Process for the moderately refractory assembling of articles made of SiC-based materials by non-reactive brazing in an oxidizing atmosphere, brazing compositions, and joint and assembly obtained by this process

Abstract

Process for the moderately refractory assembly of at least two articles made of silicon carbide-based materials by non-reactive brazing in an oxidizing atmosphere, in which the articles are placed in contact with a non-reactive brazing composition and the assembly formed by the articles and the brazing composition is heated in an oxidizing atmosphere at a brazing temperature sufficient to melt the brazing composition so as to form a moderately refractory joint, wherein the non-reactive brazing composition is a composition A composed of silica (SiO.sub.2), alumina (Al.sub.2O.sub.3) and calcium oxide (CaO), or alternatively a composition B composed of alumina (Al.sub.2O.sub.3), calcium oxide (Cao) and magnesium oxide (MgO). Brazing suspension, paste comprising a powder of said brazing composition and an organic binder. Refractory joint and assembly.

Claims

1. A process for moderately refractory assembly of at least two articles made of silicon carbide-based materials by non-reactive brazing in an oxidizing atmosphere, the process consisting of: placing the at least two articles in contact with a non-reactive brazing composition to form an assembly; and heating the assembly formed by the at least two articles and the non-reactive brazing composition under an oxidizing atmosphere to a brazing temperature that is sufficient to melt the non-reactive brazing composition in order to form a moderately refractory joint, wherein said oxidizing atmosphere remains present during said melting of the non-reactive brazing composition, and 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 an addition of titanium oxide or an addition of boron oxide or both, or alternatively a composition B consisting of alumina (Al.sub.2O.sub.3), calcium oxide (CaO) and magnesium oxide (MgO) and optionally an addition of titanium oxide or an addition of boron oxide or both.

2. The process according to claim 1, wherein the oxidizing atmosphere is an atmosphere containing oxygen comprising an atmosphere of air, of oxygenated argon or of oxygenated helium.

3. The process according to claim 1, in which composition A is composed, as mass percentages, of 75% to 7% SiO.sub.2, 60% to 6% Al.sub.2O.sub.3 and 60% to 10% CaO.

4. The process according to claim 3, in which composition A is composed, as mass percentages, of 70% to 55% SiO.sub.2, 22% to 8% Al.sub.2O.sub.3 and 35% to 15% CaO.

5. The process according to claim 4, in which composition A is composed, as mass percentages, of 62% SiO.sub.2, 15% Al.sub.2O.sub.3 and 23% CaO.

6. The process according to claim 3, in which composition A is composed, as mass percentages, of 55% to 38% SiO.sub.2, 25% to 12% Al.sub.2O.sub.3 and 45% to 30% CaO.

7. The process according to claim 6, in which composition A is composed, as mass percentages, of 42% SiO.sub.2, 20% Al.sub.2O.sub.3 and 38% CaO.

8. The process according to claim 3, in which composition A is composed, as mass percentages, of 38% to 8% SiO.sub.2, 55% to 8% Al.sub.2O.sub.3 and 55% to 28% CaO.

9. The process according to claim 8, in which composition A is composed, as mass percentages, of 22% SiO.sub.2, 37% Al.sub.2O.sub.3 and 41% CaO.

10. The process according to claim 1, in which composition B is composed, as mass percentages, of 70% to 35% Al.sub.2O.sub.3, 65% to 25% CaO and 20% to 1% MgO.

11. The process according to claim 10, in which composition B is composed, as mass percentages, of 50.5% Al.sub.2O.sub.3, 44.0% CaO and 5.5% MgO.

12. The process according to claim 1, in which the brazing temperature is 1100 C. to 1650 C., and the heating is performed at the brazing temperature for a time of 1 to 240 minutes.

13. The process according to claim 12, in which the brazing temperature is 1350 C. to 1650 C., and the heating is performed at the brazing temperature for a time of 1 to 240 minutes.

14. The process according to claim 12, in which the heating is performed for a time of 2 to 30 minutes.

15. The process according to claim 1, in which the heating of the assembly formed by the at least two articles and the non-reactive brazing composition under the oxidizing atmosphere to the brazing temperature is achieved by introducing the assembly directly into a device already brought to the brazing temperature.

16. The process according to claim 15, in which the device is an oven.

17. The process according to claim 1, in which the heating of the assembly formed by the at least two articles and the non-reactive brazing composition under the oxidizing atmosphere to the brazing temperature is achieved by applying a temperature rise from room temperature.

