Film Thickness by Electrical Capacitance

In principle the theory of electrical capacitance can be applied to the determination of lubricant film thickness between the bounding surfaces of two solids, but in practice the geometrical difficulties described in Section 5.1.1 for electrical resistance methods are even more troublesome for capacitance. If the solids are bounded by two flat surfaces parallel to each other, the electrical capacitance problem becomes that of a parallel-plate condenser, for which the formula is 

When the rings are deformed elastically, the lubricant-filled gap in the loaded zone can be regarded as a parallel-plate condenser to which, ignoring the capacitances of the entry and the recess regions, Eqn 6-3 is applied. From elastic theory is proportional to, and hence 

Since fe is obtained empirically and K can be evaluated by the Hertzian calculation, the thiciiness of the lubricant film can thus be found from capacitance,measurements. 

The following technique for scanning the oil-filled gap between loaded disks was developed by Crook [7], A chromium electrode is evaporated onto a glass disk, as shown in Fig, 6-5. As the electrode traverses the region of the oil-filled gap, the potential across the resistor R is proportional to e/h. The potential as a function of the position of the electrode in the gap is displayed on an oscilloscope and a plot of the contour of the gap can be made. The gap thickness is calibrated statically by using measured spacers. If the width of the electrode relative to the length of the conjunction zone is small enough, no elaborate theory or calculation is required. 

Crook reported comparison of the results obtained by the scanning method [8] with those obtained by another, more complicated version of the capacitance technique [9]. For average film thicknesses in the range 50-150 tile results by both methods were substantially the same. 

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