| Waterborne polyurethane(WPU)has gradually become the focus of research due to its low pollution and no volatile organic solvents.In addition,non-toxic,low cost and high gloss are the advantages of waterborne polyurethane.Graphene is a two-dimensional(2D)material with atomically thin layers of carbon atoms arranged in a hexagonal shape.However,graphene sheets are prone to agglomeration due to their large specific surface area.In order to study the tribology of the composite coatings in a magnetic field environment,it is necessary to magnetize graphene to improve its dispersion stability in the WPU and the tribological properties of the composite coatings.Many researchers have added graphene as a nano additive phase to various polymer materials,hoping to obtain graphene/polymer composite materials with excellent tribological properties.However,the lubrication mechanism of organic composite coatings in a magnetic field has not been studied yet.In this study,the ferromagnetism of Fe3O4 was first used to magnetize GO to obtain magnetized graphene oxide(MGO),using X-ray diffractometer(XRD),infrared spectrometer(FT-IR),and transmission electron microscope(TEM)and vibrating sample magnetometer(VSM)are used to characterize and analyze MGO nanocomposites.Then different content of MGO was added to WPU to prepare MGO/WPU composite coating.Finally,the tribological test of the MGO/WPU composite coating in different magnetic field strength environments was carried out.The scanning electron microscope(SEM)was used to observe the micro morphology of the wear scar and the transfer film,and it was explained that the magnetic field improves the wear resistance of the composite coating.The frictional energy system model in a magnetic field environment is proposed.(1)Fe3O4 was successfully loaded onto the GO surface,and MGO is ferromagnetic.The MGO surface introduces a large number of functional groups,which reduces the intermolecular force and obviously improves the dispersion of GO in WPU.(2)MGO has a certain anti-friction and wear-resisting effect.In a non-magnetic field environment,the MGO/WPU composite coating with a MGO content of 0.5wt%has the best antifriction and wear resistance,with the friction coefficient and wear rate being0.121 and 0.5×10-4mm~3/N·m,respectively.When the content of MGO exceeds 0.5wt%,MGO has a tendency to agglomerate in the matrix and the friction performance becomes worse,but it is still better than pure WPU coating.(3)The main mechanism for MGO to improve the tribological properties of composite coatings is that the interlayer spacing increases after GO is magnetized,which reduces the intermolecular forces between MGOs.At the same time,the surface of MGO has a large number of functional groups,which enhances the bonding force of interaction between nano additives and waterborne polyurethane.The relative sliding between GO sheets and the effect of Fe3O4 transforming sliding friction into rolling friction improve the anti-friction performance.On the other hand,the evenly dispersed MGO can prevent the expansion of microcracks in the coating and promote the generation of an effective transfer film,preventing the coating from peeling off under external force,thereby reducing the friction property of the composite coatings and improving the wear-resistance of the composite coatings.(4)The addition of magnetic field can further improve the tribological properties of the MGO/WPU composite coatings.In a 40m T magnetic field,when the addition amount of MGO is 0.5wt%,the anti-friction and wear resistance of the composite coating is the best,and the friction coefficient and wear rate are 0.094 and 0.24×10-4mm~3/N·m,respectively.Compared with the composite coating under the 0m T magnetic field,the wear rate is reduced by 55%.(5)The main mechanism for the magnetic field to improve the tribological performance of the organic composite coating is that the thermal effect of the magnetic field prevents the generation of microcracks,and the polymer is softened and filled in the depressions on the surface to participate in the formation of the transfer film.By analyzing the microscopic morphology of the transfer film,it can be found that the formation of a stable transfer film is a dynamic equilibrium process.The force exerted by the magnetic field on the magnetic transfer film increases the binding force between the transfer film and the counterpart,so the polymer is like a thin flexible film covering the wear surface evenly.(6)This study proposes a model of frictional energy system in a magnetic field environment.Adding MGO to WPU can increase the critical value of the energy system of the composite coating;the application of a magnetic field can further increase the critical value of the energy system of the composite coating,thereby reducing energy loss. |
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