ac impedance measurement of environmental water in adhesive interfaces

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Abstract
An in situ method based on interfacial electrical impendance measurements is described for following the time-dependent intrusion of water in an adhesive interface that is exposed to a humid environment. Water ingress is measured by its effect on bulk and interfacial conductivity, while regions of liquid water are identified by the presence of aqueous double-layer capacitances. Preliminary measurements are made using aluminum triple-track electrodes to study the environmental stability of adhesion between oxidized silicon (SiO2, 4% P) substrates and three coating materials (RTV silicone, amine-cured epoxy, and mercaptan-cured novolac epoxy) at 80 °C and 94% relative humidity. Moisture-induced changes in conductivity of all coatings are measured, and the presence of condensed water in the mercaptan-cured epoxy interface is clearly identified. Based on static potential field calculations, a modified experimental procedure is outlined that can be used to quantitatively isolate interfacial conduction processes from ac conduction in bulk substrate and coating phases. By indicating the state and quantity of interfacial moisture, capacitance and conductance measurements will allow interface displacement processes to be monitored.