Method for assembling parts made of SiC materials by means of non-reactive brazing in an oxidizing atmosphere, brazing compositions, and gasket and assembly obtained by said method

Abstract

A method is described for assembling at least two parts made of silicon carbide based materials by non-reactive brazing in an oxidizing atmosphere, each of the parts comprising a surface to be assembled, wherein the parts are placed in contact with a non-reactive brazing composition, the assembly formed by the parts and the brazing composition is heated to a brazing temperature sufficient for completely or at least partially melting the brazing composition, or rendering the brazing composition viscous, and the parts and the brazing composition are cooled so as to form, after cooling the latter to ambient temperature, a moderately refractory joint. The non-reactive brazing composition is a composition A consisting of silica (SiO.sub.2), alumina (Al.sub.2O.sub.3), and calcium oxide (CaO), or a composition B consisting of alumina (Al.sub.2O.sub.3), calcium oxide (CaO), and magnesium oxide (MgO), and, before heating the assembly formed by the parts and the brazing composition to the brazing temperature, a supply of silicon in a non-oxidized form is carried out on the surfaces to be assembled of the parts to be assembled, and/or on the surface layers comprising the surfaces to be assembled of the parts to be assembled, and/or in the brazing composition.

Claims

1. Method for assembling at least two parts made of silicon carbide based materials by non-reactive brazing in an oxidizing atmosphere, each of the parts comprising a surface to be assembled, wherein the parts are placed in contact with a non-reactive brazing composition to form an assembly, the assembly formed by the parts and the brazing composition is heated to a brazing temperature sufficient for completely or at least partially melting the brazing composition, or rendering the brazing composition viscous, and the parts and the brazing composition are cooled so as to form after the cooling of the latter to ambient temperature a moderately refractory joint; wherein the non-reactive brazing composition is a composition A consisting of silica (SiO.sub.2), alumina (Al.sub.2O.sub.3), and calcium oxide (CaO), and, optionally, titanium oxide, boron oxide and/or CaF.sub.2, or a composition B, consisting of alumina (Al.sub.2O.sub.3), calcium oxide (CaO), and magnesium oxide (MgO), and, optionally, titanium oxide, boron oxide and/or CaF.sub.2, and wherein, before heating the assembly formed by the parts and the brazing composition to the brazing temperature, a supply of silicon in a non-oxidized form is carried out on the surfaces to be assembled of the parts to be assembled and/or a supply of silicon in a non-oxidized form is carried out in surface layers comprising the surfaces to be assembled of the parts to be assembled to form surface layers consisting of said silicon carbide based materials enriched with Si, and/or a supply of silicon in a non-oxidized form is added to the brazing composition.

2. Method according to claim 1, wherein the oxidizing atmosphere is an atmosphere containing oxygen.

3. Method according to claim 1, wherein the supply of silicon is carried out by preparing a silicon coating on the surfaces to be assembled of the parts.

4. Method according to claim 3, wherein the silicon coating is prepared by a chemical vapour deposition (CVD) method, a physical vapour deposition (PVD) method, an electron beam physical vapour deposition (EBPVD) method, or by a liquid phase deposition method.

5. Method according to claim 1, wherein the supply of silicon is carried out in the surface layers comprising the surfaces to be assembled of the parts, the surface layers consisting of silicon enriched SiC compared to the SiC stoichiometry, or of pure silicon.

6. Method according to claim 1, wherein the supply of non-oxidized silicon is carried out in added to the brazing composition.

7. Method according to claim 6, wherein the brazing composition is enriched with non-oxidized silicon dissolved at a concentration of non-oxidized dissolved silicon extending up to the concentration corresponding to the saturation of the brazing composition in non-oxidized dissolved silicon.

8. Method according to claim 7, wherein the brazing composition is enriched with non-oxidized dissolved silicon at a concentration that is close to the concentration corresponding to the saturation of the brazing composition in non-oxidized dissolved silicon but without nevertheless attaining the latter, or which is equal to the concentration corresponding to the saturation of the brazing composition in non-oxidized dissolved silicon, wherein the non-oxidized dissolved silicon content of the brazing composition enriched with non-oxidized dissolved silicon is 0.1% to 2.6% by weight, compared to the total weight of the brazing composition and silicon.

9. Method according to claim 1, wherein composition A consists, in weight percentages, of 75% to 7% of SiO.sub.2, 60% to 6% of Al.sub.2O.sub.3 and 60% to 10% of CaO.

10. Method according to claim 9, wherein composition A consists, in weight percentages, of 70% to 55% of SiO.sub.2, 22% to 8% of Al.sub.2O.sub.3, and 35% to 15% of CaO.

11. Method according to claim 10, wherein composition A consists, in weight percentages, of 62% of SiO.sub.2, 15% of Al.sub.2O.sub.3, and 23% of CaO.

12. Method according to claim 9, wherein composition A consists, in weight percentages, of 55% to 38% of SiO.sub.2, 25% to 12% of Al.sub.2O.sub.3, and 45% to 30% of CaO.

13. Method according to claim 12, wherein composition A consists, in weight percentages, of 42% of SiO.sub.2, 20% of Al.sub.2O.sub.3, and 38% of CaO.

14. Method according to claim 9, wherein composition A consists, in weight percentages, of 38% to 8% of SiO.sub.2, 55% to 8% of Al.sub.2O.sub.3, and 55% to 28% of CaO.

15. Method according to claim 14, wherein composition A consists, in weight percentages, of 22% of SiO.sub.2, 37% of Al.sub.2O.sub.3, and 41% of CaO.

16. Method according to claim 1, wherein composition B consists, in weight percentages, of 70% to 35% of Al.sub.2O.sub.3, 65% to 25% of CaO, and 20% to 1% of MgO.

17. Method according to claim 16, wherein composition B consists, in weight percentages, of 50.5% of Al.sub.2O.sub.3, 44.0% of CaO, and 5.5% of MgO.

18. Method according to claim 1, wherein titanium oxide is added to the brazing composition.

19. Method according to claim 1, wherein boron oxide is added to the brazing composition.

20. Method according to claim 1, wherein CaF.sub.2 is added to the brazing composition.

21. Method according to claim 1, wherein a powder of brazing composition is formed, said powder is suspended in an organic binder in order to obtain a brazing composition suspension or paste, and the brazing composition suspension or paste obtained is deposited on at least one surface of at least one of the parts to be assembled.

