Fretting wear behavior of controlled ball impact treated aluminium alloy under dry sliding condition

摘要

Fretting wear occurs when two contacting solid surfaces are subjected to a relatively small amplitude oscillatory motion in the order of few microns. In addition to the introduction of compressive residual stresses and increased substrate strength controlled ball impact treatment results in the formation of nanostructured grains at the surface. Fretting wear studies were performed on the untreated and controlled ball impact treated aluminium samples using a steel counterbody at constant slip amplitude and at different applied normal loads using a fretting wear test rig. Displacement amplitude and normal force determine the nature of the slip regime. The tangential force coefficient decreases with increasing normal loads under fretting conditions. The contact between the fretting surfaces makes the asperities interlock with each other at low applied normal loads, and results in a high tangential force coefficient, whereas at high applied normal loads tangential force coefficient decreases. Crack initiation and debris formation are the predominant types of damage observed in the fretting specimens due to micro-displacement between the junctions of two contacting members. The steady state tangential force coefficient, wear volume and specific wear rate of the ball impact treated samples were lower than those of the untreated coarse grain aluminium samples. The improvement in the tribological properties of the treated sample is attributed to high dislocation density, more number of grain boundaries, presence of compressive residual stresses and increase in substrate strength with associated grain refinement. The increased substrate strength and the presence of compressive residual stresses prolonged the crack initiation time and crack tip blunting retards the crack propagation resulting in decreased wear debris formation and wear volume. The surface morphology of the wear scars was analyzed using an optical microscopy and scanning electron microscopy, to identify the failure modes and fretting wear mechanisms. At low applied normal loads a complex adhesion and oxidation type of wear mechanism was observed and abrasion was found to be a dominant wear mechanism at high applied normal loads.