ROLE OF DIELECTRIC-PROPERTIES IN THE TRIBOLOGICAL BEHAVIOR OF INSULATORS

The friction properties of single-crystal alumina were investigated under dry conditions. The relative humidity was kept to less than 1% and the contact pressure was chosen to be very small to prevent the formation of wear debris at the interface. The tangential force and acoustic emission were recorded continuously during tests. The dielectric properties of samples were characterized before and after tribological tests using scanning electron microscopy. The effect of X-ray irradiation on the dielectric and friction properties was investigated. Sapphire samples annealed at 1500-degrees-C did not charge, but after X-ray irradiation the charging capacity was highly increased, as was the friction coefficient (by a factor of 4). After the friction tests, the charging capacity of sapphire was observed both inside and outside the wear track. X-ray irradiation of the samples outside the wear track also modified the friction behaviour. Based on the results, others from the literature and on the increasingly understood correlation between the mechanical and electrical properties of dielectrics, an energetic explanation of the friction and wear behaviour of insulator materials is proposed. Friction and wear are shown to be related to the mechanisms of storage and dissipation of energy. Because of the dielectric properties of insulators, this energy results from electrostatic interactions: polarization of the material and displacements of electrical charges. Polarization is quickly achieved and increases during friction because of the build-up of a space charge in the material owing to the trapping of charge carriers. This trapping of defects already present in materials before testing or created during friction allows the storage in the lattice of very high amounts of energy (5 eV or more per charge) which, when dissipated, can lead to catastrophic failure.