Compositions for joining and assembling parts made of SiC-based materials

Abstract

A method for joining, assembling, at least two parts made of silicon carbide-based materials by non-reactive brazing is provided. According to the method, the parts are contacted with a non-reactive brazing composition, the assembly formed by the parts and the brazing composition is heated to a brazing temperature sufficient to melt the brazing composition totally or at least partly, and the parts and brazing composition are cooled to that, after solidification of the brazing composition, a moderately refractory joint is formed; wherein the non-reactive brazing composition is an alloy comprising, in atomic percentages, 45% to 65% silicon, 28% to 45% nickel and 5% to 15% aluminum. A brazing composition as defined above is provided. A brazing paste, suspension comprising a powder of said brazing composition and an organic binder as well as a joint and assembly obtained the foregoing method are also provided.

Claims

1. A refractory joint joining parts comprising silicon carbide-based materials and formed by non-reactive brazing of a non-reactive brazing composition which is a ternary alloy consisting of, in atomic percentages, 45% to 65% silicon, 28% to 45% nickel and 5% to 15% aluminium, the joint having substantially the same composition as the non-reactive brazing composition.

2. The refractory joint according to claim 1, wherein said non-reactive brazing composition consists of, in atomic percentages, 55% to 60% silicon, 30% to 34% nickel and 9% to 11% aluminium.

3. The refractory joint according to claim 1, wherein said non-reactive brazing composition consists of, in atomic percentages, 57.51% silicon, 32.51% nickel and 100.5% aluminium.

4. The refractory joint according to claim 1, wherein the refractory joint further comprises a reinforcement.

5. The refractory joint according to claim 4, wherein the reinforcement comprises ceramics.

6. The refractory joint according to claim 4, wherein the reinforcement is in form of particles, of fibres, of a non-woven fabric of fibres, of a woven fabric of fibres, of a felt, or of a foam.

7. The refractory joint according to claim 4, wherein the reinforcement is at most 50% by volume relative to a total volume of the non-reactive brazing composition.

8. The refractory joint according to claim 1, wherein said refractory joint is capable of withstanding a temperature up to 980 C.

9. The refractory joint according to claim 1, wherein the refractory joint is formed by capillary brazing of the non-reactive brazing composition and therefore substantially fills pores present on the parts.

10. An assembly comprising: at least two parts comprising silicon carbide-based materials; and the refractory joint according to claim 1 joining said at least two parts.

11. The assembly according to claim 10, wherein the silicon carbide-based materials are selected from the group consisting of pure silicon carbide and composite SiC-based materials.

12. The assembly according to claim 10, wherein the silicon carbide-based materials are selected from the group consisting of sintered pressureless silicon carbide (PLS SiC), Si infiltrated silicon carbide (SiSiC or RBSC), porous recrystallized silicon carbide (RSiC), graphite silicon (C SiC) composed of graphite coated with a SiC layer, SiC/SiC composites, SiC/SiC composites with self-healing matrix, C/SiC composites, SiC monocrystals, and SiC composites with another ceramic.

13. The assembly according to claim 10, wherein the silicon carbide-based materials comprise a silicon carbide content of at least 50% by mass relative to a total mass of the silicon carbide-based materials.

14. The assembly according to claim 10, wherein said non-reactive brazing composition consists of, in atomic percentages, 55% to 60% silicon, 30% to 34% nickel and 9% to 11% aluminium.

15. The assembly according to claim 10, wherein said brazing composition consists of, in atomic percentages, 57.51% silicon, 32.51% nickel and 100.5% aluminium.

16. The assembly according to claim 10, wherein the assembly further comprises a reinforcement.

17. The assembly according to claim 16, wherein the reinforcement comprises ceramics.

18. The assembly according to claim 16, wherein the reinforcement is in form of particles, of fibres, of a non-woven fabric of fibres, of a woven fabric of fibres, of a felt, or of a foam.

19. The assembly according to claim 16, wherein the reinforcement is at most 50% by volume relative to a total volume of the non-reactive brazing composition.

20. The refractory joint according to claim 1, wherein said refractory joint comprises, in atomic percentages, 55% to 60% silicon, 30% to 34% nickel and 9% to 11% aluminium.

21. The refractory joint according to claim 1, wherein said refractory joint comprises, in atomic percentages, 57.51% silicon, 32.51% nickel and 100.5% aluminium.

22. The assembly according to claim 10, wherein said refractory joint comprises, in atomic percentages, 55% to 60% silicon, 30% to 34% nickel and 9% to 11% aluminium.

23. The assembly according to claim 10, wherein said refractory joint comprises, in atomic percentages, 57.51% silicon, 32.51% nickel and 100.5% aluminium.

24. An assembly comprising at least two substrates made of silicon carbide-based material and a the non-reactive brazing composition which is a ternary alloy consisting of, in atomic percentages, 45% to 65% silicon, 28% to 45% nickel and 5% to 15% aluminium, wherein the at least two substrates respectively comprises silicon carbide by at least 50% relative to total amount of the substrate by mass.

25. The assembly of claim 24, wherein the at least two substrates respectively comprises silicon carbide by at least 80% relative to total amount of the substrate by mass.

26. The assembly of claim 24, wherein the non-reactive brazing composition consists of, in atomic percentages, 55% to 60% silicon, 30% to 34% nickel and 9% to 11% aluminium.

27. A method of producing an assembly comprising at least two substrates made of silicon carbide-based material and a non-reactive brazing composition which is a ternary alloy consisting of, in atomic percentages, 45% to 65% silicon, 28% to 45% nickel and 5% to 15% aluminium comprising, heating the assembly at temperature of 1150 C. or lower, and forming a refractory joint between the two substrates by cooling the assembly.

28. The method of producing the assembly of claim 27, further comprising; disposing the non-reactive brazing composition between the two substrates before heating the assembly.

29. The method of producing the assembly of claim 27, wherein the temperature of heating the assembly is in a range from 1040 to 1150 C.

