Method for protecting a porous material against oxidation

Abstract

METHOD FOR PROTECTING A POROUS MATERIAL AGAINST OXIDATION The invention relates to a method for protecting a porous material part against oxidation, comprising at least: - a step of impregnating the internal porosity of the part with an impregnation composition comprising a carrier fluid, a wetting agent, and an oxidation-protective glass powder; then - a step of colonizing a porosity of the porous material with glass by carrying out a heat treatment (30) enabling the melting of the glass powder present in the internal porosity of the porous material and the removal of the carrier fluid; and wherein the glass has a dynamic viscosity less than or equal to 1500 mPa.s during the heat treatment. Figure for the abstract: Fig. 1

Description

Title of the invention: Method for protecting a porous material against oxidation Technical field

[0001] The invention relates to the field of porous materials and more precisely to methods of impregnating the internal porosity of these materials in order to provide protection against oxidation. Prior art

[0002] Porous materials have many industrial advantages, particularly when weight reduction issues arise.

[0003] However, when used in an oxidizing environment, it is observed that the internal porosity of a porous material can be a preferred site of oxidation and the oxidation of the material can lead to its premature wear. This is why it is often necessary to protect the internal porosity of porous materials.

[0004] Solutions are generally proposed in which an external glass coating is formed on the surface of the material to be protected. The glass constitutes a barrier against the diffusion of oxidizing species into the internal porosity. This solution gives satisfactory results which can nevertheless be improved. Indeed, the external coating degrades during use of the material, which affects the protection provided. The degradation of the external coating can be a chemical degradation linked to the evolution of the composition of the glass over time or to the volatilization of part of the constituents, a physical degradation linked to leaching phenomena in a humid environment, or even a mechanical degradation due to friction, mechanical shocks or centrifugal force.Depending on the use made of the material, these different modes of degradation can apply in combination, further affecting the lifespan of the protective external coating. The degradation of the protective coating leads to premature wear of the porous material, because its internal porosity then becomes accessible to oxidizing species.

[0005] It is desirable to improve the protection of porous materials, to allow a longer life of these materials. Statement of the invention

[0006] The invention aims precisely to meet this need.

[0007] To this end, it proposes a method of protecting a part made of porous material against oxidation, comprising at least:

[0008] - a step of impregnating the internal porosity of the part with an impregnation composition comprising a vector fluid, a wetting agent, and a protective glass powder against oxidation; then - a step of colonization of a porosity of the porous material by the glass by carrying out a heat treatment allowing the fusion of the glass powder present in the internal porosity of the porous material and the elimination of the vector fluid;

[0009] and in which the glass has a dynamic viscosity less than or equal to 1500 mPa.s during the heat treatment.

[0010] Unlike the methods of the prior art, the method of the invention allows the impregnation of the internal porosity of the porous material and the protective glass is not only present on the surface of the porous material.

[0011] On the contrary, the impregnation composition allows the glass powder to impregnate all of the internal porosity of the porous material and the heat treatment allows the glass to form a protective barrier in the internal porosity and no longer only on the surface or at low depth.

[0012] Such protective coatings arranged in the internal porosity of the porous material allow improved protection compared to a surface coating because they are less subject to degradation than the surface coatings of the prior art. The internal porosity is in fact less subject to chemical degradation, because it is less exposed to the external environment. It is also less subject to physical degradation because it is not subject to friction and mechanical shocks.

[0013] In addition, the heat treatment allows the melting of the glass powder. This melting of the glass allows the molten glass powder to colonize the internal porosity even better by capillary phenomena ensured by the low viscosity of the glass during the heat treatment and favored by the presence of the wetting agent.

[0014] The viscosity which is considered in the context of the present invention is the dynamic viscosity of the molten glass during the heat treatment of the colonization step. This viscosity can be measured by means of a Brookfield viscometer of the RVT type at a speed of 200 revolutions / minute.

[0015] Viscosity values ​​are a function of temperature. In the application, and unless otherwise stated, viscosity is understood to be at the temperature imposed during heat treatment.

[0016] The viscosity indicated for the molten glass powder makes it possible to obtain improved impregnation of the internal porosity of the porous material. In particular, the viscosity and the presence of the wetting agent make it possible to ensure that the glass powder deposited during the impregnation step can, after melting, be distributed by capillarity more uniformly during the heat treatment, despite the elimination of the carrier fluid during the heat treatment.

