Method for protecting against oxidation a part made of composite material comprising carbon

Abstract

Method for protecting against oxidation a part made of composite material comprising carbon The invention relates to a method for protecting against oxidation a part made of composite material comprising carbon, comprising:- impregnating an internal porosity of the part with an impregnation composition comprising in mass percentages (i) between 1% and 60% of colloidal silica, (ii) between 0.5% and 20% of a flux precursor comprising at least one nitrate of alkali metal M1, (iii) between 0.5% and 20% of a network modifier precursor comprising at least one nitrate or oxynitrate of an element M2 which is chosen from metals or lanthanides, and (iv) between 20% and 88% of water, and- carrying out a vitrification heat treatment on the part impregnated with the impregnation composition so as to obtain an oxidation protection glass comprising at least silica, a oxide of M1 and an oxide of M2. Figure for abstract: Fig. 1.

Description

Title of the invention: Method for protecting against oxidation a part made of composite material comprising carbon Technical field

[0001] The invention relates to the protection against oxidation of parts made of composite material containing carbon, that is to say made of a material comprising a fibrous reinforcement densified by a matrix and in which the fibrous reinforcement and / or the matrix and / or an interphase coating between fibers of the reinforcement and the matrix is made of carbon. A particular field of application of the invention is the protection against oxidation of parts made of carbon / carbon (C / C) composite material, in particular brake discs made of C / C composite, and in particular aircraft brake discs. Prior art

[0002] The materials used for aeronautical braking are C / C composites. These materials were chosen for this application because of their lightness, their heat absorption capacity and their good tribological properties. However, they are likely to be oxidized, particularly by atmospheric oxygen, at operating temperatures which are typically above 450°C. This oxidation can also be catalyzed by de-icing products used in airports. Several solutions have been developed to extend the service life of a material comprising carbon exposed to high temperatures in an oxidizing atmosphere. In particular, a two-layer protection solution can be implemented comprising an internal anti-catalytic layer based on aluminum metaphosphate A1(PO3)3 and an external layer preventing the diffusion of oxygen into the material.Such a solution provides satisfactory protection but it remains desirable to provide new protection solutions that are simpler to implement and which provide protection against thermal, and possibly catalytic, oxidation that is at least equivalent. Statement of the invention

[0003] The invention relates to a method for protecting against oxidation a part made of composite material comprising carbon, comprising: - the impregnation of an internal porosity of the part with an impregnation composition comprising in mass percentages (i) between 1% and 60% of colloidal silica, (ii) between 0.5% and 20% of a flux precursor comprising at least one nitrate of alkali metal Ml, (iii) between 0.5% and 20% of a network modifier precursor comprising at least one nitrate or oxynitrate of an element M2 which is chosen from metals or lanthanides, and (iv) between 20% and 88% of water, and - carrying out a vitrification heat treatment on the part impregnated with the impregnation composition so as to obtain a protective glass against oxidation comprising at least silica, an oxide of Ml and an oxide of M2.

[0004] The invention proposes the use of an impregnation composition of particular formulation which makes it possible to obtain good protection against oxidation at high temperature while allowing implementation in a single protective layer and the carrying out of a single heat treatment, thus simplifying the implementation of the protection compared to the two-layer solution described above. In particular, the invention makes it possible to dispense with a prior step of impregnation with an aluminum phosphate so as to form an internal protective layer and the associated heat treatment for forming the anti-oxidation protection, the supply of the protective compounds being carried out in a single impregnation step and a single vitrification heat treatment being carried out.

[0005] In one exemplary embodiment, the flux precursor comprises at least sodium nitrate, potassium nitrate, or a mixture of these compounds.

[0006] The choice of such flux precursors is particularly suitable for the treatment of friction parts.

[0007] In an exemplary embodiment, the network modifier precursor comprises at least one nitrate of an alkaline earth metal, in particular calcium nitrate and / or magnesium nitrate. The phase diagram of these compounds is known and easily accessible, which makes it possible to optimize the heat treatment temperatures by allowing them to be greatly reduced from approximately 1400°C to approximately 1000°C.

[0008] Alternatively or in combination, the network modifier precursor comprises at least one nitrate of a transition metal or a poor metal. The choice of such a compound allows optimization of the heat treatment temperature by allowing it to be greatly reduced from approximately 1400°C to approximately 1000°C.

