ON THE STRENGTH OF TRIBO-INDUCED EMISSION OF CHARGED PARTICLES FROM MONO-CRYSTALLINE SILICON SURFACES SCRATCHED WITH DIAMOND STYLI

Particle emission observations during scratching of solid surfaces were undertaken by many researchers, and generally it was observed that the intensity of tribo-induced emission of the electrons, ions, and photons decrease in the order: insulator>semiconductor> conductor. Many experiments conducted to compare the emission of negatively-charged particles in case of the semiconductors Si and Ge with that from selected insulators have reported a clear decreasing trend of the tribo-emission intensity as contact progressed over the same wear track for diamond-on-Si and diamond-on-Ge. Despite that all of these experiments were performed in vacuum, the origin of the weak signals and the decrease of signal strength in the case of Si- and Ge- centered around the presence of dielectric silicon oxide films formed in air during surface preparation or because of the samples being of mono-crystalline nature. This paper offers an alternative explanation to the behaviour of tribo-emitted particles as observed by Molina and Nakayama. This is based on the documented pressure induced semi-conductor-to- metallic phase transformation that takes place in Si and Ge during sliding. It is shown that due to repeated sliding, the wear tracks experience a semiconductor-to- metal transformation that renders the bulk of material immediately under the diamond slider conducting. The results of Nakayama are also explained based on our measurements of the electrical resistivity of the metallic (conducting) silicon phase Si-II.