22. Method according to claim 21, wherein a surface to be assembled of at least one of the parts to be assembled is coated with the brazing composition suspension or paste, then the surfaces to be assembled of the parts are placed in contact so that the suspension or paste is intercalated between them.

23. Method according to claim 21, wherein the parts to be assembled are placed in contact while observing an offset between them so as to create a free surface capable of receiving the suspension or paste near to the joint formed by the surfaces to be assembled of the parts to be assembled, then the suspension or paste is deposited on said free surface.

24. Method according to claim 1, wherein the brazing is carried out at a brazing temperature of 1050° C. to 1350° C., for a duration of 1 to 240 minutes.

25. Method according to claim 1, wherein the assembly formed by the parts and the brazing composition is taken to the brazing temperature by introducing it directly into a device already taken to the brazing temperature.

26. Method according to claim 1, wherein the assembly formed by the parts and the brazing composition is taken to the brazing temperature while observing a rise in temperature from ambient temperature.

27. Method according to claim 1, wherein the silicon carbide based materials are selected from the group consisting of pure silicon carbides and composite SiC based materials.

28. Method according to claim 1, wherein the silicon carbide based materials are selected from the group consisting of pressureless sintered silicon carbide (“PLS SiC”); Si infiltrated silicon carbide (“SiSiC” or “RBSC”); recrystallized porous silicon carbide (“RSiC”); silicon graphite (“C SiC”) consisting of graphite and covered with a layer of SiC; SiC/SiC composites; SiC/SiC composites with self-healing matrix; C/SiC composites; monocrystals of SiC; composites of SiC with another ceramic; and SiC/TiN composites.

29. Method according to claim 1, wherein said silicon carbide based materials have a silicon carbide content at least equal to 50% by weight.

30. Method according to claim 1, wherein the parts to be assembled are formed from the silicon carbide based materials prior to carrying out the supply of silicon in a non-oxidized form on the surfaces to be assembled of the parts to be assembled and/or in the surface layers comprising the surfaces to be assembled of the parts to be assembled.

31. Method according to claim 1, wherein the surface layers consisting of silicon enriched silicon carbide have thickness of 1 to 150 μm.

32. Method for assembling at least two parts made of silicon carbide based materials by non-reactive brazing in an oxidizing atmosphere, each of the parts comprising a surface to be assembled, said method comprising providing the at least two parts made of silicon carbide based materials, placing the at least two parts in contact with a non-reactive brazing composition to form an assembly, heating the assembly formed by the parts and the brazing composition to a brazing temperature sufficient for completely or at least partially melting the brazing composition, or rendering the brazing composition viscous, and cooling the assembly formed by the parts and the brazing composition to form, after the cooling of the brazing composition to ambient temperature, a moderately refractory joint; wherein the non-reactive brazing composition is a composition A consisting of silica (SiO.sub.2), alumina (Al.sub.2O.sub.3), and calcium oxide (CaO), and, optionally, titanium oxide, boron oxide and/or CaF.sub.2, or a composition B, consisting of alumina (Al.sub.2O.sub.3), calcium oxide (CaO), and magnesium oxide (MgO), and, optionally, titanium oxide, boron oxide and/or CaF.sub.2, and wherein before heating the assembly formed by the parts and the brazing composition to the brazing temperature, a supply of silicon in a non-oxidized form is carried out on the surfaces to be assembled of the parts to be assembled and/or a supply of silicon in a non-oxidized form is carried out in surface layers comprising the surfaces to be assembled of the parts to be assembled, and/or a supply of silicon in non-oxidized form is added to the non-reactive brazing composition.

33. Method according to claim 28, wherein the silicon carbide based materials are SiC/SiC composites, said SiC/SiC composites with fibres or with “whiskers”.

34. Method according to claim 28, wherein the silicon carbide based materials are C/SiC composites, said C/SiC composites with carbon fibres or “whiskers” and with SiC matrix.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing the layout of the plates of SiC based material and of the brazing composition paste for brazing in “sandwich” configuration.

(2) FIG. 2 is a schematic view which shows a plate of SiC based material covered on one of its faces with a brazing composition paste with a view to a brazing in “sandwich” configuration with offset plates.

(3) FIG. 3 is a schematic view showing the layout of plates of SiC based material and of the brazing composition paste for brazing in “sandwich” configuration with offset plates.

(4) FIG. 4 is a schematic view showing the layout of plates of SiC based material and of the brazing composition paste for brazing in capillary configuration.

(5) FIG. 5 is a graph that represents the brazing thermal cycle used in example 1 to saturate the mixture of oxides with silicon.

(6) On the X-axis is plotted the duration in minutes from the start of the thermal treatment, and on the Y-axis is plotted the temperature T in ° C.

(7) FIG. 6 is a photograph taken with a scanning electron microscope (SEM) of the interface of the glass-composite assembly of Lot A (Batch) formed in example 2.

(8) The scale indicated in FIG. 6 represents 20 μm.

(9) FIG. 7 is a photograph taken with a scanning electron microscope (SEM) of the interface of the glass-composite assembly of Lot B (Batch) formed in example 2.

(10) The scale indicated in FIG. 7 represents 50 μm.

(11) FIG. 8 is a schematic view of the test specimens used during a compression/shear test of the joints and assemblies prepared in examples 3, 4, and 5.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(12) The first step of the method according to the invention consists, firstly, generally, in preparing, elaborating a brazing composition, in other words a brazing alloy, without added silicon, without input, supply, addition of silicon.

(13) The brazing alloy according to the invention is a ternary alloy, either of the system silica (SiO.sub.2)-alumina (Al.sub.2O.sub.3)-calcium oxide (CaO) (brazing composition A), or of the system alumina (Al.sub.2O.sub.3)-calcium oxide (CaO)-magnesium oxide (MgO) (brazing composition B). The preferred proportions by weight for each of compositions A and B have been mentioned above.

(14) The brazing composition is, generally, a powdery composition that may be prepared, by synthesising, firstly, from different pure oxides and/or compounds consisting of several of said oxides a glass containing said oxides.

(15) Examples of such compounds consisting of several oxides are mullite, which is the compound 3Al.sub.2O.sub.3-2SiO.sub.2, the compound CaO—Al.sub.2O.sub.3, and the compound CaO—SiO.sub.2.