30. The method of producing the assembly of claim 27, wherein the temperature of heating the assembly is in a range from 1080 to 1100 C.

31. The method of producing the assembly of claim 27, wherein time period of the heating is from 30 to 150 minutes.

32. The method of producing the assembly of claim 27, further comprising; heating the assembly at temperature of 950 to 1000 C. before heating the assembly at temperature of 1150 C. or lower.

33. The method of producing the assembly of claim 32, wherein time period of the heating at temperature of 950 to 1000 C. is from 30 to 180 minutes.

34. The method of producing the assembly of claim 27, wherein the substrate comprises silicon carbide by at least 50% relative to total amount of the substrate by mass.

35. The method of producing the assembly of claim 27, wherein the substrate comprises silicon carbide by at least 80% relative to total amount of the substrate by mass.

36. The refractory joint joining parts of claim 1, wherein a temperature for brazing the non-reactive brazing composition is equal to or lower than 1150 C.

37. The assembly of claim 10, wherein a temperature for brazing the non-reactive brazing composition is equal to or lower than 1150 C.

38. The assembly of claim 24, wherein a temperature for brazing the non-reactive brazing composition is equal to or lower than 1150 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing the arrangement of the plates of SiC-based material and of the paste of brazing composition for brazing in <<sandwich>> configuration.

(2) FIG. 2 is a schematic view showing the arrangement of the plates of SiC-based material and of the paste of brazing composition for brazing in capillary configuration.

(3) FIG. 3 is a graph illustrating the brazing heat cycle recommended for brazing a joint with a reinforcement of SiC particles or SiC fibres which is also the brazing heat cycle used in Example 6. The time in minutes as from the start of heat treatment is given along the X-axis, and the temperature T in C. is given along the Y-axis.

(4) FIG. 4 is a schematic view illustrating the arrangement of the plates of SiC-based material and of the paste of brazing composition for the brazing in capillary configuration, such as especially conducted in particular in the example of a joint with a reinforcement of SiC particles or SiC fibres emerging from the joint.

(5) FIG. 5 is a graph illustrating the brazing heat cycle used in Example 3. The time in minutes as from the start of heat treatment is given along the X-axis, and the temperature T in C. is given along the Y-axis.

(6) FIG. 6 is a schematic view of the test pieces used for mechanical testing, in particular compression/shear testing of the joints and the assemblies prepared in the Examples.

(7) FIG. 7 is a schematic view illustrating the arrangement of the plates of SiC-based material and of the paste of brazing composition for the brazing in capillary configuration, such as conducted in Example 5, of a joint with a reinforcement of SiC particles or SiC fibres emerging from the joint.

DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

(8) The first step of the method according to the invention generally consists of preparing, forming a braze, brazing composition, in other words a braze, brazing, alloy containing silicon, nickel and aluminium.

(9) The braze, brazing, alloy of the invention is a ternary Silicon (Si)-Nickel (Ni)-Aluminium (Al) alloy.

(10) The melting point of the braze alloy of the invention is generally from 1060 C. (solidus at 1030 C. and liquidus at 1060 C.). The predominant element of the alloy is silicon.

(11) The mass proportions of the ternary SiNiAl alloy, in atomic percentages, are 45% to 65% silicon, 28% to 45% nickel and 5% to 15% aluminium.

(12) Advantageous proportions and particularly advantageous or preferred proportions were indicated in the foregoing.

(13) The brazing composition is generally a powder composition which can be prepared, for example, by first synthesizing, from pure Si, Ni and Al elements, an intermetallic compound containing silicon, nickel and aluminium.

(14) The synthesis of said intermetallic compound is carried out, for example, by adding silicon (e.g. in the form of pieces), nickel (e.g. in the form of pieces or other forms) and aluminium (e.g. in the form of pieces or other forms) in the desired proportions for the brazing composition, in a refractory crucible made of alumina, for example.

(15) The assembly formed by the crucible, silicon, nickel and aluminium is placed in an heating apparatus such as a graphite furnace and is heated to a temperature generally of 1300 C., preferably under a high vacuum, for a time of 30 minutes for example, to melt the different constituents of the brazing composition and to obtain, after cooling, the final desired intermetallic compound that is homogeneous and in ingot form. The heating temperature is preferably 1300 C. for the preferred composition of the invention.

(16) The fabrication of the ingot can also be carried out in a cold crucible. This contactless melting technique (copper crucible cooled by circulating water and placed in an inductor) allows the alloy to be melted without contacting the crucible and hence the recovery thereof without machining the crucible.

(17) The ingot of intermetallic compound obtained is then ground using any suitable apparatus for example in a mortar to obtain a powder of adequate particle size, i.e. whose particles have a diameter of 1 to 300 m for example, and which constitutes the brazing composition.

(18) The second step of the method of the invention generally entails the actual joining, assembling, by brazing.

(19) Prior to assembling, joining, the two (or more) surfaces of the parts made of SiC materials to be joined are generally degreased, cleaned in an organic solvent for example of ketone, ester, ether, alcohol type, or a mixture thereof, etc.

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

(21) The parts made of SiC-based materials to be assembled are generally two in number, but it is also possible to join simultaneously a larger number of parts of possibly up to 100.

(22) By part made of SiC-based material is generally meant any element or entity of any shape and size which, after assembly with one or more other parts, enters into structures of larger size.

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

(24) By silicon carbide-based material is generally meant herein any materials containing at least 50% by mass of silicon carbide, preferably at least 80% by mass of silicon carbide, further preferably 100% by mass of silicon carbide; in this latter case, the material consists, is composed solely of silicon carbide.

(25) The silicon carbide-based materials may notably be in the form of a sintered or infiltrated powder or of fibres bound by a ceramic matrix.

(26) The silicon carbide-based materials may be chosen from among pure silicon carbides such as pure silicon carbide (-SiC) or pure silicon carbide (-SiC) and SiC-based composite materials such as composites with silicon carbide fibres and/or matrix.