[0009] In an exemplary embodiment, the impregnation composition further comprises an acid stabilizer present in a mass content of between 0.05% and 10%.

[0010] The acid stabilizer helps to prevent gelling of the impregnation composition, and therefore to facilitate the impregnation of the internal porosity of the part.

[0011] In particular, the acid stabilizer may be chosen from boric acid H3BO3, phosphoric acid H3PO4, nitric acid HNO3 or a mixture of these compounds.

[0012] The choice of boric acid H3BO3 or phosphoric acid H3PO4 makes it possible to further improve the protection provided by the glass by allowing the formation of protective phosphorus oxide or boron oxide after the vitrification heat treatment.

[0013] In an exemplary embodiment, the impregnation composition comprises in mass percentages: - between 10% and 60% colloidal silica, - between 0.5% and 20% of flux precursor, - between 0.5% and 20% of network modifier precursor, - between 0.05% and 10% acid stabilizer, and - water between 20% and 88%.

[0014] According to this example, the impregnation composition may be free of metal phosphate, in particular aluminum phosphate.

[0015] According to one variant, the impregnation composition comprises in mass percentages: - between 1% and 10% colloidal silica, - between 20% and 35% of an aluminum phosphate, - between 1% and 7% of flux precursor, - between 1% and 5% of network modifier precursor, and - water between 50% and 77%, and after heat treatment, the anti-oxidation glass and an anti-catalytic aluminophosphate phase are obtained. The anti-catalytic aluminophosphate phase can be an A1(PO3)3 phase.

[0016] This case corresponds to impregnation compositions comprising dissolved aluminum phosphate. According to this variant, it is possible to dispense with an acid stabilizer because the aluminum phosphate is itself acidic and prevents gelling of the impregnation composition. Advantageously, the aluminophosphate phase obtained in addition to the glass in this variant makes it possible to improve the resistance to catalytic oxidation.

[0017] In an exemplary embodiment, the part is a friction part, for example a brake disc. In particular, the friction part may be made of carbon / carbon composite material. Brief description of the drawings

[0018] [Fig.l] [Fig.l] is a flowchart showing a succession of steps that can be implemented within the framework of an example of a method according to the invention. Description of the embodiments

[0019] In the following description, the protection against oxidation of parts made of C / C composite material, more particularly brake discs, such as aeronautical brake discs, is envisaged. More generally, the invention is applicable to the protection against oxidation of all parts made of composite material containing carbon.

[0020] We will first describe an example of a method according to the invention in connection with the flowchart in [Fig. 1].

[0021] The part made of composite material comprising carbon can be obtained in a manner known per se, for example by densifying a fibrous reinforcement with a carbon matrix phase. This densification can be carried out by liquid means by impregnation and pyrolysis of a polymer precursor or by gaseous means by chemical vapor infiltration. The part obtained has a surface porosity and an internal porosity in communication with the surface porosity and located under the surface of the part.

[0022] A first step consists of impregnating the accessible porosity of the composite material with a wetting agent (step 10). This step is known per se. For this purpose, an aqueous solution of a wetting agent such as, for example, the product marketed by the German company Sasol Gmbh under the name “Marlophen NP9” can typically be used. The presence of the wetting agent makes it easier for the impregnation composition to penetrate into the accessible porosity of the composite material.

[0023] After impregnation with the wetting agent and drying (step 20), the impregnation composition is applied to the outer surface of the part (step 30). The impregnation composition can be applied by brush or by projection (i.e. spraying with a gun). Typically, an amount of impregnation composition per unit area of the treated part of between 5 mg / cm2 and 200 mg / cm2 can be applied.

[0024] The impregnation composition comprises a particular combination of compounds which makes it possible, as indicated above, to obtain good protection against oxidation at high temperature while allowing implementation in a single protective layer and the carrying out of a single heat treatment, thus simplifying the implementation of the protection.

[0025] The impregnation composition is in the form of a suspension comprising an aqueous phase in which the flux precursor and the network modifier precursor are dissolved, and a solid phase suspended in the aqueous phase comprising the colloidal silica.

