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6/28/03

6:49 PM

Page 1647

Solid-State Diffusion Bonding of Silicon Nitride Using Titanium Foils MASAKATSU MAEDA, RYOZO OOMOTO, TOSHIYA SHIBAYANAGI, and MASAAKI NAKA This article presents an effective way to control the interfacial reaction during solid-state diffusion bonding of silicon nitride (Si3N4) using titanium foils. The interfacial structure and its growth kinetics were analyzed in detail with scanning electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). The actual phase sequence of the joint interfaces bonded at temperatures between 1473 and 1673 K is concluded to be Si3N4/Ti5Si3(N)/a-Ti(N)  Ti5Si3(N), which is different from the phase sequence observed at room temperature after bonding. The joints are very weak due to the formation of a brittle Ti5Si3(N) layer at the interface. To suppress the growth of the Ti5Si3 layer, a nitrogen-solution treatment of titanium foils prior to each bonding experiment is implemented. Although a perfect prevention of the Ti5Si3(N) layer formation is not achieved with this treatment, it is shown that the growth of the layer is effectively suppressed enough to improve the joint strength to a level 3 times higher than the case in which pure titanium is employed.

I. INTRODUCTION

SILICON nitride (Si3N4) provides a low density of 3.196  103 kg m3 and good thermal shock resistance and retains its high strength and creep resistance at high temperatures.[1] These features make it attractive for a number of industrial fields, such as high-temperature structural components. Especially in the energy industry, the application of Si3N4 is expected to improve the thermal efficiency of turbine generators.[1] For practical use, a big problem has to be solved: that is, how to join Si3N4 with itself and/or with metallic materials infallibly. However, in spite of numerous studies with great achievements and inventions,[2–10] it is still difficult to obtain sound joints that endure the high temperatures at which Si3N4 exhibits its merits. For Si3N4 bonding, titanium plays an important role; that is, to enhance adsorption and wetting between Si3N4 and metals. For this purpose, titanium is commonly used as an active additive of filler metals or brazes for ceramics bonding.[2–6] On the other hand, it is well known that an inappropriate bonding of Si3N4 with metals containing titanium causes the formation of brittle phases and the reduction of joint strength. Especially for solid-state diffusion-bonded Si3N4 joints using a pure titanium insert metal, it has been reported that they reveal very poor strength.[7,8,9] These reports suggest that controlling the interfacial reaction and structure between Si3N4 and titanium in an appropriate state, in which the formation of such brittle phases is suppressed, is one of the most important techniques for successful bonding. Moreover, such a control technique for interfacial reactions will allow researchers to carry out a more accurate residual-stress analysis and life assessment of the joints, which is essentia