Thickness dependency of creep crack propagation mechanisms in submicrometer-thick gold films investigated using in situ FESEM and EBSD analysis

Abstract

To clarify the thickness dependency of creep crack propagation mechanisms, creep crack propagation experiments were carried out on submicrometer-thick metallic films; freestanding gold films with thicknesses of ~120 nm (thinner) and ~340 nm (thicker) were subjected to in situ field emission scanning electron microscopy (FESEM) and electron backscatter diffraction (EBSD) analysis. In both the thicker and thinner films, creep crack stably propagated at room temperature by repeating the processes of (i) stretch and local thinning ahead of the creep crack, (ii) void formation at grain boundaries in the creep-damaged region, and (iii) void growth and coalescence of voids or that between the main crack and voids. The creep-damaged region ahead of the creep crack tip was narrower, shorter and shallower in the thinner films compared to that of the thicker films. In the thicker films, the creep cracks propagated through transgranular creep fracture, whereas in the thinner films, the creep cracks propagated by transgranular and intergranular creep fracture, indicating that creep crack propagation mechanisms depend on film thickness.