[0026] The colloidal silica particles may have an average size less than or equal to 1000 nm, for example less than or equal to 100 nm. The use of such fine particles facilitates impregnation within the internal porosity of the composite material part. In this way, the protective glass is formed, after the vitrification heat treatment, on the external surface of the composite material but also inside the material to be protected, under this external surface, which makes it possible to protect it from external aggressions encountered during operation, thus improving the protection. Unless otherwise stated, by "average size" is meant the dimension given by the statistical particle size distribution at half of the population, called D50.

[0027] The flux is obtained from the flux precursor during the heat treatment and makes it possible to reduce the melting temperature of the glass, which allows it to soften, during the formation heat treatment, so as to form the glass inside the part at temperatures compatible with industrial implementation. According to one example, the flux precursor is chosen from: lithium nitrate LiNO3, sodium nitrate NaNO3, potassium nitrate KNO3, rubidium nitrate RbNO3, cesium nitrate CsNO3, and mixtures of these compounds. In the case of a mixture, the sum of the mass contents of the compounds present is included in the content range indicated above for the flux precursor. In particular, the flux precursor may be chosen from: sodium nitrate, potassium nitrate, and mixtures of these compounds.

[0028] The flux precursor nitrate(s) may be used in a hydrated or non-hydrated form.

[0029] The network modifier precursor transforms during heat treatment into a network modifier which is a compound that allows the properties of the glass to be adjusted, such as resistance to hydrolysis, resistance to acid or basic attack, melting temperature or viscosity. According to one example, the network modifier precursor is selected from: magnesium nitrate Mg(NO3)2, calcium nitrate Ca(NO3)2, strontium nitrate Sr(NO3)2, barium nitrate Ba(NO3)2, vanadium oxynitrate VO(NO3)3, manganese nitrate Mn(NO3)2, iron nitrate Fe(NO3)3, cobalt nitrate Co(NO3)2, copper nitrate Cu(NO3)2, zirconium nitrate Zr(NO3)4, zinc nitrate Zn(NO3)2, cadmium nitrate Cd(NO3)2, lead nitrate Pb(NO3)2, lanthanum nitrate La(NO3)3, cerium nitrate Ce(NO 3)3, and mixtures of these compounds.As for the flux precursor, in the case of a mixture, the sum of the mass contents of the compounds present is included in the content range indicated above for the network modifier precursor. In particular, the network modifier precursor can be chosen from: magnesium nitrate, calcium nitrate, and mixtures of these compounds. The use of these compounds is advantageous due to their ease of supply. According to a variant, the network modifier precursor can be a nitrate of a poor metal, for example zinc nitrate.

[0030] The network modifier precursor nitrate(s) may be used in a hydrated or non-hydrated form.

[0031] Generally speaking, the following combinations can be used in the impregnation composition:

[0032] - the flux precursor is sodium nitrate and the modifier precursor is calcium nitrate network,

[0033] - the flux precursor is sodium nitrate and the modifier precursor is zinc nitrate network,

[0034] - the flux precursor is a mixture of sodium nitrate and sodium nitrate. potassium and the network modifier precursor is calcium nitrate, or

[0035] - the flux precursor is sodium nitrate and the network modifier precursor is network a mixture of calcium nitrate and magnesium nitrate.

[0036] The impregnation composition may comprise other compounds such as an acid stabilizer which may be chosen from boric acid H3BO3, phosphoric acid H3PO4, nitric acid HNO3, or a mixture of these compounds.

[0037] The impregnation composition may be free of sodium silicate (Na2O)x SiO2 with x strictly positive. The impregnation composition separately provides colloidal silica and the alkali metal Ml, which makes it possible to modulate the Ml / Si ratio more finely and thus improve the effectiveness of the protective glass against oxidation compared to the use of a sodium silicate-based solution. The fact of avoiding sodium silicate also helps to improve the stability over time of the impregnation composition (less risk of gelling). Generally speaking, the impregnation composition consists essentially of colloidal silica, the flux precursor, the network modifier precursor and water, with the optional presence of aluminium phosphate and / or the acid stabilizer, each of the constituents being present in the contents indicated above.

[0038] The impregnation composition is obtained by mixing the different constituents, it should be noted that in the case where the impregnation composition is free of metal phosphate, it may be preferable to add the acid stabilizer to the colloidal silica before adding the flux precursor and the network modifier precursor, so as to obtain a pH lower than the isoelectric point of the silica so as not to have a reaction between the silica and the other cations.

