Measurement of Surface Stress in Aluminum Induced by Organic Monolayer Films

Adsorbed films on metal surfaces can trigger large-scale phenomena such as embrittlement and changes in surface plasticity. More recently, we have observed local embrittlement at the surface in large-strain deformation of metals triggered by nanometer-scale films. Some of these phenomena at least appear to be related to changes in surface stress (not surface energy) induced by the films. We describe a macroscale technique for measuring surface stress produced by organic monolayer films. Monomolecular films were deposited onto a thin aluminum foil cantilever (substrate) by molecular self-assembly. The foil, with thickness of 0.254 mm and length of 80 mm, is immersed in an organic solvent for the self-assembly. As the molecule adsorbs onto one side of the substrate, the cantilever tip deflects due to a change in surface stress induced by the film. The surface stress is estimated from in situ measurements of the tip deflection. Besides demonstrating the measurement technique for surface stress, our results show that this stress is strongly dependent on the molecule chain length. The rate of cantilever deflection can also be used to characterize adsorption kinetics. Implications for environmentally-assisted cracking and material removal processes for metals are discussed.