Simulating the lubrication behavior of glycerol - A green lubricant
||Simulating the lubrication behavior of glycerol - A green lubricant|
||SNIC Medium Compute|
||Roland Larsson <Roland.Larsson@ltu.se>|
||Luleå tekniska universitet|
||2020-11-01 – 2021-11-01|
||20308 10403 10407|
The importance of reducing friction to improve the efficiency of industrial systems becomes apparent when considering that wear and friction are responsible for about a quarter of the total global energy losses. Conventional green lubricants, such as vegetable oils, offer less environmental hazards than oil-based lubricants. On the other hand, vegetable oils have low thermal and oxidation stability and poor flow properties at low temperatures, which has led us to use glycerol as a green alternative. Hence, in recent years, glycerol-based lubricants have been utilized as a highly desirable candidate with low cost and better low-temperature characteristics than vegetable oils. Glycerol also show significantly lower friction than vegetable and mineral oils; however, the reason for this is not quite understood. Some studies have claimed that water is formed due to the severe shearing of the lubricant film, while other studies claims it is a rheological effect.
Our research aims to develop a computational framework to simulate the glycerol-aqueous solution lubricant between two metal oxide surfaces and investigate its atomic-scale behavior under real application relevant thermodynamic conditions. Our goal is to understand the considerably low friction characteristics of glycerol-based lubricants at the molecular and atomic scale using molecular dynamics (MD) simulation. Furthermore, we will develop the computational framework to describe lubrication at high temperatures and pressure in order to test the influence of new lubricant-additive compounds in a mechanistic context. As a target, we will evaluate the lubricating performance of different mixtures that lead to improved properties in reducing wear and friction.
To establish a maximum proficiency between the computational and the experimental efforts, our model systems will consist of hundreds of thousands to millions of atoms. The central part of atomic models consists of a lubricant layer confined between two metal oxide surfaces. To implement shear deformation in the lubricant, we will require movement of the metal oxide slabs using steered non-equilibrium MD.
In each series of simulations, different chemical compositions of the lubricant are used to take the concentrations and additives effects into account. In addition, through the use of reactive force field in our classical MD simulations, the chemical adsorption of glycerol deterioration byproducts on the metal oxide surface and its consequences of surface films with regard to sliding will be studied. The simulation of lubrication at the molecular level will help to discover how glycerol/aqueous solutions reduce the friction coefficient between surfaces, and at the same time, maintains the wear volume loss at a low level. At the atomic scale, some of the physical and functional parameters of the system, such as lubricant viscosity and friction coefficient, can be obtained through Green-Kubo expression, which relates the ensemble average of the auto-correlation of the stress/pressure tensor to viscosity. We will simulate it using non-equilibrium MD using a shearing simulation box coupled to an appropriate thermostat.