Fracture of metal coated elastomers
Polydimethylsiloxane (PDMS) substrates were coated with thin layers of gold varying in thickness between 40 nm and 160 nm. Arrays of parallel cracks formed when a tensile strain was applied to the coated system, with the spacing between the cracks being approximately inversely proportional to the strain. When the crack profiles were examined, it was noted that the cracks extended deep into the substrate—to depths up to two orders of magnitude greater than the film thickness. This extension of the cracks into the substrate is the result of the large mismatch in elastic properties between the metal and soft substrate, and plays a role in the development of the cracking pattern. Despite the relatively large strains that can be applied through the elastomeric substrate, there was no evidence of macroscopic plasticity in the metal films, and numerical analyses showed that the crack profiles were consistent with elastic deformation of the metal. A quantitative comparison of the crack depth and spacing with the predictions of a companion mechanics analysis indicated that the observed spacing and depth were consistent with the metal film being much tougher than the elastomeric substrate. However, for this level of toughness to be exhibited in a metal film would require plastic deformation over a scale much larger than would be expected in such a geometry. This inconsistency may be resolved by recognizing that rupture of a thin metal film can be associated with shear localization that results in a mode-II failure, rather than by classical mode-I crack propagation. This results in a failure mechanism for a metal film that is a high-strain, but low-energy process, with substrate cracking absorbing the excess elastic energy available upon rupture of the film. Such a failure mechanism is consistent with earlier observations for the failure of thin metal films, and this work suggests that failure of an elastomeric substrate may contribute to an additional loss of constraint enhancing this localization.