(16) These pure oxides or compounds are generally in the form of powders. They are weighed out while respecting the desired proportions in the final brazing composition that it is wished to obtain, then they are mixed and ground in any suitable apparatus, such as a mortar.

(17) For an alloy of the system SiO.sub.2—Al.sub.2O.sub.3—CaO, the braze is prepared either from powders of silica, mullite (compound defined 3Al.sub.2O.sub.3-2SiO.sub.2) and powder of the compound CaO—SiO.sub.2 (for example for composition A1), or from alumina powder and powder of the compound CaO—SiO.sub.2 (for example for composition A2), or from powder of the compound CaO—SiO.sub.2 and the compound CaO—Al.sub.2O.sub.3 (for example for composition A3).

(18) For an alloy of the system Al.sub.2O.sub.3—CaO—MgO, the braze is prepared from CaO, MgO, Al.sub.2O.sub.3 and CaO—Al.sub.2O.sub.3 powders.

(19) It is possible to improve the properties of the glasses of the system SiO.sub.2—Al.sub.2O.sub.3—CaO (brazing composition A) by making them crystallize by addition of titanium oxide TiO.sub.2, which is a nucleation agent. Typically, several weight % of TiO.sub.2 are added, for example 1% to 10% by weight of TiO.sub.2 compared to the total weight of the brazing composition and TiO.sub.2. On the other hand, glasses of the system Al.sub.2O.sub.3—CaO—MgO generally crystallise without this addition.

(20) It should also be noted that the melting temperature of these brazing alloys, whether it is brazing composition A or brazing composition B, may be reduced if necessary by addition of boron oxide B.sub.2O.sub.3, at a rate for example of 1% to 10% by weight of B.sub.2O.sub.3 compared to the total weight of the brazing composition and boron oxide, and/or by addition of CaF.sub.2, at a rate for example of 1% to 10% by weight of CaF.sub.2 compared to the total weight of the brazing composition and CaF.sub.2.

(21) Boron oxide and CaF.sub.2 lower the viscosity of the glass forming the brazing composition, increase the rate of filling of the joints, particularly when the brazing is carried out in capillary configuration.

(22) The mixture of ground powders is then introduced into a crucible generally made of platinum, and the different constituents of the mixture of powders are made to melt by subjecting them for example to a plateau of 4 hours in air at 1590° C. or 1640° C. according to the composition of the brazing alloy. After cooling, a homogeneous glass is obtained.

(23) The glass obtained is recovered.

(24) Two embodiments of the method according to the invention are then possible to form the assembly by brazing of parts made of silicon carbide based materials.

(25) Each of these embodiments is defined by a different manner of carrying out the supply, addition, of silicon.

(26) According to a first embodiment of the method according to the invention, the supply of silicon is carried out on the surfaces to be assembled of the parts to be assembled and/or in the surface layers comprising the surfaces to be assembled of the parts to be assembled.

(27) The glass obtained as described above is ground in a mortar or any other suitable device to obtain a powder of suitable particle size, the grains of which, for example, have a diameter of 10 to 250 μm.

(28) The ground glass, which thus constitutes the brazing composition powder, is then suspended in a conventional manner in a cement, binder, liquid organic gel, generally both viscous and sticky in order to obtain a brazing composition paste, suspension enabling homogeneous spreading on the surfaces of the silicon carbide based parts, substrates to be brazed.

(29) The binder, cement, gel decomposes generally for example between 100° C. and 300° C. without leaving traces. It may for example be a cement of NICROBRAZ® type or a gel of VITTA® gel type.

(30) The parts made of SiC based materials to be assembled are in general two in number, but a large number of parts that can extend up to 100 may also be assembled simultaneously.

(31) Part made of SiC based materials is generally taken to mean any element, or entity of any shape or size entering for example, after assembly with one or more other parts, into structures of larger dimensions.

(32) According to the invention, it is possible to assemble with each time excellent results parts of complex geometry, shape and/or large size, for example with 0.5 m.sup.2 of surface to be brazed or more.

(33) Silicon carbide based material is generally taken to mean here all materials comprising at least 50% by weight of silicon carbide, preferably at least 80% by weight of silicon carbide, even more preferably 100% by weight of silicon carbide, in the latter case the material consists of, is composed of, uniquely silicon carbide.

(34) The materials made of silicon carbide may be particularly in the form of sintered or infiltrated powder or of fibres bound by a ceramic matrix.

(35) The silicon carbide based materials may be selected from pure silicon carbides such as pure α (SiCα) or β (SiCβ) silicon carbide and SiC based composite materials such as composites with fibres and/or with silicon carbide matrix.

(36) As examples of SiC based materials may be cited pure dense silicon carbide or pressureless sintered silicon carbide (“PLS-SiC”); Si infiltrated silicon carbide (known as SiSiC or RBSC containing 5 to 20% Si); porous recrystallized silicon carbide (known as RSiC); graphite silicon (C—SiC) consisting of graphite and covered with a layer of SiC for example 0.1 to 1 mm thickness; as well as SiC/SiC composites, for example, with fibres or with “whiskers”; SiC/SiC composites with self-healing matrix; C/SiC composites, for example, with carbon fibres or “whiskers” and with SiC matrix; and instead SiC monocrystals; and composites of SiC with another ceramic, for example, SiC/Si.sub.3N.sub.4 and SiC/TiN composites.

(37) Preferably, the silicon based material of the substrates, parts to be assembled according to the invention consists of 100% silicon carbide, selected for example from grades of sintered pure α (SiCα) or β (SiCβ) silicon carbide or from SiC/SiC composites with a silicon carbide matrix and silicon carbide fibres like the composite Cerasep A40C® available from the firm Snecma Propulsion Solide.

(38) It has been observed, in a surprising manner, that the method of the invention enables brazing of composites with excellent results.

(39) In fact, the method according to the invention may be carried out at a moderate brazing temperature, close to 1100° C., which does not cause degradation of the composite materials, such as Cerasep A40C®, which do not tolerate higher temperatures.

(40) The two or more parts to be assembled may be made of a same material, based on silicon carbide, for example PLS (“Pressureless Sintered”)-SiC, or SiC—SiC composite or each of the parts may be made of a different material based on silicon carbide.

