Measurement uncertainty in optical surface profiling of discontinuous polymer transfer films

Sliding a polymer against a metal often forms transparent polymer transfer films on the metal counterface. Accurate surface topography measurements of these films are essential for understanding tribological performance. Optical profilometry is commonly used for this purpose but requires knowledge of the film’s refractive index, which is often assumed to match that of the bulk polymer. This study examines the validity of this assumption for discontinuous polytetrafluoroethylene (PTFE) transfer films using two optical profilometry techniques: laser scanning confocal microscopy (LSCM) and digital holographic microscopy (DHM). Results show that the film’s refractive index differs from that of the bulk and varies within and between films, leading to significant measurement errors. Potential contributing factors and suggestions to improve measurement accuracy are discussed. • Refractive indices of PTFE transfer films differ from their bulk polymer • Refractive indices of PTFE transfer films vary within and between films • The variation leads to significant optical surface profile measurement errors • The variation is likely due to the tribochemical contamination within the films • Different profilometry methods vary in sensitivity to refractive index variation