A novel technique for mixed-mode thin film adhesion measurement

Guided by recent parametric studies of laser-induced tensile spallation, we successfully developed a new test method for studying mixed-mode interfacial failure of thin films. By allowing an initial longitudinal wave to mode convert at an oblique surface, a high amplitude shear wave is generated in a fused silica substrate and propagated toward the thin film surface. A shear wave was generated with amplitude large enough to fail an Al film/fused silica interface and the corresponding shear stress was calculated from high-speed interferometric displacement measurements. Examination of the interfaces failed under mixed-mode conditions revealed significant wrinkling and tearing of the film, in great contrast to blister patterns observed in similar Al films failed under tensile loading. Slightly higher interfacial strength was measured for the same Al film/fused silica substrate under mixed-mode loading than under tensile loading. The mixed-mode loading experiment developed in the current work provides a new tool to explore a range of thin film interfacial failure modes as well as the role of extrinsic effects such as residual stress, surface roughness, and imperfections.