Discussion: “A Deterministic-Chaos Study of Electron Triboemission Outputs” (Molina, G. J., Furey, M. J., and Kajdas C., 2007, ASME J. Tribol., 129(3), pp. 679–683)

Abstract

The authors observed that the strength of triboemission, for diamond-Si and diamond-Ge pairs, decreased as contact progressed over the same wear track. Nakayama and Fujimoto (1) observed similar tendencies although accompanied by occasional high strength emission peaks. Nakayama and Fujimoto proposed that the weak signals are due to their samples being mono-crystalline silicon, whereas they suggested that the strong signals are probably due to contamination of samples by formation of oxides during pre-experimental surface preparation in air. This author suggests that the observed triboemission behavior in the present work and the work of Nakayama is a consequence of a pressure-induced metallization of silicon. Repeated passing of the diamond stylus on the same wear track, under suitable conditions, would induce metallization (2). Such a notion is supported by the observation of plastically extruded silicon plastically extruded silicon (PES) along the wear track (3). The presence of PES suggests a pressure induced semi-conductor-to-metal phase transformation, i.e., the formation of the metallic phase Si-II (β-tin) which has low electrical resistivity (4). This low electrical resistivity can explain the observed emission behavior. For mono-crystalline silicon transformation from Si-I to the metallic phase Si-II takes place in the pressure range 8.5 GPa <P< 12 GPa. Using 1200 GPa and 110 GPa for the moduli of elasticity, 0.2 and 0.28 for Poisson's ratio for diamond and silicon, respectively, and considering the experimental conditions of Nakayama, calculation of the Hertzian contact stress yields a value of 9.6 GPa. This value favors a transition from the semi-conductor phase to a metallic phase. Further using the Johnson solution for pressure under a conical indenter considering the experimental conditions, reported by the current authors, we reach a similar conclusion.