Contribution on understanding the friction film development in the performance of a dry automotive clutch system

• Friction films observed on clutches were compared to those from laboratory tests.
• Sliding conditions influence the stability of multi-layer friction films.
• Effects of debris removal on pin-on-disc wear data were investigated.
• Removing wear debris increases the friction level and reduces the wear rate.

To investigate the effects of a developed friction film on clutch system performance, clutch samples tested in three different field conditions were characterized. The wear mechanisms identified were simulated in laboratory. For this, a commercial truck friction material and cast iron were tested in a pin-on-disc tribometer. Tribological tests were carried out at standard procedures and by cleaning the wear track of the counter-face. For both lab test conditions, the normal load ranged from 200 N to 450 N at constant sliding speed of 2.05 ms−1 and 3.27 ms−1. These parameters correspond to PV values between 3.08 and 11.08 (MPa ms−1), which cover part of the power release in real clutch systems (between 1.3 and 20 MPa ms−1). The friction film simulated in laboratory showed similar characteristics with those observed in field and its development and morphology depend on the test severity. Standard tribometer tests presented a transition in the wear rate at temperatures of 250 °C. Removing the wear debris, the wear rate decreased almost 31% at the lowest level and 45% at the highest level of energy. The friction material tested traditionally revealed a stable friction film for tests performed to 7.88 (MPa ms−1) and a damaged film was found in severe wear regimes. When removing the wear debris under the same conditions, a stable friction film was observed and a less developed friction film was noted on the disc surfaces by eliminating the wear debris. The friction coefficient decreased with increasing testing temperature and cleaning the sliding surface increased the friction levels.

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