Creep and Fracture of Structured Nanocolumns Grown on a Substrate

The mechanics of room-temperature creep fracture on the nanoscale was evaluated on Ti oblique nanocolumns grown on a Si substrate using glancing angle deposition. An experimental method for the nanocolumn arrays using a micro-brick specimen (2 μm high and 2 μm wide) was used for the creep experiments. To clarify whether the nanoscale stress concentration dominated the creep fracture, two types of specimens were prepared: a forward specimen (loading with the column tilt direction) and a reverse specimen (loading against the column tilt direction), where the reverse specimen had a higher stress singularity at the interface edge than the forward specimen. The Ti nanocolumns deformed in a time-dependent manner under a constant applied force and then fractured at the interface. The forward specimens required a higher applied force than the reverse specimens for a similar fracture life. The local stress distribution along the Ti/Si interface during the creep experiments was analyzed using finite element method. The Mises stresses near the edge in the region of about 5 nm were very close in forward and reverse specimens with similar fracture lives. This suggested that the local stress field in the nanoscale region dominated the creep fracture. In addition, the room temperature creep of the nanocolumns greatly accelerated in comparison with their bulk counterparts.