Interfaces and Structural Integrity of Ceramic/Metal Joints

Abstract

According to the expansion of the applications of ceramics as both electronics and structural materials, many methods have been established for joining ceramics to metals. For the electronics applications, Al₂O₃ ceramics have been used as insulator materials represented by the substrates for electronics packages for many years, on which metallic electrodes are patterned and joined with tight interfaces. In addition to them a variety of ceramics have been applied as various sensors connected with metallic electrodes. As structural materials, non-oxide ceramics such as Si₃N₄ have been successfully applied to vehicle engine components replacing the heavy metallic materials since the beginning of the 1980’s. Table 1 summarizes the typical Si₃N₄ parts established in the last 15 years in the commercial fields [1]. Although many methods have been already established for joining ceramics and metals, one has to notice the fact that no ceramic/metal joint structure is stable because of the big gaps both in chemical and in physical nature between two materials. There are always some sorts of latent defects in an interfacial region. They are, for example, a brittle reaction layer, voids, unjoined area, microcracks from thermal stress, and etc., which may not only exist just after fabrication but also will be formed in service. Therefore, in practical uses of ceramic/metal joints, one has to take the influence of all defect structures in a joint into account and should design some optimized joint structure that can compensate for these potentially exiting defects. The commercial products in vehicles and also in electronics are usually required to have a high reliability as 1 ppm fracture probability through their life times. This requirement is quite severe even for ceramics materials themselves and moreover for joint structures. Thus life time prediction and insurance test becomes very important.

TABLE 1 Commercial Si₃N₄ products for vehicles in Japan [1].