Adhesion and Friction Force Measurements Using an Optical Micro-Apparatus

An optical micro-apparatus was designed and developed to visualize and measure adhesion and friction forces for a steel ball in contact with a sapphire window. The apparatus allows for in situ optical investigation of the contact interface during the initiation, separation and simultaneous measurement of the normal force generated within the contact. A high precision z-stage was utilized to move the steel ball at low velocities (0.1 μm/s) to bring it into contact with the fixed sapphire window. In this study, the adhesion force between a 1–mm-diameter steel ball and sapphire window was measured and visualized when the surfaces approach and retract from each other. The experimental results demonstrated the presence of the adhesion force and stick area during the retraction of the contacting surfaces. The optical micro-apparatus is also equipped with a piezoelectric actuator to reciprocate the ball against the sapphire window to conduct sliding friction experiments at various normal loads. In this investigation, a new optical technique was also developed to measure the tangential displacement of the contacting interfaces during sliding friction tests. In this technique, the video of the contacting bodies during sliding friction cycle was processed to trace the center of the contact area and, consequently, measure the relative tangential displacement of the contact. The friction loops obtained using the measured tangential displacement exhibited a rectangular shape thus eliminating the system stiffness commonly observed in friction loop studies. The vertical and horizontal segments of the rectangular friction loops characterize the stick and slip regions of the contact, respectively.