Novel Materials for Transient Liquid Phase Ceramics and Metal Joining
Dr. Darryl Butt
Joining is an enabling technology for many ceramics applications. Often ceramics are only useful in a system of components, requiring that they be bonded in some fashion to other ceramic components of the same composition or dissimilar materials such as metals or other ceramics. This is particularly true in the practical applications of fuel cells, gas separation membranes, and sensors, where a wide variety of ceramic-ceramic and ceramic-metal joints are required. The objective of this project is to bond a metal to a ceramic by the formation of a liquid-phase ceramic that wets to the surface of the metal and the ceramic. This liquid phase diffuses into the metal and bulk ceramic, and as the chemical composition of the liquid changes, it becomes a solid. This process is known as transient liquid phase (TLP) sintering. Ideally, this solid will be the bond between a metal and a ceramic. There are a variety of existing methods for joining ceramics to themselves or other materials. These joining methods have the disadvantage of leaving behind an interfacial phase with thermal and physical properties inferior to that of the materials being joined and may degrade the environmental stability of the parent material. Consequently, industry and academia have sought for many years to develop joining methods which leave behind effectively no interfacial phase, or a compatible, refractory phase with virtually the same thermal expansion coefficient as the joined parts. Although this work has not yet yielded a successfully novel metal to ceramic joint, a new transient liquid phase ceramic, SrMoO4, has been synthesized. This ceramic was sintered at 830°C and possesses a melting temperature of over 1200°C. Additionally a baseline study of the environmental stability of metal-to-metal brazes was conducted. This work is in collaboration with the Central Metallurgical Research Institute of Egypt with the goal of extending the operating range of high temperature materials and increasing their operational life.