Study on the Boundary-Lubricated Sliding Contact with Subsurface Plastic Deformation

Abstract

In this study, low-velocity oblique plastic impact-testing experiments were conducted at different angles of incidence to investigate the consequences of the short-distance sliding interaction between a very hard (En31) ball and a relatively softer (mild steel) metal specimen. The purpose was to understand the mechanism of boundary lubrication in the metalworking processes. The specimen was mounted on a specially designed inclined-plane-type fixture so that its surface could be oriented at any desired angle against the free-falling hard ball from a predetermined height. The experimental setup included sufficient details of instrumentation to record the post-impact travel distance and time from which the average coefficient of friction was calculated using a simple methodology. The specimen surfaces were studied using the SEM for different cases of sliding experiments with and without lubricants and two different additives in the lubricants. Marked difference was observed in the nature of surfaces produced in different cases. The oblique impact process was modeled using the equations of motion of the ball and its interaction with plastically deforming specimen material. A fourth-order Runge-Kutta method was used and variations of shear and normal forces during the sliding contact were estimated. The friction behavior showed by this model is in conformance with the experimental results. In addition to that, it has been shown by this model that the coefficient of friction cannot exceed the value of one in sliding. A finite element model has been prepared to estimate the plastic deformation component of friction. Considering the soft asperities of the workpiece deforming as a wave in front of hard asperities, the steady-state Galerkin finite-element model enabled estimation of friction. The trend of the results of the FEM model seems to substantiate the experimental results.