18. The process according to claim 1, in which the silicon carbide-based materials are selected from among the group consisting of pure silicon carbides and SiC-based composite materials.

19. The process of claim 18, wherein the pure silicon carbides are pure silicon carbide (-SiC) or pure silicon carbide (-SiC).

20. The process of claim 18, wherein the SiC-based composite materials are a composite material with at least one selected from silicon carbide fibers and a silicon carbide matrix.

21. The process according to claim 1, in which the silicon carbide-based materials are selected from among the group consisting of pressureless sintered silicon carbide (PLSSiC); Si-infiltrated silicon carbide (SiSiC or RBSC); porous recrystallized silicon carbide (RSiC); silicon graphite (CSiC) composed of graphite and covered with a layer of SiC; SiC/SiC composites; C/SiC composites; SiC monocrystals; and SiC composites with another ceramic.

22. The process of claim 21, wherein the SiC/SiC composites contain fibers or whiskers.

23. The process of claim 21, wherein the C/SiC composites contain carbon fibers or whiskers and an SiC matrix.

24. The process of claim 21, wherein the SiC composites with another ceramic are SiC/Si.sub.3N.sub.4 or SiC/TiN.

25. The process according to claim 1, in which said silicon carbide-based materials have a silicon carbide content at least equal to 50% by mass.

26. The process according to claim 25, in which said silicon carbide-based materials have a silicon carbide content at least equal to 80% by mass.

27. The process according to claim 26, in which said silicon carbide-based materials have a silicon carbide content equal to 100% by mass.

28. The process of claim 1, wherein the at least two articles are shaped articles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of the specimens, test pieces, used for the mechanical tests, especially shear tests, of the joints and assemblies prepared in the examples.

(2) FIG. 2 is a graph that gives the wetting angle (in degrees) as a function of the duration of the heating stage (in minutes) observed in Example 1b). For curve 1, the brazing stage temperature is 1590 C. For curve 2, the brazing stage temperature is 1400 C. For curve 3, the brazing stage temperature is 1300 C.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(3) The first step of the process according to the invention generally consists, first, in preparing or producing a brazing composition, in other words a brazing alloy. The brazing alloy according to the invention is a ternary alloy, either of the silica (SiO.sub.2)-alumina (Al.sub.2O.sub.3)-calcium oxide (CaO) system (brazing composition A), or of the alumina (Al.sub.2O.sub.3)-calcium oxide (CaO)-magnesium oxide (MgO) system (brazing composition B). The preferred mass proportions for each of the compositions A and B have been mentioned above.

(4) The brazing composition is generally a pulverulent composition, which may be prepared by first synthesizing, from the various pure oxides and/or from compounds composed of several of these oxides, a glass containing these oxides.

(5) Examples of such compounds formed from several oxides are mullite, which is the compound 3Al.sub.2O.sub.3-2SiO.sub.2, the compound CaOAl.sub.2O.sub.3 and the compound CaOSiO.sub.2.

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

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

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

(9) It is possible to improve the properties of the glasses of the SiO.sub.2Al.sub.2O.sub.3CaO system (brazing composition A) by crystallizing them by adding titanium oxide TiO.sub.2, which is a nucleating agent. Typically, a few mass % of TiO.sub.2 will be added, for example from 1% to 10% by mass of TiO.sub.2 relative to the total mass of the brazing composition. On the other hand, glasses of the Al.sub.2O.sub.3CaOMgO system generally crystallize without this addition. It should also be noted that the melting point of these brazing alloys, whether it is the brazing composition A or the brazing composition B, may be reduced if need be by adding boron oxide B.sub.2O.sub.3, in a proportion, for example, of from 1% to 10% by mass of B.sub.2O.sub.3 relative to the total mass of the brazing composition.

(10) The mixture of ground powders is then introduced into a crucible generally made of platinum, and the various constituents of the powder mixture are melted by subjecting them, for example, to a stage of 4 hours in air at 1590 C. or 1640 C. depending on the composition of the brazing alloy. After cooling, a homogeneous glass whose melting point generally ranges between 1200 C. and 1600 C., depending on its composition, is obtained.

(11) The glass obtained is recovered and then ground in a mortar or any other device suitable for obtaining a powder of suitable particle size whose grains have, for example, a diameter of from 10 to 250 m.

