Controlling the shape of Al/Ni multilayer foils using variations in stress

Al/Ni multilayer foils were sputter-deposited with an in-plane residual stress state that was altered midway through the thickness of the foils by changing the bilayer spacing. The difference in stress between the top and bottom halves of the foil caused these systems to curl when they were removed from their substrates. As predicted, the radius of curvature increased linearly as the difference in stress between the upper and lower halves decreased and as foil thickness increased, demonstrating the ability to fabricate layered foils with specific curvatures. Unexpectedly, however, the radii of curvature of all the free-standing foils decreased with time after removal from their substrates, suggesting that a time-dependent relaxation mechanism was operating. An explanation based on stress driven, time-dependent deformation is offered to explain the relaxation, and an elasticity-based curvature model is presented for comparison with the measured steady state curvatures.