PSI - Issue 12

Francesca Curà et al. / Procedia Structural Integrity 12 (2018) 44–51 Author name / StructuralIntegrity Procedia 00 (2018) 000 – 000

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High friction between surfaces in contact may cause noisiness, malfunction and also low efficiency and low durability: therefore, reducing the coefficient of friction would increase both machine performance and reliability. To improve the effective functioning of mechanical components or surfaces in contact, better quality of lubricants is required. The addition of nanomaterials as additives (carbon nanotubes, fullerenes and graphite nanosheets) in base lubricants is a recent field of research. As an example, graphene has received a great interest by researchers in a wide field of applications. Referring to tribological aspects, graphene has been considered as an additive in lubricants, in order to reduce components friction and machine losses. Some papers are available in literature about graphene utilized as additive in solids or liquid lubricants. A very interesting review about the tribology of graphene is presented by Penkov et al. (2014); in particular, the advantages and limitations of using graphene in nano- and micro-tribological applications have been deeply discussed. Graphene, in a form of modified graphene platelets (MGP), has been used as a lubricant oil additive by Lin et al. (2011), showing an improving of oil performance in terms of reducing wear resistance. The reduction of friction and wear obtained using mineral oils formulated with graphene oxide (GO) nanosheets has been investigated by Senatore et al. (2013). Recent developments in graphene based nanofluids an nanolubricants have been reported by Rasheed et al. (2016), presenting an overview of the remarkable research progress. In particular, graphene flakes have been investigated worldwide as an additive for coolants and lubricants due to their excellent thermo-physical and tribological properties. Yang et al. (2017) described the excellent solubility of a liquid-like graphene in a wide spectrum of solvents for a long time. Some other authors, as Yua et al. (2014), Cheng et al. (2014), Song et al. (2011), Fan et al. (2015), studied similar phenomena exploring the possibility to reduce friction simply introducing graphene additives in classical lubricants. Despite the big potential of graphene in this field, its use as a lubricant or a lubricant additive on macro-meso scale remains relatively unexplored. In particular, the literature is lacking about specific applications of graphene added lubricants on mechanical systems. In this paper, the effect graphene added to standard lubricants to create high performance compounds has been investigated. Firstly, the Coefficient of Friction (CoF) of different compositions of lubricant-graphene has been experimentally evaluated. In particular, two commercial oils with different viscosity and a commercial grease have been chosen to be used as basic lubricants. Results in terms of Coefficient of Friction values of all compounds have been compared each other. Finally, characterization results of graphene added grease have been related to that available from previous studies performed by the authors on a mechanical component (a spline coupling), commonly used in many industrial applications, lubricated by the same graphene-grease compounds. Coefficient of Friction (CoF) of all lubricants has been evaluated using a Pin-on-Disk Tribometer (TRB - Anton Paar) for a selected sliding distance, normal load and a linear sliding velocity. The Pin-on-Disk apparatus measures instantaneous values of CoF during the tests and calculates the average value at the end of the test. Table 1 shows the chosen parameters for the Pin-on-Disk tests. Both Pin Specimen and Disk specimen are made in C40 steel. Base lubricants used in the tests are: two commercial oils with different viscosity (as reported in Table2), respectively identified as Oil 1 and oil 2, and a commercial grease. Oil 1 is the synthetic oil Mobil Jet Oil II, characterized by high thermal and chemical stability, able to provide a good protection against deterioration, deposit and foam formation; its use is mainly in the aeronautical field, due to the possibility to operate in extreme conditions (between -40°C and 204°C) and to dissipate significant amount of heat contributing to the cooling of components. Oil 2 is the synthetic oil Renolin Unisyn 320, characterized in turn by a high viscosity, a good thermal stability 2. Materials and methods