Ceramic-Metal Interfaces

Abstract

ABSTRACT. High integrity ceramic-metal interfaces are required for several areas of advancing technology but are difficult to produce because of the markedly differing lattice bonding characteristics of ceramics and metals. If the required properties are to be achieved, the chemistries of interfaces must be changed and this paper considers to what extent simple chemical concepts can account for the behaviour of brazed and diffusion bonded ceramic-metal systems. Particular attention is paid to the role of Ti promoting the wetting of ceramics through the formation of metal rich, hypostoichrometric, reaction product and to the beneficial effects of incorporating such metals as Sn and In in Ti containing brazes. The need to control the extent as well as the nature of the reactions if strong bonds are to be produced is high lighted, and comparisons are made between the fabrication processes and properties of brazed and diffusion bonded interfaces. INTRODUCTIONA wide range of ceramic-metal interfaces occur naturally and are applied in technology. Thus the surface of almost every metal is covered by an oxide film, and the properties of the oxide-metal interfaces can exert a dominant influence on film characeristics. Further the interfaces formed by carbide precipitates with their matrices control important properties of steels and other metal alloys. This presentation, however, is concerned with the synthetic interfaces created in response to demands for ceramic-metal components and composite materials. In particular, attention is devoted to the interfaces formed with ceramics by liquid metals during brazing and by solid metals during diffusion bonding.Compared to metals, ceramics are usually refractory and brittle electrical and thermal insulators that have low thermal expansivities, and these differences reflect the bonding characteristics of their lattices. While the bonding electrons in metals are delocalised, the movement of electrons in ceramics is restricted by the ionicity or covalency of their lattices. Thus the nature of the lattice bonding 393