Tribo-electrification mechanisms for dissimilar metal pairs in dry severe wear process: Part II. Effect of load

The effect of normal load on the tribo-electrification mechanism is investigated for the five soft metals, namely Pb, Ag, Cu, Zn and Al, sliding on iron for complete contact under dry severe wear in a reciprocating friction tester. Results showed that the amplitude of the resultant tribo-electrification increased with increasing normal load. As the dynamic real contact area increased with increasing normal load, the tribo-electrification voltage was therefore not linearly proportional to the normal load. It was revealed that the tribo-electrification caused by material transfer was proportional to the (1 + n) power of normal load, and that caused by Seebeck effect was proportional to the square root of normal load. Consequently, the average interface temperature rise caused by Seebeck effect was also proportional to the square root of normal load. The error in estimating the interface temperature rise was found to increase with decreasing normal load. When the normal load exceeded 40 N, the temperature rise reached saturation in the range from 15% to 45%. The main finding was thus that the effect of material transfer on the interface temperature rise was critical and cannot be neglected. An empirical formulation was proposed to predict the average interface temperature rise TS for the reciprocating friction process in terms of the friction coefficient f, the normal load L, and the average speed image as image, where b is 15 for the first cycle and 35 for the quasi-steady-state cycle.