(41) According to the first embodiment, the supply of silicon is carried out on the surfaces to be assembled of the parts to be assembled and/or in the surface layers comprising the surfaces to be assembled of the parts to be assembled.

(42) Thus, the surfaces to be assembled of the (at least two) parts to be assembled could be coated with silicon.

(43) The surfaces to be assembled of the parts to be assembled must be coated with silicon, but other surfaces of the parts to be assembled may optionally be coated.

(44) This coating may be prepared by a chemical vapour deposition (CVD) method, a physical vapour deposition (PVD) method, an electron beam physical vapour deposition (EBPVD) method, or by a liquid phase deposition method.

(45) This coating generally has a thickness of 0.5 to 10 μm; a thickness of around 1 μm is recommended.

(46) The supply of silicon may also be carried out in surface layers comprising the surfaces to be assembled of the parts to be assembled, the surface layers consisting of silicon enriched SiC compared to the stoichiometry or of pure silicon.

(47) “Silicon enriched” SiC compared to the stoichiometry is generally taken to mean that the silicon content is greater than 0.5% at. This surface layer generally has a thickness, depth of 1 to 150 μm.

(48) Said supply of silicon may be carried out in surface layers comprising only the surfaces to be assembled of the parts to be assembled, but said supply of silicon may be carried out in surface layers further comprising the surfaces to be assembled of each of the parts to be assembled of other surfaces of the parts, or even all the external surfaces of the parts.

(49) Thus, in the case of a SiC based composite material, the method for elaborating the composite is carried out so that the surface of the composite and several μm in depth in the composite is not stoichiometric SiC but silicon enriched SiC or pure silicon.

(50) Among the methods making it possible to enrich with silicon the surface of a composite material, the chemical vapour infiltration (CVI) method or the liquid phase silicon infiltration method may be cited.

(51) The parts in which, according to the invention, a supply of silicon has been carried out on the surfaces or in the surface layers to be assembled may then be assembled.

(52) The second step of the method according to the invention consists generally in forming the assembly by actual brazing.

(53) Prior to the assembly, the two (or more) surfaces of the parts made of SiC based materials to be assembled are generally degreased, cleaned, in an organic solvent for example of the ketone, ester, ether, alcohol type, or a mixture thereof.

(54) A preferred solvent is acetone or an acetone-ethyl alcohol-ether mixture for example in the proportions 1/3, 1/3, 1/3; it is also possible to clean the parts successively with several different solvents, for example with acetone then ethanol. The parts are then dried.

(55) The brazing composition suspension, paste prepared as has been described previously, is spread, coated, applied, preferably in a homogenous, uniform manner, for example with a brush, a spatula, a paint brush, or using a syringe potentially fixed to a robotized system, or using any other means making it possible to deposit a uniform layer of brazing paste on the surface of at least one of the parts made of silicon carbide based material to be assembled.

(56) Then, the surface(s) coated with paste of the two parts (1, 2) to be assembled are placed in contact. This brazing configuration, illustrated in FIG. 1, is known as “sandwich configuration” because the brazing composition paste (3) is placed directly between the surfaces (4, 5) of the parts to be assembled.

(57) The “sandwich” configuration applies equally well to “thin” joints, in other words of a thickness below 500 micrometers, as to “thick” joints, in other words a thickness greater than or equal to 500 micrometers.

(58) The amount of brazing composition paste, suspension to implement in this configuration is generally 10 mg/cm.sup.2 to 100 mg/cm.sup.2, for example 50 mg/cm.sup.2. This amount obviously depends on the joint thickness.

(59) In an embodiment of the brazing in “sandwich configuration”, the parts to be assembled, such as plates, are offset.

(60) Thus, one begins by covering in a homogenous manner a surface to be assembled (21) of a first plate to be assembled (22) with the brazing paste (23) (FIG. 2), while leaving a free space (24) exempt of paste, for example in the form of a strip along one of the edges of the plate (22).

(61) Then, the surface to be assembled (25) of a second plate to be assembled (26) is placed in contact with the paste deposited on the surface to be assembled (21) of the first plate (FIG. 3).

(62) Due to the presence of free space not covered with paste on the surface of the first plate, the two composite plates are offset (27) whereas the brazing composition paste is intercalated between the surfaces to be assembled of the plates (22, 26).

(63) Or instead the surfaces to be assembled of the parts to be assembled are brought together so as to leave between them an interval, generally 1 to 200 μm, which will be filled by capillarity effect by the brazing composition, the latter being arranged near to the interval to be filled in a space or reservoir fashioned for this purpose, said reservoir being able to have millimetric dimensions in accordance with the knowledge of the man skilled in the art in this field.

(64) Thus, as is represented in FIG. 4, the surfaces to be assembled of the parts to be assembled, for example in the form of plates (41, 42), may be placed in contact, without having placed brazing composition between them and leaving an interval between them. Moreover, it is made sure that a gap, offset (43) exists between the parts, generally of several mm, for example 1 mm, 2 mm, to 10 mm so as to create a free surface (44) capable of receiving the suspension or paste near to the joint (45) formed by the surfaces to be assembled of the parts to be assembled, then the brazing composition suspension or paste is deposited for example in the form of a brazing bead (46) on this surface near to the joint, in the vicinity of the joint, or at the edge of the joint. During the thermal brazing cycle, the liquid brazing composition infiltrates into the joint.

(65) This brazing configuration is known as “capillary configuration”. With the brazing compositions according to the invention, it is possible to carry out such capillary brazing, namely an infiltration of the braze into the brazing joint, without arranging directly the brazing composition between the parts to be assembled as in the case of the “sandwich configuration”.

(66) In the case where it is chosen to carry out the brazing in capillary configuration, the addition of a compound such as CaF.sub.2 or B.sub.2O.sub.3 may increase the speed of filling of the joints.

(67) The amount of brazing composition paste, suspension to implement in this capillary configuration is generally 10 mg/cm.sup.2 to 50 mg/cm.sup.2, for example 20 mg/cm.sup.2.

(68) The parts ready to be brazed are then arranged in a heating device such as a furnace or subjected to heating by any other suitable means.

(69) The furnace is a furnace operating in air, generally a furnace with alumina refractories.

