed, each comprising: - grafting of the first precursor by exposing the adhesion promoter to the first precursor for a duration of at least 1 second, - 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 duration of at least 1 second.

8. 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.

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 fibrous reinforcing elements according to claim 8.

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

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