Complex Impedance Measurement Applied to Short-Time Contact Between Colliding Steel Surfaces

Abstract

The complex impedance method was developed for measuring the transient behaviors of short-time contacts between colliding steel surfaces, by using an alternating current circuit with a high frequency of 500 kHz at a high sampling rate of 20 MHz. The method was applied to estimate the lubrication effects of simple test lubricants acting at a short-time contact with a duration of 110 μs occurring between a free-falling steel ball and a stationary steel plate, where to minimize the hydrodynamic lubrication effect, the test lubricant was provided on the steel plate as a homogeneous thin film with a thickness of 30 μm. An advantage of the method is in the detection of the beginning and end of the contact. Particularly for wet conditions, although they were apparently unclear in the output voltage signal from the circuit, the modulus of the complex impedance detected them with high accuracy. In addition, using the modulus and argument of the complex impedance, the metallic contact ratio and residual film thickness at the short-time contact can be estimated quantitatively. By using the metallic contact duration determined by the metallic contact ratio, the differences in the boundary lubrication effects were quantified for the selected base oils (i.e., n-decane, n-dodecane, n-tetradecane, and n-hexadecane) and the 30-mM oleic acid solutions dissolved in the base oils at the short-time contact, although the differences in the lubricants did not change the shape of the plastic indentation formed on the steel plate.