Challenges in Joining Advanced Ceramic Materials: Interface Formation of Ceramic/Metal High-Temperature Brazes

Abstract

This paper describes the metallurgical interfacial reactions at elevated temperatures between reactive zirconium metal and stable oxide ceramics, specifically beryllia, yttria, and magnesia- zirconia composite ceramic. The ceramic/metal systems were preheated at 600°C, and then heated to peak temperatures of 1800°C or above, depending of the system, in ultra pure Argon atmosphere. After a short stay at the peak temperature, each system was cooled to room. The interaction was monitored during heating by a video camera and the interfaces were microscopically examined after the thermal cycle. The microstructure and chemical changes at the interface were evaluated via SEM and EDS. During heating of the beryllia/Zr system, the ceramic was initially reduced and Be alloyed the Zr metal in solid solution, causing Zr to melt locally at the interface at about 1600°C instead of 1855°C. The alloy Zr-Be liquid is what later dissolved the beryllia and infiltrated partially into the ceramic substrate. It seems that there was no solid state reaction between the Zr metal and yttria since Zr melted at its melting temperature of 1855°C; it is evident, however, that the liquid Zr partially dissolved yttria at the interface; yttrium and oxygen segregated to the grain boundaries. The solidified metal tightly bonded to the ceramic substrate as the system cooled to room temperature. In the Zr-MgO/ZrO2 system, Zr melted at 1855°C and it reduced the magnesia, but at the same time the magnesium was volatilized.