(27) As examples of SiC-based materials, mention may be made of pure dense silicon carbide or pressureless sintered silicon carbide (PLS-SiC); Si infiltrated silicon carbide (SiSiC or RBSC containing 5 to 20% Si); porous recrystallized silicon carbide (RSiC); graphite silicon (C-SiC) composed of graphite coated with a layer of SiC for example to a thickness of 0.1 to 1 mm; and SiC/SiC composites, for example with fibres or whiskers; SiC/SiC composites with self-healing matrix; C/SiC composites, for example with carbon fibres or whiskers and a SiC matrix; and also SiC monocrystals; SiC composites with another ceramic, for example SiC/Si.sub.3N.sub.4 and SiC/TiN composites.

(28) Preferably, the silicon-based material of the substrates, parts to be joined according to the invention is composed of 100% silicon carbide chosen for example from among sintered pure silicon carbide (-SiC) or pure silicon carbide (-SiC) or from among composites with silicon carbide fibres and a silicon carbide matrix.

(29) It has surprisingly been ascertained that the method of the invention allows the brazing of composites with excellent results (cf. Examples 4 to 6).

(30) The two or more parts to be joined may be made of one same silicon carbide-based material for example of PLS (<<Pressureless Sintered>>)-SiC, or of a SiC-SiC composite, or each of the parts may be made of a different silicon carbide-based material.

(31) The suspension, paste of the brazing composition prepared as described previously is spread, coated, applied homogeneously, uniformly, using for example a brush or spatula or a syringe optionally fixed to a robotized system, or using any other means allowing a uniform layer of brazing paste to be deposited on the surface of at least one of the parts made of silicon carbide-based material to be joined.

(32) The paste-coated surface(s) of the two parts (1, 2) to be joined are then placed in contact. This brazing configuration illustrated in FIG. 1 is called a <<sandwich configuration>> since the paste of brazing composition (3) is placed directly between the surfaces (4, 5) of the parts to be joined.

(33) Preferably in this <<sandwich>> configuration it is recommended, for the brazing composition of the invention, that the brazing paste should not be uniformly distributed but applied in the form of beads of brazing composition, braze alloy, which do not touch each other to avoid a configuration that is too confined.

(34) The quantity of paste, suspension of brazing composition to be used in this configuration is generally 10 mg/cm.sup.2 to 50 mg/cm.sup.2, for example 20 mg/cm.sup.2.

(35) The <<sandwich>> configuration applies both to <<thin>> joints i.e. having a thickness of less than 500 micrometers, and to <<thick>> joints i.e. having a thickness of 500 micrometers or more.

(36) Or else, as is illustrated in FIG. 2, the parts to be joined, for example in the form of plates (21, 22), are placed in contact without having put the brazing composition between them but by leaving a gap, an offset (23) therebetween generally of a few mm, for example of 1 mm, 2 mm, to 10 mm so as to create a free surface (24) able to receive the suspension or paste in the vicinity of the joint (25) formed by the surfaces to be joined, assembled, of the parts to be assembled, joined, then the suspension or paste of brazing composition is deposited for example in the form of a bead of brazing composition, braze alloy, (26) on this surface in the vicinity of the joint or on the edge of the joint. During the brazing heat cycle, the liquid brazing composition infiltrates into the joint.

(37) This brazing configuration is called a <<capillary configuration>>. With the brazing compositions of the invention it is possible to conduct said capillary brazing, with infiltration of the liquid brazing composition, braze alloy into the brazed joint during the brazing cycle, without depositing the brazing composition directly between the parts to be assembled as in the <<sandwich>> configuration.

(38) This capillary configuration is even preferred for the NiSiAl system since it allows better filling of the joint to be obtained with the brazing composition, braze.

(39) The quantity of paste, suspension of brazing composition to be used in this capillary configuration is generally 10 mg/cm.sup.2 to 30 mg/cm.sup.2, for example 20 mg/cm.sup.2.

(40) Capillary brazing is possible for <<thin>> joints having a thickness of less than 500 m, without reinforcements previously placed in the joint. Capillary brazing led to a good filling of the joints by the NiSiAl braze alloy, the joint thickness possibly varying from a few microns to almost 500 m for parts made of SiC/SiC composite having surface defects.

(41) Capillary brazing is also possible for joint thicknesses much higher than 500 m possibly reaching a few millimeters, for joints in which a <<wetting reinforcement>> (i.e. the braze alloy provides good wetting of the surface of the reinforcement, this being the case with SiC-based reinforcements for example) by the brazing composition has been placed between the surfaces to be brazed.

(42) This reinforcement may be in the form of ceramic particles for example of a ceramic powder such as SiC, of fibres for example of ceramic fibres such as SiC, or C particles, of SiC fibres, of woven fibres for example of SiC, of non-woven fibres; of a felt or of a foam. The SiC powder may be a commercial powder for example such as the powder of the trade name STARCK having a purity of 98.5% and a particle size of less than 10 m, or the powder of trade name Neyco having a purity of 98.5% and a particle size of 50 m. For thicknesses higher than 500 m, the quality of the joint is better with reinforcements of SiC particles or of SiC fibres which reduce cracks in the joint.

(43) Regarding the nature of the reinforcement and the implementation thereof, reference can be made to the corresponding pages in the description of document [2].

(44) The braze alloy placed on the edge of the joint changes to the liquid state during the brazing cycle, infiltrates the joint and wets the reinforcements which allows a joint to be obtained that is well filled with the braze alloy.

(45) The reinforcements therefore allow infiltration into thick joints.

(46) It was evidenced, in accordance with the invention, that the infiltration of the braze alloy into the joint and the wetting of the reinforcements are possible and occur optimally under certain conditions.

(47) In other words, it was evidenced that to obtain good filling without any vacancies of braze alloy in particular in the centre of the joint, several specific steps had to be heeded.

