Triboelectrification of Metals brought into Sliding Contact with Polytetrafluoroethylene: Influence of Metal Oxide Formation Energy

In fields such as adhesion, coatings, corrosion, and catalysis, the electron or charge transfer at practical metal surfaces and interfaces has been of great significance. The occurrence of such electron transfer pertains to materials used (metals and polymers) and the interactions with the environment. The surfaces can be excited by various methods, corresponding to applications to surface chemical technology.

To begin with, this chapter outlines recently published literatures related to electronic properties of materials at the surfaces: triboelectron emission (TriboEE) and triboelectrification leading to nanogenerators (TENG). TriboEE has been of great interest in tribochemistry, which pertains to friction, wear, and lubrication, causing deterioration of lubricants, but little is known about the mechanism. TENG has been a very active field of research. Next, the study of TriboEE during sliding a polytetrafluoroethylene (PTFE) rider on real metal surfaces is introduced in detail using the thermodynamic data of the formation of metal oxides, electrical conductivity of metals, and the X-ray photoelectron spectroscopy (XPS) intensity ratio of oxygen/metal on the surfaces. Rolled metal sheets of 18 types were used. The metal-oxygen bond energy calculated from the heat of the formation of metal oxide, (D(M-O)), was shown to be a key factor in dividing the TriboEE into two routes, the so-called Schottky effect and the tunnel effect, due to the surface oxide layer. The metals in periodic groups 4 (Ti and Zr), 5 (V, Nb, and Ta), and 6 (Mo and W) maintained higher values of D(M-O), while, moving down the groups, the TriboEE intensity increased, being ascribed to the former route. In groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au), the D(M-O) values decreased moving down the groups, but the TriboEE intensity increased significantly, which can be attributed to the latter route. Furthermore, with the metals in groups 4 (Ti and Zr) and 5 (V, Nb, and Ta), the TriboEE intensity increases with the increase in electrical conductivity, and in group 6 (Mo and W), the TriboEE intensity increases despite having almost the same value of electrical conductivity, while in groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au), the TriboEE intensity tends to increase with decreasing electrical conductivity. With the increase in the electrical conductivity of metals, the D(M-O) value fell rapidly and became almost constant. The XPS results showed that the dependence of the D(M-O) and XPS metal core intensity on the O1s intensity and the XPS intensity ratio of the O1s/metal core was different between groups 10 and 11 and groups 4, 5, and 6. It was concluded that, under the electric field caused on the real metal surface by the friction with PTFE, the electron from metals with small D(M-O) values predominantly tunnels the surface oxide layer as a surface barrier, while with large D(M-O) values, the electron passes over the top of the barrier.