Polyamide 66 without additives exhibits mechanical properties that are too weak for many structural applications. Young’s modulus and yield stress can be improved by incorporating stiff glass fibers; however, this simultaneously increases the friction coefficient—an undesirable effect in mechanical components such as bearings or gear wheels. Solid lubricants, especially PTFE, are widely used to reduce and stabilize friction and wear. Due to environmental concerns, PTFE is under legislative pressure and is expected to be replaced. This systematic study includes ball-on-disk experiments with Hertzian pressure close to yield stress, using different solid lubricants (e.g. PTFE, graphite, molybdenum disulfide, and a further PTFE-free lubricant inferred to be polymer-based). While friction coefficients are recorded, the interfacial dynamics during sliding are observed using a microscope camera. Synchronizing the camera’s frame rate with the rotational speed of the sample enables capturing images of precisely the same surface location over time. The resulting pictures reveal the transport of wear particles, their agglomeration into a thin layer and its removal. These surface dynamics differ significantly depending on the solid lubricant employed. The optical observations can be correlated with the coefficient of friction. The friction coefficient is governed by converging running-in behavior and a quasi-stationary state. This quasi-stationary state consistently falls into one of two characteristic levels—a lower or higher plateau—depending on the lubricant applied. A comprehensive understanding of the friction mechanisms thus offers a basis for identifying appropriate friction modifiers.
Graf et al. (2026) studied this question.