The plasma generated and photons emitted in an oil-lubricated sliding contact

Intensive work has long been going on to find out the unknown origin that sets off curious tribo-physicochemical phenomena and that causes various kinds of problems in oil-lubricated sliding contacts in mechanical and processing systems. The strange tribochemical reaction is one of the such curious chemical phenomena observed in the degradation of perfluoropolyether (PFPE) lubricating oil film in a hard disk drive. Plasma (triboplasma) (Nakayama and Mirza 2006 Tribol. Trans. 49 17) would be one of the most probable origins of the problems if it were generated sufficiently intensely in oil-lubricated sliding contacts, as it is in such a highly energetic state. The generation of plasma was predicted in both dry and oil-lubricated sliding (Nakayama 1997 Japan. J. Tribol. 42 1077, Nakayama 2004 Surf. Coat. Technol. 188-189 599). However, plasma generation in industrially important oil-lubricated contacts has not yet been proven, though it has been found in dry sliding (Nakayama and Nevshupa 2002 J. Phys. D: Appl. Phys. 35 L53). Here the author reports the discovery of the predicted plasma generated in the rear part of an oil-lubricated sliding contact, together with a new finding on photon emission phenomena from inside and outside of the sliding contact in the tribosystem of a diamond pin sliding against a sapphire disc. The plasma in oil-lubricated sliding has a simple ellipse shape, while that in dry sliding has a horseshoe pattern. Visible and infrared (IR) photons are emitted from oil-lubricated sliding contact, while ultraviolet (UV), visible and IR photons are emitted from the region of plasma generation as well as from dry sliding. The UV photon spectra emitted under oil-lubrication completely coincided with those in dry sliding and air discharge, demonstrating that plasma is generated by air discharge even under oil-lubrication. Two kinds of new concepts have been proposed. One is plasma generation by discharge of air through bubble formation in decompressed oil in the rear part of the sliding contact. The other is photon emission from surface atoms and oil molecules excited by the attack of electrons accelerated in the intense tribocharge-induced electric field of the asperity gap inside the sliding contact under dry and oil-lubricated slidings. The size and intensity of the plasma and photon emission from the sliding contact under oil-lubrication increases greatly with the applied normal force and the sliding velocity