Joining pre-oxidized dense Si3N4 to porous Si3N4 with β-spodumene based glass-ceramic interlayer

A novel two-step joining method was adopted to achieve the reliable bond of dense Si3N4 (D-Si3N4) with porous Si3N4 (P-Si3N4) components for high temperature applications using Li2O-MgO-Al2O3-SiO2-B2O3 glass-ceramic as interlayer. The joining process involved a pre-oxidation of D-Si3N4 in air for a good wettability, followed by a joining process at 1240 degrees C-1300 degrees C for 10 min in vacuum. The microstructure evolution, interface structure, fracture characteristics and joining mechanism of the joints were elucidated. The beta-spodumene solid solution based glass-ceramic was formed in the seam, which could effectively moderate the thermal expansion mismatch between interlayer and Si3N4 substrates and ensure the high-temperature resistance of joint. Compared with one-step joining, two-step joining method could obtain more reliable D-Si3N4/glass-ceramic/P-Si3N4 joints. A favorable shear strength of 43 MPa was achieved within 10 min at 1280 degrees C, which was 15 MPa (similar to 54%) higher than that of the joints by one-step joining method. As a result, good GTE match as well as strong bonding to Si3N4 substrates made the investigated glass-ceramic a suitable candidate for joining Si3N4 ceramics.