High temperature interaction behavior at liquid metal-ceramic interfaces

Liquid metal/ceramic interaction experiments were undertaken at elevated temperatures with the purpose of developing reusable crucibles for melting reactive metals. The metals used in this work included zirconium (Zr), Zr-8 wt.% stainless steel, and stainless steel containing 15 wt.% Zr. The ceramic substrates include yttria, Zr carbide, and hafnium (Hf) carbide. The metal-ceramic samples were placed on top of a tungsten (W) dish. These experiments were conducted with the temperature increasing at a controlled rate until reaching set points above 2000 °C; the systems were held at the peak temperature for about five min and then cooled. The atmosphere in the furnace was argon (Ar). An outside video recording system was used to monitor the changes on heating up and cooling down. All samples underwent a post-test metallurgical examination. Pure Zr was found to react with yttria, resulting in oxygen (O) evolution at the liquid metal-ceramic interface. In addition, dissolved O was observed in the as-cooled Zr metal. Yttrium (Y) was also present in the Zr metal, but it had segregated to the grain boundaries on cooling. Despite the normal expectations for reactive wetting, no transition interface was developed, but the Zr metal was tightly bound to yttria ceramic. Similar reactions occurred between the yttria and the Zr-stainless steel alloys. Two other ceramic samples were Zr carbide and Hf carbide; both carbide substrates were wetted readily by the molten Zr, which flowed easily to the sides of the substrates. The molten Zr caused a very limited dissolution of the Zr carbide, and it reacted more strongly with the Hf carbide. These reactive wetting results are relevant to the design of interfaces and the development of reactive filler metals for the fabrication of high temperature components through metal-ceramic joining. Parameters that have a marked impact on this interface reaction include the thermodynamic stability of the substrate, the properties of the modified interface, the temperature-dependent solubility limits of the liquids and solid phases, and the high-temperature stoichiometry of the ceramic.