Some studies on scuffing in boundary lubricated sliding contact with subsurface plastic deformation

摘要

Purpose - To study the nature of scuffing in boundary lubricated sliding contacts with subsurface plastic deformation, as it occurs in plastic deformation processing. Design/methodology/approach - Low speed oblique plastic impact testing (LOSOPIT) has been conducted on copper specimen with a hard En31 ball in a test rig that has facility to measure the coefficient of friction. Based on the findings of friction coefficient in these experiments, friction power has been estimated and was found to be in the typical range. Scuffing studies were undertaken both by observation of the slid surface of En31 sphere in a ferrographic microscope with camera facility as well as by calculation of the friction power. Findings - The boundary lubricant was found to have profound role in safeguarding the surface from severe deformation and micro-cracks. Scanning electron microscope (SEM) examination of the craters produced by LOSOPIT has given evidence that using the boundary lubricant resulted in smooth transfer of shear stress from the sphere to the specimen surface through the boundary lubricant layer. Owing to this, the asperities have been found flattened in a smooth manner instead of metal at the surface being scuffed. A limited amount of reduction was found in the coefficient of friction due to the use of boundary lubricant from that in the dry testing. Research limitations/implications - The model used to estimate the friction power is predominantly governed by the friction coefficient itself rather than either the normal load or the sliding speed. Friction coefficient itself may be contributed by various mechanisms all of which may not equally contribute to scuffing. Study is underway to carefully glean out those components of friction that exactly result in scuffing, and to use more effective criteria for scuffing. Practical implications - The knowledge and data developed in the paper give a clear explanation of conditions under which scuffing can take place in sliding contacts operating under boundary regime. The most important applications are metalforming and metal cutting. It is relevant to mechanical engineering machinery in which intense contact pressures are expected. Originality/value - This paper fills the gap of lack of scuffing studies in plastic deformation processing. All earlier studies focused on elastic conditions prevailing at the contact. Since, industry has been witnessing a need to tackle the severe problems related to formed product quality and certain defects hitherto unexplained, this paper gives a new direction to explain the defects in products from scuffing point of view. In this paper, it has been shown that friction power can be a good criterion to represent scuffing intensity in boundary lubrication.