(70) According to the invention, the brazing is carried out in an oxidizing atmosphere such as an atmosphere containing oxygen, for example in an air atmosphere, and the heating device such as a furnace, for example, is in an oxidizing atmosphere such as an atmosphere containing oxygen, for example in an air atmosphere.

(71) The parts to be assembled are subjected for example in the furnace to a thermal brazing cycle in an oxidizing atmosphere, particularly in air.

(72) Thus the assembly formed by the parts and the brazing composition may be taken to the brazing temperature (brazing plateau) while observing a preferably “slow”, rise in temperature, with one or more temperature ramp(s) from ambient temperature.

(73) This temperature rise may be a slow temperature rise for example with a temperature ramp of 400° C./minute or a rapid temperature rise for example with a temperature ramp of 50° C./minute.

(74) When a rapid temperature rise is carried out, the part is introduced into an already hot, preheated furnace.

(75) The brazing plateau is realised at a temperature, which is the brazing temperature. This is in general at least 20° C. above the melting temperature, or liquidus temperature, of the chosen brazing composition.

(76) According to the invention, the glass may enable the assembly with a plateau slightly below the melting point, domain where the glass is viscous and thus the brazing temperature may go from Tmelting (100° C.) to Tmelting+20° C. or even+50° C. or even 50° C.

(77) The brazing temperature is moreover generally selected as a function of the temperature that the parts can withstand, and as a function of the temperature that it is possible to attain with the heating device.

(78) The brazing temperature recommended according to the invention is thus, for example, 1050° C. to 1350° C., preferably 1100° C. to 1200° C.

(79) Such a brazing temperature enables use of the assembly, particularly, in air for example up to 850° C. and even up to 1000° C., or even 1200° C. or 1250° C. and for certain brazing compositions and certain SiC based materials.

(80) The duration of the brazing, in other words the thermal cycle for forming the assembly is, according to the invention, generally of short duration.

(81) The duration of the brazing plateau is generally 10 to 120 minutes.

(82) The duration of the brazing plateau depends on the brazing temperature, and it is typically 120 minutes for a temperature of 1100° C. and it may be reduced for higher brazing temperatures.

(83) This duration may also be a little increased for very large parts with large surfaces to be brazed, namely typically at least 50×50 mm.sup.2. In this case, the duration of the brazing plateau may extend up to 200 minutes, or even 240 minutes.

(84) It is also possible to introduce directly, “rapidly” the parts to be assembled, ready to be brazed, in a device such as a furnace already taken to the temperature of the brazing plateau, generally 1050° C. to 1350° C. in order to reduce the duration of the thermal cycle.

(85) At the end of the brazing cycle, following the brazing plateau, the assembly is cooled to ambient temperature at a rate for example of 5° C. to 6° C. per minute. This cooling is generally a “natural” cooling.

(86) During the cooling, the brazing solidifies and the assembly of the parts made of silicon carbide based material is effective not just in the case where a “sandwich” configuration has been used but also in the case where a “capillary” configuration is used.

(87) In a surprising manner, the assemblies of parts made of silicon carbide based materials brazed in air at moderate temperature (for example at 1100° C.), according to the first embodiment of the method according to the invention wherein a supply of silicon is provided on the surfaces to be assembled of the parts and/or in the surface layers comprising the surfaces to be assembled of the parts, have much better mechanical behaviour than assemblies brazed in air but without carrying out a supply of silicon, as demonstrated by the examples given hereafter (Examples 2, 4, and 5).

(88) According to the second embodiment of the method according to the invention, a supply of silicon is carried out in the brazing composition.

(89) The brazing composition may thus be enriched with non-oxidized dissolved (in solution) silicon up to the concentration corresponding to the saturation of the brazing composition with non-oxidized dissolved silicon.

(90) Preferably, the brazing composition is enriched with non-oxidized dissolved silicon at a concentration which is close (namely 1% to 2% by weight) to the concentration corresponding to the saturation of the brazing composition with non-oxidized silicon but without nevertheless attaining the latter, or which is equal to the concentration corresponding to the saturation of the brazing composition with non-oxidized dissolved silicon.

(91) In order to obtain this silicon saturation, it is possible for example to melt the glass obtained as described above (by melting of the constituents and cooling) at 1360° C. in argon in the presence of silicon for 6 hours.

(92) It is possible to proceed in different ways to achieve this melting of the glass in argon in the presence of silicon leading to saturation of the glass: either the glass is placed in a crucible made of silicon or coated with silicon. The assembly is placed in a furnace in argon and maintained at 1360° C. for 6 hours. or the glass is placed in a crucible not reactive or very little reactive with the glass (for example silicon carbide or graphite) and a silicon stem is immersed into the liquid glass maintained at 1360° C. for 6 hours in argon.

(93) In both cases, the glass may be poured out of its crucible to recover the glass without breaking the crucible. If the glass solidifies in its crucible, the latter risks being broken to recover the glass saturated with silicon. or the glass is placed on silicon plates. The assembly is placed in a furnace in argon and maintained at 1360° C. for 6 hours. The glass may be cast in the liquid state to be recovered.

(94) The glass saturated with silicon is ground in a mortar or any other suitable device to obtain a powder of suitable particle size, the grains of which have, for example, a diameter of 10 to 250 μm.

(95) Then, all the steps of the method according to the invention in this second embodiment are then identical to the steps of the first embodiment described above.

(96) Nevertheless, in this second embodiment, generally no silicon is provided on the surfaces to be assembled of the parts and/or in the surface layers comprising the surfaces to be assembled of the parts.

(97) In a surprising manner, the assemblies of parts made of silicon carbide based materials brazed in air at moderate temperature (for example close to 1100° C.) according to the second embodiment of the method according to the invention, wherein silicon is provided in the brazing composition, have much better mechanical behaviour than assemblies brazed in air but without providing silicon, as demonstrated by the examples provided hereafter (Example 3).

(98) The assemblies of parts made of silicon carbide comprising joints prepared by the method according to the invention make it possible to form structures, apparatuses, components of complex shapes having high operating temperatures, which can go generally up to 1000° C. or even 1250° C., with great precision.

(99) It is known in fact that the mechanical properties of silicon carbide, namely: great hardness; low coefficient of expansion; high breaking strength; good resistance to thermal shock; as well as its very good conductivity, make it a key material for present and future industrial applications at high temperature.

