Vacuum brazing of alumina to stainless steel using femtosecond laser patterned periodic surface structure

Ceramic surface patterning is a promising method to reduce the residual stress of ceramic-metal joints, since it needs no complex multiple interlayer and is cost effective. However, literature shows that surface patterns previously examined for ceramic-metal brazing were at a millimeter scale, which limits the application and the degree of enhancement. In this study, femtosecond laser ablation was used to fabricate microscale periodic surface patterns (grooves) on ceramic surfaces. No defects or damage were induced during the ablation. The groove dimension was tunable by controlling ablation parameters. High temperature vacuum brazing of 304 stainless steel to the Al2O3 ceramic with surface grooves was performed using a commercial AgCuTi brazing filler metal. With properly designed surface grooves, the average joint strength was 66 MPa, 2.75 times of that of the flat-interface joints. Finite Element Analysis models combined with fracture path examination indicated that the redistribution of residual stress induced by periodic surface pattern accounted for the joint strength enhancement.