(12) The ground glass is then conventionally mixed with a cement or a liquid organic binder that is generally both viscous and tacky so as to obtain a paste that allows uniform spreading onto the surfaces of the silicone carbide-based substrates to be brazed.

(13) Such a binder or cement generally decomposes, for example, between 100 and 300 C. without leaving traces. It may be, for example, a cement of Nicrobraz type.

(14) Experiments involving applying a drop (goutte pose in French) of this glass onto SiC were performed in air between 1100 C. and 1590 C. They showed, surprisingly, firstly, that it is possible to reach within a few minutes contact angles below 60 and, secondly, that the SiC/glass bonds formed are strong.

(15) The second step of the process according to the invention generally consists in performing the actual brazing assembly.

(16) Prior to assembling, the two (or more) surfaces of the articles made of SiC-based materials to be assembled are generally degreased in an organic solvent, for instance a ketone, an ester, an ether, an alcohol or a mixture thereof, etc., a preferred solvent being acetone or an acetone-ethyl alcohol-ether mixture, for example in , , proportions; and then dried.

(17) There are generally two articles, parts, made of SiC-based materials to be assembled, but a larger number of articles, which may be up to 100, may also be simultaneously assembled.

(18) The expression article, part, made of SiC-based materials generally means any element or species of any size or shape included, for example after assembly with one or more other articles, parts, in structures of larger size.

(19) According to the invention, it is possible to assemble, with excellent results each time, articles of complex geometry or shape and/or of large size.

(20) The term silicon carbide-based material generally means herein any material comprising at least 50% by mass of silicon carbide, preferably at least 80% by mass of silicon carbide and more preferably 100% by mass of silicon carbide; in the latter case, the material is made, composed, consists solely of silicon carbide.

(21) As has already been stated hereinabove, the silicon carbide-based materials may be chosen from pure silicon carbides such as pure silicon carbide (-SiC) or silicon carbide (-SiC) and SiC-based composite materials such as composites with silicon carbide fibres and/or with a silicon carbide matrix.

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

(23) The two or more articles to be assembled may be made of the same material, based on silicon carbide, for example made of PLS (pressureless sintered) -SiC, or made of SiCSiC composite, or each of the articles may be made of a different material.

(24) The suspension, slurry or paste of the brazing composition described previously is spread, coated or applied, preferably uniformly, onto the surface of at least one of the articles made of silicon carbide-based material to be assembled, and the surfaces of the two articles to be assembled are then placed in contact. This brazing configuration is known as the sandwich configuration since the paste of the brazing composition is placed directly between the surfaces of the articles, parts, to be assembled.

(25) The amount of paste, slurry or suspension of brazing composition to be used in this configuration is generally from 4 mg/cm.sup.2 to 17 mg/cm.sup.2, for example 11 mg/cm.sup.2.

(26) Alternatively, the surfaces of the articles to be assembled are brought together so as to leave a gap generally of 1 to 200 m, which will be filled by capillary effect with the brazing composition, the latter being placed close to the gap to be filled, in a space or reservoir made for this purpose, the said reservoir possibly being of millimetric size in accordance with the knowledge of a person skilled in the art in this field.

(27) This brazing configuration is known as the capillary configuration. With the brazing compositions according to the invention, it is possible to perform such capillary brazing, i.e. infiltration of the braze into the brazing joint, without directly placing the brazing composition between the articles to be assembled as in the case of the sandwich configuration.

(28) The amount of paste or suspension of brazing composition to be used in this capillary configuration is generally from 4 mg/cm.sup.2 to 17 mg/cm.sup.2, for example 11 mg/cm.sup.2.

(29) The articles ready to be brazed are then placed in a heating device such as an oven or subjected to heating by any other suitable means.

(30) In accordance with the invention, the brazing is performed in an oxidizing atmosphere such as an atmosphere containing oxygen, for example in air, and the heating device such as an oven, for example, is in an atmosphere of air, oxygenated argon or oxygenated helium.

(31) The articles to be assembled are subjected, for example in the oven, to a brazing heating cycle in an oxidizing atmosphere, especially in air.

(32) Thus, the assembly formed by the articles and the brazing composition may be brought to the brazing temperature (brazing stage, plateau) by applying a temperature rise that is preferably slow, with one or more temperature ramps from room temperature.

(33) This temperature rise may take place, for example, with a temperature ramp at a rate of 400 C./minute.