(48) These specific steps are the following: first, optionally heat treatment of the reinforcement at a temperature generally from 1300 C. to 1500 C. e.g. 1400 C., under a high vacuum in a graphite furnace generally for 2 to 4 hours, before use; after heat treatment of the reinforcement, if it is not to be used immediately, it must be stored preferably under argon; the brazing cycle must necessarily comprise 2 plateaux as described below (see FIG. 3): a first plateau at a temperature of 950 C. to 1000 C., for example 980 C., for a time of 2 to 4 hours, for example of 3 hours; followed by a second plateau which is the brazing plateau such as described below and which is conducted in particular at a temperature of 1080 C. to 1100 C. for 90 to 150 minutes, for example at a temperature of 1100 C. for 90 or 120 minutes to fill typically a joint length of 3 cm of a joint composed of SiC-based reinforcements.

(49) It may also, optionally, be of advantage to <<bring out>> the reinforcements (41) of the joint (42) between the surfaces (43, 44) to be joined, assembled, of the parts (45, 46) to be joined, assembled in order to facilitate initiation of the infiltration of the braze alloy into the joint (42) as illustrated in FIG. 4.

(50) This method is particularly recommended for composite materials such as CMC materials which are porous, in particular on their edges.

(51) The brazing composition may be deposited for example in the form of a bead of braze alloy (47) distant from the edge (48) of the part (46) i.e. at a distance of 2 to 5 mm away from the edge to allow initiation by means of the reinforcements which are brought out of, emerge (49), from the joint (42) without the risk of the braze alloy infiltrating into the porosities of the composite material such as CMC.

(52) The joint generally consists of at least 50% by volume of the SiNiAl alloy, this alloy having a composition of between 60% and 55 atomic % of silicon, 30% and 34 atomic % of nickel, and 11% and 9 atomic % of aluminium, and in general at most 50% by volume of reinforcements such as ceramic particles or ceramic fibres (SiC or C for example).

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

(54) The furnace is generally a graphite furnace, under a vacuum or in a neutral gas atmosphere, but a metal furnace may also be used.

(55) In general the vacuum is a high vacuum i.e. the pressure is 10.sup.3 to 10.sup.5 Pa, for example 10.sup.4 Pa.

(56) Preferably, the neutral gas is argon.

(57) With the invention it is even possible to use argon of commercial quality (generally having 5 ppm O.sub.2).

(58) The parts to be joined are subjected to a heat cycle, in the furnace for example.

(59) For example, the assembly formed by the parts and the brazing composition can be brought to the brazing temperature by observing a preferably <<slow>> temperature rise, with one or more temperature ramps from ambient temperature.

(60) This temperature rise can be obtained for example using a temperature ramp of 1 C. to 5 C./minute.

(61) The brazing plateau is generally conducted at a temperature, which is the brazing temperature, that is preferably at least 15, more preferably at least 30 C. higher than the melting point or liquidus temperature of the chosen brazing composition, braze alloy.

(62) For the brazing of porous surfaces to be brazed, for example for composite materials whose SiC surface coating is insufficiently thick, it may be useful to conduct brazing at a temperature between the liquidus and the solidus to obtain a braze alloy in the semi-solid state during the brazing (temperature) plateau. The braze alloy is then viscous and infiltration thereof into the porosities can be better controlled.

(63) This brazing temperature is generally from 1040 C. to 1150 C., preferably 1080 C. to 1100 C., depending on the brazing composition and the relative proportions of Ni, Al and Si in this composition.

(64) The melting temperature of the compositions, according to another advantage of the method of the invention, allows the use of the assembly at up to 950 C. and even up to 980 C.

(65) Surprisingly, although the brazing temperature of the brazing compositions according to the invention is lower than 1150 C., excellent adhesion and good wetting of the silicon carbide are obtained with rapid wetting kinematics, as shown by the sessile drop tests performed with these brazing compositions, and it is therefore possible (see Example 1) to obtain a contact angle smaller than 45 after brazing for 30 minutes at 1100 C.

(66) This excellent wetting is indispensable to achieve good quality of the formed joints, since it ensures good quality of the filling of the joint, but it does not always allow to ensure a good mechanical behaviour since this latter property is unpredictable. Yet, surprisingly, the joints prepared with the brazing compositions of the invention also have excellent mechanical properties (cf. Example 3).

(67) The above-defined brazing temperature (1040 C. to 1150 C., preferably 1080 C. to 1100 C.), is held for a time of 1 to 150 minutes, preferably 30 to 150 minutes, more preferably 60 to 120 minutes, most preferably 90 to 120 minutes, this being called the brazing plateau.

(68) For materials having relatively porous brazing surfaces such as composite materials, it may be useful to reduce the usual brazing time which is generally from 30 to 150 minutes, to a time of a few minutes namely a time of between 1 and 30 minutes for example, to prevent too much infiltration of the brazing composition into the porosities of the material to the detriment of filling of the joint.

(69) The duration of the brazing plateau is dependent on the size of the parts to be joined, the thickness of the joint and more specifically on the size of the surfaces to be brazed. It is effectively possible for this duration to reach 150 even 180 minutes for very large parts having large surfaces areas to be brazed, namely typically at least 5050 mm.sup.2.

(70) A brazing plateau for the method of the invention may for example be conducted at a brazing temperature of 1100 C. for 90 minutes.

(71) The specific temperature of the chosen brazing plateau is a function of the composition of the braze alloy.

(72) A homogenizing plateau at 980 C. for example is recommended even essential for large-size parts (typically on and after 5050 mm.sup.2) to guarantee the thermal homogeneity of the parts to be joined.

(73) It is to be noted that since the wetting kinetics are good, it is not necessary to accelerate the already excellent wetting, and this optional first temperature plateau for the NiAlSi compositions of the invention is essentially even solely a homogenization plateau. This is generally valid for joints without reinforcement. On the other hand, this plateau is essential if reinforcements are pre-positioned between the surfaces to be brazed

(74) This plateau can be replaced by a slow temperature rise for example around 1000 C.

