Tribocharging mechanisms explained with an experimental and theoretical approach

Alessandra Ciniero¹, Giulio Fatti², Maria Clelia Righi², Daniele Dini¹, Tom Reddyhoff¹

 1.Mechanical Engineering Department, Imperial College, London, UK

2. Physics Department, UNIMORE, Modena, IT

INTRODUCTION: The term tribocharging refers to the transfer of charge between rubbing components. A wide range of factors influences the transfer and the generation of charge, at a molecular level. For instance, material stress can cause bond scission that results in a rearrangement of the charge of the surface and the release of ions and electrons⁴. The importance of tribocharging has led to the development of new nanoscale techniques to study this phenomenon and better understand its fundamental mechanisms. In industry, tribocharging is employed in many applications, including pharmaceutical, space, electrophotography and green energy harvesting and it has the potential to be at the forefront of technological innovation. In particular, in the formation of boundary films and in certain cases in their degradation tribocharging and a phenomenon known collectively as triboemission it has been suggested as playing a key role. Triboemission consists of emission of charged particles such as electrons, protons, positive and negative ions and also acoustic emission that occur when hard, solid surfaces are rubbed together². If a lubricant is present, the charge of the surface and these particles may promote chemical reactions that lead to both degradation³, and formation of protective films on the surface of components⁴. Since this two phenomena are not yet well understood this study aimed at elucidating the underlying mechanisms. To achieve this, a recently developed measurement system is used to obtain information of the charging of the surface and emission events during the sliding of a simulate asperity contact. Here, results from silica specimens are reported in order to explain how the charging and emission depends on the wear. Furthermore, a theoretical approach provides insights into the mechanism of tribocharging at atomic level.  

METHODS: We report the use of a unique vacuum measurement system, comprising an electrode coupled with an external electrometer and a system of microchannel plates and a phosphor-screen. The set up enables to acquire surface charge variations while simultaneously record triboemission events. Charge and emission measurements obtained in this way, while a diamond tip scratches silica surfaces, lead to a detailed understanding of tribocharging mechanisms. For instance, the variation of surface charge is shown to depend directly on the dominant wear occurring. Results also show for the first time how tribocharging and triboemission behaviour are linked.

The two phenomena are then described by means of density functional theory (DFT) - Quantum Espresso package. The electronic structure of the silica surface and its changes due to the formation and breaking of chemical bonds are described by the first-principle approach based on DFT.

RESULTS & DISCUSSION: Sliding tests where no fractures were observed on the surface, as proven by white light interferometry scans of the specimen after the tests (Figure 1a)), generates positive charge of the surface and low negative emission. The surface charge measurements and the simultaneously average of emission intensity plotted as a function of the time show a correlation between charging and emission. This comparison reveals that the positive charging of the surface corresponds to negative particles leaving the specimen. In tests when fractures were observed on the sliding surface, the comparison between surface charge and the average intensity of the emission, shown in Figure 1a), reveals a different trend. Here it can be noticed that, unlike the case of sliding without fracture, the surface charges negatively. This suggests that during fracture both negative charged and positive charged particles are emitted with the number of positive particles being higher compared to negative particles.

The theoretical analysis conducted to unravel the tribocharging mechanisms suggested by the experimental tests shows (Figure1b)) that at the interface between the diamond and alpha-quartz silica slab there is a transfer of charge due to the contact between the two surfaces. In particular, silicon atoms lose electrons in favour of carbons atoms, leaving the silica slab positively charged as expected. Additional dynamics analysis show that disruption mechanisms generated by the sliding between the two slabs cause breaking of bonds and removal of material influencing the variation of the charge of the surfaces.

Figure 1 a) Surface charging and average intensity emission over time during rubbing and fracture, b) Charge distribution at diamond-alpha quartz silica interface after relaxing.

REFERENCES: 1. Burgo, Langmuir, (2012), 2. Ciniero, Wear, (2017), 3. Zhao, J. Eng. Tribol., (2001), 4. Spikes, Tribology Letters (2004).