Electron Transfer through a Natural Oxide Layer on Real Metal Surfaces Occurring during Sliding with Polytetrafluoroethylene: Dependence on Heat of Formation of Metal Oxides

Y. Momose
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Abstract

Electron emission (EE) from real metal surfaces occurring during sliding contact with a polytetrafluoroethylene (PTFE) rider has been investigated using the thermodynamic data of metal oxides and the X-ray photoelectron spectroscopy (XPS) intensity ratio of oxygen/metal on the surfaces. EE was termed triboelectron emission (TriboEE). 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 EE 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 increase in the electrical conductivity of metals, the TriboEE intensity became remarkably high, while 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.
聚四氟乙烯滑动过程中通过真实金属表面天然氧化层的电子转移:对金属氧化物形成热的依赖
利用金属氧化物的热力学数据和表面上氧/金属的x射线光电子能谱(XPS)强度比,研究了实际金属表面与聚四氟乙烯(PTFE)衬垫滑动接触时发生的电子发射(EE)。EE被称为摩擦电子发射(TriboEE)。使用了18种类型的轧制金属板。根据金属氧化物的生成热计算的金属-氧键能(D(M-O))被证明是将EE分为两种路线的关键因素,即所谓的肖特基效应和隧道效应,由于表面氧化层的存在。第4族(Ti和Zr)、第5族(V、Nb和Ta)和第6族(Mo和W)的金属保持较高的D(M-O)值,而向下移动的金属的TriboEE强度增加,归因于前者的路线。在第10组(Ni, Pd, Pt)和第11组(Cu, Ag, Au)中,D(M-O)值随组的变化而降低,但TriboEE强度显著增加,可归因于后一种途径。此外,随着金属电导率的增加,TriboEE强度变得非常高,而D(M-O)值迅速下降并趋于稳定。XPS结果表明,D(M-O)和XPS金属芯强度对O1s强度的依赖性以及O1s/金属芯的XPS强度比值在第10、11组和第4、5、6组之间存在差异。结果表明,在实际金属表面与PTFE摩擦产生的电场作用下,D(M-O)值小的金属的电子主要通过表面氧化层形成表面势垒,而D(M-O)值大的金属的电子主要通过势垒顶部。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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