A new device for measuring the real area of contact of polymeric material by the perturbation of total internal reflection

Abstract

An optical device has been developed to measure precisely and directly the real area of contact (RAC) between a glass prism and a machined polymeric surface. It is based on the perturbation of total internal reflection of a light beam in the region of contact between the two solid surfaces. The optical and electronic components of equipment intended for the measurement of the RAC, given in terms of percentage of the apparent surface of contact, were selected to provide improvements in the way of using this method and have permitted to reach a precision of about 1.5%, and a resolution of 0.05%, for finishes rougher than 1.0μm centre line average roughness value (CLA). These components also ensure measurements with a calibration factor independent of the material considered and its colouration. The design concepts are outlined, and typical experimental results for a thermoplastic material are presented, which show a good performance of repeatability. The variation of RAC with normal load was investigated with a large range of surface roughness of polymeric specimens. The results are included, and show that the real area of contact varies as 0.88 power of the normal load, with a proportionality constant dependent on the roughness of the contacting polymer surface.