Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
2001-06-22
2003-09-09
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S121000, C228S122100, C228S246000, C228S245000, C228S262100, C228S262900, C228S124600, C148S022000, C148S023000, C148S024000
Reexamination Certificate
active
06616032
ABSTRACT:
The present invention relates to a brazing composition, and to a process for assembling parts in alumina-containing materials either with themselves or with parts in metal or a metal alloy, by reactive or non-reactive refractory brazing, using said braze composition, in order to produce assemblies entirely in alumina or containing alumina and a metal or metal alloy.
The invention also concerns the refractory joint and the assembly so obtained.
By “alumina-containing” materials is generally meant all materials whose alumina content is 90% by weight or higher, these materials also comprising alumina-containing composites.
The technical field of the invention may be defined as high temperature brazing, that is to say which involves temperatures generally higher than 1200° C., which enables the assembly obtained to be used for applications requiring temperature rises which may exceed 900° C. for example and may reach 1600° C. and even higher.
It is known that it is difficult to manufacture large-size parts in ceramics, in Al
2
O
3
in particular. After the sintering of large-size primary components in alumina, tolerances are poorly controlled and the machining of these components is unacceptable for cost-related reasons.
In addition, and for the same reasons, it is generally difficult to manufacture parts of complex shape with ceramics of oxide type such as alumina.
It is therefore often preferable to produce large-scale and/or complex shape parts or structures from ceramic parts of simple shape and/or of small size, and then to assemble these parts to produce the end structure.
On account of the high temperatures, close to 1000° C. for example, that are used in the applications of ceramics such as alumina, the assembly of these ceramics by bonding with organic products is excluded since these products do not withstand temperatures above 200° C.
Also, conventional assembly techniques by welding which use a beam of energy with or without filler metal (Tungsten Inert Gas welding, electron beam or laser welding) involving partial fusion of the parts to be assembled cannot be used for the assembly of ceramics since a substrate or a part in ceramic cannot be directly melted without causing its destruction since it may decompose before fusion.
In consequence, welding by solid phase diffusion, assembly-sintering and reactive brazing are currently the most frequently used techniques to produce refractory assemblies of ceramics.
Solid phase diffusion welding and assembly-sintering have the disadvantage of being restrictive in their application.
In respect of solid phase diffusion, the shape of the parts must remain simple if uniaxial pressing is used or else it requires complex tooling and preparation comprising for example the manufacture of a casing, vacuum sealing, hot isostatic compression, and final machining of the casing if hot isostatic compression is used. This technique is therefore not economically viable.
In respect of assembly-sintering, the same problems arise (shape of the 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 assembled.
These two techniques also require the use of temperature stages or cycles of long duration (one lasting several hours) at high temperature since the processes involve solid state diffusion which could, in respect of the diffusion mixture, promote the enlarging of the metal alloy grains for metal-ceramic assemblies.
Brazing is a cheap technique, easy to implement and is moreover the most frequently used technique. Complex-shaped parts may be produced by capillary brazing and operations are limited to placing the filler metal or alloy, called “braze”, between or in the vicinity of the parts to be assembled, and to melt the braze to obtain the joining of the parts after cooling.
Brazing of ceramics must overcome the fundamental problem of the poor wettability of ceramics by metals. Indeed, the usual braze compositions which contain low melting-point metals such as silver, copper, gold and their alloys do not wet ceramics, even though they can wet metal materials. This problem is overcome by conducting surface treatment prior to brazing by choosing special alloy compositions and/or by optionally adding a reactive element as is the case for so-called “reactive brazing”.
For this last technique, a metal alloy composition is used, most often containing copper or silver, and a reactive element is added such as Ti, Zr, V, Hf, Nb. . . .
The reactive element acts by decomposing the surface of the ceramic and reacting with it to form a very stable nitride, oxide, silicide or carbide compound depending upon the type of ceramic involved. This layer generally imparts very good wetting properties to the ceramic and satisfactory bonding.
If we focus particularly on oxide ceramics, the latter, in particular alumina, are only scarcely reactive and consequently, aside from the very reactive chemical elements such as Ti, Zr and Hf, most of the transition metal elements do not wet and do not adhere to an alumina surface, as is the case in particular for the elements Ni, Fe, Cu, Mn, Co, Pt, Au, Ag, Pd, etc.
It is therefore generally necessary to conduct a surface treatment in order to metallize alumina before it is brazed, that is to say that a thin metal layer is deposited that adheres strongly to the ceramic and will act as an interface between the braze and the ceramic.
The process the most frequently used for metallizing manganese ceramics is the so-called moly-manganese process in which a suspension or paste containing a powder mixture of molybdenum and manganese is applied to the ceramic and annealed in a wet reducing atmosphere of hydrogen or ammonia. This reducing atmosphere is necessary to maintain the molybdenum in the metal state, while a certain water vapour content leads to oxidizing the metal manganese.
Even though this process functions relatively well for ceramics containing 94-96% alumina, it cannot be applied to ceramics with an alumina content of 99.5%, which is why in the document by K. WHITE, D. KRAMER, Materials Science and Engineering, 75 (1985) 207-213 “Microstructure and seal strength relation in the molybdenum-manganese glass metallization of alumina ceramics”, the description is given of the addition of manganese glass (Mno—SiO
2
—Al
2
O
3
) to molybdenum powder instead of pure metal.
The glass which forms during annealing at a temperature of 1500° C. penetrates inside the grain joints of the ceramic and creates a vitreous matrix in which the metal particles are trapped, which promotes metallization.
The deposit is then metallized by electrolysis with a layer of nickel or palladium, and the actual so-called brazing is carried out using a silver-based braze composition.
This process is heavy and complex to implement owing to the use of a hydrogenated reducing atmosphere and since it comprises four successive stages.
A second technique, which is the reactive brazing technique, consists generally of causing the alumina surface to react with a very reactive element chosen, for example, from among Ti, Zr and Hf, Nb, Al and Cr.
Reactive brazing is suitable for the assembly of oxide ceramics such as alumina, since reactivity is limited and the mechanical performance of the formed oxides is satisfactory.
This second technique itself comprises several variants among which mention may be made of indirect reactive brazing and direct reactive brazing.
For indirect brazing (which is similar to the metallization technique) a layer of reactive metal is previously deposited on the alumina, such as titanium for example, either in the form of a metal layer made by PVD (Physical Vapour Deposit) or CVD (Chemical Vapour Deposit), or in the form of titanium hydride.
Indirect brazing is a costly method that is difficult to implement since it requires two steps, and in addition the handling of titanium hydride is delicate since this product is very unstable. Finally, brazing is conducted at temperatures generally lower than 950° C. and the operating temperature of the
Eustathopoulos Nicolas
Gasse Adrien
Commissariat a l'Energie Atomique
Dunn Tom
Edmondson L.
LandOfFree
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