Slip zone model for interfacial failures of stiff film/soft substrate composite system in flexible electronics

• Slip zone model is used to mechanically explain the slippage in composite system.
• The stress evolution is completely acquired with the diverse failures models.
• The criteria to determine the failure modes have been established.
• Our theoretical results can give the critical loading for the composite system.
• Our analysis and discussion can guide the design and evaluation of layered devices.

Delicate designs of flexible inorganic electronics are based on the composite structure of the stiff film/soft substrate system, which substantially relieves stresses in the stiff inorganic functional units and utilizes the soft substrate to realize flexibility and stretchability. Resulted from the large mismatch of the mechanical properties of each layer in such system, the interface between the stiff electronic film and the soft substrate suffers serious stress concentration so that interfacial failures become more dangerous than material cracks during operating. The existing approaches for interfacial failures do not work well to explain the diverse failure modes, nor give the detailed stress evolution as the increasing of applied loading. In this paper, we employ the slip zone model in composite structure and strength theory to acquire the stress evolution, and then fundamentally and quantitatively explain the interfacial failure behaviors of such flexible electronic devices. The consistence between our theoretical prediction and the experimental results gives adequate supports to our cohesive constitutive relations in the slip zone. The criteria to determine the failure modes have also been established, which are important to guide the design and evaluation of flexible electronic devices.