FLOW AND FRICTION OF THIN LUBRICANT FILMS IN HIGH PRESSURE CONTACTS

A key and urgent challenge in mechanical engineering is to increase the efficiency of machine components and thereby reduce energy consumption. Nationally this is needed to meet CO2 emission limits, to help cope with rising fuel costs, and to reduce dependence on imported energy supplies. In global terms, the environmental impact of increased machine use in countries such as China and India as these become prosperous can only be mitigated by large increases in machine efficiency.

One of the main ways to increase machine efficiency is to reduce friction between moving surfaces. A very important source of friction in machine components originates in elastohydrodynamic (EHD) lubricated contacts. These occur in components based on elements that both roll and slide together, including in all rolling bearings, gears and cam/follower systems. EHD friction is of growing importance since, in combination with churning losses, it controls the efficiency of mechanical transmissions. It thus contributes directly to vehicle efficiency but also to the efficiency of many other machines, such as wind turbines and industrial gearboxes. We need to understand EHD friction both to predict it (during machine design) and to reduce it significantly via lubricant and surface design.

The conditions within an EHD lubricant contact are extraordinarily severe; the pressure is usually > 1 GPa; the shear rate is typically 106 to 108 s-1; film temperature can rise by > 100 degree Celcius within the contact. Under these conditions even the simplest liquids are piezoviscous and highly non-Newtonian, exhibiting both viscoelastic behavior and extensive shear thinning. The EHD friction is determined by this non-Newtonian response. Hence, to predict EHD friction and thus the efficiency of machine components, rheological equations are needed that describe the way that shear stress depends on strain rate for lubricant films in EHD contacts. Unfortunately there is currently a fundamental disagreement in the tribology community as to the form of these constitutive equations. The uncertainty arises because we are unable to probe in any detail the shear stress/strain rate behavior of thin lubricant films under the very severe conditions present in EHD contacts. This disagreement and confusion about the flow behavior of lubricants in EHD contacts is unfortunate and damaging since it has impeded the development and acceptance of computer-based models to predict EHD friction of engineering components, as well as diverting attention from the challenge of devising molecular structures that minimize this friction.

It is thus clear that we need an experimental method of studying and quantifying the local flow behavior of thin lubricant films at the extreme conditions present in EHD contacts. The research team has very recently developed a laser-induced imaging approach to obtain the through-thickness velocity profiles of confined viscous fluids and has shown that the rheology of such fluids in EHD contact is non-Newtonian and highly complex. The proposed project builds on research experience in the previous work and the goal of the current proposal is to develop such a new methodology to examine the rheology of realistic, low viscosity lubricants in high stress, high shear rate EHD contacts. The newly developed method will then be applied to explore the impact of lubricant molecular structure, experimental conditions and surface conditions on EHD flow behaviour. Fluorescence spectroscopy will also be used to measure local viscosity, pressure and temperature in EHD contacts. These results will be combined to check the validity of existing EHD rheological models will be tested and new models developed if necessary.

1. Academic Impact: The proposed work will have direct impact on our understanding of and on future directions of research in elastohydrodynamic (EHD) lubrication but also extends beyond this into other areas of tribology and fluid dynamics in general. This is fully described in 'Academic Beneficiaries'.

2. Economic and Societal Impact: The proposed work will resolve current confusion about the rheology of EHD lubricants (as present in gears, rolling bearings and cams). This will directly impact the development and application of reliable models to predict friction and identify how low friction can be obtained via lubricant and surface design in EHD contacts . These will contribute to industry and the national economy by enabling design of energy efficient machine systems and components. These will benefit the Society by reducing both CO2 emissions and the consumption of expensive energy resources. It has been estimated that a 10% reduction in friction in passenger cars world-wide would reduce global CO2 emissions by 160M tonnes (with a saving in fuel cost of 100 billion Euros) per annum.

The project will provide essential input and validation to molecular dynamic simulation of thin lubricant films under EHD conditions. Such models have the potential in future to enable the a-priori molecular design of low (and high) friction lubricants, which will be of immense value in lubricant design.

As well as lubrication and tribology, the proposed work and the methodology developed therein will have economic and societal impact on other areas of application such as polymer and food processing, crude oil extraction, microfluidics, micro-/nano-reactors, and 3-D printing. A few examples are as follows.
Co-extrusion of multi-layer polymer films: These films can consist of tens of layers, each of which is extruded through a sub-micron orifice under high shear rate. The interest in producing co-extruded multi-layer films lies on its superior mechanical and more importantly barrier properties, which are crucial for food and medicine preservation. The proposed work will provide information on how the structure and rheology of complex fluids are affected by the shear process and confinement. This will aid establishment of the process-structure-property relationships necessary for successful product production.

Crude oil extraction: Oil is now being extracted in more extreme conditions than ever and the transport of oils from reservoir involves changes in pressure which can lead to formation of aggregates. The fluorescence methods developed in this proposed project can be used to study confinement and pressure-induced morphological changes, such as phase inversion, and micro- and nano-phase separation, which should lead to better dispersants for crude oil.

Foam forming and food processing: Foams are frequently used to create lightweight materials. Snacks, such as cheese puff, are also made of foam. The structure of the foams determines the density and mechanical properties/crunchiness of the materials. It has been shown that foams have peculiar rheology under the application of shear stress. Hence the knowledge obtained from proposed work can be transferred to understand how processing controls foam formations.

3. People Impact: Tribology is interdisciplinary by nature, and this is very much the case in this proposed project. During this project the PDRA will develop advanced skills in tribology, rheology, complex fluids, surface engineering and materials science and hence become very versatile and employable. Many industries will find the availability of such a trained expert of great value. These include the oil and gas, lubricants and additives, manufacturing automotive, aerospace, pharmaceuticals and healthcare and food etc. The PDRA will also work closely with other PDRAs and students in the Tribology Group and hence members of the Group will learn and benefit from the PDRA's own initial and developing expertise.