(100) In addition, SiC has very good chemical resistance to various acids, including hydrofluoric acid and very good resistance to oxidation in air.

(101) In other words, the method according to the invention may particularly apply to the manufacture of any device, apparatus, structure, component, requiring a moderately refractory assembly between at least two substrates, parts based on silicon carbide while guaranteeing both good mechanical strength and satisfactory sealing at the level of the assembly.

(102) This type of device, apparatus, structure, component can meet needs in different fields: the field of thermal engineering, particularly for designing high performance heat exchangers because silicon carbide has good thermal conductivity and good resistance at high temperatures in extreme environments. the field of mechanical engineering for forming, in on-board devices, components that are light, rigid, refractory, abrasion resistant and resistant to mechanical stresses. the field of chemical engineering, because silicon carbide is resistant to numerous corrosive chemicals for example like strong bases and acids. the field of nuclear engineering for the production of fuel cladding. the fields of space optics (SiC telescope mirrors) and aeronautics (SiC/SiC composite parts). power electronics, which uses SiC or silicon.

(103) The invention will now be described by means of the following examples obviously given for illustrative and non-limiting purposes.

EXAMPLES

Example 1

(104) This example describes the preparation of a brazing composition, alloy consisting of a mixture of oxides of composition 62% by weight of SiO.sub.2-15% by weight of Al.sub.2O.sub.3— and 23% by weight of CaO, and the saturation with silicon of a part of said mixture.

(105) a) Preparation of the Brazing Composition.

(106) The targeted brazing composition of 62% by weight of SiO.sub.2-15% by weight of Al.sub.2O.sub.3-23% by weight of CaO is prepared from SiO.sub.2 powder, mullite (compound defined as 3Al.sub.2O.sub.3-2SiO.sub.2) and the compound CaO—SiO.sub.2.

(107) These powders are weighed while respecting the proportions, then mixed and ground in a mortar. The powder mixture is then subjected to a plateau of 4 hours in air at 1590° C. After cooling, a glass is obtained. Analyses carried out by X-microprobe indicate that the mixture is homogenous and that the composition by weight is 61.2% SiO.sub.2-15.0% Al.sub.2O.sub.3-23.4% CaO.

(108) The glass obtained is recovered then crushed in a mortar.

(109) A portion of the glass is conserved for the tests without saturation with silicon of the glass, it is noted Glass no 1, and the remainder of the glass is saturated with silicon according to the method described hereafter, and it is noted Glass no 2.

(110) b) Saturation of the Mixture of Oxides with Silicon.

(111) To saturate the mixture of oxides with silicon, it is placed in a silicon crucible.

(112) The assembly consisting of the silicon crucible and the mixture of oxides is placed in a metal furnace where it undergoes the thermal cycle represented in FIG. 5.

(113) This thermal cycle comprises the following steps: Rise in temperature from ambient temperature to 1000° C., in 90 minutes, in secondary vacuum; Plateau at 1000° C., for 30 minutes, in dynamic argon; Rise in temperature from 1000° C. to 1360° C., in 45 minutes in static argon; Plateau at 1360° C., for 6 hours (360 minutes), in static argon; Decrease in temperature from 1360° C. down to 1000° C., in 30 minutes, in static argon; Plateau at 1000° C., for 15 minutes, in dynamic argon; Decrease in temperature from 1000° C. down to ambient temperature, in 120 minutes in dynamic argon for 100 minutes, then in secondary vacuum.

(114) At the end of the cycle, a glass saturated with silicon is recovered.

(115) Microprobe analysis shows that the dissolved silicon content in the brazing is 1% by weight.

Example 2

(116) This example is a comparative example that describes the preparation of composite/composite bonds, assemblies between two parts made of CeraSep A40C® composite material by implementing a brazing method (brazing in sandwich configuration) that uses a brazing composition consisting of 62% by weight of SiO.sub.2-15% by weight of Al.sub.2O.sub.3 and 23% by weight of CaO.

(117) On the one hand, an assembly is prepared between two parts made of CeraSep C® composite material belonging to a first lot (batch) designated lot A and, on the other hand, an assembly is prepared between two parts made of Cerasep A40C® composite material belonging to a second lot (batch) designated lot B.

(118) The interfaces of the assemblies obtained respectively between the two parts made of composite material belonging to lot A and the two parts made of composite material belonging to lot B are then characterised and compared.

(119) a) Composite Materials

(120) Cerasep A40C® composite material is a SiC/SiC composite with a SiC matrix and SiC fibres. Such a material is available from the firm Snecma Propulsion Solide (Groupe Safran). Two different lots, designated lot A and lot B, have been elaborated and exhibit differences in terms of surface chemistry. Lot A is the reference in terms of stoichiometry, it is coated with a deposition of stoichiometric SiC (also known as “seal coat”). Lot B has been modified and the deposition of SiC is not stoichiometric but silicon enriched, of composition 75 at. % Si-25 at. % C, the thickness of the deposition is of the order of 150 μm.

(121) b) Preparation of the Brazing Composition and of the Parts to be Assembled.

(122) The glass is elaborated according to the procedure described in example 1.

(123) According to the notation specified in example 1, it is glass no 1, not saturated with silicon, which is used.

(124) Then, it is recovered, then ground in a mortar. Then it is mixed with NICROBRAZ® organic cement, which is both viscous and sticky, in order to obtain an easy to spread paste.

(125) The parts made of composite material are plates of dimensions 25×25 mm.sup.2 and of thickness 1.5.

(126) The two surfaces made of composite material to be assembled are degreased in an organic solvent, then dried.

(127) The paste is spread in a uniform manner on the surface of one of the substrates, parts made of composite material to be assembled.

(128) The amount placed on the 2 cm.sup.2 of surface to be assembled during this test is of the order of 50 mg/cm.sup.2.

(129) Then, the substrates, parts are placed in contact (this configuration is known as sandwich configuration).

(130) c) Brazing.

(131) The parts, placed in contact and thus ready to be brazed, are placed in a furnace and subjected to a thermal cycle of brazing in air.

(132) The cycle comprises a temperature rise ramp from ambient temperature up to 1100° C. at a rate of 400° C./h, then a plateau of 120 minutes at 1100° C. followed by natural cooling.

(133) d) Characterisation of the Composite Material/Glass Interfaces.

