The nonlinear nature of friction coefficient in lubricated sliding friction

• There exists exponential decay in the power spectrum of friction coefficient.
• The two-order Renyi entropy of friction coefficient is finite and positive.
• The friction coefficient has the nature of chaos.
• The friction process can be described by two-order Renyi entropy and phase trajectory.

The nonlinear nature of friction coefficient in the lubricated sliding friction is studied. The exponential decay is found in the power spectrum of friction coefficient, which is the characteristic of chaos. Further, the two-order Renyi entropy K2 is calculated and phase trajectory is plotted. K2 increases gradually in the running-in friction, stabilizes at a high value in the steady state, and decreases in the increasing friction. The phase trajectory contracts to a smaller size, then moves in a finite space, finally expands rapidly. What׳s more, the contraction, stabilization and expansion of phase trajectory correspond to the increase, stabilization and decrease of K2, respectively. Therefore, the friction coefficient in the sliding friction can be described by the chaos theory.