Effect of expanded graphite (EG) dispersion on the mechanical and tribological properties of nitrile rubber/EG composites

In this work, in order to prepare expanded graphite (EG) and acrylonitrile-butadiene rubber (NBR) composites, two preparation methods were adopted, namely directly mechanical blending (micro-composites) and latex compounding technique (nanocomposites), and their resulting mechanical and tribological properties were compared. Meanwhile, the effects of the graphite dispersion state, volume fraction, load and sliding velocity on the friction and wear properties were also explored. As expected, nanocomposite showed better tensile properties, hardness, dynamic storage modulus and lower hysteresis loss peak. The coefficient of friction (COF) for both composites decreased as the load and sliding velocity increased, while the effect of the graphite volume fraction was subtle. The wear rate of nanocomposites decreased as the load and sliding velocity increased. As the load increased, the wear rate of micro-composites increased, but with increasing sliding velocity, the wear rate first increased, reached a maximum, and then decreased. The wear mechanism could be described as graphite sheets de-bonded from matrix, lubricant film formation and breaking, matrix wear, and lubricant film re-formation in cycles. Compared with micro dispersion, the nano-sized dispersion of graphite was beneficial to the formation of a continuous, uniform, and stable lubricant film under high sliding velocity and heavy load, contributing to reduce the COF and improve the wear resistance.