Tribological Behavior And Mechanism Of NBR-based Water-lubricated Bearings Materials

As important support components for propeller shafts and tail shafts on surface ships and underwater vehicles,tail bearings(water-lubricated bearings)are essential for the stable output of propeller power and the safe navigation of ships.But due to the low viscosity of water,the bearing capacity of water-lubricated bearings in service is much lower than that of oil-lubricated bearings,which is easy to cause direct contact between water-lubricated bearings and tail shafts,and lead to rapid wear of bearing materials with vibration and noise,thus reducing the service life of the bearings,and increasing maintenance costs.Therefore,it has been a hotspot in domestic and overseas research and a key issue in engineering applications to investigate the properties and tribological mechanism of water-lubricated tail bearings.The investigations are academically significant and practically valuable not only for developing and designing high-performance water-lubricated rubber bearings,improving the service life of the bearings,but also for ensuring the navigation safety of ships.The present study investigates the tribological behavior and tribological mechanism of nitrile rubber(NBR)-based bearing materials under water lubrication to solve the problem of rapid wear of water-lubricated rubber bearings due to poor lubrication.NBR as the matrix material was filled with silica of different sizes and with carbon nanomaterials of different dimensions(two-dimensional graphene and one-dimensional carbon nanotubes)to prepare NBR-based composites with the method of emulsion blending and mechanical blending.The Rtec multi-functional friction and wear testing machine(in the ring-block mode)bench test and the molecular dynamics simulation method were also employed to study the effects of particle size,interfacial interaction between particles and the NBR matrix,and particle dimension on the tribological behavior of water-lubricated NBR.The main research contents are as follows.Firstly,for the poor dispersion of nano-silica in NBR,the surface of nano-silica was modified by silane coupling agents to improve its dispersion in the matrix material and enhance its interfacial interaction with the matrix.The effects of particle size and interaction between particles and the matrix on the mechanical properties and tribological behavior of water-lubricated NBR were then studied.It was showed that the particle size of silica significantly affected its dispersion in NBR;the enhanced interaction between nano-silica and the matrix notably improved the wear performance and increased the friction coefficient of NBR-based composites.Different interfacial interaction between particles and the matrix material led to different wear mechanisms of NBR composites.Secondly,molecular models of NBR filled with nano-silica and modified nano-silica were constructed respectively with the molecular dynamics simulation method to explore how different interfacial interaction could affect the mechanical properties and tribological behavior of NBR from the molecular scale.The relevant parameters under observation included the rotation radius of NBR molecular chains,the speed of NBR atoms,the temperature at the friction interface,the concentration distribution of NBR atoms,the concentration of carbon atoms around the copper bar,the binding energy between the copper bar and NBR molecular chains,and van der Waals in NBR.It was found that the increase of the binding energy between silica particles and NBR molecular chains could restrain the speed of NBR atoms,decrease the temperature peaks at the friction interface,enhance rigidity of NBR molecular chains and improve concentration distribution of NBR atoms.All these were conducive to the decreased atomic wear rate of NBR and to the significant enhancement of NBR’s wear performance.Finally,the present study probed into whether the dimension of carbon nanomaterials could affect the tribological behavior of NBR composite and how graphene and carbon nanotubes could affect the mechanical properties,hydrophilic/hydrophobic properties and tribological behavior of NBR.The results indicated that both carbon nanomaterials improved the friction coefficient of NBR and its wear performance.Graphene with a two-dimensional sheet structure formed greater interfacial interactions with the NBR matrix,but it was more likely to detach from the NBR matrix in the wear test.One-dimensional carbon nanotubes on the other hand had a great length-to-diameter ratio,which promoted the “winding” of NBR molecular chains and better “trapped” the chains during the wear process,thereby preventing the detachment of carbon nanotubes from the matrix and significantly reducing the wear amount of the composite.Therefore,graphene could better improve the friction coefficient of NBR while carbon nanotubes exerted a better influence on the wear properties of NBR.This thesis concluded that the enhanced tribological behavior of water-lubricated NBR was related to the particle size,the interfacial interaction between particles and the matrix material,and the structural dimension of particles.Nanoparticles and their strong interfacial interaction with the NBR matrix were key factors for enhancing the wear performance of water-lubricated NBR bearings.Meanwhile,the dimension of nanoparticles played a better role than the interfacial interaction in enhancing the wear properties of NBR.