[0039] As an example of an impregnation composition that can be used in the context of the present invention, the following formulation may be cited in which the percentages are by mass: 4% SiO2, 2% Ca(NO3)2, 24% aluminum phosphate A1(H2PO4)3 without water, 67% H2O and 3% NaNO3.

[0040] The applied impregnation composition impregnates the internal porosity of the part and may be present inside the part to a depth greater than or equal to 1 mm, for example greater than or equal to 2 mm, for example between 2 mm and 10 mm. The depth is measured relative to the external surface of the part.

[0041] The vitrification heat treatment is then carried out on the part impregnated with the impregnation composition (step 40). The vitrification heat treatment is carried out by raising the temperature to a temperature between 700°C and 1200°C so as to obtain the glass for protection against oxidation. This temperature may be imposed for one or more hours, for example for a period ranging from 10 hours to 15 hours. The vitrification heat treatment may be carried out in a neutral atmosphere, for example under nitrogen (N2). Following this heat treatment, the oxidation protection glass is present in the pores present inside the part. The oxidation protection glass may be present inside the part to a depth greater than or equal to 1 mm, for example greater than or equal to 2 mm, measured relative to the external surface of the part. This depth may be between 2 mm and 10 mm.

[0042] According to one example, the oxidation protection glass may have the following composition, in molar percentages, SiO2 between 68% and 73%, Na2O between 15% and 23% and CaO between 5% and 11%. Other examples of glass compositions are indicated below (molar percentages): - 67% SiO2 - 22% Na2O - 11% ZnO, - 70% SiO2 - 15% Na2O - 5% K2O - 10% CaO, - 70% SiO2 - 20% Na2O - 5% CaO - 5% MgO.

[0043] The expression “between ... and ...” must be understood as including the limits.

Claims

Claims

1. Method for protecting against oxidation a part made of composite material comprising carbon, comprising: - impregnating (30) an internal porosity of the part with an impregnation composition comprising in mass percentages (i) between 1% and 60% of colloidal silica, (ii) between 0.5% and 20% of a flux precursor comprising at least one nitrate of alkali metal M1, (iii) between 0.5% and 20% of a network modifier precursor comprising at least one nitrate of an element M2 which is chosen from alkaline earth metals, transition metals, poor metals or lanthanides or an oxynitrate of an element M2 which is chosen from metals or lanthanides, and (iv) between 20% and 88% of water, and - carrying out a vitrification heat treatment (40) on the part impregnated with the composition impregnation so as to obtain an oxidation protection glass comprising at least silica, an Ml oxide and an M2 oxide.

2. The method of claim 1, wherein the flux precursor comprises at least sodium nitrate, potassium nitrate, or a mixture of these compounds.

3. The method of claim 1 or 2, wherein the network modifier precursor comprises at least one nitrate of an alkaline earth metal.

4. The method of claim 3, wherein the network modifier precursor comprises at least one of calcium nitrate and / or magnesium nitrate.

5. A method according to any one of claims 1 to 4, wherein the lattice modifier precursor comprises at least one nitrate of a transition metal or a poor metal.

6. A method according to any one of claims 1 to 5, wherein the impregnation composition further comprises an acid stabilizer present in a mass content of between 0.05% and 10%.

7. A method according to claim 6, wherein the acid stabilizer is selected from boric acid H3BO3, phosphoric acid H3PO4, nitric acid HNO3 or a mixture of these compounds.

8. Method according to any one of claims 6 or 7, in which the impregnation composition comprises in mass percentages: - between 10% and 60% of colloidal silica, - between 0.5% and 20% of flux precursor, - between 0.5% and 20% of network modifier precursor, - between 0.05% and 10% of acid stabilizer, and - water between 20% and 88%.

9. Method according to any one of claims 1 to 7, in which the impregnation composition comprises in mass percentages: - between 1% and 10% of colloidal silica, - between 20% and 35% of an aluminum phosphate, - between 1% and 7% of flux precursor, - between 1% and 5% of network modifier precursor, and - water between 50% and 77%, and in which, after the heat treatment, the oxidation protection glass and an anticatalytic aluminophosphate phase are obtained.

10. A method according to any one of claims 1 to 9, wherein the part is a friction part.