Torsional behaviour of glass-joined, laser-processed Crofer 22 APU interconnect: Unravelling the effect of surface roughness on the shear strength

Abstract

Glass-to-metal interfaces play a crucial role in the robustness and the mechanical integrity of solid oxide cells, and it is well known that a sound interface improves the mechanical reliability of the whole stack. The present work focuses on the torsional behaviour of hourglass-shaped Crofer 22 APU stainless steel joined by a glass sealant specifically designed for this application.Specific focus was given to the Crofer surface modification by laser processing, namely, the laser fluence was varied to find suitable roughness parameters; a laser fluence of 14.1 J cm−2, leading to a Crofer surface roughness of about 4.6 μm, was selected as the optimal Crofer surface processing before the joining process.The torsional shear strength of glass-joined as-received Crofer was measured as 24 ± 7 MPa with mainly adhesive fracture mode, with failure jumping from one interface to the other, while the laser-processed Crofer gave 32 ± 5 MPa with cohesive failure. The approximate 30% increase of torsional shear strength is due to the mechanical interlocking effect given by glass infiltration inside the laser-induced protrusions on Crofer as evidenced by SEM/EDS on cross-sections and fracture surfaces after torsion tests.