(75) The duration of the first plateau and the duration of the brazing plateau are dependent on the size of the furnace, the size of the parts to be brazed and the tooling supporting the parts to be brazed.

(76) This first plateau which is therefore a homogenization plateau is generally observed at a temperature of 950 C. to 1000 C., for example 980 C. for a minimum recommended time of one hour, for example a time of 90 to 180 minutes, before conducting the actual brazing plateau under the conditions already indicated above.

(77) Both in the capillary configuration and in the <<sandwich>> configuration, the said first plateau is not indispensable for parts of small size without particle reinforcements placed in the joint.

(78) The said first plateau is generally recommended even indispensable in both these configurations for large-size parts, namely and in general parts which have surfaces to be brazed of more than 5050 mm.sup.2, to guarantee thermal homogeneity at the parts to be joined. It is also compulsory for joints with particle reinforcements.

(79) The duration of these (temperature) plateaus can be increased, and for example can be set at 180 minutes for the first plateau and 150 minutes for the second plateau for parts of very large size for example having a surface area of 0.5 m.sup.2 or more to be brazed.

(80) Or else thermal homogenization may be also obtained by omitting this first plateau and conducting a slow temperature rise (at the rate of 0.5 C./minute for example) generally between 900 C. and 1000 C., preferably at 980 C., so that the exposure time of the assembly in this temperature range is for example of the order of 90 to 120 minutes.

(81) Like the first plateau, the said slow temperature rise is advisable even indispensable for large-size parts in both configurations.

(82) On completion of the brazing cycle, after the brazing plateau, the assembly is cooled down to ambient temperature, at a rate of 5 C. or 6 C. per minute for example.

(83) During cool-down, the braze alloy solidifies and the joining of the parts made of silicon carbide-based materials becomes effective whether a <<sandwich>> configuration or a <<capillary>> configuration is used.

(84) The assemblies formed with the method of the invention were subjected to compression/shear tests (see FIG. 6) at ambient temperature.

(85) For sintered SiC/NiSiAl braze alloy of the invention without reinforcement/sintered SiC joints, the mean breaking stress value obtained was 48 MPa which is an excellent result, much higher than those obtained in document [3] with a NiSi braze alloy.

(86) For joints, substrates made of CMC composite of Cerasep A40C type (SiC matrix, SiC fibres)/NiSiAl braze alloy of the invention without reinforcement/CMC composite, the mean breaking stress value obtained was of the order of 15 MPa, the weak point of the assembly between the braze alloy and the CMC being located at the CMC seal coat which is SiC prepared by chemical vapour deposition (CVD).

(87) As already pointed out, this mechanical strength can be improved by adding reinforcements to the brazing composition. These reinforcements may be reinforcements of particle type for example in the form of a SiC powder, or of ceramic fibre type for example in the form of fibres alone or woven e.g. made of SiC. The reinforcement content is generally at most 50% by volume, and may generally range from one or a few % by volume e.g. 5% by volume up to 49% by volume of the brazing composition. As already indicated above, to obtain good filling of the joint by capillary brazing with reinforcements pre-positioned in the joint, it is necessary to follow a certain number of specific steps.

(88) The assemblies of parts made of silicon carbide comprising joints prepared using the method of the invention allow to obtain structures, apparatus, components of complex shapes having high temperatures of use which may reach 950 C., even 980 C., with great precision.

(89) It is effectively known that the properties of silicon carbide: high hardness; high rigidity; low density; low coefficient of expansion; high breaking stress; good resistance to heat shock; and very good conductivity makes this material an indispensable material for present and future industrial applications, in particular at high temperature.

(90) In addition, SiC has very good chemical resistance to various acids including hydrofluoric acid, and very good resistance to oxidation in air at high temperature of up to 1300 C.

(91) In other words, the method of the invention can notably be applied to the manufacture of any device, apparatus, structure, component requiring moderately refractory joining between at least two substrates, parts made of silicon carbide, by guaranteeing both good mechanical strength and a satisfactory sealing, leak tightness at the joint.

(92) This type of device, apparatus, structure, component is able to meet the needs in various fields: the field of heat engineering, in particular for the designing of high performing heat exchangers since silicon carbide has good thermal conductivity and good resistance to high temperatures in extreme environments; the field of mechanical engineering, to manufacture on-board devices to obtain light weight, rigid, refractory components resisting to abrasion and mechanical stresses; the field of chemical engineering, since silicon carbide is resistant to numerous corrosive chemical products such as bases and strong acids; the field of nuclear engineering, for the manufacturing of cladding for nuclear fuel; the fields of spatial optics (telescope mirror in SiC) and aeronautics (parts made of SiC/SiC composite); power electronics which use SiC substrates.

(93) The invention will now be described using the following examples evidently given as non-limiting illustrations.

EXAMPLES

Example 1

(94) This example describes sessile drop tests performed with a brazing composition, braze alloy of the invention of composition 58% Si, 32% Ni and 10% Al (atomic percentages) on sintered pure -SiC, observing a single brazing plateau at 1100 C. for 30 minutes.

(95) a) Preparation of the Brazing Composition and Brazing Paste

(96) The brazing composition concerned: 58 atomic % Si, 32 atomic % Ni and 10 atomic % Al was prepared from pieces of pure Si, pieces of pure Ni and pieces of pure Al.

(97) These pieces were weighed paying heed to the proportions of the brazing composition and placed in an alumina crucible. The assembly was placed in a graphite furnace and subjected to a heat cycle with a plateau at 1300 C. under a high vacuum for 30 minutes.

(98) After cooling, this gave an ingot. This ingot was crushed to obtain a powder.

(99) An organic binder (NICROBRAZ cement) was added to this mixture of powders to form a viscous paste.

(100) b) Sessile Drop Test at 1100 C.