(134) The interfaces of the assemblies obtained are characterised by observing them with a scanning electron microscope (SEM).

(135) The assembly formed with lot A leads to a discontinuous interface between the glass and the composite, which is characteristic of a lack of adherence (FIG. 6). On the other hand, with lot B, a continuous interface is observed (FIG. 7).

(136) The silicon enrichment, according to the invention, of the surface of lot B has enabled better adherence.

Example 3

(137) This example describes the preparation of bonds, assemblies between two parts made of CeraSep A40C® composite material, and between two parts made of pure sintered a silicon carbide SiC, by implementing a method of brazing (brazing in sandwich configuration) which uses a brazing composition consisting of 62% by weight of SiO.sub.2-15% by weight of Al.sub.2O.sub.3 and 23% by weight of CaO not saturated with silicon or the same brazing composition but saturated with silicon.

(138) This example moreover describes tests, mechanical tests carried out on these assemblies in order to compare the mechanical behaviour thereof.

(139) a) The Composite Material, and Pure Sintered α SiC.

(140) The composite material selected in this example 3 is the composite Cerasep A40C® (already described above in example 2, paragraph a)).

(141) The lot selected in this example 3 is lot A, in other words the reference in terms of stoichiometry (stoichiometric SiC).

(142) The pure sintered α SiC is alpha SiC (Hexyloy® of St Gobain).

(143) b) Preparation of the Brazing Composition and the Parts to be Assembled.

(144) Glass no 1 not saturated with silicon, and glass no 2 saturated with silicon have been elaborated as described in example 1.

(145) Each of the glasses is recovered, then ground in a mortar. Then it is mixed, as in example 2, with NICROBRAZ® organic cement, which is both viscous and sticky, in order to obtain an easy to spread paste.

(146) The parts made of composite and sintered SiC are plates of dimensions 20×10 mm.sup.2 and of thickness 1.5 mm.

(147) The parts are cleaned with acetone, then with ethanol and finally dried.

(148) The paste is spread in a uniform manner on the surface of one of the substrates, parts to be assembled while leaving free a strip of around 2 mm from the edge of the part (in the sense of the width=10 mm) as is indicated in FIG. 2. The amount deposited is: comprised between 30 and 50 mg for the plate made of sintered SiC; comprised between 90 and 110 mg for the plate made of composite material. In fact, the joint being thicker on account of the surface state of the composite material, the amount of brazing required to fill the joint is greater than for sintered SiC.

(149) Then the substrates, parts are placed in contact (FIG. 3) with an offset of 2 mm (this configuration is known as sandwich configuration).

(150) c) Brazing.

(151) The parts placed in contact and ready to be brazed are placed in a furnace and subjected to a thermal cycle of brazing in air.

(152) The cycle comprises a temperature rise ramp from ambient temperature up to 1100° C. at a rate of 400° C./h, then a plateau of 120 minutes at 1100° C. followed by natural cooling.

(153) The following assemblies, following test specimens have thus been prepared: SiC test specimens assembled with glass no 1 not saturated with Si. 2 SiC test specimens assembled with glass no 2 saturated with Si. 2 composite test specimens with glass no 1 not saturated with Si. 2 composite test specimens with glass no 2 saturated with Si.

(154) d) Mechanical Tests.

(155) The mechanical tests are carried out with the test specimens of mechanical tests prepared in c) by brazing of 2 parts each of dimensions 20×10×1.5 mm.sup.3 (the thickness of the brazed test specimen is thus 1.5+1.5=3 mm) (81, 82).

(156) In fact, the mechanics of ceramics being statistical, with a view to the tests more than one test specimen is prepared but with the same manufacturing method.

(157) The test specimens are shown schematically in FIG. 8. They are fixed in a mounting and subjected to shear during a compression/shear test (83) at ambient temperature.

(158) It should be noted that this test does not make it possible to guarantee pure shear but it is the preferential mode. This test makes it possible however to compare the assemblies with each other.

(159) Results of Mechanical Tests:

(160) The average tensile strengths determined for these test specimens are given in table 1 as well as the type of rupture: adhesive (at the interface), cohesive (within the SiC substrate or within the composite material or in the “seal coat” of the composite material), or mixed (intermediate between the 2 modes).

(161) TABLE-US-00001 TABLE 1 Tensile strength and mode of rupture for the compression/shear tested test specimens. Sintered SiC Cerasep A40C ® composite Tensile Tensile strength Type of strength Type of Glass (MPa) rupture (MPa) rupture Glass no 1 6 adhesive 6 adhesive non saturated Glass no 2 14 mixed 10 Cohesive, pull saturated off (stripping) of the seal coat of the composite

(162) It should be noted that silicon enrichment of the glass makes the mode of rupture change from adhesive (rupture at the interface) to a mixed mode of rupture (rupture within the substrate and in the joint) or cohesive (rupture within the substrate): this is the case of the composite with pull off of the SiC “seal coat”.

Example 4

(163) This example describes the preparation of bonds, assemblies between two parts made of CeraSep A40C® composite material (lot B), by implementing the brazing method according to the invention—the brazing being carried out in “sandwich configuration”—and using a brazing composition consisting of 62% by weight of SiO.sub.2-15% by weight of Al.sub.2O.sub.3 and 23% by weight of CaO, not saturated with silicon.

(164) This example moreover describes tests, mechanical tests carried out on these assemblies in order to study the mechanical behaviour thereof.

(165) a) The Composite Material.

(166) The composite material selected in this example 4 is the composite Cerasep A40C® (already described above in example 2, paragraph a)).

(167) The lot selected in this example 4 is lot B, in other words that the deposition of SIC of the “seal coat” is not stoichiometric but silicon enriched. The deposition is of composition 75 at. % Si-25 at. % C, and the thickness of the deposition is of the order of 150 μm.

(168) This deposition of silicon enriched SiC has been carried out by “CVI” for the phase of elaboration of the composite.

(169) b) Preparation of the Brazing Composition and the Parts to be Assembled.

(170) Glass no 1 not saturated with silicon has been elaborated as described in example 1.

(171) The glass is recovered, then ground in a mortar. Then it is mixed as in example 2, with NICROBRAZ® organic cement, which is both viscous and sticky, in order to obtain an easy to spread paste.

(172) The parts made of composite and made of sintered SiC are plates of dimensions 20×10 mm.sup.2 and of thickness 1.5 mm.

