Electric Field Effect on Friction Coefficient of Diamond-Like Carbon Film Heated by Laser Irradiation

Diamond-like carbon (DLC) coatings have undergone a significant amount of research over the last several years and shows promise as a coating material in reducing the friction between moving components of machinery and various other applications. This study investigated the reduction of friction coefficients on these surfaces by applying an external electric field under the assumption that thermal plasma would be generated under an inert gas atmosphere and that the number of hydrocarbons generated by thermal decomposition could also be controlled. Results show that the friction coefficient of DLC heated to 200 °C by laser irradiation was indeed controlled by an external electric field. Although the friction coefficient without the bias voltage between the DLC and leaf spring was approximately 0.35, applying a positive bias voltage to DLC of + 10 and + 20 V corresponding to an electric field of 80 and 160 kV m⁻¹ decreased to 0.1. Meanwhile, the negative bias voltage did not show a reduction in the friction coefficient. Raman spectroscopy of the pin surface after the friction test indicated that the hydrocarbons generated by the thermal decomposition of DLC were transferred onto the pin surface by applying a positive bias voltage of + 10 and + 20 V. Meanwhile, we measured the current from DLC heated using a hotplate on the counter surface of the Si wafer insulated with the DLC using a ceramic ring by applying a bias voltage to the DLC. The current from the DLC forward Si wafer was changed by the applied electric field. When a positive voltage is applied to the DLC, the current flows from the DLC to the Si wafer, and when a negative voltage is applied, the current flows from the Si wafer to the DLC. This dependence on the electric field intensity shows the opposite tendency to that of the friction coefficient on the electric field intensity. Therefore, the hydrocarbon ions generated by the thermal decomposition of the DLC are attracted to the pin surface as the counter surface side by the electric field when an appropriate positive bias voltage is applied to the DLC. Subsequently, liquid hydrocarbons are transferred and deposited on the pin surface, and the friction coefficient decreases owing to the lubrication effect of the adhered hydrocarbons. If this voltage becomes too large, thermal electron emissions from the DLC will be suppressed, and the amount of hydrocarbon generation will decrease. In contrast, when a negative voltage is applied to the DLC, the thermal electron emissions are increased by the work function reduction due to the electric field; however, the hydrocarbon ions are attracted to the DLC and redeposited to the DLC. Finally, we estimated that the number of hydrocarbons transferred to the pin surface could be controlled by the external electric field, and these hydrocarbons changed the friction coefficient of the DLC.