Use of aluminum in air-brazing aluminum oxide
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A commercial aluminum foil was used to braze alumina plates in air. Although the outer surface of the aluminum oxidizes in air, the majority of the aluminum underneath remains unoxidized during brazing, allowing the ceramic pieces to be joined together with adequate strength. Joint strength testing and subsequent examination of the fracture surfaces of the joints indicate that the joints are inherently ductile, even after long-term, high-temperature air exposure.
I. INTRODUCTION
As the operating temperatures of advanced power generation equipment continue to be pushed upward due to thermal efficiency considerations, there is rapidly growing interest in joining techniques that can be used to economically manufacture complex, high-temperature ceramic components from inexpensive, simple-shaped parts. Although a number of ceramic joining techniques currently exist, as is inherent with most materials technologies each requires some form of trade-off or exhibits some penalty in terms of joint properties, ease of processing, and/or cost. Glass joining, for example, is a costeffective and relatively simple method of bonding ceramics. However, the maximum temperature to which a glass joint may be exposed is limited by the softening point of the glass. Additional complications arise if glass devitrification occurs during service, as its thermomechanical properties will begin to deviate from the original carefully engineered state.1 An alternative joining technique, diffusion bonding, is conducted at high temperatures and under high pressures. Because of the pressure requirement, however, components fabricated by diffusion bonding are typically limited to simple shapes. Reaction bonding, also a high-temperature joining process, often yields joints that contain residual porosity, unconverted reactants, and undesired secondary product phases, any of which can reduce joint strength by acting as sites for crack initiation.2 Joints formed by converting a polymeric precursor to the final ceramic bonding phase often experience cracking during processing because of the large volumetric shrinkage that accompanies pyrolysis. The use of a ceramic filler material can partially mitigate this problem, but the joint often retains a significant
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0221 J. Mater. Res., Vol. 19, No. 6, Jun 2004
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amount of porosity, which reduces its strength and reliability.3 Active metal brazing requires a stringent firing atmosphere, either high vacuum or reducing-gas conditions, to prevent the active species, typically titanium, from pre-oxidizing. This represents a high capital expense and higher operating costs relative to air-fired processes. In addition, recent studies on the oxidation behavior of active metal brazes have shown that they are unreliable at temperatures beyond 500 °C, at which point they eventually oxidize completely, conferring little or no strength to the joint.4,5 Recently, we have deve
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