EFFECT OF NICKEL, TIN, OR INDIUM DOPING OF THE EUTECTIC SILVER–COPPER FILLER ON CAPILLARY AND CONTACT PROCESSES IN BRAZING OF NONMETALLIC ALUMINA-BASED MATERIALS WITH TITANIUM 

Abstract

The effect of nickel, tin or indium additions in the eutectic Ag–28 wt.% Cu filler on capillary and contact processes in brazing of alumina ceramics, VK94-1 and VK100 grades, and leucosapphire with titanium, VT1-0, was examined. A wetting scheme for a non-metallic substrate was used, when a chemically active element from an “unlimited” source, such as titanium, was dissolved in a melt-inactive base of non-metals. A special technique to study such processes was developed. This technique allows identifying, as much as possible, the effect of different process parameters (heating rate, holding time, degree of vacuum, etc.) on the contact angles formed by metallic melts with different compositions on nonmetals in one experiment. High-temperature capillary and optical and scanning microscopy established that even small additions of nickel, tin, or indium to the eutectic Ag–Cu melt noticeably influenced the titanium dissolution rate and incipient wetting temperature of the nonmetallic substrate. Nickel additions were shown to enhance melt separation in the Ag–Cu–Ti system, in particular, the amount of the copper-based phase, eutectically melting with titanium above 870 °С, increased. The effect of these additions in the Ag–28 wt.% Cu filler on the strength of brazed ceramic–ceramic joints produced using a titanium insert was studied. The results were discussed involving thermodynamic characteristics of the contacting structures. The role played by the type of chemical bonds in products resulting from interaction of the melt components and solid substrate on the contact angle was shown. Brazed joints of alumina ceramics were made and used to determine the three-point-bending strength. The effect of various additions to the Ag–28 wt.% Cu filler on brazing strength was examined. Indium was found to be most promising addition to the silver–copper filler as it alloys the brazing temperature to be controlled without a significant change in the mechanical strength of brazed joints.