Study On Freeze-drying Graphene/Copper Nano-additives And Its Friction Property

Both graphene and nano-copper can improve the friction property of lubricating oil as nano-additives,but nano-additives generally have the problem of high surface energy and prone to agglomeration.The wrinkles on the surface of graphene,which can load nano-particles and play a synergistic effect.In this thesis,it is intends to attach nano-copper particles to the graphene sheet to restrain its agglomeration,and exert its synergistic anti-frictional and anti-worn effects.Since freeze-dried nano-particles have the advantages of uniform particle size and improved agglomeration.If graphene and copper are compounded during the preparation in the precursor phase,graphene-based copper can be prepared by one-step method,which is expected to improve its agglomeration and the uniformity of dispersion,further the friction property of lubricating oil can be improved,the specific is as follows:The precursor solution of graphene was obtained by reduction of graphene oxide with hydrazine hydrate as reducing agent,then graphene was prepared by freeze-drying method and the microscopic morphology and phase composition of the powder were characterized.The results show that the graphene prepared by the freeze-drying method has a lateral size of 2-3?m,a thickness of 3.7 nm,the number of graphene layers is 4-7 and no hard agglomeration.Hydrazine hydrate was used to reduce graphene oxide and copper sulfate pentahydrate by one-step method to obtain a precursor solution of graphene-based copper,and then graphene-based copper was prepared by freeze-drying,and the microscopic morphology and substance of the powder phase were compared to be characterized.In order to explore the influence of technological parameters on the particle size of graphene-based copper,orthogonal experiments with four factors and three levels were carried out.Through the analysis of the experimental results,the order of the influence of each factor was:freezing method>reductant addition>reaction temperature>material thickness.Meanwhile,a better preparation process of graphene-based copper was obtained.The results show that the lateral size of the freeze-dried graphene-based copper is about 0.7-4.4?m,and the thickness is about 2.84-3.47 nm,the number of layer is 3-6,the thickness and the number of layers are less than that of the graphene prepared,respectively,and the copper particles are uniformly dispersed and attached to the graphene sheet,indicating that the preparation of graphene-based copper by freeze-drying process can uniformly attach nano-copper to graphene sheets,and reduce agglomeration and stacking for graphene.The dispersion stability and friction properties of freeze-dried graphene and graphene-based copper were studied,which are found that the freeze-dried graphene and graphene-based copper have better dispersion stability than commercially available graphene.The preferred concentration of graphene and graphene-based copper are 0.05wt%and 0.1 Owt%,respectively.At a better additive concentration,the average friction coefficient of the graphene-based copper oil sample is 23.1%and 6.1%lower than that of the base oil and the graphene oil sample,respectively,and the wear scar width is reduced by 62.3%and 55.3%,respectively.It has a better lubricating effect than graphene single agent,which is mainly due to the synergistic effect of graphene-based copper.Through friction experiments under different working conditions,it is found that single Graphene is suitable for medium-load and high-speed working conditions,and graphene-based copper can better exert its anti-friction and anti-wear performance under heavy-load and high-speed working conditions.In order to study the anti-friction and anti-wear mechanisms of nano-scale graphene/copper,molecular dynamics theory was used,the models of copper,graphene and graphene-based copper nanofluid were established respectively to simulate the friction behavior of the nanoparticle in the basic fluid.The results show that the movement of nano-copper particles between the friction interfaces has both translation and rotation,which can play a self-healing and ball bearing effects;graphene can be adsorbed on the sliding surface of the friction pair to form a deposited film;graphene-based copper is suitable to be applied under heavy load.It is easier to form a protective film on the surface of the friction pair,which has a synergistic effect with the repairing effect for the nano-copper particles and the ball bearing effect to improve the lubrication performance of the base fluid.The results show that graphene-based copper play a better friction-reducing and anti-wear roles under heavy load conditions,which are consistent of the experimental results.