Vincenzo Petrone

Graphene Oxide Nanosheets as Effective Friction Modifier for Oil Lubricant: Materials, Methods, and Tribological Results

The tribological behaviour of graphene oxide nanosheets in mineral oil was investigated under a wide spectrum of conditions, from boundary and mixed lubrication to elastohydrodynamic regimes. A ball-on-disc setup tribometer has been used to verify the friction reduction due to nanosheets prepared by a modified Hummers method and dispersed in mineral oil. Their good friction and antiwear properties may possibly be attributed to their small structure and extremely thin laminated structure, which offer lower shear stress and prevent interaction between metal interfaces. Furthermore, the results clearly prove that graphene platelets in oil easily form protective film to prevent the direct contact between steel surfaces and, thereby, improve the frictional behaviour of the base oil. This evidence is also related to the frictional coefficient trend in boundary regime.

New 'chimie douce' approach to the synthesis of hybrid nanosheets of MoS2 on CNT and their anti-friction and anti-wear properties.

Hybrid organic-inorganic oleylamine@MoS2-CNT nanocomposites with different compositions were obtained by thermal decomposition of tetrathiomolybdate in the presence of oleylamine and high quality multiwalled carbon nanotubes (CNTs) previously prepared by the CCVD technique. The nanocomposite samples were characterized by the TEM, SEM TG-MS, Raman and XRD techniques and successfully tested as anti-friction and anti-wear additives for grease lubricants.

Effects of the Lubricant Piezo-Viscous Properties on EHL Line and Point Contact Problems

This paper presents the application of the free-volume viscosity model in a Newtonian elastohydrodynamic line and point contact simulation using a more effective multigrid approach. According to recent experimental studies using high pressure viscometers, the free volume-based pressure–viscosity relationship closely represents the realistic piezo-viscous behavior for the high pressure typically encountered in elastohydrodynamic applications [1]. The effects of different pressure–viscosity relationships, including the exponential model, the Roelands model, and the free-volume model are investigated through an example with poly-alpha-olefin lubricant. It is found that the real pressure–viscosity behavior predicted by the free-volume model yields a higher viscosity at the low-pressure area, which results in a larger central film thickness. The fact that film thickness is formed mainly by the entraining action at the inlet area significantly weighs the importance of viscosity variation from different models in this area. The inlet area is a low-pressure area, and accordingly, the real viscosity of the lubricant predicted by Doolittle model undergoes a rapid increase in a convex function, being apparently larger than the Roelands one. Furthermore, the Doolittle model leads to higher pressure spike amplitude than that observed using the Roelands model. To solve the problem, a full multigrid approach has been used upon the assumptions of isothermal condition. Multigrid is more effective because it uses coarser grid levels to remove errors of different frequencies, which could be more quickly smoothed away than those on simply the fine grid alone. The developed coarse grid correction cycle proves to be an efficient tool to solve the EHL problem for a wide range of load conditions.

Effect of an Improved Yasutomi Pressure-Viscosity Relationship on the Elastohydrodynamic Line Contact Problem

This paper presents the application of an improved Yasutomi correlation for lubricant viscosity at high pressure in a Newtonian elastohydrodynamic line contact simulation. According to recent experimental studies using high pressure viscometers, the Yasutomi pressure-viscosity relationship derived from the free-volume model closely represents the real lubricant piezoviscous behavior for the high pressure typically encountered in elastohydrodynamic applications. However, the original Yasutomi correlation suffers from the appearance of a zero in the function describing the pressure dependence of the relative free volume thermal expansivity. In order to overcome this drawback, a new formulation of the Yasutomi relation was recently developed by Bair et al. This new function removes these concerns and provides improved precision without the need for an equation of state. Numerical simulations have been performed using the improved Yasutomi model to predict the lubricant pressure-viscosity, the pressure distribution, and the film thickness behavior in a Newtonian EHL simulation of a squalane-lubricated line contact. This work also shows that this model yields a higher viscosity at the low-pressure area, which results in a larger central film thickness compared with the previous piezoviscous relations.

The Influence of the Roughness Parameters on the EHL Line Contact Using the Free Volume Model

A numerical solution of elastohydrodynamic lubrication (EHL) contact between two rough surface cylinders is presented. In the theoretical approach the free-volume viscosity model is used to describe the piezo-viscous behavior of the lubricant in a Newtonian Elastohydrodynamic line contact [1,2]. Random rough surfaces with Gaussian and exponential statistics have been generated using a method outlined by Garcia and Stoll [3], where an uncorrelated distribution of surface points using a random number generator is convolved with a Gaussian filter to achieve correlation. This convolution is most efficiently performed using the discrete Fast Fourier Transform (FFT) algorithm, which in MATLAB is based on the FFTW library [4]. The maximum pressure and average film thickness are studied at different values of RMS, skewness, kurtosis, autocorrelation function and correlation length. Numerical examples show that skewness and kurtosis have a great effect on the parameters of EHD lubrication. Surface roughness, indeed, tends to reduce the minimum film thickness and it produces pressure fluctuations inside the conjunction which tend to increase the maximum stress. In this way the dynamic stress increases and tends to reduce the fatigue life of the components.It can be seen that the pressures developed in the fluid film in the case of rough surfaces fluctuate with the same frequency of the surface roughness. These pressure ripples correspond to the asperity peaks. This indicates that surface roughness causes very high local contact pressures which may lead to local thinning of the film. A significant reduction has been also observed in the minimum film thickness due to surface roughness.Copyright

The Influence of Piezo-Viscous Lubricants on EHL Line and Point Contact Problems

A good and accurate prediction of the elastohydrodynamic lubrication behaviour requires consideration of the constitutive equation for the lubricant. In particular, for applications involving synthetic oils or mineral oil with polymeric additives that exhibit shear-thinning behaviour, the use of an appropriate pressure-viscosity relationship for the lubricant is required to predict the EHL behaviour more accurately [1–3]. For this reason, this paper aims to emphasize the importance of implementing piezo-viscous models with accurate treatment methods in EHL applications. Due to the high pressure in an EHL contact, in fact, the elastic deformation of the surfaces and pressure dependence of viscosity play the pivotal role and in many applications, the lubricant exhibits a shear-thinning behaviour which significantly affects the film thickness [4–6]. The effects of different pressure–viscosity relationships, including the exponential model, the Roelands’ model and specifically, the Doolittle model are investigated and a generalized formulation that can efficiently treat shear-thinning fluids with provision for compressibility in the EHL contact is presented.In the light of above facts, models for 1D and 2D EHL contacts for simulating the behaviour of the pressure distribution and the shape of the film thickness using a generalized Reynolds equation and shear-thinning fluids is developed. In particular for EHL 2D problem a more accurate full multigrid approach has been used and both the analysis is based upon the assumptions of isothermal condition. In this work, in fact, we show that the piezo-viscous rheology of the lubricant plays an important role in determining the value of pressure peaks. Pressure profiles and film shapes are showed and variations of the minimum and central film thickness with dimensionless parameters are also presented. It is found that the real pressure–viscosity behaviour predicted by the free-volume model yields a higher viscosity at the low-pressure area which results in a larger central film thickness. Therefore, due to use of the free-volume model, the presented results are more consistent with literature experimental observations and the Doolittle model effectively predicts the film thickness that closely matches experiments and properly characterizes the behaviour of shear-thinning lubricants.Copyright