What is the link between a grain silo, a nylon sleeping bag and a flying helicopter? They can all accumulate triboelectric charges! The charges accumulate because the materials in relative motion are insulating materials (respectively grain against grain, nylon against skin and composite blade against air) and the material pairs are constituted of materials with different triboelectric properties. While rubbing your legs against the sleeping bag nylon and observing the little corona discharges in the dark is fun, other tribo-pairs can be the seat of more dramatic events if not handled properly.Indeed, the grain silo destructions are numerous in the last decades even though it is not always clear whether the fermentation gases were ignited by electric discharges or other heat sources. Besides, the helicopters (and other large flying vehicles by the way) are always connected to the ground before anyone gets in or out of it: it is better to leave kV discharges for a good old piece of copper than someone’s body.
To better understand how the electric potential difference builds in the contacts and how the charges affect the contact behaviour, Luo et al.  have upgraded a pin on disc machine. They decided to simultaneously make electric and tribological measurements. On their experimental test-rig they are able to track:
- contact friction
- and electric potential.
Moreover, after the experiment they investigate the contact wear track on the disc to assess the worn material quantity with a profilometer.
The disc is either made of Nylon or Polyvinyl Chloride (PVC) while the ball is a copper ball which can be grounded or left insulated from the test rig by its polymer support. Because the Nylon is electropositive and the PVC electronegative, the tribo-pair formed between the disc and the conductive ball could be varied importantly.
After each rotation of the disc, the charge measured on the disc and the pin ball increases more and more until it reaches a threshold. The potential on the discs can reach up to +800 V for the Nylon and -800V for the PVC depending on the load and the velocity. The larger the load and the velocity, the larger the threshold. When the threshold is reached, the charges dissipated balance the charge added by the rubbing contact.
On the tribological side, the more the charges accumulate, the larger the friction coefficient and the larger the wear. The relationship is clearly demonstrated by Luo et al. and they are able to not only undergo the relationship but also to use it to vary the contact behaviour. Indeed, the charge accumulation can be cancelled by different means that Luo et al. test in this study: grounding at least one of the contact bodies or using an ionizing blower to neutralise the surface charges. Both methods show reduction of wear and friction compared to the purely insulated contact, but the ionizing blower seems the best of the two.
Luo et al. demonstrate that a basic contact with insulated bodies can lead to large electric potential build up. Such potential can lead to electric discharges on the one hand, but it also leads to a degraded contact behaviour with larger wear and larger friction coefficient. Moreover, they show the influence of load and velocity. Thus, they give a good starting point for future studies on the topic and for investigations on industrial systems where tribo-electric failures are suspected.
 Luo, N., Feng, Y., Zhang, L., Sun, W., Wang, D., Sun, X., … & Liu, W. (2021). Controlling the tribological behavior at the friction interface by regulating the triboelectrification. Nano Energy, 106183, https://doi.org/10.1016/j.nanoen.2021.106183.
- grain silo: Photo by David Ballew on Unsplash
- sleeping bag: Photo by Felix M. Dorn on Unsplash
- helicopter: Photo by SPACEDEZERT on Unsplash