Comparison between thermophysical and tribological properties of two engine lubricant additives: electrochemically exfoliated graphene and molybdenum disulfide nanoplatelets.

Recently graphene and other 2D materials were suggested as nano additives to enhance the performance of nanolubricants and reducing friction and wear-related failures in moving mechanical parts. Nevertheless, to our knowledge there are no previous studies on electrochemical exfoliated nanomaterials as lubricant additives. In this work, engine oil-based nanolubricants were developed via two-steps method using two different 2D nanomaterials: a carbon-based nano additive, graphene nanoplatelets (GNP) and a sulphide nanomaterial, molybdenum disulfide (MoS2) nanoplatelets (MSNP). The influence of these nano additives on the thermophysical properties of the nanolubricants, such as viscosity index, density and wettability, was investigated. The unique features of the electrochemical exfoliated GNP and MSNP allow the formulation of nanolubricant with unusual thermophysical properties. Both the viscosity and density of the nanolubricants decreased by increasing the nanoplatelets loading. The effect of the nano additives loading and temperature on the tribological properties of nanolubricants was investigated using two different test configurations: reciprocating ball-on-plate and rotational ball-on-three-pins. The tribological specimens were analysed by scanning electron microscopy (SEM) and 3D profiler in order to evaluate the wear. The results showed significant improvement in the antifriction and anti-wear properties, for the 2D-materials-based nanolubricants as compared with the engine oil, using different contact conditions. For the reciprocal friction tests, maximum friction and worn area reductions of 20% and 22% were achieved for the concentrations of 0.10 wt% and 0.20 wt% GNP, respectively. Besides, the best anti-wear performance was found for the nanolubricant containing 0.05 wt% MSNP in rotational configuration test, with reductions of 42% and 60% in the scar width and depth, respectively, with respect to the engine oil.

Tribological Performance of a Paraffinic Base Oil Additive with Coated and Uncoated SiO2 Nanoparticles.

Electric vehicles (EVs) have emerged as a technology that can replace internal combustion vehicles and reduce greenhouse gas emissions. Therefore, it is necessary to develop novel low-viscosity lubricants that can serve as potential transmission fluids for electric vehicles. Thus, this work analyzes the influence of both SiO2 and SiO2-SA (coated with stearic acid) nanomaterials on the tribological behavior of a paraffinic base oil with an ISO VG viscosity grade of 32 and a 133 viscosity index. A traditional two-step process through ultrasonic agitation was utilized to formulate eight nanolubricants of paraffinic oil + SiO2 and paraffinic base oil + SiO2-SA with nanopowder mass concentrations ranging from 0.15 wt% to 0.60 wt%. Visual control was utilized to investigate the stability of the nanolubricants. An experimental study of different properties (viscosity, viscosity index, density, friction coefficient, and wear) was performed. Friction analyses were carried out in pure sliding contacts at 393.15 K, and a 3D optical profilometer was used to quantify the wear. The friction results showed that, for the SiO2-SA nanolubricants, the friction coefficients were much lower than those obtained with the neat paraffinic base oil. The optimal nanoparticle mass concentration was 0.60 wt% SiO2-SA, with which the friction coefficient decreased by around 43%. Regarding wear, the greatest decreases in width, depth, and area were also found with the addition of 0.60 wt% SiO2-SA; thus, reductions of 21, 22, and 54% were obtained, respectively, compared with the neat paraffinic base oil.

Performance and Antiwear Mechanism of 1D and 2D Nanoparticles as Additives in a Polyalphaolefin.

This work is focused on the thermophysical and tribological study of eight nanolubricant compositions based on a polyalphaolefin (PAO 20) and two different nanoadditives: multi-walled carbon nanotubes (MWCNTs) and hexagonal boron nitride (h-BN). Regarding the thermophysical properties, density and dynamic viscosity of the base oil and the nanolubricants were measured in the range of 278.15-373.15 K, as well as their viscosity index, with the aim of evaluating the variation of these properties with the addition of the nanoadditives. On the other hand, their lubricant properties, such as contact angle, coefficient of friction, and wear surface, were determined to analyze the influence of the nanoadditives on the tribological performance of the base oil. The results showed that MWCNTs and h-BN nanoadditives improved the wear area by 29% and 37%, respectively, at a 0.05 wt% concentration. The density and dynamic viscosity increased compared with the base oil as the nanoadditive concentration increased. The addition of MWCNTs and h-BN nanoparticles enhanced the tribological properties of PAO 20 base oil.

