Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1999-07-02
2001-04-24
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S450000, C428S615000, C428S698000, C420S578000, C423S326000
Reexamination Certificate
active
06221499
ABSTRACT:
DESCRIPTION
The present invention relates to a method for joining parts of silicon carbide based materials by refractory brazing using an intermetallic Co—Si braze alloy in order to produce components entirely of silicon carbide, and to the refractory thick joint thus obtained.
The term “silicon carbide based materials” is generally understood to mean all materials whose SiC content is 80% by weight or more.
The technical field of the invention may be defined as high temperature brazing, that is to say which uses temperatures that are generally over 1200° C., allowing the joints obtained to be used in applications requiring temperature rises which may for example exceed 900° C. reaching as high as 1000° C. and even beyond.
As for all ceramics, it is generally difficult to manufacture parts of complex shape with silicon-based compounds such as silicon carbide.
It is therefore often preferable to manufacture structures using ceramic parts of simple shape, and subsequently to join these parts in order to achieve the final structure.
Owing to the high temperatures, for example in the region of 1000° C., used in applications of ceramics such as silicon carbide, joining these ceramics by gluing with organic products is excluded.
Also, conventional joining techniques by welding using a power beam with or without filler metal (tungsten inert gas welding(TIG), electron or laser welding) implying partial fusion of the parts to be joined cannot be used for joining ceramics since it is not possible to melt a ceramic substrate or part and because in particular silicon carbide dissociates before melting.
Consequently, solid phase diffusion welding, joining-sintering and reactive brazing are currently the most frequently used techniques to carry out refractory joining of ceramics.
Welding by solid phase diffusion and joining-sintering have the disadvantage of being restrictive regarding their implementation.
For solid phase diffusion welding, the shape of the parts must remain simple if uniaxial pressing is used, otherwise complex tooling and preparation are required comprising for example the making of a casing, sealed closing in a vacuum, hot isostatic compression, final machine-finishing of the casing if Hot Isostatic Compression (HIC) is used.
For joining-sintering, the same problems arise (shape of parts, complex implementation) with in addition the need to control the sintering of a filler powder to be inserted between the two materials to be joined.
These two techniques also require the use of long heating stages (one to several hours) at high temperature since the processes involved use solid phase diffusion; these long periods may contribute to grain enlargement of the refractory alloys making them fragile.
Reactive brazing is a cheap technique, easy to set in operation which is the most commonly used. Parts of complex shape may be made using capillary brazing and operations are restricted to placing the filler metal between or next to the joint and melting the braze alloy.
Ceramic brazing has to overcome the fundamental problem of the poor wetting of ceramics by metals. This problem is overcome by selecting particular alloy compositions and/or by optionally adding a reactive element as is precisely the case for reactive brazing.
For this latter technique a metal alloy composition is used, most often copper and silver based to which is added a reactive agent such as Ti, Zr, V, HF, Nb . . .
The reactive element acts by dissociating the surface of the ceramic and reacting with the latter to form a very stable nitride, oxide, silicide or carbide compound depending upon the nature of the ceramic involved. This very stable compound layer offers properties analogous to metals and therefore allows very good wetting of the ceramic and satisfactory adhesion to the latter.
Document EP-A-0 135 603 (GTE Products Corporation) thus describes a ductile brazing alloy comprising from 0.25% to 4% of a reactive metal selected from titanium, vanadium, zirconium, and their mixtures, from 20 to 85% of silver, from 1 to 70% of copper or aluminium, and from 3 to 30% of palladium.
The document published by E. Lugscheider and W. Tillman “Development of New Active Filler metal for Joining Silicon Carbide & Nitride”—Babs 6
th
International Conference, Sep. 3-5, 1991, Stratford-upon-Avon—indicates that ceramics of non-oxide type such as Si
3
N
4
and SiC may be joined together using braze alloys containing copper, silver and a reactive metal selected from titanium, zirconium, hafnium or niobium.
The document by W. Tillman et al. “Herstellungsmöglichkeiten von hochtemperaturbeständigen Verbunden nichtoxidischer Ingenieurkeramiken mittels verschiedener Lotkonzepte” “Brazing-High Temperature Brazing and Diffusion Welding”; Aachen, June 27-29, DVS Verlag GmbH, Düsseldorf, p. 110-114, 1995, describes mixtures for brazing of non-oxide ceramics that essentially contain palladium and platinum with additions of iron, nickel or cobalt.
The reactive metal such as titanium may also be previously deposited on the silicon carbide using techniques such as “CVD” (Chemical Vapour Deposit) or “PVD” (Physical Vapour Deposit) as described in the document by M. Naka et al: “Ti-precoating effect on Wetting and Joining of Cu to SiC”; ISIJ International; Vol. 30 (1990), N° 12, p. 1108-1113) relating to the joining of parts in SiC with copper braze alloys.
Reactive brazing is adapted to the bonding of oxide ceramics such as alumina, as the reactivity is limited and the mechanical properties of the oxides formed is satisfactory.
In the case of non-oxide ceramics such as silicon nitride or carbide, the reactivity between the active elements and the ceramic is heightened; the latter induces the formation of fragile intermetallic compounds such as silicides and carbides in the case of SiC, of extensive porosity and fissure formation extending within the ceramic, which very seriously restricts the mechanical resistance of the bonds thus formed. These phenomena are mentioned in the document published by J. K. Boadi, T. Yano, T. Iseki: “Brazing of Pressureless Sintered (PLS) SiC using AgCuTi alloy”, J of Materials Science, Vol. 22, 1987, p. 2431-2434 relating to the brazing of pressureless-sintered SiC or PLS SiC with itself using a braze alloy essentially containing Ag—Cu with titanium as reactive metal. In this document particular stress is laid upon the fragility of bonds with high titanium content and the formation of intermetallic compounds when temperature and contact time are increased.
Also, the melting point of the above-described braze alloys restricts the temperature of use to 600-700° C., which is largely insufficient for applications at high temperatures in the region of 1000° C. and over.
Another important problem encountered when joining ceramics is that these ceramics are brittle and practically lack any deformability even at high temperatures;
It is therefore most important, when joining ceramics, to limit the residual stresses which develop during cooling, due to a differential in dilation coefficient between the two parts to be joined if they are of a different nature, but also between the ceramic and the braze alloy when joining two ceramics of the same nature. Therefore, the thermal dilation coefficient of the braze alloy must correspond closely to that of the ceramic parts to be joined.
The Cu and Ag based alloy compositions commonly used are ductile but have a limited range of temperature of use as mentioned above and do not therefore provide a satisfactory answer to this problem.
In order to bring a solution to this problem and to allow satisfactory joining of ceramics containing silicon that braze alloys have been developed containing metal silicides. In this respect, document U.S. Pat. No. 4,499,360 (DORNIER SYSTEM GmbH) which corresponds to patent documents DE-A-3 230 320; JP-A-59 (84) 09 7580 and to patent EP-A0 100 835 therefore describes a braze alloy for joining SiC based elements consisting of 20 to 45% by weight of cobalt and of 80 to 55% by weight of silicon. Although this document mentions refractory brazi
Bourgeois Gerard
Coing-Boyat Gisele
Gasse Adrien
Commissariat A l'Energie Atomique
Jones Deborah
McNeil Jennifer
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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