The Mechanism Of Triboluminescence In The Sliding Contact

Triboluminescence(TL)is the light emitted by the application of mechanical action to solid materials.Since TL was first recorded by Francis Bacon in 1605,it has attracted extensive attentions and researches from many scholars at home and abroad.However,due to the fact that TL can be affected by many factors such as material properties,friction conditions and surrounding environment,the phenomenon of TL is complex and thus the underlying mechanism is still unclear.In order to study the phenomenon of TL and its mechanism,a device for detecting TL has been designed and then fabricated in the present work.By using this device,the TL phenomenon under different experimental conditions and different configarations of friction pairs have been deeply studied and analyzed.To obtain quantitative observations on TL,a real-time detection system for TL in the disk-on-disk sliding contact was developed.The system consists of three parts:the mechanical subsystem,the control subsystem and the measurement subsystem.By using the dection system,the intensity and the spectrum of TL for different sliding speeds,different loads and different configarations of friction pairs can be measured.In this system,the wavelength range of observed light is 200?900 nm,the load range is 0?200 g,and the accuracy of the load is about 0.4 g.By using the dection system,the influence of the relative sliding velocity on the TL has been investagated.The results show that the TL intensity increases with the increase of the sliding speed.Analyses on the spectrum of TL showed that TL is generated by the air discharge which is in turn induced by the triboelectrification(TE).In order to further clarify the relationship between the TL intensity and the sliding speed,a"capacitor model"for the charge transfer dynamics in TE was proposed.The model believes that the electron transfer between contacting surfaces driven by the contact potential difference is the the dominant mechanism for TE,and TE can be considered as the charging process of a capacitor formed by the contacting surfaces.By comparing the theoretical model with the experimental results,the time constant of the equivalent RC circuit and the distance between the contacting surfaces have been calculated,which verify the validity of the present theoretical model.Furthermore,the experimental results show that the position reversal of friction pair has a dramatic impact on the TL:when a stationary Al₂O₃ crystal disk is pressed on a rotating Si O₂crystal disk,apparent light is produced,but contrarily,there is nearly no light emission if the positions of the two disks are reversed.Such a position dependence was then explained by the combined effect of the contact potential difference and the temperature difference between the contacting surfaces on the charge transfer during the friction process.The effect of the crystal orientation on the intensity of the TL has also been investagated.The results show that the TL intensities of silica crystal with different crystal directions are different.This phenomenon may be related to the different work functions for different crystal orientations.Finally,the TL of different kinds of friction pairs,including metal-metal pair,metal-insulator pair and insulator-insulator pair,have been experimentally studied.Because the charge can not stay on the metal surface,no luminescence was observed for the metal-metal friction pair.When two insulators of the same materials contact with each other,TL was detected.The reason may be the temperature difference between the two contacting surfaces caused by the asymmetric friction.