Frictional Characteristics and Mechanical Model of PTFE Against Stainless Steel Under Low Loads

ObjectivePolytetrafluoroethylene (PTFE) is a highly effective solid lubricating material that has been widely applied in engineering, particularly in sliding isolation bearings. Its friction pair with stainless steel effectively controls the upward transmission of seismic energy and reduces the seismic response of the superstructure through frictional energy dissipation. However, existing research and experiments have primarily concentrated on the friction characteristics under heavy load conditions, with limited understanding of the friction behavior under low load conditions. With growing awareness of preventive protection, seismic isolation technology has been increasingly employed to safeguard various free-standing objects, including cultural relics and precision instruments, which are typically small in volume and light in weight. Given the close relationship between friction characteristics and contact surface pressure, it is of practical significance to investigate the friction characteristics of PTFE and stainless steel under low load conditions. This research contributes to enhancing the accuracy of isolation effect predictions and optimizing the design of seismic isolators.MethodsThe specimens of PTFE and 304 stainless steel were prepared to ensure a smooth surface and accurate size, and a test device was designed and constructed. When measuring the static friction coefficient, the relative moving speed of the specimen was set at 1.5 mm/s. A series of vertical loads were applied to the specimen, and the corresponding static friction force was recorded. The relationship between the static friction coefficient and the pressure was then obtained. A simple harmonic wave was employed as the displacement excitation, with an amplitude of 6 cm and a frequency of 0.4 Hz, to measure the dynamic friction coefficient. The relationships between friction and displacement were recorded under different vertical load conditions. The variation of the dynamic friction coefficient with velocity and pressure was analyzed. Based on the test results, a mechanical model describing the friction and velocity of PTFE and 304 stainless steel friction pairs was established. The established friction model was compared to the classical Coulomb friction model. The accuracy and differences between the two models in predicting friction behavior were evaluated by comparing the test results with the numerical analysis results.Results and DiscussionsThe static and dynamic friction coefficients are correlated with the vertical load. When the pressure is between 0 and 0.5 MPa, the static friction coefficient exhibits a pronounced decline with increasing pressure. When the pressure is between 0.5 and 1.2 MPa, the rate of decline of the static friction coefficient slows down and stabilizes at 0.045. Regarding the dynamic friction coefficient, when the pressure is between 0 and 2 MPa, there is a trend of initial decrease followed by a subsequent increase. However, the growth rate continues to slow down. In addition, under the same pressure, the dynamic friction coefficient increases with increasing velocity and tends to stabilize after the velocity reaches a certain threshold. The upper limit values under different pressures are close to 0.202. In the multiple vibration response tests, the coincidence degree of the acceleration time-history curves is better than that of the relative displacement. Regarding the acceleration response, the time-history curves exhibit consistent trends and minimal differences in the extreme values, and the coincidence degree between the curves is exceptionally high. For the displacement response, the repeatability of test results is relatively poor, with differences in the displacement values of each curve at a specific time. However, the overall shape or trend of the curves remains relatively consistent, with local segments exhibiting considerable similarity. As the number of tests increases, the coincidence degree of the displacement response curve also increases. The theoretical analysis results based on the novel model are in good agreement with the experimental results. When the Coulomb model is used, if the friction coefficient is relatively low, the isolation efficiency and displacement response of friction slip will be overestimated. In contrast, if the coefficient is higher, these results will be underestimated.ConclusionsThe static and dynamic friction coefficients of the PTFE‒stainless steel friction pair are found to be significantly influenced by pressure under low-load conditions. Based on the test results concerning the relationship between friction coefficient and sliding velocity, the proposed exponential function model proves to be more suitable for simulating the friction behavior under low loads and for accurately evaluating the isolation effect of sliding isolation bearings. This holds substantial importance for the development and enhancement of sliding isolation bearings designed for free-standing objects.