Chang, Yuh-Ping

With the development of MEMS and nano-technology, effects of tribo-electrification on the size accuracy and quality of micro-element will be more sensitive. The electrification in the order of mV is also important in the nano-machining process. Therefore, the tribo-electrification mechanisms and characteristics between the metal surfaces are investigated in this study. The experiments are conducted on a reciprocating friction tester with a measuring system, and the tribo-electrification behavior is studied for eleven pure metals, namely, Platinum (Pt), Ferrous (Fe), Molybdenum (Mo), Titanium (Ti), Tungsten (W) and Lead (Pb), Zinc (Zn), Aluminum (Al), Silver (Ag), Aurum (Au), Copper (Cu), in dry severe wear process. According to the SEM and EDS observations on the wear particles and the worn surfaces, the tribo-electrification mechanisms between the metal surfaces are proposed.
Concerning the study of self-mated pure metal pairs; a model of the tribo-electrification mechanism by asperity removal for five hard metal pairs of Pt/Pt, Fe/Fe, Mo/Mo, Ti/Ti, and W/W is proposed. In this model, the wear for the hard self-mated metals is mainly caused by the asperity removal with small wear particle. When the material transfers from pin specimen to plate specimen, the polarity of tribo-electrification for pin specimen becomes positive, and vice versa. Another model of the tribo-electrification mechanism by junction growth for six soft metal pairs of Pb/Pb, Zn/Zn, Al/Al, Ag/Ag, Au/Au and Cu/Cu is proposed. In this model, the wear mechanism of the soft self-mated metals is the flake-like wear particles that are formed by the particle aggregation with junction growth. The polarity of tribo-electrification for the upper specimen keeps negative due to the wear loss of the upper specimen always less than the plate specimen. Furthermore, the transition mechanisms of tribo-electrification are investigated with changing normal load, hence a map has been established to predict the polarity of tribo-electrification for self-mated metal pairs. That is, with increasing normal load, the polarity of tribo-electrification varies from the random, through tending to negative, to negative, and the formation mechanism of wear particle from the micro-asperity removal, through the transition, to the particle aggregation with junction growth. Moreover, an equation is proposed to predict the average magnitude of tribo-electrification. Results show that the average magnitude of electrification voltage is linearly proportional to the electric resistivity and the relative wear rate, but inversely to the real contact area.
Concerning the study of dissimilar metal pairs of Pb/Fe, Ag/Fe, Cu/Fe, Zn/Fe and Al/Fe; the total voltage of tribo-electrification Vt for dissimilar metal pairs consists of three components: (a) tribo-electrification by material transfer Vw, Vw is independent of the reciprocating speed, and is proportional to the (1+n) power of normal load, where n is in the range from \ue20.5 to \ue20.9 for lead, silver, copper, zinc, and aluminum. (b) tribo-electrification by friction heat Vf, Vf is linearly proportional to the reciprocating speed, and is proportional to the square root of normal load. (c) tribo-electrification by residual heat Vr, Vr is linearly proportional to the reciprocating speed, and is proportional to the square root of normal load. Moreover, Temperature rise Tt between the contact surfaces can be calculated by Vf and Vr. Hence, Tt consists of two components: (a) temperature rise by friction heat Tf, an equation is proposed to predict Tf. Results show that Tf is a function of friction coefficient, normal load and speed. (b) temperature rise by residual heat Tr, Tr is linearly proportional to the reciprocating speed, and is proportional to the square root of normal load. Finally, a model of tribo-electrification mechanism for dissimilar metal pairs is proposed to describe the tribo-electrification phenomenon for sliding pairs with low to high mutual solubility

Studies on the Tribo-electrification Mechanisms between the Metal Surfaces

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