(173) The parts are cleaned with acetone, then with ethanol and finally dried.

(174) The paste is spread in a uniform manner on the surface of one of the substrates, parts to be assembled while leaving free a strip of around 2 mm from the edge of the part (in the sense of the width) as is indicated in FIG. 2. The amount deposited is comprised between 90 and 110 mg for the plate made of composite material.

(175) Then the substrates, parts are placed in contact with an offset of 2 mm (this configuration is known as sandwich configuration: FIG. 3).

(176) c) Brazing.

(177) The parts placed in contact and ready to be brazed are placed in a furnace and subjected to a thermal cycle of brazing in air.

(178) The cycle comprises a temperature rise ramp from ambient temperature up to 1100° C. at a rate of 400° C./h, then a plateau of 120 minutes at 1100° C. followed by natural cooling.

(179) Three assemblies, test specimens have thus been manufactured.

(180) d) Mechanical Tests.

(181) The mechanical tests are carried out with the test specimens of mechanical tests prepared in c) by brazing of 2 parts each of dimensions 20×10×1.5 mm.sup.3 (the thickness of the brazed test specimen is thus 1.5+1.5=3 mm) (81, 82).

(182) The test specimens are shown schematically in FIG. 8. They are fixed in a mounting and subjected to shear during a compression/shear test (83) at ambient temperature.

(183) Results of Mechanical Tests:

(184) The average tensile strengths determined for these three test specimens are 12 MPa.

(185) The type of rupture is mixed with rupture in the “seal coat” and in the joint.

(186) It should be noted that silicon enrichment of the surface of the composite makes it go from an adhesive mode of rupture (rupture at the interface) to a mixed mode of rupture (rupture in the “seal coat” and in the joint).

Example 5

(187) This example describes the preparation of bonds, assemblies, between two parts made of CeraSep A40C® composite material (lot A) coated with a deposition of silicon of a thickness of 1 μm, and between two parts made of CeraSep A40C® composite material (lot A), while implementing a brazing method (brazing in sandwich configuration) which uses a brazing composition consisting of 62% by weight of SiO.sub.2-15% by weight of Al.sub.2O.sub.3 and 23% by weight of CaO not saturated with silicon.

(188) This example moreover describes tests, mechanical tests carried out on these assemblies in order to study the mechanical behaviour thereof.

(189) a) The Composite Material.

(190) The composite material is Cerasep A40® coated with silicon.

(191) The composite material selected in this example 5 is the composite Cerasep A40C® (already described above in example 2, paragraph a)).

(192) The lot selected in this example 5 is lot A, in other words the reference in terms of stoichiometry (stoichiometric SiC).

(193) This composite material has been coated with a deposition of silicon of a thickness of 1 μm by the electron beam physical vapour deposition method.

(194) b) Preparation of the Brazing Composition and the Parts to be Assembled.

(195) The targeted brazing composition of 62% by weight of SiO.sub.2-15% by weight of Al.sub.2O.sub.3-23% by weight of CaO has been prepared from SiO.sub.2 powder (1.412 g), mullite (compound defined as 3Al.sub.2O.sub.3-2SiO.sub.2) (0.940 g), the compound CaO—SiO.sub.2 (2.142 g), and B.sub.2O.sub.3 powder (0.500 g).

(196) These powders are weighed while respecting the proportions, then mixed and ground in a mortar.

(197) The mixture of powder is then subjected to a plateau of 4 hours in air at 1590° C. After cooling, glass is obtained.

(198) The glass obtained is recovered then crushed in a mortar.

(199) The glass thereby elaborated is then mixed as in example 2 with NICROBRAZ® organic cement, which is both viscous and sticky, in order to obtain an easy to spread paste.

(200) The parts made of composite are plates of dimensions 20×10 mm.sup.2 and of thickness 1.5 mm. They have been coated with a layer of 1 μm of silicon by the electron beam physical vapour deposition method.

(201) The plates made of composite material are cleaned with acetone, then with ethanol and finally dried.

(202) The paste is spread in a uniform manner on the surface of one of the substrates, parts to be assembled while leaving free a strip of around 2 mm from the edge of the part (in the sense of the width) as is indicated in FIG. 2.

(203) The amount deposited is comprised between 90 and 110 mg.

(204) Then the substrates, parts are placed in contact with an offset of 2 mm (this configuration is known as sandwich configuration—FIG. 3).

(205) c) Brazing.

(206) The parts placed in contact and ready to be brazed are placed in a furnace and subjected to a thermal cycle of brazing in air.

(207) The cycle comprises a temperature rise ramp from ambient temperature up to 1100° C. at a rate of 400° C./h, then a plateau of 120 minutes at 1100° C. followed by natural cooling.

(208) Two test specimens have thus been manufactured.

(209) d) Two other test specimens have been manufactured exactly in the same way as in paragraphs a) to c) of example 5 with the sole difference that the parts, substrates are made of Cerasep A40© composite material but without silicon coating.

(210) e) Mechanical Tests.

(211) The mechanical tests are carried out with the test specimens of mechanical tests prepared in c) and in d) by brazing of 2 parts each of dimensions 20×10×1.5 mm.sup.3 (the thickness of the brazed test specimen is thus 1.5+1.5=3 mm) (81, 82).

(212) The test specimens are shown schematically in FIG. 8. They are fixed in a mounting and subjected to shear during a compression/shear test (83) at ambient temperature.

(213) Results of Mechanical Tests:

(214) The tensile strengths determined for these 4 test specimens are given in table 2 as well as the type of rupture.

(215) TABLE-US-00002 TABLE 2 Tensile strength and mode of rupture for the test specimens tested for compression/shear in the case of the composite CeraSep A40C ® with and without Si coating. Cerasep A40C ® composite Cerasep A40C ® without Si coating composite with Si coating Tensile Tensile strength Type of strength Type of Glass (MPa) rupture (MPa) rupture Non 1-4 Adhesive 13-19 Cohesive, saturated pull off of glass the seal coat

(216) It should be noted that silicon enrichment of the surface of the composite material makes the mode of rupture change from adhesive (rupture at the interface) to a cohesive mode of rupture (rupture within the substrate): this is the case of the composite material with pull off (stripping) of the SiC “seal coat”.

REFERENCES

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