Influence of mild and severe wear condition in the formation and stability of friction film in clutch system

Pin-on-disc tests frequently shows a transition from mild to severe wear when steel slides against steel in unlubricated condition. Studies have shown that the transition is influenced by hardness, normal load, roughness, sliding velocity, temperature and time. This phenomenon was also observed when a brake friction material was tested against perlitic cast iron. In this work the same regime transition of standard clutch friction material was evaluated for varying loads and sliding velocity. For this purpose tribometric tests were carried out varying the normal load from 200 N to 450 N in two different sliding speeds: 750 rpm (2.05 m/s) and 1200 rpm (3.27 m/s). Each test was run continuously for 3 h at room temperature. The transitions from mild to severe wear have been observed only at 1200 rpm and loads ranging from 380 N to 450 N. Characterization of sliding surface and cross-section of the disc by scanning electron microscopy and dispersive energy X-ray has shown that material transferred from friction material to sliding surface of disc. The characterization has shown that the quantity of friction material found on the surface of disc after the tests decreased when the transition from mild to severe wear occurred. In the mild wear the friction film is relatively thick and continuous. However, in the severe wear regime the film is thinner and less continuous and the severity may influence the formation and stability of friction film. The friction film may act as a solid lubricant reducing the wear rate.