Diffusion bonding of silicon nitride to austenitic stainless steel with metallic interlayers
- PDF / 787,835 Bytes
- 6 Pages / 613 x 788.28 pts Page_size
- 110 Downloads / 218 Views
I.
INTRODUCTION
IN a previous article, the results were reported for experiments that were aimed at joining silicon nitride and austenitic stainless steel by means of diffusion bonding without using interlayers.r~l It was found that under specific process conditions, joints can be obtained. However, the strength of the joints obtained was relatively small and the strength values were characterized by large scatter. This poor mechanical behavior was found to be due to the presence of thermal residual stresses, which develop during cooling after the diffusion bonding process as a result of the difference in thermal shrinkage of the ceramic and the metal. It was suggested that more reliable joints might be obtained by utilizing interlayers in the form of thin metal foils inserted between the ceramic and the metal. The present article deals with the diffusion bonding of silicon nitride to austenitic stainless steel using metallic interlayers. Candidate interlayer materials were selected on the basis of their mechanical and physical properties, taking into account information provided by relevant phase diagrams. Candidate interlayer materials need to be ductile and should have a low yield stress and a thermal expansion coefficient matching the material combination to be joined. Due to the brittle nature of silicides, a prerequisite is avoidance of silicide formation at the diffusionbonding temperature (about 1000 ~ On this basis, it may be expected that positive results should be obtained when using iron, nickel, and ironnickel alloys. In this work, four alloys were chosen as interlayer material: pure iron, pure nickel, invar, and kovar. B.T.J. STOOP and G. DEN OUDEN, Professor, are with the Department of Materials Science and Engineering, Delft University of Technology, Delft, The Netherlands. Manuscript submitted January 25, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
A. Materials The materials to be joined were austenitic stainless steel AISI 316 and hot-pressed silicon nitride (HPSN) in the form of discs having a diameter of 10 mm and a height of 5 mm. The stainless steel discs were ground and polished, using 1-/xm diamond paste as the last polishing step. Prior to bonding, the steel samples were ultrasonically cleaned in alcohol. The silicon-nitride discs were lapped using diamond paste, yielding a roughness R, of typically 0.04/xm. After polishing, the silicon-nitride discs were cleaned in alcohol. Firing in air at 500 ~ for 30 minutes was used to evaporate contaminating compounds still present at the ceramic surface. The interlayer materials used were pure Fe (99.95 wt pct), pure Ni (Wiggins Alloy 200, Hereford, United Kingdom), invar (Fe-36 wt pct Ni), and kovar (Fe-29 wt pct Ni-17 wt pct Co). Interlayer foils were 250-/xm thick in the case of Fe, invar, and kovar and 200-/xm thick in the case of Ni. The chemical composition of the materials is given in Tables I and II.
B. Diffusion Bonding Equipment Diffusion bonding experiments were carried out using the furnace described
Data Loading...
