Cracking and decohesion of a thin Al2O3 film on a ductile Al-5%Mg substrate

A ductile metal substrate (1 mm thick Al-5%Mg) with a brittle thin film coating (0.1 μm thick Al2O3) was quasi-statically stretched to induce an array of parallel cracks in the coating. Additional cracking, decohesion, and buckling of the coating with increasing straining were observed using optical, scanning electron, and atomic force microscopes. The number of cracks per unit axial distance and the average opening gap of each crack were found to increase with increasing axial strain. A 2D finite element analysis incorporating a cohesive interface model of cracked coating segments was carried out to obtain both the normal stress distribution in coating segments with various widths and the shear stress distribution along coating-substrate interfaces. Both the fracture strength of the coating and the shear strength of the interface were estimated. Several theoretical analyses appeared in the literature on the cracking and decohesion of thin film coatings were critically discussed.