Deformation and cracking of thin second-phase layers on deforming metals at elevated temperature

Deformation, cracking, and spalling of oxide layers or protective coatings on high-temperature components during creep may have a significant influence on the lifetime of the components. The continuum-mechanical equations for thin second-phase layers on deforming metals are formulated for plane stress taking into account elastic and creep deformation, as well as stress-independent straining such as thermal expansion. Linear or non-linear viscous sliding of the scale on the metal is allowed, and microscopic models for sliding are discussed. Cracks in the scale are assumed to form once a critical tensile stress is attained. The theory is especially applied to the stress fields and crack formation in scales on round creep-test specimens. The dependence of the calculated crack spacing on the applied strain rate compares well with experimental results reported in the literature for oxide scales on ferritic steel.