| Being one of most-recently found nanostructures in 2D dimension,Graphene is intrinsically featured of nanoscaled thickness,high aspect ratio,and easily sliding layer-structure,along with its superior properties of mechanical strength,toughness,chemical stability,conductivity of electron and heat,etc.Due to all these structure and performances,graphene is generally esteemed as the most perfect candidate for solid lubrication,or otherwise,is suitable for the utilization of high-performance additive for lubricant oil to improve tribological properties.On the other hand,graphene is also prone to severe aggregation in liquid matrix because of its enormous surface energy,which inevitably makes a negative effect to the dispersibility and suspension stability of graphene in lubricant oil,and lowers the corresponding properties in friction elimination and loss reduction.It is also unclear up to now the influence of the preparation method upon the tribological properties of graphene.In order to resolve the problems to some extent mentioned above,the graphene prepared by a method of Inserting Catalytic Expansion(ICE)was systematic investigated in this work to determine its suspensible dispersibility and tribological properties,which were compared with that of graphene oxide(GO),reduced graphene oxide(rGO).On the base of microstructural characterizations,the dispersibility and suspension stability of graphene is base oil was measured and assessed to evaluate the contribution of pre-treatment of graphene including particle size segregation,surface modification and so on.Thereafter,a method of four-ball friction was used to characterize and compare the tribological properties of graphene-improved lubricant oil in friction coefficient and size of worn scar.Microstructural characterizations showed that ICE graphene is composed of the nanoscaled sheets with a lay number of 5～10,a lateral dimension of 1～5 μm,and the layer structure in high graphitization.The atomic ratio of C/O is more than 20 in ICE graphene,of which the impurity is mainly iron chloride.On the other hand,it is also found that ICE graphene is very high in specific surface area and aspect ratio,and thus is easy to generate the aggregates in a size of several decades of micrometers.In this work,ICE graphene was preliminarily treated by using a mechanical crushing method to obtain the specimen with uniform particle size,following by a measurement by sedimentation balance to reveal the deposition characterization of graphene.The ICE graphene having been graded in particle size was separated and dried,and then employed to prepare ICE graphene-improved base oil by a combined process of electrically-drived stirring,ultrasonic treatment,while oleic acid in desired concentration was used as the dispersion additive to realize high dispersibility and excellent suspension stability of ICE Graphene in base oil.In a test of suspensible dispersion for 30 days,the graphene-improved base oil generally remained excellent dispersibility.As an effective dispersion agent,oleic acid(OE,C18H34O2)was used to realize the surface modification of ICE graphene,which was subsequently helpful to restrict the aggregation and deposition of graphene particles in base oil,to obtained much more excellent dispersibility and long-term suspension,and to make a great positive contribution to the friction elimination and loss reduction of graphene-improved base oil.In order to explore the effect of OE dispersant onto graphene-improved base oil,ICE graphene was used to prepare graphene-improved base oil with a concentration of 0.1 mg-ml-1,in which OE dispersant was added with a volume content of 3%-15%.Tribological properties of the as-prepared graphene-improved base oil were investigated by a four-ball method.The results showed that the friction coefficient of graphene-improved base oil decreased apparently with the increasing of OE content,giving rise of a minimum value of friction coefficient with a OE content of 10%,which is 57.36%lower than that of the base oil without the addition of OE dispersant.But further increasing of OE content resulted in the increase of friction coefficient.On the other hand,the addition of OE dispersant was useful to reduce the friction loss of base oil,and as a result,on the condition of 10%OE content,the size of corresponding worn scar decreased-from 737.21 μm to 423.23μm,suggesting an improvement up to 42.59%.All the experimental results mentioned above suggests that with the combination of particle size separation and selected dispersant in optimized content,ICE graphene can reaches to well dispersion and long-term suspension in base oil,as well as an apparent improvement to the friction elimination and abrasion resistance.So ICE graphene is capable of being employed as the novel nanoscaled additive to outstandingly improve the comprehensive performances of lubricant products,which must have a great prospective in business. |
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