Nitrile butadiene rubber (NBR) and its various composite materials are widely employed as friction materials in mechanical equipment. The use of carbon nanotube (CNT) reinforcement in NBR for improved friction and wear characteristics has become a major research focus. However, the mechanisms underlying the improvement in the friction and wear characteristics of NBR with different CNT contents remain insufficiently elucidated. Therefore, we conducted a combined analysis of NBR reinforced with varying CNT contents through molecular dynamics (MD) simulations and ring–block friction experiments. The aim is to analyze the extent to which CNTs enhance the water-lubricated friction and dry wear properties of NBR and explore the improvement mechanisms through molecular chain characteristics. The results of this study demonstrate that as the mass fraction of CNTs (0%, 1.25%, 2.5%, 5%) increases, the water-lubricated friction coefficient of NBR continuously decreases. Under water-lubricated conditions, CNTs improve the water storage capacity of the NBR surface and enhance lubrication efficiency. In the dry wear state, CNTs help reduce scratch depth and dry wear volume.

The synergistic analysis of friction properties of carbon nanotube (CNT)-reinforced polymers at the nanoscale and macroscale can help to obtain the intrinsic mechanism of carbon nanotubes to reduce the friction coefficient of polymers, which is important to guide the modification of polymer friction properties. However, the huge gap in spatial scales makes it difficult for molecular dynamics simulations at the nanoscale to predict the friction coefficient of virtual contact interfaces, and conducting a large number of macroscopic experiments to obtain natural frictional laws could be more efficient. This study proposes a multi-scale model to investigate the frictional behavior of copper (Cu)-CNT/polytetrafluoroethylene (PTFE). By using the micromechanics Mori-Tanaka homogenization method as a bridge, the nanoscale simulations of the CNT/PTFE elasticity and frictional behaviour and the macroscopic finite element simulation of the Cu ring-CNT/PTFE block contact are coupled, thus integrating the nanoscale frictional laws of Cu-CNT/PTFE obtained from molecular dynamics simulations into the actual contact interface. The results of multi-scale friction simulations show that the filling of CNTs can effectively improve the elastic and frictional properties of the PTFE matrix, and the degree of improvement is related to the orientation and mass fraction of the CNTs. Under a normal load of 0.5 MPa and a rotating speed of 30 rpm, the friction coefficient continuously decreases (from 0.198 to 0.156) with increasing CNTs mass fraction (0%, 1.25%, 2.5%, 5%). The simulation results were verified by copper ring-CNT/PTFE block friction experiments.

Nitrile butadiene rubber (NBR) and its various composites are extensively studied as matrix materials for oil-free lubricated mechanical friction surfaces. While numerous macroscopic friction experiments can compare the frictional performance of different NBR composites, the complex material preparation and low experimental efficiency pose challenges. Nanoscale molecular dynamics (MD) simulations can help develop materials with improved friction properties, but they cannot directly provide the friction coefficients of actual macroscopic friction surfaces. Therefore, it is important to integrate the advantages of macroscopic and nanoscale friction studies and perform a synergistic analysis to modify the friction properties of composite materials. In this study, a multi-scale approach is proposed to simulate the frictional characteristics of carbon nanotube (CNT)-reinforced NBR by combining the MD friction simulation， micromechanical bridging model, and macroscopic finite element analysis (FEA) methods. The results of the multi-scale friction simulation of copper (Cu)-CNT/NBR composites show that the addition of CNTs significantly improves the frictional properties of NBR, and the extent of the improvement is related to the mass fraction of CNTs. As the mass fraction of CNTs increases (0%, 1.25%, 2.5%, 5%), the coefficient of friction decreases from 0.50 to 0.38. The results are validated by ring-block friction experiments at the same scale as the simulations.