Electrical contact resistance or capacitance as measured between two
interfaces of a lubricated contact has been used in tribometers, partially
reflecting the lubrication condition. In contrast, the electrical impedance
spectroscopy (EIS) provides rich information of magnitude/phase spectrum,
which is thoroughly investigated using a combination of electrical circuit
models (equivalent to the lubricated contact) and in-situ measurements with a
ball-on-disc contact. Results indicate a promising potential of EIS in
lubrication condition monitoring, including the variation of lubricant film
thickness as estimated using high-frequency magnitude response; the
transition between full-film, mixed, and boundary lubrication regimes, as
differentiated using extracted electrical resistance together with phase
spectrum; the forming of anti-wear boundary film, where extra
resistor/capacitor are added; and the degradation of lubricant, such as fuel
dilution, oil oxidization, and water emulsifying.