Study on tribological properties of polyamide 66 in different states based on molecular dynamics simulation

To investigate the micro-mechanisms of tribology for polyamide 66 (PA66) and glass fibre-reinforced polyamide 66 (PA66/GF) composites at the molecular level, initial models of PA66, PA66/GF, and PA66 with different water contents were established via molecular dynamics simulations. The deformation and wear processes of different materials in contact with iron were analysed to explain the effects of temperature and humidity on the tribological properties of PA66, as well as the effect of temperature on PA66/GF composites from a microscopic perspective. Results indicate that the friction coefficients of both materials exhibit a non-monotonic ‘decrease-then-increase' trend with rising temperature, consistent with experimental observations. Elevated temperatures enhance molecular chain mobility, promoting shear deformation and reducing adhesive forces, thereby decreasing the friction coefficient. Beyond a critical temperature, molecular chains compact, increasing intermolecular adhesion and causing friction coefficient rebound. For the effect of humidity, the trend is opposite: low water content leads to hydrogen bond formation between water molecules and amide groups of PA66 chains, breaking inter-chain hydrogen bonds and enhancing chain relaxation. This increases the actual contact area with the Fe layer, elevating the friction coefficient. Elevated water content lowers friction due to the formation of a lubricating film by excess water.