Adhesion hysteresis of silane coated microcantilevers

We have developed a new experimental approach for measuring hysteresis in t he adhesion between a free standing thin film and a substrate. By accuratel y measuring and modeling the deformations in micromachined cantilever beams that are subject to combined interfacial adhesive and applied electrostati c forces, we determine adhesion energies for advancing and receding contact s. We examined adhesion hysteresis for silane coated cantilevers and found no hysteresis at low relative humidity (RH) conditions. The dominant contri bution to inter facial energy at low RH is van der Waals attraction between portions of the surfaces that are separated by nanometer asperities. In co ntrast, significant hysteresis was observed for surfaces that were exposed to high RH conditions. Atomic force microscopy studies of these surfaces sh owed spontaneous formation of silane mounds that have irreversibly transfor med from initially uniform hydrophobic surface layers. Contact mechanics co nsiderations show that the compliance of the mounds can reasonably allow mi crocapillaries in surrounding hydrophilic areas to bridge at high RH as the surfaces are forced into contact by an externally applied load, leading to the adhesion hysteresis. (C) 2000 Acta Metallurgica Inc. Published by Else vier Science Ltd. All rights reserved.