Effects of Thickness and Particle Size on Tribological Properties of Graphene as Lubricant Additive

Abstract

Increasing studies have demonstrated excellent tribological properties of graphene as lubricant additive. Thus, extended explanations of the relationship between microstructure and tribological properties are needed. This paper reported the effects of thickness and particle size of graphene on its frictional performance, using a ball-on-plate tribotester under reciprocating condition. Graphene with few-layer (G2) or multi-layer (G10) structure was added to PAO4 as lubricant additive. The particle of G10 was further separated through centrifugation into two types, large size G10 (G10L) and small size G10 (G10S). G10S exhibits excellent reduction in friction (37%) and wear (47%) relative to the neat base oil, indicating the superior tribological properties of graphene with more layers and smaller particle size. Wear scar analysis shows that the surface lubricated with G10 is flatter and glossier than that of G2. It can be hypothesized that the interlaminar shear slip is more likely to occur in multi-layer graphene, leading to better friction reduction and wear resistance performance. Graphene with smaller particle size is not only less prone to structural defects and wrinkles, but also more easily adsorbed onto the sliding surface to form a lubricating film, providing enhanced wear and frictional performance. The present work suggests that the thickness and size may have direct influence on the tribological properties of graphene, providing theoretical and experimental guidance for the application of graphene as lubricant additive.

Carbon Nanoparticles of Varying Shapes as Additives in Mineral Oil Assessment of Comparative Performance Potential

“…Nearly ∼50% reduction in wear was due to 2 wt % MLG in group III base oil. A similar enhancement in AW performance (∼47%) has been reported by Kong et al 32 for MLG flakes added in PAO 4 base oil. They explored the effect of thickness and size of graphene sheets on the friction performance as an additive in PAO 4 base oil.…”

“…Some NPs, in particular, having layer-lattice structures transfer the beneficial nanofilm on the surfaces, while hard and spherical NPs work as nanobearings, thereby transforming sliding contact into rolling contact and thus reducing friction and wear. Also, most NPs are chemically inert and relatively less toxic and thus have less environmental implications than conventional additives such as ZDDP and have been reported to offer better fuel efficacy. , In order to cater to the need for new generation machine components mainly for high thermal efficiency, several types of NPs from carbon family-like C 60 (buckyballs), nanographite (NG), nanodiamond, carbon nanotubes (CNTs), graphene, and so forth have been explored. , Especially, since the inception of graphene in 2004 by Geim, various forms of graphene and its chemical derivatives, viz., graphene oxide (GO), reduced graphene oxide (rGO), and chemically modified graphene, have been extensively explored as lubricant additives. − Graphene nanoflakes refer to one single layer of atomic carbon (0.34 nm thickness) atoms arranged in a honeycomb 2-D hexagonal lattice structure. Although, in multilayer graphene (MLG), several graphene planes are weakly bonded together through van der Waal forces in the thickness direction .…”

Graphene Nanosheets as Lubricant Additives: Effects of Nature and Size on Lubricating Performance

“…Graphene shows remarkable tribological properties as solid lubricant and lubricant additives in lubricating oils, owing to its large surface area, weak interlayer interaction, excellent chemical stability, and versatile surface modification, as well as thermal conductivity and heat transfer performance. − Especially for the most widely used liquid lubricants, graphene-based additives have been widely reported for the aqueous or oil system, after atom doping or in the form of composites . It has been well-recognized that the tribological performance of the nanolubricants was mainly determined by the following four factors: the micromorphology, , number of layers, − size, − and surface nature of the graphene.…”

“…The smaller size makes the graphene-based lubricant additives easy to be dispersed in the lubricating oils with better stability . The smaller graphene-based lubricant additives also provide more edge-functionalization of polar groups, which is favorable for the formation of a lubricating film on the sliding surfaces, , but goes against the dispersibility in the widely used nonpolar lubricating oils (such as poly-α-olefin (PAO)) and low polar ones (esters). In addition, it has been reported that graphene nanosheets could be ground into smaller pieces during the friction process .…”

“…In addition, it has been reported that graphene nanosheets could be ground into smaller pieces during the friction process . Based on the reported works, , the question arises: why did the lubricating performance of the nanolubricant with larger graphene after friction differ from that with the smaller ones?…”