Stress in porous thin films through absorption of polar molecules (and relevance to optical coatings)

Films of magnesium fluoride are found to be under large tensile stress following exposure to water vapour. It is shown that this stress, which varies reversibly in magnitude with the adsorption equilibrium, can be attributed to electrostatic dipole interactions between the adsorbed water molecules. The effect is not confined to magnesium fluoride, but can occur in other materials of an appropriate porous structure. Its relevance to the stability of optical coatings is discussed, together with some other applications.