Journal of Student Research 2014

Characterization of Vacuum Brazed Advance Ceramic & Composite Joints

Silicon Nitride has a very low expansion coefficient, which causes the residual stress in the samples to be much higher than in ZrB 2 -based ceramic joints. The zirconium diboride-based ceramics exhibit much lower residual stresses, due in large part to their higher coefficients of thermal expansion (over twice that of silicon nitride). This allows the zirconium diboride samples to more consistently match the alloys’ expansions that they are bonded to, giving less prevalence of major cracking. Conclusions Joint microstructure and micro-hardness were characterized in a number of vacuum brazed ceramic/metal joints made using a wide variety of braze alloys and combinations of technical ceramic and ceramic/metal joints made using silicon nitride, zirconium diboride, titanium, Inconel 625, and copper-clad molybdenum with and without stress absorbing interlayers of tungsten, molybdenum, and tantalum. Directly bonded zirconium diboride-to-metal joints exhibited better integrity and less interfacial defects than directly bonded silicon nitride to-metal joints with the latter exhibiting a greater propensity for cracking because of residual stresses arising out of a large mismatch of thermal expansion coefficients of bonded materials. Residual stress management with the aid of stress absorbing metal interlayers yielded better quality silicon nitride-to-metal joints. Further research shall be needed to optimize interlayer thickness and interlayer ordering in order to create perfectly bonded joints. The interfaces in joints that were made using Ti-containing brazes exhibited Ti enrichment. The distribution of

79