Processing of alumina-niobium interfaces via liquid-film-assistedjoining Page: 2 of 36
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Processing of Alumina-Niobium Interfaces via Liquid-Film-Assisted Joining
Bonded ceramic-metal interfaces play a vital role in modern materials applications. Precise control
of interfacial microstructure through processing is therefore essential, and the development of
processing-microstructure-properties correlations is of sound fundamental value. Among the more
widely-studied ceramic-metal systems is alumina-niobium, which, due to closely matched thermal
expansion coefficients, results in bonded interfaces that are virtually free of thermal stresses. Considerable
research has previously been conducted on the mechanical properties and interfacial characterization of this
system (Refs. 1-9). Niobium and alumina are also chemically compatible, resulting in interfaces with no
chemical reaction layer when bonded in vacuum (Refs. 2,7,10).
In the present study, alumina was joined using copper/niobium/copper interlayers via
liquid-film-assisted joining (LFAJ). The LFAJ approach to joining ceramics employs a multilayer metallic
interlayer composed of two thin cladding layers of a low-melting-point metal (copper) and a thick core of a
high-melting-point or refractory metal (niobium) between sections of the ceramic to be joined (alumina).
Copper was chosen as the liquid former because of its low melting point, ease of deposition, previous
research on the joining of copper and alumina via diffusion brazing and partial transient liquid phase
(PTLP) bonding, previous fracture studies of alumina-copper interfaces, and the past success of PTLP
bonding with copper (see Ref. 7 and references therein). The joining temperature is above the melting point
of copper but below that of niobium; consequently during the initial stages of bonding, a thin, copper-rich
liquid film develops between the alumina and niobium, resulting in heterophase liquid-phase sintering.
Redistribution of this liquid layer fills voids at the interface and provides a path for the rapid diffusion of
niobium, which, in turn, accelerates contact formation between the alumina and niobium. Fractography of
the interfaces indicates that the liquid copper film results in more extensive alumina-niobium contact
compared to solid-state diffusion brazing, and concomitantly improved strength (Refs. 11,12). The copper
film becomes discontinuous, and upon cooling from the bonding temperature, discrete particles of copper
remain at the interface due to the limited solubility and slow diffusion of copper in niobium. Plastic
deformation of the ductile copper particles increases the toughness of the interface (Ref. 13), and tearing of
J. T. McKeown et al.
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McKeown, Joseph T.; Sugar, Joshua D.; Gronsky, Ronald & Glaeser,Andreas M. Processing of alumina-niobium interfaces via liquid-film-assistedjoining, article, February 15, 2005; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc873254/m1/2/: accessed May 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.