(101) The brazing paste thus prepared was used to form a small mound of braze alloy having a mass of approximately 50 mg. This mound of braze alloy was deposited on a previously cleaned SiC plate.

(102) The assembly of the braze alloy mound and plate was placed in a brazing furnace and subjected to a brazing heat cycle under a high vacuum with only a single plateau which was the brazing plateau at 1100 C. for a time of 30 minutes.

(103) The mound of braze alloy melts during this heat treatment and forms a drop that is called a sessile drop.

(104) After cooling the wetting, contact angle of the drop was measured on the solidified drop.

(105) The wetting angle was of the order of 30 which corresponds to good wetting.

(106) The SiC and its drop of solidified braze alloy were then cross-sectioned, prepared and polished and observed under scanning electron microscope.

(107) The SiC/braze alloy interface did not show any reactivity on the scale of scanning electron microscopy i.e. there was no formation of a new compound. In particular, there was no formation of fragile compounds at the interface.

Example 2

(108) This example describes sessile drop tests performed with a brazing composition, braze alloy of the invention having the composition 58% Si, 32% Ni and 10% Al (atomic percentage) on sintered pure -SiC observing a single brazing plateau at 1100 C. for 5 minutes.

(109) a) Preparation of the Brazing Composition and Brazing Paste

(110) The brazing composition concerned i.e. 58% Si, 32% Ni and 10% Al (atomic percentages) was prepared from pieces of pure Si, pieces of pure Ni and pieces of pure Al.

(111) These pieces were weighed paying heed to the proportions of the brazing composition and placed in an alumina crucible. The assembly was placed in a graphite furnace and subjected to a heat cycle with a plateau at 1300 C. under a high vacuum for 30 minutes.

(112) After cooling, this gave an ingot. This ingot was crushed to obtain a powder.

(113) An organic binder (NICROBRAZ cement) was added to this mixture of powders to form a viscous paste.

(114) b) Sessile Drop Test at 1100 C.

(115) The brazing paste thus prepared was used to form a small mound of braze alloy having a mass in the order of 50 mg. This mound of braze alloy was deposited on a SiC plate that was previously cleaned.

(116) The assembly of the mound of braze alloy and plate was placed in a brazing furnace and subjected to a brazing heat cycle under a high vacuum with a single plateau, which was the brazing plateau at 1100 C. for a time of 5 minutes.

(117) The mound of braze alloy melts during this heat treatment and forms a so-called sessile drop.

(118) After cooling, the wetting, contact, angle of the drop was measured on the solidified drop.

(119) The wetting angle was of the order of 50 which corresponds to good wetting.

Example 3

(120) This example describes the preparation of bonds, joining between two parts made of sintered pure -SiC silicon carbide using the brazing method according to the invention, the brazing being conducted in capillary configuration using a brazing composition, braze alloy of the invention composed of 58 atomic % Si, 32 atomic % Ni and 10 atomic % aluminium.

(121) This example also describes tests, mechanical testing performed on these assemblies.

(122) a) Preparation of the Brazing Composition, of the Brazing Paste and of the Parts to be Assembled

(123) The brazing composition concerned, namely 58 atomic % Si, 32 atomic % Ni and 10 atomic % aluminium was prepared in the manner described in Example 1.

(124) An organic binder (NICROBRAZ cement) was added to the mixture of powders obtained to form a viscous brazing paste.

(125) The parts made of sintered SiC to be assembled were plates of size 2010 mm.sup.2 and thickness of 1.5 mm.

(126) The parts were cleaned with acetone then ethanol and finally dried.

(127) The substrates, parts were placed in contact leaving a small offset of 1 to 2 mm, so as to leave a space for depositing the brazing paste in the vicinity of the joint (this configuration is called the capillary configuration). The paste was deposited with a spatula on the available surface on the edge of the joint, in the form of a bead of braze alloy (see FIG. 2). The quantity of braze alloy was between 20 and 40 mg for this assembly.

(128) b) Brazing

(129) The contacted parts ready to be brazed were placed in a brazing furnace (graphite furnace) under a high vacuum and subjected to a brazing heat cycle under a vacuum which comprised a single temperature plateau of 90 minutes at 1100 C., which was the brazing plateau.

(130) The heat cycle is illustrated in FIG. 5.

(131) c) Observation of the Joint

(132) After cooling, the assembly was well joined. The joint was characterized by scanning electron microscopy. There was no <<void>> and a reactivity between the SiC and the braze alloy was not evidenced on the scale of observation under scanning electron microscopy.

(133) d) Preparation of Mechanical Test Pieces and Results of Mechanical Testing

(134) Assemblies, test pieces (5 test pieces) for mechanical testing were prepared by brazing 2 parts each of size 20101.5 mm.sup.3 (the thickness of the brazed test piece was therefore 1.5+1.5=3 mm) (61, 62) with the brazing paste prepared at a) above and under the brazing conditions described at b) above. Since the mechanics of ceramics are statistical, more than one test piece was prepared for testing but following the same method of fabrication.

(135) The test pieces are schematized in FIG. 6. They were held on a mount and subjected to shearing during a compression/shear test (63) at ambient temperature.

(136) It is to be noted that this test does not allow the guaranteeing of pure shear but it is the preferred mode. However this test does allow a comparison between the assemblies.

(137) Results of the Mechanical Tests

(138) The breaking stresses determined for each of the 5 test pieces were 33 MPa; 67 MPa; 35 MPa; 53 MPa and 54 MPa i.e. a mean of 48 MPa.

(139) Breaking occurred in the SiC, which is characteristic of strong bonds between the braze alloy and the substrate in SiC.

(140) It is to be noted that the breaking stress values of joints, assemblies of the type SiC/braze alloy with high Si content/SiC can be more or less dispersed on account of the fragile nature of ceramic materials.

Example 4

(141) This example describes the preparation of bonds, joining between two parts made of CMC, more specifically made of SiC/SiC composite with a SiC matrix and SiC fibres, using the brazing method of the invention, the brazing being conducted in capillary configuration using a brazing composition, braze alloy of the invention composed of 58 atomic % Si, 32 atomic % Ni and 10 atomic % aluminium.

