THE NONVIBRATING KELVIN PROBE AND ITS APPLICATION FOR MONITORING SURFACE WEAR

This paper reports on the design and development of the nonvibrating K elvin probe that could be used as a noncontact sensor for tribological damage. This device detects surface charge through temporal variation in the electron work function of a material. Experiments are performe d to demonstrate the operation of the probe on a rotating aluminum sha ft. The probe, made of lead, is placed adjacent (<0.5-mm distance) to the shaft. The two surfaces, which are electrically connected, form a capacitor. Benchmark experiments on the variation of the work function with changes in surface chemistry were performed by coating a segment along the shaft circumference with a colloidal silver paint. As the s haft rotated, the probe senses changes in the contact potential differ ence with the shaft surface, due to the compositional variation. The t emporal variation in the contact potential difference induces a curren t in an external electrical circuit. This current is amplified and con verted to a voltage signal. The magnitude of the signal decreases asym ptotically with the electrode-shaft distance and increases linearly wi th the rotational frequency, as expected. These results are consistent with the theoretical. Preliminary tests to detect mechanical surface damage, using the probe, were also done. These results show that the K elvin probe can distinguish geometrical features on the surface.