(34) The brazing stage, plateau, is generally effected at a temperature corresponding to the melting point of the chosen brazing composition, but this temperature is preferably a temperature at least 25 C. higher than this melting point.

(35) Depending on the compositions, the brazing temperature will thus range, for example, from 1350 C. to 1650 C.

(36) Such a melting point of the compositions allows, according to another advantage of the process of the invention, use of the assembly, especially, in air, for example up to 850 C. and even up to 1200 C.

(37) The brazing time, i.e. the heating cycle for performing the assembling, is, according to the invention, generally short. The duration of the brazing stage is generally from 2 to 30 minutes.

(38) This duration may be increased slightly for very large articles to be brazed, and may then be up to 200 minutes, or even 240 minutes.

(39) However, it is possible, for certain compositions, to form a brazed joint at a lower temperature than the particularly recommended temperature mentioned above, below the melting point, in the viscous range of the glass, for example at 1100 C. for a brazing stage time that is then longer and that may be up to 120 minutes. However, the mechanical strength of the joint will be lower than in the case of brazing at higher temperature (see Example 2). This brazing at lower temperature may satisfy certain applications that require brazing at moderate temperature.

(40) It is also possible to introduce directly, rapidly, the articles to be assembled, ready to be brazed, into a device such as an oven already brought to the brazing stage temperature, generally from 1350 C. to 1650 C., so as to reduce the duration of the heating cycle.

(41) At the end of the brazing cycle, after the brazing stage, the assembly is cooled to room temperature, for example at a rate of 5 C. per minute.

(42) During cooling, the braze solidifies.

(43) Surprisingly, the SiC/SiC assemblies brazed in air in accordance with the process of the invention show very little or no porosity, with filling of the joint by the braze that is generally greater than 90% by volume.

(44) The assembly is satisfactory with little or no porosity, both when heating is performed with a rapid temperature rise, i.e. with introduction of the articles to be brazed directly into the already-hot oven, and when a slow temperature rise is performed, i.e. with introduction of the articles to be brazed into a cold oven.

(45) The assemblies of articles made of silicon carbide comprising joints prepared by the process according to the invention make it possible to produce structures, apparatus or components of complex shapes having high working, use, temperatures that may generally be up to 1200 C., with great precision.

(46) Specifically, it is known that the mechanical properties of silicon carbide: great hardness; low coefficient of expansion; high breaking strength; good thermal shock strength;
and also its very good conductivity make it an indispensable material for the present and future industrial applications at high temperature.

(47) Furthermore, SiC shows very good chemical resistance to various acids, including hydrofluoric acid, and very good resistance to oxidation in air at high temperature up to 1300 C.

(48) In other words, the process according to the invention may be applied especially to the manufacture of any device, apparatus, structure or component requiring moderately refractory assembly between at least two silicon carbide-based substrates, parts or articles while ensuring both good mechanical strength and satisfactory leaktightness of the assembly.

(49) This type of device, apparatus, structure or component may satisfy needs in various fields: the field of heat engineering, especially for designing highly efficient heat exchangers, since silicon carbide has good heat conductivity and good resistance to high temperatures in extreme environments; the field of mechanical engineering for producing in embarked devices light, rigid, refractory components that are resistant to abrasion and resistant to mechanical stresses; the field of chemical engineering, since silicon carbide is resistant to numerous corrosive chemical products, for instance strong acids and bases; the field of nuclear engineering, for the production of sheathing for fuel rods; the fields of spatial optics (telescope mirror made of SiC) and of aeronautics (article, part, made of SiC/SiC composite); power electronics using SiC.

(50) The invention will now be described by means of the examples that follow, which, needless to say, are given as non-limiting illustrations.

Example 1

(51) This example describes applied drop tests (Essais de goutte pose in French) and bond strength tests performed with a brazing alloy or brazing composition according to the invention having the composition: 62% by mass of SiO.sub.2-15% by mass of Al.sub.2O.sub.3-23% by mass of CaO on sintered pure -SiC.

(52) a) Preparation of the Brazing Composition

(53) The targeted brazing composition: 62% by mass of SiO.sub.2-15% by mass of Al.sub.2O.sub.3-23% by mass of CaO, was prepared from SiO.sub.2 powder, mullite powder (defined compound 3Al.sub.2O.sub.3-2SiO.sub.2) and powder of the compound CaOSiO.sub.2.

