High-Temperature Metallizing of Alumina

Abstract

In the high-temperature metallizing process a thin layer of molybdenum paint is fired on to the surface of a debased alumina so that the alumina can be brazed to a metal component. During the firing, glass from the alumina migrates into the molybdenum layer and helps it to adhere to the alumina. In general it is desirable to form a dense glass/metal composite structure in order to form a strong seal. Fig. 1. shows a typical seal structure. Tensile-strength measurements on such seals often result in the seals failing in the alumina and it is sometimes suggested that the seal is therefore stronger than the alumina. This is a rather loose interpretation as seals which fracture in the alumina may cover a very wide range of strengths depending upon the seal components and process conditions. One example of strength variation was described in a previous paper¹ in which it was found that ASTM² test-pieces metallized at 1400°C failed in the alumina at an average tensile stress of 43 MN/m² compared with an average of 71 MN/m² for samples metallized at 1500°C. Fairly large variations in strength have also been reported by other workers. Thus Cole and Hynes³ found that the average strength of samples metallized with a Mo/Mn paint was 55 MN/m² compared with 83 MN/m² for a Mo/Ti paint.