The structural integrity of the sealing material is critical for the reliability of solid oxide fuel cell stacks. In this respect failure and deformation are aspects which need to be assessed in particular for glass ceramic sealant materials. Bending tests were carried at room temperature and typical stack operation temperature for glass ceramic sealants with different crystallization levels. Elastic moduli, fracture stresses, and viscosity values are reported. In addition to sintered bars some bending testing were carried out for steel specimens that were head‐to‐head joined with the glass ceramics similar as in a stack application. The ceramic particle reinforced sealant material was screen printed onto the steel. The results reveal a decrease of the strength for the partially crystallized sealant at operation relevant temperatures that can be associated with the viscous deformation of the material.