A novel two-dimensional ultrasonic surface burnishing process (2D-USBP) is proposed. 7075-T6 aluminum samples are processed by a custom-designed 2D-USBP setup. Parameter optimization of 2D-USBP is conducted to determine the best processing strategy of 7075-T6 aluminum. A uniform design method is utilized to optimize the 2D-USBP process. U ₁₃ (13 ³ ) and U ₇ (7 ² ) tables are established to conduct parameter optimization. Burnishing depth, spindle speed, and feed rate are taken as the control parameters. The surface roughness and Vickers hardness are taken as the evaluation indicators. It establishes the active control models for surface quality. Dry wear tests are conducted to compare the wear-resistance of the 2D-USBP treated sample and the original sample. Results show that the machining quality of 2D-USBP is best under 0.24 mm burnishing depth, 5000 r/min spindle speed, and 25 mm/min feed rate. The surface roughness S _a of the sample is reduced from 2517.758 to 50.878 nm, and the hardness of the sample surface is improved from 167 to 252 HV. Under the lower load, the wear mechanism of the 2D-USBP treated sample is mainly abrasive wear accompanied by delamination wear, while the wear mechanism of the original sample is mainly delamination wear. Under the higher load, the accumulation of frictional heat on the sample surface transforms the wear mechanisms of the original and the 2D-USBP treated samples into thermal wear.

To investigate the influence of surface roughnesses on the dynamic characteristics of AISI 5120 steel in sliding friction process, a suite of running-in experiments are performed on a pin-on-disk tribometer. The running-in attractor is used to analyze the high-dimensional features of a friction system under different surface roughnesses. The experimental results show that the roughnesses of different original surfaces evolve to the same value in stable wear stage. The pin-on-disk friction system has a chaotic nature and exists a running-in attractor. The original surface of a disk of Ra = 0.953 μm is the most conducive to shorten the running-in duration, reduce the friction coefficient value, and improve the stability of the friction system. This roughness value is termed as the optimum surface roughness. It is an ideal working surface for the friction pair in the running-in design.

Purpose: The purpose of this work is to comprehensively reveal the spatial distribution and evolution features of a running-in attractor. Design/methodology/approach: The friction coefficient signals extracted from wear experiments are reconstructed. A projected matrix is obtained based on the reconstructed matrix. Then the approach of three-dimensional (3D) histogram of phase points is proposed, which is used to intuitively characterize the complex properties of the running-in attractor. Findings: The space occupied by the running-in attractor gradually contracts, then stabilizes and finally expands; the maximum of phase points number in a certain bin initially decreases, then keeps stable and finally increases rapidly; yet the percentage of bins number storing phase points shows an inverse variation tendency. Consequently, 3D histogram evolves from a nonuniform state to a uniform state then returns back to the nonuniform state, which indicates the evolution rule of “formation, stabilization and disappearance” of the running-in attractor. Originality/value: Characterization on the features of the running-in attractor can provide valuable information about friction systems and their dynamic behaviors.