Triboplasma Generation and Triboluminescence: Influence of Stationary Sliding Partner

Abstract

In this work, the characteristics of triboplasma have been investigated from the point of view of how the kinds of the stationary partner influence the photon energy and the plasma distribution. The energy spectrum and two-dimensional images of the emitted photons were measured during the sliding of a diamond pin with a tip radius of 1.5 and 3 mm on the three kinds of disk made of Al2O3, MgO, and SiO2 in dry sliding in air. The results showed that all the three kinds of solid tested had narrow bands of air-discharge plasma in the ultraviolet region, demonstrating that the triboplasma generation does not depend on the kinds of insulating solids. Triboplasma was distributed having a ring with one or two tails in all the three kinds of solid Al2O3, MgO, and SiO2. The mechanisms of UV, visible, and IR photon emissions are discussed. The UV photons are emitted from the ionized plasma gas produced by discharging of air, while the visible photons are emitted from the defect and impurity centers excited by the triboplasma-photons, triboplasma-electrons, and frictional temperature rise together with the photons from the plasma itself. A 696-nm narrow band appeared only with Al2O3, and it was attributed to the Cr3+ ions excited by the UV photons and electrons from the triboplasma. The IR photons should be emitted from the sliding contact by the thermal emissions consisting of thermoluminescence and blackbody radiation, as well as from the triboplasma. The visible photon emission was strongest at the pair points facing across the frictional track in Al2O3 and MgO. To explain the photon emission at the pair points, a new model has been proposed, in which the visible photons are emitted from the surface defect and/or impurity centers excited by the back electrons accelerated with the inverse electric field caused by the negatively charged patches produced by the electron bombardment to the diamond surface. The negative charges on the frictional track repelled the back electrons, so that the back-electrons attack just outside the frictional track.