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 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.
The energy of the electron emission depends on the strength of the electric field generated by tribo-charging (surface potential). This depends, principally, on the electric resistivity of the solid. If the resistivity remains constant for the duration of the scratch, then the strength and the counts of the emission will remain fairly constant and donators of the emission will remain unaffected. Experimental observations, however, suggest that the resistivity changes during scratching as evidenced by the reported short-lived emission bursts, strength of emission that rapidly grows weak in time, in addition to the changes in the work function as pointed out by the authors. Using the resistivity values for Si-II (5), we can verify that the strength of emission of the metallic phase to that of the semi-conductor phase is detectable only in the time range 1E8 s1E04 s where the ratio of strengths is in the range of 0.999 to 0.75. Thus beyond the few initial ten thousandths of a second the emission will not be of detectable strength. This is because the transformed phase, which is metallic, is incapable of emitting particles of detectable strength. Any contribution to particle counts at this point would likely originate from the recovering material in the wake of the slider. The phase of the recovering material, the PES, is a-Si (amorphous silicon). This phase is capable of emitting particles of detectable strength and is the source of the observed emission since it rubs against the sides of the diamond slider.