(54) These powders are weighed out, adhering to the proportions, and are then mixed together and ground in a mortar. The powder mixture is then subjected to a stage of 4 hours in air at 1590 C. After cooling, a glass with a melting point of about 1200 C. is obtained. The analyses performed with an X-ray microprobe indicate that the mixture is homogeneous and that the mass composition is 61.2% SiO.sub.2-15.0% Al.sub.2O.sub.3-23.4% CaO.

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

(56) b) Applied Drop and Bond Strength Tests

(57) A small piece of the glass prepared as described above, with a mass of between 10 mg and 40 mg, is placed on a silicon carbide substrate.

(58) This substrate and its small piece of glass are introduced into an oven under air and subjected to a heating cycle with a heating stage. The glass melts during this heat treatment and forms a drop referred to as an applied drop (goutte pose in French).

(59) All the tests performed with stages between 1300 C. and 1590 C. (1300, 1400 and 1590 C.) led to very rapid wetting of the SiC (a few minutes) with contact angles of between 20 and 50.

(60) The results of these tests are given in FIG. 2.

(61) It is also important to note that the SiC/glass bonds thus formed are strong (cohesive break, no cracking at the SiC/glass interface), as may be observed visually in the course of cooling on removal from the oven.

Example 2

(62) This example describes the preparation of bonds or assemblies between two articles made of sintered pure -SiC silicon carbide, by performing the brazing process according to the invention (brazing in sandwich configuration), and using a brazing composition according to the invention composed of (consisting of) 62% by mass of SiO.sub.2-15% by mass of Al.sub.2O.sub.3-23% by mass of CaO, and mechanical tests or trials performed on these assemblies.

(63) a) Preparation of the Brazing Composition and of the Articles to be Assembled

(64) The glass is produced according to the procedure described in Example 1. It is then recovered and ground in a mortar. It is then mixed with an organic cement, Nicrobraz, which is both viscous and tacky, so as to obtain a paste that is easy to spread.

(65) The articles made of sintered SiC are plates 2020 mm.sup.2 in size and 1 mm thick or discs 15 mm in diameter and 5 mm thick.

(66) The two SiC surfaces to be assembled are degreased in an organic solvent, and then dried.

(67) The paste is spread uniformly onto the surface of one of the silicon carbide-based substrates, parts or articles to be assembled. The amount applied during this test is about 11 mg/cm.sup.2, the minimum being 4 mg/cm.sup.2 and the maximum 17 mg/cm.sup.2. The substrates or articles are then placed in contact (this configuration is known as the sandwich configuration).

(68) b) Brazing

(69) The articles placed in contact and thus ready to be brazed are placed in an oven and subjected to a brazing heating cycle in air. The assembly obtained is then cooled to room temperature, at a rate of 6 C. per minute.

(70) Various heating cycles were performed: A first series was performed by directly introducing the articles into the hot oven at the brazing stage temperature (rapid heating); a heating cycle with a brazing stage at 1590 C. for 5 minutes allowed a defect-free homogeneous joint to be obtained; similarly, a cycle with a stage at 1590 C. for 10 minutes led to a defect-free homogeneous joint; a heating cycle with a brazing stage at 1400 C. for 5 minutes gives a joint that is satisfactorily filled with braze (filling of greater than 90% by volume). A second series was performed by introducing the articles into the oven that is initially cold, and then subjecting them to slow heating up to the brazing stage (typically 4 hours to reach the brazing stage); under these slow heating conditions, a heating cycle with a brazing stage at 1500 C. for 10 minutes made it possible to obtain a satisfactorily filled homogeneous joint (filling of greater than 90% by volume).

(71) c) Preparation of the Mechanical Test Specimens and Results of the Mechanical Tests Mechanical test specimens are prepared by brazing two articles 15156 mm.sup.3 in size (1,2) with the glass paste described above in a).

(72) The specimens are represented schematically in FIG. 1. They are fixed in a mount and subjected to shear (3) at room temperature. in a first series of tests, five specimens are prepared by brazing with the glass paste described above in a), using a brazing cycle at 1590 C. for 10 minutes. in a second series of tests, two other mechanical test specimens are prepared with the glass paste described above in b), using a brazing cycle at 1100 C. for 120 minutes. Results of the mechanical tests:

(73) First series of tests: the breaking stresses determined for each of the five specimens are 43.4 MPa; 23.5 MPa; 45.7 MPa; 32.0 MPa; 65.3 MPa; i.e. an average of 42 MPa.

