Reaction layers and mechanisms for a Ti-activated braze on sapphire
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11/7/03
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Reaction Layers and Mechanisms for a Ti-Activated Braze on Sapphire J.J. STEPHENS, F.M. HOSKING, T.J. HEADLEY, P.F. HLAVA, and F.G. YOST A study was conducted to understand the wetting phenomena observed in brazing of a Ti-containing active filler metal on sapphire substrates. The goal of the study was to understand the interfacial reactions that permit wetting of commercial Ag-Cu-Ti active filler metal to pure alumina, despite the lower thermodynamic stability of TiO2 relative to Al2O3. Based upon transmission electron microscope, electron microprobe, and Auger analyses, it is proposed that two coupled reactions and diffusion of reactants take place. The oxides TiO, Ti2O, and Cu3Ti3O were observed at the braze/ceramic interface. It is suggested that the complex oxide Cu3Ti3O grows at its interface with TiO, and the oxide TiO is produced by reaction of Ti and sapphire and is subsequently consumed at its interface with Cu3Ti3O. It is also suggested that Ti2O forms from Ti and TiO while cooling from the brazing cycle.
I. INTRODUCTION
BRAZE alloys containing small amounts of an active metal, commonly referred to as active braze alloys (ABAs), are often used to join ceramics to metals and ceramics to ceramics. Active metal brazing results in considerable cost savings by means of simplified production sequences and more efficient mechanical designs. Most notably, ABA filler metals have the potential to eliminate the need for Mo-Mn metallization and the subsequent Ni plating of alumina surfaces prior to brazing, which is a standard industrial process. The active constituent is often Ti, but other reactive metals such as Nb, V, Zr, or Hf are also suitable constituents. The bond developed by these braze alloys stems from the reaction that occurs between the active constituent and the ceramic while the alloy is in the molten state. That thin reaction layer allows the molten alloy to wet the ceramic surface, resulting in the joining of mating surfaces. Note that ABAs spread only as a result of chemical reaction with the ceramic surface. Several commercially available filler metals exist, based upon Cu, Ag, Ni, or Au, that contain one of the active constituents for joining ceramic materials. The successful ABA joint requires the selection of an active constituent such that the chemical reaction with the mating ceramic surface yields a uniform and continuous reaction layer. In addition, the brazing-process thermal cycle and atmosphere must be properly controlled, in particular to prevent unwanted dissolution and chemical reactions at the mating metal or ceramic surface. At present, commercial ABA filler metals typically have Ti as the active constituent. The most widely used composition is 63 wt pct Ag, 35.25 wt pct Cu, and 1.75 wt pct Ti (known by the trade name CusilABA, which is produced by WESGO Metals, Inc. (Hayward, CA)). For many applications, Ti is an excellent active element because it reacts with J.J. STEPHENS, F.M. HOSKING, T.J. HEADLEY, and P.F. HLAVA, Principal Member of Technical S
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