[0017] For a glass of given composition, the evolution of the viscosity as a function of the temperature can be determined by numerical modeling, for example by means of what is described in the article "Glass viscosity calculation based on a global statistical modeling approach" by Alexander Fluegel, in Glass. Technol.: Europ. J. Glass Sci. Technol. A, vol. 48, 2007, 1, p 13-30. In particular, this article provides a calculation method for determining, as a function of the composition of a glass, the parameters of the Vogel-Fulcher-Tammann equation establishing an equation for the evolution of the viscosity of the glass as a function of temperature.

[0018] Furthermore, it is known that the viscosity of a glass decreases with temperature. In particular, the viscosity of a molten glass is generally of the order of 50,000 mPa.s.

[0019] A sufficient but not necessary criterion for choosing a glass satisfying the viscosity criterion imposed by the definition of the invention is to choose a glass whose melting temperature is at least 200°C lower than the heat treatment temperature, in order to ensure that the viscosity of the glass, at the heat treatment temperature, is less than or equal to 1500 mPa.s.

[0020] In one embodiment, the glass powder of the impregnation composition may comprise boron, for example in the form of B2O3. This compound in fact makes it possible to lower the melting point of the glass and consequently makes it possible to obtain a glass whose viscosity is reduced at the temperature of the heat treatment.

[0021] In one embodiment, the glass powder of the impregnation composition may also comprise one or more alkaline element(s), for example sodium or potassium, for example in the forms Na2O or K2O, which are also elements lowering the melting temperature of the glass.

[0022] In one embodiment, the glass powder may comprise boron, sodium, potassium or several of these elements, for example in the forms B2O3, Na2O or K2O.

[0023] In one embodiment, the glass powder of the impregnation composition may be selected from the glass compositions provided by Schott, the trade names of which are as follows: G017-002, G017-339, G017-712, G017-393, G017-340, G018-228, 8465, G018-229, G018-256, G017-052, G018-250, G018-255, 8470 or G018-249.

[0024] In one embodiment, the glass powder may comprise, or even consist essentially of:

[0025] - a compound chosen from boron sesquioxide, B2O3, sodium oxide Na2O, potassium oxide K2O, or lead monoxide PbO or a mixture of several of these compounds, in a mass content of between 20% and 80%, and

[0026] - a compound chosen from silica SiO2, alumina A12O3, phosphorus pentoxide P2O5, sodium metaphosphate NaO3P, potassium metaphosphate KO3P, sodium pyrophosphate Na4P2O7, potassium pyrophosphate K4P2O7, sodium hydrogen phosphate Na2HPO4, potassium hydrogen phosphate HK2O 4P or a mixture of several of these compounds. In one embodiment, the glass has a dynamic viscosity of between 500 mPa.s and 1500 mPa.s during the heat treatment, and preferably between 600 mPa.s and 1100 mPa.s.

[0027] The use of an impregnation composition makes it possible to deposit the glass powder throughout the internal porosity of the porous material.

[0028] In one embodiment, the impregnation step allows the impregnation composition and the glass powder it contains to reach areas of internal porosity at a depth greater than or equal to 20 mm, measured from the external surface of the part.

[0029] In one embodiment, the impregnation composition may comprise additives in a mass content of less than or equal to 30%, for example less than or equal to 15% relative to the total weight of the impregnation composition.

[0030] For example, such additives may be infusible elements, for example comprising silicon carbide, silicon nitride or any other infusible oxide or carbide.

[0031] The infusible elements make it possible to contribute even more effectively to the blocking of the porosities and to provide the porous material with a refractory character.

[0032] In a different embodiment or in association with the previous embodiment, the additives may be oxygen-trapping elements, for example a metal powder comprising silicon, aluminum, tungsten or a mixture of these compounds. These compounds make it possible, by their oxidation, to cause an increase in the volume of the glass present in the porosity, and they then contribute to the blocking of the internal porosity of the porous material.

[0033] Such oxygen trapping elements make it possible to form compounds in the internal porosity of the porous material which can combine with the oxygen which manages to diffuse into the internal porosity despite the protective glass. The quantity of oxygen available to oxidize the internal porosity of the porous material is thus further reduced.