(74) Second series of tests: the breaking stresses determined for each of the two specimens are 11 MPa and 6 MPa.

Example 3

(75) This example describes the preparation of bonds or assemblies between two articles made of sintered pure -SiC silicon carbide, by performing the brazing process according to the invention (brazing in capillary configuration) using a brazing composition according to the invention composed of 62% by mass of SiO.sub.2-15% by mass of Al.sub.2O.sub.3 and 23% by mass of CaO.

(76) a) Preparation of the Brazing Composition and of the Articles to be Assembled

(77) The brazing paste is prepared as described in Example 2. The sintered SiC articles are plates 1515 mm.sup.2 in size and 5 mm thick.

(78) The two SiC surfaces to be assembled are degreased in an organic solvent, and then dried.

(79) The substrates or articles are placed in contact with an offset of a few mm so as to leave a space to apply the brazing paste close to the joint (this configuration is known as the capillary configuration). The paste is applied with a spatula to the available surface at the edge of the joint. The amount applied is about 11 mg/cm.sup.2, the minimum being 4 mg/cm.sup.2 and the maximum 17 mg/cm.sup.2.

(80) b) Brazing

(81) The articles, parts, placed in contact and ready to be brazed are placed in an oven and subjected to a brazing heating cycle in air. The articles to be brazed are introduced directly into the hot oven at the brazing stage temperature (rapid heating), i.e: at 1590 C., for 15 minutes. This treatment made it possible to obtain a defect-free homogeneous joint.

Example 4

(82) This example describes the preparation of bonds or assemblies between two articles made of sintered pure -SiC silicon carbide, by performing the brazing process according to the inventionthe brazing being performed in sandwich configurationand using a brazing composition according to the invention composed of (consisting of) 42% by mass of SiO.sub.2-20% by mass of Al.sub.2O.sub.3 and 38% by mass of CaO.

(83) This example also describes mechanical trials or tests performed on these assemblies.

(84) a) Preparation of the Brazing Composition and of the Articles to be Assembled

(85) The braze having the composition: 42% by mass of SiO.sub.2-20% by mass of Al.sub.2O.sub.3-38% by mass of CaO, was prepared from alumina powder Al.sub.2O.sub.3 and powder of the compound CaOSiO.sub.2. These powders are weighed out, adhering to the proportions, and are then mixed together and ground in a mortar. The powder mixture is then subjected to a stage of 4 hours in air at 1590 C. After cooling, a glass with a melting point of about 1200 C. is obtained.

(86) The glass obtained is recovered and then ground in a mortar. It is then mixed with an organic cement, Nicrobraz, which is both viscous and tacky, so as to obtain a paste that allows uniform spreading onto the surfaces to be brazed.

(87) The sintered SiC articles are discs 15 mm in diameter with a thickness or height of 5 mm.

(88) The two SiC surfaces to be assembled are degreased in an organic solvent and then dried.

(89) The paste is spread uniformly onto the surface of one of the silicon carbide-based substrates, parts or articles to be assembled. The amount applied during this test is about 11 mg/cm.sup.2, the minimum being 4 mg/cm.sup.2 and the maximum 17 mg/cm.sup.2. The substrates, parts, or articles are then placed in contact (this configuration is known as the sandwich configuration).

(90) b) Brazing

(91) The articles placed in contact and thus ready to be brazed are placed in an oven and subjected to a brazing heating cycle in air. The article to be brazed was introduced directly into the hot oven at the brazing stage temperature (rapid heating), i.e. at 1610 C., for 10 minutes. This treatment allowed a defect-free homogeneous joint to be obtained.

(92) The assembly obtained is then cooled to room temperature, at a rate of 6 C. per minute.

(93) c) Preparation of the Mechanical Test Specimens and Results of the Mechanical Tests Mechanical test specimens are prepared by brazing two articles 15156 mm.sup.3 in size with the glass paste described above in a).

(94) The specimens are represented schematically in FIG. 1. They are fixed in a mount and subjected to shear at room temperature.

(95) Two specimens are prepared by brazing with the glass paste described above in a) using a brazing cycle at 1590 C. for 10 minutes.

(96) Results of the Mechanical Tests:

(97) The breaking stresses determined for each of the specimens are 59 MPa and 56 MPa.

