Advances in lubrication technology and modelling. Novel nanoscale friction modifiers - Piezoviscosity effect in EHL contacts

Abstract

Lubricant additives have an important influence on the lubrication performances. These additives are active ingredients which can be added during a blending process to base oils in order either to enhance the existing performance of the base fluids or to impart new properties that the base fluids lack. In modern industry, the ever growing demand on the duration and efficiency of machineries stimulates the research for lubricant additives with better performance. At the same time, industry is facing increasingly rigorous environmental regulations. Compared with the traditional organic lubricant additives that contain P, S and Cl elements, novel additives with environmental friendly feature are certainly becoming more desirable in the future, and research for the novel lubricant additives with good tribological properties and low environmental impact becomes important. Most of lubricant oils at present contain several critical additives, including antiwear additive, dispersant, detergent, friction modifier, viscosity index improver and antioxidant. Traditionally, lubricant oils are presented as a single phase material in order to maintain a good consistency and dispersibility of the additives in the base oil. However, a great amount of research has been focused on introducing solid particles as a friction reduction or antiwear additive over recent years due to a number of incomparable advantages of the two-phase lubricant oils (liquid-solid), such as the superior thermal conductivity, the high pressure standing ability, high resistance to decomposition at temperature, low environmental impact, etc. Some of the solid lubricant additives, particularly in nano or submicron size, have even demonstrated better tribological performances than the traditional organic additives, Zinc dialkyldithiophosphates (ZDDP) for instance. Due to the diversity of the materials, there are still many controversies about their behaviour in a base lubricant and their lubricating mechanisms although many potential candidates have been tested as the solid lubricant additives and many of them has shown the excellent tribological properties. The major drawback of solid lubricant additives, the intrinsic poor stability in liquid base lubricant systems, has considerably restrained them from applications. Therefore the research on exploring novel solid lubricant additives and the techniques that would improve their dispersibility in base lubricants is certainly required. Furthermore, many machine elements, lubricated by fluid film have surfaces that do not conform to each other, so small lubricated areas must then carry the load. The lubrication area of a nonconformal conjunction is typically three orders of magnitude less than that of a conformal conjunction. In general, the lubrication area between nonconformal surfaces enlarges considerably with increasing load, but it is still smaller than the lubrication area between conformal surfaces. Accordingly, the lubrication regime, known as elastohydrodynamic lubrication (EHL) is the typical regime of lubrication characterized by high contact pressures and, therefore, significant elastic deformation and piezoviscous increase in lubricant viscosity. For this reason, a good and accurate prediction of the EHL behaviour, in several different applications, requires consideration of the constitutive equation for the lubricant. Actually, the EHL has two primary aspects: the increase of viscosity with pressure, and the elastic deformation, caused by high pressure, which is comparable to that of the film thickness. Due to the high pressure and the limited contact area, elastic deformation of the surfaces will occur and it is not negligible, as well as the pressure dependence of viscosity play a crucial role in EHL simulation because the viscosity at the inlet has crucial influence on film formation. In particular, for applications involving lubricants 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. Hence, this Ph.D. thesis aims, also, to emphasize the importance of implementing piezoviscous models with accurate treatment methods in EHL applications. In order to a better modelling of the lubricant rheology Doolittle developed the first free-volume model based on a physical meaning, that the resistance to flow in a liquid depends upon the relative volume of molecules present per unit of free volume. Using an exponential function, Doolittle related viscosity to the fractional free volume. Summarizing, this research aimed to study:  Firstly, the influence of different solid lubricant additives (organic additives, such as graphene nanosheets, and inorganic additives, such as inorganic fullerene-like tungsten and molybdenum disulphide nanoparticles) on friction and wear properties of the base lubricants.  Secondly, by means of numerical simulations: The effects of different pressure–viscosity relationships (including the exponential model, the Roelands model and overall the so called Yasutomi-Doolittle free-volume model) for different operating parameters and surface characterization in EHL lubrication The lubrication mechanism of a perfectly spherical nanoparticle, with size and shape close to those of the inorganic fullerene like nanoparticle (IF-NPs), in order to understand the lubrication mechanism and how this kind of solid additive could affect the tribological performance of mating surfaces. The above presented research fields have been studied in different ways:  Exploring the influence of different types of solid lubricant additives on the friction and wear properties of the base lubricants.  Interpreting the mechanisms of the effects of these solid lubricant additives on tribological performances through tribochemical analysis. Using numerical simulations techniques (such as full multigrid numerical code and finite element simulations) to investigate the lubricant piezoviscous property in EHL regime and to understand IF-NPs deformation and lubrication mechanism at nanoscale. [edited by author]