Journal of Student Research 2014

Journal of Student Research

Characterization of Vacuum Brazed Advanced Ceramic and Composite Joints Logan Jacobson | Senior B.S. in Manufacturing Engineering Abstract The microstructure and hardness of nineteen vacuum brazed joints of zirconium diboride and silicon nitride made using a variety of braze alloys were evaluated. Samples were cut and mounted in epoxy, ground and polished, tested for Knoop hardness, and examined for joint microstructure using optical microscopy. Select multilayer joints were examined in depth for microstructure and composition using scanning electron microscopy and energy dispersive spectroscopy. Directly bonded zirconium diboride (ZrB 2 ) led to better quality joints than directly bonded silicon nitride (Si 3 N 4 ). The larger thermal expansion coefficient of Si 3 N 4 than ZrB 2 led to greater expansion mismatch and a higher propensity for cracking in Si 3 N 4 joints. The incidence of cracking in Si 3 N 4 joints decreased with the use of tungsten, molybdenum and tantalum interlayers. The joints made using Ti containing braze alloys exhibited Ti enrichment of the interface. The hardness distribution mimicked the stacking sequence of the interlayers. Introduction Because of their high hardness and brittle nature, ceramics are rarely machined into finished parts. Ceramic parts are either cast to net-shape or assembled from simpler units by joining. Ceramics also need to be joined to metallic alloys in applications involving implantable electronics (e.g., neuro-stimulators), seals for vacuum tubes, microwave reflectors, spark plugs, nozzles and cutting tools. Because of the very different physical and chemical natures of metals and ceramics, joining ceramics and metals is technically more difficult than brazing, soldering or welding of like materials. In the following paragraphs, we first review the current state of research on joining of ZrB 2 and Si 3 N 4 ceramics Logan was in the Honors College of UW-Stout (Ed.)

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