(142) This example also describes mechanical tests performed on these assemblies.

(143) a) Preparation of the Brazing Composition, of the Brazing Paste and of the Parts to be Joined

(144) The brazing composition concerned, namely 58 atomic % Si, 32 atomic % Ni and 10% aluminium was prepared in the manner described in Example 1.

(145) An organic binder (NICROBRAZ cement) was added to the mixture of powders obtained to form a viscous brazing paste.

(146) The parts, substrates to be brazed, joined were plates of SiC/SiC composite with a SiC matrix and SiC fibres. The said composite material is available from Snecma Propulsion Solide under the trade name Cerasep A40C. These plates were of size 2010 mm.sup.2 and of thickness 1.5 mm.

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

(148) The substrates, parts were placed in contact leaving a small offset of 3 mm, so as to leave a space for depositing the brazing paste in the vicinity of the joint (this configuration is called the capillary configuration). The paste was deposited with a spatula on the free surface at the edge of the joint, in the form of a bead of braze alloy (see FIG. 2), as described in Example 2. The quantity of deposited braze alloy was between 180 and 200 mg for this assembly.

(149) This quantity of braze alloy is much higher than in Example 2 since the clearance between the plates made of CMC was much greater than for the plates of sintered SiC in Example 2.

(150) For example the thickness of the joint may reach 500 m for the CMC plates on account of the planarity defects, whereas it is generally less than 100 m for the SiC plates.

(151) b) Brazing

(152) The parts placed in contact and ready to be brazed were placed in a brazing furnace (graphite furnace) under a high vacuum and subjected to a vacuum brazing heat cycle which, as for Example 2, comprised a single plateau for 90 minutes at 1100 C., which was the brazing plateau.

(153) The heat cycle is illustrated in FIG. 5.

(154) c) Observation of the Joint

(155) After cooling, the assembly was well joined. The joint was characterized under scanning electron microscopy. There was no <<void>>, and no reactivity between the SiC and the braze alloy was evidenced on the scale of observation under scanning electron microscopy.

(156) The thickness of the joint was between 100 and 500 m depending on the observed zones owing to local coating defects of the CMC and planarity defects.

(157) d) Preparation of Mechanical Test Pieces and Results of Mechanical Tests

(158) Assemblies, test pieces (4 test pieces) for mechanical testing were prepared by brazing 2 parts each of size 20101.5 mm.sup.3 with the brazing paste prepared at a) above and under the brazing conditions described at b) above.

(159) The test pieces were of similar size to those in Example 2 and were similarly tested under compression/shear.

(160) Results of the Mechanical Tests:

(161) The breaking stresses determined for each of the 4 test pieces were 14 MPa; 12 MPa; 13 MPa and 20 MPa i.e. a mean of about 15 MPa.

(162) For three test pieces, yield occurred by detachment of the SiC coating seal coat from the CMC. This coating therefore proves to be the weak point of the CMC/braze alloy/CMC assembly.

(163) For the fourth test piece, the measured stress corresponded to the onset of degradation of the composite.

Example 5

(164) This example describes the preparation of bonds, joining, assemblies, between two parts made of CMC, more specifically made of SiC/SiC composite with SiC matrix and SiC fibres, using the brazing method of the invention, brazing being conducted in capillary configuration using a brazing composition, braze alloy of the invention composed of 58 atomic % Si, 32 atomic % Ni and 10 atomic % aluminium and reinforcements of SiC particles not heat treated at 1400 C.

(165) This example further describes tests, mechanical testing conducted on these assemblies.

(166) a) Preparation of the Brazing Composition, of the Brazing Paste and of the Parts to be Joined

(167) The brazing composition concerned, namely 58 atomic % Si, 32 atomic % Ni and 10 atomic % aluminium was prepared in the manner described in Example 1.

(168) An organic binder (NICROBRAZ cement) was added to the mixture of powders to form a viscous brazing paste.

(169) The parts, substrates to be brazed were plates (71, 72) made of SiC/SiC composite with a SiC matrix and SiC fibres. Said composite material is available from Snecma Propulsion Solide under the trade name Cerasep A40C. These plates (71, 72) were of size 2010 mm.sup.2 with a thickness of 1.5 mm.

(170) The parts (71, 72) were cleaned with acetone followed by ethanol and then dried.

(171) The plates (71, 72) were coated with SiC particles of particle size 50 m. These particles were not heat treated at 1400 C.

(172) For depositing on the composite plates, the SiC particles were bonded together using an organic binder such as a cement of NICROBRAZ type, which allows the obtaining of a paste easy to deposit on the CMC plates. Depositing was carried out as indicated in FIG. 7 and the amount of deposited particles was 87 mg1 mg.

(173) The plates of CMC (71, 72) were then contacted leaving a small offset (73) of 3 mm so as to leave a space (74) to deposit the brazing paste in the vicinity of the joint (75) (this configuration is called a capillary configuration).

(174) The joint (75) was filled with the paste of SiC reinforcement particles (76) which projected beyond the joint (75) over the available, free, surface (74) offset from the lower plate (72).

(175) The paste of brazing composition (77) was deposited using a spatula over the available surface (74) at the edge of the joint, in the form of a bead of braze alloy (77) (see FIG. 7), as described in Example 2. The amount of braze alloy deposited was between 195 and 220 mg for this assembly.

(176) This amount of braze alloy was much higher than in Example 2 since the clearance between the CMC plates was much greater than between the plates of sintered SiC in Example 2.

(177) For example, the thickness of the joint may reach 700 m for CMC plates owing to planarity defects whereas it is generally less than 100 m for SiC plates.

(178) b) Brazing

(179) The parts placed in contact and ready to be brazed were placed in a brazing furnace (graphite furnace) under a high vacuum and subjected to a vacuum heat cycle which, as for Example 2, comprised a single plateau of 90 minutes at 1100 C., which was the brazing plateau.

