No abstract

It has been hypothesized that plasma is generated at a sliding contact. However, it has not yet been found. Here, we report on a discovery of plasma generated in the microscopic gap around a sliding contact, having an elliptical shape with a horseshoe pattern and with a size beyond a hundred micrometers. It emits mostly invisible ultraviolet (UV) photons and, to a lesser extent, infrared (IR) photons. It must be a main source of the curious tribophysical and chemical phenomena. The origin, characteristics and the relation to these curious phenomena are discussed.

Processes of mechanochemical degradation of imidazolium bis((trifluoromethyl)sulfonyl)imide ionic liquids with alkyl and monomethoxypoly(ethylene glycol) (mPEG) side groups were studied using the novel technique of mechanically stimulated gas emission spectroscopy. Mechanical stimulus caused degradation of both the anionic and cationic moieties. For the latter, the degradation was concentrated on the alkyl and ether chains rather than on the imidazole. For the anionic moiety, various degradation steps associated with the emission of CH x F y volatile products followed by SO2 and SO3 emission were identified. Simulation of frictional heat dissipation revealed that tribochemical reactivity was induced by the mechanical energy supply rather than the temperature increase. Thermal degradation of the same ionic liquids studied using thermogravimetric analysis and Fourier transform infrared spectroscopy was concentrated mainly on the cationic moiety. Thermal stability significantly depended on the side chains. The decomposition of mPEG-functionalized ionic liquids was a two-step process, where the lower temperature step corresponded to mPEG decomposition.

To verify the tribo-microplasma concept proposed by Nakayama, who suggested that a microplasma is generated in the gap of a sliding contact due to electrical discharge of the ambient gas in the electric field caused by tribo-charging, we challenged to observe directly the tribo-microplasma and to measure spectral characteristics of the emitted photons. In experiments to observe plasma image (with a diamond hemispherical pin sliding on a sapphire disk) it was found that the plasma was generated in the several micrometer gap of the sliding contact. The plasma had a shape of an ellipse with a tail, surrounding the contact and spread to the rear of the sliding contact. The plasma image observed through the UV transmittable filter (UV image) had a horseshoe pattern, while the IR image had a shape of a ring on the ellipse. The strongest UV emission was observed in the center of the horseshoe pattern outside the sliding contact, while the IR photon image showed that the most intense emission occurred at the sliding contact. The electrical discharge origin of the photon triboemission was proved by comparing spectra of tribophotons with spectra of photons emitted from plasma by electrical discharge in parallel electrodes in various gases. The results showed that the spectra of photons emitted from the sliding contact and those of gas-discharge completely coincided for all gases tested, i.e., dry air, N2,O2,H2, He, CH4,C2H4 and C3H8, except peaks originated from the excited atoms of the sliding surfaces. It was concluded that microplasma is produced by electrical breakdown of ambient gas at sliding contact. These results corroborate the tribomicroplasma concept.