Effect of Normal Force on the Triboplasma Generation Under Oil Lubrication

Previously, triboplasma image was successfully taken under oil lubrication, and the mechanism of triboplasma generation was clarified to be caused by air discharge in the cavity or bubble produced in the divergent rear clearance gap of sliding contact. In the present study, the effect of normal force on the triboplasma generation was investigated in the tribosystem of a diamond pin sliding on a sapphire disk under perfluoropolyether oil lubrication. Two-dimensional images and the intensities of the photons emitted from the sliding contact and the rear divergent clearance of the plasma region were measured. UV photons are emitted mostly from the plasma region, whereas visible and IR photons are emitted from both the sliding contact and the plasma regions. The visible and IR photon emission intensity from the sliding contact increases linearly with the normal force, i.e., proportional to the real area of contact. On the other hand, photon emission intensity of the UV, visible and IR photons from the plasma region is nothing or negligibly small below a critical normal force, above which it increases greatly with the normal force. This means that there exists critical tribocharge-induced electric field determined by real area contact to accelerate the electrons passing through the thin oil film into the bubbles. Then, to avoid or utilize the plasma, the normal force should be applied below or above the critical normal force, which depend on the type of tribosystem, respectively.