(180) The heat cycle is illustrated in FIG. 5.

(181) c) Preparation of the Mechanical Test Pieces and Results of Mechanical Testing

(182) Assemblies, test pieces (5 test pieces) for mechanical testing were prepared by brazing 2 parts each of size 20101.5 mm.sup.3 with the brazing paste prepared at a) above with the coating of SiC particles described above, and under the brazing conditions described at b) above.

(183) The test pieces were of similar size to those in Example 2 and were tested in the same manner under compression/shear.

(184) Results of Mechanical Tests:

(185) The breaking stresses determined for each of the 5 test pieces were 13 MPa; 15 MPa; 14 MPa; 11 MPa and 22 MPa i.e. a mean of 15 MPa.

(186) For four test pieces breaking occurred by detachment of the SiC coating seal coat from the CMC. This coating therefore proves to be the weak point of the CMC/braze alloy/CMC assembly.

(187) For the fifth test piece, the measured stress corresponded to the onset of degradation of the composite.

(188) After these tests, the test pieces were cross-sectioned. A lack of braze alloy was observed in the centre of the test pieces.

(189) d) Observation of the Joints

(190) After the mechanical tests, the test pieces were cross-sectioned. A lack of braze alloy was observed (under SEM, but also visually) in the centre of the test pieces.

(191) The thickness of the joint was between 100 and 700 m depending on the zones observed owing to local defects of the CMC coating and planarity defects.

Example 6

(192) This example describes the preparation of bonds, joining, assemblies, between two parts made of CMC, more specifically made of SiC/SiC composite with a SiC matrix and SiC fibres, using a brazing method of the invention, brazing being conducted in capillary configuration using a brazing composition, braze alloy of the invention composed of 58 atomic % Si, 32 atomic % Ni and 10 atomic % aluminium, and SiC particle reinforcements that were heat treated at 1460 C.

(193) a) Preparation of the Brazing Composition, of the Brazing Paste and of the Parts to be Joined

(194) The brazing composition concerned, namely 58 atomic % Si, 32 atomic % Ni and 10 atomic % aluminium was prepared in the manner described in Example 1.

(195) An organic binder (NICROBRAZ cement) was added to the mixture of powders obtained to form a viscous brazing paste.

(196) The parts, substrates to be brazed, assembled were two plates made of SiC/SiC composite with a SiC matrix and SiC fibres. Said composite material is available from Snecma Propulsion Solide under the trade name Cerasep A40C.

(197) The size of these plates was respectively 2030 mm.sup.2 and 2040 mm.sup.2 and they each had a thickness of 1.5 mm.

(198) The parts were cleaned with acetone followed by ethanol and then dried.

(199) The plates were coated with SiC particles of particle size 50 m.

(200) These particles were heat treated at 1460 C. under a high vacuum for two hours. After this heat treatment, the SiC particles were stored under argon until use.

(201) For depositing on the composite plates, the SiC particles were bonded together with an organic binder such as a cement of NICROBRAZ type, which allows the obtaining of a paste easy to deposit on the CMC plates. Depositing was carried out as indicated in FIG. 4, and the quantity of deposited particles was 194 mg1 mg, this quantity being distributed between the two plates.

(202) The CMC plates were then placed in contact leaving a slight offset of 3 mm so as to leave a space for depositing the brazing paste in the vicinity of the joint (this configuration is called a capillary configuration).

(203) The joint (42) was filled with the paste of SiC reinforcing particles (41) which projected beyond the joint (42) over the available surface, offset from the lower plate (45).

(204) The paste was deposited with a spatula over the available surface at the edge of the joint, in the form of a bead of braze alloy (47) (see FIG. 4), as described in Example 2. The quantity of deposited braze alloy was 1280 mg for this assembly.

(205) This amount of paste is high since there was a large clearance between the CMC plates.

(206) b) Brazing

(207) The parts placed in contact and ready to be brazed were placed in a brazing furnace (graphite furnace) under a high vacuum and subjected to a vacuum brazing heat cycle which comprised two temperature plateaus, namely: a first plateau at 980 C. for 180 minutes, a second plateau at 1100 C. for 90 minutes. The heat cycle is illustrated in FIG. 3.

(208) c) Observation of the Joints

(209) The assembly thus prepared after step b) was cross-sectioned and characterized under scanning electron microscopy.

(210) The joint was fully filled with the braze alloy, even in the centre.

(211) This example shows that filling of the centre of the joint is controlled in the presence of reinforcement.

REFERENCES

(212) [1] Gasse A., Coing-Boyat G., Bourgeois G., Method using a thick joint for joining parts in SiC-based materials by refractory brazing and refractory thick joint thus obtained, U.S. Pat. No. 5,975,407, 1999. [2] Gasse A., Method for assembling parts made of materials based on SiC by non-reactive refractory brazing, brazing composition, and joint and assembly obtained by said method, Patent application US-A1-2003/0038166. [3] Heap H., Method of Brazing, U.S. Pat. No. 3,813,759, 1974. [4] S. Kalogeropoulou, L. Baud, N. Eustathopoulos., Relationship between wettability and reactivity, Acta. Metall. Mater., Vol. 43, N.sup.o 3, pp. 907-912, 1995. [5] C. Rado, S. Kalogeropoulou, N. Eustathopoulos., Wetting and bonding of NiSi alloys on silicon carbide, Acta. Metall. Mater., Vol. 47, N.sup.o 2, pp. 461-473, 1999. [6] J. R. Mc Dermid, R. A. L. Drew., Thermodynamic brazing alloy design for joining silicon carbide, J. Am. Ceram. Soc., Vol. 74, N.sup.o 8, pp. 1855-1860, 1991. [7] Montgomery F. C., Streckert H. H., Braze for Silicon Carbide bodies, U.S. Pat. No. 5,447,683, 1995.