The AC impedance measurement of environmental water in adhesive interfaces.

A method is described in which the AC impedance spectra between parallel metal lines in an adhesive interface exposed to a humid environment can be used to: i)follow the time-dependent adsorption of water by measurement of interfacial conductivity, and ii)identify the presence of regions of liquid water in the interface by the detection of double-layer capacitances. Preliminary experiments, in which "triple-track" test patterns were used to study the environmental stability of adhesion between three different coating materials (RTV silicone, and amine- and mercaptan-cured epoxies) to the silicon substrates,demonstrate the ability of the technique to measure moisture-induced changes in conductivity (either interfacial or bulk) of moisture resistant coatings, and to differentiate between the formation of aqueous double layers in the interfacial region and changes in the bulk permittivity of the coating material. A modified experimental procedure is outlined through which interfacial impedances can be quantitatively isolated from the inherently coupled AC conduction in the bulk substrate and coating media. The procedure is based on the assumptions of ohmic conduction and frequency-independent permittivities in the bulk media, but allows for time-dependent properties of the coating material.