High-temperature metallizing

Abstract

Metal-ceramic seals have been prepared with six debased aluminas using molybdenum metallizing paints and with one alumina using tungsten paints. The metallized layers in strong seals (ASTM tensile strength >55 MN m⁻²) consisted of a dense metal/glass composite layer which was formed by glass migrating from the alumina into the metallizing layer. Observations on the glass migration agree with the predictions made from the hypothesis suggested in Part 1. The glass migration is favoured by a high metallizing temperature, large grain size of the alumina relative to the particle size of the metallizing, and the use of a reducing atmosphere containing a suitable concentration of water vapour. (The dew point required to form optimum conditions in H₂ is higher than that for 90% N₂+10% H₂.) Addition of MnO₂ to the metallizing paint did not appear to aid glass migration nor increase seal strengths. The metallizing layer in weak seals contained comparatively little glass except on one coarse-grained alumina which had been metallized at relatively low temperature (1400‡ C). This metallizing layer was well filled with glass and tensile failure occurred in the alumina at <44 MN m⁻². Increasing the metallizing temperature to 1500‡ C increased the strength of the seals to 70 MN m⁻² without causing any apparent change to the metallizing layer.