Resistance to interface sliding and effects on detachment of directly-bonded pillars

Surface force-mediated adhesion, e.g. via van der Waals intermolecular forces, can lead to direct bonding between two bulk solids, but many analyses of this phenomenon only consider normal surface stresses. However, when finite size effects are accounted for, an interface shear traction generally arises from a mismatch in Poisson contraction, which can reduce the interface adhesion significantly from its ideal strength. The underlying mechanism of detachment is crack propagation along the interface. The understanding of the interplay between normal and shear surface stresses unlocks opportunities to control interfacial strength and toughness in various applications, including micro-transfer printing, MEMS/NEMS, manufacturing advanced 3D integrated circuits, and robotic grippers. We propose potential-based, coupled normal and shear traction-separation-sliding relations (TSSRs) and show that cohesive shear stresses have a significant effect of the detachment force and behavior of adhered pillars. Detailed finite element simulations utilizing cohesive elements based on the coupled TSSR are used to study the failure mechanism of an elastic pillar adhered to a rigid substrate. The effects of coupled normal and shear cohesive stresses are investigated in detail. A non-dimensional parameter, which incorporates the effect of cohesive shear stresses, is defined to describe the transition between strength-based failure and fracture-based failure. Preliminary experiments demonstrate how the TSSR properties can be determined.