Interfacial microstructure characterization and strength evaluation of Si3N4/Si3N4 joints with Si-Mg composite filler for high-temperature applications

Silicon-nitride (Si3N4) components were joined under vacuum at 1100 degrees C for 10 min using Si-Mg composite fillers with Mg contents (XMg) that ranged from 0 at.% to 59 at.%. The Si3N4/Si3N4 joints were fabricated via Si layer formation at the joint interface; the molten Si-Mg liquid was transformed into a solid Si layer after Mgevaporation-induced isothermal solidification. The joint tensile strength at room temperature increased considerably when XMg exceeded the liquidus composition of 37 at.% because of the enhanced densification/ thinning of the Si layer. In these cases, some Mg atoms reacted with Si3N4 to form a fine-grained MgSiN2-based layer, whereas relatively large ( 0.1 mu m) and smaller MgO precipitates (<10 nm) were observed in the Si layer. At a high XMg, the MgO precipitates were arranged in a network-like structure, which improved the fracture strength of the Si layer. The joints with a high strength at room temperature were examined using a three-point bending test at 1200 degrees C in air and endured a maximum fracture stress of -200 MPa, which confirmed their potential for use in oxidizing atmospheres at least 100 degrees C above the bonding temperature.