Effect of surface topography on formation of squeal under reciprocating sliding

Under reciprocating sliding there are at least four states available to distinguish whether squeal occurs or not and how squeal evolves if it is present: (1) no squeal is emitted in the whole reciprocating sliding as many as 3800 cycles or more; (2) squeal is not emitted when the friction coefficient is smaller in the beginning stage of reciprocating sliding; (3) squeal is emitted when the friction coefficient arrives at a larger (critical) value; (4) sometimes squeal disappears after more cycles. These four states are found to be to some extent in correlation with the topography characteristics of the wear scars. Acoustic and dynamic measuring techniques were applied to ascertain the area where squeal was present on a wear scar. By means of SEM observations of wear scars, a comparative investigation into the characteristics of scar surfaces was carried out under both silent and squealing conditions. The result shows that for flat–flat contact the area associated with squeal on a wear scar is characterized by uneven serious pit-like detachment of the specimen material. For flat–ball contact it is found that the area related to squeal is characterized by adhesively joined asperities. Moreover, the effect of a negative friction–velocity slope on squeal formation is examined. The result seems to demonstrate that not all negative friction–velocity slopes can cause squeal. A possible alternative mechanism of squeal formation is proposed and dependence of squeal generation on the friction coefficient are also discussed based on the detachment of the material or formation of adhesively joined asperities.