Electrochemically-stimulated nanoscale mechanochemical wear of silicon

Mechanochemical reactions at the sliding interface between a single-crystalline silicon (Si) wafer and a silica (SiO₂) microsphere were studied in three environmental conditions: humid air, potassium chloride (KCl) solution, and KCl solution with an applied voltage. Compared to that from humid air, mechanochemical material removal from the silicon surface increased substantially in the KCl-immersed condition, and further increased when electrochemistry was introduced into the tribological system. By measuring the load dependence of the material removal rate and analyzing the results using a mechanically assisted Arrhenius-type kinetic model, the activation energy (E _a) and the mechanical energy (E _m), by which this energy is reduced by mechanical activation, were compared qualitatively under different environmental conditions. In the KCl-immersed condition, mechanochemistry may decrease the required effective energy of reactions (E _eff = E _a − E _m) and promote material removal mainly through improved catalysis of the mechanochemical reactions facilitated by greater availability of water molecules compared to the humid air condition. Thus, the effectiveness of the mechanochemistry is improved. In the electrochemical condition, electrochemically-accelerated oxidation of the silicon surface was confirmed by the X-ray photoelectron spectroscopy (XPS) characterization. The results strongly suggest that electrochemistry further stimulates mechanochemical reactions primarily by increasing the initial energy state of the surface via the facilitated formation of interfacial bonding bridges, i.e., a surface oxidation/hydroxylation process.