A multiscale, integrated experimental–analytical approach to mitigate boundary friction using lubricant–surface system perspective

An overarching challenge for the automotive industry is the improvement in fuel efficiency and reduction of harmful emissions. The automotive industry has seen considerable technological improvements over the recent decades, enhancing efficiency and reducing emissions. However, there have also been some unintended effects as many sub-systems, in particular the load bearing conjunctions, are now required to operate at more severe conditions (load, temperature, speed, and with lubricants of lower viscosity). The frictional losses in these conjunctions account for 15-20 % of all Internal Combustion (IC) the engine power losses, 45% of which is attributed to the piston-cylinder system. This translates to 7-8% losses at the piston compression rings. Therefore, the optimisation of the tribological performance of piston ring-cylinder liner interface in internal combustion engines is clearly quite important. The frictional performance at the piston rings conjunctions can be optimised through selection of suitable rings’ and liner surface materials and topography with compatible engine lubricant, which should be viewed as lubricant-surface systems. Selecting the cylinder liner surfaces is a significant challenge for engine specialists, tribologists and material scientists alike. Identification of appropriate surfaces is typically conducted in an empirical manner or through mimicry of tried and tested exemplars found in the current engineering practice. Similarly, the engine lubricants used in the automotive sector are formulated with various additive packages, largely through empirical knowledge and market requirements.This thesis presents a lubricant-surface integrated multi-scale system approach, incorporating atomic force microscopy (nano and meso-scales), microscale tribometry and continuum contact mechanics models to characterise the typical cylinder liner surfaces and lubricant formulations. The component level investigation at microscale is conducted through sliding-strip tribometry with atomic force microscopy in lateral force mode, employed to analyse the nanoscale characteristics of asperity level contacts. The relevant continuum contact mechanics models are used for nanoscale interactions and an analytical contact model, considering boundary and viscous contributions to generated friction are used to represent tribometric tests. Tribofilms are activated through tribometric tests, operating in mixed and boundary regimes of lubrication. Activation occurs with different lubricant formulations due to application of pressure, shear and control of surface temperature of contacting solids. Tribometry is also employed to measure in-situ microscale coefficient of friction. The samples are then subjected to lateral force microscopy in AFM to measure friction of samples pre and post-tribometry. Changes in friction are due to the formation of tribofilms on sample surfaces, composition of which is ascertained through use of X-ray photoelectron spectroscopy (XPS). AFM is also used with stiff cantilevers to determine the modulus of elasticity of the sample surfaces, which provides information about the formed tribofilms as well as input data for nanoscopic contact mechanics.The thesis shows asperity-scale friction of cylinder liner surfaces is dependent on the modulus of elasticity of surfaces, interfacial shear strength and free surface energy. When a tribofilm is generated on a sample surface, asperity level measurements provide the pressure coefficient of boundary shear strength of the tribofilm, which is required for accurate prediction of friction at the microscale contact analysis. In this manner an integrated multi-scale analysis is also achieved. The contact behaviour of combination of dispersants, anti-wear, organic and inorganic friction modifiers are investigated. The content of dispersant is shown to affect Zinc DialkylDithioPhosphate (ZDDP) and organic friction modifier tribofilm formation on the surface, whereas the inorganic friction modifiers show insignificant dependence on the concentration of the dispersant. The major contribution to knowledge is the development of experimental and analytical procedure to characterise the frictional performance of cylinder liners and tribofilm, formed by the mutual interaction of important additives, using a combined experimental and analytical approach at multi-scale is not hitherto reported in literature.