[0034] In one embodiment, the impregnation composition comprises, in mass content relative to the total weight of the composition:

[0035] - between 0.5% and 5% of wetting agent; - between 30% and 70% glass powder; - optionally up to 30% additives,

[0036] the complement being the vector fluid, for example chosen from water or an organic solvent, for example ethanol or propanol.

[0037] In one embodiment, the porous material may be a carbon / carbon composite material, or may be alumina.

[0038] In one embodiment, the porous material initially comprises an internal porosity greater than or equal to 10%, for example between 10% and 30%, or even between 10% and 25%.

[0039] This is for example the case when the porous material is a carbon / carbon composite material.

[0040] The oxidation resistance properties of the porous material are improved by forming a protective glass in the internal porosity of the porous material.

[0041] For example, the remaining internal porosity after impregnation is less than or equal to 5%. In particular, the reduced porosity makes it possible to limit the reaction surface available for oxidation and thus to limit the degradation of the porous material, so that the lifetime of the porous material is increased.

[0042] In one embodiment, the quantity of glass powder provided during the impregnation step may be greater than or equal to 150 mg.cm3. This quantity is understood as a mass of dry glass powder per unit volume of impregnated part.

[0043] This quantity makes it possible to provide enough glass to form a continuous film around the part and to fill all the porosities in the volume of the material.

[0044] As described above, the impregnation composition comprises a wetting agent which makes it possible to improve the wettability of the internal porosity by the impregnation composition. Thus, it makes it possible to ensure that the impregnation composition has good penetration into the internal porosity of the porous material and this up to the heart of the internal porosity.

[0045] In one embodiment, the wetting agent may comprise at least one of a boron-containing element, polyethoxylated nonylphenol, a silicone surfactant, or a succinate.

[0046] In one embodiment, the wetting agent may be present in a mass content of between 0.5% and 5%, or even between 1% and 5%, relative to the total weight of the impregnation composition.

[0047] The impregnation composition also comprises a carrier fluid. The expression “carrier fluid” is intended to characterize a component of the impregnation composition whose role is to allow the suspension of the glass powder and to allow its conveyance inside the internal porosity of the part. The carrier fluid does not contribute to the final composition of the protective glass. As indicated, the carrier fluid is removed during the heat treatment, for example by evaporation.

[0048] For example, the carrier fluid may comprise an aqueous medium or an organic medium.

[0049] More specifically, the carrier fluid may comprise water, ethanol or propanol.

[0050] In one embodiment, the method may comprise, before the heat treatment of the glass powder, a heat treatment step of drying at a temperature above the boiling point of the carrier fluid, but at a temperature below 700°C.

[0051] The method comprises a heat treatment to melt the glass powder.

[0052] In one embodiment, the heat treatment is carried out at a temperature greater than or equal to 700°C, for example between 700°C and 1500°C.

[0053] This step must allow the melting of the glass powder which impregnates the internal porosity after the impregnation step, without however causing significant oxidation of the porous material.

[0054] The heat treatment conditions are therefore advantageously chosen according to the nature of the porous material.

[0055] In particular, when the porous material is oxidizable, for example when it is made of carbon / carbon composite, the heat treatment step is preferably carried out under an inert atmosphere.

[0056] On the other hand, when the porous material is not oxidizable, for example when it is made of alumina, the heat treatment step can be carried out in an oxidizing atmosphere, for example in air.

[0057] However, it should be noted that the heat treatment step can also be carried out in an oxidizing atmosphere in the presence of an oxidizable porous material, since the access of oxygen to the internal porosity of the substrate is limited as soon as the glass begins to soften during the heat treatment. In practice, after only a few minutes after the start of the heat treatment, the possibility of oxidation of the porous material is greatly limited.

[0058] A temperature below 1500°C ensures that the heat treatment does not damage the impregnated part. Brief description of the drawings

[0059] [Fig.l] [Fig.l] is a flowchart showing different successive steps of an example of a method according to the invention. Description of the embodiments

[0060] The invention is now described by means of a figure describing particular embodiments which should not be considered as limiting the scope of the invention.

[0061] [Fig.l] illustrates a flowchart showing different successive steps of an example of a method according to the invention.

[0062] Step 10 corresponds to the step of impregnation of the internal porosity of the material porous by the impregnation composition.

