Study: Electrical Field Alters Film Thickness

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The remote application of an electric field between a metal part and electrodes can change the thickness of pre-existing lubricant film, opening the possibility of adjusting lubrication in applications considered inaccessible, according to a recently published scientific report by researchers representing organizations and universities in Sweden, France, Belgium and Australia.

“This discovery provides the possibility of condition monitoring-driven rescue of lubrication by the tribotronic system,” the authors said in their scientific report, “Tribotronic control of an ionic boundary layer in operando extends the limits of lubrication,” published on Nov. 28. “On detection of deterioration of lubrication, a short burst of high electric potential, outside the electrochemical window, can be applied to form a thick, surface-protecting multiplayer ionic film.” When an object moves against an electric field, it gains some amount of energy, which is defined as the electric potential energy.

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Tribotronics is the fusion of tribology and electronics that aims to enhance machine efficiency and durability through in-place control of loss outputs, such as friction and wear. The report defines a tribotronic system as having a tribological contact, sensors that monitor factors such as friction and temperature, actuators and a control unit.

The researchers studied the effect of electric potential on the lubrication of a non-halogenated phosphonium orthoborate ionic liquid used as an additive in biodegradable oil.

They built an in-house tribotronic system around an instrument designed to measure lubricant film thickness between a rolling steel ball and a rotating, silica-coated glass disc, the researchers explained in their abstract. “Applying an electric field between the steel ball and a set of customized counter-electrodes clearly induced changes in the thickness of the lubricant film: a marked decrease at negative potentials and increase at positive potentials,” they said.

The report notes that ionic liquids, defined as organic salts with metaling points below a certain nominal temperature, have emerged as viable candidates for tribotronic actuation systems. Ionic liquids can undergo changes to their ion dynamics under the influence of external factors, such as temperature, pressure and electric field, the reported noted. Much research and development in recent years has taken place into ionic liquid additives. They are defined as salts, but because of their large and asymmetric cations and anions that prevent them from forming stable crystals, they remain in liquid form even at unusually low temperatures.

They said that although a contribution to the film thickness from electrochemical reaction products cannot be completely ruled out, “This may nonetheless play a positive role analogous to the formation of beneficial tribofilms through the chemical breakdown of some conventional lubricant additives like [zinc dialkyldithiophosphate]. Thus, the [ionic liquids] not only provide an absorbed electrically addressable film, which may also have benefits for electrical applications and preventing electric discharge erosion, but they may also provide a ‘red button’ rescue function that can be operated remotely.”

The pressing need for such remote control is in inaccessible offshore wind and wave power installations, as well as aeronautics and space applications, they said.

The report was authored by Akepati Bhaskar Reddy, Georgia A. Pilkington, Mark W. Rutland and Sergei Glavatskih. Affiliations included the KTH Royal Institute of Technology in Sweden, the Tribology and System Dynamics Laboratory of Ecole Centrale de Lyon in France, the University of New South Wales in Australia and Ghent University in Belgium.

The full academic paper is available to view on the Nature.com website.

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