Characterization of ceramic/TA6V titanium alloy brazed joints
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A12O3 and Si3N4 ceramics were joined to titanium alloy (TA6V) and niobium by active brazing. Chemical reactions between the Cu-40Ag-5Ti braze alloy and these materials were examined, and interfacial reaction products were characterized by scanning electron microscopy and electron probe microanalysis. Results indicate the importance of studying the microstructural features of the joints in order to understand better their effect on the mechanical properties of the interface.
I. INTRODUCTION
Ceramic materials are being increasingly employed in various fields such as automotive engineering, aeronautics, electronics, medicine, and others. They are used as electronic insulators, pump and valve components, heat engines, thermal insulators in combustion chambers, and prostheses. Such applications require that ceramics be joined to metallic components, and the joints must be strong, vacuum-tight, and reliable. Successful fabrication of joints is not easy because ceramics and metals exhibit a wide range of differing properties (crystallographic, electronic, mechanical, thermodynamic), and thus there is an abrupt discontinuity at the ceramic/ metal interface. The properties of a joint depend on how this discontinuity is accommodated. Consequently, the understanding of the processes taking place during joint formation is the subject of considerable research. Some of the fundamental issues and techniques of ceramic joining have been discussed over the years in several papers.1"8 Ceramics can be joined to metals by mechanical, direct, and indirect techniques. Among the indirect techniques, the brazing process — reported in the present work — is widely used. It involves the introduction and subsequent melting of a metallic alloy; however, brazing of ceramics presents difficulties because liquid metals and conventional brazes do not easily wet ceramics such as alumina and zirconia. Wetting of ceramics by metals, which seems to be only qualitatively understood,9"11 is related to the tendency of the metal to react with the ceramic surface. In order to promote wetting, the chemistry of the ceramic surface must be modified, for example, by metallization before brazing.12'13 Such a process, however, presents technical and economical disadvantages which have led to the development of direct brazing using active metal alloys.14"20 These are obtained by adding elements (e.g., Ti, Zr,Ta, Nb) having a high affinity for oxygen (in the case of oxide ceramics) or for niJ. Mater. Res., Vol. 5, No. 1, Jan 1990
trogen (in the case of nitrides) to braze material. In practice, braze alloys based on copper, gold, silver, or nickel are commonly employed, and titanium is often recommended as reactive element. The literature suggests that Ti reacts with the ceramic surface to form wettable compounds such as TiO or TiN.21"24 However, in most cases, these active metal alloys are very brittle, difficult to fabricate into shapes suitable for brazing, and therefore, often used in powder form. A new class of active brazing materials in the form of ductil
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