[0063] The latter can be carried out, for example, by dipping the porous material in a bath comprising the impregnation composition. Alternatively, the composition can be applied by injection or infusion or by vacuum impregnation. Whatever the embodiment chosen, it is important that the impregnation composition can penetrate to the heart of the internal porosity of the part.

[0064] Indeed, good dispersion of the impregnation composition in the internal porosity of the part allows a more homogeneous dispersion of the glass in this porosity, and therefore ultimately allows more effective protection against oxidation to be obtained.

[0065] For example, the impregnation composition may comprise a mass content of glass particles of between 30% and 70% relative to the total weight of the impregnation composition.

[0066] Step 20, optional and shown in dotted lines, may be a drying which comprises exposing the impregnated part obtained at the end of step 10 to a temperature above the boiling point of the vector fluid of the impregnation composition but below the melting temperature of the glass powder.

[0067] Such a drying step 20 makes it possible to eliminate the vector fluid before the heat treatment.

[0068] Step 30 of [Fig.l] represents a heat treatment step allowing the melting of the glass powder infiltrated into the internal porosity of the porous material and, if necessary, the elimination of the vector fluid.

[0069] This step 30 allows the glass powder, dispersed in the internal porosity by the impregnation composition during step 10, to be able to fill by capillarity the entire internal porosity of the part in an even more uniform manner than the powder allowed.

[0070] Indeed, the initial step of impregnation 10 with the impregnation composition makes it possible to disperse the glass powder throughout the internal porosity of the part, and in particular in the internal porosity up to the core of the part. However, the mere distribution of the glass powder is not sufficient to obtain the desired protection against oxidation. Indeed, the glass powder does not cover the preform at the end of the impregnation step.

[0071] The heat treatment step 30 makes it possible precisely to obtain that the glass powder covers the internal porosity of the part due to the melting of the glass powder. In addition, this melting allows the glass to extend into the internal porosity of the part by capillarity.

[0072] At the end of the heat treatment step 30, the glass powder which has been melted occupies the internal porosity of the part even more uniformly than at the end of the impregnation step 10 and covers the internal surface, thus forming a barrier to the oxidation reactions and increasing the life of the part.

Claims

Claims

1. Method for protecting a part made of porous material against oxidation, comprising at least: - a step of impregnating (10) the internal porosity of the part with an impregnation composition comprising a vector fluid, a wetting agent, and a glass powder for protection against oxidation; then - a step of colonizing a porosity of the porous material with the glass by carrying out a heat treatment (30) allowing the melting of the glass powder present in the internal porosity of the porous material and the elimination of the vector fluid; and in which the glass has a dynamic viscosity less than or equal to 1500 mPa.s during the heat treatment.

2. A protection method according to claim 1, wherein the wetting agent comprises at least one of a boron-containing element, polyethoxylated nonylphenol, a silicone surfactant or a succinate.

3. A protection method according to any one of claims 1 to 2, wherein the porous material is a carbon / carbon composite material, or is made of alumina.

4. A protection method according to any one of claims 1 to 3, wherein the heat treatment is carried out at a temperature between 700°C and 1500°C.

5. Protection method according to any one of claims 1 to 4, in which the quantity of glass powder provided during the impregnation step is greater than or equal to 150 mg.cm3.

6. A protection method according to any one of claims 1 to 5, wherein the glass powder comprises boron, sodium, potassium or more of these elements.

7. Protection method according to claim 6, in which the glass powder comprises: - a compound chosen from boron sesquioxide, B2O3, sodium oxide Na2O, potassium oxide K2O, or lead monoxide PbO or a mixture of several of these compounds, in a mass content of between 20% and 80%, and

8. - a compound chosen from silica SiO2, alumina A12O3, phosphorus pentoxide P2O5, sodium metaphosphate NaO3P, potassium metaphosphate KO3P, sodium pyrophosphate Na4P2O7, potassium pyrophosphate K4P2O7, sodium hydrogen phosphate Na2HPO4, potassium hydrogen phosphate HK2O4P or a mixture of several of these compounds. Protection method according to any one of claims 1 to 7, in which the impregnation composition comprises, in mass content relative to the total weight of the composition: - between 0.5% and 5% wetting agent; - between 30% and 70% glass powder; - optionally up to 30% additives; the complement being the vector fluid.

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