Wetting and capillary driven crack self-healing in metals revealed through Al/Sn/Al thin film model architectures

Thermally-activated, liquid-based crack healing was investigated using a magnetron sputtered Al / Sn / Al thin film architecture with Sn acting as the healing agent. A brittle Al2O3 layer was sputtered prior to the deposition of the Al and Sn layers enabling the formation of cracks with varying widths in the Al layers (2–4 μm) using a 3-point bending technique. Thermal activation was achieved through annealing at 250°C for 10 min and 400°C for 10 and 30 min. Scanning transmission electron microscopy imaging revealed that Sn mobility was activated only after annealing at 400°C for 30 min. resulting in efficient healing of ~2 μm wide cracks, while >3 μm wide cracks showed a reduced healing efficiency in comparison. The here observed variations in healing efficiency are governed by the temperature-dependent wetting behavior of Al by Sn as well as by crack-width-dependent capillary forces and the healing time. This study provides insights into the fundamental mechanisms governing liquid-based self-healing in metals, contributing towards future knowledge-driven design of self-healing bulk metals and metallic thin films.