Deep Indentation of Soft Thin Films Beyond the 10% Rule

Area of Science:

• Materials Science
• Mechanics of Materials
• Nanotechnology

Background:

• Indentation is a common method for measuring material stiffness.
• Substrate effects can significantly artifactually increase the apparent stiffness of thin films.
• The empirical 10% rule of thumb for indentation depth is unreliable for ultrathin films.

Purpose of the Study:

• To explore the mechanics of thin films under deep indentation, a regime traditionally avoided.
• To develop a theoretical framework for interpreting indentation in this deep regime.
• To provide a more accessible experimental method for characterizing ultrathin films.

Main Methods:

• Development of an asymptotic theory incorporating nonlinear geometry and elasticity.
• Analysis of indentation force-depth relationships in the deep indentation regime.
• Investigation of pull-off force behavior under deep indentation.

Main Results:

• A simple indentation force-depth relation was elucidated for deep indentation.
• The pull-off force was found to be independent of material law and geometric nonlinearity.
• The study provides a unified framework for interpreting deep indentation of thin films.

Conclusions:

• Deep indentation offers a viable and accessible method for characterizing thin film mechanics, especially for ultrathin films.
• The established theoretical framework simplifies the interpretation of experimental data in this regime.
• This work overcomes limitations of traditional indentation methods for thin films.