Tribological properties of graphene-reinforced polyurethane bearing material

For water-lubricated stern tube bearing in ships, bearing material with low friction can prevent severe damage on the shaft surface when lubrication failure occurs, and hence improve operation reliability of the whole propeller system. In this work, 0.25-5 wt% of solid lubricant graphene platelets were blended with commonly used bearing matrix thermoplastic polyurethane (TPU) via melting injection, and tribological performance of TPU composites were investigated at a set of applied loads and sliding speeds which mimics the actual operating conditions. Graphene inclusion at a low content (< 0.5 wt%) reduced friction and wear compared to neat TPU, while maintaining mechanical properties. Graphene-induced tribofilm is formed and it is believed to offer low shear strength at the sliding surfaces while also regulating the size of wear particles. Since more platelets are exfoliated due to the greater extent of surface damage under high loads, it presents a pressure-dependent anti-friction ability. Higher concentration of graphene led to further friction reduction but deteriorated both the strength and ductility of materials due to the agglomeration. Results from Raman spectroscopies show that graphene-reinforced TPU composites underwent no chemical changes but mechanical deformation during rubbing, and this resulted in wear particles and tribofilm, containing graphene platelets with modified structure.