Fractal Theory Applied to Evaluate the Tribological Performances of Two Greases Demonstrated in Four-Ball Tests

In the present study, two commercial greases with different rheological properties were subjected to four-ball tests to identify their performance in anti-wear and anti-scuffing. A wear test machine equipped with a data acquisition system was used to collect and analyze the experimental data of electrical contact resistance (ECR) and friction torque (T_f). Fractal theory was used to deal with the signals of the above two parameters simultaneously. The fractal dimension (D_s) and topothesy (G) of the signals were used to establish their magnitude in relation to the tribological parameters, such as worn surface roughness and friction coefficients. The variations in the fractal parameters can be used to determine the possibility of surface scuffing under the given operating conditions. The frictional energy required for surface scuffing decreases with increasing normal load. Worn surface roughness (R_a) that varies with test time depends strongly on the amount of oxide residual on the worn surface. If the oxide amount increases with time, the surface roughness decreases, which increases the fractal dimension and topothesy of ECR. For grease, the time starting the net growth of oxides is thus the governing factor for variations in worn surface roughness. The fractal dimension of friction coefficients varied in a narrow range regardless of scuffing. However, scuffing in the wear process affected the topothesy of the friction coefficient. The fractal analysis of friction coefficients is an efficient method for determining the possibility of scuffing that arises at contact surfaces during the wear testing processes.