Example 5

(98) This example describes the preparation of bonds or assemblies between two articles made of sintered pure -SiC silicon carbide, by performing the brazing process according to the inventionthe brazing being performed in sandwich configurationand using a brazing composition according to the invention composed of (consisting of) 22% by mass of SiO.sub.2-37% by mass of Al.sub.2O.sub.3 and 41% by mass of CaO.

(99) a) Preparation of the Brazing Composition and of the Articles to be Assembled

(100) The braze having the composition: 22% by mass of SiO.sub.2-37% by mass of Al.sub.2O.sub.3-41% by mass of CaO, was prepared from powder of the compound CaOAl.sub.2O.sub.3 and powder of the compound CaOSiO.sub.2. These powders are weighed out, adhering to the proportions, and are then mixed together and ground in a mortar. The powder mixture is then subjected to a stage of 4 hours in air at 1640 C. After cooling, a glass with a melting point of about 1600 C. is obtained.

(101) The glass obtained is recovered and then ground in a mortar. It is then mixed with an organic cement, Nicrobraz, which is both viscous and tacky, so as to obtain a paste that allows uniform spreading onto the surfaces to be brazed.

(102) The sintered SiC articles are discs 15 mm in diameter with a thickness or height of 5 mm.

(103) The two SiC surfaces to be assembled are degreased in an organic solvent and then dried.

(104) The paste is spread uniformly with a brush onto the surface of one of the silicon carbide-based substrates or articles to be assembled. The amount applied during this test is about 11 mg/cm.sup.2, the minimum being 4 mg/cm.sup.2 and the maximum 17 mg/cm.sup.2. The substrates or articles are then placed in contact (this configuration is known as the sandwich configuration).

(105) b) Brazing

(106) The articles placed in contact and thus ready to be brazed are placed in an oven and subjected to a brazing heating cycle in air. The article to be brazed was introduced directly into the hot oven at the brazing stage temperature (rapid heating), i.e. at 1610 C., for 10 minutes. This treatment made it possible to obtain a homogeneous joint that is satisfactorily filled, i.e. with filling of greater than 90% by volume.

(107) The assembly obtained was then cooled to room temperature, at a rate of 6 C. per minute.

Example 6

(108) This example describes the preparation of bonds or assemblies between two articles made of sintered pure -SiC silicon carbide, by performing the brazing process according to the inventionthe brazing being performed in sandwich configurationand using a brazing composition according to the invention composed of (consisting of) 50.5% by mass of Al.sub.2O.sub.3, 44% by mass of CaO and 5.5% by mass of MgO.

(109) a) Preparation of the Brazing Composition and of the Articles to be Assembled:

(110) The braze having the composition: 50.5% by mass of Al.sub.2O.sub.3-44% by mass of CaO-5.5% by mass of MgO, was prepared from powder of the compound CaOAl.sub.2O.sub.3 and powders of CaO, MgO and Al.sub.2O.sub.3. These powders are weighed out, adhering to the proportions, and then mixed together and ground in a mortar. The powder mixture is then subjected to a stage of 4 hours in air at 1590 C. After cooling, a glass with a melting point of about 1350 C. is obtained.

(111) The glass obtained is recovered and then ground in a mortar. It is then mixed with an organic cement, Nicrobraz, which is both viscous and tacky, so as to obtain a paste that allows uniform spreading onto the surfaces to be brazed.

(112) The sintered SiC articles are discs 15 mm in diameter with a thickness or height of 5 mm.

(113) The two SiC surfaces to be assembled are degreased in an organic solvent and then dried.

(114) The paste is spread uniformly with a spatula onto the surface of one of the silicon carbide-based substrates or articles to be assembled. The amount applied during this test is about 11 mg/cm.sup.2, the minimum being 4 mg/cm.sup.2 and the maximum 17 mg/cm.sup.2. The substrates or articles are then placed in contact (this configuration is known as the sandwich configuration).

(115) b) Brazing

(116) The articles placed in contact and thus ready to be brazed are placed in an oven and subjected to a brazing heating cycle in air. The article to be brazed was introduced directly into the hot oven at the brazing stage temperature (rapid heating), i.e. at 1610 C. for 10 minutes. This treatment made it possible to obtain a homogeneous joint that is satisfactorily filled, i.e. with filling of greater than 90% by volume.

(117) Crystallization at the centre of the joint is noted.

(118) The assembly obtained is then cooled to room temperature, at a rate of 6 C. per minute.

REFERENCES

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