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Effects of Copper Oxide Content in AgCuO Braze Alloy on Microstructure and Mechanical Properties of Reactive-Air-Brazed Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF)

A. Kaletsch, A. Bezold, E.M. Pfaff, C. Broeckmann

Journal of Ceramic Science and Technology, Vol. 3, No. 2 (2012), pp. 95-104

DOI: 10.4416/JCST2012-00001

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Abstract

For the joining of ceramic materials as well as ceramics and metals, reactive air brazing (RAB) is an important improvement compared to existing brazing technologies. In contrast to commonly used active brazing processes, no vacuum atmosphere is needed in RAB. This is a crucial factor for certain materials, such as some functional ceramics, which cannot be brazed in vacuum easily because they are thermodynamically unstable in atmospheres of low oxygen partial pressure. Sometimes for those materials, RAB is the only possibility to obtain a gas-tight sealing, which works safely up to high temperatures. This paper presents current research findings concerning the influence of the copper oxide (CuO) content in AgCuO braze on the microstructure of the brazing zone and the mechanical properties of reactive-air-brazed ceramic/metal joints. The joining components are the perovskite-type ceramic Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and the heat-resistant austenitic steel AISI 314. As braze materials, mixtures of silver with CuO contents between 1 mol% and 16 mol% were used. The present study shows that a higher CuO content in the braze results in better wettability. However, a change in the microstructure of the ceramic can be observed, which becomes more pronounced with increased CuO concentration. This change in microstructure is characterized by higher and irregularly distributed porosity and increased grain sizes with copper cobalt oxide phases at the grain boundaries. Mechanical tests show that this reaction area leads to lower mechanical strength of the bonded specimens.

Keywords

RAB, reactive air brazing, perovskite, BSCF, CuO

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