Spectral analysis of photons emitted during scratching of an insulator surface by a diamond in air

Spectral analysis of photons emitted during scratching of insulator surfaces of Si3N4, Al2O3, ZrO2 and soda-lime glass with a diamond was performed under relatively slight frictional conditions, i.e., with a normal force of less than 1 N and a sliding velocity of less than 16 cm/s in ambient air. All the spectra showed similar profiles with strong sharp peaks in the ultraviolet region and photon energies of 2.8–4 eV. The wavelengths of the photons were perfectly matched to those for the second positive band of N2, demonstrating that an electric discharge of N2 gas occurred at the frictional contact.