Stock material or miscellaneous articles – All metal or with adjacent metals – Having composition – density – or hardness gradient
Patent
1996-01-22
1998-07-07
Ramsey, Kenneth J.
Stock material or miscellaneous articles
All metal or with adjacent metals
Having composition, density, or hardness gradient
428469, 428472, 4284722, 2281211, 2281221, B22F 704
Patent
active
057766207
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the assembly of parts intended to withstand elevated temperatures, and more particularly of a ceramic part and a metal or refractory alloy part or superalloy part.
A metal or an alloy is described as refractory when it withstands elevated temperatures--while being if necessary protected against corrosion--on condition that it is not subjected to significant mechanical loads. A superalloy is distinguished from a refractory alloy in that it has good mechanical, in particular creep, strength at elevated temperatures.
A process currently used for the assembly of a ceramic and a metal is that called brazing, which consists in applying a metal in the molten state (brazing alloy), which is then solidified by cooling. The most common brazing alloys are the alloys Cu--Ti--Sn and Ag--Cu. Alloys possessing a relatively low melting point of between 600.degree. and 850.degree. C., and inert with respect to ceramics, are involved. The assembly obtained cannot be used at high temperature, since the brazing alloy melts again as soon as the corresponding temperature is reached, causing the parts to desolidify.
This remains true for the particular brazing processes described in FR-A-2 271 644 and FR-A-2 619 562, the latter using as brazing alloy aluminium, which melts at 660.degree. C. and in addition burns at this temperature, that is to say oxidizes very violently, in the presence of oxygen. FR-A-2 636 621 and EP-A-0 422 343 propose brazing alloys having melting points of more than 900.degree. C. Although the assemblies obtained withstand relatively elevated temperatures momentarily, oxidation or corrosion of the brazing alloys would lead to deterioration of the joint in the long term.
The only known technique allowing a stable connection to be obtained at high temperature between a metal part and a ceramic entails the formation of the ceramic by projection at the surface of the part. This technique is intended to protect a metallic superalloy substrate having phases .gamma. and .gamma.' from the thermal environment of the air stream of an aeronautical turbo-engine. First of all the superalloy is coated, by a physical method such as plasma projection or vapor phase deposition under electron bombardment, with a layer of MCrAlY alloy, where M represents the base metal of the superalloy, namely nickel and/or cobalt and/or iron, with a thickness of about 100 .mu.m. The superalloy thus protected is then coated, by plasma projection, with about 300 .mu.m of zirconium partly stabilized with yttria, that is to say containing about 8% by weight of yttria (yttrium oxide Y.sub.2 O.sub.3). The good mechanical coupling of the yttrified zirconium deposit to the MCrAlY, which is combined with physico-chemical phenomena, permits a high degree of resistance to thermal shocks of this unit (1000 cycles of one hour at 1100.degree. C. with cooling in five minutes to about 200.degree. C.), despite a high differential of the expansion coefficients. This technique, described in detail in the literature, naturally does not allow the metal part to be assembled to a pre-existing ceramic part.
The aim of the invention is to provide a process permitting such an assembly which is stable at elevated temperatures.
The invention concerns in particular a process for assembling a first ceramic part and a second part of a material chosen from ceramics and refractory metallic materials and superalloys based on at least one metal chosen from nickel, cobalt, iron and platinum metals, wherein the ceramic which constitutes the first part or the two parts is capable of syncrystallizing, directly or indirectly, with mullite, in which process the two parts are juxtaposed, the gap which separates them being filled with a sealing composition containing fine particles of aluminium and of a moderating metal capable of forming an alloy with the latter so as to limit the high temperature reactivity of the aluminium in relation to the materials of said parts, and/or of a pre-formed alloy of these two metals, and optionally of at least one constitu
REFERENCES:
patent: 3284174 (1966-11-01), Zimmer
patent: 4109031 (1978-08-01), Marscher
patent: 5561321 (1996-10-01), Hirano et al.
Alperine Serge
Josso Pierre
Office National d'Etudes et de Recherches Aerospatiales
Ramsey Kenneth J.
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