Real-Time Observation of the Evolution of Contact Area Under Boundary Lubrication in Sliding Contact

To study the evolution of asperity contact in the sliding contact, a new in situ observation system of sheet metal surface is built up. The workpiece is stationary so that the continuous deformation of a specific group of asperities can be traced. In the case of no strain rate in the substrate of workpiece, the experiments for various materials, roughness patterns, sliding velocities, and normal loads all show that the fractional contact area will increase from its initial state of pure flattening without shear stress, to a saturated value with sliding distance in different rates. The sliding velocity has minor influence on the variation of contact area for same sliding distance. The maximum increasing rate of the contact area is found in the case of medium load. In most cases, the increase in contact area takes place with simultaneously decreasing shear stress and pressure on the flattened asperity peak. The development of adhesive wear can be clearly observed in the case of high normal load and long sliding distance.