Contact conditions and surface degradation mechanisms in low amplitude fretting

Abstract

The characteristic contact conditions and mechanisms of surface degradation in low amplitude (1–16 μm) fretting have been investigated. A crossed cylinder geometry set-up was used with pure niobium as test material. Fretting experiments, varying displacement amplitude, normal force, frequency of vibration and number of cycles, have been performed. Dynamic tangential force measurements and metallographic examination revealed the existence of three different fretting regimes: stick, mixed stick and slip, and gross slip. In the study, the influence of normal force and frequency on the regime boundaries was determined. Niobium was selected as test material since it is a pure metal with strongly strain-rate-dependent mechanical properties and, therefore, is expected to be sensitive to frequency-related effects in fretting. The size of the fretting wear scar was found to increase rapidly with increasing amplitude. The wear rates were significantly higher in the gross slip regime in which material removal predominantly occurred by delamination and crushing of surface oxides. Surface degradation and wear were also observed under partial stick contact conditions. In this case, wear was mainly due to the generation and intersection of surface cracks. The extensive crack formation is caused by the cyclic shearing of material in the near-surface region during fretting. The frequency of vibration is found to influence both the contact conditions and the rate of surface damage. The limiting displacement amplitude to yield partial stick conditions is reduced and the size of the wear scar is increased with increasing frequency (in the range 10–20 000 Hz). This is an effect of the increase in both the interfacial strain rate and the temperature with increasing frequency.