Density scaling of the transport properties of molecular and ionic liquids.

Casalini and Roland [Phys. Rev. E 69, 062501 (2004); J. Non-Cryst. Solids 353, 3936 (2007)] and other authors have found that both the dielectric relaxation times and the viscosity, η, of liquids can be expressed solely as functions of the group (TV (γ)), where T is the temperature, V is the molar volume, and γ a state-independent scaling exponent. Here we report scaling exponents γ, for the viscosities of 46 compounds, including 11 ionic liquids. A generalization of this thermodynamic scaling to other transport properties, namely, the self-diffusion coefficients for ionic and molecular liquids and the electrical conductivity for ionic liquids is examined. Scaling exponents, γ, for the electrical conductivities of six ionic liquids for which viscosity data are available, are found to be quite close to those obtained from viscosities. Using the scaling exponents obtained from viscosities it was possible to correlate molar conductivity over broad ranges of temperature and pressure. However, application of the same procedures to the self-diffusion coefficients, D, of six ionic and 13 molecular liquids leads to superpositioning of poorer quality, as the scaling yields different exponents from those obtained with viscosities and, in the case of the ionic liquids, slightly different values for the anion and the cation. This situation can be improved by using the ratio (D∕T), consistent with the Stokes-Einstein relation, yielding γ values closer to those of viscosity.

Relationship between viscosity coefficients and volumetric properties using a scaling concept for molecular and ionic liquids.

In this work, a scaling concept based on relaxation theories of the liquid state was combined with a relation previously proposed by the authors to provide a general framework describing the dependency of viscosity on pressure and temperature. Namely, the viscosity-pressure coefficient (partial differentialeta/partial differentialp)T was expressed in terms of a state-independent scaling exponent, gamma. This scaling factor was determined empirically from viscosity versus Tvgamma curves. New equations for the pressure- and temperature-viscosity coefficients were derived, which are of considerable technological interest when searching for appropriate lubricants for elastohydrodynamic lubrication. These relations can be applied over a broad range of thermodynamic conditions. The fluids considered in the present study are linear alkanes, pentaerythritol ester lubricants, polar liquids, associated fluids, and several ionic liquids, compounds selected to represent molecules of different sizes and with diverse intermolecular interactions. The values of the gamma exponent determined for the fluids analyzed in this work range from 1.45 for ethanol to 13 for n-hexane. In general, the pressure-viscosity derivative is well-reproduced with the values obtained for the scaling coefficient. Furthermore, the effects of volume and temperature on viscosity can be quantified from the ratio of the isochoric activation energy to the isobaric activation energy, Ev/Ep. The values of gamma and of the ratio Ev/Ep allow a classification of the compounds according to the effects of density and temperature on the behavior of the viscosity.

PEG 400-Based Phase Change Materials Nano-Enhanced with Functionalized Graphene Nanoplatelets.

This study presents new Nano-enhanced Phase Change Materials, NePCMs, formulated as dispersions of functionalized graphene nanoplatelets in a poly(ethylene glycol) with a mass-average molecular mass of 400 g·mol-1 for possible use in Thermal Energy Storage. Morphology, functionalization, purity, molecular mass and thermal stability of the graphene nanomaterial and/or the poly(ethylene glycol) were characterized. Design parameters of NePCMs were defined on the basis of a temporal stability study of nanoplatelet dispersions using dynamic light scattering. Influence of graphene loading on solid-liquid phase change transition temperature, latent heat of fusion, isobaric heat capacity, thermal conductivity, density, isobaric thermal expansivity, thermal diffusivity and dynamic viscosity were also investigated for designed dispersions. Graphene nanoplatelet loading leads to thermal conductivity enhancements up to 23% while the crystallization temperature reduces up to in 4 K. Finally, the heat storage capacities of base fluid and new designed NePCMs were examined by means of the thermophysical properties through Stefan and Rayleigh numbers. Functionalized graphene nanoplatelets leads to a slight increase in the Stefan number.