Gas discharge caused by triboelectricity around a contact during friction between insulators

Gas breakdown caused by triboelectricity during friction between two insulators was observed in measuring two-dimensional spatial distributions of luminescence from gas discharge plasma. The insulators were chosen among diamond, quartz, sapphire, MgO single crystal, and quartz glass. The discharge between identical materials was also observed. The patterns of the gas discharge luminescence had a shape of a ring for all frictions between insulators. By using a gold-coated diamond pin for friction with a quartz disk, the discharge pattern differed from the ring pattern. In the case of insulator pin, electric field induced by a localized charge at the off-track area near the tip of the pin during friction accounts for formation of the ring shape of the gas discharge. The gold coating makes it possible to monitor an amount of charge transferred from the quartz surface during friction. The negative charge density at the frictional track on the quartz disk is calculated to be -2.2times10^-4 C/m², which leads the gas discharge in a micro-gap near the contact between the diamond pin and the quartz disk.