What is Oil Efficiency?
In the average car, friction occurs between the moving parts of the engine and within the gearbox. This wastes about 15% of the fuel used by the vehicle, amounting to millions of tons of fossil fuel oil. This is especially so during engine startup or when driving at low speeds, because then the film of lubricant is very thin, and significant metal-metal contact does occur.
Oil efficiency is achieved by minimizing this wastage of fuel caused by the production of heat and other useless forms of energy. Among many efforts to improve the efficiency of lubricant oil in the car engine, nanomaterials have been experimented with because of their small size, which lets them slide into tiny spaces and lubricate them against direct grating friction.
This is the field of nanolubricants, which means lubricants that have had nanoparticles added to them to improve their performance. However, nanoadditive particles show a tendency to clump together, and this not only reduces their efficiency but causes jamming of surfaces because the particle agglomerates tend to be trapped between them. In such cases, the use of nanolubricants produces greater rather than less friction and wear on the machinery.
Different approaches have been adopted to solve the problem of clumping, including the addition of surfactants to reduce the surface tension between the nanoadditives. These chemicals can, however, wear off with friction, and eventually decompose, causing recurrence of the same phenomenon.
Novel Approach – Crumpled Graphene Balls
One team of scientists used crumpled graphene instead. This refers to nanoparticles made of graphene, in the shape of crumpled paper balls. They observed the effects of such addition to the tribologic properties of the oil.
What are crumpled graphene balls? Crumpled graphene balls are the result of evaporating water from miniscule droplets which contain sheets of graphene inside them. The drying out process causes capillary forces which causes the sheets to form crumpled balls.
The effectiveness of these balls is due to their rolling action, which is similar to nanoscale ball bearings. Moreover, their unique shape lends them self-dispersive characteristics, since they cannot easily form close aggregations (“stable against graphitization”) due to the numerous vertices and angles formed by the crumples. This ensures the van der Waals attractions between them are weak. This obviates the need to add chemical functional groups to enhance dispersion, which is already high, unlike graphite platelets, reduced graphene oxide sheets and carbon black which all aggregate more readily.
Even when compressive forces are exerted on these crumpled graphene balls, they easily disperse again once the containing liquid is stirred up. They are stable both as solids and when added to liquids, refusing to collapse or to unfold into flat graphene sheets even when they are pelletized or heated. This resistance to deformation or damage is because of strain-hardening, which means that compression causes it to form more folds than before. This makes them stiffer with compression, while retaining the crumpled shape.
Do Crumpled Graphene Balls in Lubricants Have Other Benefits?
Another advantage is the high free volume and the amount of surface area within the crumpled graphene balls that is accessible to solvent. This property is exploited to absorb oil into the crevices, so that it can be released for future use when the balls are compressed. This is meant to ensure that the lubricated surfaces in contact remain wetted all the time.
The addition of crumpled graphene balls to lubricant oils causes a 15% fall in friction and the same degree of reduction in metal surface degradation. Even better, a low concentration of these crumpled graphene balls, at 0.01-0.1%, is sufficient to confer these benefits, since their effects are not proportional to their concentration.
In future, the irregular surface of crumpled graphene balls and the resulting high surface area could lead to their use as carriers for other useful additives as well, such as materials that inhibit corrosion, adding still more value to the oil and improving its efficiency further.
The use of crumpled graphene balls at 0.1 weight % as an additive to lubricant oil allows an 85% improvement in wear reduction, protecting the surfaces against the formation of wear tracks and reducing friction by about 20% because of their resistance to agglomeration. The concentration of crumpled graphene balls is also not sensitively related to their effect on the tribological properties of the base oil because of their high self-dispersion characteristics.